Amphibian melanophores become photosensitive when treated with retinal

Culture Xenopus melanophores treated with all-trans retinal disperse their pigment granules when exposed to light, but fail to do so when maintained in retinoid-free medium. Two similar, but distinct, responses to retinal are described. One cellular cohort (fast cells) requires less than 4 h of retinal treatment to become photosensitive; the other (slow cells) requires up to 7 days of retinal treatment. Fast cells are usually, but not always, more sensitive to retinal, responding to concentrations as low as 1 nM retinal. Slow cells require 10–100 nM retinal before they become photosensitive. In both slow and fast cells, retinal isomers are more potent activators of photosensitivity than retinols. Retinoic acid is not effective when given alone. Pretreatment with retinoic acid, however, enhances the cellular sensitivity to retinal so that slow cells tend to assume the characteristics of fast cells. These findings demonstrate that melanophore responses to light in vitro vary as a function of culture conditions and that retinal can activate melanophore photosensitivity.