Strophanthidin-induced gain of Ca2+ occurs during diastole and not systolein guinea-pig ventricular myocytes

We have investigated the effects of inhibiting the Na-K pump with strophant hidin on the intracellular Ca2+ concentration ([Ca2+](i)), sarcoplasmic ret iculum (s.r.) Ca2+ content and membrane currents. s.r. Ca2+ content was mea sured by integrating the Na-Ca exchange current resulting from application of 10 mM caffeine. The application of strophanthidin increased both diastol ic and systolic [Ca2+](i). This was accompanied by an increase of s.r. Ca2 content from a resting value of 17.9+/-1.5 mu mol/l to 36.9+/-3.3 mu mol/l (n=16) after 5 min. Systolic fluxes of Ca2+ into and out of the cell befor e and during strophanthidin application were also measured. Ca2+ efflux (me asured as the integral of the Na-Ca exchange tail current) rose steadily in the presence of strophanthidin, while Ca2+ influx (the integral of the L-t ype Ca2+ current) was reduced. In spite of this, s.r. Ca2+ content rose sub stantially. In the presence of Cd2+ (100 mu M), which inhibits the L-type C a2+ current, strophanthidin had negligible effects on current suggesting th at Ca2+ influx via Na-Ca exchange during depolarization does not account fo r the increase of s.r. Ca2+ content. This suggests that changes of Ca2+ flu x during systole are not responsible for the strophanthidin-induced increas e of s.r. Ca2+. We conclude that the primary mechanism by which the cardiac cell gains Ca2+ when the Na-K pump is inhibited is by a net influx during diastole.