Kinetics of serotonin uptake in the dog lung.

A bolous containing [3H]serotonin and indocyanine green dye was rapidly introduced into the pulmonary artery on an isolated blood-perfused left lower dog lung lobe. Sequential samples of the venous effluent were collected for approximately 25 s following the injections. The lung intake of the injected [3H]serotonin was inversely proportional to the injected dose, ranging from about 70% for a 10-nmol injection to about 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 [3H]5-hydroxyindoleacetic acid; and during the bolous transit through the lung lobe, uptake of serotonin by platelets was negligible. The serotonin extraction ratio curves exhibited dose-dependent characteristic shapes that we have interpreted using a model in which it is assumed that serotonin uptake follows Michaelis-Menten kinetics and flow through the capillary bed is heterogeneous. Using this model and a 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.