Renal metabolism of atrial natriuretic peptide in the rat.

The purpose of this study was to identify the nephron and cell sites involved in the renal metabolism of alpha-rat atrial natriuretic peptide (alpha-rANP) and to examine the degradation products of the peptide. In micro-dissected nephrons 125I-labeled ANP degradation rate was highest in proximal convoluted (PCT) and straight tubules and lowest in glomeruli and papillary collecting tubules, indicating that the sites of ANP degradation and of the receptors that mediate its biological activity in the nephron do not coincide. Among subcellular fractions of cortical homogenates, the luminal membranes were the most active in metabolizing ANP. In contrast, ANP degradation by isolated basolateral membranes was negligible, and the basolateral uptake route in intact tubules did not contribute significantly to its catabolism. Cortical homogenates, luminal membranes, and isolated PCT degraded ANP without evidence of saturation up to pharmacological concentrations (10(-6) M) of the peptide. A major intermediate metabolite was rapidly formed by luminal membranes and was identified with use of a sequence and compositional analysis. This metabolite had the same amino acid sequence as ANP with a cleavage at position Cys7-Phe8, and the disulfide bridge was preserved. These results demonstrate a rapid degradation of ANP by kidney tissue and suggest that the luminal membrane of the proximal tubule is a major nephron site of ANP catabolism.