Expression of inhibin subunit mRNAs and inhibin levels in the testes of rats with stage-synchronized spermatogenesis.

Inhibin alpha- and beta B-subunit mRNA expression, and levels of bioactive and immunoreactive inhibin were studied in rat testes, synchronized for the stage of the cycle of the seminiferous epithelium by treating vitamin A-deficient rats with vitamin A. Measurement of inhibin subunit mRNA expression and inhibin levels was started directly after the start of vitamin A treatment, and continued for 65 days. Inhibin subunit mRNA expression, and testicular bioactive and immunoreactive inhibin levels increased after the start of vitamin A treatment, reaching maximum values after 9 days, when B spermatogonia and preleptotene spermatocytes had appeared in the stage-synchronized testes. The ratio between beta B- and alpha-subunit mRNA expression was high at that time-point, whereas the ratio between bioactive and immunoreactive inhibin remained low. These data suggest a relatively high production of activin at that moment, and this may play a role in the development of B spermatogonia into preleptotene spermatocytes during the initiation of spermatogenesis. Stage-dependency was demonstrated for inhibin subunit mRNA expression, and for the levels of bioactive and immunoreactive inhibin, in rats with complete spermatogenesis. Inhibin alpha-subunit mRNA expression was relatively high at stages V and XIII of the spermatogenic cycle, whereas beta B-subunit mRNA expression was high at stage XIII but not at stage V. This resulted in a high beta B/alpha subunit mRNA ratio at stage XIII. Since it has been shown that expression of the activin receptor is high at stages XIII-I, locally formed activin might play a role in the regulation of meiosis. Bioactive and immunoreactive inhibin were highly correlated during the cycle, with maximum levels at stages XIV-I. It was concluded that the production of inhibin, and possibly activin, is dependent on the stage of the cycle of the seminiferous epithelium; these growth factors might play a paracrine role in the differentiation of spermatogenic cells.