Changes in the Activities of Steroidogenic Enzymes during the Development of Ovarian Follicles in Rana nigromaculata

Abstract

Earlier studies showed that the predominant steroid (estradiol [E 2], testosterone [T], progesterone [P 4]) secreted by in vitro cultured amphibian ( Rana nigromaculata) ovarian follicles varied with their size. E 2 was mainly produced by medium-sized follicles, T by intermediate-sized follicles, and P 4 by the largest follicles. Experiments were carried out to ascertain whether the activities of steroidogenic enzymes changed during follicle development and were responsive to gonadotropic stimulation. Enzyme activities were measured indirectly by monitoring conversion of exogenously added substrates to products during in vitro culture of isolated follicles. Different stage follicles were cultured in the presence or absence of frog pituitary homogenate (FPH, 0.1 pituitary/2 ml) and/or various steroid precursors (25-200 ng/2 ml). Amounts of E 2, T, androstenedione (AD), 17α-hydroxyprogesterone (17α-OHP 4), or P 4 secreted into the medium were measured by RIA. Exogenous pregnenolone (P 5) was converted to P 4 by all types of follicles in a dose-dependent manner in the absence of FPH. Addition of FPH markedly enhanced medium levels of P 4 in all sized follicles. Highest levels of P 4 were presented in cultures containing the largest follicles. Such follicles were much less efficient than intermediate follicles in metabolizing P 4 to AD or T. EPH suppressed conversion of exogenous 17α-OHP 4 but not androstenedione to testosterone by the largest follicles. Exogenous T was converted to E 2 only by medium-sized follicles and FPH had little or no stimulating or inhibiting effect on this process in either medium- or intermediate-sized follicles. Taken together, the data suggest that (1) the transition from E 2 to T secretion (Class III to IV) is associated with a loss of aromatase activity, (2) the transition from T to P 4 production (Class IV to V) is correlated with a decrease in the activity of enzymes which metabolize P 4, and (3) activities of steroidogenic enzymes varied in their responsiveness to EPH stimulation.