FURTHER STUDIES OF THE HORMONAL INDUCTION OF MELANOGENESIS IN THE KILLIFISH,FUNDULUS HETEROCLITUS

Abstract

New experiments confirm the respective roles of prolactin and intermedin in the in vivo melanin pigmentation of Fundulus heteroclitus: prolactin promotes melanin synthesis, intermedin stimulates proliferation of melanocytes. Prolactin potentiates the response to intermedin, but intermedin has no reciprocal influence on the response to prolactin. Treatment with alkali enhances the activity of intermedin about tenfold. The response to alkali-treated intermedin increases in proportion to the dose. The slight melanocyte stimulating action of ACTH, which is potentiated by prolactin, is concordant with its inherent MSH-like activity. In vitro studies of fin samples incubated with dopa indicate a significant correlation between O2 consumption and degree of darkening. After hypophysectomy, which leads to progressive in vivo depigmentation, dopa tyrosinase activity declined. Enzyme activity was restored, to a greater or lesser extent, by chronic administration of any of the 3 hormones under investigation: prolactin, intermedin, and ACTH. Only prolactin restored in vivo melanin pigmentation, possibly by increasing the supply of endogenous precursor. Since new pigment cells are being formed, the stimulating action of intermedin on the enzyme system may be different from that of prolactin in which only an intracellular increase in already existing cells can be postulated. The influence of prolactin in potentiating the response to intermedin may be correlated with its hypothetical action on chromagen precursor. ACTH appears to stimulate both dopa tyrosinase and also a non-tyrosinase oxidative enzyme system.