[The mechanisms of serotonin participation in the smooth-muscle reactions of the trachea].

Serotonin was shown to induce a dose-dependent enhancement of the tonus of the rat and guinea pig trachea smooth muscle: low concentrations facilitated the contractile activity, whereas high doses relaxed the muscle via transmural stimulation of nervous fibres. The serotonin receptors were found to be located at the postganglionic cholinergic fibres and metasympathetic neurons of the trachea intramural ganglia. Presence of the receptors at the trachea smooth muscle cells seems to be highly improbable.