Mechanisms Regulating Prostaglandin F2α Secretion from the Bovine Endometrium

Abstract

The mechanism that regulates luteolytic PGF2α secretion as stimulated by oxytocin is thought to involve induction of the inositol (1,4,5)-trisphosphate-diacylglycerol second messenger system, which mobilizes intracellular calcium and activates protein kinase C. In Experiment 1, endometrial explants taken from heifers on d 18.5 to 19.5 postestrus had increased PGF2α secretion after treatment with 1μM calcium ionophore A23187 to increase intracellular calcium, 100 nM phorbol 12-myristate 13-acetate to activate protein kinase C, and 100 nM oxytocin. The stimulatory effects of oxytocin and calcium ionophore A23187 plus phorbol 12-myristate 13-acetate did not differ from each other. In Experiment 2, endometrial explants taken from cows on d 18.5 to 19.5 postestrus had increased PGF2α secretion after treatment with 0.2 and 2μM thapsigargin to mobilize intracellular calcium that was sensitive to inositol (1,4,5)-trisphosphate. Secretion of PGF2α was also increased by 100 nM oxytocin and was influenced by the interaction of thapsigargin and oxytocin such that 100 nM oxytocin did not further increase the secretion of PGF2α in the presence of 2μM thapsigargin. In Experiment 3, 100 nM oxytocin stimulated greater production of inositol trisphosphate and total inositol phosphates in the endometrium of cyclic cows than in the endometrium of pregnant cows on d 16.5 to 17.0 postestrus, although luteolysis was not yet initiated in the cyclic cows. These results are consistent with the hypothesis that the activation of the inositol (1,4,5)-trisphosphate-diacylglycerol second messenger system by oxytocin is involved in the stimulation of PGF2α secretion from the endometrium during late diestrus in cows.