Altered Cardiovascular Responses in Mice Lacking the M₁Muscarinic Acetylcholine Receptor

Abstract

Although the M 2 muscarinic acetylcholine receptor (mAChR) is the predominant functional mAChR subtype in the heart, some responses of the cardiovascular system to acetylcholine (ACh) may be mediated by other mAChR subtypes. The potential effect of M 1 mAChR on heart function was investigated using M 1 knockout (M 1 -KO) mice. In vivo cardiodynamic analysis showed that basal values of heart rate (HR), developed left ventricular pressure (DLVP), left ventricular dP/dt max (LV dP/ dt max ), and mean blood pressure (MBP) were similar between wild-type (WT) and M 1 -KO mice. Injection of the putative M 1 -selective agonist 4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium (McN-A-343) produced an increase in LV dP/dt max , DLVP, HR, and MBP in WT mice but did not affect hemodynamic function in the M 1 -KO mice. The stimulatory effect of McN-A-343 in WT mice was blocked by pretreatment with propranolol, indicating that stimulation of the M 1 mAChRs on sympathetic postganglionic neurons evoked release of catecholamines. Intravenous injection of ACh in both WT and M 1 -KO mice caused atrioventricular conduction block, without a significant change in the frequency of atrial depolarization, or atrial fibrillation. Immunoprecipitation and reverse transcriptase-polymerase chain reaction failed to detect the expression of M 1 mAChR in cardiac tissue from WT mice. The carbacholinduced increase of phospholipase C activity in cardiac tissues was not different between WT and M 1 -KO mice. These results demonstrate that 1) activation of M 1 mAChR subtype on sympathetic postganglionic cells results in catecholamine-mediated cardiac stimulation, 2) M 1 mAChR is not expressed in mouse heart, and 3) administration of ACh to mice induces arrhythmia.Stimulation of the parasympathetic postganglionic neurons causes the release of acetylcholine (ACh) that acts on muscarinic ACh receptors (mAChRs) on the myocardial cell membrane. There are five mammalian mAChR subtypes that preferentially couple to either the inhibition of adenylyl cyclase activity (M 2 and M 4 subtypes) or to the stimulation of phospholipase C (PLC) activity (M 1 , M 3 , and M 5 subtypes) (Bonner, 1989). The M 2 mAChR is the predominant muscarinic receptor subtype in mammalian heart (Brann et al., 1993;Caulfield, 1993). Activation of these M 2 mAChRs inhibits the activity of adenylyl cyclase, closes calcium channels, lowers the hyperpolarization-activated pacemaker current, and activates an inwardly rectifying potassium channel. These changes cause both negative chronotropic and inotropic effects on the heart (Caulfield, 1993).Accumulating evidence indicates that other mAChR subtypes also contribute to the regulation of heart rate and contractility. The novel muscarinic agonist McN-A-343 has pressor activity in mammals (Roszkowski, 1961). When injected in dogs and cats, McN-A-343 raised blood pressure, an effect blocked by pretreatment with the muscarinic antagonist atropine. The McN-A-343-mediated hypertension in vivo was also antagonized by adrenergic b...

Acute desensitization of acetylcholine and endothelin-1 activated inward rectifier K+ current in myocytes from the cardiac atrioventricular node

“…1C), demonstrating M2 receptor activation to be responsible for generation of AVN I KACh . This is concordant with: (i) prior work on anaesthetised dogs in which AFDX-116 inhibited chronotropic and dromotropic responses to intracardiac vagal nerve stimulation [29]; (ii) the persistence of AV conduction block in response to intravenous ACh in mice deficient in M1-receptors [30], and (iii) presumed M2-receptor mediated conduction effects of propofol on guinea-pig hearts [31]. SAN I KACh is sensitive to the bee venom toxin tertiapin [32,33].…”

Contribution of both M1 and M4 receptors to muscarinic agonist-mediated attenuation of the cocaine discriminative stimulus in mice

“…However, this appears unlikely for several reasons. First, M 1 receptor stimulation moderately increases heart rate in rodents, and would thus be expected to enhance cocaine’s cardiovascular effects, not attenuate them (Hardouin et al 2002; Ma et al 2009). Second, we previously showed that muscarinic ligands with poor brain penetration had lower potency or lacked effects in the cocaine discrimination assay, suggesting that this modulation is centrally mediated (Thomsen et al 2010).…”

RGS proteins in heart: brakes on the vagus

“…Amongst these disorders are sick sinus syndrome and AV block (Fu et al, 2007). Excessive vagal stimulation leads directly to AV block, an effect that is primarily dependent on increased GIRK current in mice (Drici et al, 2000; Hardouin et al, 2002) but can also result from inhibition of I Ca,L by maternal auto-antibodies in cases of congenital heart block in humans (Garcia et al, 1994). Activation of RGS6 or RGS4 activity in heart would be expected to dampen parasympathetic stimulation and relieve these symptoms.…”