Pulmonary capillary endothelium-bound angiotensin-converting enzyme activity in humans

Background-Pulmonary endothelium has metabolic functions including the conversion of angiotensin I to angiotensin II by angiotensin-converting ectoenzyme (ACE), In this study, we have validated an indicator-dilution technique that provides estimations of dynamically perfused capillary surface area (DPCSA) in humans, and we have characterized pulmonary endothelial ACE in vivo. Methods and Results-In 12 adults, single-pass transpulmonary (one or both lungs) hydrolysis of the specific ACE substrate H-3-benzoyl-Phe-Ala-Pro (H-3-BPAP) was measured and expressed as % metabolism (%M) and v = -1n(l-M). We also calculated A(max)/K-m, an index of DPCSA. %M (70.1+/-3.2 vs 67.9+/-3.1) and v (1.29+/-0.14 vs 1.20+/-0.12) were similar in both lungs and the right lung, respectively, whereas A(max)/K-m/body surface area decreased from 2460+/-193 to 1318+/-115 mL/min per square meter. Conclusions-Pulmonary endothelial ACE activity can be assessed in humans at the bedside by means of indicator-dilution techniques, Our data suggest homogeneous pulmonary capillary ACE concentrations and capillary transit times (t(c)) in both human lungs, and similar t(c) within the normal range of cardiac index. A(max)/K-m in the right lung is 54% of total A(max)/K-m in both lungs, suggesting that A(max)/K-m is a reliable and quantifiable index of DPCSA in humans.