Possible Relaxant Effects of Thymoquinone on Guinea Pig Tracheal Chains

In the previous studies the relaxant, anti-cholinergic (functional antagonism) and anti-histaminic effects of Nigella sativa have been demonstrated on guinea pig tracheal chains. To elucidate the main substance responsible for the relaxant effect of this plant, the effect of thymoquinone, one of the main constituent of Nigella sativa was examined in this study. The bronchodilatory effects of three cumulative concentrations (40, 80, and 120 μM) of thymoquinone were examined by their relaxant effects on precontracted tracheal chains of guinea pigs using (1) 10 μM methacholine and (2) 60 mM KCl. The results were compared with the effects of saline, theophylline, and extracts (macerated and aqueous) of Nigella sativa (n = 5 for each group). The results showed significant relaxant effects of theophylline and extracts from Nigella sativa as compared to saline in group 1 experiments (P<0.05 and P<0.005). There were no significant differences between the effects of two higher concentrations of extracts with those of theophylline. However, none of the three thymoquinone concentrations showed any relaxant effect in both groups. There were also significant differences between the relaxant effect of theophylline and extracts with those of thymoquinone concentrations (P<0.05 and P<0.001). In group 2, only theophylline showed a significant relaxant effect (P<0.05 and P<0.001). The effects of two higher concentrations of both extracts and thymoquinone were significantly lower than those of theophylline in group 2 (P<0.05 to P<0.001). These results indicated that the relaxant effect of Nigella sativa is not due to its constituent thymoquinone. Iran. Biomed. J. 9 (3): 123-128, 2005