The role of serotonin in mucociliary transport system in the ciliated epithelium of frog palatine mucosa

The effects of serotonin (5-hydroxytryptamine, 5-HT) on mucociliary transport and mucus secretion were investigated pharmacologically. The presence of 5-HT in the ciliated epithelium of frog palatine mucosa was examined chromatographically. Mucociliary transport was accelerated by the application of 10^?6 M 5-HT. Mucus secretion was also stimulated significantly by 5-HT (30 mg/kg). 5-HT in the mucosa was detected by a chromatographic technique when a large amount of 5-HT precursor, 5-hydroxytryptophan (5-HTP), was given previously. Phenylhydrazine was used as MAO inhibitor. When phenylhydrazine (20 mg/kg) was given before 5-HTP, 5-HT was markedly concentrated in the mucosa. Radioactive 5-HT was also detected in the mucosa after incubation with [¹⁴C]5-HTP. One hour after systemic administration of [³H]5-HTP, a large number of silver grains appeared in the autoradiograms of some epithelial cells. These results suggest the possibility that epithelial cells with the ability to synthesize 5-HT from 5-HTP exist in ciliated epithelium. A possible functional relationship is proposed between the regulation of mucociliary transport with mucus secretion and 5-HT-containing cells.