Quercetin, one of the most widely distributed flavonoids in the plant kingdom, inhibits various enzymes. This study examined its inhibitory effect on the angiotensin-converting enzyme activity through the cardiovascular response to bradykinin and angiotensin I. Quercetin pretreatment (88.7 µmol/kg p.o., 45 min; 14.7 µmol/kg i.v., 5 min) significantly potentiated the hypotensive effect of bradykinin (10 nmol/kg i.v.). This association was significantly attenuated by an antagonist of the B2 receptor. In addition, the hypertensive response to angiotensin I (0.1 nmol/kg i.v.) was significantly reduced by quercetin pretreatment using the same parameters as before. These results suggest an inhibitory effect of quercetin on the angiotensin-converting enzyme activity, similar to that of captopril. Quercetin was equally effective when given orally or intravenously.

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A crude aqueous alcoholic extract of Mandevilla velutina (Apocynaceae) rhizomes produced a concentration-dependent displacement to the right of the concentrationresponse curve to bradykinin (Bk) in the isolated uterus of the rat. Schild analysis of the data revealed a linear relationship (r = 0.99) and yielded a pA2 value of 3.3 + 0.08 (-log g ml-') but the slope differed significantly from unity. The anti-Bk action was of rapid onset and was reversible upon washout. Contractions induced by acetylcholine, oxytocin, angiotensin II and BaCl2 were unaffected by the extract. This represents the first report of a selective Bk antagonist of plant origin. The isolation of the active principle(s) may result in a useful pharmacological tool for elucidating the physiological and physiopathological roles of endogenous Bk.

The effect of a fraction (Bc2) from the venom of the sea anemone Bunodosoma caissarum on [3H]glutamate release from rat cortical synaptosomes was investigated. Bc2 (2-20 microg/ml) provoked massive glutamate release without causing synaptosome disruption. Glutamate release stimulated by Bc2 was independent of extracellular Ca2+ and of voltage-sensitive Na+ channels, and it was completely abolished by the addition of sphingomyelin. No definitive evidence about the mechanism underlying the stimulatory effect of Bc2 is available as yet. However, a direct interaction with the exocytotic machinery cannot be ruled out.

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 wg/ml) enhanced inotropism, higher concentrations (>300 sglml) 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 tannie-rich plant extracts. Isolated uterus Uterine strips, 15-20 mm long, were suspended in 10 ml of aerated de Jalon solution (composition in mM: NaCI 154, KCI 5.