The effect of bovine activin and follicle-stimulating hormone (FSH) suppressing protein/follistatin on FSH-induced differentiation of rat granulosa cells in vitro.

The time- and dose-dependent effects of bovine activin A and bovine follicle stimulating hormone (FSH) suppressing protein (FSP) or follistatin on basal and FSH-induced steroidogenesis and inhibin production were studied in granulosa cells from immature, diethylstilbestrol (DES)-treated rats. In the presence of rat FSH (20 ng/ml) which stimulates aromatase activity and the production of progesterone and inhibin, activin (0.3-100 ng/ml) augmented all three parameters, whereas FSP (0.3-100 ng/ml) enhanced progesterone production and attenuated the other two parameters. In the absence of FSH, the basal parameters were unaffected by treatment with either activin or FSP alone, except for a statistically significant increase in basal inhibin in the presence of activin alone (P less than 0.05, at doses of 30 and 100 ng/ml). Neither activin nor FSP influenced the timing of the maxima of FSH-induced activities over 5 days. These findings suggest that activin and FSP, both present in follicular fluid, may play an important role in the local regulation of granulosa cell differentiation.