Progesterone-induced changes in membrane phospholipids in Rana oocytes: The role of phospholipids in the release of the prophase block

Abstract

The amphibian oocyte is a system in which a steroid hormone acts at receptors on the plasma membrane. Brief exposure to exogenous progesterone triggers the resumption of meiosis in the prophase-arrested oocyte. Progesterone causes an immediate transient Ca{dollar}\sp{lcub}2+{rcub}{dollar} release, produces a rise in intracellular pH, and down-regulates membrane adenylate cyclase.;The results outlined in this thesis show that progesterone stimulation of Rana oocytes results in a transient increase in N-methylation of the phosphatidylethanolamine pool(s) in the oocyte plasma membrane within the first minute. The increase in N-methylation coincides with the transient release of Ca{dollar}\sp{lcub}2+{rcub}{dollar} from the oocyte surface. The first product of phospholipid-N-methylation, phosphatidylmonomethylethanolamine (PME), will itself release Ca{dollar}\sp{lcub}2+{rcub}{dollar} from the oocyte surface and induce meiosis, when added to the external medium.;The transient increase in N-methylation is followed by a rise in cytosolic IP{dollar}\sb3{dollar} beginning 5-10 minutes after exposure to progesterone. This in turn is followed by increased arachidonic acid incorporation in phosphatidylcholine. The rise in IP{dollar}\sb3{dollar} occurs after the transient release of Ca{dollar}\sp{lcub}2+{rcub}{dollar} from the oocyte surface but coincides with a second prolonged elevation of cytosolic Ca{dollar}\sp{lcub}2+{rcub}{dollar} and Ca{dollar}\sp{lcub}2+{rcub}{dollar} efflux from the oocyte. The methyl- and arachidonate-labeled phospholipids disappear sequentially 5-90 min after the hormone stimulus, suggesting that activation of phospholipases A{dollar}\sb2{dollar} and/or C occur as part of a cascade of membrane events.;These findings indicate that progesterone acts at surface receptors to stimulate N-methyl transferase I to form PME. PME in turn acts to release membrane Ca{dollar}\sp{lcub}2+{rcub}{dollar} which is critical to activation of phospholipase C (release of IP{dollar}\sb3{dollar}) and phospholipase A{dollar}\sb2{dollar} (release of arachidonic acid).