Kinetics of serotonin uptake in the dog lung

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Abstract
A bolus containing [3H]serotonin and indocyanine green dye was rapidly introduced into the pulmonary artery of an isolated, blood-perfused [dog] left lower lung lobe. Sequential samples of the venous effluent were collected for .apprx. 25 s following the injections. The lung uptake of the injected [3H]serotonin was inversely proportional to the injected dose, ranging from 70% for a 10-nmol injection to .apprx. 38% for a 100-nmol injection. Less than 2% of the injected 3H, which appeared in the venous effluent over the sampling interval, was in the form of [3]5-hydroxyindoleacetic acid; and during the bolus transit through the lung lobe, uptake of serotonin by platelets was negligible. The serotonin extraction ratio curves exhibited dose-dependent characteristic shapes that were interpreted using a model which assumes that serotonin uptake follows Michaelis-Menten kinetics and that flow through the capillary bed is heterogeneous. Using this model and multiple regression analysis, the outflow dye and 3H concentration vs. time curves were used to estimate Km and Vmax for serotonin uptake by the lungs.