STIMULATORY EFFECT OF Nigella sativa   ON β2-ADRENOCEPTORS OF GUINEA PIG TRACHEAL CHAINS

Our previous studies have demonstrated the relaxant, anticholinergic (functional antagonism), antihistaminic, and inhibitory effect of calcium channels of Nigella sativa on guinea pig tracheal chains. To investigate the other mechanism responsible for the relaxant effect of this plant, the stimulatory effect of Nigella sativa on β₂-adrenergic receptors in tracheal chains of the guinea pig was examined in this study.

The β₂-Adrenergic effects of macerated and aqueous extracts from Nigella sativa, 50 nM propranolol, and saline were tested by performing the cumulative Log concentration-response curves of isoprenaline induced relaxation of precontracted isolated guinea pig tracheal chains with three different conditions including: non-incubated (group 1, n=8); incubated with 1 μM chlorpheniramine, and 1 μM atropine (group 2, n=5). The effective concentration of isoprenaline, causing 50% of maximum response (EC50), maximum response and the slope of isoprenaline curves obtained in the presence of extracts, and propranolol were compared with those of saline.

The results showed clear leftward shifts in isoprenaline curves obtained in the presence of both macerated and aqueous extracts compared with that of saline in group 2. The EC50 obtained in the presence of aqueous extract in group 2 was significantlylower than that of saline (p<0.05). The maximum response obtained in the presence of aqueous extract in group 2 was non-significantly greater than that of saline. However, in group 1 experiments there was no significant difference between EC50 and maximum responses obtained in the presence of two extracts and saline, although isoprenaline curves obtained in the presence of both macerated and aqueous extracts showed clear rightward shifts compared to that of saline. Isoprenaline curves were obtained in the presence of propranolol in both groups of experiments. The EC50 obtained in the presence of propranolol was significantly greater than that obtained in the presence of saline and extracts in both groups of experiments (p<0.001 for all cases).

The results of this study indicated a stimulatory effect of aqueous extract and a possible stimulatory effect of macerated extract from Nigella sativa on β₂-adrenoceptors.