Mullerian inhibiting substance inhibits testosterone synthesis in adult rats

Mullerian inhibiting substance (MIS) is a gonadal hormone that causes regre ssion of the Mullerian ducts during male sexual differentiation. Postnatall y, MIS inhibits the proliferation and differentiation of immature Leydig ce lls, and transgenic mice that overexpress MIS have decreased serum testoste rone concentrations. To elucidate the effects of MIS on androgen regulation in the postnatal testis, we examined testosterone synthesis in adult Sprag ue-Dawley rats following intratesticular and intraperitoneal injections of MIS. Intratesticular MIS injection achieved high local concentrations of MI S (574.0 +/- 60.0 ng/mL) at 4 hours, with a corresponding decline in serum testosterone concentrations to 0.7 +/-0.1 ng/mL, compared to 1.1 +/-0.2 ng/ mL with intraperitoneal MIS and 1.6 +/-0.1 ng/mL with intratesticular vehic le (IT-Veh) (P<.001). Intratesticular administration of MIS (IT-MIS) result ed in much higher serum and testicular interstitial fluid MIS concentration s than the intraperitoneal route. To directly examine the testosterone prod uction rate in MIS-treated animals, we isolated Leydig cells from MIS and v ehicle-injected testes. Primary Leydig cells exposed to MIS had a lower tes tosterone production rate and decreased expression of p450c17 (hydroxylase/ lyase) and luteinizing hormone (LH) receptor mRNAs than that of vehicle-inj ected controls or the noninjected contralateral testis. In conclusion, intr atesticular administration of MIS caused a decline in serum testosterone co ncentrations by decreasing the rate of testosterone biosynthesis, confirmin g that MIS can regulate adult Leydig cell androgen production. The ability of MIS to down-regulate mRNA expression of the p450c17 and LH receptor gene s suggests that this effect is mediated transcriptionally. These data indic ate that, in addition to its role in embryonic differentiation of the male reproductive tract, MIS has a regulatory function in the postnatal testis. We conclude that one such function is for MIS to directly inhibit adult Ley dig cell steroidogenesis.