Effects of baicalein on prostanoid generation from the lung and contractile responses of the trachea in guinea pig

Effects of baicalein on release of slow reacting substance of anaphylaxis (SRS-A) or leukotriene (LT) from the sensitized guinea pig lung after antigen challenge and tonus of guinea pig tracheal muscles were studied. Baicalein inhibited release of SRS-A from sensitized guinea pig lung after antigen challenge. High-performance liquid chromatography (HPLC) analysis revealed that released SRS-A consisted of LTC4 and D-4. Baicalein also reduced release of LTC4 and D-4 from the sensitized lung after antigen challenge. Baicalein relaxed the isolated guinea pig tracheal smooth muscle contracted by LTD4, carbachol or histamine. However, this compound produced a contraction when the tracheal muscle was contracted by prostaglandin F-2 alpha(PGF(2 alpha)). This contraction by baicalein was abolished by pretreatment with indomethacin, a cyclooxygenase inhibitor. Baicalein elicited a relaxation in the normal non-sensitized tracheal preparation but a contraction in the tissue isolated from actively sensitized guinea pig in 4 among 7 cases. Baicalein also produced a contraction in the trachea pretreated with phorbol dibutyrate and contracted by carbachol, which was eliminated after treatment with indomethacin. The results suggest that baicalein exerts action via, at least, two different mechanisms, the inhibition of releasing SRS-A (LTs) and direct relaxing effects on the trachea. Besides, baicalein seems to produce contraction under certain conditions, which may involve stimulation of the cyclooxygenase pathway.