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.
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