Neomysin inhibits Ca2+-stimulated phosphatidylinositol hydrolysis and protects cultured rat cardiomyocytes from Ca2+-dependent cell injury

Exposure of cultured rat cardiomyocytes to ionomycin and extracellular Ca2+ leads to a rapid, sustained increase in intracellular free Ca2+ as monitored by Ca2+-dependent phosphorylase a activation and to a subsequent loss of cardiomyocyte viability as determined by lactate dehydrogenase (LDH) leakage. The intracellular free Ca2+ increase coincided with a rapid hydrolysis of phosphatidylinositol that preceded cell death. Phosphatidylinositol hydrolysis was monitored by the release of radiolabeled phosphoinositides from cardiomyocytes prelabeled with [2-3H]-myo-inositol. Neomycin, a known inhibitor of phospholipase C, inhibited the phosphatidylinositol hydrolysis and markedly reduced the extent of cell injury. Inhibitors of other Ca2+-activated processes, including intracellular proteases and phospholipase A2, had no effect on ionomycin-mediated cell injury. These data suggest that ionomycin-induced Ca2+-dependent cell injury in cultured cardiomyocytes may be due in part to the stimulation of phosphatidylinositol hydrolysis, presumably catalyzed by a Ca2+-dependent phospholipase C