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

In order to investigate the involvement of phosphoinositide-specific p hospholipase C (PLC), an enzyme associated with phosphoinositide signa l transduction pathway, for the occurrence of Ca2+-paradox (loss of co ntractile activity associated with contracture), rat hearts perfused w ith Ca2+-free medium (I to 5 min) were reperfused (5 to 10 min) with m edium containing 1.25 mM Ca2+. Crude membranes isolated from hearts pe rfused with Ca2+-free medium exhibited a significantly increased activ ity of PLC, whereas normal activity was detected in hearts reperfused with Ca2+-containing medium. A significant rise in PLC activity was ob served at 1 min of Ca2+-free perfusion; maximal increase was seen at 4 min of Ca2+-free perfusion. Minimal concentration of Ca2+ in the perfu sion medium required for showing an increase in PLC activity was 10muM , whereas that required for the occurrence of Ca2+-paradoxic changes i n 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 othe r enzymes. namely the phosphatidylinositol (PI) 4-kinase and the PI-4- monophosphate (PIP) 5-kinase, which synthesize the PLC substrate, phos phatidylinositol 4,5-bisphosphate (PIP2). No alteration of the alpha-a drenoreceptors was observed after 5 min of Ca2+-free perfusion. On the other hand, the observed changes in PLC activity during Ca2+-free per fusion appear to be due to some redistribution of the enzyme in the my ocardium. These results suggest a possible role of the phosphoinositid e/PLC pathway in the induction of Ca2+-paradox via mechanisms which do not appear to be associated with changes in the characteristics of al pha-adrenergic receptors.