Potentiating effects on contractions by purified baicalin and baicalein in the 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. Both baicalin (1-100 μM) and baicalein (1-50 μM) potentiated the contractile response to phenylephrine in a concentration-related manner. Both flavonoids (10 μM) also enhanced the U46619- or 40 mM K⁺-induced contractions. Baicalein (100-300 μM) reduced the phenylephrine-induced tone. Prazosin at 1 μM did not affect U46619-induced contraction in the absence and presence of baicalein or baicalin. Neither baicalin (1-100 μM) nor baicalein (1-100 μM) affected the basal tension. Removal of the functional endothelium abolished the potentiating effects of baicalin and baicalein in arteries preconstricted by both constrictors. Pretreatment of endothelium-intact rings with 100 μMN^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 effect of baicalin but not of baicalein on phenylephrine-evoked contraction. Pretreatment with 10 μM baicalin or 10 μM baicalein significantly reduced the endothelium-dependent relaxation induced by acetylcholine or ionomycin. These results indicate that both baicalin and baicalein potentiated the evoked contractile response, likely through inhibition of nitric oxide formation and/or release in the endothelium.