Deinhibition of cardiac Na+-K+-ATPase after exposure to exogenous phospholipase A2.

Abstract

After 2 h of exogenous phospholipase A2 (PLA2) exposure, membrane phospholipid decreased from 3.22 +/- 0.31 to 1.06 +/- 0.13 mumol/mg (33% of control). All classes of phospholipid, except sphingomyelin, were hydrolyzed, whereas total cholesterol content was unaffected. Increases in nonesterified fatty acids (NEFA) were reflected primarily in oleic (18:1), linoleic (18:2), and arachidonic (20:4). Na+-K+-adenosinetriphosphatase (ATPase) activity was inhibited to 29% of control by 2 h of PLA2 treatment, and this inhibition was reversed (albeit, not completely after 5 min of PLA2 treatment) by removal of the hydrolysis products with 0.1% bovine serum albumin (BSA). In contrast, the apparent binding capacity for [3H]ouabain was not affected by PLA2 treatment. Unmasking of latent [3H]ouabain binding by alamethicin was utilized to estimate changes in the proportion of sealed vesicles present before and after PLA2 treatment. PLA2 treatment resulted in a time-dependent loss of sealed vesicles that paralleled the time course of phospholipid hydrolysis and was not reversed by washing with BSA. These studies demonstrate that cardiac Na+-K+-ATPase activity is inhibited by accumulation of endogenously produced lysophospholipids and NEFA. In contrast, loss of vesicle integrity may result from both accumulation of endogenously produced hydrolysis products and membrane phospholipid depletion.