the relaxant effect of plantago major on rat tracheal smooth muscles and its possible mechanisms

This study was conducted to evaluate the possible mechanisms of the relaxant effects ofhydroalcoholic extract of plantago major (p. major) on tracheal smooth muscle (tsm) in rats.the effects of cumulative concentrations of p. major (5, 10, 20 and 40 mg/ml) andtheophylline (0.2, 0.4, 0.6 and 0.8 mm) were evaluated on pre-contracted tsm with 10 μμmethacholine or 60 mm kcl. to determine the possible mechanisms, the relaxant effect ofthe plant was also examined on incubated tsm with atropine, indomethacin,chlorpheniramine, glibenclamide, diltiazem, papaverine, and propranolol.the results indicated concentration-dependent relaxant effects for p. major in nonincubatedtsm contracted by methacholine or kcl. there was no statistically significantdifference in the relaxant effects of p. major between non-incubated and incubated tissueswith indomethacin, papaverine, and propranolol. however, the relaxant effects of p. major inincubated tissues with atropine (p<0.01 to p<0.001), chlorpheniramine (p<0.05 to p<0.001),glibenclamide (p<0.05), or diltiazem (p<0.01) were significantly lower than non-incubatedtsm.p. major indicated relatively potent relaxant effects which were lower than those oftheophylline. muscarinic and histamine (h1) receptors inhibition, as well as calcium channelblocking and potassium channel opening effects are suggested to contribute to the tsmrelaxant effect of the plant.