Removal and release of recirculating atrial natriuretic peptide (ANP) by rabbit lungs perfused in situ

The finding that radioiodinated, synthetic ANP (125I-ANP), analogous to the ANP secreted by the right atrium, binds to specific sites of endothelial cells, smooth muscle, adrenals and lungs (1-3) raised the importance of analyzing lung-ANP interactions. Rabbit lungs were accordingly loaded with 125I-ANP, 2000 Ci/mM (2.5 pM) in Krebs-albumin recirculating perfusion medium at a rate of 20 ml/min, over 27 min. 125I-ANP binding was estimated by taking samples at 2-min intervals from the perfusate. Fourteen min after recirculation, uptake was maximal (66.4 +/- 5.2%, N = 8) (X +/- SEM) and not different from that observed after a single passage through the lungs and was dose-dependent, antagonized by unlabelled ANP. The system was then changed to single-pass washout and efflux curves were obtained, collecting the entire efflux every 6 s per sample, over 15 min. Efflux curves after 125I-ANP loading fitted a bi-exponential decay model (Y(to) = Y(t)e-b11t + Y(t)e-b2t). Lung ANP uptake/release may be involved in regulating systemic concentrations and hence renal and other actions of ANP.