Control of organelle transport in melanophores: regulation of Ca2+ and cAMP levels.

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 alpha 1 receptors results in a rise in intracellular free Ca2+, while alpha 2 stimulation results in decreased cAMP levels [Exton, 1985: Am. J. Physiol. 248:E633-E647]. Each alpha 1 or alpha 2 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 alpha 1 or an alpha 2 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.