Effects of cyclic 3′,5′‐AMP and other adenine nucleotides on the melanophores of the lizard (Anolis carolinensis)

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Abstract
1 . Cyclic 3′,5′-AMP has been reported to darken skins of the frog, Rana pipiens. This suggests that cyclic 3′,5′-AMP may mediate the action of MSH on amphibian chromatophores. Since MSH also darkens skins of the lizard, Anolis carolinensis, we investigated the effects of cyclic 3′,5′-AMP and other nucleotides on Anolis melanophores to determine whether cyclic 3′,5′-AMP may be the intracellular mediator of hormone action on melanophores of another vertebrate class. 2 . Cyclic 3′,5′-AMP, itself, causes a rapid melanin granule aggregation within melanophores of Anolis. This response is, however, somewhat nonspecific in that both 5′-ATP and 5′-ADP also lighten the skins by aggregating the melanin granules. Another nucleotide, 5′-AMP, darkens the skins by dispersing melanin granules. Cyclic 2′,3′-AMP does not darken or lighten Anolis skins. 3 . Dibutyryl cyclic 3′,5′-AMP, which is considered to be better able to penetrate membranes and resist degradation by a specific phosphodiesterase, maximally darkens Anolis skins, as does MSH. This darkening by the potent dibutyryl cyclic 3′,5′-AMP suggests that cyclic 3′,5′-AMP may be the intracellular mediator of melanin granule dispersion within Anolis melanophores leading to skin darkening. 4 . Other evidence supporting the first-messenger—second-messenger hypothesis for melanophore regulation is discussed. 5 . The differences in responses of Anolis melanophores to adenine nucleotides may relate to the ability of these agents to penetrate melanophore membranes; thus, the nucleotides could exert their effects either intracellularly or extracellularly on the plasma membrane.