DOWN-REGULATION OF PULMONARY ATRIAL-NATRIURETIC-PEPTIDE RECEPTORS IN RATS EXPOSED TO CHRONIC HYPOXIA

We hypothesized that a downregulation in pulmonary atrial natriuretic peptide (ANP) receptors helps raise plasma ANP levels during chronic hypoxia. We measured in vivo pulmonary uptake and plasma clearance of I-125-ANP and in vitro pulmonary binding kinetics of I-125-ANP, normoxic and chronically hypoxic rats. Exposure to 21 days of hypobaric (0.5 atm) hypoxia did not decrease specific binding of I-125-ANP in the kidney, but pulmonary binding decreased 35 and 75% after 1 and 3 days of hypoxia, respectively, and increased 200% after 3 days of normoxic recovery from 21 days of hypoxia. The total binding capacity for ANP to lung membrane fractions from normoxic rats, chronically hypoxic rats, and rats that had recovered from hypoxia was 488 +/- 59, 109 +/- 17, and 338 +/- 48 fmol/mg, respectively (P < 0.05 for hypoxic vs. normoxic or recovered lung membranes). The area under the I-125-ANP plasma concentration curve for normoxic and hypoxic rats and normoxic rats that were infused with the ANP C-receptor ligand C-ANF(4-23) was 3,292 +/- 216, 5,022 +/- 466, and 8,205 +/- 1,059 disintegrations.min(-1).ml(-1), respectively [P < 0.05 for hypoxic vs. normoxic or C-ANF-(4-23)-infused rats]. We conclude that pulmonary ANP clearance is reduced during chronic hypoxia secondary to a downregulation in pulmonary ANP clearance receptors. Reduced pulmonary clearance of ANP may represent an adaptation that contributes to increased plasma ANP levels during chronic hypoxia.