Recruitment in regionally

of myocardial work and metabolism stunned porcine myocardium

EDWARD 0. McFALLS, ANJA HOOGENDOORN,

DIRK J. DUNCKER, ROB KRAMS, AND PIETER D. VERDOUW

Laboratory for Experimental Cardiology 3000 DR Rotterdam, The Netherlands

(Thoraxcenter),

McFalls, Edward O., Dirk J. Duncker, Rob Krams, Loes M. A. Sassen, Anja Hoogendoorn, and Pieter D. Verdouw. Recruitment of myocardial work and metabolism in regionally stunned porcine myocardium. Am. J. Physiol. 263 (Heart Circ. Physiol. 32): H1724-H1731, 1992.-We characterized postischemic changes in myocardial metabolism and regional external work, as measured by the integral of left ventricular pressure-segment-length loops. In 12 anesthetized swine, the left anterior descending coronary artery (LAD) was occluded for 10 min and reperfused for 30 min for two successive cycles. Before ischemia, regional work was 16,920 k 5,630 mmHg-mm/min and after stunning, work was reduced to 50 & 14% (P < 0.05). At baseline, oxygen and lactate consumption were 4.80 & 1.40 and 1.02 t 0.46 pmol . min-’ g-l, respectively, and after stunning they were reduced to 3.24 t 0.80 (P < 0.05) and 0.16 & 0.21 pmol. min-’ l g-l (P < 0.05), respectively. The atria were then paced 50 beats/min higher than the reperfusion heart rate, during and without an infusion of dobutamine (2 pg. kg-l emin - l). During dobutamine, both regional external work and oxygen consumption returned to 98% of preischemic values, but lactate utilization remained depressed. We conclude that regional external work and oxygen consumption remain coupled during inotropic stimulation after stunning, with a preferential shift toward nonlactate substrates. myocardial stunning; oxygen consumption; dobutamine; external work l

WELL ESTABLISHED that the contractile state of the myocardium can be depressed after brief periods of ischemia and reperfusion in the absence of necrosis. This downregulation in function has been termed “myocardial stunning,” and its reversibility has been observed both chronically, following prolonged periods of reperfusion (Z), and acutely, during the administration of inotropic agents or calcium (1, 13). Although persistent coronary blood flow and metabolic abnormalities are unlikely to be the mechanism of stunning, such alterations have stimulated interest in understanding how the reperfused myocardium can adjust to subsequent myocardial oxygen demands. Prolonged periods of ischemia followed by reperfusion have been shown to alter the coronary vasculature, particularly in regard to endothelium-dependent mechanisms of vasodilation (19). However, in models of stunning where necrosis is not present, it is unlikely that alterations in the coronary vasculature could account for perfusion abnormalities to viable, nonfunctioning myocardium (5, 31). Because there is evidence that oxidative phosphorylation and electron transport in the myocardium remain tightly coupled (26), changes in myocardial blood flow following stunning most likely reflect changes in myocardial oxygen demand. Myocardial oxygen consumption has been shown to be highly influenced by the contractile state of the myocardium (10) and as such would be expected to be lower IT HAS BEEN

H1724

0363-6135/92

$2.00

Erasmus

LOES M. A. SASSEN, University

Rotterdam,

after reperfusion. Observations in certain models of stunning, however, have shown that myocardial oxygen consumption is unchanged (17) or even increased (29) despite severe reductions in function. It has been postulated that this increased oxygen utilization could be accounted for by a number of factors, including the increased energy demands associated with cellular repair or calcium homeostasis (4). Alternatively, the reperfused myocardium has been shown to perform more external work than might be expected from measurements of systolic shortening and might partially account for the paradoxical increase in oxygen consumption relative to function (32). Therefore, in this study, we have provided measures of regional external work, based on pressure-segment length areas, to relate changes in function with metabolism before and after stunning, and during subsequent chronotropic and inotropic stimulation. The integral of the left ventricular (LV) pressure-segment length loop not only incorporates changes in pressure, segment length, and heart rate but also changes in waveform and the timing of shortening. We hypothesized that changes in both external work and oxygen consumption would parallel one another after reperfusion and both would normalize with the addition of low-dose dobutamine. This would suggest close coupling between work and oxygen consumption despite severe reductions in both following reperfusion. We also postulated that lactate consumption would increase during increased heart rate or contractility following stunning to support the contention that reperfused myocardium retains metabolic reserve. MATERIALS

AND METHODS

Studies were performed in accordance with the position of the American Heart Association on research animal use and under regulations of the Erasmus University. General preparation. After an overnight fast, 12 cross-bred Landrace-Yorkshire pigs of either sex (25-32 kg) were sedated with intramuscular ketamine (20 mg/kg) (A.U.V. Cuijk, The Netherlands) and intravenous metomidate (5 mg/kg) (Jansen Pharmaceutical, Beerse, Belgium). Animals were intubated and connected to a respirator for intermittent positive pressure ventilation with a mixture of oxygen and nitrous oxide (1:2). Ventilator settings were adjusted during the experiments to maintain normal arterial pH (7.35-7.45), PCO~ (35-45 mmHg) and PO, (X00 mmHg). The external jugular vein was cannulated with two 7-Fr catheters for administration of either the anesthetics, fluids, or the dobutamine. Anesthesia was maintained throughout the experimental period with a continuous infusion of sodium pentobarbitone (20-30 mg= kg-l h-l, Apharmo, Arnhem, The Netherlands). The femoral arteries were cannulated with 7-Fr catheters and were used for either aortic blood pressure measurements, arterial blood gas collections, or reference

Copyright 0 1992 The American Physiological

l

Society

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POSTISCHEMIC

MYOCARDIAL

WORK

sampling for microsphere determination. A micro-tipped catheter (7-Fr Millar) was advanced into the left ventricle via the left carotid artery and used to monitor LV pressure and its first derivative (LV dP/dt). Rectal temperature was monitored throughout the experiment and maintained near 37°C with external heating pads. After administration of pancuronium bromide (4 mg iv, Organon Teknika, Boxtel, The Netherlands), a midline thoracotomy was performed, and the heart was suspended in a pericardial cradle. The left mammary vessels were ligated, and the second left rib was removed for ease of further instrumentation. The adventitia surrounding the aorta was dissected free, and an aortic flow probe (15 mm; Skalar, Delft, The Netherlands) was placed for measurement of cardiac output. A 7-F catheter was secured in the left atrium and used for the administration of radiolabeled microspheres. On the proximal third of the left anterior descending coronary artery (LAD), a segment was dissected free of its adventitia to position the arterial clamp. A sampling catheter was inserted into the great cardiac vein draining the myocardium perfused distal to the occlusion site of the LAD. Pacing leads were attached to the left atria1 appendage and connected to a pacing stimulator. The animals were not systemically heparinized. Regional segment lengths. To measure regional segment length changes in the distribution of the ischemic segment of the LAD, a pair of ultrasonic crystals (Triton, Technology, San Diego, CA) were placed into the endocardial layer, -10 mm apart (LAD region). Another pair of crystals were similarly placed in the distribution of the circumflex artery, remote from the ischemic myocardium (non-LAD region). Systolic shortening in each of the regions was calculated from the difference between lengths at end diastole (time of onset of positive dP/dt) and end systole (time of peak negative dP/dt) and expressed as a percent of end-diastolic length. Regional external work. In addition to segment length changes, regional function was determined by calculating indexes of external work, in both myocardial regions. Such measures of regional work have been successfully applied using the integral of the pressure-segment length loops (8, 24, 32). LV pressure-segment length loops were collected with pressure plotted on the y-axis and segment length on the x-axis. Data from 12 beats from each period were stored on computer for later off-line analysis. The area from all 12 beats were averaged and reported as indexes of regional myocardial work. Regional myocardial blood flow. For each flow measurement, l-2 million microspheres (15 pm) labeled with either 141Ce, YSn, lo3Ru, or g”Nb (NEN Chemicals, Dreieich, Germany) were injected into the left atrium. Reference arterial blood samples were withdrawn from the femoral arterial catheter at a fixed rate of 10 ml/min, from 15 s before until 1 min after microsphere injection. At the conclusion of the experiment, the distribution of the postischemic myocardium was identified by injecting patent violet dye into the left atrium during LAD

AND

H1725

METABOLISM

occlusion, and the animal was killed. Hearts were fixed in 10% Formalin for 48 h and separated into LAD and non-LAD regions. Each region was then divided into three layers of equal thickness (inner, mid, and outer) and placed in 1- to 2-g samples. Myocardial and reference blood samples were counted in a multichannel analyzer (Gamma Counter-5000, Packard Instrument), and regional blood flows were determined. Arterial-coronary venous oxygen and lactate. Oxygen saturations and hemoglobins from arterial and venous blood samples were measured by an OSM2 system (Radiometer; Copenhagen, Denmark). Lactate concentrations were determined from 1 -ml aliquots of blood that were transferred into ice-cold glass tubes containing 2 ml of 0.6 M HC104 and promptly vortexed. At the conclusion of the experiment, samples were centrifuged for later analysis by an enzymatic technique (3). Percent lactate and oxygen extractions were calculated by the arterial-coronary venous differences divided by arterial levels times 100. Consumptions were calculated from the product of myocardial blood flows and the arterial-venous differences. Experimental protocol. Animals were allowed to stabilize for 30 min before the experimental protocol. Baseline recordings of aortic and left ventricular pressures, maximal first derivative of LV pressure (dP/dt,,,), cardiac output, and segment length changes in the two myocardial regions were obtained. Whole blood samples were withdrawn from the aorta and great cardiac vein for oxygen and lactate determinations, and radiolabeled microspheres were injected for blood flow measurements. With the use of a small clamp, the LAD was completely occluded for 10 min (occlusion 1) and allowed to fully reperfuse for 30 min (reperfusion 1). The cycle was then repeated (occlusion 2 and reperfusion 2). This model has been previously reported to induce severe alterations in function in the absence of necrosis (27). After the final 30-min reperfusion period (reperfusion Z), simultaneous aortic and coronary venous samples were obtained, and microspheres were injected. The heart was then paced 50 beats/min higher than the heart rate recorded during reperfusion 2. This was done to provide chronotropic stimulation to the reperfused myocardium without changing the inotropic state. After 5 min of pacing, recordings, blood sampling, and microsphere injection were repeated (stun + atria1 pacing). While pacing was continued, an intravenous infusion of dobutamine (2 pg. kg-l emin-‘) was then begun, and recordings, samplings, and injections of microspheres were repeated after 10 min (stun + dobutamine). This dose was chosen because in pilot studies, it was found to recruit postischemic function with less effect on systemic hemodynamics compared with higher doses. Recordings were made at 50 mm/s during each experimental period. Statistics. Results are expressed as arithmetic means t SD. Changes were tested for significance at the P < 0.05 level by

Table 1. Systemic hemodynamics before and after stunning Time

Period

Baseline Occlusion 1 Reperfusion 1 Occlusion 2 Reperfusion 2 Pacing Pacing + Dob

HR

104t13 103t15 106&19 102t12 103t15 153&16*-f 153&16**

MAP

LVEDP

89t13 74t20* 76&14* 77*11* 80&11* 83&10* 85klO

7t2 12t5* 9*4* 9t3* 7t2 7k3 5t3*:$

LV

dP/dt,,,

2,020+440 1,560+400* 1,630&250* 1,600+310* 1,550+330* 1,620&300* 3,320&700*t

co

2.90t0.77 2.24t0.77 2.66t0.67* 2.25*0.59* 2.28t0.68* 2.36t0.72* 2.88k0.93ti

sv

27.4t6.2 23.3t5.4* 23.3t5.1* 21.5&5.5* 22.1&5.7* 15.3t4.2*+ 18.7t5.6”~l:

Data are means & SD; n, 12 anesthetized swine. The atria were paced at 50 beats/min above the heart rate (HR) measured at the end of reperfusion 2; Dobutamine (Dob) was infused intravenously at 2 pg. kg-l min- ‘; MAP, mean arterial pressure (mmHg); LVEDP, left ventricular end-diastolic pressure (mmHg); LV dP/dt,,,, maximal left ventricular pressure first derivative (mmHg/s); CO, cardiac output (l/min); SV, stroke volume (ml/beat). * P < 0.05 vs. baseline; t P < 0.05 vs. reperfusion 2; $ P < 0.05 vs. atria1 pacing. l

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H1726

POSTISCHEMIC

MYOCARDIAL

WORK

AND

METABOLISM

Table 2. Segment length changes in LAD and non-LAD regions before and after stunning LAD Time

Region

Non-LAD

Region

Period EDL

Baseline Occlwion 1 Reperfusion 1 Occlusion 2 Reperfusion 2 Pacing Pacing + Dob

12.2k2.2 13.4t2.3* 12.5-t-2.3 13.3t2.2* 12.6t2.2 12.0+2.0t 11.7t2.1*?

%SS

ESL

9.8t1.9 13.9*2.6* 11.6*2.4* 13.9*2.6* ll.Bt2.4* 11.6t2.2* 10.0+2.Ot$

EDL

18.3k5.2 -3.7t2.5* 7.2t5.0* -4.1*2.6* 6.6&4.6* 4.3t3.7* 14.0+5.O*t~

10.7tl.l 1 l.lkl.1” 10.9k1.2 11.2t1.2* 10.8d.l 10.1+1.1? 9.8*1.2*t

ESL

%SS

9.3tl.l 9.4& 1.2 9.5tl.l 9.6t1.2 9.4tl.O 9.3kl.O 8.8t 1 .o*=Q

13.2k3.7 15.4k6.2” 13.2t4.1 14JW4.9* 12.9k3.8 8.2t3.7=?10.2+3.5*Q

Data are means & SD; n, 12 anesthetized swine. The atria were paced at 50 beats/min above the heart rate measured at the end of reperfusion length (mm); ESL, end-systolic length (mm); %SS, %systolic shortening. 2; Dob was infused intravenously at 2 pg. kg-l . min +; EDL, end-diastolic * P < 0.05 vs. baseline, t P < 0.05 vs. reperfusion 2; $ P < 0.05 vs. atria1 pacing.

analysis of variance with repeated measurements (Fisher’s protected least significant difference plus F-test). In addition, changes in microsphere flows in the LAD and non-LAD regions were compared by unpaired Student’s t test. RESULTS

Systemic hemodynamics (Table 1). At baseline, heart rate was 104 t 13 beats/min and remained unchanged through the reperfusion 2 period. Mean arterial pressure was 89 t 13 mmHg at baseline and dropped to 74 t 20 mmHg (P < 0.05) during occlusion 1. By the reperfusion 2 period, it had increased to 80 t 11 mmHg and did not significantly change during either of the interventions following stunning. LV end-diastolic pressure was 7 t 2 mmHg at baseline and increased to 12 t 5 mmHg (P c 0.05) during the occlusion 1 period. It returned to baseline values at reperfusion 2 and remained unchanged with atria1 pacing but decreased to 5 t 3 mmHg (P < 0.05) during the atria1 pacing and dobutamine intervention. At was 2,020 t 440 mmHg/s and baseline, LV dP/dt,,, cardiac output was 2.90 t 0.77 l/min. Both decreased 20-25% during occlusion 1 and remained depressed after reperfusion 2. During the atria1 pacing intervention, stroke volume was lowered from 22.1 t 5.7 ml during reperfusion 2 to 15.3 t 4.2 ml (P < 0.05). The addition of dobutamine to pacing caused an increase in cardiac output of 22%, a doubling in LV dP/dt,,,, and an increase in stroke volume. Regional segment length changes (Table 2). At baseline, segment length shortening in the control region was 13.2 + 3.7%, and in the LAD region this shortening was 18.3 I 5.2%. During each of the arterial occlusions, systolic bulging occurred in the LAD region, while an increase in both end-diastolic length and shortening was observed in the nonischemic region. After reperfusion 1, systolic shortening was reduced to 7.2 t 5.0% (P c 0.05) in the LAD region and remained unchanged in the control region. The degree of stunning was similar after the second reperfusion period. Both myocardial regions responded similarly to the increased heart rate during pacing with a mild reduction in systolic shortening. The addition of dobutamine recruited shortening in both regions, however, the changes were much greater in the postischemic myocardium. During the dobutamine intervention, segment length shortening in the non-LAD and LAD regions were 10.2 t 3.5 and 14.0 t 5.0% respectively. Regional external work. Representative examples of LV

pressure-segment length loops for both the control and LAD regions during each of the interventions are shown in Fig. 1. The integrals of these loops represent indexes of work per contraction and are expressed as measures of regional work per minute. Figure 2 summarizes the data for both the LAD and non-LAD regions during each of the interventions. At baseline, external work was 16,920 t 5,630 mmHg-mm/min and 19,590 t 5,560 mmHgmm/min in the non-LAD and LAD regions, respectively. During ischemia, regional work in the LAD region was reduced to 3% of baseline, whereas it remained unchanged in the control region. This demonstrates that even with net systolic lengthening during ischemia, the

Control region

lschemic region .-i!i Q, 2

m ‘,

125’

0. 1257

.h 30 8

1,In

0J

nJ 1251

0J 125

0

125

$0

1 1

1* 5I5

I

Fig. 1. Representative examples of left ventricular length loops in both left anterior descending coronary non-LAD regions at baseline, occlusion I, reperfusion of the interventions following stunning.

1 (mm) l5 pressure-segment artery (LAD) and 2, and during each

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POSTISCHEMIC 30000 n

C L *II C

MYOCARDIAL

WORK

1 q LAD q NON-LAD

I

BL

OCC-1

H1727

METABOLISM

Regional myocardial blood flows (Table 3). At baseline, myocardial blood flows in the control and LAD regions were 1.49 t 0.43 and 1.48 t 0.42 ml-g-l min-l, respectively. At reperfusion 2, blood flow in both myocardial regions were lower than their respective baseline values, however, the decrease in the postischemic region was significantly lower than that of the control region. During atria1 pacing, the increments in blood flows for the two regions were comparable and suggest that pacing induced similar oxygen demands in both the control and reperfused myocardial regions. The addition of dobutamine to pacing, however, caused blood flow to increase to a greater degree in the LAD region compared with the control region in parallel with the increased oxygen demand of improved external work. Oxygen and lactate metabolism (Table 4). At baseline, myocardial oxygen consumption was 4.80 t 1.40 pm01 min-l g-l and after reperfusion 2, this consumption was lowered to 3.24 t 0.80 prnol mine1 l g-l (P < 0.05). It increased to 3.84 t 1.20 pmol.rnin-‘*g-l (P < 0.05) with atria1 pacing and then returned to preischemic baseline values with the addition of dobutamine. The relationship between myocardial oxygen consumption and external work is displayed in Fig. 3 for all animals during the four interventions. This relates the recruitment of both external work and myocardial oxygen consumption after stunning with dobutamine, suggesting that both remain coupled during interventions, which recruit function after reperfusion. Lactate consumption was 1.02 t 0.46 pmol min-l g-l at baseline and was lowered to 0.16 t 0.21 prnolm min-l l g-l (P < 0.05) after reperfusion 2. During the subsequent atria1 pacing intervention, it increased to 0.29 t 0.20 prnol. mine1 g-l (P < 0.05) and further increased l

REP-2

P

P+DOB

l

EXPERIMENTAL PERIOD Fig. 2. Regional external work or left length areas in both LAD and non-LAD chsion I (OCC-l), reperfusion 2 (REP-2), during atria1 pacing plus dobutamine (P stunning. Data are expressed as means t vs. baseline value.

AND

ventricular pressure-segment regions at baseline (BL), ocduring atria1 pacing (P), and + DOB) interventions after SD; n = 12 swine. * P < 0.05

LAD region performed work with each mechanical contraction. Compared with baseline values at reperfusion 2 external work was reduced to 50 t 14% (P < 0.05) in the LAD region and 75 t 10% (not significant) in the nonLAD region. During atria1 pacing, external work was lower than reperfusion 2 measurements for both regions, and with the addition of dobutamine, external work increased back to baseline values in the postischemic LAD region.

l

l

l

l

Table 3. Regional myocardial blood flows before and after stunning LAD Time

Region

Non-LAD

Region

Period Transmural

Endocardial

Baseline 1.48t0.42 Reperfusion 2 0.94t0.27* Pacing 1.07&0.35* Pacing + Dob 1.50&0.44t$ Data are means t SD in ml. min-l end of reperfusion 2; Dob was infused pacing.

Epicardial

1.44kO.44

1.52t1.49

0.93*0.28* 1.05t0.36*

0.94*0.27*

1.44+0.42-Q

Transmural

1.08t0.34*-f 1.56+0.42?$

Endocardial

1.49kO.43 1.27t0.52 1.47t0.64 1.76+0.60*1-j:

1.54kO.47 1.23t0.53* 1.37t0.60 1.65+0.57-Q

Epicardial

1.42t0.41 1.32k0.53 1.55+0.28f1.9lk0.66*t$

*g-l; n; 12 anesthetized swine. The atria were paced at 50 beats/min above the heart rate measured at the intravenously at 2 pg. kg+ . min- l. * P < 0.05 vs. baseline, t P < 0.05 vs. reperjusion 2; $ P < 0.05 vs. atria1

Table 4. Metabolism in LAD region before and after stunning Oxygen Time

Period Arterial,

Baseline Reperfusion 2 Pacing Pacing + Dob Data are means 2; Dob was infused pacing.

mM

4.45t0.80 4.72t0.88* 4.70k0.89 4.68k0.92

A-V,

mM

3.28t0.47 3.48t0.47* 3.62&0.66*t 3.20&0.69-i-$

Lactate Consumption, pm01 . mine1

- g:-’

Arterial,

mM

4.80-t-1.40 3.24t0.80*

1.89k0.92 1.91t0.81

3.84&1.20*t 4.70+1.36t$

1.79t0.68 1.60&0.63*

A-V,

t

mM

Consumption, pm01 +min- l - g-l

0.67t0.22 0.16*0.21* 0.27&0.14*

1.0220.46 0.16&0.21* 0.29t0.20*

0.30&0.17*t

0.44t0.27*

t

t SD; n, 12 anesthetized swine. The atria were paced at 50 beats/min above the heart rate measured at the end of reperfusion intravenously at 2 pg. kg-l. min-I; A-V, arterial-venous. * P < 0.05 vs. baseline, t P < 0.05 vs. reperfusion 2; $ P < 0.05 vs. atria1

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H1728

POSTISCHEMIC

MYOCARDIAL

WORK

AND

METABOLISM

vention based on a 6:l molecular conversion of oxygen, and the contribution from lactate was determined based on an 18:1 molecular conversion (23). Total mean ATP values, as well as the contributions from lactate and nonlactate substrates, are shown in Fig. 5 for all 12 animals. Figure 5 emphasizes that after reperfusion, ATP production via lactate oxidation is reduced compared with baseline but increases during the chronotropic and inotropic interventions. 0 stun + dobutamine W stun + pacing

c 00

2-

0

n

:

.

I

EXTERNAL

.

I

10000

0

20000

WORK

+ pacing

1

30000

(mmHg-mm/min)

Fig. 3. Regional external work or left ventricular pressure-segment length area vs. myocardial oxygen consumption in LAD region at baseline, after reperfusion 2, and during atria1 pacing and pacing plus dobutamine interventions. Regression line of mean data points during 4 periods is included and shows that a 3O-50% reduction in oxygen consumption would be expected with no external work performed. Data are expressed as means & SD; n = 12 swine.

G6 .-c

4 .-5 z. t

DISCUSSION

In this study, we have used a model of myocardial stunning that has been shown to induce severe reductions in function following reperfusion without evidence of necrosis (27). The data demonstrate that myocardial blood flow and oxygen consumption may be reduced at a time when postischemic function is severely depressed but return to preischemic levels as external work is recruited during inotropic stimulation. The findings support the concept that external work is a good predictor of the amount of energy consumed during mechanical contraction and is an important parameter for relating changes in metabolism following reperfusion. Myocardial oxygen consumption and stunning. The interpretation of altered myocardial blood flow and metabolism in models of ischemia and reperfusion has been controversial. There is some evidence that the coronary vasculature may not respond to normal stimuli after prolonged periods of ischemia and reperfusion, either because of mechanical obstruction to coronary blood flow (16) or altered endothelial-dependent mechanisms of vasodilation (19). Although shorter periods of ischemia and reperfusion may be associated with a reduction in reactive hyperemia (l4), the capacity to maximally vasodilate remains intact (5, 14). Therefore, in this model of stunning induced by two cycles of 10 min of ischemia and 30

2 c

SUBSTRATE 0

stun + dobutamine n stun + pacing Cl stun

0

f 0.0

I

0.4

lactate

1

I

0.8

consumption

.

+ pacing

I

1.2

I

q q

non-lactate lactate

1

1.6

(umoI/min/g)

Fig. 4. Lactate vs. oxygen consumption in LAD region at baseline, after reperfusion 2, and during atria1 pacing and pacing plus dobutamine interventions. Mean data points (+SD; n = 12 swine) are shown for each time period and demonstrate that metabolic reserve for both oxygen and lactate utilization exist following stunning. Of interest, lactate utilization remains depressed during dobutamine infusion at a time when both external work and oxygen consumption have returned to normal.

to 0.44 t 0.27 prnol. min-l l g-l (P < 0.05) with the addition of dobutamine. Although oxygen consumption returned to baseline values during the dobutamine and pacing intervention, lactate consumption remained depressed (Fig. 4). Total ATP production was calculated for each inter-

BL

REP-2 EXPERIMENTAL

P PERIOD

P+DOB

Fig. 5. Calculated ATP production during each experimental period is shown from mean values for all 1‘2 animals. Total ATP was determined from a 6:l molecular conversion from oxygen, and ATP from lactate was determined from an l&l molecular conversion from the amount of lactate utilized.

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POSTISCHEMIC

MYOCARDIAL

min of reperfusion, it is unlikely that changes in the vasculature would have accounted for the changes in myocardial blood flow after reperfusion. A number of studies have shown that myocardial oxygen consumption in postischemic myocardial regions are unchanged (17) or even increased (29) despite severe reductions in systolic shortening. Recent evidence shows that ATP production and electron transport in mitochondria of stunned myocardium remain tightly coupled, making it unlikely that the increased oxygen consumption is caused by a decreased efficiency of oxidative phosphorylation (26). In light of this, a number of mechanisms have been proposed to explain the observed increase in energy requirements, including the increased oxygen demands associated with cellular repair and ionic homeostasis (4). External

work and myocardial oxygen consumption.

Changes in mechanical contraction, as determined from the left ventricular pressure-segment length area, may provide an alternative explanation for the increased oxygen consumption in stunning as observed by others (29). The basic concept of time-varying elastance was introduced as global measures of function, based on the pressure-volume relationship. Myocardial oxygen consumption has been related to the principal components of that relationship, namely external mechanical work, potential energy, and the end-systolic and end-diastolic pressurevolume curves (21, 30). The concept of external mechanical work has more recently been validated in regional segments of myocardium, as defined by pressure-segment length relations (9). Using such regional measurements of function expressed as external work, the energy expended during mechanical contraction has been shown to be greater than might be expected from changes in systolic segment length, which could account for the paradoxical increase in oxygen consumption after reperfusion (32). Consistent with the above studies using regional pressure-segment length loops, our data show that external work following stunning is higher than might be expected from systolic segment length changes. For example, during occlusion, there was net systolic lengthening (dyskinesis) at a time when regional myocardial work was performed as calculated from the pressure-segment length loops. It has recently been shown that external work from the pressure-volume trajectory during constant contractility consumes equivalent amounts of oxygen, regardless of whether the work occurs during systole or relaxation (12). Therefore, the postsystolic shortening that often occurs after stunning is work performed by the myocardium and an energy-consuming process not predicted from systolic shortening. This supports the contention that the pattern of mechanical contraction, related to regional pressure-volume or pressure-segment length areas can explain some of the paradoxically increased oxygen consumption, despite severe reductions in segment length shortening. We have shown that myocardial oxygen consumption was lower in the postischemic regions compared with baseline at a time when other determinants of oxygen consumption were relatively unchanged. This is consistent with findings in anesthetized swine (25) and in anes-

WORK

AND

METABOLISM

H1729

thetized dogs after four repetitive occlusion-reperfusion periods (32). In the latter study, the reductions in myocardial oxygen consumption in the postischemic regions were lower than baseline following each of the four occlusion-reperfusion periods; however, they were lower than the control region only after the final period. Although LAD venous oxygen samples were taken from the interventricular LAD vein, venous oxygen blood for the control region was sampled from the coronary sinus, which is less specific for that region. It is possible that contamination from noncontrol myocardial regions masked early differences in oxygen consumption between the postischemic and control regions. In our study, oxygen consumption was not measured in the control region, and therefore we cannot be certain that it was higher than that of the LAD region. However, because the reduction in myocardial blood flow was less in the non-LAD region, we would speculate that reductions in oxygen consumption would also be less compared with the postischemic region. Regional external work has been shown to be a better predictor of changes in oxygen consumption than systolic shortening after reperfusion, particularly in akinetic segments (32). The present study extends this previous work by showing that myocardial oxygen consumption returns to preischemic values as external work is recruited to normal during infusion of low-dose dobutamine. This suggests that external work and oxygen consumption are coupled during interventions, which recruit function following reperfusion. Although external work is a major determinant of myocardial oxygen consumption, it does not account for the entire energy consumed related to mechanical contraction. The pressure-segment length area is also composed of potential energy, which may increase after reperfusion, and in turn, increase myocardial oxygen consumption (30). Myocardial segments, which perform no external work, consume 50-70% of the oxygen consumption in the basal state, much of which is dependent on basal energy requirements and potential energy (6). This is consistent with the relationship between myocardial oxygen consumption and external work in this study, where a 3O50% reduction in oxygen consumption would be expected at a time when no regional external work was performed (Fig. 3). Pacing the reperfused myocardium has been shown to increase myocardial oxygen consumption (15), which is consistent with our results. However, this was associated with a decrease in external work per minute in both the control and postischemic regions. It is not clear from these data what might account for this reduction in energy transfer to external work but may involve an increase in potential energy. Potential energy has been shown to degrade into heat production, if not utilized for external work (3). In our experiments, the addition of dobutamine while pacing recruited both external work and myocardial oxygen consumption, implying that energy transfer was also increased. Me ltabolic reserve following stunning. Stunned myocardium has been shown to retain functional reserve (1, 13),

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POSTISCHEMIC

MYOCARDIAL

and therefore the findings of recruitable systolic shortening in this study are not new. The degree of metabolic reserve after stunning, however, has not been well characterized, particularly in combination with parameters of external work. The pattern of substrate utilization is altered following reperfusion, with a reduction in the metabolic utilization of fatty acids or acetate (28). Utilization of these substrates has been shown to decrease in other models of ischemia and reperfusion (20) but has the capacity to increase with increased myocardial oxygen demands (8). We have calculated the amount of ATP utilized during each intervention, based on mean data for oxygen and lactate consumptions. In anesthetized dogs, net lactate oxidation as determined from regional Fick measurements correlated well with values obtained by radioactive tracer studies, justifying our calculations of net myocardial lactate oxidation (11). In the latter study, lactate substrate accounted for 25% of the oxygen consumed, which is lower than our calculated values. One of two possibilities may have accounted for this discrepancy. The arterial lactate values in our data were nearly twice that of the prior study, which may have caused a preferential shift toward the oxidation of that substrate. Alternatively, during exercise or increased stress, lactate can provide as much as 60% of the energy requirements of the heart (23), and therefore our findings may reflect different experimental conditions. After stunning, oxygen utilization and ATP production via lactate oxidation decreased at a time when arterial lactate remained unchanged. The interpretation of this may be confounded by the fact that transmural production and extraction rates of lactate may differ (23). However, there is evidence suggesting that the utilization of carbohydrate substrates such as pyruvate and lactate are reduced after reperfusion (25). It has been suggested that regulatory inhibition of carbohydrate oxidation such as with the enzyme pyruvate dehydrogenase may account for these observations (22). In addition, lactate dehydrogenase could conceivably be depleted following ischemia and reperfusion, limiting the degree of lactate oxidation in our model. Against these possibilities, lactate consumption following stunning increased during both the pacing and dobutamine interventions, implying that enzyme inhibition was not a limiting factor for lactate oxidation. Interestingly, lactate consumption did not completely return to baseline values at a time when both function and myocardial oxygen consumption had normalized. We did not measure free fatty acids or insulin, and therefore we can not exclude that other regulators of lactate utilization were altered during the catecholamine infusion. In addition, arterial lactates were lower during this intervention, which could have reduced overall utilization. However, the data do suggest that the utilization of noncarbohydrate substrates plays an important role during increased oxygen demands following reperfusion (8, 25). Limitations. The load-independent measure of contractility comprises another important determinant of the entire pressure-segment length area, which affects myocardial oxygen consumption (30). After reperfusion,

WORK AND METABOLISM

changes in this parameter play a major role in altering the amount of energy utilized (21) and as such may explain some of the variability between studies, relating postischemic reductions in myocardial function and oxygen consumption. Future emphasis should be directed at measuring all components of the pressure-segment length area, including contractility and potential energy from in tact preparations. Arterial-venous differences in lactate concentration represent the sum of overall extraction and production and thus may not represent true measures of substrate consumption. For instance, this heterogeneity of myocardial metabolism has been demonstrated in humans with coronary arterial disease where global net extraction of lactate can occur at a time of increased production (7). Therefore, after reperfusion, our observations of reduced consumption of lactate cannot discriminate between differences in regional production and extraction rates, particularly regarding subendocardial and subepicardial layers. Conchsion. In conclusion, we have shown in this model of myocardial stunning that both external work and myocardial oxygen consumption can be recruited to preischemic baseline values following reperfusion. The findings support the hypothesis that external work is an accurate measure of myocardial oxygen demands associated with contraction. The findings also demonstrate significant metabolic reserve in stunned myocardium, both during inotropic and chronotropic stimuli. Present address of E. 0. McFalls: Dept. of Cardiology, VA Medical Center, Univ. of Minnesota, 1 Veterans Dr., Minneapolis, MN 55417. Address for reprint requests: P. D. Verdouw, Laboratory for Experimental Cardiology, Thoraxcenter, Erasmus Univ. Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands. Received 6 April 1992; accepted in final form 30 July 1992. REFERENCES 1. Becker, L. C., J. H. Levine, A. F. DiPaula, T. Guarnieri, and T. Aversano. Reversal of dysfunction in postischemic stunned myocardium by epinephrine and postextrasystolic potentiation. J. Am. Coil. Cardiol. 7: 580-589, 1986. 2. Bolli, R., W. X. Zhu, J. I. Thornby, P. G. O’Neill, and R. Roberts. Time-course and determinants of recovery of function after reversible ischemia in conscious dogs. Am. J. Physiol. 254 (Heart

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MYOCARDIAL

9. Goto, Y., Y. Igarashi, Y. Yasumura, T. Nozawa, S. Futaki, K. Hiramori, and H. Suga. Integrated regional work equals total left ventricular work in regionally ischemic canine heart. Am. J. Physiol. 254 (Heart Circ. Physiol. 23): H894-H904, 1988. T. P., J. W. Covell, E. H. Sonnenblick, J. Ross, 10. Graham, and E. Braunwald. Control of myocardial oxygen consumption: relative influence of contractile state and tension development. J. Clin. Invest. 56: 978-985, 1968. 11. Griggs, D., S. Nagano, J. Lipana, and P. Novack. Myocardial lactate oxidation in situ and the effect thereon of reduced coronary flow. Am. J. Physiol. 211: 335-340, 1966. 12. Hata, K., Y. Goto, and H. Suga. External mechanical work during relaxation period does not affect myocardial oxygen consumption. Am. J. Physiol. 261 (Heart Circ. Physiol. 29): H1778H1784, 1991. 13. Ito, B. R., H. Tate, M. Kobayashi, and W. Shaper. Reversibly injured, postischemic canine myocardium retains normal contractile reserve. Circ. Res. 61: 834-846, 1987. 14. Jeremy, R. W., L. Stahl, M. Gillinov, M. Litt, T. R. Aversano, and L. C. Becker. Preservation of coronary flow reserve in stunned myocardium. Am. J. Physiol. 256 (Heart Circ. Physiol. 25): H1303-H1310, 1989. J., B. Acad, and H. R. Weiss. Pacing during reperfu15. Kedem, sion elevates regional myocardial oxygen consumption Am. J. Physiol. 259 (Heart Circ. Physiol. 28): H872-H878, 1990. R. A., C. E. Ganote, and R. B. Jennings. The “no16. Kloner, reflow” phenomenon after temporary coronary occlusion in the dog. J. Clin. Invest. 54: 1496-1508, 1974. D. D., D. C. Homans, X. Dai, E. Sublett, and R. J. 17. Laxson, Bach. Oxygen consumption and coronary reactivity in postischemic myocardium Circ. Res. 64: 9-20, 1989. F., and G. Elzinzga. Heat released during relaxation 18. Mast, / equals force-length area in isometric contractions of rabbit papillary muscle. Circ. Res. 67: 893-901, 1990. 19. Mehta, J. L., W. H. Nichols, W. L. Donnelly, D. L. Lawson, and T. G. P. Saldeen. Impaired canine coronary vasodilator response to acetylcholine and bradykinin after occlusion-reperfusion. Circ. Res. 64: 43-54, 1989. 20. Myears, D. W., B. E. Sobel, and S. R. Bergmann. Substrate use in ischemic and reperfused canine myocardium: quantitative considerations. Am. J. Physiol. 253 (Heart Circ. Physiol. 21): H107-H114, 1987. 21. Ohgoshi, Y., Y. Goto, S. Futaki, H. Yaku, 0. Kawaguchi, and H. Suga. Increased oxvgen cost of contractilitv in stunned

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myocardium of dog. Circ. Res. 69: 975-988, 1991. M. S., S. C. Dennis, C. A. Routh, and M. S. Debuy22. Olson, sere. The regulation of pyruvate dehydrogenase by fatty acids in isolated rabbit heart mitochondria. Arch. Biochem. Biophys. 187: 121-131, 1978. L. Metabolism of the heart in health and disease. Part II. 23. Opie, Am. Heart J. 77: 100-122, 1969. M., S. F. Vatner, H. Baig, and E. Braunwald. Ini24. Pagani, tial myocardial adjustments to brief periods of ischemia and reperfusion in the conscious dog. Circ. Res. 43: 83-92, 1978. R., S. H. Nellis, and J. A. Liedtke. Metabolic 25. Renstrom, oxidation of pyruvate and lactate during early myocardial reperfusion Circ. Res. 66: 282-288, 1990. E., P. Kingsley-Hickman, A. From, J. Foker, and K. 26. Sako, Ugurbil. ATP synthesis kinetics and mitochondrial function in the post-ischemic myocardium as studied by NMR. J. Biol. Chem. 263: 10600- 10607, 1988. R. J., S. Rohmann, E. R. Braun, and W. Schaper. 27. Schott, Ischemic preconditioning reduces infarct size in swine myocardium. Circ. Res. 66: 1133-1142, 1990. M., R. A. Neese, L. Araujo, W. Wyns, J. A. 28. Schwaiger, Wisneski, H. Sochor, S. Swank, D. Kulber, C. Selin, M. Phelps, H. R. Schelbert, M. C. Fishbein, and E. W. Gertz. Sustained nonoxidative glucose utilization and depletion of glycogen in reperfused canine myocardium. J. Am. Coil. Cardiol. 13: 745-754, 1989. L. D., H. R. Weiss, and L. C. Becker. Myocardial 29. Stahl, oxygen consumption, oxygen supply/demand herogeneity, and microvascular patency in regionally stunned myocardium. Circulation 77: 865-872, 1988. H., T. Hayashi, M. Shirahata, S. Suehiro, and R. 30. Suga, Hisano. Regression of cardiac oxygen consumption on ventricular pressure-volume area in dog. Am. J. Physiol. 240 (Heart Circ. Physiol. 9): H320-H325, 1981. 31. Vanhaecke, J., W. Flameng, M. Borgers, I. Jang, F. Van de Werf, and H. De Geest. Evidence for decreased coronary flow reserve in viable postischemic myocardium. Circ. Res. 67: 12Ol1210, 1990. 32. Vinten-Johansen, J., P. Gayeart, W. Johnston, J. Julian, and R. Cordell. Regional function, blood flow and oxygen utilization relations in repetitively occluded-reperfused canine myocardium. Am. J. Physiol. 261 (Heart Circ. Physiol. 30): H538H547, 1991.

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