NA+ CA2+ EXCHANGE CURRENTS AND SR CA2+ CONTENTS IN POSTINFARCTION MYOCYTES/

Myocytes isolated from rat hearts 3 wk after myocardial infarction (MI ) had lower peak cytosolic free Ca2+ concentration ([Ca2+](i)) and red uced maximal extent of cell shortening during contraction, but Ca2+ en try via L-type Ca2+ channels was normal. In the current study using wh ole cell patch-clamp technique, reverse Na+/Ca2+ exchange current (I-N a/Ca; 3 Na+ out:1 Ca2+ in) was measured in myocytes in which Na+, K+, and Ca2+ currents were blocked or minimized. Steady-state outward curr ents measured under these conditions increased with depolarization or with elevation of extracellular Ca2+ concentration ([Ca2+](o)) from 1. 8 to 5.0 mM, but were inhibited by 5 mM Ni2+ or by reduction of [Ca2+] (i) to near zero. In addition, reduction of cytosolic free Naf concent ration or of [Ca2+](i) also decreased the amplitude of the outward cur rent. These characteristics indicate the outward current was I-Na/Ca o perating in reverse mode. Reverse I-Na/Ca was significantly lower in M I myocytes, especially at more positive voltages. In addition, sarcopl asmic reticulum (SR)-releasable Ca2+ content as estimated by integrati ng forward I-Na/Ca during caffeine-induced SR Ca2+ release was also si gnificantly lower in MI myocytes. Depressed Na+/Ca2+ exchange activity may contribute to abnormal [Ca2+](i) dynamics in MI myocytes.