Cytochalasin B and Pigment Granule Translocation

The redistribution of melanin granules within dermal and epidermal melanocytes of the frog is responsible for changes in skin color . Surviving frog skin can be lightened and darkened by many different compounds (I). When the granules are dispersed into the branched processes of individual melanocytes, the skin appears dark . When melanin granules are aggregated in the perikaryon, the melanocyte appears rounded or punctate by light microscopy and the skin is light . Melanocyte stimulating hormone (MSH) causes melanin granule dispersion, and an increase in cyclic adenosine 3,' ,5'-monophosphate (cyclic AMP) in frog skin (2, 3) . Cyclic AMP itself causes pigment granule dispersion ; accordingly, it has been postulated that the pigment granule dispersive effect of MSH is mediated by cyclic AMP which functions within the melanocyte as a second messenger (4, 5). The transducer between cyclic AMP and pigment granule dispersion has not been identified ; however, recent observations have suggested that microfilaments are the intermediary between MSH and granule dispersion (6, 7) . We have reported that microfilaments are abundant in epidermal melanocytes of frog skin darkened by MSH and that cytochalasin B, a compound that interferes with microfilaments (8), prevents MSHdarkening. When cytochalasin B was added to a skin previously darkened by MSH, the pigment granules of both dermal and epidermal melanocytes rapidly aggregated . It was concluded from these observations that microfilaments were the structural elements responsible for pigment granule dispersion caused by MSH . If this conclusion is correct, then the pigment granule dispersive effect of cyclic AMP should be prevented by cytochalasin B . In this report, we will show that the darkening of dermal and epidermal melanocytes by dibutyryl cyclic AMP (DBcAMP) and theophylline is prevented and reversed by cytochalasin B . It was necessary in these and previous experi-