Effects of Temperature on Norepinephrine-Induced Sinus Acceleration and Overdrive Suppression in the Isolated Dog Atrium Shigetoshi CHIBA, M.D.,* Tony W. SIMMONS, and Matthew N. LEVY, M.D. SUMMA Using an isolated, blood-perfused atrium preparation, the effects of temperature on SA nodal pacemaker activity were investigated in 9 preparations. The PP interval decreased as temperature was raised. Regular sinus rhythm and atrial contraction were maintained above approximately 26 Ž. Below 26 Ž, sinus depolarization still showed a regular rate, although atrial contractions had ceased. At about 24 Ž, atrial rhythm became irregular. Below 20 Ž, atrial depolarization disappeared. Chronotropic responses to norepinephrine were suppressed at decreased temperatures, not only with respect to maximum PP shortening but also to the threshold dose for inducing sinus acceleration. Overdrive suppression was not influenced significantly by decreasing temperature. These results indicate that a temperature decrease causes suppression of SA nodal pacemaker activity, although the SA node continues to function regularly until about 25 Ž. Additional Indexing Words: Isolated dog atrium Norepinephrine Temperature Overdrive suppression T is generally accepted that sinus rate is reduced by hypothermia. In 1951, Badeer1) reported that the relationship between heart rate and temperature is linear within the range of 25 Ž to 38 Ž in the heart-lung preparation of the dog. It has also been reported that the epinephrine-induced positive chronotropic response was reduced by cooling in isolated hearts of rats2) and rabbits.3) Recently, the phenomenon of postpacing depression of cardiac pacemakers has been utilized to evaluate sino-atrial (SA) node func- From the Department of Investigative Medicine, Mt. Sinai Hospital, Cleveland, Ohio 44106, U.S.A. * Dr. Chiba was a post Doctoral Fellow of the Samuel T. Haas Trust Fund, Mt. Sinai Hospital. The work was supported by Grant HL 15758 and General Research Support Grant PRO5658 from the National Heart and Lung Institute, Bethesda, Maryland. Dr. Chiba's present address is the Department of Pharmacology, Faculty of Medicine, Shinshu University, Matsumoto 390, Japan. Received for publication November 25, 1975. 656
Vol. 17 No. 5 TEMPERATURE EFFECT ON SINUS RATE 657 tion by analyzing the SA nodal recovery time ; that is, the interval between the last paced P wave and the following sinus P wave. In this study, the following problems were investigated utilizing the isolated, blood-perfused SA node preparation, developed by Chiba et al in 1972:4) 1) the relationship between atrial temperature and sinus rate, 2) the effect of temperature on the positive chronotropic response to norepinephrine, and 3) the influence of temperature on postdrive suppression in the blood-perfused, isolated SA node preparation of the dog. METHODS Nine mongrel dogs, weighing 15 to 22Kg, were anesthetized with morphine sulfate, 2mg/Kg i.m., followed in 30min by an i.v. injection of alpha chloralose, 100mg/Kg. After i.v. administration of sodium heparin, 500units/Kg, the right atrium was quickly excised and immersed in saline solution at about 4 Ž. The right coronary artery near the sinus node artery was carefully isolated and cannulated by means of polyethylene tubing. The isolated atrium was perfused with blood that was pumped from a femoral artery of a heparinized support dog, which had been similarly anesthetized. The perfusion was instituted at constant pressure of 100mmHg, using the technique that has previously been described.4) The isolated atrium was enclosed in a temperature-controlled polyethylene chamber and was perfused by blood at a controlled temperature. The temperatures of the chamber and of the blood were measured by a scanning telethermometer (YSI Model 47, Yellow Springs Instrument Co). To record the electrical activity of the atrium, a bipolar electrode catheter was affixed to the atrial surface near the SA node. The arterial blood pressure of the support dog and the electrogram of the isolated atrium were recorded on a Brush Mark 200 oscillograph. The PP interval was measured on a beat-by-beat basis by means of a parallel logic analog computer (EAI 580). In 4 experiments, the atrium was paced at a frequency of 1.5 to 4Hz with external clip electrodes placed near the SA node. All drug solutions were injected into the perfusion tubing in a volume of 0.01 to 0.03ml over a period of 4sec by mens of microinjectors (Hamilton Co). The drugs used in these experiments were norepinephrine bitartrate (Winthrop) and atropine sulfate (Lilly). RESULTS 1. Effect of temperature on SA nodal pacemaker activity Fig. 1 shows the effect of 4 different temperatures on the isolated, bloodperfused SA node preparation of the dog. The PP interval became shorter as temperature was increased. The results of this experiment show a stable PP interval of 1,720, 1,490, 970, and 520msec for temperatures of 26, 29, 32, and 36 Ž, respectively. Below a temperature of 24 Ž, a variety of irregular configuration of atrial depolarization were usually observed, so it
658 CHIBA, SIMMONS, AND LEVY Fig.1. Effects of temperatures on SA nodal pacemaker activity. Art Press: arterial blood pressure of the support dog. Atrial dep: atrial depolarization of the isolated dog atrium. P-P: interval between atrial depolarizations. Time scale shows 1sec intervals. Table I. Effect of Temperature on Isolated, Blood- Perfused SA Node Preparation of the Dog Values are mean }SEM for 8 dogs. was difficult to keep a constant sinus rhythm. Below 20 Ž, atrial depolarization frequently disappeared. Above 25 Ž, a constant atrial rate was usually observed at any given temperature level and the rate gradually increased with an increase in temperature up to 40 Ž. Below about 26 Ž, atrial contraction ceased, although atrial depolarization was still observed. Table I shows the relationship between temperature and PP interval in 8 isolated SA node preparations.
Vol. 17 TEMPERATURE EFFECT ON SINUS RATE 659 No. 5 Fig.2. Positive chronotropic responses to various amounts of norepinephrine injected into the blood perfusing the isolated SA node preparation at temperatures of 26, 28, 30, and 34 Ž. Fig.3. Effect of temperature on dose-response curve of norepinephrine. The response measured was the percent reduction in PP interval. 2. Effect of temperature on the response to norepinephrine Fig. 2 shows an example of the effect of various doses of norepinephrine on the SA node at different temperatures. At a low temperature, the effect of norepinephrine was not clear at a small dose level, such as 0.01ƒÊg. The threshold dose for inducing a positive chronotropic response became less as the temperature was increased. Also, the maximum heart rate with a given dose of norepinephrine was progressively increased as the temperature was elevated. However, the duration of the norepinephrine effect became shorter with increasing temperature. Fig. 3 shows dose-response curves illustrating the relationship between PP interval and the dose of norepinephrine at various
660 CHIBA, SIMMONS, AND LEVY temperatures. This indicates that the SA node is more sensitive to norepinephrine at higher temperatures. 3. Effect of change in temperature on postdrive suppression It is well known that, following termination of rapid pacing, automaticity in an intrinsic pacemaker is temporarily suppressed. This postdrive suppression was compared at different temperatures. Fig. 4 shows a typical experiment of the effect of temperature on postdrive suppression. At each temperature (27 to 39 Ž), postdrive suppression was clearly observed and its Fig.4. Effect of electric pacing (5v, 1msec, 1.5Hz) on an isolated atrium at various temperatures from 27 to 39 Ž. Time scale shows sec intervals. Table II. Relationship Between Temperature and PP Interval by Postdrive Suppression Values are mean }SEM in 4 preparations. Comparisons with control values of 35-37 Ž (t-test): *P>0.1
Vol. 17 TEMPERATURE EFFECT ON SINUS RATE 661 No. 5 absolute duration gradually but slightly decreased with increasing temperatures. However, the percentage of the duration of postdrive suppression [100 ~(maximum PP-control PP)/control] was not significantly different (P>0.1). Table II shows the relationship between temperature and percentage of postdrive suppression in 4 isolated atrium preparations. DISCUSSION In the isolated, blood-perfused canine SA node preparation, the relationship between temperature and sinus rate was studied. Within the range of 24 to 37 Ž, each degree of rise of temperature shortened PP interval as shown in Fig.1 and Table I. These results confirmed the observation of Badeer.1) In our study, below a temperature of 26 Ž a complete arrest of atrial contraction was observed, but regular atrial depolarization continued. Below 24 Ž, atrial rhythm was usually irregular and atrial contraction never occurred. However, atrial depolarization was still observed frequently below 23 Ž, although it was markedly irregular. There has been no report of the effect of temperatures below 24 Ž on the SA node of the dog. The rhythm disturbance by low temperature may be due to SA exit block while the SA node may keep its functional activity. In general, pacemaker fibers are very resistant to cooling even though all mechanical activity and propagated electric activity has ceased.5)-7) It has also been reported that cooling from 37 Ž reduces the positive chronotropic response induced by catecholamines.2),3),8) However, there is no available report in the dog heart. In the present study, it was demonstrated that the maximum positive chronotropic response to norepinephrine and the percent reduction in PP interval became less as temperature was lowered in the isolated dog atrium. Many investigators have studied the effects of electrical pacing on intrinsic cardiac pacemaker funetion.9),10) Recently, SA nodal function was evaluated in patients with the sick sinus syndrome. A greater degree of suppression of SA node function than in normal subjects was observed.11),12) In another experimental study using the isolated, blocd-perfused SA ncde preparations of the dog, the present authors found that occlusion of the sinus node artery caused an augmentation of postdrive suppression, and suggested that ischemia depressed SA node unction.13) In this study, SA nodal pacemaker activity was markedly depressed at low temperatures. However, postdrive suppression was not influenced significantly in terms of percent decrease in sinus rate in the temperature range from 25 to 37 Ž. The fact that Fleming14) found no significant changes in plasma potas-
662 CHIBA, SIMMONS, AND LEVY Jap. Heart J. S eptember, 1976 sium concentration during cooling and our inability to show any changes in postdrive suppression may be related to a discovery made by Harris.15) Harris was able to show that plasma potassium levels do increase during myocardial ischemia. Chiba et al13) showed that myocardial ischemia increases postdrive suppression. Thus it seems possible that increased levels of potassium may play a role in both phenomena. Smith. ACKNOWLEDGEMENTS We are grateful for the assistance of Mr. Andrew R. Nara and Mrs. Debra REFERENCES 1. Badeer H: Influence of temperature on S-A rate of dog's heart in denervated heart-lung preparation. Am J Physiol 167: 76, 1951 2. Nayler WG, Wright JE: Effect of epinephrine on the mechanical and phosphorylase activity of normo- and hypothermic hearts. Circulat Res 13: 199, 1963 3. Szekeres L, Lenard G: Die Wirkung von Adrenalin auf Herztatigkeit and Kreislauf in Hypothermie. Hung Acta Physiol 16: 221, 1959 4. Chiba S, Kubota K, Hashimoto K: Double peaked positive chronotropic response of the isolated blood-perfused S-A node to caffeine. Tohoku J exp Med 107: 101, 1972 5. Weidmann S: Elektrophysiologie der Herzmuskelfasern. Bern, Hans Huber, 1956 6. Yamagishi S, Sano T: Effect of temperature on pacemaker activity of rabbit sinus node. Am J Physiol 212: 829, 1967 7. Marshall JM: Effects of low temperature on transmembrane potential of single fibers of the rabbit atrium. Circulat Res 5: 664, 1957 8. Brown TG Jr, Cotten MDeV : Responses to cardiovascular drugs during extreme hypothermia. J Pharmacol exp Ther 110: 8, 1954 9. Lange G : The action of driving stimuli from intrinsic and extrinsic sources on in situ cardiac pacemaker tissue. Circulat Res 17: 449, 1965 10. Alanis J, Benitez D: The decrease in the automatism of the Purkinje pacemaker fibers provoked by high frequencies of stimulation. Jap J Physiol 17: 556, 1967 11. Mandel W, Hayakawa H, Danzig R, Marcus HS: Evaluation of sino-atrial node function in man by overdrive suppression. Circulation 44: 59, 1971 12. Narula OS, Samet P, Javier RP: Significance of the sinus-node recovery time. Circulation 45: 140, 1972 13. Chiba S, Simmons TW, Levy MN, Zieske H: Effect of ischemia on overdrive suppression in isolated, blood-perfused atrial preparation of the dogs. Cardiovasc Res (in press) 14. Fleming R: Acid-base balance of the blood in dogs at reduced body temperature. Arch Surg 68: 145, 1954 15. Harris AS, Bisteni A, Russel RA, Brigham JC, Firestone JE: Excitatory factors in ventricular tachycardia resulting from myocardial ischemia: Potassium a major excitant. Science 119: 200, 1954