Effects of changes in pulmonary perfusion and surface area on endothelial ACE activity

Measurement of transport or enzymatic processes associated with pulmonary endothelial cells could provide unique information regarding the physiologic function of the microcirculation as well as data describing the biochemical integrity of these cells in acute lung injury. Since an important goal of such measurements is to quantify the kinetics of reactions of substrates (indicators) with the endothelium, it would be highly advantageous to account for (convective) phenomena related to blood flow and its distribution which also influence whole organ metabolism. In the present report, we describe studies that have utilized a nonlinear model of organ metabolism to estimate Vmax, the apparent maximal velocity and Km, concentration at one-half Vmax of angiotensin-converting enzyme (ACE) activity. Our hypotheses have been that (a) there is sufficient data to calculate apparent kinetic constants from indicator-dilution outflow curves after injection into the pulmonary circulation of a radiolabelled synthetic substrate for ACE, and (b) Vmax for ACE is a property related to the amount of enzyme located on the endothelial cells, and as such, should be directly proportional to perfused surface area of an organ. In isolated perfused rabbit lungs, when flow was increased over a two-fold range, but surface area was unchanged, neither Vmax nor Km for ACE activity was significantly altered. When indicator-dilution measurements of pulmonary ACE activity were made in lambs from 1-171 days of age there was a progressive increase in Vmax with no significant change in Km as a function of age. The increase in Vmax was closely correlated with an independent measure of surface area, carbon monoxide diffusing capacity, and a morphometric measure of capillary endothelial cell surface area (stereology at electron microscopic level) made at post-mortem. These experimental observations in whole organs support our hypotheses and predictions regarding the metabolism or removal of substrates from the circulation by means of a saturable process.