Low-Affinity M2 Receptor Binding State Mediates Mouse Atrial Bradycardia: Comparative Effects of Carbamylcholine and the M1 Receptor Agonists Sabcomeline and Xanomeline

Carbamylcholine, a nonselective muscarinic receptor agonist, and sabcomeline and xanomeline, functional M1receptor-selective agonists with high M2 receptor affinities, were used to explore the relationship of the M2receptor affinity of these agonists to mouse atrial bradycardia and to understand the relationship of the high and low M2 receptor affinity states to carbamylcholine-induced mouse atrial bradycardia. All three agonists produced bradycardia with sabcomeline (pEC50 = 6.7) more potent than either carbamylcholine (pEC50 = 5.9) or xanomeline (pEC50 = 5.1). Sabcomeline and carbamylcholine produced a rapid, concentration-related bradycardia, which was antagonized by atropine with p K B values of 8.6 and 8.9, respectively. In addition, sabcomeline antagonized carbamylcholine-induced bradycardia (p K B = 7.48), indicating that sabcomeline was a partial agonist at M2 receptors. In contrast, xanomeline (up to 10−5 M), did not antagonize carbamylcholine-induced bradycardia, and atropine (3.0 × 10−8 M) did not antagonize xanomeline-induced bradycardia, suggesting that xanomeline-induced bradycardia was not mediated by M2 receptors. Analysis of receptor occupancy curves indicated that bradycardia resulted from the interaction of carbamylcholine with the low- rather than high-affinity state of the M2 receptor and that sabcomeline was a partial agonist at M2 receptors in mouse atria. In contrast, similar analysis for xanomeline using the receptor affinity of xanomeline at M2 receptors (1.8 × 10−8 M) was not consistent with classical receptor theory. These data document that 1) the low-affinity state of the M2 receptor is responsible for muscarinic-induced atrial bradycardia, 2) sabcomeline was an M2 receptor partial agonist, and 3) xanomeline-induced bradycardia was not mediated by activation of M2 muscarinic receptors.