JOURNAL OF APPLIED PHYSIOLOGY Vol. 41, No. 3, September 1976. Printed

Evaluation

in U.S.A.

of the isolated

paced rat heart

PAUL B. TAYLOR AND FRANK J. CERNY Faculty of Human Kinetics, University of Windsor,

Windsor,

Ontario

N9B 3P4 Canada

ligature. Retrograde perfusion was begun from a reservoir 75 cm above the heart. After 10 min of preliminary washout perfusion, the heart was transferred to a recirculating system in which the perfusion pressure was adjusted by a peristaltic pump to either 60, 90, or 120 mmHg. The perfusion medium was a Krebs-Henseleit bicarbonate buffer, containing the following salts in millimoles per liter: NaCl, 118; KCl, 4.7; CaCl,, 2.5; MgS04, 1.2; KH2P04, 1.2; NaHCO,, 25; Na,EDTA, 0.5; and glucose, 5.5. The perfusate was maintained at 37OC and oxygenated with 95% 0,.5% CO, gas mixture equilibrated with water at 37°C. Left ventricular pressures were measured through a 20-gauge needle inserted through the apex of the heart. The needle was attached to a Statham P23Db pressure transducer and the pressure changes monitored on a Beckman RS dynograph recorder. Ventricular systolic Langendorff perfused heart; work overload; electrical pacing; and diastolic pressures and dP/& were determined from left ventricular function three consecutive ventricular pressure curves. Coronary flow was estimated by taking timed, volumetric collections of the effluent from the pulmonary artery. NUMEROUS STUDIES HAVE EMPLOYED an isolated rat To induce tachycardia, the isoated hearts were paced heart to observe both hemodynamic and biochemical (4-10 V; 4 ms) with a Grass SD5 stimulator. The active changes induced by acute forms of work overload. In lead was placed on the left ventricle and the ground lead general, most experiments have altered work capacity on the aortic cannula. Hearts were paced from 300 to 600 by either increasing or decreasing the pressure load beats/min with 5O-beat increments. At each rate, venwhile the heart rate has remained unchanged. Re- tricular pressure and coronary flow were monitored at cently, Barnard et al. (2) have shown that both young 60, 90, and 120 mmHg afterload. Ventricles unable to and old animals respond to sudden exhaustive exercise respond to a systematic increase in contraction rate and/ by increasing their heart rates to approximately 600 or perfusion pressure were not used. To determine the beatslmin. During steady-state exercise, these animals effect of this protocol, three hearts were stimulated in maintained a heart rate of 554 t 5 beatslmin in excess of reverse order (600 to 300 beatslmin) immediately follow1 h of continuous running. ing the above procedures; no differences were noted in In the present investigation, an isolated perfused rat any of the parameters. All data were analyzed with a heart model was used to evaluate the effects of high one-way analysis of variance. heart rates as an additional means of inducing acute work overload.

TAYLOR, PAUL, B., ANDFRANK J. CERNY.EU&~~~O~ of the isoZated paced rat heart. J. Appl. Physiol. 41(3): 328331. 1976. -Ventricular performance and coronary flow in Langendorff perfused rat hearts were measured over a wide range of perfusion pressures and heart rates. A change in aortic pressure from 60 to 120 mmHg induced a linear increase in coronary flow, ventricular systolic pressure, and contractility. Ventricular pacing from 300 to 600 beats/min under a constant afterload had no effect on coronary flow. Systolic pressure remained stable up to 400-450 beats/min and then decreased 14% at 600 beats/min compared to the nonpaced controls. When contraction rate exceeded 450 beats/min diastolic pressure progressively increased as the heart rate was elevated. Contractility decreased rapidly between 450 and 600 beatslmin under all perfusion pressures. These data indicate that this heart model is physiologically stable with heart rates less than 450 beats/min and may be useful in studying tachycardia-induced work overload.

RESULTS

METHODS

Male Wistar rats weighing 350-450 g with free access to water and laboratory chow were used for all experiments. The animals were anesthetized with ether and the abdominal cavity opened by making a midline incision with scissors. The inferior vena cava was located and 150-200 U of heparin injected. The hearts were quickly removed and placed into ice-cold saline to arrest the contractions. With fine-tipped forceps, the hearts were removed from the cold saline and the aorta slipped onto a grooved perfusion cannula of a modified LangendorfY perfusion apparatus (9) and secured with a silk

Effects of perfusion pressure on ventricular performance. The data on hearts perfused by this modification of the Langendorff technique under different perfusion pressures are presented in Table 1. When the perfusion pressure was increased from 60 to 120 mmHg, coronary flow increased linearly from 70 to 136 ml/min per g. Heart rate showed a slight increase as the perfusion was raised and may be accounted for as suggested by Neely et al. (10) by an increase in the temperature of the perfusate in the aortic cannula at the higher flow rates. Intraventricular peak systolic pressure increased as the afterload was raised and was found to exceed the perfu-

328

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 17, 2019.

PACED

ISOLATED

329

HEART

1. Effect of perfusion pressure on coronary flow, ventricular pressure, and contractility TABLE

Perfusion

Pressure,

60 Heart rate, beats/min Coronary flow, ml/min per g Ventricular pressure, mmHg Systolic Diastolic Contractility, dP/&, mmHg/s

180-

87 + 2 0 3,250 Y.!I200

120 286 + 11 136 + 9

125 2 3 0 4,937 2 185

159 + 4 0 7,008 + 210

T

5 r 5

i20

mm Hg

60

mm Hg

80

cr s

SOiI

I

I

I

I

300

I

I

600

HEART RATE (beats/min) 1. Coronary

flow of 60, 90, and 120 mmHg. to each contraction rate. FIG.

s 1

60

mm Hg

30

2-3 min to adjust to each perfusion flow is expressed as ml/min per g dry

sion pressure by 20-25 mmHg. Ventricular diastolic pressure was stable over the range of perfusion pressures used. Contractility as measured by dP/dt also showed a linear increase from 3250 to 7000 mmHg/s as the perfusion pressure was elevated from 60 to 120 mmHg. These data indicate that hearts perfused in this system appeared stable and were able to adapt to acute changes in afterload. Effect of contraction rate on coronary flow. In an attempt to examine the influence of ventricular contraction on coronary flow, hearts were perfused under a constant aortic pressure of 60, 90, and 120 mmHg and the heart rate progressively increased to 600 beats/min. In the present study a 2.5fold increase in the rate of muscle contraction had no systematic effect on coronary flow (Fig. 1). However, adjusting the aortic pressure by increasing the output of the peristaltic pump significantly increased the flow through the myocardium, indicating that coronary flow in this system was more responsive to changes in aortic pressure than to an increase in contraction rate. Effect of contraction rate on ventricular pressure. The ability of the myocardium to develop ventricular pressure was studied over a wide range of aortic pressures and contraction rates (Fig. 2). At any given perfusion

160 c

mm Hg

60

276 + 11 105 2 9

Values are means + SEM. Hearts were allowed pressure before measurements were taken. Coronary tissue. Nine hearts were used.

120

mmHg

90

245 + 6 70 zt 6

,

measurements under a constant afterload The hearts were allowed 2-3 min to adjust Values are means + SEM of 10 hearts.

SYSTOLIC

5 loo

'80:'-N--44

2

p

I

I

604020o-

DIASTOLIC 0

I

300

I

1

400

I

1

HEART RATE (beatslmin) 2. Effects of varying contraction rate on ventricular systolic and diastolic pressure while the aortic pressure was maintaned constant at three different pressures. Values are means + SEM of 811 hearts. FIG.

pressure as rate of muscle contraction increased, peak ventricular systolic pressure appeared stable up to 450 beats/min. A further increase in heart rate produced a small but statistically significant (P < 0.05) decrease in systolic pressure which was observed during low (60 mmHg) and high (120 mmHg) perfusion conditions. Although these hearts showed a decrease in peak pressure development during the higher rates of contraction, all hearts were able to develop ventricular pressure in excess of the aortic perfusion pressure. In contrast to the small change in systolic pressure, ventricular diastolic pressure rapidly increased as the heart rate was elevated from 500 to 600 beats/min. Under all perfusion conditions at 600 beats/min the average increase in diastolic pressure was 55 mmHg above the nonpaced hearts. Effect of heart rate on contractility. Using the assumption that a change in ventricular pressure reflects a change in muscle tension (8), the capacity of the contractile apparatus to shorten and develop tension was evaluated from the isovolumetric portion of the ventricular function curve. Figure 3 shows that myocardial contractility was maintained under all perfusion pressures with heart rates up to 400-450 beatslmin. Forcing the hearts to contract at 600 beats/min under 60, 90, and 120 mmHg decreased muscle contractility by 53%, 56%, and 72%, respectively, of the nonpaced controls. These experiments indicate that a simple increase in heart rate did not enhance the rate of pressure devel-

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 17, 2019.

330

P. B. TAYLOR

60

04



mm Hg

I

I

1 400

HEART FIG.

while mean

I

I 500

1

I 600

RATE (beats/min)

3. Effects of ventricular contraction aortic pressure was maintained constant. of 9-11 determinations + SEM.

opment. At the higher contraction was severely compromised.

rate on contractility Values represent the

rates

contractility

DISCUSSION

Experiments with isolated heart muscle have clearly shown that the rate of myocardial contraction markedly affects the mechanical response of the heart (7, 12, 13). In general, the influence of contraction rate on isolated myocardial performance has been studied under conditions where the heart is forced to contract below the normal in vivo range of 330-600 beats/min for the rat heart (1, 5, 14). The recent in vivo experiments of Barnard et al. (2) and Wranne and Woodson (16) have shown that rats respond to forced exercise by increasing their heart rates to at least 600 beats/min. In the present investigation, an isolated perfused rat heart was used to determine muscle stability to tachycardia-induced work overload within the expected physiological range. In this model, ventricular function and coronary flow may be studied in the absence of circulating hormones and neurotropic factors. When hearts were perfused under a constant pressure and the contraction rate progressively increased, coronary flow did not change. These results are in contrast to blood-perfused hearts where increased heart rates induced an increased coronary flow (6, 11). In the present experiments, coronary flow was most responsive to an increase in perfusion pressure. The lack of change in coronary flow with tachycardia may be explained by the absence of autoregulation known to occur in hearts perfused with nonplasma perfusates. In addition, when

AND

F. J. CERNY

the heart rate was systematically raised, the pressure in the bubble trap above the aortic cannula increased, suggesting that there was some ventricular emptying with each systole. To maintain a constant afterload, the output of the peristaltic pump was reduced. This manipulation may also have masked any change in coronary flow induced by tachycardia. The enhanced coronary flow response to increased perfusion pressure is consistent with the observation of Neely et al. (10) and is similar to the results of Weisfeldt and Shock (X5), who used a higher viscosity perfusion medium. Since coronary flow failed to increase with tachycardia, it may be postulated that the increased contractile activity may have initiated an ischemic response. In support of this hypothesis, Buckburg et al. (4) have shown that the subendocardium receives coronary flow during diastole and that tachycardia may induce regional myocardial ischemia by shortening the diastolic coronary filling time. In addition, the lack of sustained inotropic response and the increased end-diastolic pressure, at the higher contraction rates, further supports the notion that these hearts may be partially ischemic. The observation that the rate of ventricular pressure development (dP/&) during the higher heart rates decreased with increased contraction rate is similar to the well-known negative inotropic response to electrical pacing of the isolated rat papillary muscle (5, 7) and the intact ventricle (1, 14). Previous studies have clearly shown that the rat myocardium differs from other mammalian species in electrophysiological characteristics (3) and is insensitive to digitalis (7), suggesting that ionic changes in the rat heart have special characteristics. The lack of a positive inotropic response to electrical pacing in the rat heart appears to be a species difference (5). The lar ge decrease in the rate of pressure development with ventricular pacing may reflect a lag between Ca2+ accumulation and sodium transport as postulated by Blesa et al. (3) for rat myocardium. The results of the present investigation confirm previous observations that the isolated perfused rat heart responds positively to an increased afterload. However, inducing work overload with tachycardia appears to compromise myocardial functional capacity as the contraction rate approaches the upper physiological limit. The present findings suggest that heart rates less than 450 beats/min would be appropriate to induce tachycardia work overload. However, in this Langendorff system, higher heart rates produce hemodynamic failure. This study was partially sor Western Hospital and ada. Received

for publication

supported the Sellers 25 November

by a donation Foundation,

from the WindWinnipeg, Can-

1975.

REFERENCES 1. BAILEY, L. E., AND J. W. DOWNIE. The effect of heart rate on the content of calcium in a pool associated with the maintenance of contractile force. Can. J. PhysioZ. PharmacoZ. 48: 498-499, 1970. 2. BARNARD, R. J., H. W. DUNCAN, AND A. T. THORSTENSSON. Heart rate responses of young and old rats to various levels of exercise. J. Appl. Physiol. 36: 472-474, 1974. 3. BLESA, E. S., G. A. LANGER, A. J. BRADY, AND S. D. SERENA. Potassium exchange in rat ventricular myocardium: its relation to rate of stimulation. Am. J. PhysioZ. 219: 747-754, 1970.

4. BUCKBERG,

G. D., D. E. FIXLER, J. P. ARCHIE, AND J. I. E. Experimental subendocardial ischemia in dogs with normal coronary arteries. CircuZation Res. 30: 67-81, 1972. 5. BUCKLEY, N. M., 2. J. PENEFSKY, AND R. S. LITWAK. Comparative force-frequency relationships in human and other mammalian ventricular myocardium. PfZuegers Arch. 332: 259-270, 1972. 6. COBB, F. R., R. J. BACHE, P. A. EBERT, J. C. REMBERG, AND J. C. GREENFIELD, JR. Effects of beta-receptor blockade on the systemic and coronary hemodyamic response to increasing ventricHOFFMAN.

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 17, 2019.

PACED

7.

8.

9.

10.

ISOLATED

HEART

ular rate in the unanesthetized dog. CircuZation Res. 25: 331-341, 1969. HOFFMAN, B. F., AND J. J. KELLY, JR. Effects of rate and rhythm on contraction of rat papillary muscle. Am. J. PhysioZ. 197: 11991204, 1959. MASON, D. T., E. BRAUNWALD, J. W. COVELL, E. H. SONNENBLICK, AND J. Ross. Assessment of cardiac contractility. The relationship between the rate of pressure rise and ventricular pressure during isovolumic systole. Circzdation Res. 44: 47-58, 1971. MORGAN, H. E., M. J. HENDERSON, D. M. REGEN, AND C. R. PARK. Regulation of glucose uptake in muscle. I. The effects of insulin and anoxia on glucose transport and phosphorylation in the isolated, perfused heart of normal rats. J. BioZ. Chem. 236: 253-261, 1961. NEELY, J. R., H. LIEBERMEISTER, E. J. BATTERSBY, AND H. E. MORGAN. Effects of pressure development on oxygen consumption by isolated rat heart. Am. J. Physiol. 212: 815-822, 1967.

331 11. PITT, B., AND D. E. GREGG. Coronary hemodynamic effects of increasing ventricular rate in the anesthetized dog. CircuZation Res. 22: 753-761, 1968. 12. RUMBERGER, E., AND H. REICHEL. The force-frequency relationship: a comparative study between warm and cold blooded animals. Pfluegers Arch. 332: 206-217, 1972. 13. SONNENBLICK, E. H. Force-velocity relations in mammalian heart muscle. Am. J. PhysioZ. 202: 931-939, 1962. 14. TOMLINSON, C. W., AND N. S. DHALLA. Myocardial contractility. II. Effects of changes in cardiac function on the subcellular distribution of calcium in the isolated perfused rat heart. Can. J. Physiol. PharmacoZ. 50: 853-859, 1972. 15. WEISFELDT, M. L., AND N. W. SHOCK. Effect of perfusion pressure on coronary flow and oxygen usage of nonworking heart. Am. J. PhysioZ. 218: 95-101, 1970. 16. WRANNE, B., AND R. D. WOODSON. A graded treadmill test for rats: maximal work performance in normal and anemic animals. J. AppZ. PhysioZ. 34: 732-735, 1973.

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 17, 2019.

Evaluation of the isolated paced rat heart.

JOURNAL OF APPLIED PHYSIOLOGY Vol. 41, No. 3, September 1976. Printed Evaluation in U.S.A. of the isolated paced rat heart PAUL B. TAYLOR AND FRA...
828KB Sizes 0 Downloads 0 Views