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

Abstract

 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 µmol/l to 36.9±3.3 µmol/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 µM), 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.