Voltage dependence of digitalis afterpotentials, aftercontractions, and inotropy

The effects of membrane potential on digitalis-induced oscillatory afterpotentials (OAP) and aftercontractions (AC) were investigated in isolated canine false tendons and canine and feline papillary muscles. Transmembrane potential and contractions were measured simultaneously. Membrane potential was varied by current applied through an extracellular pipet. The amplitudes of OAP and AC induced by acetylstrophanthidin (0.5-1.0 X 10(-7) g/ml) or actodigin (0.5-1.0 X 10(-6) g/ml) were increased by depolarization and decreased or abolished by hyperpolarization in all tissues. Prior to the appearance of OAP and AC in preparations exposed to the drugs, depolarization caused both phenomena to appear. As in muscle, the strength of beats superimposed on the ascending limb of AC was potentiated, and the strength of beats superimposed on the relaxation phase was depotentiated. When the amplitude of AC was decreased by hyperpolarization, both potentiation and depotentiation were diminished. This effect was accompanied by a partial reversal of the changes induced by acetylstrophanthidin in the course of restitution of contractility and the configuration of the force-frequency relation.