Uptake of N-[1 (S)-carboxy-(4-OH-3-125I-phenyl)ethyl]-L-Ala-L-Pro, an inhibitor of angiotensin-converting enzyme by rabbit lungs in situ.

Single pass extraction of a new iodinated inhibitor of angiotensin-converting enzyme (ACE) was measured by means of indicator-dilution techniques applied to rabbit lungs, perfused in situ at 20 ml/min with Krebs bicarbonate solution containing 3% bovine serum albumin. A bolus containing the inhibitor, N-[1(S)-carboxy-(4-OH-3-125I-phenyl)ethyl]-L-Ala-L-Pro (CPAP), and an intravascular marker, [14C]dextran, was injected and extraction calculated at the peak of the resulting venous outflow-time curve. In 13 of 21 lungs used, a synthetic substrate for ACE, [3H]benzoyl-phenylalanyl-alanyl-proline (BPAP), was added to the bolus and appearance of its hydrolysis product, [3H]benzoyl-Phe, measured in effluent samples. When low amounts (0.15 nmol) of [125I]CPAP were injected, pulmonary extraction (E) of CPAP was 67 +/- 14% (X +/- S.D; n = 21) and metabolism (M) of BPAP was 56 +/- 9% (n = 13). Addition of unlabeled CPAP (3, 34 or 340 nmol) to the Addition of unlabeled CPAP (3, 34 or 340 nmol) to the injected bolus caused dose-dependent reduction of E(CPAP) and M(BPAP) that was no longer evident 10 min after the largest dose of CPAP. Coadministration of the ACE inhibitor, captopril (3, 6, 8 and 28 nmol), also caused dose-dependent, reversible depression of both E(CPAP) and M(BPAP). Accordingly, extraction of CPAP by perfused rabbit lung is saturable. Inasmuch as CPAP inhibits ACE activity (as reflected by BPAP metabolism) and CPAP uptake is inhibited by captopril (which also inhibits BPAP hydrolysis), it appears that a large portion of this saturable process probably reflects binding to vascular ACE.(ABSTRACT TRUNCATED AT 250 WORDS)