ALTERATION OF THE CARDIAC EFFECTS OF ISOPROTERENOL AND PROPRANOLOL BYHYPOTHERMIA IN ISOLATED RAT ATRIUM

1. Hypothermia alters the myocardial response to some inotropic maneuv ers. By measuring developed force and effective refractory period in i solated left atrial preparations, we determined whether hypothermia af fected the cardiac response to isoproterenol and propranolol. 2. Twelv e experimental groups were formed, each consisting of 6 atrial prepara tions. Three groups maintained at either 35, 28 or 20 degrees C served to determine the effects of hypothermia alone. 3. At each temperature , 3 additional groups were exposed to 1.0 mu M isoproterenol alone or in combination with either 0.3 or 10.0 mu M propranolol. At 35 degrees C, isoproterenol produced an increase in developed force and decrease d effective refractory period, Propranolol reversed these isoprotereno l induced effects in a concentration dependent manner. 4. Decreasing t emperature to either 28 or 20 degrees C significantly increased develo ped force and effective refractory period, At 28 degrees C, isoprotere nol no longer produced a significant increase in developed force, alth ough effective refractory period was still decreased, At 20 degrees C, isoproterenol significantly reduced both developed force and effectiv e refractory period. These effects of isoproterenol were reversed by t he addition of propranolol, so that the effective refractory period wa s increased and developed force was not different from that observed a t 20 degrees C in the absence of isoproterenol. 5. These effects of is oproterenol might be explained by effects on Na+/Ca2+ exchange.