The hormone-sensitive hepatic Na+-pump. Evidence for regulation by diacylglycerol and tumor promoters

Abstract

Ouabain-sensitive 86Rb+ uptake by isolated rat hepatocytes was studied to elucidate how Ca2+-mobilizing hormones stimulate the Na+-pump. Stimulation of this uptake was observed with concentrations of vasopressin ([8-arginine]vasopressin, AVP), angiotensin II, and norepinephrine which elicited Ca2+ mobilization and phosphorylase activation. These results suggested that changes in cytosolic Ca2+, mediated by inositol trisphosphate, might trigger sodium pump stimulation by AVP. However, in hepatocytes incubated in Ca2+-free Krebs-Henseleit buffer, Na+-pump activity was not altered over 15 min by either 1.5 mM EGTA or 1.5 mM Ca2+. Furthermore, incubation of cells in 5 mM EGTA for 15-30 min drastically impaired the ability of AVP to increase cytosolic Ca2+, but only modestly attenuated AVP-stimulated Na+-pump activity. Two tumor promoters, phorbol myristate acetate (PMA) and mezerein, stimulated Na+/K+-ATPase-mediated transport activity. Similarly, addition of synthetic diacylglycerols or of exogenous phospholipase C from Clostridium perfringens to increase endogenous diacylglycerol levels also resulted in a stimulation of the Na+-pump in the absence of changes in cytosolic or total cellular Ca2+ levels. Stimulation of the Na+-pump by the combination of maximal concentrations of PMA and AVP did not produce an additive response, and both agents displayed a transient time course, suggesting that the two agents share a common mechanism. Stimulation of the Na+-pump by AVP and PMA was not blocked by amiloride analogs which inhibit Na+/H+ exchange, but these compounds blocked the action of insulin. These data suggest that the elevated Na+/K+-ATPase-mediated transport activity observed in hepatocytes following exposure to Ca2+-mobilizing hormones is a consequence of stimulated diacylglycerol formation and may involve protein kinase C.