Androgen stimulation of nucleotide pyrophosphatase during mullerian duct regression

Mullerian inhibiting substance (MIS) causes regression of the embryonic Mullerian duct. In the fetal rat urogenital ridge, extracellular nucleotide pyrophosphatase (NPPase) can be detected by histochemical staining on the regressing male Mullerian duct, with no corresponding enzyme localization on developing female Mullerian ducts. In vivo results in male embryos can be confirmed in vitro by incubating 14.5-day-gestation female urogenital ridges with MIS and testosterone for 72 h before enzyme localization. Since the addition of testosterone to MIS is obligatory to detect NPPase activity in vitro, and certain steroids enhance Mullerian duct regression, additional steroids were tested in vitro alone or in combination with MIS for their abilities to stimulate NPPase. NPPase induction occurred only with the combinations of MIS and testosterone or MIS and medroxyprogesterone acetate. Neither MIS alone nor any steroid used alone stimulated NPPase activity. The effect of exogenous NPPase added alone to the developing urogenital ridge was also assessed. Incubation of the female urogenital ridge for 72 h with exogenous NPPase caused marked hyperplasia of the Mullerian duct epithelial cells and early mesenchymal cell condensation, without the basement membrane breakdown normally seen in regression. Since NPPase activity is present in the Mullerian duct only during regression, these findings suggest that MIS and fetal androgens are synergistically modulating the activity of this enzyme. Its role in the Mullerian duct, as suggested by its cytological effects, may be to stimulate cellular responses before migration during regression.