Action of melanophore-stimulating hormone on melanophores of the cyprinid fish Zacco temmincki.

1. Alpha MSH was extremely effective in inducing melanosome dispersion in both innervated and denervated melanophores in isolated scales of Zacco termmincki. 2. The sensitivity of the melanophores to MSH did not change after denervation. 3. The MSH action was blocked by mersalyl, a SH inhibitor, but not by any of alpha and beta adrenergic blockers. 4. Ca2+ was required for the MSH action, but not for melanosome dispersion itself, since the beta adrenergic response was normally produced in the absence of this ion. 5. Mg2+, Sr2+ and Ba2+ could not replace the Ca2+. 6. Mn2+ reversibly inhibited the MSH action.