Black tea flavonoid mediated endothelial dependent relaxation in rabbits

Abstract

This study was done to test the hypothesis that theoflavones (Theo) and catechines (Cat) derived from black tea caused endothelium dependent relaxation (EDR). Tea leaves were homogenized in 70% methanol and filtered. Chlorophyll was removed by extraction with chloroform and Cat was extracted into ethyl acetate and evaporated by freeze drying in water. Theo was extracted from tea leaves into isobutylmethylketone (IBMK). After chlorophyll was removed by extraction with chloroform, IBMK was evaporated by freeze drying in water to yield Theo. New Zealand White rabbits were sacrificed and the aorta harvested for measuring EDR. Aortic rings were suspended in organ baths containing oxygenated Krebs buffer at 37°C. EDR evoked by acetylcholine (Ac), Theo and Cat were measured after pre-contracting the rings with 10−5 M noradrenaline. The maximum relaxations observed with the three agonists are given in the table. The relaxations to both Theo and Cat were significantly lower than that induced by Ac. Denuding the rings of endothelium abolished all the relaxations. Prior incubation of rings with L-NAME also abolished the responses. After abolition of EDR, sodium nitroprusside (10−5) caused a relaxation of the rings (85%). EDR was lost after repeated exposure to tea flavones and restored partially upon incubation with L-arginine. It is concluded that (i) the mechanism of relaxation is an endothelial dependent one (ii) repeated exposure to Theo and Cat probably depletes the substrate for endothelial nitric oxide synthase.