Loss of vagally mediated bradycardia and bronchoconstriction in mice lacking M ₂ or M ₃ muscarinic acetylcholine receptors

Abstract

The presence of multiple muscarinic acetylcholine receptor (mAChR) subtypes in the heart and lung, combined with the lack of mAChR subtype-selective ligands, have complicated the task of identifying the mAChR subtypes mediating cardiac slowing (bradycardia) and airway narrowing (bronchoconstriction) due to vagal innervation. To determine which of the five mAChRs are responsible for the cholinergic control of heart rate and airway caliber in vivo, we performed experiments on mutant mice lacking the two prime candidates for such control, the M2 or M3 mAChR. Here, we report that in vivo, bradycardia caused by vagal stimulation or administration of the muscarinic agonist methacholine (MCh) was abolished in mice lacking functional M2 mAChRs (M2-/- mice). In contrast, heart rate responses remained unchanged in M3 receptor-deficient mice (M3-/- mice). The reduced hypotensive response of M3-/- mice to MCh suggests M3 mAChRs contribute to peripheral vasodilation. The M2-/- mice showed significantly enhanced in vivo bronchoconstrictor responses to vagal stimulation or MCh administration. In contrast, bronchoconstrictor responses were totally abolished in M3-/- mice. Because altered cardiac or pulmonary vagal tone is involved in a number of pathophysiological conditions, including cardiac arrhythmias, chronic obstructive pulmonary disease and asthma, these results should be of considerable therapeutic relevance.

Prolonged exposure to NT-3 attenuates cholinergic nerve-mediated contractions in cultured murine airways

“…The lack of capsaicin reactivity is probably due to the absence of sensory nerve cell bodies in the cultured tracheal preparation. Thus, in the present organ culture assay EFS responsiveness in the mouse tracheal segments is essentially cholinergic in accordance with the lack of response to vagal stimulation in muscarinic M3 receptor-deficient mice (M3-/-mice) (Fisher et al 2004). …”

Male sex hormones promote vagally mediated reflex airway responsiveness to cholinergic stimulation

“…The M2AchR mediates negative feedback inhibition of acetylcholine release from parasympathetic nerve terminals in the airways (9), and its inhibition or absence promotes airway responsiveness to vagal stimulation (8,31). Furthermore, we have observed that the airways of female estrogen receptor-␣-deficient mice are male-like (i.e., more sensitive than those of wild-type females to methacholine aerosol) and that this phenotype is associated with decreased M2AchR expression and function (5).…”

“…The M2AchR is responsible for negative feedback regulation of acetylcholine release in the (9), and mice genetically deficient in this receptor display enhanced airway reactivity to cholinergic and vagal stimulation (8). Hence, pulmonary M2AchR protein levels were determined in naïve male and female mice to determine if differences in M2AchR expression might underlie the in vivo sex differences in airway responsiveness.…”

Potassium channels in the sinoatrial node and their role in heart rate control

“…The pathway delineated by genetic, physiological and pharmacological studies involved the activation of M 2 muscarinic receptors and the dissociation of the inhibitory heterotrimer with the G βγ activating GIRK channels in the SAN [47–50]. However the GTP bound inhibitory G α subunit can also inhibit adenylate cyclase and reduce levels of cAMP reducing I f [51].…”