In vitro bioactivity of gonadotrophin surge attenuating factor is not affected by an antibody to human inhibin

The effect of anti-inhibin antibodies on gonadotrophin surge attenuating/inhibiting factor (GnSAF/GnSIF) and its effect on gonadotrophin secretion in a pituitary cell bioassay were determined by culturing rat pituitary cells in a serum-free medium to which inhibin and a partially purified preparation of gonadotrophin surge inhibiting factor (GnSIF) were added. The samples were treated with an anti-inhibin antiserum and basal and GnRH-stimulated FSH and LH secretion were measured by radioimmunoassay of the culture supernatant. Inhibin had a dose-dependent inhibitory effect on both basal and GnRH-stimulated FSH secretion and also on GnRH-stimulated LH secretion. Anti-inhibin antibody blocked the inhibition of FSH secretion, except at the highest two doses of human inhibin (1.25 ng ml-1, P = 0.04; 2.5 ng ml-1, P = 0.0004). The GnSIF preparation inhibited basal and GnRH-stimulated FSH and GnRH-stimulated LH secretion and was not affected by the anti-inhibin antibody. The novel hormone, GnSIF, is different from inhibin.