A19. Modification of intracellular calcium by a Na+-H+ exchange inhibitor, methyl isobutyl amiloride

Abstract

Although the inhibition of Na+-H+ exchange is considered to reduce the development of intracellular Ca2+-overload in myocardium upon ischemia-reperfusion, the effects of Na+-H+ exchange inhibitors including methyl isobutyl amiloride (MIA) on Ca2+-handling by cardiomyocytes have not been well examined. In the present study, we employed isolated rat heart cardiomyocytes loaded with fura-2 acetoxymethyl ester for monitoring changes in the intracellular concentration of Ca2+ ([Ca2+]i) spectrofluorometrically. MIA was found to produce a marked increase in basal [Ca2+]i and augmented the KCl-induced increase in [Ca2+]i in a dose-dependent manner. The action of MIA was independent of the extracellular concentration of Ca2+ and was not affected by sarcolemmel (SL) Ca2+-channel antagonist, verapamil and Na+-Ca2+ exchange inhibitor, KB-R7943. On the other hand, Na+-K+ ATPase inhibitor, ouabain, caused augmentation of basal as well as KCl-induced increase in [Ca2+]i. Sarcoplasmic reticulum (SR) Ca2+-release channel blocker, ryanodine, SR Ca2+ store depleting agent, caffeine, and SR Ca2+-pump ATPase inhibitor, cyclopiazonic acid, decreased the MIA-induced incease in basal and KCl-induced increase in [Ca2+]i. In contrast, mitochondrial Ca2+-uptake inhibitors, sodium azide and ruthenium red as well as SL Ca2+-ATPase inhibitor, vanadate, did not affect the MIA mediated alteration in [Ca2+]i. These results suggest that, unlike SL Na+-Ca2+ exchanger and L-type Ca2+-channels, SR Ca2+ stores are involved in the MIA-induced increase in basal and KCl-induced increase in [Ca2+]i in adult cardiomyocytes (Supported by a grant from CIHR).