Comparison of serotonin uptake from the alveolar and capillary spaces of isolated rat lung.

The role of pulmonary epithelium in the clearance of 5-hydroxytryptamine (5-HT) was evaluated by measuring uptake of the amine by the isolated rat lung during vascular perfusion or after endotracheal instillation. During perfusion with 0.5 microM of [14C]5-HT, the initial rate of uptake was 5.3 +/- 0.14 nmol/min (mean +/- SE; n = 19). Autoradiography with [3H]5-HT demonstrated localization of grains predominantly over the pulmonary endothelium with a grain density/mu2 of 0.78 compared with 0.05 to 0.21 for other structures of the alveolar septum. When 0.5 microM of [14C]5-HT was instilled into the air spaces, the rate of uptake was 0.15 +/- 0.005 nmol/min (n = 4), and increased to 0.25 +/- 0.05 nmol/min (n = 6) with instillation of 1 microM of 5-HT. Uptake from the air spaces was inhibited by 72% in the presence of 0.5 mM of imipramine. Autoradiography after instillation of [3H]5-HT into the trachea showed localization of developed silver grains over both alveolar epithelium and endothelium; the grain density/mu2 was greatest in endothelial cells, and significantly exceeded the predicted value based on uniform distribution only for this compartment. The results indicate that alveolar epithelium compared with pulmonary capillary endothelium has very limited capacity to accumulate 5-HT.