THE INFLUENCE OF RESERPINE ON THE CALCIUMINDUCED RESPONSES OF SPONTANEOUSLY BEATING ISOLATED RABBIT ATRIA

¹ Department of Pharmacology, The University of Texas Medical School at San Antonio, San Antonio, Texas

Atria from rabbits, four hours after administration of either 1 or 3 mg of reserpine per kg, exhibited a significantly greater base-line tension than controls. Although absolute tension responses to cumulative concentrations of calcium were significantly greater in reserpine-treated than in control atria, when the data were plotted as percent maximum tension there were no significant differences between the two groups. Propranolol did not prevent the tension responses to calcium. Base-line tension and absolute tension responses to calcium of atria, 24 hours after reserpine treatment (1 or 3 mg/kg), were not different from those of control atria. When control and reserpine-treated atria (3 mg/kg, four hours) were incubated in calcium-free Ringer's solution, the reserpine-treated atria demonstrated 1) a decrease in the threshold concentration of calcium required to elicit a response, 2) an increase in the rate and magnitute of tension development in response to calcium and 3) a greater incidence of calcium-induced arrhythmia. The rate of tension decline of atria placed in calcium-free Ringer's solution was significantly delayed after reserpine-treatment (T1/2 = 63.1 ± 7.0 seconds for control atria and 185.2 ± 38.1 seconds for reserpine-treated atria). There were no significant changes in atrial rate in response to calcium of any of the reserpine-treated or control atria. The results of this study suggest that the increase in sensitivity to calcium in reserpine-treated atria is due to an increase in membrane permeability to calcium and an alteration in the intracellular homeostasis of calcium.