Immunohistochemical and functional studies for M₃ muscarinic receptors and cyclo‐oxygenase‐2 expressed in the mouse atrium

Abstract

In mouse atrium, M₂ and M₃ muscarinic receptors (M₂R and M₃R) are involved in biphasic (negative and positive) inotropic actions of muscarinic agonists, and the positive inotropic action is reduced by indomethacin. The aim of our study was to determine the localization of M₂R, M₃R and cyclo-oxygenase (COX) in mouse atrium and to characterize muscarinic receptor-mediated positive inotropy. M₂R immunoreactivity was found only on atrial myocardium, but M₃R immunoreactivity was localized on both the myocardium and endocardial endothelium. COX-1 and COX-2 immunoreactivities were identified in both myocardial and endocardial endothelium. In electrically stimulated left atria, carbachol caused M₂R-mediated negative inotropy followed by M₃R-mediated positive inotropy. Removal of atrial endothelium reduced the positive inotropy without affecting the negative inotropy, suggesting that stimulation of endothelial M₃R mediates the positive inotropy. N-[2-(cyclohexyloxy)-4-nitrophenyl]-methanesulfonamide (NS398, COX-2 inhibitor) decreased the carbachol-induced positive inotropy; however, 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole (SC560, COX-1 inhibitor), 1-[[4,5-bis(4-methoxyphenyl)-2-thiazolyl]carbonyl]-4-methylpiperazine (FR122047, COX-1 inhibitor) and L-nitroarginine methylester did not affect the inotropic response. M₃R activation caused positive chronotropy in spontaneously beating right atria when M₂R-mediated negative chronotropy was suppressed and rate of contraction was low, <350 beats min⁻¹. Our results indicate that although M₃Rs are located on both myocardial cells and endocardial endothelial cells, only endothelial M₃Rs mediate positive inotropy in response to muscarinic agonists via activation of COX-2 in the mouse atrium. M₃R-mediated positive chronotropy counteracting M₂R-mediated negative chronotropy was also demonstrated.

An Inotropic Action Caused by Muscarinic Receptor Subtype 3 in Canine Cardiac Purkinje Fibers

“…In the studies using atria and ventricles, several studies demonstrated that M 3 receptor stimulation increased prostaglandin (PG) production in the endocardial endothelium which caused positive inotropy in the mouse atria [14–17]. In contrast, Ateş and Kaygisiz reported that the positive inotropy was not dependent on PG production in rat hearts [18].…”

M3 muscarinic acetylcholine receptor in cardiology and oncology

“…It has also been thought that coronary artery spasm was related to the decrease of M 3 -mAChR ′s density [32] and it has demonstrated both an endothelium-dependent relaxation to acetylcholine in coronary circulation mediated predominantly by the activation of M3-mAChR [33] and a M 3 -mAChR mediated vasoconstriction in vascular smooth muscle cells in the absence of endothelium [19]. It has also demonstrated an endocardial endothelial M Rs mediate positive inotropy in response to muscarinic agonists via activation of COX-2 [34]. Further study is needed to evaluate differences in M 3 -mAChR signaling caused by changes in receptor desensitization, sequestration [35][36][37][38][39], and up-or down-regulation and to elucidate the role of M 3 -mAChR in cardiovascular system as well in inflammation and in cancer while new findings are emerging as regards the use of cardiovascular drugs [38,55,68,69,74,75,[77][78][79]82].…”

The possible proarrhythmic effects of some non-antiarrhythmic drugs

“…In experimental conditions, acetylcholine (ACh) and carbachol (CCh) produces positive inotropic effects in isolated rat hearts (Ates et Kaygisiz, 1998) and biphasic inotropic response transient decrease in contractility followed by an increasein isolated mice left atria (Tanaka et al, 2001;Hara et al, 2009). The increase in contractile force is mediated by type 3 muscarinic acetylcholine receptors via activation of COX-2 enzyme (Harada et al, 2012).…”