In anterior pituitary reaggregate cell culture gamma3-MSH has a mitogenic effect on lactotrophs and augments PRL mRNA levels. Moreover, the peptide increases intracellular free calcium levels ([Ca2+]i) in a subpopulation of cells which is heterogenous with respect to expressed pituitary hormone mRNAs. The effect of gamma3-MSH on ([Ca2+]i is blocked by SHU9119, an MC3- and MC4R antagonist, in only 50% of the responsive cells, suggesting that in half of these cells the mediating receptor is not the MC3R. gamma3-MSH also increases ([Ca2+]i in the GH- and PRL-secreting GH3 cell line. Increasing doses recruit an increasing number of responding cells. gamma2-MSH and alpha-MSH display a similar effect. SHU9119, does not affect the gamma3-MSH-induced ([Ca2+]i response and has also no effect on its own. MTII, a potent synthetic agonist of the MC3-, MC4- and MC5R, also show low potency in increasing ([Ca2+]i in GH3 cells. By means of RT-PCR the mRNA of the MC3R, MC4R and MC5R is detectable in the anterior pituitary but undetectable in GH3 cells. gamma2-MSH analogues with single alanine-replacements in the His5-Phe6-Arg7-Trp8 sequence retain full potency and activity on ([Ca2+]i as well as on PRL mRNA level. Low nanomolar doses of gamma3-MSH increase intracellular cAMP levels, an effect blocked by pertussis toxin. Blockade of protein kinase A abolishes the ([Ca2+]i responses to gamma3-MSH. gamma2-MSH increases binding of [S35]GTP-gamma-S to membrane preparations of GH3 cells.
The present data suggest the existence of a receptor mediating effects of gamma3-MSH that is different from hitherto characterized MCR's.
03 - 05 Nov 2003
Society for Endocrinology