title: Calcium sites in MSH stimulation of xenopus melanophores: studies with photoreactive alpha-MSH.
abstract: Photo-affinity labelling of MSH receptors on tail-fin melanophores of Xenopus tadpoles with p-azidophenylalanine 13-alpha-MSH (Pap13)-alpha-MSH) or p-azidophenylacetyl-serine1-alpha-MSH ([Apac-Ser1]-alpha-MSH) resulted in a long-lasting stimulation of the melanophores which cannot be reversed despite continuous washing. The generation of this irreversible response is inhibited when photo-affinity labelling is performed in a Ca2+-free medium or in the presence of Ca2+ antagonists. The irreversible stimulation produced in normal medium is completely reversed upon removal of Ca2+ , but is not affected by Ca2+ antagonists or melatonin. Re-addition of Ca2+ after temporary removal restores to irreversible stimulation, even in the presence of Ca2+ antagonists or melatonin. This proves that covalent alpha-MSH-receptor complexes remain fully functional despite temporary deprivation of ca2+. Racemized alpha-MSH, which binds 'tightly' to the receptor and produces a long-lasting effect, is removed from the receptor in Ca2+-free medium, but not by Ca2+ antagonists or melatonin. These results confirm earlier results showing that at least 2 Ca2+ sites are involved in alpha-MSH action, one associated with MSH-receptor binding and the other with the subsequent generation of the effect. The dual role of Ca2+ is not the result of EGTA present; it is specific (Mg2+ has no effect) and is identical for the two different photoreactive alpha-MSH derivatives.
mesh_terms: D000345:Affinity Labels; D000818:Animals; D001386:Azides; D002118:Calcium / Q000494:pharmacology*; D007536:Isomerism; D007814:Larva / Q000502:physiology; D009074:Melanocyte-Stimulating Hormones / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D008547:Melanophores / Q000187:drug effects* / Q000502:physiology / Q000528:radiation effects; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011979:Receptors, Pituitary Hormone; D013623:Tail; D014466:Ultraviolet Rays; D014982:Xenopus laevis; D000521:alpha-MSH / Q000031:analogs & derivatives*
chemical_list: D000345:Affinity Labels; D001386:Azides; D011956:Receptors, Cell Surface; D011979:Receptors, Pituitary Hormone; D000521:alpha-MSH; C026817:MSH, 13-(4-azido-Phe)alpha-; C029446:MSH receptor; D009074:Melanocyte-Stimulating Hormones; D002118:Calcium