Ontogeny of mesenchymal androgen receptors in the embryonic mouse mammary gland.

We demonstrate the presence of a high affinity androgen-binding site in the embryonic mouse mammary gland, and describe its appearance and development from the initial formation of the gland bud (day 12) through the androgen-responsive stage (day 14) until term (day 19). Binding assays were done on entire organ rudiments exposed to the ligand [( 3H] testosterone, [3H]5 alpha-dihydrotestosterone, or [3H]methyltrienolone) in vitro, and receptor levels are expressed per gland. In competition experiments, the binding site shows a ligand specificity of a typical rodent androgen receptor. Scatchard analysis yielded a figure of 90-100 million binding sites for one 14-day gland, or approximately 30,000 per target cell. The apparent dissociation constant Kd for ligand binding by the intact tissue was between 0.55 and 0.75 X 10(-9) M. The first binding sites become detectable at the time of mammary bud formation (day 12); their number increases about 20-fold towards the responsive stage (day 14), and they persist at high levels at least until birth. The loss of androgen responsiveness on day 15 is neither accompanied by a loss of receptors nor by a change of their binding affinity.