European Journal of Pharmacology, 249 (1993) 235-238 235 © 1993 Elsevier Science Publishers B.V. All rights reserved 0014-2999/93/$06.00 EJP 21360 Short communication Muscarinic M 2 receptors do not participate in the functional antagonism between methacholine and isoprenaline in guinea pig tracheal smooth muscle Ad F. Roffel *, Herman Meurs, Carolina R.S. Elzinga and Johan Zaagsma Department of Pharmacology and Therapeutics, University of Groningen, A. Deusinglaan 2, 9713 A W Groningen, Netherlands Received 10 June 1993, accepted 14 September 1993 We investigated whether muscarinic M 2 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 p.M), and cumulative isoprenaline-relaxation curves were obtained. It was found that muscarinic M2 receptor blockade had no significant effect on isoprenaline pD z and Emax values, neither with histamine nor with methacholine. The results show that, in guinea pig trachea, muscarinic M z receptors do not significantly influence the functional antagonism of cholinergic smooth muscle contraction by isoprenaline. Muscarinic M 2 receptor; Trachea (guinea-pig); Functional antagonism; fl-Adrenoceptor agonist; Adenylyl cyclase inhibition 1. Introduction Cholinergic contraction of airway smooth muscle preparations from various species, including guinea pig, cow, dog and man is mediated by muscarinic M 3 receptors (Mutschler et al., 1988; Roffel et al., 1988; Yang et al., 1991; Roffel et al., 1990a, respectively), presumably via activation of phospholipase C (Roffel et al., 1990b; Yang et al., 1991). Several studies, how- ever, have indicated the additional presence of a major population of muscarinic M 2 binding sites in bovine (Roffel et al., 1988; Lucchesi et al., 1990; Schaefer et al., 1992), canine (Fernandes et al., 1992) and guinea pig airway smooth muscle (Mak and Barnes, 1990; Haddad et al., 1991). These muscarinic M 2 receptors have been shown to inhibit adenylyl cyclase activity in intact airway smooth muscle cells (Schaefer et al., 1992; Sankary et al., 1988; Yang et al., 1991), which might be the result of receptor cross-talk, but also in membranes (Meurs et al., 1992; Jones et al., 1987; Pyne et al., 1992), which indicates a direct inhibitory effect on adenylyl cyclase, presumably via Gi (Sankary et al., * Corresponding author. Tel. (050) 633323, fax (050) 633311. 1988; Pyne et al., 1992; Lucchesi et al., 1990). Recently, it has been described that muscarinic M 2 receptor- mediated inhibition of adenylyl cyclase activity plays an inhibitory role in isoprenaline-induced relaxation of canine tracheal smooth muscle (Fernandes et al., 1992), and also that pretreatment of these preparations with pertussis toxin increases to some extent the ability of isoprenaline to relax cholinergic tone, presumably via inactivation of G i (Mitchell et al., 1993). These obser- vations might explain the phenomenon that /3-adreno- ceptor-mediated relaxation of airway smooth muscle depends on the nature of the contractile agonist used (e.g. histamine vs. muscarinic agonists), as has been shown for guinea pig airway smooth muscle (Van Ams- terdam et al., 1989), in addition to the fact that the relaxant response depends on the concentration of the contractile agonist, which has been shown to be related to the ability of this agonist to induce phosphoinositide turnover via histamine H 1 and muscarinic M 3 recep- tors, respectively (Van Amsterdam et al., 1989). There- fore, in the present study we investigated whether muscarinic M 2 receptors also participate in the func- tional antagonism 'of cholinergic but not histaminergic contraction by isoprenaline in guinea pig tracheal smooth muscle.