Reduced lung endothelial angiotensin-converting enzyme activity in Watanabe hyperlipidemic rabbits in vivo

We investigated pulmonary endothelial function in vivo in 12- to 18-mo- old male Watanabe heritable hyperlipidemic (WHHL; n = 7) and age- and sex- matched New Zealand White (n = 8) rabbits. The animals were anesthetized and artificially ventilated, and the chest was opened and put in total heart bypass. The single-pass transpulmonary utilizations of the angiotensin- converting enzyme (ACE) substrate [³H]benzoyl-Phe-Ala-Pro (BPAP) and the 5'- nucleotidase (NCT) substrate [¹⁴C]AMP were estimated, and the first-order reaction parameter A(max)/K(m), where A(max) is the product of enzyme mass and the catalytic rate constant and K(m) is the Michaelis-Menten constant, was calculated. BPAP transpulmonary utilization and A(max)/K(m) were reduced in WHHL (1.69 ± 0.16 vs, 2.9 ± 0.44 and 599 ± 69vs. 987 ± 153 ml/min in WHHL and control rabbits, respectively; P < 0.05 for both). No differences were observed in the AMP parameters. BPAP K(m) and A(max) values were estimated separately under mixed-order reaction conditions. No differences in K(m) values were found (9.79 ± 1 vs. 9.9 ± 1.31 μM), whereas WHHL rabbit A(max) was significantly decreased (5.29 ± 0.88 vs. 7.93 ± 0.8 μmol/min in WHHL and control rabbits, respectively; P < 0.05). We conclude that the observed pulmonary endothelial ACE activity reduction in WHHL rabbits appears related to a decrease in enzyme mass rather than to alterations in enzyme affinity.