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

We have investigated the effects of inhibiting the Na-K pump with strophanthidin on the intracellular Ca2+ concentration ([Ca2+]i), sarcoplasmic reticulum (s.r.) Ca2+ content and membrane currents. s. r. Ca2+ content was measured by integrating the Na-Ca exchange current resulting from application of 10 mM caffeine. The application of strophanthidin increased both diastolic and systolic [Ca2+]i. This was accompanied by an increase of s.r. Ca2+ content from a resting value of 17.9+/-1.5 micromol/l to 36.9+/-3.3 micromol/l (n=16) after 5 min. Systolic fluxes of Ca2+ into and out of the cell before and during strophanthidin application were also measured. Ca2+ efflux (measured 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-type Ca2+ current) was reduced. In spite of this, s.r. Ca2+ content rose substantially. In the presence of Cd2+ (100 microM), which inhibits the L-type Ca2+ current, strophanthidin had negligible effects on current suggesting that Ca2+ influx via Na-Ca exchange during depolarization does not account for the increase of s.r. Ca2+ content. This suggests that changes of Ca2+ flux during systole are not responsible for the strophanthidin-induced increase 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.