Tissue-specific effects of physiological ANP infusion on blood-tissue albumin transport

Blood-tissue transport of 131I-labeled bovine serum albumin (BSA) during intravenous infusion of synthetic atrial natriuretic peptide (ANP) was examined in anesthetized male Wistar rats. Plasma volumes were maintained at pre-ANP levels by infusion of 2% BSA in lactated Ringer solution (LR) to minimize compensatory responses to ANP-induced hypovolemia. 131I-BSA clearance was measured over 30 min, and 125I-BSA was injected terminally to correct for intravascular volume. Thirty-minute infusion of 20 ng.kg-1.min-1 ANP resulted in a tissue-selective increase in 131I-BSA clearance in jejunum and colon compared with controls given LR only. Smaller but significant increases in tracer clearance also were observed in fat, kidney, left ventricle, and skeletal muscle exposed to 400 ng.kg-1.min-1 ANP. The observed elevation in tracer albumin extravasation was not associated with any measurable increase in tissue extravascular water content. Furthermore, it was shown that coupling of 131I-BSA transport to filtration induced by hindlimb venous congestion was similar in control and ANP-treated rats. In a second series of experiments, plasma ANP levels were determined after 30-min ANP infusions from 0 to 180 ng.kg-1.min-1. Significant linear associations between physiological ANP levels (62-578 pg/ml) and 131I-BSA clearance were demonstrable for small intestine, colon, fat, kidney, and skeletal muscle but not for skin, heart, diaphragm, and lung. We conclude that raising plasma ANP by infusion of the synthetic peptide results in a filtration-independent, tissue-selective increase in albumin transport. Tissue uptake of albumin is a potential mechanism for extrarenal fluid shift during circulatory volume overload.