An Increase in Extracellular Ca2+ Concentration Induces Pigment Aggregation in Teleostean Melanophores

An increase in the concentration of Ca2+ ions in the external medium ([Ca2+]o) induced pigment aggregation in melanophores of three species of freshwater teleosts examined. Denervated melanophores were refractory to elevations of [Ca2+]o. The pigment-aggregating action was inhibited by the sympathetic blocking agents, phentolamine, prazosin and yohimbine. Bretylium, an agent known to block the release of the neurotransmitter, interfered with the response effectively. Ca2+ blockers, such as Mn2+, verapamil and gallopamil, also inhibited the response, possibly by inhibiting Ca2+ entry into the presynaptic elements of melanosome-aggregating fibers. The conclusion is that the increase in [Ca2+]o may induce membrane depolarization of presynaptic nervous elements around the melanophores, which open the voltage-dependent Ca2+ channels there. The liberation of adrenergic neurotransmitter follows, which induces the aggregation of pigment in melanophores.