Inhibitory effect of Carum copticum on histamine (H1) receptors of isolated guinea-pig tracheal chains.

In a previous study, the relaxant and anti-cholinergic (functional antagonism) effects of Carum copticum were demonstrated on guinea-pig tracheal chains. To elucidate the other mechanisms responsible for this relaxant effect, the inhibitory effect of this plant on histamine H1 receptors was examined in this study. The anti-histaminic effects of extracts, essential oil, 5 nM chlorpheniramine, and saline were tested by performing the cumulative log concentration-response curves of histamine induced contraction of isolated guinea-pig tracheal chains incubated with three different conditions: 1.4 microM indomethacin (group 1, n = 9); indomethacin, 1 microM propranolol, and 10 nM atropine (group 2, n = 8); and indomethacin and propranolol (group 3, n = 7). The results showed clear rightward shifts in histamine response curves obtained in the presence of extracts, essential oil, and chlorpheniramine in all three sets of experiments compared with the curves obtained in the presence of saline. The effective concentrations of histamine causing 50% of maximum response (EC50) obtained in the presence of extracts, essential oil, and chlorpheniramine in all three sets of experiments were significantly higher than those of saline (P < 0.05- < 0.001) except EC50 of macerated extract in group 1. However, maximum response to histamine obtained in the presence of extracts and essential oil were lower (P = 0.005- < 0.001) except maximum response of essential oil in group 2. In addition, the maximum responses obtained in the presence of extracts in group 2 experiments compared to the other two sets of experiments were improved. Comparison of the slope of histamine-response curves showed parallel shifts of the curves obtained in the presence of all extracts and essential oil in group 2 experiments. The results of this study indicated a competitive antagonism effect of C. copticum at histamine H1 receptors. A beta-adrenergic stimulatory effect of essential oil and an anti-cholinergic property of the plant were also suggested.