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 of hydroalcoholic 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) and theophylline (0.2, 0.4, 0.6 and 0.8 mM) were evaluated on pre-contracted TSM with 10 μΜ methacholine or 60 mM KCl. To determine the possible mechanisms, the relaxant effect of the 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 non-incubated TSM contracted by methacholine or KCl. There was no statistically significant difference in the relaxant effects of P. major between non-incubated and incubated tissues with indomethacin, papaverine, and propranolol. However, the relaxant effects of P. major in incubated 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-incubated TSM. P. major indicated relatively potent relaxant effects which were lower than those of theophylline. Muscarinic and histamine (H1) receptors inhibition, as well as calcium channel blocking and potassium channel opening effects are suggested to contribute to the TSM relaxant effect of the plant.