Identification of Gonadotropin Surge-Inhibiting Factor (GnSIF) in Follicular Fluid and its Differentiation from Inhibin1

Abstract

Exposure of rat pituitary cell cultures to charcoal-extracted porcine follicular fluid (pFF) inhibits gonadotropin-releasing hormone (GnRH) stimulation of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release in a dose-dependent manner. The inhibition of GnRH-stimulated gonadotropin secretion (gonadotropin surge-inhibiting factor [GnSIF] activity) by pituitary cells in vitro requires up to 24-48 h preexposure to pFF. One microliter of pFF inhibits approximately 50% of the GnRH-stimulated LH release and is defined as 1 unit of GnSIF activity. Basal LH secretion is unaltered under these conditions. GnSIF activity is distinct from that of inhibin, which selectively suppresses basal release of FSH but not LH. A partially purified preparation of inhibin contains less than 1% as much GnSIF activity as inhibin activity. GnSIF activity is resistant to moderate heat treatment (60 degrees C for 60 min) and is fully recovered after acetone precipitation. Chromatography of pFF on heparin/Sepharose affinity matrix effectively separates inhibin from GnSIF. Whereas inhibin has a high affinity for heparin, GnSIF activity does not associate with this affinity matrix and is recovered in the column void volume. In summary, we have developed an in vitro bioassay for the detection of GnSIF activity in pFF. Moreover GnSIF activity seemingly derives from a molecular entity distinct from inhibin, having different physicochemical characteristics and differential effects on pituitary gonadotropin secretion.

Immunoneutralization of Inhibin in Cycling Rats Increases Follicle-Stimulating Hormone Secretion, Stimulates the Ovary and Attenuates Progesterone Receptor-Dependent Preovulatory Luteinizing Hormone Secretion

“…Several authors report that inhibin could be the FSH-dependent GnSAF [36,43,44]. The present results, however, agree with others [3,5,6,8,42,45,46,47] indicating that, regardless of whether inhibin might have some GnSAF activity [43], an ovarian factor other than inhibin is involved in the reduction in LH levels on the proestrous afternoon [1,2]. This is because after an intravenous injection of 400 µl AIS at 11:00 h in metestrus, free AIS levels remained high throughout the cycle until the morning of estrus [22], and the serum levels of FSH were elevated after the injection of AIS, at least until the proestrous afternoon.…”

Changes in pulsatile secretion patterns of LH, FSH, progesterone, androstenedione and oestradiol in cows after superovulation with PMSG

“…Deviations in concentrations of PMSG, cortisol or prolactin are not responsible for a missing LH surge . It is possible that a so-called gonadotrophin-surge inhibiting factor, as demonstrated in the sow (Danforth et al 1987), is also secreted by developing bovine follicles.…”

The recombinant subdomain IIIB of human serum albumin displays activity of gonadotrophin surge-attenuating factor

“…GnSAF is distinct from inhibin (Ying, 1988), a gonadal protein factor which acts primarily by suppressing the basal secretion of FSH (Fowler et al, 1990;Byrne et al, 1995;Mroueh et al, 1996;Pappa et al, 1999a,b). GnSAF bioactivity has been demonstrated, by monitoring changes in the GnRH-induced LH secretion levels of rat pituitary primary cell cultures, in the ovarian follicular¯uid (FF) of human (Busbridge et al, 1990;Fowler et al, 1990;Knight et al, 1990;Mroueh et al, 1996) and other mammalian species (Schenken and Hodgen, 1986;Danforth et al, 1987;Busbridge et al, 1988;Danforth and Cheng, 1995), as well as in culture supernatants from human ovarian granulosa cells (Fowler et al, 2002).…”