[Functional compartmentation of the endocrine action of cardiac natriuretic peptides]

The endocrine function of the heart is to secrete Atrial and Brain natriuretic -peptides (ANP and BNP). These peptides are biologically active via particulate guanylate cyclases which generate cyclic GMP, the second intracellular messenger. A polysaccharide antagonist, HS-142-1 has been recently described by a Japanese Group. Cyclic GMP is partly secreted from the target cells into the extra cellular medium in which its accumulation is proportional to the concentration of the natriuretic peptide. Neutral Endopeptidase (NEP) is a zinc ectoenzyme involved in the catabolism of natriuretic peptides. NEP is absent in plasma but present on the surface of endothelial and smooth muscle cells. NEP is mainly expressed at the apical pole of the epithelial cells of the proximal tubule in the nephron. Chronic increase in volume and pressure within the cardiac cavities is associated with the oversecretion of natriuretic peptides. This chronic phenomenon involves the recruitment of all the cardiac myocytes to express natriuretic peptide genes. The clinical application of this hyperplasic phenomenon is congestive heart failure, in which the plasma levels of natriuretic peptides correlate with the level of the -hemodynamic stress. Therefore the plasma levels of natriuretic peptides are good pronostic markers in both experimental and human heart failure. The degree of congestive heart failure as well as the plasma levels of ANP and BNP are also -correlated with the plasma and urinary levels of cyclic GMP. The plasma level of -cyclic GMP is correlated with the endothelial concentration of cyclic GMP but not with the cyclic GMP concentration in smooth muscle cells. From these experimental data, we can conclude that plasma cyclic GMP originates from endothelial cells and is related to particulate guanylate cyclase activity. In contrast natriuretic peptides do not modulate vascular wall cyclic GMP content. The natriuretic action of ANP is probably due to the interaction of the filtered peptide with the particulate guanylate cyclase at the apical pole of the epithelial cells. The apparition of peptiduria associated with natriuresis during NEP inhibition provides evidence of the action of the peptide in the urinary compartment. It is also by a urinary pathway via the macula densa that ANP, and its potentiation by NEP inhibition, decreases renin secretion. The fact that plasma levels of ANP and plasma and urine levels of cyclic GMP correlate with the degree of salt retention in congestive heart failure, provides evidence for chronic desensitization of the system. An up-regulation of Na(+), K(+), 2Cl(-) expression associated with experimental congestive heart failure has recently been shown. Similarly, a modulation of the different sodium transporter systems along the nephron could be one of the counter-regulations leading to desensitization to natriuretic peptides. In conclusion, natriuretic peptides are true endocrine peptides, secreted by the heart, transported in the plasma, filtered by the glomeruli and active at the nephron level. The molecular effector of ANP and cyclic GMP in the epithelial cells is probably the G-kinase II, isoform phosphorylating the cystic fibrosis transmembrane conductance regulator (CFTR). The exact mechanism of desensitization remains to be elucidated.