Radioimmunoassay of inhibin in various mammals.

A sensitive radioimmunoassay (RIA) for the determination of inhibin in peripheral plasma and tissue homogenates of different species has been developed using antisera to partially purified bovine follicular fluid (bFF) inhibin and 125I-labelled bFF 32 kDa inhibin. Antisera were produced by immunization of rabbits with partially purified bFF inhibin prepared by immunoaffinity chromatography. Increasing doses of a high titre antiserum could neutralize the suppressing effect of bFF, porcine follicular fluid and rat ovarian homogenate on FSH secretion from rat anterior pituitary cells in culture. Sensitivity of the assay was 3.1 ng International Research Standard of porcine inhibin per tube. Parallel inhibition curves were obtained for inhibin preparations from female and male animals of ten species, i.e. cattle, goats, sheep, cats, dogs, monkeys, pigs, horses, rats and man. Inhibin subunits and related proteins cross-reacted minimally with the antiserum used in the study. Plasma concentrations of inhibin in adult male and female rats were measured by the RIA before and at various times after gonadectomy. Inhibin levels in peripheral plasma before gonadectomy were significantly higher in adult female than in adult male rats. Inhibin levels decreased abruptly after gonadectomy in both sexes and they correlated negatively with plasma concentrations of FSH. This inhibin RIA will facilitate studies of the physiology of inhibin in various species of animals.