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

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.