The regulation of inhibin secretion has not been elucidated fully in male ruminants. The aim of this study was to determine the relative importance of FSH and testosterone concentrations, and FSH receptors, in the control of secretion of immunoactive inhibin in rams. In Expt 1, temporal changes in hormone concentrations and testicular FSH binding were determined for two groups of rams (n = 4) kept under opposite, alternating 4 month periods of long (16 h light:8 h dark) and short (8 h light:16 h dark) days. Testicular biopsies (1-2 g) were collected when the testes were regressed, redeveloping, redeveloped and regressing. In Expt 2, separate groups of rams (n = 4) kept under natural photoperiod (latitude 45 degrees 48 minutes N) were designated as controls or passively immunized (for 3 weeks) with sufficient oestradiol antiserum to increase testosterone secretion without altering LH and FSH; this was done when the testes were regressed (non-breeding season) and redeveloped (breeding season). In both groups of rams (Expt 1), 'seasonal' increases in FSH concentrations began a few weeks earlier than did increases in inhibin concentrations. FSH reached maximum concentrations during testicular recrudescence, whereas numbers of FSH receptors in the testis and circulatory inhibin concentrations did not reach peak values until the testes were fully developed. Numbers of FSH receptors per testis, but not FSH concentration, were positively correlated (r = 0.65) with inhibin concentrations across the four stages of the testicular cycle. Near the end of testicular recrudescence early in the breeding season (Expt 2), relatively high FSH concentration was associated with increased abundance of FSH receptor mRNA (90%) and number of receptors (45%) in the testis and increased inhibin concentrations (50%), compared with when the testes were regressed. Moderate, physiological increases in testosterone secretion in immunized rams did not affect inhibin in either season. These results indicate that: (i) FSH stimulation of immunoactive inhibin secretion by Sertoli cells as testes recrudesce is via increases in secretion (early) and cognate receptors (late); (ii) FSH upregulates the synthesis of its own receptor late in recrudescence; and (iii) the positive correlation (r = 0.70) observed between circulatory testosterone and immunoactive inhibin does not reflect a causal relationship.

Gonadotropin receptors in previously frozen (-70 degrees C) sheep testicular tissue were characterized, and methods of assessment of receptor binding activity were established and applied to an investigation of testicular function in the short scrotum ram. Binding of 125I-labelled ovine luteinizing hormone (125I-oLH) and 125I-labelled ovine follicle-stimulating hormone (125I-oFSH) to testicular membranes was highly specific and saturable. Uptake of labelled gonadotropins was proportional to the amount of membrane protein, with 125I-oFSH showing greater specific binding. Initial association of 125I-oLH with binding sites was comparable at 4, 25, and 34 degrees C; with prolonged incubation, maximal binding occurred at 4 degrees C. Equilibrium was achieved in 8 h at 34 degrees C and in 16 h at 25 and 4 degrees C. In contrast, the temperature-dependent association of LH with rat testicular membranes was greater at 25 than at 4 degrees C. The rate of association of 125I-oFSH to binding sites was proportional to incubation temperature, with equilibrium being achieved in 2 h at 34 degrees C and in 16 h at 25 degrees C; binding at 4 degrees C; was slow and still increasing by 48 h. Binding of radioactive and nonradioactive oLH and oFSH was hormone specific and increased in a dose-dependent manner until saturation occurred. Shortening the scrotum of adult rams led to reductions (p less than 0.05) in testicular weight (60%) and in the number of LH (55%) and FSH (90%) binding sites per testis, with no apparent change in serum testosterone concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Membranes derived from free floating granulosa cells in porcine ovarian follicular fluid were used as a starting material for structural characterization of both LH/hCG and FSH receptors. The receptors were highly hormone-specific and showed single classes of high-affinity binding sites (Kd = 19-74 pM). Their molecular weights as determined by affinity cross-linking with their respective 125I-ligands were similarly 70,000. The membrane-localized receptors could be solubilized with reduced Triton X-100 in the presence of 20% glycerol with good retention of hormone binding activity. The Triton extracts of membranes also showed hormone specificity and equilibrium binding constants similar to the membrane receptors (Kd = 32-48 pM). Affinity chromatography on divinylsulfonyl-Sepharose-oLH columns was utilized to purify the solubilized LH/hCG receptor to a specific activity of 2000 pmol/mg of protein. The purified receptor exhibited a high specificity for hCG and hLH but not for hFSH nor bTSH. The purified receptor was iodinated and visualized to be composed of a major protein of Mr approximately 70,000 and other minor proteins of molecular weights ranging from 14,000 to 40,000. Except for the Mr 14,000 protein, all other protein species bound to the concanavalin A-Sepharose column. The data suggest that the ovarian LH/hCG and FSH receptors are structurally similar and consist of a single polypeptide chain, as recently documented for the LH/hCG receptor (Loosefelt et al., 1989; McFarland et al., 1989).