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Estradiol-17 beta has a biphasic effect on oxytocin secretion by bovine granulosa cells


Biology of Reproduction 48(6): 1404-1409
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- 17 beta production is high before and declines after the LH surge. The objective of this study was to determine whether steroid hormones, particularly estradiol-17 beta, influence OT secretion by bovine granulosa cells. Heifers (n = 5) were injected with prostaglandin F2 alpha (PGF2 alpha) on Day 7 of the estrous cycle to induce luteolysis. Preovulatory follicles were obtained during the ensuing follicular phase, 24 h after PGF2 alpha 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 micrograms/ml) of estradiol-17 beta, testosterone, or progesterone. Media were collected daily arid assayed for OT by radioimmunoassay. Addition of 0.001 or 0.01 micrograms estradiol-17 beta/ml stimulated OT secretion by granulosa cells by about 75% during the last 2 days of culture (p < 0.01). In contrast, estradiol-17 beta at high doses (1 and 10 micrograms/ml) inhibited OT secretion by 45-85% (p < 0.01 ). Therefore, estradiol-17 beta 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-17 beta is high in follicular fluid (approximately 1 to 1.5 micrograms/ ml), and with high OT secretion after the LH/FSH surge, when estradiol-17 beta in follicular fluid is low. Testosterone, which is utilized by granulosa cells as a precursor for estradiol-17 beta production, increased OT secretion during Days 3-5 of culture, with doses of 0.01 and 0.1 micrograms testosterone/ml stimulating OT secretion by about 74 and 100% (p < 0.01). The highest dose of testosterone (10 micrograms/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-17 beta in the follicular fluid may counteract the stimulatory effect of rising LH on OT secretion. Subsequently, the decline in estradiol-17 beta 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.


Accession: 002373549

PMID: 8318593

DOI: 10.1095/biolreprod48.6.1404



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