THE ROLE OF PHOSPHOINOSITIDE-DERIVED 2ND-MESSENGERS IN OXYTOCIN-STIMULATED PROSTAGLANDIN-F2-ALPHA RELEASE FROM ENDOMETRIUM OF PIGS

The mechanism for prostaglandin (PG) F-2 alpha release from pig endome trium after oxytocin (OT) treatment is unknown. OT may rapidly stimula te inositol (1,4,5)-trisphosphate (IP3) and diacylglycerol (DAG) forma tion, consistent with the concept of rapid activation of a second-mess enger system. In support of this hypothesis, endometrial IP3 levels we re increased (P < 0.05) within 0.5 min after treatment with 0.1 mu M O T. In contrast, increased DAG formation was not detected after treatme nt with OT. However, similar to the stimulation of endometrial PGF(2 a lpha) secretion observed after OT treatment (P < 0.001), PGF(2 alpha) release was increased (P < 0.01) after treatment with phorbol-12-myris tate-1 3-acetate (PMA), which mimics DAG activation of protein kinase C. Further, stimulation of endometrial PGF(2 alpha) secretion did not result from cell death induced by Ph?A or OT because lactate dehydroge nase, a cytosolic marker of cellular integrity, did not leak into the medium after PMA or OT treatment. In contrast. 0.5% saponin (positive control for cell death and concomitant release of lactate dehydrogenas e) increased PGF(2 alpha) secretion (P < 0.05) and lactate dehydrogena se release (P < 0.001). These results indicate that OT induces endomet rial IP3 production in a rapid manner indicative of a second-messenger system. The finding that increased DAG was not also detected after OT treatment may reflect rapid metabolism or compartmentalized productio n of DAG involved in the second-messenger stimulation of phospholipase C. The high background of DAG used in the biosynthesis of cellular li pids would obscure the rather small spatially localized changes in DAG levels resulting from the activation of phospholipase C. The finding that DAG was present at approximately 10 to 20-fold higher levels than IP3 in resting cells was consistent with this conclusion.