Effect of partial blockade of the Na +/Ca 2+-exchanger on Ca 2+ handling in isolated rat ventricular myocytes

Abstract

SEA0400 is a selective inhibitor of the Na +/Ca 2+ exchanger having equal potencies to suppress both the forward and reverse mode operation of the Na +/Ca 2+ exchanger. Present experiments were designed to study the effect of partial blockade of Na +/Ca 2+ exchanger on Ca 2+ handling in isolated rat ventricular myocytes. Intracellular Ca 2+ transient and cell shortening were measured in ventricular myocytes loaded with Fura-2-AM fluorescent dye. Partial blockade of Na +/Ca 2+ exchanger was induced by superfusion of the cells with SEA0400 at a concentration of 0.3 μM. Amplitude of the intracellular Ca 2+ transient and cell shortening was significantly increased by SEA0400 in both field stimulated and voltage clamped myocytes, without significant elevation of diastolic Ca 2+ level and the decay time constant of the Ca 2+ transient. In patch clamped myocytes the SEA0400 induced increase in the Ca 2+ transient and cell shortening was accompanied by significant reduction of peak L-type Ca 2+ current. These effects can be explained by the autoregulative nature of cardiac Ca 2+ handling, as the reduced Ca 2+ efflux from the cell results in an increased Ca 2+ load to the sarcoplasmic reticulum leading to increased Ca 2+ release, which in turn may decrease the L-type Ca 2+ current by accelaration of Ca 2+ dependent inactivation of L-type Ca 2+ current. Our results suggest that complex changes in the Ca 2+ cycling can occur after selective pharmacological inhibition of the Na +/Ca 2+ exchanger.