Calcium requirement for alpha-MSH action on tail-fin melanophores of xenopus tadpoles.

The role of Ca2+ in alpha-MSH action on melanophores was studied, in vitro, with a bioassay on ventral tail-fin pieces from tadpoles of Xenopus laevis. Melanosome dispersion induced by alpha-MSH required 1-2 mM extracellular Ca2+. Gradual lowering of the extracellular Ca2+ levels produced a concentration-dependent inhibition of the alpha-MSH response; complete inhibition was obtained in a Ca2+-free medium containing 10-4 M EGTA. In Mg2+-free medium, normal dispersion was observed. The Ca2+ antagonists verapamil (10-4 M), methoxy-verapamil (10-4 M) and La3+ (10-3 M) inhibited the dispersion induced by 3 X 10-9 M alpha-MSH, whereas ruthenium red (10-3 M) was without effect. The ionophore A23187 mimicked the effect of the hormone. Melanosome movement per se was evidently independent of Ca2+, because cAMP and dibutyryl-cAMP induced a full dispersion in the absence of Ca2+. These results show that extracellular Ca2+ is specifically required for alpha-MSH action on tail-fin melanophores in vitro and suggests a Ca2+ influx concomitant with the action of the hormone. Possible intra- and extra-cellular Ca2+ sites are discussed.