Thymoquinone-induced relaxation of guinea-pig isolated trachea.

The effect of thymoquinone (TQ), the main constituent of the volatile oil of black seed (Nigella sativa), on the guinea-pig isolated tracheal zig-zag preparation was investigated. TQ caused a concentration-dependent decrease in the tension of the tracheal smooth muscle precontracted by carbachol. The effects of TQ were significantly potentiated by pretreatment of the tracheal preparations with quinacrine, a phospholipase A2 inhibitor, nordihydroguiaretic acid, a lipoxygenase inhibitor and by pretreatment with methylene blue, an inhibitor of soluble guanylyl cyclase. On the other hand, the effects of TQ were not influenced by pretreatment of the tracheal preparations with indomethacin, a cyclooxygenase inhibitor, propranolol, a non-selective beta-adrenoceptor blocker or by the pretreatment with theophylline, an adenosine receptors antagonist TQ totally abolished the pressor effects of histamine and serotonin on the guinea-pig isolated tracheal and ileum smooth muscles. The results of the present study suggest that TQ induced relaxation of precontracted tracheal preparation is probably mediated, at least in part, by inhibition of lipoxygenase products of arachidonic acid metabolism and possibly by non-selective blocking of the histamine and serotonin receptors. This relaxant effect of TQ, further support the traditional use of black seeds either alone or in combination with honey to treat bronchial asthma.