Divergent effects of adrenocorticotropin and melanotropin on isolated rat and rabbit adipocytes

The stimulation of lipolysis in isolated rat and rabbit fat cells by adrenocorticotropin (ACTH) and α-melanotropin has been studied. The concentration of α-melanotropin required for half maximal stimulation is 0.23 times that of ACTH in rabbit adipocytes but as high as 1140 times that of ACTH in rat fat cells. Chemical modification of the tryptophan residue in ACTH and melanotropin resulted in a loss of lipolytic activity in rat adipocytes and an increase in lipolytic potency in rabbit fat cells. These differences between rat and rabbit fat cells were evident when stimulation of cyclic AMP synthesis was measured in isolated cells or ghosts. The results are discussed in terms of the difference in the hormone receptors of the fat cells of the two species.