ESTRADIOL-17-BETA HAS A BIPHASIC EFFECT ON OXYTOCIN SECRETION BY BOVINE GRANULOSA-CELLS

The peptide hormone oxytocin (OT) and its mRNA are synthesized by bovine granulosa cells. Bovine granulosa cells isolated before the preovulatory LH/FSH surge secrete low amounts of OT and respond to gonadotropins with an increase in OT secretion. After the LH/FSH surge, basal OT secretion increases 20-fold. At the same time, dramatic changes in steroidogenesis occur in the preovulatory follicle. Estradiol-17beta production is high before and declines after the LH surge. The objective of this study was to determine whether steroid hormones, particularly estradiol-17beta, influence OT secretion by bovine granulosa cells. Heifers (n = 5) were injected with prostaglandin F2alpha (PGF2alpha) on Day 7 of the estrous cycle to induce luteolysis. Preovulatory follicles were obtained during the ensuing follicular phase, 24 h after PGF2alpha injection (about 24-36 h before the expected time of the preovulatory LH/FSH surge). Granulosa cells were isolated and cultured for 5 days in defined medium containing graded doses (0, 0.001, 0.01, 0.1, 1, 10 mug/ml) of estradiol-17beta, testosterone, or progesterone. Media were collected daily and assayed for OT by radioimmunoassay. Addition of 0.001 or 0.01 mug estradiol-17beta/ml stimulated OT secretion by granulosa cells by about 75% during the last 2 days of culture (p < 0.01 ). In contrast, estradiol-17beta at high doses (1 and 10 mug/ml) inhibited OT secretion by 45-85% (p < 0.01). Therefore, estradiol-17beta had a biphasic effect on OT secretion by granulosa cells. These results are consistent with low OT secretion by bovine granulosa cells before the LH/FSH surge, when estradiol-17beta is high in follicular fluid (approximately 1 to 1.5 mug/ml), and with high OT secretion after the LH/FSH surge, when estradiol-17beta in follicular fluid is low. Testosterone, which is utilized by granulosa cells as a precursor for estradiol-17beta production, increased OT secretion during Days 3-5 of culture, with doses of 0.01 and 0.1 mug testosterone/ml stimulating OT secretion by about 74 and 100% (p < 0.01). The highest dose of testosterone (10 mug/ml) inhibited OT secretion by 66% (p < 0.01). Physiological doses of progesterone stimulated total OT secretion by 40-85% during the last 2 days of culture (p < 0.01). The data suggest that during the early follicular phase in cattle, high concentrations of estradiol-17beta in the follicular fluid may counteract the stimulatory effect of rising LH on OT secretion. Subsequently, the decline in estradiol-17beta levels in the follicular fluid after the LH surge may allow stimulation of OT secretion by LH/FSH and progesterone. This study suggests that steroids may have an important role in modulating follicular OT secretion.