PARENT SESSION
Corpus Luteum

OVARIAN/LUTEAL LOCALIZATION OF GnRH RECEPTORS DURING PREGNANCY AND CHANGES IN INTRACELLULAR CALCIUM LEVELS IN GnRH SUPPRESSION OF LUTEAL PROGESTERONE.

Bhat, Ganapathy¹, Yang, Hyunwon¹, Scanlon, Mary¹, Singh, Udai¹, Sridaran, Rajagopala¹, ¹

ABSTRACT- Previous studies from our laboratory have demonstrated the presence of GnRH in the ovary /corpus luteum (CL) of the rat with increased levels on days 22 and 23 of pregnancy compared to day 8 and further demonstrated an inverse correlation between the levels of GnRH in CL and serum progesterone levels during pregnancy and parturition (Biol. Reprod. Suppl.1. 60:101, A26, 1999). The purpose of this study was to demonstrate the presence of GnRH receptors in the CL of pregnant rats using Western blotting and to determine if the GnRH effect on luteal cell production of progesterone is mediated via changes in the intracellular calcium levels using calcium imaging techniques. Ovaries and CL were collected from pregnant rats. Luteal cells were isolated for culture by enzymatic dissociation of the CL. Our results show that GnRH receptors are present in the ovary and CL of the pregnant rat. When we treated the cultured luteal cells with a GnRH agonist (Wyeth 40972, 1M) for 12 h, there was a significant reduction in the progesterone levels as compared to the saline treated control (22.92 vs 15.82 ng/ml). Further, GnRH agonist (1M) induced an increase in the intracellular calcium in the luteal cells suspended in the medium with calcium (from 87.37 to 333.57 nM) or without calcium (from 94.72 to 385.5 nM). Prior treatment with thapsigargin (1M) was able to substantially block the increase in the intracellular calcium following GnRH agonist treatment (from 85.92 to 182.93 nM) suggesting that the intracellular stores are the main source for the increase in the intracellular calcium following GnRH agonist treatment. Taken together, our earlier observations of endogenous GnRH and the present findings of its receptor in the corpus luteum plus the action of GnRH agonist in increasing the intracellular calcium and reducing the progesterone production suggest a paracrine/autocrine mechanism of its action in the CL. (Supported by grants SO6-GM08248 from NIH and NAG 9-963 from NASA).

KEY WORDS: GnRH receptors, Progesterone, Corpus luteum, Calcium