This study investigates the effects of spermatogenic germ cells on inhibin alpha-subunit and beta B-subunit expression, and inhibin alpha-subunit and inhibin B production by rat Sertoli cells in vitro. Sertoli cells isolated from 19-day-old rats were cultured for 48 h at 32 degrees C, in the presence or absence of FSH (2.3-2350 mIU/ml), and in the presence of pachytene spermatocytes, round spermatids or cytoplasts of elongated spermatids purified from adult rat testis by elutriation and density gradient separation. Sertoli cell secretion of inhibin alpha-subunit and inhibin B, as measured by immunoassay, was dose-dependently stimulated by FSH (maximal stimulation 13- and 2-fold, respectively). Round spermatids or cytoplasts co-cultured with Sertoli cells had no effect on basal or FSH-induced secretion of inhibin alpha-subunit or inhibin B. When Sertoli cells were co-cultured with pachytene spermatocytes, inhibin alpha-subunit secretion was unaltered, while inhibin B secretion was suppressed in a cell concentration-dependent manner to reach a maximal suppression of 45% compared with Sertoli cells alone (P<0.01). A similar suppression in inhibin B was still observed (64% of Sertoli cells alone) when the pachytene spermatocytes were separated from Sertoli cells by a 0.45 microm pore membrane barrier in bicameral chambers. Pachytene spermatocytes also suppressed FSH-induced inhibin B levels in Sertoli cell co-cultures and this suppression was attributed to a decrease in basal inhibin B production rather than a change in FSH responsiveness. Quantitation of Sertoli cell inhibin alpha- and beta B-subunit mRNA by quantitative (real-time) PCR demonstrated that pachytene spermatocytes did not alter Sertoli cell alpha-subunit mRNA expression, but significantly (P<0.01) suppressed basal and FSH-induced beta B-subunit mRNA expression to a similar degree to that seen with inhibin B protein levels. It is concluded that pachytene spermatocytes in vitro suppress Sertoli cell inhibin B secretion via factor-mediated suppression of inhibin beta B-subunit expression. These findings support the hypothesis that specific germ cell types can influence inhibin B secretion by the testis independent of FSH regulation.

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