Control of organelle transport in melanophores: Regulation of CA2+ and cAMP levels

Abstract

Melanophores of the cichlid Tilapia mossambica can be induced to aggregate pigment by addition of epinephrine to the medium, suggesting adrenergic control of this transport. The melanophore response to adrenergic stimulation was examined using agonists and antagonists that are highly specific for each alpha-adrenoceptor subclass. The signal transduction mechanism of each subclass is unique: stimulation of alpha₁ receptors results in a rise in intracellular free Ca²⁺, while alpha₂ stimulation results in decreased cAMP levels [Exton, 1985: Am. J. Physiol. 248:E633–E647 ]. Each alpha₁ or alpha₂ specific agonist tested showed a dose dependent ability to induce aggregation and each was able to effect complete aggregation of pigment, suggesting that aggregation can be mediated either by elevating Ca²⁺ or by lowering cAMP. However, in the presence of either an alpha₁ or an alpha₂ receptor antagonist, none of the agonists were able to induce significant aggregation, suggesting that changes in levels of both messengers are required for pigment aggregation in the melanophores. Moreover, experiments in which intracellular levels of Ca²⁺ or cAMP were perturbed, using BAPTA and forskolin, respectively, indicated that elevating Ca²⁺ in the presence of high cAMP is not sufficient to induce aggregation and, conversely, that lowering cAMP levels in the presence of reduced Ca²⁺ is not sufficient to induce pigment aggregation. These data indicate that the concentrations of both cAMP and Ca²⁺ are important in regulating pigment aggregation in teleost melanophores, and suggest that maximal aggregation of pigment requires altering the levels of both messengers.