Effects of Beta Blockers on Cardiac Function and Myocardial Oxygen Consumption in the Isolated Supported Heart Preparation of the Dog

Seiji KUMAKURA,Ph.D. and Takeshi OSHIMA,Ph.D.

SUMMARY Effectsof 2 different types of beta blockers,CS-359 and practolol, on me- chanical performance and myocardial oxygen consumption (MVO,) in the isovolumiccontraction of the isolated, blood-perfusedcanine heart with a support dog were studied. Two dose levels of the beta blockers were given intravenously to the support dog, one of which was enough to pro- duce a significantbeta adrenergic blocking action and other of which was a higher dose to reveal a direct depressant action on the isolated heart. CS-359 in the lower dose produced significantdecreases in heart rate (HR), peak ventricular systolic pressure (peak VSP), and peak dp/dt while practolol caused no change in these parameters. The HR of the support dog was significantly decreased to the same degree in the lower dose of respectiveblocker. In the higher dose CS-359decreased all para- meters more definitely whereas practolol diminished peak VSP and peak dp/dt but did not significantly HR. MVO, was decreased dose-relatedly by CS-359,but not affected significantlyby practolol. From the analysis of relationshipsof MVO, to peak VSP and HR in the isolated heart it is concluded that the effectivereduction in MVO, by CS-359 resulted mainly from a reduction in the peak VSP in the lower dose and additional decrease in HR in the higher dose of CS-359. Additional Indexing Words: Isovolumic contraction Constant volume perfusion Practolol CS-359 Intrinsic stimulant action Controlled cardiac parameters

T HERE agents is forno the doubt ischemicon the heartclinical disease.usefulness The of principalbeta adrenergic usefuln( to be based on the lollowing reason. sympathetic excitation augments me- chanical activity of the heart by increasing heart rate and contractile force against elevated aortic pressure which results in an increase in myocardial oxygen consumption.') In the normal heart the coronary arteries dilate to allow an increase in the coronary flow enough to meet this requirement. Beta blockers antagonize catecholamines at their receptor sites preventing the

From the Central Research Laboratories, Sankyo Co., Ltd., 2-58, 1-chome Hiromachi, Shina- gawa-ku, Tokyo 140, Japan. Received for publication January 10, 1975. 592 Vol. 16 No. 5 B-BLOCKERS AND MYOCARDIAL OXYGEN CONSUMPTIOP 593 augmentation of heart rate and contractile force which tends to reduce nutri- tional circulation ,2l and thus restored the impaired balance in the ischemic heart between oxygen supply and oxygen requirement to respond to the imposed extra cardiac work on the adrenergic excitation. A fairly large number of beta blockers have been reported, but each one has its own pharmacological characteristics such as intrinsic stimulant, mem- brane stabilizing, and cardiac depressing actions and relaxation of vascular smooth muscle. Practolol is characterized by its intrinsic stimulant action 3),4> whereas CS-359 is devoid of it.) In this study we used an isolated blood-perfused caine heart with a support dog which was originally designed by Monroe et als> but extensively modified by us for the more precise determination of oxygen consumption in order to analyse the effect of 2 characteristic beta blockers above mentioned on the myocardial oxygen consumption.

METHODS

Procedure to prepare an isolated supported heart performing isovolumiccontractions Mongrel dogs of either sex weighing 10 to 18 Kg were used. Two dogs anes- thetized with sodium pentobarbital (30 mg/Kg i.v.) were used in each experiment, a smaller one from 8 to 10 Kg for isolation of the heart, another one over 15 Kg as

Fig. 1. Illustration of the isolated supported heart provided with can- nulae and a balloon. A screen attached to a supporting cannula was intruded into the ventricle to protect the aortic valve against slip-out of the balloon when compressed. A mitral cannula was secured at the level of the mitral annulus. rap. Heart J S 59, KUMAKURA AND OSHIMi eptember, 197

Fig. 2. Schematic diagram to show perfusion circuit for the isolated heart and instruments to measure cardiac parameters. AT; air trap, B; balloon, BP; blood pressure transducer for support dog, BR; blood reservoir for venous return, CBF; magnetic flowmeter, dp/dt; differentiator of left ventricular pressure, HE; heat exchanger, HR; cardiotachometer, ISHP; isolated supported heart preparation, LVP; left ventricular pressure trans- ducer, MC; mitral cannula, P; peristaltic pump, PAO2 and PVO2; oxygen electrodes for arterial and venous blood, PC; perfusion cannulae, PP; perfusion pressure transducer, R; pneumatic resistance, S; electrical stimulator, SD; support dog, T; thermometer, VV; syringe to adjust amount of water in the balloon. a support dog. The chest of the smaller dog was opened by midsternal incision under artificial respiration and 1,000 units/Kg of heparin was administered. Im- mediately after the superior and inferior vena cava and the vena azygos were ligated, the heart was excised and then quickly transferred into cold saline (0°-2°C) saturated with a gas mixture of 95% 0, and 5% CO, The trunk of pulmonary artery was separated free from the aorta so that the common left coronary artery was easily approached for isolation. The coronary artery was dissected from the surrounding tissues as closely to its orifice as possible. The right coronary artery was also carefully dissected from the fat tissue where the artery was embedded. The left atrium was opened so wide that the mitral valves were visualized directly enough to be excised along the annulus. A thin latex balloon attached to a mitral cannula was intruded through the annulus and placed in the left ventricular cavity. The mitral cannula was then fixed around the annulus by a purse suture (Fig. 1). Additional purse suture of the cut margin of the open left atrium was made firmly around the mitral cannula to prevent bleeding. Next, a supporting cannula was fixed in the aorta, inserting a screening plate attached to the cannula in the ventricle through the aortic valves which served for prevention of herniation Vol. 16 No. 5 fl-BLOCKERS AND MYOCARDIAL OXYGEN CONSUMPTION 595

)t the inflated balloon toward the aorta when the lest ventricle contractea vigor- 3usly. The right and left coronary cannulae were advanced to the orifices if both arteries and carefully fixed with silk threads without disturbing the natural blood stream. The heart prepared with the aortic and mitral cannulae in the cold saline was hen mounted on a circulation system as shown in Fig. 2. Arterial blood led from :he femoral artery of the support dog was perfused via the coronary cannulae by i peristaltic pump. The support dog was artificially respired with air (Aika model [2-60). Coronary venous blood was collected from the pulmonary artery and returned to the femoral vein of the support dog.

Measurementof cardiac parametersof the isolated heart The coronary blood flow was measured with a magnetic flowmeter (Nihon- koden model MF-2) as total coronary inflow. The left ventricular pressure generated in the water-filled balloon was measured with a pressure transducer. (Nihonkoden model LPU-0.5). Pressure rise in the left ventricle was differen- tiated with a CR circuit of time constant 2 msec to obtain dp/dt. Arterial and venous oxygen tensions were continuously measured with oxygen electrodes (Beck- man macrotype) which were placed amidst the blood stream. The oxygen tension was determined in a steady state because these electrodes did not respond to quick change in oxygen tension. Myocardial oxygen consumption (MVO, in ml/100 Gm/ min) was then calculated as follows. MVO,= (sAO2-sVO2) xCBF x HW x0.013 where CBF is coronary flow rate in ml/min, sAO2 and sVO2 are percent oxygen saturation in arterial and venous blood respectively which are found in the oxyger dissociation curve of the dog,') Hb is hemoglobin content in Gm/100 ml in the per. fusing blood determined with a hemoglobinometer (Elmer model 303), and HW ih wet weight of the isolated heart in grams before the perfusion experiment. Hear,,, rate was measured with a cardiotachometer (Nihonkoden model RT-2) triggered by the pulse pressure developed in the left ventricle. The heart rate of the supponl dog was also measured with another tachometer triggered by the carotid arteria pressure. In the present study the heart was initially perfused by a constant pressure fol about 30 min which was provided by allowing over-flow of the perfusing blood viz a Starling resistance (R in Fig. 2). When the stable state was obtained, it wag changed to the constant volume perfusion by closing the bypass via the resistance After closing the bypass the perfusion pressure was adjusted to 100 mmHg by con• trolling the perfusion volume. The perfusion pressure was measured by a pressure transducer (PP in Fig. 2) which was non-pulsatile because of the use of a large air. cushion (AT in Fig. 2). Thus the heart received the constant volume of arteria blood throughout the subsequent experiment. In the present experiments 2 doses of CS-359 were employed; a lower dose 0.03 mg/Kg, producing about 85% reduction in the positive chronotropic actior induced by intravenous isoproterenol, 0.3 ug/Kg in the anesthetized dog and higher one, 1.0 mg/Kg, inducing a slight cardiodepression of approximately 25% increase in the right arterial pressure of canine heart-lung preparation .5) Corres Jap. Heart J 596 KUMAKURA AND OSHIMA September, 197. ponding doses of practolol, 0.3 and 10.0 mg/Kg, were selected according to the ratio of beta blocking potencies between practolol and CS-359.4) Two cumulative doses of each beta blocker were administered intravenously to the support dog so as to demonstrate simultaneously effects of the blocker on the isolated heart and the in vivo heart.

RESULTS Controlvalue of coronaryflow When the heart prepared in the cold saline solution was perfused with arterial blood of the support dog, fibrillation occurred to all preparations. Normal sinus rhythm, however, was readily restored by a counter shock. The amount of water filled in the balloon to develop peak systolic pressure of 100 mmHg was 14.5 ±1.8 ml/ 100 Gm HW (n=10). Total coronary inflow was 75±9 ml/100 Gm HW/min (n=12).

Influenceof changingvolume in the balloon on the peak systolicpressure (peak VSP) of the ventricle A relationship between peak VSP and amount of filling water is presented in Fig. 3, which indicates almost linear relation within a range from a little more than 12 ml to over 20 ml. Further increase revealed an extra augmenta- tion in compliance of the ventricle with increase in diastolic pressure. The figure also presents a reproducibility of the volume-pressure relationship when the same procedure was repeated twice.

Fig. 3. Volume-pressure relationship in the isolated heart and reproduci- bility. Filled circles show the first trial and open ones the second trial 30 min later. Vol. 1t No. 5 a-BLOCKERS AND MYOCARDIAL OXYGEN CONSUMPTION 597 Influenceof changingpeak VSP or heart rate on myocardialoxygen consumption (MV02) Influence of changing peak VSP on MVO, was studied by increasing the ventricular volume, i.e. the amount of water in the balloon, in the electrically driven heart (Fig. 4). Any increase in peak VSP was accompanied by a certain increase in peak dp/dt while the level of oxygen tension in the coronary outflow diminished, indicating an increase in MVO,. On the other hand, relationship between heart rate and MVO, was de- termined by increasing heart rate while peak VSP was kept constant at 100 mmHg by adjusting the amount of water in the balloon (Fig. 5). This proce-

Fig. 4. Influence of elevation of peak systolic pressure on myocardial oxygen consumption in the isolated heart with constant heart rate . Peak VSP was changed stepwise as illustrated in LVP.

Fig. 5. Influence of elevation of heart rate on myocardial oxygen con- sumption in the isolated heart under constant peak systolic pressure as shown in LVP. Jap. Heart J S 59E KUMAKURA AND OSHIMA eptember, 197

Fig. 6. Relationships between myocardial oxygen consumption and peak systolic pressure, and heart rate. Heart rate was fixed in A, whereas peak systolic pressure was held constant in B.

Fig. 7. Changes in cardiac parameters produced by CS-359 in A and by practolol in B. Cardiac parameters of the isolated heart were affected more definitely by CS-359 than by practolol while the heart rate of the sup- port dog was decreased in a similar manner by both blockers. * and ** indicate P<0.10 and <0.05 respectively by paired group comparison. Vol. 16 No. 5 19-BLOCKERS AND MYOCARDIAL OXYGEN CONSUMPTIOI\ 599 dure was also accompanied by an inevitable increase in peak dp/dt. MVO2 increased in proportion to the elevation of heart rate. The MVO,-peak VSP curve seems to be convex to the peak VSP axis (Fig. 6A) while the MVO, -HR curve is almost linear (Fig. 6B).

Elect of bata blockerson cardiacfunction and MVO2 The heart rate of the support dog was markedly reduced following the lower dose of CS-359, 0.03 mg/Kg i.v., and the sinus rate of the isolated heart was also reduced but less significantly. All other cardiac parameters measured were significantly decreased in parallel with the change in heart rate and more definitely influenced by the higher dose, 1.0 mg/Kg i.v. Changes in the cardiac parameters of 5 experiments are presented in Fig. 7A. Practolol was applied to 5 other preparations in 2 doses just 10 times as much as corresponding doses of CS-359. The heart rate of the support dog decreased with 0.3 mg/Kg i.v. of the lower dose of practolol to the similar extent with that of CS-359, 0.03 mg/Kg (Fig. 7B). In the isolated heart no significant change in the cardiac parameters was observed with the lower dose of practolol. Both peak VSP and peak dp/dt decreased when 10 mg/Kg of practolol was given, but any significant change in MVO, was not induced.

Fig. 8. Influence of elevation of peak systolic pressure and heart rate in the presence of CS-359 (left) and practolol (right). A and A'; direct effect of the blockers on spontaneously beating heart. B and B'; influence of adjusted peak systolic pressure by increasing vertricular volume. C and C'; additional influence of adjusted heart rate by pacing the isolated heart. * and ** denote P<0 .10 and <0.05 respectively from the preceding value. Jap. Heart J S 600 KUMAKURA AND OSHIMA eptember, 197°. What parameter will contribute to the beta blockade-induceddecrease in MVO,? Heart rate and peak VSP of the isolated heart were separately control- led to the initial level in the presence of the beta blocker. When the maximum decrease was observed with the higher dose of CS-569, the decreased peak VSP was restored to the initial level of 100 mmHg by adding water to the balloon in the ventricle. The decreased MVO, presented at A in Fig. 8 increased from 4.2± 1.1 to 4.8 ± 1.0 ml/ 100Gm/min under this treatment (B in Fig. 8). Then the lowered heart rate by the drug was elevated to the initial sinus rate by electrical pacing as shown at C in Fig. 8 while the peak VSP was maintained at 100 mmHg. Under both treatments MVO, restored almost the control level, though there was no significant difference in MVO, between these 2 procedures (B and C in Fig. 8). On the contrary, practolol even in the higher dose did not affect the level of MVO, significantly after the similar procedures as shown at B' and C' in Fig. 8. In the presence of either beta blocker peak dp/dt decreased consistently when the heart rate was elevated by electrical pacing, even though peak VSP was held at 100 mmHg during the stimulation.

DIscussior' In the present study much attention was paid to the heart which per- forms isovolumic contraction which requires more energy consumption." A thin latex balloon was placed in the left ventricle to impose isometric work load on the ventricle. The device enabled us to control peak VSP at will by adjusting amount of water in the balloon. The volume-pressure relationship in the heart of the present study re- veals that a change in compliance is observed at the ventricular volume of more than 20 ml with concomitant increase in diastolic pressure. MVO, cannot remain unaffected by the change in the diastolic pressure, which, however, is to lesser extent than by change in tension development in systole.') The lower doses of the 2 beta blockers employed in the present experi- ment were almost equipotent for producing beta blocking activity because reductions in the heart rate of the support dog by them were almost same in magnitude. The heart rate in the isolated heart was little affected by use of the beta blocking dose, suggesting that very slight sympathetic tone was involved in maintenance of cardiac function of the isolated heart. CS-359 in the lower dose, however, affected contractile force as evidenced by a reduc- tion in peak VSP while practolol did not appear to have any influence on it, which is considered to have induced the difference in the change of Vol. 16 No. 5 fl-BLOCKERS AND MYOCARDIAL OXYGEN CONSUMPTIOI' 60'. MVO, between CS-359 and practolol in their lower dose. A direct action of both beta blockers was clearly demonstrated on the isolated heart in the higher doses. It seems very important that CS-359 reduced further the heart rate of the isolated heart as well as peak VSP, and the difference in the reduction of MVO, between both beta blockers was manifested more distinctly. The significant reduction in MVO, by CS-359 is thus ascribed partly to a decrease in peak VSP in the lower dose and partly to a decrease in the heart rate in the higher dose as shown in the MVO,-peak VSP and -heart rate relationships (Fig. 6A and B). Effective reduction in MVO, would be obtained when both parameters of peak VSP and heart rate move downward along these relationship curves mentioned above, and this is exactly the case with CS-359 as evidenced in Fig. 7A. A heart rate-dependent increase in peak dp/dt was abolished after both beta blockers. Even when peak VSP was adjusted at 100 mmHg, a decrease in peak dp/dt was observed. Byon and Fleckenstein could not demonstrate any contribution of dp/dt to MVO, in the isolated rabbit papillary muscles where isometric tension development was controlled in the presence of stimulant agents.') The drug-induced decrease in peak dp/dt, however, will reduce extravascular compression during systole and allow much time for the heart to improve nutritional circulation through hypoxic capillary beds. Some beta blockers have been reported to possess a cardiac stimulant property,3),4>,11)which may supplement intrinsic negative chronotropic and inotropic actions of these drugs,") indicating a therapeutic advantage. 12),'3) However, since any mechanical augmentation of the heart during cardiac cycles is related to elevation of MVO2i1>,14)effective reduction in MVO, is based to some extent on a decrease in the mechanical activity at the expense of depressed function of the heart. In the present study it was demonstrated that the difference in the effect between both beta blockers on MVO, is mainly due to the extent of reduc- tion in heart rate and peak VSP, and thus this is considered to be an essential difference in MVO, between a beta blacker characterized by an intrinsic stimulant action and other blocker free from such character.

ACKNOWLEDGEMENTS

Authors express their cordial thanks to Prof. K. Hashimoto, Department of Pharmacology, Tohoku University School of Medicine for advice and discussion, and to Dr. Y. Sakai for much encouragement, to Messrs. H. Nishino, H. Mizuno, and K. Iwamoto for technical assistance, and to Mr. T. Takahashi for excellent workmanship. 602 KUMAKURA AND OSHIMA September,Jap. Heart1975 J,

REFERENCES

1. Klocke FJ, Kaiser GA, Jr, Braunwald E: Mechanism of increase of myocardial oxygen up- take produced by catecholamines. Am J Physiol 209:913, 1965 2. Somani P, Laddu AR, Hardman HF: Nutritional circulation in the heart. III Effect of isoproterenol and beta adrenergic blockade on myocardial hemodynamics and Rubidium86 extraction in the isolated supported heart preparation. J Pharmacol Exp Ther 175:577, 1970 3. Barret AM, Carter J: Comparative chronotropic activity of beta adrenoceptive antagonists. Br J Pharmac 40:373, 1970 4. Hashimoto K, Kubota K, Chiba S, Taira N: Comparison of beta adrenergic blocking activity of eight blockers in the excised and blood-perfused canine sino-atrial node preparation. Ex- perientia 28:822, 1972 5. Oshima T, Kumakura S, Koike H, Nakayama K: Pharmacology of dl-5-methyl-8-(2-hydroxy- 3-t-butylaminopropoxy) coumarin hydrochloride (CS-359), a new beta-adrenergic blocking agent. Jap J Pharmacol 23:497, 1973 6. Monroe RG, French GN: Left ventricular pressure-volume relationships and myocardial oxygen consumption in the isolated heart. Circulat Res 9:362, 1961 7. Bartels H, Harms H: Sauerstoffdissoziationskurven des Blutes von Saugetieren (Mensch, Kaninchen, Meerschweichen, Hund, Katze, Schwein, Rind and Schaf). Pflugers Archiv 268:334, 1959 8. Burns JW, Covell JW: Myocardial oxygen consumption during isotonic and isovolumic con- tractions in the intact heart. Am J Physiol 223:1491, 1972 9. Byon YK, Fleckenstein A: Myocardial oxygen consumption at various rates of tension de- velopment. Das chronisch kranke Herz, FK Schattauer Verlag, Stuttgart-New York, 53, 1973 10. Merin RG: Myocardial metabolism and dynamics after practolol during light halothane anaesthesia. Br J Anaesth 44:437, 1972 11. Fitzgerald JD, O'Donnell SR: A comparison of the haemodynamic effects of propranolol, 4-hydroxy propranolol and practolol in anaesthetized dogs. Br J Pharmac 45:207, 1972 12. Sowton E, Balcon R, Cross D, Frick H: Haemodynamic effects of I.C.I. 50172 in patients with ischaemic heart disease. Br Med J 1:215, 1968 13. Smith RD, Nash CB: Ventricular function curve analysis of the myocardial effects of three beta blockers. Arch int Pharmacodyn 203:151, 1973 14. Sonnenblick EH, Ross J, Braunwald E: Oxygen consumption of the heart. Newer concepts of its multifactoral determination. Am J Cardiol 22:328, 1968