Cardio-selective inhibitory effect of the betel nut extract: possible explanation

Chewing of betel nut, the seed of Areca catechu, is associated with a host of physical and psychological effects while it is also traditionally used in constipation and hypertension. In this study, we report the cardio-selective cholinomimetic activity of the betel nut crude extract (Ac.Cr). Ac.Cr, that tested positive for saponins, tannins, phenols, alkaloids and terpenes, exhibited dose-dependent atropine-sensitive inhibition of isolated guinea-pig atrial contractility with an EC50 value of 0.93 microg/ml (0.57-1.51, 95% CI). In rabbit jejunum, Ac.Cr showed atropine-sensitive spasmogenicity with an EC50 of 7.31 microg/ml (5.41-9.88, 95% CI) showing that it is around 8 times more potent in the cardiac than the intestinal preparation. Both carbachol and physostigmine exhibited acetylcholine-like stimulant activity in jejunum with the latter being more potent in jejunum than in atrial tissues. Activity-directed fractionation of Ac.Cr yielded fractions with similar cholinergic activity in atria and jejunum except the aqueous fraction being 6 times more potent in the atria. Arecoline, the known betel nut compound with cholinergic activity showed similar potency in both tissues while catechin and tannic acid exhibited intestinal spasmolytic effect but were inactive in atria. The results show the cardio-selective inhibitory effect of Ac.Cr which might possibly be due to selective gut-spasmolytic behaviour of catechin and tannic acid thus reducing the cholinomimetic activity of Ac.Cr in the gut though the preferential binding of the constituents of betel nut extract at muscarinic receptor subtypes in heart cannot be ignored.