Vol. 296, Issue 3, 818-824, March 2001

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

Peter W. Stengel and Marlene L. Cohen

Eli Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana

Carbamylcholine, a nonselective muscarinic receptor agonist, and sabcomeline and xanomeline, functional M₁ receptor-selective agonists with high M₂ receptor affinities, were used to explore the relationship of the M₂ receptor 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 pK_B values of 8.6 and 8.9, respectively. In addition, sabcomeline antagonized carbamylcholine-induced bradycardia (pK_B = 7.48), indicating that sabcomeline was a partial agonist at M₂ receptors. In contrast, xanomeline (up to 10⁵ M), did not antagonize carbamylcholine-induced bradycardia, and atropine (3.0 × 10⁸ 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 × 10⁸ 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.