Muscarinic M2 receptors do not participate in the functional antagonism between methacholine and isoprenaline in guinea pig tracheal smooth muscle.

We investigated whether muscarinic M2 receptors, known to inhibit adenylyl cyclase activity in airway smooth muscle, also inhibit isoprenaline-induced relaxation of guinea pig tracheal smooth muscle, as has recently been described for the dog (Fernandes et al., 1992, J. Pharmacol. Exp. Ther. 262, 119). Smooth muscle strips were contracted with various concentrations of methacholine or histamine (which served as a control) in the absence or presence of the M2-selective muscarinic receptor antagonist, gallamine (30 microM), and cumulative isoprenaline-relaxation curves were obtained. It was found that muscarinic M2 receptor blockade had no significant effect on isoprenaline pD2 and Emax values, neither with histamine nor with methacholine. The results show that, in guinea pig trachea, muscarinic M2 receptors do not significantly influence the functional antagonism of cholinergic smooth muscle contraction by isoprenaline.