Estradiol Inhibition of Luteinizing Hormone-Stimulated Progesterone Synthesis in Isolated Bovine Luteal Cells*

Abstract

The effects of several steroid hormones on progesterone synthesis and c[cyclic]AMP accumulation in isolated bovine corpora luteal cells were investigated in an attempt to determine if any of the steroids would affect the basal level of these processes or their response to gonadotropin. Isolated bovine corpora luteal cells responded to LH [lutropin] with a significant (P < 0.05) increase in progesterone synthesis and cAMP accumulation when incubated at 37.degree. C for up to 1 h. Exogenous cAMP and analogs of cAMP also significantly stimulated steroidogenesis in these incubated cells. Stimulation of progesterone synthesis by 1 .mu.g/ml LH was significantly suppressed (P < 0.05) in the presence of 5-10 .mu.g/ml estradiol. This inhibition appeared to be largely specific for 17.beta.-estradiol, in that other steroids, e.g., estrone, estriol, 17.alpha.-estradiol, cortisol and dihydrotestosterone, were not inhibitory. Testosterone was inhibitory, but it is uncertain if this effect was due to the androgen itself or to its conversion to estradiol. Estradiol did not affect the increase in endogenous cAMP caused by LH in these cells, but did inhibit the effect of exogenous dibutyryl (Bu)2 cAMP on progesterone synthesis. The magnitude of this inhibition of the effect of (Bu)2 cAMP was not equal to the estradiol inhibition of the stimulation of progesterone synthesis by LH. Estradiol, a possible physiological luteolytic agent, may have a direct inhibitory action on the corpus luteum and produce its suppression by blocking the stimulatory effect of LH at a step after cAMP.