Mechanism of Cellular Metabolic Responses of Cultured Rat Luteal Cells to Stimulation with Prostaglandin F2α and Gonadotropin Releasing Hormone

Cellular metabolic responses, determined as the indications of inositol phosphate (IPs) formation, arachidonic acid release and intracellular Ca2+ concentration, to stimulation with prostaglandin F2α (PGF2α) or gonadotropin releasing hormone (GnRH), were studied in monolayer- cultured rat luteal cells which maintained a function of progesterone production and secretion. The results were as follows: 1. In 5 day-old luteal cells, IPs formation, i. e., activation of phospholipase C, was significantly enhanced by stimulation with PGF2α or GnRH. Arachidonic acid release, i. e., activation of phospholipase A2, was also stimulated by these agonists. The agonist stimulation of [14C] glycerollabeled cells induced a marked decrease in the labels of phosphatidylcholine (PC) and phosphatidylinositol (PI), and concomitantly an increase in lyso-phosphatidylcholine and lyso-phosphatidylinositol. In mepacrine-pretreated cells, arachidonic acid release from the cells was significantly decreased. Thus, arachidonic acid release from luteal cells by stimulation with PGF2α or GnRH seems to be mainly derived from PC and PI by the action of phospholipase A2. 2. In 9 day-old luteal cells, IPs formation and intracellular Ca2+ were not enhanced by GnRH stimulation, while, in contrast, arachidonic acid release was enhanced in the cells. These findings indicate that the degradation of phosphatidylinositol 4, 5-bisphosphate to inositol triphosphate and diacylglycerol may not be essential in the activation of phospholipase A2, and the activation of phospholipase A2 and phospholipase C by GnRH stimulation may occur independently. 3. In experiments with GTPγS added to saponin-permeabilized cells, it was demonstrated that G protein might be involved in the activation of phospholipase C and phospholipase A2 induced by stimulation of PGF2α or GnRH. Pertussis toxin (IAP) treatment of luteal cells, which induced ADP-ribosylation of 41K membrane protein, did not block the stimulation effects. Therefore, G proteins ?other than Gi may be involved in the activation of the phospholipases