Cellular Aspects of Hormonally Controlled Pigment Translocations Within Chromatophores of Poikilothermic Vertebrates

Pigment movements triggered by the vertebrate melanocyte-stimulatinghormone (MSH) are known to be mediated at the cellular levelby cyclic adenosine monophosphate (cAMP). A similar role forcAMP is indicated in the melanosome-dispersing action of catecholaminesacting via rß3-adrenergic receptors. Thus, MSH andrß3-adrenergic agonists cause an elevation of cAMPwithin chromatophores. On the other hand, a lowering of cAMPlevel seems to be associated with the paling action of melatoninand -adrenergic agonists. Depending upon the cell type and speciestested, cyclic guanosme monophosphate (cGMP) may have no effector cause either darkening or paling. Although cGMP induces palingin frog skin, the lack of evidence for a physiological palingagent makes it difficult to assign a role for cGMP The mechanismof action of cyclic nucleotides and the details of the pigment-translocatingmechanism are far from clear, but recent studies indicate thatchanges in free calcium ion levels play a critical role. Thisarticle will primarily review three areas' the paling actionof cGMP, the darkening action of ionophore A23187 GenBank] , and the actionsof enkephalms and endorphins on chromatophores. In addition,new data are presented to support the concept that the A23187 GenBank] -induceddarkening of frog skin does not involve either prostaglandinbiosynthesis or adenylate cyclase stimulation.