Effect of glutamate on basal steroidogenesis by ovarian follicles of rainbow trout (Oncorhynchus mykiss).

The effects of glutamate on the in vitro basal steroid production of three maturational stages of rainbow trout (Oncorhynchus mykiss) ovarian follicles were investigated. Radioimmunoassays were used to measure the rates of synthesis of testosterone (T) and 17-estradiol (E2). High performance liquid chromatography (HPLC) was used to examine the steroid metabolites produced from a tritium labeled precursor, pregnenolone (P5). The glutamate agonist, N-methyl-d,l-aspartate (NMA) had a dose-dependent suppressive effect on T and E2 synthesis in mid-vitellogenic (MV) follicles, but had no significant effect on early- (EV) and late-vitellogenic (LV) follicles. l-glutamic acid (GA) had a dose-related suppressive effect on T synthesis by MV follicles, suppressing both T and E2 synthesis by LV follicles, but having no effect on EV follicles. HPLC separation of the steroid metabolites synthesized from P5 showed clear evidence of differences in rates of overall steroidogenesis of the three follicular stages, but no effect of either NMA or GA on the nature or the amount of the metabolites produced by the three developmental stages examined. The findings suggest that glutamate may act via a reduction in the production of P5, which is the principal rate-limiting step in the steroidogenic cascade, and not via modulation of steroidogenic enzyme activities.