Evidence for an autocrine-paracrine role of adrenomedullin in the cultured rat adrenal zona glomerulosa cells

Rat adrenomedullin (ADM) is a 50-amino acid hypotensive and vasodilating peptide, which derives from the posttranslational proteolytic cleavage of pro(p)ADM. ADM acts via at least two subtypes of receptors, named L1-receptor (L1-R) and calcitonin receptor-like receptor (CRLR). CRLR functions as a calcitonin gene-related peptide or a selective ADM receptor depending on the expression of the subtype 1 or the subtypes 2 and 3 of a family of receptor-activity-modifying proteins (RAMPs). Adrenal zona glomerulosa (ZG) is one of the main target tissues of ADM, which has been shown to exert a potent inhibitory effect on aldosterone secretion acting through ADM[22-52]-sensitive receptors. Reverse transcription (RT)-polymerase chain reaction (PCR) consistently allowed the detection of pADM mRNA in the ZG, but not zona fasciculata-reticularis (ZF/R) cells of the rat adrenal cortex. Immunocytochemistry and radioimmune assay showed a weak but sizeable expression of ADM protein in the ZG, but not inner adrenocortical layers. ZG cells expressed peptidyl-glycine alpha-amidating monooxigenase, the enzyme converting immature ADM to the mature peptide, thereby suggesting their potential ability to produce active ADM. RT-PCR demonstrated the presence in ZG, but not ZF/R cells, of the specific mRNAs of L1-R, CRLR and RAMPs (especially RAMP2). ZG cells were cultured in vitro for 24 or 48 h in the presence of ADM (10(-8) M) and/or its receptor antagonist ADM[22-52] (10(-6) M). ADM increased proliferation index and lowered apoptotic index of cultured cells, and the effects were annulled by ADM[22-52]. ADM[22-52] alone was ineffective in 24 h cultures, but moderately decreased proliferation index and raised apoptotic index in 48 h cultures. In conclusion, our study provides evidence that i) rat ZG cells express ADM and ADM receptor of L1 and CRLR/RAMP2 subtypes, which both are sensitive to ADM[22-52]; and ii) endogenous ADM system modulates in an autocrine/paracrine manner ZG growth, by stimulating cell proliferation and reducing cell apoptotic deletion.