Cellular Mechanism of the Antigonadotropic Action of Luteinizing Hormone-Releasing Hormone in the Corpus Luteum*

Abstract

In incubations of rat luteal cells, LHRH or an agonistic analog (D-Trp⁶)LHRH showed a dose-response inhibition of LH-stimulated cAMP accumulation and progesterone secretion. The dose of LHRH estimated to produce 50% inhibition of LH-dependent cAMP accumulation was 3 × 10^-7 M, and the LHRH dose estimated to inhibit progesterone secretion by 50% was 8 × 10^-6 M. (D-Trp6)LHRH was 4- and 40-fold more potent than LHRH on LH-dependent cAMP accumulation and progesterone secretion, respectively. Although LHRH (8 × 10^-6 M) inhibited LH-stimulated cAMP accumulation at 100 and 1000 ng/ml LH, inhibition of progesterone secretion by LHRH was evident at 100 ng/ml, but not at 1000 ng/ml LH. Thus, high levels of gonadotropin may overide the inhibitory effect of LHRH on progesterone secretion. LHRH did not inhibit the stimulation of progesterone secretion induced by dibutyryl cAMP or a combination of dibutyryl cAMP and LH (100 ng/ ml). However, LHRH inhibited LH-dependent cAMP accumulation in both the absence and presence of isobutylmethylxanthine (10^-4M), which indicates that LHRH may not reduce cAMP accumulation by increasing cAMP phosphodiesterase activity. LHRH did not inhibit LH stimulation of adenylate cyclase activity in isolated luteal membranes, and LHRH did not inhibit LH receptor-binding activity in luteal membranes or prevent binding of [¹²⁵I]hCG to luteal cells. It is concluded that LHRH inhibits LH-stimulated progesterone secretion in dispersed rat luteal cells by an inhibitory action on LH activation of adenylate cyclase activity, and this action of LHRH appears to require the intact cell. This functional uncoupling of the LH receptor complex and adenylate cyclase by LHRH in the intact luteal cell is identical to the action of prostaglandin F_2α (PGF_2α) shown earlier. However, LHRH did not increase PGF_2α accumulation when incubated with luteal cells, and coincubation with indomethacin did not prevent the expression of the antigonadotropic activity of LHRH. Since LHRH did not inhibit the binding activity of the PGF_2α receptor in luteal membranes or prevent binding of [³H]PGF_2α to isolated luteal cells, it is concluded that the antigonadotropic activity of LHRH may not be PG dependent, but the two luteolytic effectors may share a common intracellular mediator.