Dietary sodium intake modulates pituitary proopiomelanocortin mRNA abundance

The pituitary prohormone proopiomelanocortin gives rise to melanocortins of α, β, and γ primary structure in addition to corticotropin. Melanocortins have a variety of actions in mammals, and each is natriuretic. In particular, γ-melanocyte-stimulating hormone has been shown to mediate reflex natriuresis after acute unilateral nephrectomy. We examined whether this peptide could play a role in longer term adjustments in sodium balance by measuring plasma γ-melanocyte-stimulating hormone and corticotropin concentrations, as well as pituitary proopiomelanocortin mRNA abundance, in Sprague-Dawley rats ingesting either a low (0.07% NaCl) or high (7.5% NaCl) sodium diet. One week after the high sodium diet, plasma γ-melanocyte-stimulating hormone concentration was double the value seen in rats on the low sodium diet (158±5 [SE] versus 76±9 fmol/mL, P<.001), a change that was accompanied by a fivefold increase in plasma atrial natriuretic peptide concentration but no change in plasma corticotropin. Whole pituitary proopiomelanocortin mRNA abundance, measured with a probe to exon 3 of the rat proopiomelanocortin gene, was significantly increased after 1 week of the high sodium diet compared with the low sodium diet and increased further at 2 and 3 weeks. This increase occurred primarily in the neurointermediate lobe as demonstrated by in situ hybridization; the content of γ-melanocyte-stimulating hormone immunoreactivity was also increased in this lobe, but not the anterior lobe, after 1 week of the high sodium diet. These results demonstrate that high dietary sodium intake increases neurointermediate lobe proopiomelanocortin mRNA abundance compared with a very low sodium diet and also suggest that proopiomelanocortin is preferentially processed into γ-melanocyte-stimulating hormone rather than corticotropin. These observations consequently raise the possibility of a role for this peptide hormone system in the adjustments to a high salt diet.