The endocrine heart and natriuretic peptides: histochemistry, cell biology, and functional aspects of the renal urodilatin system

This review focuses on some selected aspects of the endocrine heart and natriuretic peptides. The endocrine heart is composed of specific myoendocrine cells of the cardiac atria. The myoendocrine cells synthesize and secrete the natriuretic peptide hormones which exhibit natriuretic, diuretic, and vasorelaxant properties. Immunohistochemical analyses show that natriuretic peptides of the A-type and B-type are localized not only in the specific granules of these myoendocrine cells but also in many other organs including the brain, adrenal medulla, and kidney. Also, their receptors are detected in many organs showing the multiple functions of these regulatory peptides. Of the members of the natriuretic peptide family, ANP (ANP for atrial natriuretic peptide; also denominated cardiodilatin, CDD), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), and the A-type, including its renal form, urodilatin, are emphasized in this review. Urodilatin is localized in the kidney, differentially processed, and secreted into the urine. The intrarenal synthesis and secretion is the basis for a paracrine system regulating water and sodium reabsorption at the level of the collecting duct. CDD/ANP-1-126, cleaved from a precursor of 126 amino acids in the heart to a 28-amino acid-containing circulating molecular form (CDD/ANP-99-126), and urodilatin (CDD/ANP-95-126) share similar biochemical features and biological functions, but urodilatin may be more involved in the regulation of body fluid volume and water-electrolyte excretion, while circulating CDD/ANP-99-126 is responsible for blood pressure regulation. The physiological and pharmacological properties of these peptides have great clinical impact, and as a consequence urodilatin is involved in drug development for the treatment of acute renal failure, cardiomyopathia, and acute asthma.