Steroid production by ovarian follicles of the viviparous guppy (Poecilia reticulata) and its regulation by precursor substrates, dibutyryl cAMP and forskolin

Abstract

Production in vitro of estradiol-17β, testosterone, 17α-20β-dihydroxy-4-pregnen-3-one (17α,20β-P), 17α-hydroxyprogesterone, and progesterone by follicles of the guppy at various stages of oocyte growth and gestation was investigated. Basal production of estradiol-17β was highest in 0.8- and 1.2-mm follicles, whereas that of testosterone and 17α,20β-P was highest in 1.6-mm (postvitellogenic) follicles. Levels of these steroids declined after fertilization and were undetectable in late gestation and postpartum follicles. 17α-Hydroxyprogesterone and progesterone levels were low at all stages. Thus, none of these steroids appears to be involved in maintaining gestation. Regulation of estradiol-17β and 17α,20β-P secretion by vitellogenic (1.0 mm) and postvitellogenic follicles by precursor substrates, dbcAMP (0.1 to 10 mM) and forskolin (1 to 100 μM), was also investigated. Vitellogenic follicles synthesized increased quantities of estradiol-17β in the presence of exogenous testosterone, whereas estradiol-17β production by postvitellogenic follicles was not altered by testosterone. These results suggest decreased aromatase activity in the postvitellogenic follicles. Dibutyryl cAMP and/or forskolin stimulated testosterone and estradiol-17β production by vitellogenic follicles but did not stimulate conversion of testosterone to estradiol-17β, suggesting that the adenylate cyclase system stimulates estradiol-17β production by stimulating testosterone production but does not mediate conversion of testosterone to estradiol-17β. Postvitellogenic follicles synthesized increased quantities of 17α,20β-P in response to 17α-hydroxyprogesterone in a dose-dependent manner. Although 1μM of forskolin stimulated 17α,20β-P production by postvitellogenic follicles in the absence of exogenous 17α-hydroxyprogesterone, 100 μM of forskolin inhibited 17α,20β-P production. Dibutyryl cAMP, however, did not affect 17α,20β-P production. In the presence of 50 ng of 17α-hydroxyprogesterone, dbcAMP (10 mM) and forskolin (1 to 100 μM) suppressed 17α,20β-P production. It is suggested that cAMP mediates 17α,20β-P production up to a certain threshold level, beyond which it inhibits 17α,20β-P production. © 1992.