Ovarian steroidogenesis in white sturgeon ( Acipenser transmontanus) during oocyte maturation and induced ovulation

Abstract

Ovarian follicles and plasma were collected from two female white sturgeon, Acipenser transmontanus, injected with sturgeon pituitary homogenate followed 12 h later with GnRHa to induce ovulation. The oocytes of one female underwent germinal vesicle breakdown (GVBD) but ovulation did not occur in response to hormonal stimulation (Female 1), while the oocytes of the other female underwent GVBD and ovulation (Female 2). Follicles collected 12 h after the first injection to induce ovulation were incubated with radioinert pregnenolone (P5) or tritiated-P5 ([ 3H]P5) plus radioinert P5. Steroids were extracted from media and intact follicles, and the extracts were fractionated by high performance liquid chromatography (HPLC). Fractions from the incubation with radioinert precursor were used in a bioassay to determine the potency of the steroid products to induce GVBD. Plasma levels of testosterone (T), estradiol, and 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) were measured by radioimmunoassay during induced ovulation, and plasma collected at the time of ovulation (actual or expected) was analyzed by HPLC. A peak in plasma 17,20β-P was detected at the time of the second injection to induce ovulation in Female 2 (the time at which follicles were collected for incubation with [ 3H]P5). The HPLC analysis revealed several progestins in the plasma of Female 2 at ovulation that were not present in Female 1 at the time of expected ovulation. A variety of C19 and C21 steroids were produced in vitro by ovarian follicles from both females. The “suggestive” identities of the major metabolites were 11-deoxycortisol, androstenedione, 17-hydroxyprogesterone (17OHP), and 17,20β-P in Female 1 and cortisol, 17,20β, 21-trihydroxyprogesterone (20β-S), 11-deoxycortisol, T, 17OHP, and 17,20β-P in Female 2. Several of the steroids were active in a GVBD bioassay, but the fractions that contained the steroid coeluting the authentic 11-deoxycortisol on the HPLC and 17,20β-P (positively identified by gas chromatography-mass spectrometry) were found to be the most potent. The results from this study combined with the results of Webb et al. (2001b) suggest the potential roles of 11-deoxycortisol, 17,20β-P, 20β-S, and P4 as maturation-inducing steroids in sturgeon.