EFFECT OF DIVALENT-CATION IONOPHORE A23187 ON AMPHIBIAN MELANOPHORES AND IRIDOPHORES

[摘要] Ca2+ ions are required in the medium for the full darkening action of melanocyte-stimulating hormone, but not for its second messenger, cyclic[c]AMP, on isolated frog skin. The possible effect of the divalent cation ionophore A23187 was studied, using the skin of Rana pipiens. It is a potent darkening agent over the range of 1-2 .mu.M, as measured by reflectance change and microscopic observation. During the darkening response to A23187, melanosome dispersion takes place in both dermal and epidermal melanophores, as well as aggregation of iridophore reflecting platelets. The darkening is slowly reversible in Ringer''s solution, thus it is not due to toxicity. The darkening is partially dependent on the presence of Ca in the medium under certain conditions, suggesting that the response involves both the uptake and release of Ca ions. Cytochalasin B reversibly inhibits the darkening produced by A23187. Since this drug is known to bring about the breakdown of microfilaments, its inhibitory action is in accord with the concept that the ionophore is stimulating microfilament function. A23187 administered to tissue cultured embryonic salamander (Ambystoma maculatum) melanophores produced an irreversible rounding up of the cells, suggesting a toxic effect. The results with frog skin are interpreted as supporting the concept that the action of melanocyte-stimulating hormone involves the production of pigment granule movements as a result of the interaction of Ca ions with intracellular microfilaments and possible also the breakdown of microtubules. The mobilization of Ca ions could be brought about by cAMP.