The effects of steroidal and non-steroidal ovarian hormones on pituitary responsiveness to gonadotrophin surge-attenuating factor

Primary pituitary cultures from adult female rats were used to investigate the effects of steroidal (oestradiol and progesterone) and non-steroidal (inhibin, follistatin) ovarian hormones on the suppressive actions of the ovarian factor gonadotrophin surge-attenuating factor (GnSAF) in the control of gonadotrophin secretion. The source of GnSAF was a chromatographic preparation from follicular fluid containing four distinct protein bands as resolved on SDS-PAGE. Oestradiol and progesterone added alone had no effect on gonadotrophin secret ion but had a wide range of effects on the suppression of both LH and FSH secretion caused by the non-steroidal factors. Oestradiol, progesterone and oestradiol + progesterone enhanced the suppressive actions of GnSAF on GnRH-induced LH secretion (causing 19.3 +/- 5.2% (P<0.05), 41.9 +/- 3.4% (P<0.001) and 32.2 +/- 5.3% (P<0.001) greater suppression than GnSAF alone). Progesterone and oestradiol + progesterone completely abolished the suppression of basal FSH secretion caused by inhibin (causing 157.1 +/- 22.2%, P<0.001, and 160.9 +/- 11.3%, P<0.001, stimulation compared with inhibin alone). Separately the steroids had no effect on the suppression of gonadotrophin secretion caused by follistatin. However, in combination, oestradiol; progesterone potentiated the suppressive actions of follistatin on GnRH-induced LH secretion causing 29.9 +/- 5.3% (P<0.05) greater suppression than follistatin alone. In combination, high-dose follistatin and GnSAF caused 31.1 +/- 6.5% (P<0.01) greater suppression than GnSAF alone. Thus in combination high-dose follistatin and GnSAF have additive effects on the suppression of GnRH-induced LH secretion. Recombinant human inhibin and GnSAF added in combination had little further effect compared with either alone suggesting that they may have a similar mechanism of action at the pituitary level. These results demonstrate that while FSH secretion in vitro is mainly controlled by inhibin and follistatin, LH secretion is affected. by the presence of a whole range of factors. We have demonstrated that oestradiol and progesterone potentiate the suppressive actions of GnSAF in vitro. These data are compatible with the suggestion that in the late follicular phase it is falling levels of GnSAF that allow positive feedback of the steroids on the pituitary to elicit the LH surge, rather than increases in the stimulatory effects of the ovarian steroids overcoming GnSAF. The actions of GnSAF on the pituitary may be modulated by follistatin but it is unlikely that inhibin has any modulatory effects oil the GnSAF-induced suppression of LH secretion.