The cellular uptake of 5-hydroxytryptamine in the rat lung in vivo--a histochemical study

The cellular uptake of 5-hydroxytryptamine (5-HT, serotonin) in the rat lung was studied in vivo after bolus injection of the amine (1 mg/100 g b.w.i.v.). The cells responsible for the 5-HT uptake were visualized by fluorescence microscopy according to the Hillarp-Falck technique and further identified as mast cells by Astra Blau staining. The quantitative aspects of the 5-HT uptake were studied by a specific cytofluorimetric method. A significantly increased intracellular content of 5-HT was found in interstitial pulmonary mast cells 1 min after injection. No 5-HT uptake was demonstrated within epithelial cells of the bronchiolar mucosa or within the pulmonary vascular endothelial cells; not even in animals pretreated with monoamine oxidase inhibitor. This finding is in contrast to the endothelial uptake earlier demonstrated by autoradiography in artificially perfused lungs. One explanation for this discrepancy may be a rapid transport of the amine across the vast endothelial area in the lung in vivo resulting in low 5-HT concentrations within the endothelium, not detectable by fluorescence microscopy. After endothelial passage the amine was rapidly taken up and stored in interstitial mast cells, which normally have the amine handling properties.