Mechanism of Stimulatory Action of Growth Hormone on Ovarian Steroidogenesis in Spotted Seatrout, Cynoscion nebulosus

Abstract

The mechanism of growth hormone's (bovine growth hormone, bGH) stimulatory action on steroidogenesis was examined using ovarian tissue of spotted seatrout ( Cynoscion nebulosus) in an in vitro incubation system and compared to that of a gonadotropin (human chorionic gonadotropin, hCG). Testosterone and estradiol production by seatrout ovaries in vitro were stimulated even with low concentrations of bGH (50 ng/ml), and increasing concentrations demonstrated a concentration-dependent relationship similar to that observed with hCG. The steroidogenic responses to combined treatments with bGH and hCG approximately equaled the sum of the stimulatory effects of the two hormones, possibly indicating that bGH does not potentiate the action of gonadotropin. Cyanoketone treatment of vitellogenic follicles inhibited bGH-induced estradiol production, but the stimulatory effect of bGH on the conversion of exogenous testosterone to estradiol was not altered by cyanoketone, which suggests that GH stimulates aromatase activity in addition to an earlier step in the biosynthetic pathway for estradiol production. The stimulatory effects of bGH and hCG on aromatase activity were mimicked by forskolin and dibutyryl cAMP, both of which are known to raise the intracellular level of cAMP. These results suggest that bGH as well as hCG-induced increases in aromatase activity are mediated through adenylate cyclase-cAMP-dependent mechanism(s). The cAMP content of ovarian follicles was markedly increased within 15 min of bGH stimulation and was maximal at 1 hr, whereas steroid production did not increase significantly until 30 min and was maximal after 3 to 9 hr of incubation. The cAMP response to bGH, like that of steroid production, was concentration-dependent. The time-course and concentration-response effects of bGH on cAMP accumulation and steroid production were similar to those of hCG and forskolin, although the magnitude of the responses was less. The addition of cycloheximide or actinomycin D to the media did not affect the basal production of estradiol but completely blocked the bGH and hCG stimulation of the aromatization of exogenous testosterone to estradiol. These results suggest that the stimulatory effects of bGH and hCG on aromatase activity depend upon the synthesis of new RNA and regulatory protein(s).