Removal of 5‐hydroxytryptamine in the pulmonary circulation of rat isolated lungs

1 . Rat isolated lungs perfused via the pulmonary artery with Krebs solution removed 92% of the 5‐hydroxytryptamine (5‐HT) infused through it. This degree of removal was independent of concentration in the range from 5 to 100 g/ml. 2 . The removal of 5‐HT by the lungs was inhibited by amitriptyline and desmethylimipramine (10−6–10−5m). 3 . The monoamine oxidase inhibitors, mebanazine and iproniazid (10−6–10−5m), inhibited the initial removal slightly, but their main effect was to preserve the 5‐HT taken up and this 5‐HT slowly reappeared in the effluent from the lungs. Tranylcypromine (5 × 10−7–10−6m) showed a combination of amitriptyline‐like and mebanazine‐like effects on the 5‐HT removal in rat lung. 4 . Experiments with 3H‐5‐HT showed that although under normal conditions only 10% of the radioactivity appeared in the lung effluent as 5‐HT within the first 5 min, the rest of radioactivity administered could be recovered in the effluent over 50 min as a metabolite, probably 5‐hydroxyindoleacetic acid. 5 . The following amines were without effect on the removal of 5‐HT by rat lungs: noradrenaline (6 × 10−7m), normetanephrine (5 × 10−6m), metaraminol (10−6m), reserpine (10−6–10−5m) and phenoxybenzamine (10−5m). 6 . We conclude that the removal of 5‐HT by rat lungs involves a process of uptake and metabolism rather than one of uptake and storage, but this process is not the catecholamine Uptake2. The cells involved in this process might be either capillary endothelial cells or septal cells.