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, MCH5-17 and MCH1-14 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 MCH1-14 increased, whereas the sensitivity of the cells to MCH5-17 decreased. Verapamil diminished the sensitivity to MCH5-17, but did not affect melanophore responses to MCH or MCH1-14. 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 MCH1-14 also exhibit MSH-like melanosome dispersion within melanophores, skin darkening activity on fish melanophores whereas MCH5-17 lacks this characteristic. Since the darkening activity of MCH and MCH1-14 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.
Departamento de Fisiologia Geral, Universidade de São Paulo, Brazil.
M A
MA
Visconti
A M
AM
Castrucci
M E
ME
Hadley
V J
VJ
Hruby
D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology*; D002449:Cell Aggregation / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D007028:Hypothalamic Hormones; D008543:Melanins / Q000494:pharmacology*; D008547:Melanophores / Q000166:cytology / Q000187:drug effects*; D009994:Osmolar Concentration; D010907:Pituitary Hormones / Q000494:pharmacology*; D011036:Poecilia; D012964:Sodium / Q000494:pharmacology / Q000502:physiology*
D007028:Hypothalamic Hormones; D008543:Melanins; D010907:Pituitary Hormones; C019463:melanin-concentrating hormone; D012964:Sodium; D002118:Calcium