Changes in the plasma concentrations of inhibin throughout the normal sheep oestrous cycle and after the infusion of FSH

Abstract

A radioimmunoassay for inhibin was developed using a peptide containing the 1-26 amino acid sequence of the N-terminus of the alpha-chain of 32 kDa porcine inhibin as immunogen, and 125I-labelled tracer. Evaluation of this assay using Sephadex column chromatography, chromatoelectrophoresis and immunoblotting confirmed that it measured all forms of inhibin present in sheep follicular fluid and was suitable for measurement of inhibin in sheep plasma. There was no evidence of the presence of free alpha-subunit in either sheep follicular fluid or ovarian vein plasma. The concentration of inhibin in jugular plasma throughout the follicular and luteal phases of four ewes with ovarian autotransplants was measured. The ovarian secretion of inhibin and oestradiol were also measured simultaneously throughout the follicular phase in a spontaneous cycle and after infusion of NIH-oFSH-S14 at 10 micrograms/h for 48 h following premature luteal regression induced by prostaglandin. The results showed: (1) no change in the peripheral concentration of inhibin throughout the cycle except an increase related to the periovulatory increase in FSH and LH. (2) Following luteal regression, the concentration of FSH fell as the secretion rate of oestradiol increased. During this time there was no significant change in the peripheral concentration of inhibin or ovarian inhibin secretion rate. (3) Following the infusion of FSH there was a marked increase in the concentration of inhibin in both ovarian and peripheral plasma and an increase in ovarian inhibin secretion rate. (4) The calculated metabolic clearance rate of inhibin, 20.3 ml/min, is similar to that of FSH. We conclude that in the ewe the ovarian inhibin secretion rate is stimulated by FSH and, although inhibin may modulate the basal secretion of FSH, a change in its secretion does not account for the fall in FSH which occurs during the follicular phase of the sheep oestrous cycle.

Effects of gonadotrophin treatments on meiotic and developmental competence of oocytes in porcine primordial follicles following xenografting to nude mice

“…After treatment with gonadotrophins, especially with FSH, the ovarian grafts showed increased inhibin production, probably in association with increased estradiol production, and this had further negative-feedback effects on mouse pituitary FSH secretion. A similar reciprocal relationship between inhibin and FSH has been observed in FSH-or eCG-treated domestic animals (McNeilly et al 1989, Kaneko et al 1992. The fact that FSH secretion was severely suppressed in mice of the eCG-3-and porcine FSH-treated groups strongly suggests that, in these groups, follicular growth is stimulated or maintained largely by exogenous gonadotrophins, not by endogenous mouse FSH.…”

Expression of Messenger Ribonucleic Acid for Inhibin Subunits and Ovarian Secretion of Inhibin and Estradiol at Various Stages of the Sheep Estrous Cycle1

“…To investigate this possibility further, levels of immunoactive and bioactive inhibin were compared at various stages of the cycle. Immunoactive inhibin was quantified through use of an antibody directed against the a-subunit [7], and bioactivity was determined by inhibition of FSH production by ovine pituitary cells in vitro [24]. These two measures of inhibin were in general agreement at all times except after the LH surge, when apparent secretion of immunoactive inhibin was high but that of bioactive inhibin was low (unpublished results).…”

“…According to this hypothesis, inhibin secreted by the preovulatory follicle acts in an endocrine fashion to depress the secretion of FSH while concurrently having paracrine actions allowing the preovulatory follicle to grow and differentiate in spite of the reduction in FSH concentrations. Although inhibin secretion increases [7,8] and FSH levels decrease [9] during the follicular phase in the sheep, this change in inhibin secretion does not seem to account for the fall in FSH [7]. Although the endocrine role of inhibin during this period is uncertain, evidence is accumulating to support the paracrine aspect of this hypothesis: in vitro studies in the rat [10] and human [11] have shown that inhibin enhances LHstimulated thecal androgen production.…”

“…In vivo experiments in the sheep have shown that the major source of inhibin is large estrogenic follicles [12,15], although unlike estradiol, inhibin is secreted in appreciable quantities by large, nonestrogenic follicles and small follicles [15]. Although inhibin secretion in the sheep can be stimulated by long-term treatment with FSH [7,16], interpretation of these effects is confounded by the stimulatory action of FSH on follicular growth. In the short term, inhibin secretion has been shown to be unaffected for up to 12 h after injection of FSH in amounts that double basal concentrations [17].…”

Inhibin A is a follicle stimulating hormone-responsive marker of granulosa cell differentiation, which has both autocrine and paracrine actions in sheep

“…Initial studies examining the follicular origin of inhibin relied on bioassays that, although potentially confounded by the high steroid concentrations in antral fluid, showed that inhibin bioactivity was correlated with both the number of granulosa cells and their oestrogenicity (Tsonis et al 1983). Until relatively recently, the available data on the synthesis of immunoactive inhibin was limited to assays that could not differentiate between inhibin A, inhibin B and non-bioactive isoforms of inhibin (McNeilly et al 1989. Early studies using these assays in sheep also indicated that the ovarian secretion of inhibin was positively correlated with oestradiol secretion ) and that, whilst immunoactive inhibin was secreted by both small and large antral follicles, the majority of the inhibin secreted was derived from large oestrogenic follicles (Campbell et al 1991, Mann et al 1992b.…”