Potentiating effects on contractions by purified baicalin and baicalein inthe rat mesenteric artery

The effects of purified baicalin and baicalein from the traditional Chinese herb, Huangqin, on contractions induced by phenylephrine, U46619, and high extracellular K+ were investigated in isolated rat mesenteric arteries. Bo th baicalin (1-100 mu M) and baicalein (1-50 mu M) potentiated the contract ile response to phenylephrine in a concentration-related manner. Both flavo noids (10 mu M) also enhanced the U46619- or 40 mM K+-induced contractions. Baicalein (100-300 mu M) reduced the phenylephrine-induced tone. Prazosin at 1 mu M did not affect U46619-induced contraction in the absence and pres ence of baicalein or baicalin. Neither baicalin (1-100 mu M) nor baicalein (1-100 mu M) affected the basal tension. Removal of the functional endothel ium abolished the potentiating effects of baicalin and baicalein in arterie s preconstricted by both constrictors. Pretreatment of endothelium-intact r ings with 100 mu M N-G-nitro-L-arginine also potentiated phenylephrine- or U46619-induced contraction but completely inhibited the effects of baicalin and baicalein. Pretreatment with 1 mM L-arginine reversed the enhancing ef fect of baicalin but not of baicalein on phenylephrine-evoked contraction. Pretreatment with 10 mu M baicalin or 10 mu M baicalein significantly reduc ed the endothelium-dependent relaxation induced by acetylcholine or ionomyc in. These results indicate that both baicalin and baicalein potentiated the evoked contractile response, likely through inhibition of nitric oxide for mation and/or release in the endothelium.