Lung angiotensin-converting enzyme kinetics from indicator-dilution and constant-infusion methods

Abstract

We compared the results of three methods used to evaluate the kinetics of benzoyl-phenylalanyl-alanyl-proline (BPAP) hydrolysis by angiotensin-converting enzyme in the isolated rabbit lung. In method A, the arteriovenous concentration differences at each of four rates of BPAP infusion were used to determine the effect of concentration on the rate of BPAP hydrolysis. In methods B1 and B2, trace doses of [3H]BPAP were injected during each constant infusion of unlabeled BPAP so that the venous [3H]BPAP concentration could be used to estimate the venous BPAP concentration and the rate of hydrolysis at each background concentration. In method C, three boluses containing different amounts of unlabeled BPAP as well as tracer [3H]BPAP were injected such that each bolus resulted in a range of concentrations and hydrolysis rates that could be estimated from the venous concentrations of [3H]BPAP. Each method provided the data needed to calculate the maximum uptake rate (Vmax) and Km, the concentration at half Vmax, assuming that the hydrolysis can be represented by the Michaelis-Menten equation. However, the mathematical model underlying each method involved different assumptions about the effects of heterogeneity of capillary transit times and bolus dispersion. The mean values of Vmax were 180, 216, 217, and 200 nmol/s and for Km 7.8, 10.4, 8.6, and 10.0 microM for methods A, B1, B2, and C, respectively. The differences between methods were not statistically significant. These results suggest that theoretical differences between the methods did not have a quantitatively important impact relative to other factors contributing random errors to each of the methods. The choice between methods can therefore be made on practical grounds.