Induction of Polyphosphoinositide Breakdown in Rat Corpus Luteum by Prostaglandin F2α*

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Abstract
The present study examines the possibility that, in the rat corpus luteum, an initial action of prostaglandin F2.alpha. (PGF2.alpha.) is to induce a ligand-stimulated breakdown of membrane inositol phospholipids. Luteal cells in primary culture were prepared from immature rats after PMSG and human CG priming. In 32P-prelabeled cells, PGF2.alpha. caused a rapid decrease in the level of radiolabel found in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, as early as 20 sec after addition of the hormones. At 1 and 2.5 min, the effect of 10-6 M PGF2.alpha. on phosphatidylinositol 4,5-bisphosphate was significantly greater than that caused by 10-6 LHRH in identical cell cultures. By contrast, the levels of the 32P-prelabeled phosphatidylinositol and phosphatidic acid were increased at 5 min by PGF2.alpha. or LHRH. Concomitant with the alterations in cellular levels of 32P-prelabeled phospholipids, PGF2.alpha. markedly enhanced the accumulation of 3H-labeled inositol phosphates, i.e. inositol 1-phosphate, inositol diphosphate, and inositol triphosphate, during a 5-min incubation. A significant increase of radiolabeled inositol diphosphate was seen as early as 1 min after the addition of either PGF2.alpha. or LHRH: PGF2.alpha. was more effective than LHRH in this regard. The stimulatory effect of LHRH on inositol phosphate accumulation could be blocked completely by the concomitant presence of a potent LHRH antagonist, and at the concentration used (10-6 M) the effects of PGF2.alpha. and LHRH were not additive. Interestingly, the addition of an exogenous phospholipase C also caused a similar enhancement of inositol phosphate accumulation in identical cell cultures. For the first time, these data suggest that, at the level of the corpus luteum, hydrolysis of phosphoinositides may immediately follow PGF2.alpha. (and to a lesser extent LHRH) receptor binding, and this in turn may lead to the generation of 1,2-diacylglycerol and inositol phosphates, resyntehsis of phosphatidic acid and phosphatidylinositol, and mobilization of Ca2+.