Time course of inhibition of synthesis and basal release of FSH from pituitary glands of female rats by bovine follicular fluid in vitro

Abstract

Inhibin-like activity in steroid-free bovine follicular fluid (bFF) is demonstrated using an in vitro technique with hemi-pituitary glands from intact female (second day of dioestrus) and ovariectomized rats: synthesis as well as basal release of FSH, but not of LH, are inhibited profoundly. The results confirm and extend data from other investigators on the action of inhibin-like material. The effect of the inhibin-like activity is shown to be reversible, as synthesis and the rise of basal release are restored when bFF is withdrawn from the incubation medium. Synthesis of FSH seems to be inhibited earlier than basal release, and it is suggested that the inhibin-like material acts only directly on FSH synthesis. Some possibilities of the mechanisms of action of inhibin-like activity are discussed.In a previous paper we reported that basal release of FSH by pituitary glands from female rats in vitro spontaneously became elevated to a much higher rate after 4 h of incubation (Jenner et al. 1982).The increased rate of release persisted up to at least 12 h of incubation and was fully dependent on protein synthesis. We suggested that this rise of basal release of FSH could be due to the withdrawal of a specific ovarian feedback control, such as exerted by inhibin-like factor(s) and/or steroid hor¬ mones.Inhibin was originally postulated by McCullagh (1932) as a non-steroidal testicular factor which specifically suppresses FSH release. Subsequently, inhibin-like activity, of protein nature, has been detected in ovarian and testicular fluid of several species (Setchell & Sirinathsinghji 1972;de Jong & Sharpe 1976;Marder et al. 1977;Welschen et al. 1977;Daume et al. 1978;Davies et al. 1978;DePaolo et al. 1979b) and in medium from cul¬ tured rat Sertoli cells (Steinberger & Steinberger 1976;de Jong et al. 1978). These observations favour a physiological role for inhibin in the repro¬ ductive system.In the present work we investigated whether inhibin-like activity, as present in charcoal ex¬ tracted bovine follicular fluid, bFF (Welschen et al. 1977), could prevent the rise of basal release and the synthesis of FSH, which is normally seen in our incubation system. These results were compared with those of bFF on LH. Materials and Methods AnimalsFemale rats from the Wistar-derived colony were housed in a temperature-and light-controlled room: 22°C, lights on from 5 a.m. till 7 p.m. Vaginal smears were taken daily at 9 a. m. and only adult rats with regular 4-or 5-day cycles were used on the second day of dioestrus (Q Di-II). Ovariectomized (OVX) rats (0) were used 14 days after gonadectomy. IncubationsThe rats were killed by decapitation. After removal of the posterior lobe, the anterior pituitary glands were hemisected and the halves were placed into different incuba¬ tion flasks containing 1 ml medium TC 199 (Difco Labo¬ ratories). Each flask ultimately received two pituitary halves. All incubations were carried out in a thermostatic shaking-bath at 37°C under continuous shaking and gassing with 95 % 02 and 5 % C02.After ...