Adrenomedullin receptor is found exclusively in noradrenaline-secreting cells of the rat adrenal medulla

Adrenomedullin, originally identified in the adrenal medulla, has binding s ites in the adrenal gland; however, its role in the adrenal medulla is uncl ear. This study was designed to characterise adrenomedullin binding sites i n the rat adrenal medulla, using ligand binding studies, immunocytochemistr y, and mRNA analysis. A single population of specific adrenomedullin recept ors was identified in adrenal medullary homogenates. I-125- Adrenomedullin was displaced only by adrenomedullin(1-50) and not by calcitonin gene-relat ed peptide or amylin at concentrations up to 100 nmol/L. The receptor K-D w as 3.64 nmol/L with a receptor density of 570 fmol/mg of protein. Analysis of mRNA revealed that the genes encoding both the putative adrenomedullin r eceptors, termed calcitonin receptor-like receptor (CRLR) and L1, were expr essed in the rat adrenal medulla. Dual-colour indirect-labelled immunofluor escence was used to localise phenylethanolamine N-methyltransferase (PNMT) and the adrenomedullin receptor in the same section. PNMT is the enzyme tha t converts noradrenaline to adrenaline and is not expressed in noradrenalin e-secreting cells. These studies revealed that both CRLR and L1 were expres sed only in cells that did not express PNMT, suggesting that adrenomedullin receptors are only found in noradrenaline-secreting cells. Further evidenc e to support this conclusion was provided by the demonstration of colocalis ation of adrenomedullin receptors with dopamine beta-hydroxylase, confirmin g the presence of the receptors in medullary chromaffin cells. Taken togeth er, these data suggest that adrenomedullin acts through a specific adrenome dullin receptor in the rat adrenal medulla. RT-PCR and northern blot analys is revealed greater abundance of mRNA for L1 than for CRLR, possibly sugges ting that L1 may be the major adrenomedullin receptor expressed in this tis sue. As it has been reported that adrenomedullin is synthesised predominant ly by adrenaline-secreting cells, it appears likely that adrenomedullin is paracrine regulator in the adrenal medulla.