Phospholipid methylation in the calcium-dependent release of arachidonate for prostaglandin synthesis in renal medulla.

Exposure of rat renal inner medullary slices to arginine vasopressin in the presence of Ca2+ or the addition of Ca2+ to Ca2+-deprived slices incubated with or without ionophore A23187 increases the release of labeled arachidonic acid and the accumulation of immunoreactive prostaglandin E by 1.5- to fivefold. The transmethylation inhibitor homocysteine thiolactone (250 to 500 mumol/L) suppresses or abolishes these actions of arginine vasopressin or Ca2+ (+/- A23187). Combined addition of a submaximal inhibitory concentration of homocysteine thiolactone and the methylation inhibitor 3-deazaadenosine also abolished the actions of arginine vasopressin or Ca2+ to stimulate release of labeled arachidonic acid and immunoreactive prostaglandin E. The effects of both homocysteine thiolactone and 3-deazaadenosine are thought to be expressed through intracellular formation of S-adenosyl-L-homocysteine, a potent competitive inhibitor of the cellular methylating moiety S-adenosyl-L-methionine. In contrast, methylation inhibitors had no effect on basal 3H-arachidonic acid or immunoreactive prostaglandin E release, the 10- to 12-fold increases in iPGE induced by exogenous arachidonic acid, or on basal and AVP-induced increases in inner medullary slice cyclic AMP. The suppressive effects of the methylation inhibitors on increases in 3H-arachidonic acid and immunoreactive prostaglandin E accumulation in slice incubates in response to arginine vasopressin or Ca2+ (+/- A23187) were correlated with their ability to inhibit the incorporation of 3H-methionine into slice phosphatidyl-L-monomethylethanolamine, phosphatidyl-N,N-dimethylethanolamine, and phosphatidylcholine. In isolated microsomes, the incorporation of label from S-adenosyl-L-methionine into microsomal phospholipids was similarly suppressed by S-adenosyl-L-homocysteine. Neither arginine vasopressin nor Ca2+ (+/- A23187) detectably influenced the incorporation of labeled methionine into inner medullary slice phospholipids or S-adenosyl-L-methionine--mediated methylation of phospholipids in isolated microsomes. However, incubation of inner medullary microsomes with S-adenosyl-L-methionine under conditions leading to phospholipid methylation increased both Ca2+ responsive phospholipase A2 and C activities approximately twofold. The increases induced by S-adenosyl-L-methionine were blocked by S-adenosyl-L-homocysteine. Our results are consistent with a permissive role for phospholipid methylation in expression of the effects of Ca2+ and arginine vasopressin on inner medullary 3H-arachidonic acid release for immunoreactive prostaglandin E synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)