Differential effects of gonadotropin and orthovanadate on oocyte maturation, ovulation, and prostaglandin synthesisby Rana ovarian follicles in vitro

Abstract

Effects of gonadotropin and sodium orthovanadate on oocyte maturation, ovulation, prostaglandin, and progesterone synthesis were examined during in vitro culture of Rana ovarian follicles obtained from hibernating animals. Frog pituitary homogenates (FPH, 0.05 gland/ml) effectively induced oocyte maturation (germinal vesicle breakdown, GVBD) and ovulation (>90%) in ovarian follicles obtained in mid-hibernation (mid-December through late January). In contrast, orthovanadate induced a limited amount (<45%) of ovulation of some oocytes without concomitant induction of maturation during mid-hibernation. In late hibernation (February to early March), considerable spontaneous maturation and ovulation occurred in cultured ovarian follicles. During this period both FPH (0.0005–0.05 gland/ml) or orthovanadate (0.01-1 mM) treatment markedly increased the incidence of ovulation and accelerated the onset of ovulation in a dose dependent manner. When examined after 12 h of culture, few (<5%) oocytes ovulated in response to orthovanadate had undergone GVBD whereas most (> 90%) of those ovulated in response to FPH had matured. However, the majority of oocytes ovulated (72%) within 6 h of exposure to FPH had intact GVs, indicating that GVBD is also not a prerequisite for ovulation induction in response to FPH. Orthovanadate stimulated PGF2α in a dose dependent manner but failed to stimulate progesterone production whereas FPH stimulated secretion of both PGF2α and progesterone. The amount and time course of PGF2α secretion in response to orthovanadate were similar to results produced with FPH stimulation. Treatment of PGF2α to follicles obtained in late hibernation also accelerated the ovulation of the oocytes. Taken together, the data suggest that orthovanadate enhanced ovulation of immature oocytes was mediated via enhanced PGF2α production and that oocyte maturation is not essential or prerequisite for in vitro oocyte ovulation in Rana. J. Exp. Zool. 277:155–165, 1997. © 1997 Wiley-Liss, Inc.