The inotropic effects of strophanthidin in Purkinje fibers and the sodium pump.

The role of the inhibition of the Na pump in strophanthidin inotropy was studied in canine Purkinje fibers by correlating changes in contractile force with changes in maximum diastolic potential caused by conditions that enhance the electrogenic extrusion of Na. It was found that a brief exposure to a zero-K or to a zero-K, zero-Ca solution (but not to a zero-Ca solution) is followed by an increase in maximum diastolic potential. This hyperpolarization is reduced if NaCl is substituted by LiCl and in the presence of tetrodotoxin. In quiescent fibers exposed to tetrodotoxin, the hyperpolarization is abolished. A low concentration of strophanthidin (5 X 10(-8)M) increases contractile force but does not modify the hyperpolarization. Larger strophanthidin concentrations (5 X 10(-7)M to 10(-6)M) increase and then decrease contractile force and reduce or abolish the hyperpolarization. Metabolic inhibitors also reduce the hyperpolarization. We conclude that the positive inotropic effect of a low (therapeutic) concentration of strophanthidin is due to a mechanism other than Na pump inhibition.