Objective: To investigate the effects and the mechanisms of synephrine (Syn) on the contraction of isolated guinea-pig tracheal smooth muscle. Method: The guinea-pig tracheal smooth muscle was prepared by isolated tracheal thermostatic perfusion experimental method and isolated in the K-H solution. The tension of Syn on the isolated tracheal smooth muscle and the effect of Syn on tracheal smooth muscle contraction incubation by four antispasmodic agents including acetylcholine (Ach)

histamine (His)

CaCl2 and β-receptor blocking agent (propranolol) were measured by BL-420S biological function experiment systems. Result: Syn produced a dose-dependent relaxant effect on the isolated guinea-pig tracheal smooth muscle. The value of PD2 was 2.22±0.14.Syn (0.021 2

0.042 4 mmol·L-1) could reduce the maximum of guinea-pig tracheal smooth muscle contraction induced by Ach and His

and Syn at 0.042 4 mmol·L-1 had significant effect (P<0.05). Syn also could reduce the value of guinea-pig tracheal smooth muscle contraction induced by CaCl2 with no significant differences. Three dosages of Syn (0.010 6

0.021 2

0.042 4 mmol·L-1) could inhibit the release of internal calcium of tracheal smooth muscle cell induced by Ach significantly in a dose-dependent manner (P<0.05

P<0.01).The propranolol and Syn had no significant reducing effect on tracheal tension. Conclusion: Syn could relax tracheal smooth muscle by acting on M-receptor and histamine receptor

excit β-receptor and inhibiting the release of internal calcium.