en Cardiovascular Physiology/Pharmacology Cardiovascular Physiology/Pharmacology Experimental research article Experimental research article Introduction: Our previous studies showed that hydroalcoholic extract of leaf of Vitis vinifera relaxes the phenylephrine-induced contraction in rat thoracic aorta. This effect was dependent on endothelial integrity and NO-cGMP system. The vasorelaxant effect of extract was much lesser on KCl-induced contraction. We, therefore, postulated that K+ channels are involved. The main aim of the present study was to determine the type of K+ channels involved in this vasorelaxant effect. Methods: Thoracic aorta with intact endothelium was removed from adult male Wistar rats (170-220g). The aorta was mounted in an organ bath containing Krebs-Henseleit (37 ºC, pH 7.4) bubbled with O2. Aortic contractions were recorded isometrically under 1 g resting tension. The aorta endothelium was considered intact if acetylcholine (1 μM) could induce more than 70% aorta relaxation on 1μM phenylephrine-induced contraction. Extract was prepared by maceration method using 70% alcohol and the solvent was then evaporated. Results: The results showed that in the presence of tetraethylammonium (10 mM), the vasorelaxant effect of extract (0.25, 0.5, 1 and 2 mg/ml) was reduced (P<0.001, n=7). In contrast, glibenclamide (1 μM) had no effect. In calcium-free (plus 0.1 mM of EDTA) Krebs-Henseleit solution, the vasorelaxant effect of extract (0.25, 0.5 and 1 mg/ml) was reduced (P<0.0001, n=8). Furthermore, the vasorelaxant effect of extract was unaffected by indomethacin (1 μM). Conclusion: These results suggest that Vitis vinifera leaf hydroalcoholic extract induces relaxation in rat aorta possibly by opening the Ca2+ -operated K+ channels but, not ATP- sensitive K+ channels and extracellular calcium was essential for inducing vasorelaxation by extract. Furthermore, cyclooxigenase was not involved in this vasorelaxant effect. Vitis vinifera, Rat, Aorta, K+ channels. 71 77 http://ppj.phypha.ir/browse.php?a_code=A-10-110-28&slc_lang=en&sid=1 Mohammad Kazem Gharib Naseri 320031947532846002892 320031947532846002892 Yes Akbar Heidari 320031947532846002893 320031947532846002893 No