POSSIBLE ROLE OF PHOSPHOLIPASE-C IN THE INDUCTION OF CA2+-PARADOX IN RAT-HEART

In order to investigate the involvement of phosphoinositide-specific phospholipase C (PLC), an enzyme associated with phosphoinositide signal transduction pathway, for the occurrence of Ca2+-paradox (loss of contractile activity associated with contracture), rat hearts perfused with Ca2+-free medium (I to 5 min) were reperfused (5 to 10 min) with medium containing 1.25 mM Ca2+. Crude membranes isolated from hearts perfused with Ca2+-free medium exhibited a significantly increased activity of PLC, whereas normal activity was detected in hearts reperfused with Ca2+-containing medium. A significant rise in PLC activity was observed at 1 min of Ca2+-free perfusion; maximal increase was seen at 4min of Ca2+-free perfusion. Minimal concentration of Ca2+ in the perfusion medium required for showing an increase in PLC activity was 10muM, whereas that required for the occurrence of Ca2+-paradoxic changes in heart function upon reperfusion was 50 muM. Perfusion of the hearts with Ca2+-free medium in the presence of low Na+ or at low temperature, which prevents the occurrence of Ca2+-paradox upon reperfusion. did not prevent the increase in PLC activity. An increase during Ca2+-free perfusion similar to that seen for PLC was also observed for two other enzymes. namely the phosphatidylinositol (PI) 4-kinase and the PI-4-monophosphate (PIP) 5-kinase, which synthesize the PLC substrate, phosphatidylinositol 4,5-bisphosphate (PIP2). No alteration of the alpha-adrenoreceptors was observed after 5 min of Ca2+-free perfusion. On the other hand, the observed changes in PLC activity during Ca2+-free perfusion appear to be due to some redistribution of the enzyme in the myocardium. These results suggest a possible role of the phosphoinositide/PLC pathway in the induction of Ca2+-paradox via mechanisms which do not appear to be associated with changes in the characteristics of alpha-adrenergic receptors.