Tetrandrine: a vasodilator of medicinal herb origin with a novel contractile effect on dog saphenous vein

During our recent pharmacological characterization of the vasodilator effects of tetrandrine on endothelium-denuded rings of dog saphenous vein, we have unexpectedly observed a slowly developing transient contractile response upon washout of KCl following the relaxation by tetrandrine of KCl-induced contraction. This washout-induced contraction in the presence of tetrandrine was most prominent in dog saphenous vein, smaller in dog mesenteric vein and not observed in dog mesenteric artery, dog aorta and rat aorta. This transient contraction induced in the presence of tetrandrine was not affected by atropine, indomethacin or prazosin, but was substantially inhibited by phentolamine and rauwolscine. Addition of amiloride or readmission of KCl inhibited the transient contraction upon washout of high KCl in the presence of tetrandrine, whereas addition of ouabain turned this transient contraction into a sustained one. Our results suggest that tetrandrine, in addition to its well known vasodilator effect via the blockade of Ca2+ channels, elicited a novel contractile effect which is specifically associated with the activation of postjunctional alpha 2-adrenoceptors functionally characteristic in venous smooth muscle.