Production and endocrine role of inhibin during the early development of bull calves

This study investigated the ontogeny of control of FSH secretion by inhibin during early prepubertal development of bulls by 1) measurements of circul ating levels of inhibin and FSH from 1 to 13 wk of age, and 2) immunoneutra lization of endogenous inhibin at 7, 21, 60, and 120 days of age. In additi on, production and localization of inhibin in testes were examined by immun ohistochemistry and Western blots at 7, 21, 60, and 120 days of age. Plasma immunoreactive inhibin levels were relatively low between 1 and 3 wk of ag e and then showed a tendency to rise (P < 0.1) from 4 wk of age. Circulatin g concentrations of FSH were low during 3 wk after birth and increased at 5 wk, remained high (P < 0.05) until 16 wk of age. Treatment with inhibin an tiserum resulted in a significant (P < 0.05) increase in plasma FSH at 7, 2 1, 60, and 120 days of age compared to those following injection of control serum; however, the magnitude of the FSH rise after inhibin immunization w as greater as bulls aged. There were no significant changes in plasma LH af ter inhibin immunization. An intense staining of inhibin alpha subunits was found in Sertoli cells within the solid seminiferous cords from 7 to 120 d ays of age, while no specific immune reaction was found in interstitial cel ls. Western blot analysis of testicular homogenates isolated from bulls 7-1 20 days of age revealed presence of a 28.5-kDa molecule that cross-reacted with inhibin or subunit and beta (B) subunit-specific antibodies. In this s tudy, before 13 wk of age in bull calves, there was no inverse relationship between plasma concentrations of immunoreactive inhibin and FSH. However, the present immunization study clearly indicates that inhibin participates in the regulation of FSH secretion from infancy to early prepubertal stage, although the endocrine significance of inhibin becomes greater in older bu lls. The results also indicate that the major production site of inhibin in the testis is Sertoli cells and that these cells produce inhibin that exer ts a negative feedback effect on FSH secretion from early stages of develop ment.