Pharmacological Actions of Tannic Acid. I. Effects on Isolated Smooth and Cardiac Muscles and on Blood Pressure

Abstract

Tannins are common constituents of medicinal plant crude extracts and have been reported to have pharmacological actions of their own. Studies on these actions are likely to be helpful in interpreting data obtained with tannin-rich crude plant extracts. In this context, the present report analyses the effects of tannic acid on responses of the isolated rat uterus and vas deferens to agonists and left atrium to electrical stimulation, as well as on blood pressure in anesthetized rats. Tannic acid dose-dependently and non-competitively antagonized contractions to different agonists in rat uterus and vas deferens. The compound produced a biphasic effect in the atrium. Whereas lower concentrations (30 to 100 µg/ml) enhanced inotropism, higher concentrations (> 300 µg/ml) promoted depression of contractility. Intravenous tannic acid led to hypotension, which was partially reduced by antihistamine pretreatment, but did not affect heart rate. The depressant effect of tannic acid in uterine fragments was potentiated by increasing calcium concentration in the bathing solution, although in the depolarized strips the compound non-competitively antagonized calcium-induced contractions. The results suggest that tannic acid can affect calcium availability for contraction of smooth and cardiac muscles. This action could well mask the effects of other active constituents of tannic-rich plant extracts.