Radioimmunoassay of rat serum inhibin: changes after PMSG stimulation and gonadectomy

A heterologous inhibin radioimmunoassay (RIA) method has been developed which is highly specific and of sufficient sensitivity to detect inhibin in female and male rat serum. Purified bovine 31 kDa inhibin was used in the generation of the antiserum and following iodination as tracer in the assay. Parallel logit-log dose-response lines were observed between a rat ovarian extract used as standard and serial dilutions of female and male serum and testicular interstitial fluid. The within-assay variation based on index of precision was 0.049 (n = 5) and the between-assay variation (n = 4) was 9.8%. The specificity of the assay was assessed from: (a) the failure of a number of structurally related proteins (activin-A, transforming growth factor-beta (TGF-beta), Müllerian inhibitory substance) as well as inhibin subunits to crossreact (less than 0.5%) in the assay relative to bovine 31 kDa inhibin; (b) nondetectable levels of immunoactivity in the serum of gonadectomised rats; and (c) a close correlation (r = 0.96) between serum levels of in vitro biological and immunological activities from rats following pregnant mare serum gonadotropin (PMSG) stimulation. Similar initial t 1/2 values (14-15 min) of serum inhibin following gonadectomy were obtained in both sexes. This RIA method will be useful in the study of the physiology of inhibin in the female and male rat.