The melanogenic response to testosterone in scrotal epidermis: effects on tyrosinase activity and protein synthesis.

Pigmentation of the scrotum of the black-pelted rat, as expressed through melanocyte melanogenic activity, is controlled by androgens. Castration decreased in vitro incorporation of [14C]tyrosine into melanin. Testosterone pre-treatment for 4 days increased malanin radioactivity over castrate cont rols; the increment in vitro was prevented by an inhibitor of protein systhesis (cycloheximide) added to the incubation. However, cycloheximide only partially blocked melanin synthesis when added to tissue from animals hromone treated for 6 days in vivo, and was ineffective in tissue from intacts. Bulk protein snthesis in vitro (incorporation of [14C]tyrosine or -leucine) was not affected by castration or testosterone treatment but was uniformly inhibited by cycloheximide. The data suggest that new synthesis of specific protein in vitro was necessary for initial hormone-stimulation of melanogenesis, but with longer exposure to hormone sufficient protein was pre-synthetized in vivo to permit melanogenesis during incubation with the inhibitor.