European Journal of Pharmacology, 177 (1990) 19-27
19
Elsevier EJP 51167
Haemodynamic and metabolic effects of xamoterol in exercising dogs G e o r g Fischer, W o l f g a n g Schneider, J o s e f G . G r o h s , Stefan E h r e n d o r f e r a n d G e r h a r d R a b e r g e r Pharmakologisches lnstitut Universit?it Wien, Wiihringerstrasse13a, 1090 Wien, Austria
Received 10 July 1989, revised MS received 20 October 1989, accepted 14 November 1989
The effects of xamoterol on the haemodynamic adaptation to graded treadmill exercise were evaluated during five subsequent cycles in chronically instrumented dogs. At rest xamoterol, 0.2 mg/kg i.v., preferentially showed a positive inotropic effect, whereas 1 mg/kg i.v. also exhibited a marked chronotropic effect. The cardiac output and left ventricular power increased dose dependently. The mean left atrial pressure and total peripheral resistance decreased concomitantly. Xamoterol did not produce a noteworthy decrease in heart rate or positive dp/dtma x during exercise, even at a dosage of 1 mg/kg. A fl-adrenoceptor blocking effect could only be seen from the diminution of the exercise-induced changes in heart rate, dp/dtm~ x, cardiac output, left ventricular power and total peripheral resistance. Determination of the blood glucose, lactate and pyruvate levels before the start of each exercise cycle revealed that the drug induced a decrease in blood glucose and an increase in blood pyruvate. Thus, xamoterol exerted a dose-dependent sympathomimetic effect in dogs at rest. However, there was little evidence for a fl-adrenoceptor blocking action even at higher work loads, although preliminary experiments in conscious dogs showed that xamoterol shifted the isoprenaline dose-response curve to the right by a factor of 1.31 (0.2 mg/kg) and 3.05 (1 mg/kg). Xamoterol; Haemodynamic effects; Metabolic effects; (Exercising dogs)
I. Introduction Xamoterol is commonly defined as a partial fll-adrenoceptor agonist. The dual action of the drug has been demonstrated experimentally under in vitro conditions and after autonomic blockade in vivo (Malta et al., 1985; Nuttall and Snow, 1982). Ohyagi et al. (1984) reported that xamoterol changed the adaptation of the cardiovascular system to graded treadmill exercise in dogs by inducing a strong inotropic and chronotropic effect at rest and a marked reduction in d p / d t m a x and heart rate at the peak rate of exercise. However, these findings were obtained with the very high dose of 10 m g / k g i.v., which exceeds the clinically applicable dose range by 50-100 fold (Sato et al.,
Correspondence to: G. Raberger, Pharmakologisches Institut Universit~it Wien, W~ihringerstrasse 13a, 1090 Wien, Austria.
1987; Sasayama et al., 1986; Jennings et al., 1983). Hence, the intention of the present study was to characterize the dualism of the effect of xamoterol on sympathetically mediated adaptation to graded treadmill exercise over a relevant range of dosages, which were assessed in preliminary experiments by isoprenaline antagonism in conscious dogs.
2. Materials and methods The animals used in this study were handled in accordance with the animal welfare regulations of the University of Vienna, and the experimental protocol was approved by the Animal Subjects Commitee of this Institution and of the Ministry of Science. The study was carried out with six mongrel dogs of either sex, weighing between 17 and 32 kg. The animals were vaccinated with Candivac D H L
0014-2999/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
20
(distemper, hepatitis, leptospirosis, rabies) and Candur P (parvovirosis) and were free of parasites. 'Loyal' dry food (Tagger, Graz) was used as the standard diet. Before instrumentation the dogs were trained to run on a treadmill (Quinton model 1854). In order to keep the animals accustomed to the specific exercise performance, identical time protocols and work loads were used in the training period and for the investigation after instrumentation. The animals were fasted overnight before the operation, with free access to water. Morphine (1 mg/kg s.c.) was given as premedication 1 h before anaesthesia was induced with pentobarbital (25 mg/kg i.v.). After endotracheal intubation with a cuffed Magill tube, the animals were ventilated with a N 2 0 / O 2 mixture (2:1) in a rebreathing system using an Engstroem respirator. Sterile thoracotomy was performed in the left fifth intercostal space and the pericardium was opened. A Koningsberg microtip manometer was inserted into the left ventricle via a stab hole in the apex. An electromagnetic flow probe was then placed around the pulmonary artery. A Tygon catheter was inserted into the left auricle for measurement of the left atrial pressure. Measurement of systolic and diastolic blood pressure was carried out by means of a catheter advanced into the descending aorta via the left carotid artery. Another catheter
advanced from the left jugular vein into the right atrium was used for drug infusions and blood sampling. All catheters and wires were exteriorized between the scapulae, and the thorax was then closed. The animals were monitored postoperatively. Propranolol, lidocaine, flunitrazepam and methadone were administered as required overnight, Ampicillin, 0.5 g, was given twice a day, for four days, beginning on the day before surgery. The dogs recovered within a couple of days, but the investigations were not started earlier than one week after surgery. The heart rate (derived from left ventricular pressure), systolic and diastolic aortic pressure, mean left atrial pressure (Statham pressure transducers), left ventricular positive dp/dtma x (Koningsberg-microtip, HSE physio-differentiator) and cardiac output (Statham flow probe and flowmeter) were recorded on a Watanabe 6 channel recorder. Pre-exercise values were taken 0.5 min before starting the treadmill. The work load was changed stepwise during the runs. The dogs were exercised at a constant elevation of 7% for 0.8 min, running at an effective speed of 4 km/h, then for 1.2 rain at 7 k m / h and for another 1.2 rain at 10 km/h. In order to complete the exercise cycle of 4 min, and for the technical purpose of resetting the programmer unit of the treadmill, the
I X A M O T E R O L i.v.
Elevation 7% Speed km/h 10-
i~i ili
0
4
kGycle []
15
11
CONTROL
lg
30
21 ~J
34
45
31
49
,7
60
4i
64
min
sl
EVALUATION OF DRUG EFFECTS
Fig. 1. The columns show the duration and grade of the five exercise cycles. The evaluated periods are dotted (control) and hatched (evaluation of drug effects).
21 final 0.8 min were used to return to the initial speed of 4 km/h. Exercise variables were recorded 0.8, 2 and 3.2 rain after the start. Exercise runs of 4 min were followed by recovery periods of 11 min, giving cycles of 15 min duration. After a warm-up cycle the dogs were subjected to the above mentioned protocol for five consecutive cycles. The first and second cycle served as control within the experiment. Xamoterol was given intravenously, over 1 rain, 9 rain after the start of the second cycle at a dosage of either 0.2 or 1 mg/kg. The third cycle was not taken into statistical evaluation to allow proper drug distribution. Drug effects were analysed during the fourth and fifth cycle (fig. 1). Biochemical variables such as blood glucose (Bergmeyer et al., 1970), lactate (Hohorst, 1970) and pyruvate (Bticher et al., 1962) were determined before and after administration of the high dose of xamoterol (1 mg/kg i.v.). For this purpose, blood samples were collected into icecooled tubes containing 20 I.U. of heparin before the start of the first, second, fourth and fifth cycle. In order to evaluate haemodynamic and biochemical changes which might occur physiologically in response to graded exercise, the dogs also underwent five control cycles and received physiological NaC1 solution instead of the drug. In preliminary experiments the fl-adrenoceptor blocking potency of xamoterol was evaluated by isoprenaline antagonism for dosages of 0.2 (n = 6) and 1 mg/kg i.v. (n = 5). Increasing concentrations of isoprenaline were administered by continous intravenous infusion. The dose-response curves obtained were averaged and statistically evaluated according to Carpenter (1986). The following formulas were used for calculations: (a) mean arterial pressure (MAP, kPa): (PAS + PAD)/2, where PAS and PAD are the systolic and diastolic blood pressure (kPa), respectively; (b) stroke volume (SV, ml): CO * 1000/HR, where HR and CO are the heart rate (beats/min) and the cardiac output (l/rain); (c) stroke work (Work, mJ): ((MAP-PLA) * C O / H R ) * 1000, where PEA is the mean left atrial pressure (kPa); (d) power (mW): ((MAP - PEA) * CO/60) * 1000; (e) total peripheral resistance (TPR, Units): MAP/CO.
2.1. Statistical analysis After a two-way analysis of variance for repeated measures had shown significant differences between the haemodynamic data recorded before and after treatment, a two-tailed Student's t-test for paired data was used for statistical evaluation of significant differences between the corresponding values. The metabolic data were evaluated in the same way by a two-way analysis of variance followed by a two-tailed Student's t-test for paired data with a-correction (Holm, 1979).
3. Results
3.1. Displacement by xamoterol of the dose-response curve for the isoprenaline-induce increase in heart rate Figure 2 shows the dose-response curve for the isoprenaline-induced increase in heart rate before and after the administration of xamoterol. The fl-adrenoceptor blocking potency of 0.2 and 1 mg/kg xamoterol i.v. shifted the dose-response curve for isoprenaline to the right and increased the EC~0 for isoprenaline by factors of 1.31 + 0.12 (S.E.M.) and 3.05 + 0.26 (S.E.M.). Comparison of the ECs0 values for isoprenaline before and after the administration of xamoterol revealed probability values of P > 0.05 (at 0.2 mg/kg) and P < 0.01 (at 1 mg/kg). The high dose of xamoterol shifted the isoprenaline dose-response curve to the right but not in a parallel manner.
3.2. Cardiovascular adaptation and metabofic response during five subsequent control cycles The average haemodynamic data for exercise cycles 1 and 2 did not significantly differ from the data for cycles 4 and 5. The resting values for glucose, lactate and pyruvate and the calculated lactate-pyruvate ratio did not significantly change throughout the experiments (table 1).
22 HR
// /
I
I
/
,d, f
150'
t
/
100
/I 7/ //I
,;o,,"
/z tO / / O' /
6"
100-
O
---.r-~,l C
i
0.1
110
ISO pg/kg/min i.v.
-'r~ff
,
C
0.1
110
ISO I.Jg/kg/min i.v.
Fig. 2. Dose-response curves for the isoprenaline-induced changes in heart rate (HR) before (o) and after (e) the administration of xamoterol (left side 0.2 and right side 1 mg/kg i.v.). The 95% confidence area before treatment is enclosed by broken lines and by solid lines thereafter.
3.3.1. Cardiovascular adaptation to exercise before drug administration
In exercisingdogs all haemodynamicvariables changed in proportion to the degree of activity TABLE 1 Arterial concentrations of glucose, lactate and pyruvate, and the calculated lactate-pyruvate ratio (L/P) in the control group (saline i.v.) and in the group treated with xamoterol (1 mg/kg i.v.). Values are means:kS.E.M., n = 6 , a P < 0.05 and b p < 0.01 with respect to cycle 1 of each group. (Cycle)
Control group
Xamoterol group
Glucose /t mol/ml
(1) (2) (4) (5)
3.83+0.08 3.90+0.13 3.77+0.21 3.76 + 0.24
3.69+ 3.59+ 3.19+ 3.21 +
0.16 0.18 0.18 b 0.21 b
Lactate /xmol/ml
(1) (2) (4) (5)
0.74+0.07 0.84+0.06 0.89+0.09 0.92+0.07
0.67+ 0.78+ 1.06+ 1.03-t-
0.08 0.11 0.18 0.18
Pyruvate nmol/ml
(1) (2) (4) (5)
58.7 72.5 79.2 76.2
-t-4.98 5:5.52 5:6.03 5:6.15
65.3 75.2 86.5 82.8
5:8.69 + 9.41 5:14.1 a +12.3 a
L/P
(1) (2) (4) (5)
12.7 11.7 11.2 12.2
5:0.83 5:0.47 5:0.32 5:0.46
10.6 10.4 12.6 12.5
5:1.03 5:0.88 5:1.50 5:1.18
(fig. 3). T h e h e a r t rate, c a r d i a c o u t p u t a n d calcul a t e d variables, such as positive left v e n t r i c u l a r d p / d t m ~ x, a n d left v e n t r i c u l a r power, g r a d u a l l y increased, r e a c h i n g their highest levels at the m a x i m u m w o r k load. T h e t o t a l p e r i p h e r a l resistance decreased. A l l v a r i a b l e s r e t u r n e d to the b a s a l values w i t h i n a few m i n u t e s o f recovery.
3.3.2. Effects of xamoterol on haemodynamic variables at rest and during exercise T h e a d m i n i s t r a t i o n of 0.2 o r I m g / k g x a m o t e r o l to resting a n i m a l s (fig. 4) i n c r e a s e d the positive d p / d t m a x ( + 3 8 % , + 5 6 % ) , h e a r t rate ( + 1 5 % , + 4 3 % ) , c a r d i a c o u t p u t ( + 2 2 % , + 2 6 % ) a n d left v e n t r i c u l a r p o w e r ( + 1 5 % , + 2 9 % ) . T h e stroke v o l u m e was i n c r e a s e d b y the low d o s e ( + 7%), b u t was d e c r e a s e d b y the high dose ( - 11%). T h e total p e r i p h e r a l resistance was d e c r e a s e d b y 22 a n d 20% b y the low a n d high doses, respectively. T h e m e a n arterial p r e s s u r e ( - 6%) was d e c r e a s e d o n l y b y the low dose. T h e m e a n left atrial p r e s s u r e was dec r e a s e d b y b o t h d o s a g e s ( - 13%, - 18%; fig. 3). A f t e r the a d m i n i s t r a t i o n o f 0.2 or 1 m g / k g x a m o t e r o l (fig. 3), the a b s o l u t e changes in the h e a r t rate, positive d p / d t m ~ , c a r d i a c o u t p u t a n d c a l c u l a t e d v a r i a b l e s e v o k e d b y increasing the w o r k l o a d were f o u n d to b e reduced. However, at the
23 XAMOTEROL 0.2 Speed Elevation
mg/kg
I.V.
km/h 7%
HR bm
_.. k--@ l
A’
0’
,’
-
:I?=7
co I mui’
1 66, ‘. I-$ ---f.
30
sv ml
e__=
U
0
PLA
11
;_z*
kPa
,
t LVWdt kPa
max
p+
/9---
S’
QL'
POWER w
j-6
response
to administration
of
The resting values for pyruvate increased after the intravenous administration of 1 mg/kg of xamoterol (table 1). The blood glucose level decreased concomitantly. The increase in lactate and calculated lactate-pyruvate ratio were not statistically significant.
4. Discussion
-&_;a
O-
TPR
3.3.3. Metabolic xamoterol
i.2
Fig. 3. Mean va1uesfS.E.M. of exercise-induced changes in the heart rate (HR), cardiac output (CO), stroke volume (SV), total peripheral resistance (TPR), mean left atrial pressure (PLA), positive left ventricular dp/dt mBx(LV dp/dt ,,), and left ventricular power (POWER) obtained from six dogs before (0) and after (0) the administration of xamoterol (0.2 and 1 md/kg). Statistical evaluation: t-test for paired data (* P < 0.05, * * P < 0.01, * * * P < 0.001).
peak exercise level none of the observed variables were significantly different from the pretreatment values following the administration of the low dose of xamoterol. After the high dose, only the cardiac output ( - 11%) was found to be markedly reduced in comparison with its corresponding pre-drug value. Increasing the work load abolished the significant drug-induced differences in the mean left atrial pressure.
The effects of xamoterol with regard to receptor affinity, intrinsic activity and P-adrenoceptorblocking potency are well defined. In an anaesthetized dog preparation, in which cardiovascular reflexes were prevented by depletion of catecholamines using syrosingopine and surgical section of vagal nerves, the agonist activity of xamoterol on the heart rate was 43% of the maximum increase produced by isoprenaline (Nuttall and Snow, 1982). Unlike other partial & selective adrenoceptor agonists with an intrinsic activity of more than 30% of that of isoprenaline, xamoterol does not possess &-adrenoceptor agonist activity (Barlow et al., 1981). The affinity ratio of xamoterol for & to & adrenoceptors in isolated tissues was approximately 40 to 1 (Cook et al., 1984). Xamoterol was shown to be 13 times more potent as an antagonist of the effects of isoprenaline at & than at & adrenoceptors (Nuttall and Snow, 1982). Both in vitro studies and in vivo experiments with autonomic blockade and anaesthesia are necessary for the classification of /3-adrenoceptor blocking agents. However, since cardiovascular reflexes can markedly modulate the effects of a drug, only in vivo experiments under physiological circumstances allow a true haemodynamic analysis. Ohyagi et al. (1984) reported a dual effect of xamoterol on cardiovascular adaptation to graded treadmill exercise in dogs. They concluded that the drug ‘buffered the heart’ from both a very low and a high sympathetic tone. According to Carpenter (1986), the maximum effect in a doseresponse relationship can be assumed to have been reached when a ten fold increase in dosage results in a less than 10% increase in the response. The
24
HEMODYNAMIC CHANGES WITH XAMOTEROL LV
HR
CO
SV
POWER
TPR
MAP
dp/dt max
zx~ 60" 40. 20. O' -20" -40 []
0.2 mg~kg i.v.
1 mg~kg i.v.
Fig. 4. Mean values+S.E.M, of the drug-induced changes (A values in %) in the resting values for the positive left ventricular dp/dtma x (LV dp/dtmax), heart rate (HR.), cardiac output (CO), stroke volume (SV), left ventricular power (POWER), total peripheral resistance (TPR) and mean arterial blood pressure (MAP) obtained from six dogs after the administration of xamoterol (0.2 mg/kg, white bars) and (1 mg/kg, hatched bars). Statistical evaluation: t-test for paired data * P < 0 . 0 5 ; ** P
19
Elsevier EJP 51167
Haemodynamic and metabolic effects of xamoterol in exercising dogs G e o r g Fischer, W o l f g a n g Schneider, J o s e f G . G r o h s , Stefan E h r e n d o r f e r a n d G e r h a r d R a b e r g e r Pharmakologisches lnstitut Universit?it Wien, Wiihringerstrasse13a, 1090 Wien, Austria
Received 10 July 1989, revised MS received 20 October 1989, accepted 14 November 1989
The effects of xamoterol on the haemodynamic adaptation to graded treadmill exercise were evaluated during five subsequent cycles in chronically instrumented dogs. At rest xamoterol, 0.2 mg/kg i.v., preferentially showed a positive inotropic effect, whereas 1 mg/kg i.v. also exhibited a marked chronotropic effect. The cardiac output and left ventricular power increased dose dependently. The mean left atrial pressure and total peripheral resistance decreased concomitantly. Xamoterol did not produce a noteworthy decrease in heart rate or positive dp/dtma x during exercise, even at a dosage of 1 mg/kg. A fl-adrenoceptor blocking effect could only be seen from the diminution of the exercise-induced changes in heart rate, dp/dtm~ x, cardiac output, left ventricular power and total peripheral resistance. Determination of the blood glucose, lactate and pyruvate levels before the start of each exercise cycle revealed that the drug induced a decrease in blood glucose and an increase in blood pyruvate. Thus, xamoterol exerted a dose-dependent sympathomimetic effect in dogs at rest. However, there was little evidence for a fl-adrenoceptor blocking action even at higher work loads, although preliminary experiments in conscious dogs showed that xamoterol shifted the isoprenaline dose-response curve to the right by a factor of 1.31 (0.2 mg/kg) and 3.05 (1 mg/kg). Xamoterol; Haemodynamic effects; Metabolic effects; (Exercising dogs)
I. Introduction Xamoterol is commonly defined as a partial fll-adrenoceptor agonist. The dual action of the drug has been demonstrated experimentally under in vitro conditions and after autonomic blockade in vivo (Malta et al., 1985; Nuttall and Snow, 1982). Ohyagi et al. (1984) reported that xamoterol changed the adaptation of the cardiovascular system to graded treadmill exercise in dogs by inducing a strong inotropic and chronotropic effect at rest and a marked reduction in d p / d t m a x and heart rate at the peak rate of exercise. However, these findings were obtained with the very high dose of 10 m g / k g i.v., which exceeds the clinically applicable dose range by 50-100 fold (Sato et al.,
Correspondence to: G. Raberger, Pharmakologisches Institut Universit~it Wien, W~ihringerstrasse 13a, 1090 Wien, Austria.
1987; Sasayama et al., 1986; Jennings et al., 1983). Hence, the intention of the present study was to characterize the dualism of the effect of xamoterol on sympathetically mediated adaptation to graded treadmill exercise over a relevant range of dosages, which were assessed in preliminary experiments by isoprenaline antagonism in conscious dogs.
2. Materials and methods The animals used in this study were handled in accordance with the animal welfare regulations of the University of Vienna, and the experimental protocol was approved by the Animal Subjects Commitee of this Institution and of the Ministry of Science. The study was carried out with six mongrel dogs of either sex, weighing between 17 and 32 kg. The animals were vaccinated with Candivac D H L
0014-2999/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
20
(distemper, hepatitis, leptospirosis, rabies) and Candur P (parvovirosis) and were free of parasites. 'Loyal' dry food (Tagger, Graz) was used as the standard diet. Before instrumentation the dogs were trained to run on a treadmill (Quinton model 1854). In order to keep the animals accustomed to the specific exercise performance, identical time protocols and work loads were used in the training period and for the investigation after instrumentation. The animals were fasted overnight before the operation, with free access to water. Morphine (1 mg/kg s.c.) was given as premedication 1 h before anaesthesia was induced with pentobarbital (25 mg/kg i.v.). After endotracheal intubation with a cuffed Magill tube, the animals were ventilated with a N 2 0 / O 2 mixture (2:1) in a rebreathing system using an Engstroem respirator. Sterile thoracotomy was performed in the left fifth intercostal space and the pericardium was opened. A Koningsberg microtip manometer was inserted into the left ventricle via a stab hole in the apex. An electromagnetic flow probe was then placed around the pulmonary artery. A Tygon catheter was inserted into the left auricle for measurement of the left atrial pressure. Measurement of systolic and diastolic blood pressure was carried out by means of a catheter advanced into the descending aorta via the left carotid artery. Another catheter
advanced from the left jugular vein into the right atrium was used for drug infusions and blood sampling. All catheters and wires were exteriorized between the scapulae, and the thorax was then closed. The animals were monitored postoperatively. Propranolol, lidocaine, flunitrazepam and methadone were administered as required overnight, Ampicillin, 0.5 g, was given twice a day, for four days, beginning on the day before surgery. The dogs recovered within a couple of days, but the investigations were not started earlier than one week after surgery. The heart rate (derived from left ventricular pressure), systolic and diastolic aortic pressure, mean left atrial pressure (Statham pressure transducers), left ventricular positive dp/dtma x (Koningsberg-microtip, HSE physio-differentiator) and cardiac output (Statham flow probe and flowmeter) were recorded on a Watanabe 6 channel recorder. Pre-exercise values were taken 0.5 min before starting the treadmill. The work load was changed stepwise during the runs. The dogs were exercised at a constant elevation of 7% for 0.8 min, running at an effective speed of 4 km/h, then for 1.2 rain at 7 k m / h and for another 1.2 rain at 10 km/h. In order to complete the exercise cycle of 4 min, and for the technical purpose of resetting the programmer unit of the treadmill, the
I X A M O T E R O L i.v.
Elevation 7% Speed km/h 10-
i~i ili
0
4
kGycle []
15
11
CONTROL
lg
30
21 ~J
34
45
31
49
,7
60
4i
64
min
sl
EVALUATION OF DRUG EFFECTS
Fig. 1. The columns show the duration and grade of the five exercise cycles. The evaluated periods are dotted (control) and hatched (evaluation of drug effects).
21 final 0.8 min were used to return to the initial speed of 4 km/h. Exercise variables were recorded 0.8, 2 and 3.2 rain after the start. Exercise runs of 4 min were followed by recovery periods of 11 min, giving cycles of 15 min duration. After a warm-up cycle the dogs were subjected to the above mentioned protocol for five consecutive cycles. The first and second cycle served as control within the experiment. Xamoterol was given intravenously, over 1 rain, 9 rain after the start of the second cycle at a dosage of either 0.2 or 1 mg/kg. The third cycle was not taken into statistical evaluation to allow proper drug distribution. Drug effects were analysed during the fourth and fifth cycle (fig. 1). Biochemical variables such as blood glucose (Bergmeyer et al., 1970), lactate (Hohorst, 1970) and pyruvate (Bticher et al., 1962) were determined before and after administration of the high dose of xamoterol (1 mg/kg i.v.). For this purpose, blood samples were collected into icecooled tubes containing 20 I.U. of heparin before the start of the first, second, fourth and fifth cycle. In order to evaluate haemodynamic and biochemical changes which might occur physiologically in response to graded exercise, the dogs also underwent five control cycles and received physiological NaC1 solution instead of the drug. In preliminary experiments the fl-adrenoceptor blocking potency of xamoterol was evaluated by isoprenaline antagonism for dosages of 0.2 (n = 6) and 1 mg/kg i.v. (n = 5). Increasing concentrations of isoprenaline were administered by continous intravenous infusion. The dose-response curves obtained were averaged and statistically evaluated according to Carpenter (1986). The following formulas were used for calculations: (a) mean arterial pressure (MAP, kPa): (PAS + PAD)/2, where PAS and PAD are the systolic and diastolic blood pressure (kPa), respectively; (b) stroke volume (SV, ml): CO * 1000/HR, where HR and CO are the heart rate (beats/min) and the cardiac output (l/rain); (c) stroke work (Work, mJ): ((MAP-PLA) * C O / H R ) * 1000, where PEA is the mean left atrial pressure (kPa); (d) power (mW): ((MAP - PEA) * CO/60) * 1000; (e) total peripheral resistance (TPR, Units): MAP/CO.
2.1. Statistical analysis After a two-way analysis of variance for repeated measures had shown significant differences between the haemodynamic data recorded before and after treatment, a two-tailed Student's t-test for paired data was used for statistical evaluation of significant differences between the corresponding values. The metabolic data were evaluated in the same way by a two-way analysis of variance followed by a two-tailed Student's t-test for paired data with a-correction (Holm, 1979).
3. Results
3.1. Displacement by xamoterol of the dose-response curve for the isoprenaline-induce increase in heart rate Figure 2 shows the dose-response curve for the isoprenaline-induced increase in heart rate before and after the administration of xamoterol. The fl-adrenoceptor blocking potency of 0.2 and 1 mg/kg xamoterol i.v. shifted the dose-response curve for isoprenaline to the right and increased the EC~0 for isoprenaline by factors of 1.31 + 0.12 (S.E.M.) and 3.05 + 0.26 (S.E.M.). Comparison of the ECs0 values for isoprenaline before and after the administration of xamoterol revealed probability values of P > 0.05 (at 0.2 mg/kg) and P < 0.01 (at 1 mg/kg). The high dose of xamoterol shifted the isoprenaline dose-response curve to the right but not in a parallel manner.
3.2. Cardiovascular adaptation and metabofic response during five subsequent control cycles The average haemodynamic data for exercise cycles 1 and 2 did not significantly differ from the data for cycles 4 and 5. The resting values for glucose, lactate and pyruvate and the calculated lactate-pyruvate ratio did not significantly change throughout the experiments (table 1).
22 HR
// /
I
I
/
,d, f
150'
t
/
100
/I 7/ //I
,;o,,"
/z tO / / O' /
6"
100-
O
---.r-~,l C
i
0.1
110
ISO pg/kg/min i.v.
-'r~ff
,
C
0.1
110
ISO I.Jg/kg/min i.v.
Fig. 2. Dose-response curves for the isoprenaline-induced changes in heart rate (HR) before (o) and after (e) the administration of xamoterol (left side 0.2 and right side 1 mg/kg i.v.). The 95% confidence area before treatment is enclosed by broken lines and by solid lines thereafter.
3.3.1. Cardiovascular adaptation to exercise before drug administration
In exercisingdogs all haemodynamicvariables changed in proportion to the degree of activity TABLE 1 Arterial concentrations of glucose, lactate and pyruvate, and the calculated lactate-pyruvate ratio (L/P) in the control group (saline i.v.) and in the group treated with xamoterol (1 mg/kg i.v.). Values are means:kS.E.M., n = 6 , a P < 0.05 and b p < 0.01 with respect to cycle 1 of each group. (Cycle)
Control group
Xamoterol group
Glucose /t mol/ml
(1) (2) (4) (5)
3.83+0.08 3.90+0.13 3.77+0.21 3.76 + 0.24
3.69+ 3.59+ 3.19+ 3.21 +
0.16 0.18 0.18 b 0.21 b
Lactate /xmol/ml
(1) (2) (4) (5)
0.74+0.07 0.84+0.06 0.89+0.09 0.92+0.07
0.67+ 0.78+ 1.06+ 1.03-t-
0.08 0.11 0.18 0.18
Pyruvate nmol/ml
(1) (2) (4) (5)
58.7 72.5 79.2 76.2
-t-4.98 5:5.52 5:6.03 5:6.15
65.3 75.2 86.5 82.8
5:8.69 + 9.41 5:14.1 a +12.3 a
L/P
(1) (2) (4) (5)
12.7 11.7 11.2 12.2
5:0.83 5:0.47 5:0.32 5:0.46
10.6 10.4 12.6 12.5
5:1.03 5:0.88 5:1.50 5:1.18
(fig. 3). T h e h e a r t rate, c a r d i a c o u t p u t a n d calcul a t e d variables, such as positive left v e n t r i c u l a r d p / d t m ~ x, a n d left v e n t r i c u l a r power, g r a d u a l l y increased, r e a c h i n g their highest levels at the m a x i m u m w o r k load. T h e t o t a l p e r i p h e r a l resistance decreased. A l l v a r i a b l e s r e t u r n e d to the b a s a l values w i t h i n a few m i n u t e s o f recovery.
3.3.2. Effects of xamoterol on haemodynamic variables at rest and during exercise T h e a d m i n i s t r a t i o n of 0.2 o r I m g / k g x a m o t e r o l to resting a n i m a l s (fig. 4) i n c r e a s e d the positive d p / d t m a x ( + 3 8 % , + 5 6 % ) , h e a r t rate ( + 1 5 % , + 4 3 % ) , c a r d i a c o u t p u t ( + 2 2 % , + 2 6 % ) a n d left v e n t r i c u l a r p o w e r ( + 1 5 % , + 2 9 % ) . T h e stroke v o l u m e was i n c r e a s e d b y the low d o s e ( + 7%), b u t was d e c r e a s e d b y the high dose ( - 11%). T h e total p e r i p h e r a l resistance was d e c r e a s e d b y 22 a n d 20% b y the low a n d high doses, respectively. T h e m e a n arterial p r e s s u r e ( - 6%) was d e c r e a s e d o n l y b y the low dose. T h e m e a n left atrial p r e s s u r e was dec r e a s e d b y b o t h d o s a g e s ( - 13%, - 18%; fig. 3). A f t e r the a d m i n i s t r a t i o n o f 0.2 or 1 m g / k g x a m o t e r o l (fig. 3), the a b s o l u t e changes in the h e a r t rate, positive d p / d t m ~ , c a r d i a c o u t p u t a n d c a l c u l a t e d v a r i a b l e s e v o k e d b y increasing the w o r k l o a d were f o u n d to b e reduced. However, at the
23 XAMOTEROL 0.2 Speed Elevation
mg/kg
I.V.
km/h 7%
HR bm
_.. k--@ l
A’
0’
,’
-
:I?=7
co I mui’
1 66, ‘. I-$ ---f.
30
sv ml
e__=
U
0
PLA
11
;_z*
kPa
,
t LVWdt kPa
max
p+
/9---
S’
QL'
POWER w
j-6
response
to administration
of
The resting values for pyruvate increased after the intravenous administration of 1 mg/kg of xamoterol (table 1). The blood glucose level decreased concomitantly. The increase in lactate and calculated lactate-pyruvate ratio were not statistically significant.
4. Discussion
-&_;a
O-
TPR
3.3.3. Metabolic xamoterol
i.2
Fig. 3. Mean va1uesfS.E.M. of exercise-induced changes in the heart rate (HR), cardiac output (CO), stroke volume (SV), total peripheral resistance (TPR), mean left atrial pressure (PLA), positive left ventricular dp/dt mBx(LV dp/dt ,,), and left ventricular power (POWER) obtained from six dogs before (0) and after (0) the administration of xamoterol (0.2 and 1 md/kg). Statistical evaluation: t-test for paired data (* P < 0.05, * * P < 0.01, * * * P < 0.001).
peak exercise level none of the observed variables were significantly different from the pretreatment values following the administration of the low dose of xamoterol. After the high dose, only the cardiac output ( - 11%) was found to be markedly reduced in comparison with its corresponding pre-drug value. Increasing the work load abolished the significant drug-induced differences in the mean left atrial pressure.
The effects of xamoterol with regard to receptor affinity, intrinsic activity and P-adrenoceptorblocking potency are well defined. In an anaesthetized dog preparation, in which cardiovascular reflexes were prevented by depletion of catecholamines using syrosingopine and surgical section of vagal nerves, the agonist activity of xamoterol on the heart rate was 43% of the maximum increase produced by isoprenaline (Nuttall and Snow, 1982). Unlike other partial & selective adrenoceptor agonists with an intrinsic activity of more than 30% of that of isoprenaline, xamoterol does not possess &-adrenoceptor agonist activity (Barlow et al., 1981). The affinity ratio of xamoterol for & to & adrenoceptors in isolated tissues was approximately 40 to 1 (Cook et al., 1984). Xamoterol was shown to be 13 times more potent as an antagonist of the effects of isoprenaline at & than at & adrenoceptors (Nuttall and Snow, 1982). Both in vitro studies and in vivo experiments with autonomic blockade and anaesthesia are necessary for the classification of /3-adrenoceptor blocking agents. However, since cardiovascular reflexes can markedly modulate the effects of a drug, only in vivo experiments under physiological circumstances allow a true haemodynamic analysis. Ohyagi et al. (1984) reported a dual effect of xamoterol on cardiovascular adaptation to graded treadmill exercise in dogs. They concluded that the drug ‘buffered the heart’ from both a very low and a high sympathetic tone. According to Carpenter (1986), the maximum effect in a doseresponse relationship can be assumed to have been reached when a ten fold increase in dosage results in a less than 10% increase in the response. The
24
HEMODYNAMIC CHANGES WITH XAMOTEROL LV
HR
CO
SV
POWER
TPR
MAP
dp/dt max
zx~ 60" 40. 20. O' -20" -40 []
0.2 mg~kg i.v.
1 mg~kg i.v.
Fig. 4. Mean values+S.E.M, of the drug-induced changes (A values in %) in the resting values for the positive left ventricular dp/dtma x (LV dp/dtmax), heart rate (HR.), cardiac output (CO), stroke volume (SV), left ventricular power (POWER), total peripheral resistance (TPR) and mean arterial blood pressure (MAP) obtained from six dogs after the administration of xamoterol (0.2 mg/kg, white bars) and (1 mg/kg, hatched bars). Statistical evaluation: t-test for paired data * P < 0 . 0 5 ; ** P
