title: Human recombinant inhibin A and testosterone act directly at the pituitary to suppress plasma concentrations of FSH in castrated rams.
abstract: The roles of inhibin and testosterone in the negative feedback control of the secretion of FSH were explored in experiments using castrated rams administered human recombinant inhibin A (hr-inhibin) and testosterone propionate (TP). Two experiments were conducted in the non-breeding season. In experiment 1, two groups of long-term castrated rams (wethers) were treated with an i.v. injection of either vehicle or hr-inhibin in two doses (25 and 50 micrograms) given 2 weeks apart. Plasma concentrations of FSH, measured by radioimmunoassay, were suppressed significantly (P < 0.01) and equally by both doses of hr-inhibin with a mean (+/- S.E.M.) maximal suppression of FSH of 19.9 +/- 2.60% occurring 6-10 h after injection. In experiment 2, hypothalamo-pituitary disconnected (HPD) wethers given 125 ng gonadotrophin-releasing hormone (GnRH) every 2 h, were treated with vehicle or 25 or 50 micrograms hr-inhibin before or after treatment (32 mg/day) with TP. A cross-over design was used so that each wether was treated with vehicle and hr-inhibin. Treatment with TP significantly (P < 0.001) suppressed plasma concentrations of FSH by 56%. Both doses of hr-inhibin were similarly effective in significantly (P < 0.05) suppressing plasma concentrations of FSH causing a mean suppression of 31.1 +/- 5.60% 6-10 h after injection. The suppressive effect of hr-inhibin was significantly (P < 0.05) increased when the wethers were treated with TP to a mean suppression of 50.7 +/- 5.6% 6-10 h after injection.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Agriculture, Werribee, Victoria, Australia.
authors.forename: A J
authors.initials: AJ
authors.lastname: Tilbrook
authors.forename: D M
authors.initials: DM
authors.lastname: de Kretser
authors.forename: I J
authors.initials: IJ
authors.lastname: Clarke
mesh_terms: D000818:Animals; D003864:Depression, Chemical; D005246:Feedback; D005640:Follicle Stimulating Hormone / Q000097:blood* / Q000378:metabolism; D007265:Inhibins / Q000494:pharmacology*; D008297:Male; D009919:Orchiectomy; D010902:Pituitary Gland / Q000187:drug effects* / Q000378:metabolism; D011994:Recombinant Proteins / Q000494:pharmacology; D012756:Sheep; D013739:Testosterone / Q000494:pharmacology*
chemical_list: D011994:Recombinant Proteins; D013739:Testosterone; D007265:Inhibins; D005640:Follicle Stimulating Hormone