Effects of gonadotrophin surge-attenuating factor on the two pools of gonadotrophin-releasing hormone-induced luteinizing hormone secretion in vitro

In order to study the in-vitro mechanism of action of gonadotrophin surge-attenuating factor (GnSAF), ovine pituitary monolayers were incubated for 48 h either with steroid-free follicular fluid from women in whom ovulation had been induced or oestradiol or progesterone. Monolayers were also incubated with 10-30 and > 30 kDa follicular fluid fractions. The effect of these preparations on gonadotrophin-releasing hormone (GnRH)-induced luteinizing hormone (LH) secretion was then determined by incubation with GnRH for between 15 and 240 min. Incubation with steroid-free follicular fluid resulted in the significant attenuation of both the acutely releasable LH pool (LH 54.3 +/- 3.0% of control secretion at 15 min of GnRH incubation) and the reserve LH pool (LH 36.8 +/- 1.6% of control secretion at 240 min GnRH incubation). The 10-30 kDa follicular fluid fraction had similar effects while the >30 kDa follicular fluid fraction significantly stimulated GnRH-induced LH secretion, with concentrations up to 134.0 +/- 8.6% of controls. Oestradiol or progesterone at doses of 1000 nmol/l/well did not significantly reduce any aspect of GnRH-induced LH secretion (never <95.1 +/- 15.1% of controls). These results demonstrate that GnSAF markedly attenuates both the acutely releasable and reserve pools of GnRH-induced LH secretion. Inhibin, oestradiol and progesterone are not responsible for the marked attenuation of either the initial or reserve pools of GnRH-induced LH secretion.