Cardiac fibroblasts are major production and target cells of adrenomedullin in the heart in vitro.

Objective: Adrenomedullin (AM) is a potent vasodilator peptide. Plasma AM concentration is increased in patients with various heart diseases, and both myocytes (MCs) and non-myocytes (NMCs) secrete AM and express its receptors. These facts suggest that cardiac cells possess an autocrine/paracrine capability mediated by AM.

Methods: MCs and NMCs were prepared from cardiac ventricles of neonatal rats. AM and endothelin-1 concentrations were measured by radioimmunoassays, and interleukin-6 level by a specific bioassay. Total nitrite/nitrate contents were measured with a fluorescence assay kit.

Results: A basal secretion rate of AM from NMCs was 2.8-fold higher than that from MCs. Interleukin-1beta, tumor necrosis factor-alpha and lipopolysaccharide stimulated AM secretion from NMCs but not from MCs. AM stimulated interleukin-6 production in the presence of these cytokines or lipopolysaccharide, which was more prominent in NMCs. In the presence of interleukin-1beta, AM augmented nitric oxide synthesis 2.7-fold in NMCs, but slightly in MCs. NMCs secreted endothelin-1 at a rate nine times higher than MCs, and AM inhibited endothelin-1 secretion from NMCs.

Conclusion: This in vitro study suggests that AM in the heart is mainly produced in NMCs and exerts its effects through NMCs, especially under inflammatory conditions.