Kinetics of Serotonin Uptake by the Dog Lung Is pH Independent in the Physiological Range

Given the strong pH dependence of platelet serotonin uptake, the many similarities between platelet and pulmonary endothelial serotonin uptake and the potential importance of a pH dependence on the interpretation of changes in pulmonary endothelial serotonin uptake in diseased lungs, we examined the influence of pH on the kinetics of serotonin uptake in isolated dog lung lobes using a multiple indicator technique. A range of pH values from about 7.2 to 8.0 were obtained by ventilating the lobes with gas containing different concentrations of CO₂. Over the pH range studied, no significant changes in serotonin uptake were observed, nor were significant changes detected in the kinetic parameters, K _m and V _max, for the uptake process. The lack of a significant pH effect represents a difference between platelet and lung endothelial serotonin uptake. It also indicates that correlations between P CO₂ and serotonin uptake which might occur in diseased lungs do not imply causality.