Regulation of oxytocin receptors in bovine granulosa cells

It is reported that oxytocin (OT) receptors in bovine granulosa cells decrease in concentration during follicular development. However, the factor or factors that regulate OT receptors are not known. In the present study, we evaluated hormonal control of OT receptors in bovine granulosa cells obtained from small antral follicles (3-5 mm in diameter). Granulosa cells were cultured for 48 h and exposed to FSH, LH, progesterone, and/or estradiol-17beta (estradiol) in the final 15 h of culture. The relative binding of OT decreased to 63% of the control value following treatment with FSH (100 ng/ml). The inhibitory effect of FSH was mimicked by an adenylate cyclase activator, forskolin. In contrast, estradiol (10(-7) M) increased the number of OT receptors by 77% compared with that in untreated controls, without changing binding affinity. The effects of estradiol were dose dependent and were diminished by an estradiol antagonist, tamoxifen (10(-6) to 10(-5) M). Although tamoxifen (10(-5) M) alone did not change OT binding, the stimulatory effects of 10(-9) M and 10(-8) M estradiol were inhibited by treatment with tamoxifen (10(-5) M). Furthermore, when the granulosa cells were exposed to FSH (10 ng/ml) and estradiol (10(-10) to 10(-7) M) in various combinations, estradiol inhibited the reduction of OT receptors by FSH. On the other hand, LH and progesterone did not affect OT binding in the cultured granulosa cells. Additionally, OT secretion from cultured granulosa cells was not changed by any treatment used in the present study. These findings suggest that both FSH and estradiol are significant regulators of OT receptors in granulosa cells during follicular development. FSH might down-regulate OT receptors in this phase, and the inhibitory effects of FSH are mediated by the adenylate cyclase-cAMP-protein kinase A system. Furthermore, estradiol seems to play a role in neutralizing the effects of FSH.