A possible mechanism of action of tetramethylpyrazine on vascular smooth muscle in rat aorta.

The vasodilatation of isolated rat aorta by tetramethylpyrazine (TMP) was studied by examining its effect on phenylephrine-induced contraction. We found no difference between the effects on intact and on endothelium-denuded preparations. The effect of TMP was similar to that of theophylline because propranolol did not block the vasodilatation. Also, there was a summation effect when the pyrazine was combined with theophylline. Furthermore, like that due to theophylline, the vasodilatation was accompanied by an increase in cyclic AMP. The pyrazine, as do other dilators, affected differently the two separate phases of the contractile response elicited with either phenylephrine or high potassium. The drug predominantly suppressed the phasic responses but both the phasic and tonic phases could be inhibited significantly if the concentration of the pyrazine was high enough. The present results suggest that intracellular accumulation of cyclic AMP and blockade of the release of calcium from internal stores may be important elements of the mechanism by which TMP reduces the development of tension in rat aortic smooth muscle.