Radioimmunoassay of Inhibin: Serum Responses to Unilateral and Bilateral Orchidectomy*

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Abstract
An overnight double antibody RIA using a rabbit antiserum to porcine inhibin .alpha.-chain [Tyr30] (1-30) NH2 [pI.alpha.(1-30)], radioiodinated pI.alpha.(1-30), and a preprecipitated second antibody complex has been developed to measure inhibin concentrations in sera and other biological fluids. The assay is accurate, precise (intraassay coefficient of variation, 4.8%), sensitive (25 pM; 2.5 fmol/tube), and specific for inhibin. The synthetic reference standard pI.alpha.(1-30) produced a displacement curve that paralleled intact male ovine and bovine sera, crude bovine follicular fluid, and a partially purified porcine follicular fluid reference preparation (WHO/NIH 86/690). Bilateral castration of prepubertal and postpubertal ram lambs resulted in a rapid decrease in serum inhibin concentrations and a subsequent increase in serum FSH. Inhibin levels were high in prepubertal lambs (.apprx. 375 pM), but these levels were not sustained near the time of puberty (.apprx. 180 pM). Intensive sampling by jugular venipuncture after castration indicated a 50% drop in circulating inhibin levels within 2 h of testes removal with chronic castrate levels (.apprx. 75 pM) achieved by 6 h postcastration. A rapid fall in circulating levels of inhibin was also observed after unilateral castration, but these values stabilized within hours to levels intermediate (i.e. .apprx. 200 pM) to those of intact and bilateral castrate rams. Hemicastrates exhibited a more subtle rise in serum FSH after testis removal, with FSH and inhibin levels of prepubertal hemicastrates returning to mature intact ram values by 15 weeks of age. Serum inhibin levels remained low and FSH levels high at 14 days in unilateral castrate postpubertal rams. Inhibin immunoreactivity increased abruptly in castrate ewes and rams injected iv with 5 ml bovine follicular fluid. Serum inhibin reached 300 pM immediately after bovine follicular fluid injection, and castrate FSH levels decreased 30% to a nadir 8 h later. These physiological findings provide support for the validity and utility of this inhibin RIA and further suggest that the Sertoli cells (not the germ cells) are the likely source of the FSH release-inhibiting factor from the testis.

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