Ionic Requirements for Melanin Concentrating Hormone (MCH) Actions on Teleost Poecilia reticulata Melanophores

Melanin concentrating hormone (MCH) is a cyclic heptadecapeptide, Asp‐Thr‐Met‐Arg‐Cys‐Met‐Val‐Gly‐Arg‐Val‐Tyr‐Arg‐Pro‐Cys‐Trp‐Glu‐Val, synthesized in the hypothalamus and released by the neurohypophysis of teleost fish. This hormone is a potent lightening agent of fish skin. This lightening results from the stimulation of a centripetal melanosome (melanin granule) migration to a perinuclear position within integumental melanophores. MCH and related fragment analogues, MCH_5‐17 and MCH_1‐14 were used to investigate the ionic requirements for receptor activation by MCH on dermal melanophores of the fish Poecilia reticulata. In calcium‐free saline, the sensitivity of the melanophores to MCH and MCH_1‐14 increased, whereas the sensitivity of the cells to MCH_5‐17 decreased. Verapamil diminished the sensitivity to MCH_5‐17, but did not affect melanophore responses to MCH or MCH_1‐14. The melanosome aggregating response to MCH was not affected in the presence of tetrodotoxin or in sodium‐ or potassium‐free (choline‐substituted) saline. These results suggest that neither TTX‐sensitive sodium channels nor extracellular sodium or potassium ions play a role in MCH‐induced melanosome aggregation. It is known that MCH and MCH_1‐14 also exhibit MSH‐like melanosome dispersion within melanophores, skin darkening activity on fish melanophores whereas MCH_5‐17 lacks this characteristic. Since the darkening activity of MCH and MCH_1‐14 requires calcium, these analogues exhibited a diminished lightening (MCH‐like) activity in the presence of the divalent cation. In the absence of the N‐terminal tetrapeptide sequence (necessary for the expression of MSH‐like activity), a role for calcium on melanosome aggregation became evident. These results demonstrate a bifunctional role of calcium on melanosome movements.