Melanocortin receptor-mediated mobilization of intracellular free calcium in HEK293 cells

Mouse melanocortin receptors, MC1-R, MC3-R, MC4-R, and MC5-R, when expresse d in HEK293 cells and stimulated with either alpha -melanocyte-stimulating hormone (alpha -MSH) or desacetyl-alpha -MSH, mediate increases in intracel lular free calcium concentration ([Ca2+](i)) with EC50 values between 0.3 a nd 4.3 nM. The increase in [Ca2+](i) is cholera toxin sensitive and pertuss is toxin insensitive. The mechanism involves calcium mobilization from intr acellular stores without a transient rise in inositol trisphosphate. Mouse agouti protein (55 nM) is a competitive antagonist of alpha -MSH (6-fold) a nd desacetyl-alpha -MSH (8-fold), coupling the mMC1-R to increased [Ca2+](i ). Agouti protein (55 nM) significantly increased the EC50 for alpha -MSH ( 3-fold), and 550 nM agouti protein significantly increased the EC50 for des acetyl-alpha -MSH (4-fold), coupling the mMC4-R to a rise in [Ca2+](i). How ever, agouti protein antagonism of the MC4-R may not be competitive since t here was a trend for the maximum response to also increase. There was no si gnificant antagonism of the MC3-R and MC5-R by agouti protein (55 nM). Unde rstanding the physiological relevance of the transduction of a calcium sign al by melanocortin peptides may be important for future development of ther apeutic targeting of the melanocortin receptors.