To investigate the inhibin-induced suppression of FSH secretion by the anterior pituitary, chronically ovariectomized heifers (three per group) were treated for 56–58 h with either steroid-free bovine follicular fluid (bFF; 8 ml i.v. every 8 h) or 0·9% (w/v) NaCl (8 ml i.v. every 8 h). Blood was withdrawn at 8-h intervals for analysis of plasma concentrations of FSH and LH by radioimmunoassay. At the end of the treatment period, heifers were slaughtered and pituitary glands recovered for determination of gonadotrophin contents and levels of mRNA encoding FSH-β, LH-β, TSH-β and common α glycoprotein hormone subunits using [32P]cDNA probes in total RNA dot and Northern blot assays. Treatment with bFF markedly suppressed plasma FSH by 85% (P<0·001 compared with pretreatment period), but did not affect plasma LH concentrations. Plasma FSH and LH concentrations did not vary significantly in the saline-injected control heifers. The level of FSH-β subunit mRNA was reduced by 60% (P<0·001) in heifers treated with bFF, whereas no significant differences between control and bFF-treated heifers were observed in the levels of mRNA encoding LH-β, TSH-β or common α subunits. Treatments with bFF, however, did not affect pituitary content of either FSH or LH.
These results support the conclusion that inhibin exerts its selective suppressive effect on the secretion of FSH by the bovine pituitary, at least in part, by directly inhibiting expression of the gene encoding the FSH-β subunit.