Inhibition of vasoconstriction by tetramethylpyrazine: does it act by blocking the voltage-dependent Ca channel?

Using dog mesenteric arterial ring preparations, we studied the in vitro vascular effects of tetramethylpyrazine (TMP, also known as Ligustrazine), a vasoactive component derived from Ligustium Wollichii Franchat; a widely used ingredient in Chinese herbal medicine for the treatment of cardiovascular diseases. TMP had no effect on the resting force of the vascular muscle, but it caused a dose-dependent inhibition of vascular contractile responses to KCl and phenylephrine (PHE). These inhibitory effects of TMP were reversible and more prominent at lower agonist concentrations. TMP also inhibited the responses to PHE in Ca-free medium containing 50 microM EGTA suggesting the action on the release of intracellular Ca2+. Therefore, TMP is not a specific Ca-channel blocker as claimed previously. Its vasodilatory effect is mediated by the inhibition of Ca influx, as well as the release of intracellular Ca2+.