Evaluation of changes in the secretion of immunoactive inhibin by adult rat seminiferous tubules in vitro as an indicator of early toxicant action on spermatogenesis

Abstract

A method for culturing isolated seminiferous tubules (ST) from adult rats for 1–3 days has been developed and optimized rigorously on the basis of the secretion of immunoactive inhibin under basal conditions and after maximal stimulation with rat FSH or dibutyryl cyclic AMP. The effect on these cultures of three known testicular toxicants was assessed. Of these, two are though to act on the Sertoli cell, meta-dinitrobenzene (mDNB) and nitrobenzene (NB), while the third, methoxy acetic acid (MAA), is thought to act on pachytene spermatocytes. In addition, the effect of a possible testicular toxicant, 3-mononitrotoluene (3-MNT), was investigated. These data were compared with those obtained using cultures of immature rat Sertoli cells (SC) or SC + germ cells and with data on the effect of equivalent doses of the compounds on the secretion of immunoactive inhibin in vivo. In studies designed to optimize conditions for the secretion of immunoactive inhibin by ST in culture, significant effects were found of the type of culture medium used, the duration of culture, the total and individual length of tubules used, etc. All subsequent studies with toxicants utilized optimal conditions. Addition of either mDNB or NB to ST cultures at 10 −5 or 10 −3 m, or MAA at 10 −4 m, stimulated basal secretion of immunoactive inhibin by two- to fourfold on Days 1, 2, or 3 of culture while FSH or dibutyryl cyclic AMP-stimulated secretion of immunoactive inhibin was either unaffected or was enhanced to a small extent. At the same doses, mDNB or NB also enhanced secretion of immunoactive inhibin by SC cultures, although these effects were more variable and of smaller magnitude than the effects on ST cultures. In contrast, addition of up to 10 −3 m MAA to cocultures of SC + germ cells had no effect on the secretion of immunoactive inhibin. Exposure of rats in vivo to levels of mDNB, NB, or MAA similar to those which stimulated secretion of immunoactive inhibin in vitro resulted in a two- to fourfold increase in the levels of immunoactive inhibin in testicular interstitial fluid (IF) at 1 and 3 days post-treatment, and this was associated with early impairment of spermatogenesis (as judged by testis weight). In contrast to these effects, addition of 3-MNT to ST or SC cultures had no effect except at 10 −3 m, when the secretion of immunoactive inhibin was increased marginally. Treatment of rats with an equivalent dose of 3-MNT in vivo resulted in death, but exposure to the highest nonlethal dose (1 g/kg) had no significant effect either on spermatogenesis or on the levels of immunoactive inhibin in testicular IF. In view of these findings, it is concluded that modulation of the secretion of immunoactive inhibin by isolated ST from adult rats has considerable potential as an in vitro screening method for investigating potential adverse effects of chemicals on spermatogenesis, and thus merits more detailed evaluation. Moreover, because of the agreement between in vitro and in vivo findings, measurement of the levels of immunoactive inhibin levels in vivo in testicular IF (or blood) may also be useful in the detection of early adverse effects of chemicals on spermatogenesis.