Hypothermia alters the myocardial response to some inotropic maneuvers. By measuring developed force and effective refractory period in isolated left atrial preparations, we determined whether hypothermia affected the cardiac response to isoproterenol and propranolol. 2. Twelve experimental groups were formed, each consisting of 6 atrial preparations. 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 microM isoproterenol alone or in combination with either 0.3 or 10.0 microM propranolol. At 35 degrees C, isoproterenol produced an increase in developed force and decreased effective refractory period. Propranolol reversed these isoproterenol-induced effects in a concentration-dependent manner. 4. Decreasing temperature to either 28 or 20 degrees C significantly increased developed force and effective refractory period. At 28 degrees C, isoproterenol no longer produced a significant increase in developed force, although effective refractory period was still decreased. At 20 degrees C, isoproterenol significantly reduced both developed force and effective refractory period. These effects of isoproterenol were reversed by the addition of propranolol, so that the effective refractory period was increased and developed force was not different from that observed at 20 degrees C in the absence of isoproterenol. 5. These effects of isoproterenol might be explained by effects on Na+/Ca(2+)-exchange.
