Hormonal control of ovarian phospholipase A2 activity in rats.

The ovulatory effect of LH is mediated by prostaglandins, the synthesis of which involves prostaglandin synthetase and phospholipase A2 (PLA2). It is known that the activity of the former is regulated by LH in the ovary and the objective of this study was to establish the presence of PLA2 in the rat ovary and to determine whether LH has a role in regulating its activity. We demonstrated the presence of a calcium-dependent PLA2 in rat ovaries and shown that, in adult rats, the activity of this enzyme rises significantly on the evening of pro-oestrus (at the expected time of the preovulatory LH surge) and remains high over the evening hours, declining by the next morning. Furthermore, pregnant mares' serum gonadotrophin (PMSG), which when injected into 28-day-old rats induces an LH surge 54 h later, caused at the same time, a rise in ovarian PLA2 activity. Administration of 5 micrograms oestradiol benzoate S.C. to 25-day-old rats stimulated ovarian PLA2 activity 54 h later, at the time of the oestrogen-induced LH surge. Human chorionic gonadotrophin (hCG) had no effect on circulating concentrations of LH, FSH or oestradiol but stimulated ovarian PLA2 activity 2 h later. Gonadotrophin-releasing hormone (GnRH) given to 25-day-old rats (either untreated or primed with 0.5 mg oestradiol benzoate) had no effect on ovarian PLA2 activity but the resulting LH surge may have been too transient.(ABSTRACT TRUNCATED AT 250 WORDS)