Abstract

The condensed tannins (CTs) from black currant (R. nigrum) leaves were isolated as a tannin fraction to investigate their effect on isolated rat duodenal contraction induced by acetylcholine (ACh). The mechanisms of CT action were compared with those of a muscarinic receptor antagonist (atropine) and a calcium channel blocker (verapamil). Duodenal segments were fixed in organ bath, and the frequency, amplitude, and tone of contraction were recorded. The experiments were performed in seven groups, including the vehicle control, CTs (10, 50, 100, and 300 µg/ml), atropine (1.55×10−5 M), and verapamil (10−6 M), with cumulative additions of ACh (10−8–10−4 M). The possibility that CTs might act by altering ion channel activity in intestinal epithelial cells was tested in a human colonic adenocarcinoma cell line by measuring the equivalent short-circuit current, compared with that induced by an adenylyl cyclase activator forskolin (10 µM). It was found that CTs at low concentrations (10 and 50 µg/ml) significantly decreased the frequency of ACh-induced duodenal contractions. Only the CTs at high concentration (300 µg/ml) were able to inhibit ACh-induced amplitude and tone of contraction, similar to those of atropine, but not verapamil. Preliminary data of equivalent short-circuit currents were unaltered after the four doses of CTs, in contrast to forskolin. It is concluded that the mechanism of CT action is more likely mediated via the muscarinic receptor signaling pathway rather than through inhibition of Ca2+ channels in intestinal pacemaker and smooth muscle cell membranes or epithelial cAMP-dependent Cl– secretion.