Carbamylcholine, a nonselective
muscarinic receptor agonist, and
sabcomeline and
xanomeline, functional M(1) receptor-selective agonists with high M(2) receptor affinities, were used to explore the relationship of the M(2) receptor affinity of these agonists to mouse atrial
bradycardia and to understand the relationship of the high and low M(2) receptor affinity states to
carbamylcholine-induced mouse atrial
bradycardia. All three agonists produced
bradycardia with
sabcomeline (pEC(50) = 6.7) more potent than either
carbamylcholine (pEC(50) = 5.9) or
xanomeline (pEC(50) = 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 M(2) receptors. In contrast,
xanomeline (up to 10(-5) M), did not antagonize
carbamylcholine-induced
bradycardia, and
atropine (3.0 x 10(-8) M) did not antagonize
xanomeline-induced
bradycardia, suggesting that
xanomeline-induced
bradycardia was not mediated by M(2) 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 M(2) receptor and that
sabcomeline was a partial agonist at M(2) receptors in mouse atria. In contrast, similar analysis for
xanomeline using the receptor affinity of
xanomeline at M(2) receptors (1.8 x 10(-8) M) was not consistent with classical receptor theory. These data document that 1) the low-affinity state of the M(2) receptor is responsible for
muscarinic-induced atrial
bradycardia, 2)
sabcomeline was an M(2) receptor partial agonist, and 3)
xanomeline-induced
bradycardia was not mediated by activation of M(2)
muscarinic receptors.