The identification of gonadotrophin surge inhibiting factor and its role in the regulation of pituitary gonadotrophin secretion.

Evidence from several laboratories suggests that the ovaries of rats, pigs, monkeys and women produce a non-steroidal factor which inhibits or attenuates the mid-cycle gonadotrophin surge. This substance is present in follicular fluid and we have called it gonadotrophin surge inhibiting factor (GnSIF). Utilizing a rat pituitary cell bioassay, we have monitored GnSIF activity during purification from porcine follicular fluid (PFF). Rat pituitary cells in short-term culture were incubated with GnSIF for 48 h, washed and then exposed to gonadotrophin releasing hormone (GnRH) plus GnSIF for 4 h. GnSIF activity was defined as suppression of GnRH-stimulated LH secretion over 4 h, and inhibin activity was expressed as suppression of basal follicle stimulating hormone (FSH) secretion over 48 h. Approximately 5 l of PFF was fractionated through heparin-Sepharose, Q-Sepharose, Mono-S, hydroxylapatite, and gel permeation chromatography steps. This fractionation removes all detectable inhibin and follistatin immunoactivity and bioactivity. The purified GnSIF inhibits GnRH-stimulated LH secretion with little or no effect on basal FSH release. In summary, we have obtained a purified preparation of GnSIF which contains undetectable inhibin and follistatin. As such, GnSIF appears to be distinct from other known gonadal proteins regulating reproductive function, and may participate along with the inhibins, activins and follistatins in the gonadal regulation of pituitary gonadotrophin secretion.