International Journal of Cardiology, 37 (1992) 131-143 0 1992 Elsevier Science Publishers B.V. All rights reserved
CARD10
131 0167-5273/92/$05.00
01539
Effects of duration of pressure overload on the reversibility of impaired coronary autoregulation in rats Fumitoshi
Sato, Shogen
Isoyama
and Tamotsu
Takishima
First Department of Internal Medicine, Tohoku University School of Medicine, Sendai, Japan (Received
10 April 1992; accepted
22 April 1992)
Sato F, Isoyama S, Takishima T. Effects of duration of pressure overload on the reversibility of impaired coronary autoregulation in rats. Int J Cardiol 1992;37:131-143. The aim of this study was to determine the effects of duration of pressure overload on the reversibility of impaired coronary autoregulation in hypertropied hearts. The experiments were performed on 38 anesthetized male Wistar rats aged 6 to 8 weeks. The ascending aorta was banded for 4 or 10 weeks, then in some rats the bands were removed for 4 weeks. We estimated coronary hemodynamics in a model consisting of isolated non-working hearts perfused with Tyrode’s solution containing bovine red blood cells and serum albumin. Myocardial mass increased significantly in 4 and lo-week banded groups compared to controls. Four weeks after debanding in 4- and lo-week banded groups, the value returned to that of controls. Autoregulation gain was significantly lower in banded groups than in controls in the range between 50 and 100 mmHg of coronary perfusion pressure. Although the gain normalized in the debanded group after 4 weeks of banding, the value in the debanded groups after 10 weeks of banding remained less than zero between 25 and 150 mmHg of perfusion pressure. In transient flow response to a stepwise increase of perfusion pressure within the autoregulatory range, promptly increased flow was followed by more rapid and greater decrease in controls than in banded groups. The flow response regressed in the debanded group after 4 weeks of banding, while it remained unchanged in the debanded group after 10 weeks of banding. Thus, duration of pressure overload alters the regression of impaired coronary autoregulation in cardiac hypertrophy. Key words: Coronary circulation; Hypertrophy;
Regression; Isolated heart; Aortic constriction
Introduction In cardiac hypertrophy produced by pressure overload, it has been shown that coronary auCorrespondence to: S. Isoyama, M.D., First Department of Internal Medicine, Tohoku University School of Medicine, 1-l Seiryo-machi, Aoba-ku, Sendai 980, Japan. Tel. (022) 274-l 1 Il. Fax (022) 233-7985. This study was partly supported by Grant-in-Aid for Scientific Research (No 63570379) from the Ministry of Education, Science and Culture of Japan.
toregulation is impaired [l-3]. Harrison et al. [l] showed an impairment of the lower range of autoregulation in the subendocardium in conscious dogs with renal hypertension. In the study of Jeremy et al. [2] the lower limit of autoregulation shifted to a higher perfusion pressure in anesthetized canine hearts. In our previous study [3], autoregulation is abolished over the wide range of perfusion pressure in isolated rat hearts with coarctation hypertension. Moreover, we showed that after relief of pressure overload the
132
impaired autoregulation is normalized in shortterm cardiac hypertrophy. Using an experimental model of coarctation hypertension and cardiac hypertrophy, we have reported that pressure overload and cardiac hypertrophy decrease coronary vasodilator capacity [3,4], and after relief of pressure overload, the decreased vasodilator capacity is normalized in short-term overload but not in long-term overload [4]. Therefore, we hypothesized that the duration of pressure overload may also affect the reversibility of impaired coronary autoregulation during regression of hypertrophy. To test this hypothesis, we employed short- and long-term ascending aortic banding and debanding methods in rats. After sacrifice of the banded and debanded rats, we evaluated steady-state coronary perfusion pressure-flow relationships and transient flow responses to stepwise changes in perfusion pressure using an isolated, beating but nonworking heart preparation perfused with oxygenated Tyrode’s solution containing bovine red blood cells and serum albumin.
Materials and Methods We used male Wistar rats of 6 to 8 weeks of age and banded the ascending aorta. After 4 or 10 weeks of banding, bands were removed in some of the rats. Using isolated hearts perfused with oxygenated Tyrode’s solution containing bovine red blood cells and serum albumin, we studied coronary hemodynamics in the following groups of rats: 4-week-banded rats (n = 6), loweek-banded rats (n = 7), rats debanded after 4 weeks of banding (n = 7), rats debanded after 10 weeks of banding (n = 6), and their respective sham-operated controls, i.e., controls for the 4week-banded rats and lo-week-banded rats, and controls for rats debanded after 4 weeks of banding and rats debanded after 10 weeks of banding. The control data for the two banded groups and for the two debanded groups of rats were combined and identified as their respective combined control groups (n = 6 and n = 6, respectively) for each group of the banded or debanded group of rats. Four or 10 weeks after banding in the banded groups and 4 weeks after debanding in the two
debanded groups in vivo left ventricular and aortic pressures and coronary hemodynamics were estimated. In the experiment, rats were treated in accordance with the Declaration of Helsinki and the Guiding Principles in the Care and Use of Animals. Also, the experimental and animal care protocol was approved by the Animal Care Committee of the Tohoku University School of Medicine. Production of pressure-overloaded cardiac hypertrophy and estimation of pressure overload
aortic banding, [4-61. Each rat was sodium (50 and ventilated artificially (Model 141, Princeton Inc., Natick, MA). left thoracotomy, aorta and a tube (1.4 mm in diameter) nylon surgical thread (3-O). The tube was quickly removed. chest was closed immediately after flated with a positive end-expiratory pressure After the endotracheal standard rat chow and ad libitum for 4 or 10 weeks. rats, the same procedures except for aortic banding. After weeks of aortic banding, bands rats. The left thorax opened artificial ventilation with room air intraperitoneal anesthesia and traintubation. Fibrotic tissue around nylon thread which aortic banding in the operation nylon thread taken off. The was closed anesthetized
ation was performed rats. Four or 10 banding in the banded groups and 4 debanding banded groups, in vivo aortic Under anesthesia
133
endotracheal tube, a polyethylene cannula (Polyethylene Tubing, SP-31, 0.50 mm i.d., 0.80 mm o.d., Natsume Instrument Co., Ltd., Tokyo, Japan) was inserted into the right carotid artery [5-71. Following thoracotomy, the left ventricular cavity was punctured at the base with a 21-gauge needle connected to a strain gauge pressure transducer (Model TP-300T, Nihon Kohden Co., Ltd., Tokyo). Left ventricular peak systolic and end-diastolic pressures and peak-to-peak pressure difference between the left ventricle and the aorta were estimated. Zero pressure reference was taken at the midlevel of the heart. The pressures were continuously displayed on a multichannel recorder (Type 8S, Rectigraph, San-ei Instrument Co., Ltd., Tokyo). The damping coefficient and undamped natural frequency of the pressure measurement system were 0.56 and 41 Hz, respectively. Preparation
of isolated perfused hearts
After in vivo pressure measurements, the pericardium was opened, and the heart was quickly isolated. A perfusion cannula was inserted into the ascending aorta (at the proximal position of the thread which had been used for aortic banding in the groups of banded rats) and was positioned immediately above the aortic valve. The heart was mounted on a perfusion apparatus [5,8,9]. The perfusion apparatus consists of a rotary pump, water-jacketed reservoir and bubble trap. The bubble trap was pressurized with a gas tank and a pressure regulator (Pressure Regulator Type 70, Bellofram, Burlington, MA). The coronary arteries were perfused via the aortic root with the solution containing bovine red blood cells and serum albumin. A drainage cannula was inserted into the left ventricular cavity through a left atria1 incision to vent the Thebesian flow. The temperature of the solution was maintained at 37°C with the water-jacketed reservoir. Heart rate was kept constant at 300 beats/min by right ventricular pacing with an electrical stimulator (Electrical Stimulator SEN 7103, Nihon Kohden Co., Ltd., Tokyo). The perfusate was composed of a modified Tyrode’s solution containing oxygenated bovine red blood cells and 15 g/l bovine
serum albumin (No. A-4503, Sigma Chemical Co., Ltd., St. Louis, MO) [3,10]. The specific electrolyte concentrations of the modified Tyrode’s solution were (in mmol/l): NaCl, 110.0; CaCl,, 2.5; KCl, 6.0; MgCl,, 1.0; NaH,PO,, 0.435; NaHCO,, 32.0; Glucose, 11.0. Bovine serum albumin was filtered through a 0.8 pm membrane filter (Advantec, Toyo Roshi Kaisha, Ltd., Tokyo) with a sufficient amount of Tyrode’s solution at 4°C. Fresh bovine blood was collected at a local slaughterhouse in polyethylene bottles with sodium heparin to prevent coagulation and transported on ice. The blood was centrifuged at 4°C for 20 min at 2600 rpm. After aspirating the plasma and buffy coat, the cells were washed with ice-cold buffer solution. The red blood cells were added to the modified Tyrode’s solution containing bovine serum albumin. The perfusate was equilibrated with a 20% O,, 3% CO,, and 77% N, gas mixture by bubbling. Final perfusate pH, P02’ Pco, and hematocrit were measured just before entering the hearts, and were 7.39 + 0.02 (mean k SEMI, 138 f 5 mmHg, 23 &-1 mmHg and 32 & l%, respectively. Coronary perfusion pressure was measured with a pressure transducer from the side arm of the perfusion cannula. Zero pressure reference was taken at the midlevel of the heart. Mean coronary flow was measured using an extracorporeal-type probe (Model FF-OlOT. Nihon Kohden Co., Ltd., Tokyo) of 1 mm internal diameter which was positioned in the perfusion line, and an electromagnetic flowmeter (Model MFV-3100, Nihon Kohden Co., Ltd., Tokyo). The time constant of the electrical circuit used to obtain mean flow was 1.0 s. Calibration was performed by timed volume sampling in each heart. Coronary
hemodynamic
studies
Coronary hemodynamic data were obtained after coronary flow had reached a steady-state at 100 mmHg of coronary perfusion pressure. The steady-state of coronary flow at 100 mmHg of perfusion pressure was achieved approximately 15 min after initiation of coronary perfusion. We elevated coronary perfusion pressure to 150 mmHg and measured coronary flow at a new
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steady state. Coronary perfusion pressure was then reduced successively to 125, 112, 100, 87, 75 and 50 mmHg in the control and experimental groups of rats. At each level of coronary perfusion pressure, mean coronary flow was measured in the steady state. Next, we evaluated transient flow response to a stepwise increase of perfusion pressure from 50 to 100 mmHg. After coronary flow reached a steady state at 100 mmHg of perfusion pressure, perfusion pressure was elevated to 150 mmHg in a stepwise fashion. The final experimental measurements of coronary flow were completed at 25 mmHg of perfusion pressure in order to avoid irreversible myocardial damage by ischemia. Preceding the final experimental measurements, to estimate reactivity of the coronary vascular system, we measured reactive hyperemic response following a 40-s &hernia at 100 mmHg of perfusion pressure in hearts of sham-operated control rats. Peak/ resting flow ratio and repayment/debt ratio were as follows: 2.8 f 0.2 and 1.8 k 0.2 in the controls for the banded groups; 2.7 IL-0.2 and 1.7 + 0.2 in the controls for the debanded groups. These values were consistent with those in our previous study [3] and greater than those in in situ rat hearts reported by Peters et al. [ll]. Initial and final coronary flows at 100 mmHg of perfusion pressure showed high stability as followings (in ml/min/g)r 2.7 f 0.3 and 2.8 + 0.3 in the sham-
operated controls for the banded groups; 2.9 f 0.2 and 3.2 + 0.3 in the 4-week banded group; 2.7 f 0.3 and 3.1 f 0.5 in the lo-week banded group; 2.6 f 0.2 and 3.0 + 0.2 in the sham-operated controls for the debanded groups; 2.1 k 0.2 and 2.3 of:0.4 in the 4-week debanded group; 2.0 + 0.2 and 2.1 f 0.2 in the lo-week debanded group. The weight of the left ventricle including the septum and the weight of the right ventricular free wall were measured. Coronary flow was expressed as the value per left ventricular weight including the septum (ml/min/g). Autoregulation gain was calculated by the following equation: Gain = 1 - [(flow change/initial flow)/ (pressure change/initial pressure)] [1,12]. Transient flow response to a stepwise increase of perfusion pressure was evaluated in terms of the following parameters: decrement of flow (ml/min/g) = peak flow - final flow at each elevated perfusion pressure; T1/3 (s) = time from the point of peak flow to that of one-third of peak flow after stepwise increase of perfusion pressure. Statistical analysis Variables measured are expressed as mean + SEM. The statistical significance of differences in mean values between sham-operated and experimental groups was assessed by using an unpaired Student’s t-test with the Bonferroni correction.
TABLE I Iq vivo hemodynamics in sham-operated,
banded and debanded groups of rats.
Variables
Sham(B) (n = 6)
4W-B(6)
low-B(7)
Sham(DBX6)
4WB-DB(7)
lOWB-DB(6)
Syst. LVP CmmHg) LVEDP (mmHg) Syst. AoP (mmHg) Diast. AoP CmmHg) Mean AoP (mmHg) AP (mmHg) HR (beats/min)
112+ 2+ 108-1 78 + 96k 5+ 389 +
177 + 4+ 107 + 82+ 92+ 71 t 377 +
170 f 13 * 3 f 0.4 108& 9 85 f 10 92 + 10 62+12* 390 * 31
120+ 3* 111+ 91+ 99+ 9+ 358 +
103+ 6 3 f 0.3 97+ 7 73* 5 83+ 5 6+ 3 342 f 23
125+ 3+ 108+ 78& 88f 17* 406 f
8 9.5 7 10 7 1 12
10 * 1.2 11 9 10 14 * 7
9 0.8 11 11 12 2 14
8 0.3 5 6 6 3 25
Values are mean f SEM. Sham(B) and Sham(DB) = sham-operated controls for banded and debanded groups of rats; 4W- and lOW-B = banded group of rats for 4 or 10 weeks; 4WB- and lOWB-DB = debanded groups of rats after 4 or 10 weeks of banding; Syst LVP = peak systolic left ventricular pressure; LVEDP = left ventricular end-diastolic pressure; Syst., Diast. and Mean AoP = systolic, diastolic and mean aortic pressure; AP = peak-to-peak difference between the LVP and the AoP; HR = heart rate. *p < 0.05 vs. sham-operated control group of rats.
135
3oo\(A)
Results
0 Sham(B)
Table 1 summarizes in vivo hemodynamic data from the 6 groups. In both the banded groups of rats, peak systolic left ventricular pressure and peak-to-peak pressure difference between the left ventricle and the aorta were significantly higher than those in the sham operated group of rats. Four weeks after debanding in the 4- and lo-week banded rats, both peak systolic left ventricular pressure and peak-to-peak pressure difference between the left ventricle and the aorta returned to the values of the sham-operated group of rats. There were no differences in systolic, diastolic and mean aortic pressures, left ventricular enddiastolic pressure, or heart rate between shamoperated and experimental groups of rats. Fig. 1 shows the degree of cardiac hypertrophy in the 6 groups. In the banded groups of rats, left ventricular weight/body weight ratios increased to 141% and 137% of the sham-operated group of rats in the 4- and IO-week banded groups of rats, respectively. Four weeks after debanding in the 4- and lo-week banded rats, the ratios decreased to 112% and 106% of the sham-operated group of rats, respectively. Right ventricular
0 4w-B
! n lOW-B
B
E : 5
loo
B 0 B ag 200
9
0
25
300
50
75 CPP
(mm
100
125
150
Hgl
(6) .
Sham(DB)
n 4WB-DB
.
lOWB-DB
z p
I /9/f
200
E 8 r
tii
loo
!! 6
ap
I
0
L
p
25
‘
50 CPP
Sham (6)
4W -6
1OW -6
;Mm (DS)
4WB -DB
1OWB -DS
Fig. 1. Left ventricular weight/body weight ratios in the sham-operated, banded and debanded groups of rats. Values are mean + SEM. LVW = left ventricular weight (mg); BW = body weight (g); Sham(B) and Sham(DB) = sham-operated controls for banded and debanded groups of rats; 4W- and lOW-B = banded groups of rats for 4 or 10 weeks; 4WB- and IOWB-DB = debanded groups of rats after 4 or 10 weeks of banding. * p < 0.05 vs. sham-operated control group of rats.
100 (mm Hg)
75
125
150
Fig. 2. Steady-state coronary perfusion pressure-flow relationships in the sham-operated, banded (A) and debanded groups of rats (B). Coronary flow is expressed as percentage of the value at 100 mmHg of perfusion pressure. Values are mean f SEM. CFR = coronary flow rate; CPP = coronary perfusion pressure; Sham(B) and Sham(DB)-sham-operated control groups for the banded and debanded groups of rats; 4Wand IOW-B = banded groups of rats for 4 or 10 weeks; 4WBand lOWB-DB = debanded groups of rats after 4 or 10 weeks of banding. * p < 0.05 vs. sham-operated control group of rats.
weight/body weight ratios did not differ between sham-operated and experimental groups of rats. Fig. 2 shows the steady-state relationships between coronary perfusion pressure and flow expressed as percentages of the values at 100 mmHg of perfusion pressure. In both the sham-operated
136
groups of rats (Figs. 2A, B), the relationships exhibited autoregulation between 50 and 100 mmHg of perfusion pressure. On the other hand, the autoregulation in both the 4- and lo-week banded groups of rats was impaired over a wide range of perfusion pressure levels, and flows were significantly lower than those of the sham-operated group of rats between 50 and 100 mmHg of perfusion pressure (Fig. 2A). In the debanded group of rats after 4-week banding, the relationship was similar to that in the sham-operated group of rats between 50 and 100 mmHg of perfusion pressure, and there were no difference in flow compared to the value in the sham-operated control group. However, in the debanded group of rats after IO-week banding, the autoregulation remained impaired over the range of perfusion pressures, and the flow was significantly lower than that in the sham-operated group of rats between 50 and 100 mmHg of perfusion pressure (Fig. 2B). Fig. 3 shows autoregulation gains obtained from steady-state coronary perfusion pressure flow relationships. In the sham-operated group for the banded groups of rats, the gains were greater than zero between 50 and 100 mmHg of perfusion pressure and significantly higher than those of both the banded groups of rats (Fig. 3A). In the debanded group of rats after 4-week banding, the gains returned to values greater than zero between 50 and 87 mmHg of perfusion pressure. In th debanded group of rats after lo-week banding, however, those values remained less than zero over the whole range of perfusion pressure, and significantly lower than those in the sham-operated group of rats between 50 and 100 mmHg of perfusion pressure (Fig. 3B). Fig. 4 shows typical tracings of transient coronary flow responses to stepwise increases of perfusion pressure from 50 to 100 mmHg (Fig. 4A) and from 100 to 150 mmHg (Fig. 4Bl in hearts of the sham-operated, banded and debanded groups of rats. As shown in Fig. 4A, in hearts of the sham-operated groups prompt increase of flow was followed by a marked and rapid decrease to a steady-state flow: AF of 2.5 ml/min/g (peak flow minus final flow at 100 mmHg of perfusion pressure) and T/13 of 36 s. In contrast, in hearts
+0.5
(A)
d
-1.01 0
1
25
50
75
4
l
100
125
150
125
150
c=(mHg)
0
25
50
75
100
cpp(fnmHg)
Fig. 3. Autoregulation gains in the sham-operated, banded (A) and debanded groups of rats (B). Values are mean + SEM. Sham(B) and Sham(DB) = sham-operated control groups for the banded and debanded groups of rats; 4W- and lOW-B = banded groups of rats for 4 or 10 weeks; 4WB- and lOWB-DB = debanded groups of rats after 4 or 10 weeks of banding. * p < 0.05 vs. sham-operated control group of rats.
of banded groups of rats the increased flow linearly and gradually decreased to a steady-state flow (AF of 1.2 ml/min/g and T1/3 of 68 s in the 4-week banded group; AF of 0.5 ml/min/g and T1/3 of 78 s in the lo-week banded group). In hearts of the debanded group of rats after 4-week banding, the response returned to that of the hearts of the sham-operated group (AF = 3.6 ml/min/g; T1/3 = 34 s); however, in hearts of
137
between the hearts of the sham-operated and experimental groups. Table 2 summarizes coronary flows following stepwise increases of perfusion pressure from 50 to 100 mmHg and from 100 to 150 mmHg. In
the debanded group of rats after lo-week banding it remained unchanged (AF = 1.9 ml/min/g; T1/3 = 66 s). As shown in Fig. 4B, coronary flow responses to a stepwise increase in perfusion pressure from 100 to 150 mmHg did not differ
(A) CPP
50+100
mm w
1.0
-’
0 i 5.0,
4w-0
6.0
F=1.2
6ham
F=l,4(mbmin-1.9-l)
5.0 4.0
6.0 5.0 4.0
Fig. 4. Typical tracings of coronary flow responses to stepwise increases of perfusion pressure from 50 to 100 mmHg (A) and from 100-150 mmHg (B) of perfusion pressure in hearts of control, banded and debanded groups of rats. CPP = coronary perfusion pressure; CFR = coronary flow rate (ml/min); AF = peak flow - final flow at elevated perfusion pressure (ml/min/g); T1/3 = time from the point of peak flow to that of one-third of peak flow; Sham = sham-operated control groups of rats; 4W- and IOW-B = banded groups of rats for 4 or 10 weeks; 4WB- and IOWB-DB = debanded groups of rats after 4 or 10 weeks of banding.
138
both the banded groups of rats, steady-state coronary flow at 50 mmHg of perfusion pressure, peak and final steady-state flows at 100 mmHg of perfusion pressure did not differ from those of the sham-operated group of rats. In both the debanded groups of rats, coronary flow at 50 mmHg of perfusion pressure was significantly lower than that of the sham-operated group of rats. In the debanded group of rats after IO-week banding, peak and final steady-state flows at 100 mmHg of perfusion pressure were lower than those of the sham-operated group of rats. How(A) CPPso+100
.5
lh shan (W
4w
low
-B
-8
(B) CPP wo+lso
ever, increment of flow (peak flow after stepwise increase of perfusion pressure minus steady-state flow before stepwise increase of perfusion pressure) did not differ between the sham-operated and experimental groups of rats. After stepwise increase of perfusion pressure to 150 mmHg, the flows at peak response and at the new steady-state and increment of flow did not differ between sham-operated and experimental groups of rats. Fig. 5 shows the coronary flow responses to stepwise increases of perfusion pressure from 50 to 100 mmHg (Fig. 5A) and from 100 to 150
mm h
mm fig
a
Fig. 5. Transient flow responses to stepwise increases of perfusion pressure from 50-100 mmHg (A) and from 100-150 mmHg (B) in control, banded and debanded groups of rats. Values are mean f SEM. CPP = coronary perfusion pressure; Decrement of flow = peak flow - final flow at elevated perfusion pressure; T1/3 = time from the point of peak flow to that of one-third of peak flow; sham(B) and Sham(DB) = sham-operated control groups for banded and debanded groups of rats; 4W- and lOW-B = banded groups of rats for 4 or 10 weeks; 4WB- and lOWB-DB = debanded groups of rats after 4 or 10 weeks of banding. * p < 0.05 vs. sham-operated control group of rats.
139
TABLE Coronary
2 flow rates following
Variables
stepwise
Sham(B)
CPP Changes from SO to 100 mmHg 1.6 f 0.2 CFR(50) 5.0 + 0.5 Peak CFR(100) 2.8 + 0.3 Final CFR(100) CPP changes from 100 to 150 mmHg CFR(10) Peak CFR(150) Final CFR(150)
2.8 jr 0.3 6.9 + 0.7 4.9 + 0.5
increases
of perfusion
pressure.
4W-B
IOW-B
Sham(DB)
4WB-DB
lOWB-DB
1.2 * 0.2 4.8 f 0.7 3.2 + 0.3
1.2 + 0.2 4.2 + 0.8 3.1 + 0.5
1.5 * 0.1 5.0 f 0.3 3.0 * 0.2
1.0 f 0.2 * 4.2 + 0.6 2.3 f 0.4
0.8+0.1* 3.2 f 0.4 * 2.1 f 0.2 *
3.2 f 0.3 5.8 + 0.5 4.7 f 0.7
3.1 + 0.5 6.1 f 0.9 4.7 f 0.7
3.0 f 0.2 6.3 f 0.6 5.0 f 0.4
2.3 f 0.4 5.0 f 0.6 4.0 + 0.4
2.1 *0.2* 4.8 f 0.8 3.8 + 0.7
Values are mean f SEM. CPP = coronary perfusion pressure; CFR(50) or (100) = steady-state coronary flow rates (ml/mitt/g) at 50 or 100 mmHg of perfusion pressure; peak CFR(100) or (150) = peak coronary flow rates (ml/mitt/g) immediately after stepwise increases of perfusion pressure to 100 or 150 mmHg; final CFR(100) or (150) = steady-state coronary flow rates (ml/min/g) at 100 or 150 mmHg of perfusion pressure. Other abbreviations as in Table 1. * p < 0.05 vs. sham-operated control group of rats.
mmHg (Fig. 5B) in the sham-operated, banded and debanded groups of rats. In the sham-operated groups of rats, the decrement of flow was significantly greater and T1/3 was smaller in the autoregulatory pressure range (50 to 100 mmHg), compared to those in the non-autoregulatory range (100 to 150 mmHg). In the banded groups of rats decrement of flow was significantly smaller and T1/3 was greater than those in the sham-operated group of rats when coronary perfusion pressure was stepwisely elevated from 50 to 100 mmHg (Fig. 5A). In the debanded group of rats after 4-week banding both parameters were similar to those in the sham-operated group of rats. In the debanded group of rats after lo-week banding, however, decrement of flow was significantly smaller and T1/3 was greater compared to the sham-operated group of rats. As shown in Fig. 5B, transient flow responses to stepwise increases in perfusion pressure from 100 to 150 mmHg did not differ between the sham-operated and experimental groups of rats. Discussion
We studied the effects of duration of pressure overload on coronary autoregulation during the progression and regression of cardiac hypertrophy. In the present study, we estimated coronary autoregulation in terms of steady-state pressureflow relationships and transient flow responses to stepwise changes of perfusion pressure. After re-
lief of pressure overload, impaired coronary autoregulation reversed to normal in short-term cardiac hypertrophy (4 weeks) but remained unchanged in long-term hypertrophy (10 weeks). Therefore, duration of pressure overload affects the regression process of impaired coronary autoregulation in hypertropied hearts. To date, there are no other reports concerning the effect of duration of pressure overload and cardiac hypertrophy on coronary autoregulation and the factors which affect the reversibility of impaired autoregulation. Advantages model
and limitations
of our experimental
In the present study, we employed the ascending aortic banding method and also the debanding method in rats. Ascending aortic banding provides pressure-overloaded cardiac hypertrophy while minimizing the effects of neurohumoral factors, and debanding discretely terminates the pressure overload resulting in a specific duration of pressure overload. Furthermore, we used an isolated non-working heart model and minimized the effects of extravascular and myocardial metabolic factors on coronary circulation. Gamble et al. 1131 reported that resting coronary flow in isolated non-working rat hearts perfused with rat whole blood from support animals was 2.0 ml/min/g. In the study of Peters et al. [ill, resting flow in in situ rat hearts of 3 months
140
of age was 4.4 ml/min/g. In the present study, resting flow at 100 mmHg of perfusion pressure was 2.7 ml/min/g in the sham-operated controls for the banded groups and 2.6 ml/min/g in those for the debanded groups, and is consistent with the values measured in in situ and isolated hearts by other investigators [11,13-151. Additionally, in the present study, coronary flow rates per unit mass were almost equal between sham-operated control and hypertropied hearts. These equal flow rates per unit mass under resting conditions are also consistent with the observations reported from other laboratories [11,15]. Furthermore, resting coronary flow at 100 mmHg of perfusion pressure was quite stable throughout the measurements. The flow increased by only 7% at the conclusion of the experiments in the sham-operated groups, 13% in the banded groups and 7% in the debanded groups. Peters et al. [ll] have reported that in small laboratory animals such as rats, reactive hyperemit response is smaller even in the in vivo experimental model because of elevated basal metabolic rates. We could observe marked reactive hyperemic responses to brief ischemia in the sham-operated groups. The peak/ resting flow ratio was near 3.0, and the value was greater than that of 2.6 reported by Peters et al. [ill. The marked response of coronary flow following stepwise changes of perfusion pressure was also observed. In the previous study, we inserted a balloon into the left ventricular cavity and measured isovolumic left ventricular pressures at end-diastolic pressure of 10 mmHg and a coronary perfusion pressure of 100 mmHg [5]. Peak systolic pressures in sham-operated, banded and debanded groups were comparable with those in studies reported from other laboratories [8-10,161. As described above, our experimental model showed resting coronary flow and left ventricular pressure generation approximately identical to those in experimental models reported from other laboratories, high stability of resting coronary flow, and great reactivity of coronary vasculature. Autoregulation gain in the sham-operated control groups was not high compared to the value in large laboratory animals such as dogs [2,12]. In
non-working guinea pig hearts perfused with crystalloid solution, the autoregulatory range was between 25 and 55 mmHg, and the gain values were less than 0.5 [17]. To date, there is no data available concerning the autoregulation gain in rat hearts perfused with blood. Autoregulation gain in small animals may be lower than that in larger animals. Small autoregulation gain in the sham-operated controls and impairments of coronary autoregulation .in hypertrophied hearts would show the characteristics of coronary vasculature per se. Comparisons
with previous studies
In the study of Harrison et al. [l] in situ heart models of dogs with renovascular hypertension and hypertrophy showed impairments of coronary autoregulation in the subendomyocardium in the lower range of perfusion pressure. Also, Jeremy et al. demonstrated a rightward shift of the lower limit of the autoregulatory range [2]. Of interest, in the present study, the steady-state pressure flow relationships in short- and long-term banded groups did not show a shift of coronary regulatory plateau to a higher range of perfusion pressure, but showed impairments of coronary autoregulation over a whole range of perfusion pressure. The degree of myocardial hypertrophy estimated by the ratio of left ventricular weight to body weight did not differ between our model and theirs [1,2]. The duration of pressure overload, however, was 4 and 10 weeks in our study, 3 to 6 months in the study of Harrison et al. [ll and 6 to 12 weeks in the study of Jeremy et al. [2]. Since the life span of rats is shorter than that of dogs, the duration of pressure overload relative to life span in our study may be construed as longer than that of their studies. Because the onset of pressure overload and the development of cardiac hypertrophy appear to be faster in our experimental models with aortic banding than in theirs with renovascular hypertension, not only the duration of hypertrophy [15,181 but also the more rapid onset of hypertension and hypertrophy might prevent the coronary vasculature from adapting and maintaining autoregulation at the higher perfusion pressure [19,20]. In addition, the
141
differences in neurohumoral factors between the renovascular hypertension model and ascending aortic banding model might cause the discrepancy in results. Although the exact reason for these different results is not clear, the differences in the experimental models are likely to be the most important. Coronary autoregulation pressure overload
before and after relief of
We estimated coronary autoregulation in terms of steady-state pressure - flow relationships and transient flow responses to stepwise changes of perfusion pressure. In steady-state pressure flow relationships, coronary autoregulation was observed in the range between 50 and 100 mmHg of perfusion pressure in controls. After debanding, the autoregulatory gains completely returned to normal in short-term banding but did not regress in long-term banding. In transient flow responses to stepwise changes of perfusion pressure, we could determine the extent and speed of flow regulation in the coronary vasculature by the amount of flow decrement and by the time from the point of peak flow to that of one-third of peak flow. Mosher et al. showed that prompt increase of flow after a step increase of perfusion pressure was followed by an immediate return to an initial flow level in the autoregulatory pressure range and was then followed by gradual flow reduction in the nonautoregulatory pressure range [21]. Our observations of flow changes in shamoperated controls are consistent with their findings. The rapid flow reduction followed by gradual reduction was observed in the autoregulatory range of perfusion pressure in the control groups and the debanded group after short-term banding. This observation suggests that the first rapid reduction in flow may be the result of active regulation by arterial trees (arterioles) and that the second gradual reduction may not reflect the active arterial function. Although the steady-state pressure - flow relationship provides unequivocal evidence for autoregulation, transient flow responses to stepwise changes of perfusion pressure could characterize the speed of flow regulation in
the coronary vasculature at a constant perfusion pressure. The speed of the regulation was always in parallel with the extent of flow regulation. The parallel changes in flow regulation were observed in all the sham-operated control, banded and debanded groups. Therefore, the extent and speed of flow regulation was determined by the presence or absence of autoregulation in the pressure level studied, but not by chronic pressure overload or myocardial hypertrophy. In the present study, total myocardial mass increased by 40% and total metabolic demand would presumably also be increased in hypertrophied hearts, even though we used non-working hearts paced at a constant rate. However, coronary flows per unit myocardial mass in the experiment on transient flow responses to stepwise increase of perfusion pressure from 50 to 100 mmHg did not differ between the control and banded groups. It is reasonable to assume that increased metabolic demand in hypertropied hearts would have been balanced by increased total coronary flows. Therefore, the metabolic effect of myocardium at 100 mmHg of perfusion pressure on the differences between the control and banded groups would be minimal, and the different transient flow responses to stepwise changes in perfusion pressure in the autoregulatory range might be the result of myogenic rather than metabolic regulation [22-251. Even if such flow responses are mainly regulated by metabolic mechanisms, similar flow in the peak response at 100 mmHg of perfusion pressure in the control and banded groups should wash out metabolic substrates in a similar manner. Furthermore, the time required for the flow regulation, even in the first rapid reduction, was longer than 30 s. Such a long period of time would not be required to wash out substrates from the myocardial tissue. Therefore, changes in autoregulation after banding and debanding may be the result of changes in the coronary arterial wall. The changes in the speed of flow regulation in response to stepwise increases of perfusion pressure especially may reflect smooth muscle function of the vascular wall. It is possible that contractile properties of smooth muscles in chronic hypertension were different from those in controls. Also, endothelial cells
142
influence the arterial vascular tone by endothelium-derived relaxing factors [26,27]. Kelm et al. [28] reported that vascular endothelial cells contribute to the maintenance of coronary vascular tone through release of endothelium-dependent relaxing factor(s). Since chronic hypertension impairs the endothelial function [29-311 and modulates arterial vascular tone [32], it is possible that modulation of coronary vascular tone by the endothelium was diminished in the banded groups. impaired autoregulation After debanding, completely regressed in short-term pressure overload and did not regress in long-term pressure overload. Increases in perfusion pressure may cause changes in myocardial oxygen demand (the Gregg phenomenon) [33]. In the present study, we did not directly measure myocardial oxygen consumption nor did we estimate the influence of the Gregg phenomenon on the coronary perfusion pressure - flow relationships. Even if, during measurements of coronary hemodynamics, the Gregg phenomenon might slightly modify the configuration of the pressure - flow relationships, it is unlikely that this phenomenon caused the differing reversibilities of the coronary autoregulation in hearts debanded after short- and long-term banding. This complete regression in short-term pressure overload might be caused by the reversal of smooth muscle hypertrophy [34,351 and/or endothelial dysfunction [36,37] if the endothelium has a role in controlling coronary vascular tone [28,38]. In long-term pressure overload, increased extracellular proteins such as collagen and elastin may alter the characteristics of the vascular wall [39-411, and the regression of collagen deposition after debanding may not be sufficient to normalize the properties of the vascular wall. Furthermore, subendothelial thickening may produce uncoupling between the endothelium and smooth muscle cells [421 even if the endothelial dysfunction regressed. From our present study it is not clear what size of arterioles mainly modulate coronary autoregulation [24,43, 441 and how coronary arterial hypertension impairs autoregulation at the microcirculation level. Further studies are needed to determine how short- and long-term pressure overload impair arteriolar function, and what changes determine
the reversibility of impaired coronary autoregulation. In conclusion, we studied the effects of duration of pressure overload on reversibility of impaired coronary autoregulation. After relief of pressure overload, impaired autoregulation completely returned to normal in short-term pressure overload, while it remained unchanged in spite of regression of myocardial hypertrophy in long-term pressure-overload. Thus, the duration of pressure overload and cardiac hypertrophy alters the regression of impaired coronary autoregulation. References DG, Florentine MS, Brooks LA, Cooper SM, 1 Harrison Marcus ML. The effect of hypertension and left ventricular hypertrophy on the lower range of coronary autoregulation. Circulation 1988;77:1108-1115. 2 Jeremy RW, Fletcher PJ, Thompson J. Coronary pressureflow relations in hypertensive left ventricular hypertrophy. Comparison of intact autoregulation with physiological and pharmacological vasodilation in the dog. Cir Res 1989;65:224-236. T. Normalization of im3 Sato F, Isoyama S, Takishima paired coronary circulation in hypertropied rat hearts. Hypertension 1990;16:26-34. 4 Ito N, lsoyama S, Takishima T. Duration of pressure-overload alters regression of coronary circulation abnormalities. Am J Physiol 1990;258:H1753-H1760. S, Ito N, Kuroha M, Takishima T. Complete 5 lsoyama reversibility of physiological coronary vascular abnormalities in hypertropied hearts produced by pressure-overload in the rat. J Clin Invest 1989;84:288-294. 6 Isoyama S, Wei JY, Izumo S, Fort P, Schoen FJ, Grossman W. Effect of age on the development of cardiac hypertrophy produced by aortic constriction in the rat. Circ Res 1987;61:337-345. Isoyama S, Grossman W, Wei JY. Effect of age on myocardial adaptation to volume overload in the rat. J Clin Invest 1988;81:1850-1857. Isoyama S, Apstein CS, Grice WM. Lore11 BH. Acute decrease in left ventricular diastolic chamber distensibility during simulated angina in isolated hearts. Circ Res 1987;61:925-933. Lore11 BH, Isoyama S, Grice WM, Weinberg EO, Apstein CS. Effect of ouabain and isoproterenol on left ventricular diastolic function during low-flow ischemia in isolated, blood-perfused rabbit hearts. Circ Res 1988;63:457-467. Marshall RC, Nash WW, Bersohn MM, Wong GA. Myocardial energy production and consumption remain balanced during positive inotropic stimulation when coronary flow is restricted to basal rates in rabbit heart. J Clin Invest 1987;80:1165-1171.
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11 Peters KG, Wangler RD, Tomanek R, Marcus ML. Effects of long-term cardiac hypertrophy on coronary vasodilator reserve in SHR rats. Am J Cardiol 1984;54:13421348. 12 Dole WP. Autoregulation of the coronary circulation. Prog Cardiovasc Dis 1987;29:293-323. 13 Gamble WJ, Conn PA, Kumar AE, Plenge R, Monroe RG. Myocardial oxygen consumption of blood perfused, isolated supported rat heart. Am J Physiol 1970;219:604612. 14 Duvelleroy MA, Duruble M, Martin JL, Teisseire B, Droulez J, Cain M. Blood-perfused working isolated rat heart. J Appl Physiol 1976;41:603-607. 15 Wangler, RD, Peters KG, Marcus ML, Tomanek RJ. Effects of duration and severity of arterial hypertension and cardiac hypertrophy on coronary vasodilator reserve. Circ Res 1982;51:10-18. 16 Lorell BH, Wexler LF, Momomura S. Weinberg EO, Apstein CS. The influence of pressure overload left ventricular hypertrophy on diastolic properties during hypoxia in isovolumically contracting hearts. Circ Res 1986;58:653663. 17 Toma BS, Wangler RD, DeWitt DF, Sparks HV Jr. Effect of development on coronary vasodilator reserve in the isolated guinea pig heart. Circ Res 1985:57:538-544. 18 Scheel KW, Eisenstein BL. Ingram LA. Coronary, collateral, and perfusion territory responses to aortic banding. Am J Physiol 1984;246:H768-H775. 19 Bathe RJ. Effects of hypertrophy on the coronary circulation. Prog Cardiovasc Dis 1988;30:403-440. 20 Manohar M, Parks CM, Busch MA et al. Regional myocardial blood flow and coronary vascular reserve in unanesthetized young calves exposed to a simulated altitude of 3500 m for 8-10 weeks. Circ Res 1982;50:714-726. 21 Mosher P, Ross J Jr, McFate PA, Shaw RF. Control of coronary blood flow by an autoregulatory mechanism. Circ Res 1964:14:250-259. 22 Griffith TM, Edwards DH. Myogenic autoregulation of flow may be inversely related to endothelium-derived relaxing factor activity. Am J Physiol 1990;258:H1171-Hl180. 23 Johnson PC. Autoregulation of blood flow. Circ Res 1986;59:483-495. 24 Kuo L, Davis MJ, Chilian WM. Myogenic activity in isolated subepicardial and subendocardial coronary arterioles. Am J Physiol 1988;255:H1558-H1562. 25 McHale PA. Dube GP. Greenfield JC Jr. Evidence for myogenic vasomotor activity in the coronary circulation. Prog Cardiovasc Dis 1987;30:139-146. 26 Gardiner SM. Compton AM, Bennett T, Palmer RMJ, Moncada S. Control of regional blood flow by endothelium-derived nitric oxide. Hypertension 1990;15:486-492. 27 Griffith TM, Edwards DH, Davies RLI, Harrison TJ, Evans KT, EDRF coordinates the behaviour of vascular resistance vessels. Nature 1987;329:442-445. 28 Kelm M. Schrader J. Control of coronary vascular tone by nitric oxide. Circ Res 1990;66:1561-1.575.
29 Diederich D, Yang Z, Buhler FR, Luscher TF. Impaired endothelium-dependent relaxations in hypertensive resistance arteries involve cyclooxygenase pathway. Am J Physiol 1990;258:H44-H45 1. 30 Lockette W, Otsuka Y, Carretero 0. The loss of endothehum-dependent vascular relaxation in hypertension. Hypertension 1986;8(suppl II):61-66. 31 Liischer TF, Vanhoutte PM. Endothelium-dependent contractions to acetylcholine in the aorta of the spontaneously hypertensive rat. Hypertension 1986;8:344-348. 32 Linder L, Kiowski W, Buhler FR, Liischer TF. Indirect evidence for release of endothelium-derived relaxing factor in human forearm circulation in viva. Blunted response in essential hypertension. Circulation 1990;81:1762-1767. 33 Greg DE. Effect of coronary perfusion pressure or coronary flow on oxygen usage of the myocardium. Circ Res 1963;13:497-500. 34 Anderson PC, Bishop SP, Stanley BD. Vascular remodeling and improvement of coronary reserve after hydralazine treatment in spontaneously hypertensive rats. Circ Res. 1989;64:1127-1136. 35 Ito N, Isoyama S, Takishima T. Effects of coronary vascular medial hypertrophy on decreased coronary perfusion in pressure-overloaded cardiac hypertrophy (abstract). Clin Res 1990;38:33lA. 36 Armer A, Malmquist U, Uvelius B. Structural and mechanical adaptation in rat aorta in response to sustained changes in arterial pressure. Acta Physiol Stand 1984;122: 119-126. 37 Liischer TF, Vanhoutte PM, Raij L. Antihypertensive treatment normalizes decreased endothelium-dependent relaxations in rats with salt-induced hypertension. Hypertension 1987;9(suppl 111):193-197. 38 Hodgson J, Marshall JJ. Mechanism of acetylcholine effects on coronary flow and arterial diameter in patients with non-stenotic coronary arteries. Circulation 1989;79: 1043-1051. 39 Ehrhart LA, Ferrario CM. Collagen metabolism and reversal of aortic medial hypertrophy in spontaneously hypertensive rats treated with methyldopa, Hypertension 1981;3:479-484. 40 Gilligan JP, Spector S. synthesis of collagen in cardiac and vascular walls. Hypertension 1984;6(suppl 111):44-49. 41 Haudenschild CC, Prescott MF, Chobanian AV. Effects of hypertension and its reversal on aortic intima lesions of the rat. Hypertension 1982;2:33-44. 42 Van de Vorde J, Leusen I. Endothelium-dependent and independent relaxation of aortic rings from hypertensive rats. Am J Physiol 1986;250:H711-H717. 43 Chilian WM. Eastham CL. Marcus ML. Microvascular distribution of coronary vascular resistance in beating left ventricle. Am J Physiol 1986;251:H779-H788. 44 Kuo L, Chilian WM. Davis MJ. Coronary arteriolar myogenie response is independent of endothelium. Circ Res 1990366:860-866.
CARD10
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01539
Effects of duration of pressure overload on the reversibility of impaired coronary autoregulation in rats Fumitoshi
Sato, Shogen
Isoyama
and Tamotsu
Takishima
First Department of Internal Medicine, Tohoku University School of Medicine, Sendai, Japan (Received
10 April 1992; accepted
22 April 1992)
Sato F, Isoyama S, Takishima T. Effects of duration of pressure overload on the reversibility of impaired coronary autoregulation in rats. Int J Cardiol 1992;37:131-143. The aim of this study was to determine the effects of duration of pressure overload on the reversibility of impaired coronary autoregulation in hypertropied hearts. The experiments were performed on 38 anesthetized male Wistar rats aged 6 to 8 weeks. The ascending aorta was banded for 4 or 10 weeks, then in some rats the bands were removed for 4 weeks. We estimated coronary hemodynamics in a model consisting of isolated non-working hearts perfused with Tyrode’s solution containing bovine red blood cells and serum albumin. Myocardial mass increased significantly in 4 and lo-week banded groups compared to controls. Four weeks after debanding in 4- and lo-week banded groups, the value returned to that of controls. Autoregulation gain was significantly lower in banded groups than in controls in the range between 50 and 100 mmHg of coronary perfusion pressure. Although the gain normalized in the debanded group after 4 weeks of banding, the value in the debanded groups after 10 weeks of banding remained less than zero between 25 and 150 mmHg of perfusion pressure. In transient flow response to a stepwise increase of perfusion pressure within the autoregulatory range, promptly increased flow was followed by more rapid and greater decrease in controls than in banded groups. The flow response regressed in the debanded group after 4 weeks of banding, while it remained unchanged in the debanded group after 10 weeks of banding. Thus, duration of pressure overload alters the regression of impaired coronary autoregulation in cardiac hypertrophy. Key words: Coronary circulation; Hypertrophy;
Regression; Isolated heart; Aortic constriction
Introduction In cardiac hypertrophy produced by pressure overload, it has been shown that coronary auCorrespondence to: S. Isoyama, M.D., First Department of Internal Medicine, Tohoku University School of Medicine, 1-l Seiryo-machi, Aoba-ku, Sendai 980, Japan. Tel. (022) 274-l 1 Il. Fax (022) 233-7985. This study was partly supported by Grant-in-Aid for Scientific Research (No 63570379) from the Ministry of Education, Science and Culture of Japan.
toregulation is impaired [l-3]. Harrison et al. [l] showed an impairment of the lower range of autoregulation in the subendocardium in conscious dogs with renal hypertension. In the study of Jeremy et al. [2] the lower limit of autoregulation shifted to a higher perfusion pressure in anesthetized canine hearts. In our previous study [3], autoregulation is abolished over the wide range of perfusion pressure in isolated rat hearts with coarctation hypertension. Moreover, we showed that after relief of pressure overload the
132
impaired autoregulation is normalized in shortterm cardiac hypertrophy. Using an experimental model of coarctation hypertension and cardiac hypertrophy, we have reported that pressure overload and cardiac hypertrophy decrease coronary vasodilator capacity [3,4], and after relief of pressure overload, the decreased vasodilator capacity is normalized in short-term overload but not in long-term overload [4]. Therefore, we hypothesized that the duration of pressure overload may also affect the reversibility of impaired coronary autoregulation during regression of hypertrophy. To test this hypothesis, we employed short- and long-term ascending aortic banding and debanding methods in rats. After sacrifice of the banded and debanded rats, we evaluated steady-state coronary perfusion pressure-flow relationships and transient flow responses to stepwise changes in perfusion pressure using an isolated, beating but nonworking heart preparation perfused with oxygenated Tyrode’s solution containing bovine red blood cells and serum albumin.
Materials and Methods We used male Wistar rats of 6 to 8 weeks of age and banded the ascending aorta. After 4 or 10 weeks of banding, bands were removed in some of the rats. Using isolated hearts perfused with oxygenated Tyrode’s solution containing bovine red blood cells and serum albumin, we studied coronary hemodynamics in the following groups of rats: 4-week-banded rats (n = 6), loweek-banded rats (n = 7), rats debanded after 4 weeks of banding (n = 7), rats debanded after 10 weeks of banding (n = 6), and their respective sham-operated controls, i.e., controls for the 4week-banded rats and lo-week-banded rats, and controls for rats debanded after 4 weeks of banding and rats debanded after 10 weeks of banding. The control data for the two banded groups and for the two debanded groups of rats were combined and identified as their respective combined control groups (n = 6 and n = 6, respectively) for each group of the banded or debanded group of rats. Four or 10 weeks after banding in the banded groups and 4 weeks after debanding in the two
debanded groups in vivo left ventricular and aortic pressures and coronary hemodynamics were estimated. In the experiment, rats were treated in accordance with the Declaration of Helsinki and the Guiding Principles in the Care and Use of Animals. Also, the experimental and animal care protocol was approved by the Animal Care Committee of the Tohoku University School of Medicine. Production of pressure-overloaded cardiac hypertrophy and estimation of pressure overload
aortic banding, [4-61. Each rat was sodium (50 and ventilated artificially (Model 141, Princeton Inc., Natick, MA). left thoracotomy, aorta and a tube (1.4 mm in diameter) nylon surgical thread (3-O). The tube was quickly removed. chest was closed immediately after flated with a positive end-expiratory pressure After the endotracheal standard rat chow and ad libitum for 4 or 10 weeks. rats, the same procedures except for aortic banding. After weeks of aortic banding, bands rats. The left thorax opened artificial ventilation with room air intraperitoneal anesthesia and traintubation. Fibrotic tissue around nylon thread which aortic banding in the operation nylon thread taken off. The was closed anesthetized
ation was performed rats. Four or 10 banding in the banded groups and 4 debanding banded groups, in vivo aortic Under anesthesia
133
endotracheal tube, a polyethylene cannula (Polyethylene Tubing, SP-31, 0.50 mm i.d., 0.80 mm o.d., Natsume Instrument Co., Ltd., Tokyo, Japan) was inserted into the right carotid artery [5-71. Following thoracotomy, the left ventricular cavity was punctured at the base with a 21-gauge needle connected to a strain gauge pressure transducer (Model TP-300T, Nihon Kohden Co., Ltd., Tokyo). Left ventricular peak systolic and end-diastolic pressures and peak-to-peak pressure difference between the left ventricle and the aorta were estimated. Zero pressure reference was taken at the midlevel of the heart. The pressures were continuously displayed on a multichannel recorder (Type 8S, Rectigraph, San-ei Instrument Co., Ltd., Tokyo). The damping coefficient and undamped natural frequency of the pressure measurement system were 0.56 and 41 Hz, respectively. Preparation
of isolated perfused hearts
After in vivo pressure measurements, the pericardium was opened, and the heart was quickly isolated. A perfusion cannula was inserted into the ascending aorta (at the proximal position of the thread which had been used for aortic banding in the groups of banded rats) and was positioned immediately above the aortic valve. The heart was mounted on a perfusion apparatus [5,8,9]. The perfusion apparatus consists of a rotary pump, water-jacketed reservoir and bubble trap. The bubble trap was pressurized with a gas tank and a pressure regulator (Pressure Regulator Type 70, Bellofram, Burlington, MA). The coronary arteries were perfused via the aortic root with the solution containing bovine red blood cells and serum albumin. A drainage cannula was inserted into the left ventricular cavity through a left atria1 incision to vent the Thebesian flow. The temperature of the solution was maintained at 37°C with the water-jacketed reservoir. Heart rate was kept constant at 300 beats/min by right ventricular pacing with an electrical stimulator (Electrical Stimulator SEN 7103, Nihon Kohden Co., Ltd., Tokyo). The perfusate was composed of a modified Tyrode’s solution containing oxygenated bovine red blood cells and 15 g/l bovine
serum albumin (No. A-4503, Sigma Chemical Co., Ltd., St. Louis, MO) [3,10]. The specific electrolyte concentrations of the modified Tyrode’s solution were (in mmol/l): NaCl, 110.0; CaCl,, 2.5; KCl, 6.0; MgCl,, 1.0; NaH,PO,, 0.435; NaHCO,, 32.0; Glucose, 11.0. Bovine serum albumin was filtered through a 0.8 pm membrane filter (Advantec, Toyo Roshi Kaisha, Ltd., Tokyo) with a sufficient amount of Tyrode’s solution at 4°C. Fresh bovine blood was collected at a local slaughterhouse in polyethylene bottles with sodium heparin to prevent coagulation and transported on ice. The blood was centrifuged at 4°C for 20 min at 2600 rpm. After aspirating the plasma and buffy coat, the cells were washed with ice-cold buffer solution. The red blood cells were added to the modified Tyrode’s solution containing bovine serum albumin. The perfusate was equilibrated with a 20% O,, 3% CO,, and 77% N, gas mixture by bubbling. Final perfusate pH, P02’ Pco, and hematocrit were measured just before entering the hearts, and were 7.39 + 0.02 (mean k SEMI, 138 f 5 mmHg, 23 &-1 mmHg and 32 & l%, respectively. Coronary perfusion pressure was measured with a pressure transducer from the side arm of the perfusion cannula. Zero pressure reference was taken at the midlevel of the heart. Mean coronary flow was measured using an extracorporeal-type probe (Model FF-OlOT. Nihon Kohden Co., Ltd., Tokyo) of 1 mm internal diameter which was positioned in the perfusion line, and an electromagnetic flowmeter (Model MFV-3100, Nihon Kohden Co., Ltd., Tokyo). The time constant of the electrical circuit used to obtain mean flow was 1.0 s. Calibration was performed by timed volume sampling in each heart. Coronary
hemodynamic
studies
Coronary hemodynamic data were obtained after coronary flow had reached a steady-state at 100 mmHg of coronary perfusion pressure. The steady-state of coronary flow at 100 mmHg of perfusion pressure was achieved approximately 15 min after initiation of coronary perfusion. We elevated coronary perfusion pressure to 150 mmHg and measured coronary flow at a new
134
steady state. Coronary perfusion pressure was then reduced successively to 125, 112, 100, 87, 75 and 50 mmHg in the control and experimental groups of rats. At each level of coronary perfusion pressure, mean coronary flow was measured in the steady state. Next, we evaluated transient flow response to a stepwise increase of perfusion pressure from 50 to 100 mmHg. After coronary flow reached a steady state at 100 mmHg of perfusion pressure, perfusion pressure was elevated to 150 mmHg in a stepwise fashion. The final experimental measurements of coronary flow were completed at 25 mmHg of perfusion pressure in order to avoid irreversible myocardial damage by ischemia. Preceding the final experimental measurements, to estimate reactivity of the coronary vascular system, we measured reactive hyperemic response following a 40-s &hernia at 100 mmHg of perfusion pressure in hearts of sham-operated control rats. Peak/ resting flow ratio and repayment/debt ratio were as follows: 2.8 f 0.2 and 1.8 k 0.2 in the controls for the banded groups; 2.7 IL-0.2 and 1.7 + 0.2 in the controls for the debanded groups. These values were consistent with those in our previous study [3] and greater than those in in situ rat hearts reported by Peters et al. [ll]. Initial and final coronary flows at 100 mmHg of perfusion pressure showed high stability as followings (in ml/min/g)r 2.7 f 0.3 and 2.8 + 0.3 in the sham-
operated controls for the banded groups; 2.9 f 0.2 and 3.2 + 0.3 in the 4-week banded group; 2.7 f 0.3 and 3.1 f 0.5 in the lo-week banded group; 2.6 f 0.2 and 3.0 + 0.2 in the sham-operated controls for the debanded groups; 2.1 k 0.2 and 2.3 of:0.4 in the 4-week debanded group; 2.0 + 0.2 and 2.1 f 0.2 in the lo-week debanded group. The weight of the left ventricle including the septum and the weight of the right ventricular free wall were measured. Coronary flow was expressed as the value per left ventricular weight including the septum (ml/min/g). Autoregulation gain was calculated by the following equation: Gain = 1 - [(flow change/initial flow)/ (pressure change/initial pressure)] [1,12]. Transient flow response to a stepwise increase of perfusion pressure was evaluated in terms of the following parameters: decrement of flow (ml/min/g) = peak flow - final flow at each elevated perfusion pressure; T1/3 (s) = time from the point of peak flow to that of one-third of peak flow after stepwise increase of perfusion pressure. Statistical analysis Variables measured are expressed as mean + SEM. The statistical significance of differences in mean values between sham-operated and experimental groups was assessed by using an unpaired Student’s t-test with the Bonferroni correction.
TABLE I Iq vivo hemodynamics in sham-operated,
banded and debanded groups of rats.
Variables
Sham(B) (n = 6)
4W-B(6)
low-B(7)
Sham(DBX6)
4WB-DB(7)
lOWB-DB(6)
Syst. LVP CmmHg) LVEDP (mmHg) Syst. AoP (mmHg) Diast. AoP CmmHg) Mean AoP (mmHg) AP (mmHg) HR (beats/min)
112+ 2+ 108-1 78 + 96k 5+ 389 +
177 + 4+ 107 + 82+ 92+ 71 t 377 +
170 f 13 * 3 f 0.4 108& 9 85 f 10 92 + 10 62+12* 390 * 31
120+ 3* 111+ 91+ 99+ 9+ 358 +
103+ 6 3 f 0.3 97+ 7 73* 5 83+ 5 6+ 3 342 f 23
125+ 3+ 108+ 78& 88f 17* 406 f
8 9.5 7 10 7 1 12
10 * 1.2 11 9 10 14 * 7
9 0.8 11 11 12 2 14
8 0.3 5 6 6 3 25
Values are mean f SEM. Sham(B) and Sham(DB) = sham-operated controls for banded and debanded groups of rats; 4W- and lOW-B = banded group of rats for 4 or 10 weeks; 4WB- and lOWB-DB = debanded groups of rats after 4 or 10 weeks of banding; Syst LVP = peak systolic left ventricular pressure; LVEDP = left ventricular end-diastolic pressure; Syst., Diast. and Mean AoP = systolic, diastolic and mean aortic pressure; AP = peak-to-peak difference between the LVP and the AoP; HR = heart rate. *p < 0.05 vs. sham-operated control group of rats.
135
3oo\(A)
Results
0 Sham(B)
Table 1 summarizes in vivo hemodynamic data from the 6 groups. In both the banded groups of rats, peak systolic left ventricular pressure and peak-to-peak pressure difference between the left ventricle and the aorta were significantly higher than those in the sham operated group of rats. Four weeks after debanding in the 4- and lo-week banded rats, both peak systolic left ventricular pressure and peak-to-peak pressure difference between the left ventricle and the aorta returned to the values of the sham-operated group of rats. There were no differences in systolic, diastolic and mean aortic pressures, left ventricular enddiastolic pressure, or heart rate between shamoperated and experimental groups of rats. Fig. 1 shows the degree of cardiac hypertrophy in the 6 groups. In the banded groups of rats, left ventricular weight/body weight ratios increased to 141% and 137% of the sham-operated group of rats in the 4- and IO-week banded groups of rats, respectively. Four weeks after debanding in the 4- and lo-week banded rats, the ratios decreased to 112% and 106% of the sham-operated group of rats, respectively. Right ventricular
0 4w-B
! n lOW-B
B
E : 5
loo
B 0 B ag 200
9
0
25
300
50
75 CPP
(mm
100
125
150
Hgl
(6) .
Sham(DB)
n 4WB-DB
.
lOWB-DB
z p
I /9/f
200
E 8 r
tii
loo
!! 6
ap
I
0
L
p
25
‘
50 CPP
Sham (6)
4W -6
1OW -6
;Mm (DS)
4WB -DB
1OWB -DS
Fig. 1. Left ventricular weight/body weight ratios in the sham-operated, banded and debanded groups of rats. Values are mean + SEM. LVW = left ventricular weight (mg); BW = body weight (g); Sham(B) and Sham(DB) = sham-operated controls for banded and debanded groups of rats; 4W- and lOW-B = banded groups of rats for 4 or 10 weeks; 4WB- and IOWB-DB = debanded groups of rats after 4 or 10 weeks of banding. * p < 0.05 vs. sham-operated control group of rats.
100 (mm Hg)
75
125
150
Fig. 2. Steady-state coronary perfusion pressure-flow relationships in the sham-operated, banded (A) and debanded groups of rats (B). Coronary flow is expressed as percentage of the value at 100 mmHg of perfusion pressure. Values are mean f SEM. CFR = coronary flow rate; CPP = coronary perfusion pressure; Sham(B) and Sham(DB)-sham-operated control groups for the banded and debanded groups of rats; 4Wand IOW-B = banded groups of rats for 4 or 10 weeks; 4WBand lOWB-DB = debanded groups of rats after 4 or 10 weeks of banding. * p < 0.05 vs. sham-operated control group of rats.
weight/body weight ratios did not differ between sham-operated and experimental groups of rats. Fig. 2 shows the steady-state relationships between coronary perfusion pressure and flow expressed as percentages of the values at 100 mmHg of perfusion pressure. In both the sham-operated
136
groups of rats (Figs. 2A, B), the relationships exhibited autoregulation between 50 and 100 mmHg of perfusion pressure. On the other hand, the autoregulation in both the 4- and lo-week banded groups of rats was impaired over a wide range of perfusion pressure levels, and flows were significantly lower than those of the sham-operated group of rats between 50 and 100 mmHg of perfusion pressure (Fig. 2A). In the debanded group of rats after 4-week banding, the relationship was similar to that in the sham-operated group of rats between 50 and 100 mmHg of perfusion pressure, and there were no difference in flow compared to the value in the sham-operated control group. However, in the debanded group of rats after IO-week banding, the autoregulation remained impaired over the range of perfusion pressures, and the flow was significantly lower than that in the sham-operated group of rats between 50 and 100 mmHg of perfusion pressure (Fig. 2B). Fig. 3 shows autoregulation gains obtained from steady-state coronary perfusion pressure flow relationships. In the sham-operated group for the banded groups of rats, the gains were greater than zero between 50 and 100 mmHg of perfusion pressure and significantly higher than those of both the banded groups of rats (Fig. 3A). In the debanded group of rats after 4-week banding, the gains returned to values greater than zero between 50 and 87 mmHg of perfusion pressure. In th debanded group of rats after lo-week banding, however, those values remained less than zero over the whole range of perfusion pressure, and significantly lower than those in the sham-operated group of rats between 50 and 100 mmHg of perfusion pressure (Fig. 3B). Fig. 4 shows typical tracings of transient coronary flow responses to stepwise increases of perfusion pressure from 50 to 100 mmHg (Fig. 4A) and from 100 to 150 mmHg (Fig. 4Bl in hearts of the sham-operated, banded and debanded groups of rats. As shown in Fig. 4A, in hearts of the sham-operated groups prompt increase of flow was followed by a marked and rapid decrease to a steady-state flow: AF of 2.5 ml/min/g (peak flow minus final flow at 100 mmHg of perfusion pressure) and T/13 of 36 s. In contrast, in hearts
+0.5
(A)
d
-1.01 0
1
25
50
75
4
l
100
125
150
125
150
c=(mHg)
0
25
50
75
100
cpp(fnmHg)
Fig. 3. Autoregulation gains in the sham-operated, banded (A) and debanded groups of rats (B). Values are mean + SEM. Sham(B) and Sham(DB) = sham-operated control groups for the banded and debanded groups of rats; 4W- and lOW-B = banded groups of rats for 4 or 10 weeks; 4WB- and lOWB-DB = debanded groups of rats after 4 or 10 weeks of banding. * p < 0.05 vs. sham-operated control group of rats.
of banded groups of rats the increased flow linearly and gradually decreased to a steady-state flow (AF of 1.2 ml/min/g and T1/3 of 68 s in the 4-week banded group; AF of 0.5 ml/min/g and T1/3 of 78 s in the lo-week banded group). In hearts of the debanded group of rats after 4-week banding, the response returned to that of the hearts of the sham-operated group (AF = 3.6 ml/min/g; T1/3 = 34 s); however, in hearts of
137
between the hearts of the sham-operated and experimental groups. Table 2 summarizes coronary flows following stepwise increases of perfusion pressure from 50 to 100 mmHg and from 100 to 150 mmHg. In
the debanded group of rats after lo-week banding it remained unchanged (AF = 1.9 ml/min/g; T1/3 = 66 s). As shown in Fig. 4B, coronary flow responses to a stepwise increase in perfusion pressure from 100 to 150 mmHg did not differ
(A) CPP
50+100
mm w
1.0
-’
0 i 5.0,
4w-0
6.0
F=1.2
6ham
F=l,4(mbmin-1.9-l)
5.0 4.0
6.0 5.0 4.0
Fig. 4. Typical tracings of coronary flow responses to stepwise increases of perfusion pressure from 50 to 100 mmHg (A) and from 100-150 mmHg (B) of perfusion pressure in hearts of control, banded and debanded groups of rats. CPP = coronary perfusion pressure; CFR = coronary flow rate (ml/min); AF = peak flow - final flow at elevated perfusion pressure (ml/min/g); T1/3 = time from the point of peak flow to that of one-third of peak flow; Sham = sham-operated control groups of rats; 4W- and IOW-B = banded groups of rats for 4 or 10 weeks; 4WB- and IOWB-DB = debanded groups of rats after 4 or 10 weeks of banding.
138
both the banded groups of rats, steady-state coronary flow at 50 mmHg of perfusion pressure, peak and final steady-state flows at 100 mmHg of perfusion pressure did not differ from those of the sham-operated group of rats. In both the debanded groups of rats, coronary flow at 50 mmHg of perfusion pressure was significantly lower than that of the sham-operated group of rats. In the debanded group of rats after IO-week banding, peak and final steady-state flows at 100 mmHg of perfusion pressure were lower than those of the sham-operated group of rats. How(A) CPPso+100
.5
lh shan (W
4w
low
-B
-8
(B) CPP wo+lso
ever, increment of flow (peak flow after stepwise increase of perfusion pressure minus steady-state flow before stepwise increase of perfusion pressure) did not differ between the sham-operated and experimental groups of rats. After stepwise increase of perfusion pressure to 150 mmHg, the flows at peak response and at the new steady-state and increment of flow did not differ between sham-operated and experimental groups of rats. Fig. 5 shows the coronary flow responses to stepwise increases of perfusion pressure from 50 to 100 mmHg (Fig. 5A) and from 100 to 150
mm h
mm fig
a
Fig. 5. Transient flow responses to stepwise increases of perfusion pressure from 50-100 mmHg (A) and from 100-150 mmHg (B) in control, banded and debanded groups of rats. Values are mean f SEM. CPP = coronary perfusion pressure; Decrement of flow = peak flow - final flow at elevated perfusion pressure; T1/3 = time from the point of peak flow to that of one-third of peak flow; sham(B) and Sham(DB) = sham-operated control groups for banded and debanded groups of rats; 4W- and lOW-B = banded groups of rats for 4 or 10 weeks; 4WB- and lOWB-DB = debanded groups of rats after 4 or 10 weeks of banding. * p < 0.05 vs. sham-operated control group of rats.
139
TABLE Coronary
2 flow rates following
Variables
stepwise
Sham(B)
CPP Changes from SO to 100 mmHg 1.6 f 0.2 CFR(50) 5.0 + 0.5 Peak CFR(100) 2.8 + 0.3 Final CFR(100) CPP changes from 100 to 150 mmHg CFR(10) Peak CFR(150) Final CFR(150)
2.8 jr 0.3 6.9 + 0.7 4.9 + 0.5
increases
of perfusion
pressure.
4W-B
IOW-B
Sham(DB)
4WB-DB
lOWB-DB
1.2 * 0.2 4.8 f 0.7 3.2 + 0.3
1.2 + 0.2 4.2 + 0.8 3.1 + 0.5
1.5 * 0.1 5.0 f 0.3 3.0 * 0.2
1.0 f 0.2 * 4.2 + 0.6 2.3 f 0.4
0.8+0.1* 3.2 f 0.4 * 2.1 f 0.2 *
3.2 f 0.3 5.8 + 0.5 4.7 f 0.7
3.1 + 0.5 6.1 f 0.9 4.7 f 0.7
3.0 f 0.2 6.3 f 0.6 5.0 f 0.4
2.3 f 0.4 5.0 f 0.6 4.0 + 0.4
2.1 *0.2* 4.8 f 0.8 3.8 + 0.7
Values are mean f SEM. CPP = coronary perfusion pressure; CFR(50) or (100) = steady-state coronary flow rates (ml/mitt/g) at 50 or 100 mmHg of perfusion pressure; peak CFR(100) or (150) = peak coronary flow rates (ml/mitt/g) immediately after stepwise increases of perfusion pressure to 100 or 150 mmHg; final CFR(100) or (150) = steady-state coronary flow rates (ml/min/g) at 100 or 150 mmHg of perfusion pressure. Other abbreviations as in Table 1. * p < 0.05 vs. sham-operated control group of rats.
mmHg (Fig. 5B) in the sham-operated, banded and debanded groups of rats. In the sham-operated groups of rats, the decrement of flow was significantly greater and T1/3 was smaller in the autoregulatory pressure range (50 to 100 mmHg), compared to those in the non-autoregulatory range (100 to 150 mmHg). In the banded groups of rats decrement of flow was significantly smaller and T1/3 was greater than those in the sham-operated group of rats when coronary perfusion pressure was stepwisely elevated from 50 to 100 mmHg (Fig. 5A). In the debanded group of rats after 4-week banding both parameters were similar to those in the sham-operated group of rats. In the debanded group of rats after lo-week banding, however, decrement of flow was significantly smaller and T1/3 was greater compared to the sham-operated group of rats. As shown in Fig. 5B, transient flow responses to stepwise increases in perfusion pressure from 100 to 150 mmHg did not differ between the sham-operated and experimental groups of rats. Discussion
We studied the effects of duration of pressure overload on coronary autoregulation during the progression and regression of cardiac hypertrophy. In the present study, we estimated coronary autoregulation in terms of steady-state pressureflow relationships and transient flow responses to stepwise changes of perfusion pressure. After re-
lief of pressure overload, impaired coronary autoregulation reversed to normal in short-term cardiac hypertrophy (4 weeks) but remained unchanged in long-term hypertrophy (10 weeks). Therefore, duration of pressure overload affects the regression process of impaired coronary autoregulation in hypertropied hearts. To date, there are no other reports concerning the effect of duration of pressure overload and cardiac hypertrophy on coronary autoregulation and the factors which affect the reversibility of impaired autoregulation. Advantages model
and limitations
of our experimental
In the present study, we employed the ascending aortic banding method and also the debanding method in rats. Ascending aortic banding provides pressure-overloaded cardiac hypertrophy while minimizing the effects of neurohumoral factors, and debanding discretely terminates the pressure overload resulting in a specific duration of pressure overload. Furthermore, we used an isolated non-working heart model and minimized the effects of extravascular and myocardial metabolic factors on coronary circulation. Gamble et al. 1131 reported that resting coronary flow in isolated non-working rat hearts perfused with rat whole blood from support animals was 2.0 ml/min/g. In the study of Peters et al. [ill, resting flow in in situ rat hearts of 3 months
140
of age was 4.4 ml/min/g. In the present study, resting flow at 100 mmHg of perfusion pressure was 2.7 ml/min/g in the sham-operated controls for the banded groups and 2.6 ml/min/g in those for the debanded groups, and is consistent with the values measured in in situ and isolated hearts by other investigators [11,13-151. Additionally, in the present study, coronary flow rates per unit mass were almost equal between sham-operated control and hypertropied hearts. These equal flow rates per unit mass under resting conditions are also consistent with the observations reported from other laboratories [11,15]. Furthermore, resting coronary flow at 100 mmHg of perfusion pressure was quite stable throughout the measurements. The flow increased by only 7% at the conclusion of the experiments in the sham-operated groups, 13% in the banded groups and 7% in the debanded groups. Peters et al. [ll] have reported that in small laboratory animals such as rats, reactive hyperemit response is smaller even in the in vivo experimental model because of elevated basal metabolic rates. We could observe marked reactive hyperemic responses to brief ischemia in the sham-operated groups. The peak/ resting flow ratio was near 3.0, and the value was greater than that of 2.6 reported by Peters et al. [ill. The marked response of coronary flow following stepwise changes of perfusion pressure was also observed. In the previous study, we inserted a balloon into the left ventricular cavity and measured isovolumic left ventricular pressures at end-diastolic pressure of 10 mmHg and a coronary perfusion pressure of 100 mmHg [5]. Peak systolic pressures in sham-operated, banded and debanded groups were comparable with those in studies reported from other laboratories [8-10,161. As described above, our experimental model showed resting coronary flow and left ventricular pressure generation approximately identical to those in experimental models reported from other laboratories, high stability of resting coronary flow, and great reactivity of coronary vasculature. Autoregulation gain in the sham-operated control groups was not high compared to the value in large laboratory animals such as dogs [2,12]. In
non-working guinea pig hearts perfused with crystalloid solution, the autoregulatory range was between 25 and 55 mmHg, and the gain values were less than 0.5 [17]. To date, there is no data available concerning the autoregulation gain in rat hearts perfused with blood. Autoregulation gain in small animals may be lower than that in larger animals. Small autoregulation gain in the sham-operated controls and impairments of coronary autoregulation .in hypertrophied hearts would show the characteristics of coronary vasculature per se. Comparisons
with previous studies
In the study of Harrison et al. [l] in situ heart models of dogs with renovascular hypertension and hypertrophy showed impairments of coronary autoregulation in the subendomyocardium in the lower range of perfusion pressure. Also, Jeremy et al. demonstrated a rightward shift of the lower limit of the autoregulatory range [2]. Of interest, in the present study, the steady-state pressure flow relationships in short- and long-term banded groups did not show a shift of coronary regulatory plateau to a higher range of perfusion pressure, but showed impairments of coronary autoregulation over a whole range of perfusion pressure. The degree of myocardial hypertrophy estimated by the ratio of left ventricular weight to body weight did not differ between our model and theirs [1,2]. The duration of pressure overload, however, was 4 and 10 weeks in our study, 3 to 6 months in the study of Harrison et al. [ll and 6 to 12 weeks in the study of Jeremy et al. [2]. Since the life span of rats is shorter than that of dogs, the duration of pressure overload relative to life span in our study may be construed as longer than that of their studies. Because the onset of pressure overload and the development of cardiac hypertrophy appear to be faster in our experimental models with aortic banding than in theirs with renovascular hypertension, not only the duration of hypertrophy [15,181 but also the more rapid onset of hypertension and hypertrophy might prevent the coronary vasculature from adapting and maintaining autoregulation at the higher perfusion pressure [19,20]. In addition, the
141
differences in neurohumoral factors between the renovascular hypertension model and ascending aortic banding model might cause the discrepancy in results. Although the exact reason for these different results is not clear, the differences in the experimental models are likely to be the most important. Coronary autoregulation pressure overload
before and after relief of
We estimated coronary autoregulation in terms of steady-state pressure - flow relationships and transient flow responses to stepwise changes of perfusion pressure. In steady-state pressure flow relationships, coronary autoregulation was observed in the range between 50 and 100 mmHg of perfusion pressure in controls. After debanding, the autoregulatory gains completely returned to normal in short-term banding but did not regress in long-term banding. In transient flow responses to stepwise changes of perfusion pressure, we could determine the extent and speed of flow regulation in the coronary vasculature by the amount of flow decrement and by the time from the point of peak flow to that of one-third of peak flow. Mosher et al. showed that prompt increase of flow after a step increase of perfusion pressure was followed by an immediate return to an initial flow level in the autoregulatory pressure range and was then followed by gradual flow reduction in the nonautoregulatory pressure range [21]. Our observations of flow changes in shamoperated controls are consistent with their findings. The rapid flow reduction followed by gradual reduction was observed in the autoregulatory range of perfusion pressure in the control groups and the debanded group after short-term banding. This observation suggests that the first rapid reduction in flow may be the result of active regulation by arterial trees (arterioles) and that the second gradual reduction may not reflect the active arterial function. Although the steady-state pressure - flow relationship provides unequivocal evidence for autoregulation, transient flow responses to stepwise changes of perfusion pressure could characterize the speed of flow regulation in
the coronary vasculature at a constant perfusion pressure. The speed of the regulation was always in parallel with the extent of flow regulation. The parallel changes in flow regulation were observed in all the sham-operated control, banded and debanded groups. Therefore, the extent and speed of flow regulation was determined by the presence or absence of autoregulation in the pressure level studied, but not by chronic pressure overload or myocardial hypertrophy. In the present study, total myocardial mass increased by 40% and total metabolic demand would presumably also be increased in hypertrophied hearts, even though we used non-working hearts paced at a constant rate. However, coronary flows per unit myocardial mass in the experiment on transient flow responses to stepwise increase of perfusion pressure from 50 to 100 mmHg did not differ between the control and banded groups. It is reasonable to assume that increased metabolic demand in hypertropied hearts would have been balanced by increased total coronary flows. Therefore, the metabolic effect of myocardium at 100 mmHg of perfusion pressure on the differences between the control and banded groups would be minimal, and the different transient flow responses to stepwise changes in perfusion pressure in the autoregulatory range might be the result of myogenic rather than metabolic regulation [22-251. Even if such flow responses are mainly regulated by metabolic mechanisms, similar flow in the peak response at 100 mmHg of perfusion pressure in the control and banded groups should wash out metabolic substrates in a similar manner. Furthermore, the time required for the flow regulation, even in the first rapid reduction, was longer than 30 s. Such a long period of time would not be required to wash out substrates from the myocardial tissue. Therefore, changes in autoregulation after banding and debanding may be the result of changes in the coronary arterial wall. The changes in the speed of flow regulation in response to stepwise increases of perfusion pressure especially may reflect smooth muscle function of the vascular wall. It is possible that contractile properties of smooth muscles in chronic hypertension were different from those in controls. Also, endothelial cells
142
influence the arterial vascular tone by endothelium-derived relaxing factors [26,27]. Kelm et al. [28] reported that vascular endothelial cells contribute to the maintenance of coronary vascular tone through release of endothelium-dependent relaxing factor(s). Since chronic hypertension impairs the endothelial function [29-311 and modulates arterial vascular tone [32], it is possible that modulation of coronary vascular tone by the endothelium was diminished in the banded groups. impaired autoregulation After debanding, completely regressed in short-term pressure overload and did not regress in long-term pressure overload. Increases in perfusion pressure may cause changes in myocardial oxygen demand (the Gregg phenomenon) [33]. In the present study, we did not directly measure myocardial oxygen consumption nor did we estimate the influence of the Gregg phenomenon on the coronary perfusion pressure - flow relationships. Even if, during measurements of coronary hemodynamics, the Gregg phenomenon might slightly modify the configuration of the pressure - flow relationships, it is unlikely that this phenomenon caused the differing reversibilities of the coronary autoregulation in hearts debanded after short- and long-term banding. This complete regression in short-term pressure overload might be caused by the reversal of smooth muscle hypertrophy [34,351 and/or endothelial dysfunction [36,37] if the endothelium has a role in controlling coronary vascular tone [28,38]. In long-term pressure overload, increased extracellular proteins such as collagen and elastin may alter the characteristics of the vascular wall [39-411, and the regression of collagen deposition after debanding may not be sufficient to normalize the properties of the vascular wall. Furthermore, subendothelial thickening may produce uncoupling between the endothelium and smooth muscle cells [421 even if the endothelial dysfunction regressed. From our present study it is not clear what size of arterioles mainly modulate coronary autoregulation [24,43, 441 and how coronary arterial hypertension impairs autoregulation at the microcirculation level. Further studies are needed to determine how short- and long-term pressure overload impair arteriolar function, and what changes determine
the reversibility of impaired coronary autoregulation. In conclusion, we studied the effects of duration of pressure overload on reversibility of impaired coronary autoregulation. After relief of pressure overload, impaired autoregulation completely returned to normal in short-term pressure overload, while it remained unchanged in spite of regression of myocardial hypertrophy in long-term pressure-overload. Thus, the duration of pressure overload and cardiac hypertrophy alters the regression of impaired coronary autoregulation. References DG, Florentine MS, Brooks LA, Cooper SM, 1 Harrison Marcus ML. The effect of hypertension and left ventricular hypertrophy on the lower range of coronary autoregulation. Circulation 1988;77:1108-1115. 2 Jeremy RW, Fletcher PJ, Thompson J. Coronary pressureflow relations in hypertensive left ventricular hypertrophy. Comparison of intact autoregulation with physiological and pharmacological vasodilation in the dog. Cir Res 1989;65:224-236. T. Normalization of im3 Sato F, Isoyama S, Takishima paired coronary circulation in hypertropied rat hearts. Hypertension 1990;16:26-34. 4 Ito N, lsoyama S, Takishima T. Duration of pressure-overload alters regression of coronary circulation abnormalities. Am J Physiol 1990;258:H1753-H1760. S, Ito N, Kuroha M, Takishima T. Complete 5 lsoyama reversibility of physiological coronary vascular abnormalities in hypertropied hearts produced by pressure-overload in the rat. J Clin Invest 1989;84:288-294. 6 Isoyama S, Wei JY, Izumo S, Fort P, Schoen FJ, Grossman W. Effect of age on the development of cardiac hypertrophy produced by aortic constriction in the rat. Circ Res 1987;61:337-345. Isoyama S, Grossman W, Wei JY. Effect of age on myocardial adaptation to volume overload in the rat. J Clin Invest 1988;81:1850-1857. Isoyama S, Apstein CS, Grice WM. Lore11 BH. Acute decrease in left ventricular diastolic chamber distensibility during simulated angina in isolated hearts. Circ Res 1987;61:925-933. Lore11 BH, Isoyama S, Grice WM, Weinberg EO, Apstein CS. Effect of ouabain and isoproterenol on left ventricular diastolic function during low-flow ischemia in isolated, blood-perfused rabbit hearts. Circ Res 1988;63:457-467. Marshall RC, Nash WW, Bersohn MM, Wong GA. Myocardial energy production and consumption remain balanced during positive inotropic stimulation when coronary flow is restricted to basal rates in rabbit heart. J Clin Invest 1987;80:1165-1171.
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