Cyclic AMP and adrenergic receptors in melanophore responses to methylxanthines

Skins of the lizard Anolis carolinensis darken in response to melanophore stimulating hormone (MSH), methylxanthines (e.g., theophylline), and catecholamines, the latter through β-adrenergic receptor stimulation. α-Adrenergic blocking agents inhibit MSH, β-adrenergic blocking agents inhibit catecholamines, but neither inhibit methylxanthines. Catecholamines lighten MSH-darkened skins through α-receptor stimulation but, in contrast, increase the darkening of methylxanthine-darkened skins through β-receptor stimulation. Apparently, α-stimulation dominates in the presence of MSH whereas, in the presence of methylxanthines, β-receptor stimulation dominates. α-Adrenergic stimulation apparently decreases cyclic AMP levels within melanophores, possibly by stimulation of cyclic AMP phosphodiesterase activity which would lead to cyclic AMP degradation. In the presence of methylxanthines which inhibit phosphodiesterase, α-stimulation would then have less effect and, therefore, β-stimulation would dominate.