Muscarinic type-1 receptors contribute to IK,ACh in human atrial cardiomyocytes and are upregulated in patients with chronic atrial fibrillation

Int J Cardiol. 2018 Mar 15:255:61-68. doi: 10.1016/j.ijcard.2017.12.050. Epub 2017 Dec 22.


Background: Basal and acetylcholine-gated inward-rectifier K+-currents (IK1 and IK,ACh, respectively) are altered in atrial fibrillation (AF). Gi-protein-coupled muscarinic (M) receptors type-2 are considered the predominant receptors activating IK,ACh. Although a role for Gq-coupled non-M2-receptor subtypes has been suggested, the precise regulation of IK,ACh by multiple M-receptor subtypes in the human atrium is unknown. Here, we investigated M1-receptor-mediated IK,ACh regulation and its remodeling in chronic AF (cAF).

Methods and results: M1-receptor mRNA and protein abundance were increased in atrial cardiomyocyte fractions and atrial homogenates from cAF patients, whereas M2-receptor levels were unchanged. The regulation of IK,ACh by M1-receptors was investigated in right-atrial cardiomyocytes using two applications of the M-receptor agonist carbachol (CCh, 2μM), with pharmacological interventions during the second application. CCh application produced a rapid current increase (Peak-IK,ACh), which declined to a quasi-steady-state level (Qss-IK,ACh). In sinus rhythm (Ctl) the selective M1-receptor antagonists pirenzepine (10nM) and muscarinic toxin-7 (MT-7, 10nM) significantly inhibited CCh-activated Peak-IK,ACh, whereas in cAF they significantly reduced both Peak- and Qss-IK,ACh, with no effects on basal inward-rectifier currents in either group. Conversely, the selective M1-receptor agonist McN-A-343 (100μM) induced a current similar to the CCh-activated current in Ctl atrial cardiomyocytes pretreated with pertussis toxin to inhibit M2-receptor-mediated Gi-protein signaling, which was abolished by MT-7. Computational modeling indicated that M1- and M2-receptors redundantly activate IK,ACh to abbreviate APD, albeit with predominant effects of M2-receptors.

Conclusion: Our data suggest that Gq-coupled M1-receptors also regulate human atrial IK,ACh and that their relative contribution to IK,ACh activation is increased in cAF patients. We provide novel insights about the role of non-M2-receptors in human atrial cardiomyocytes, which may have important implications for understanding AF pathophysiology.

Keywords: Atrial fibrillation; Inward-rectifier K(+)-channel; Muscarinic receptor subtypes.

MeSH terms

  • Acetylcholine / pharmacology*
  • Atrial Fibrillation / metabolism*
  • Atrial Fibrillation / pathology
  • Cells, Cultured
  • Chronic Disease
  • Dose-Response Relationship, Drug
  • Heart Atria / metabolism
  • Heart Atria / pathology
  • Humans
  • Muscarinic Antagonists / pharmacology
  • Myocytes, Cardiac / drug effects
  • Myocytes, Cardiac / physiology*
  • Potassium Channels, Inwardly Rectifying / biosynthesis*
  • Receptor, Muscarinic M1 / antagonists & inhibitors
  • Receptor, Muscarinic M1 / biosynthesis*
  • Up-Regulation / drug effects
  • Up-Regulation / physiology*


  • Muscarinic Antagonists
  • Potassium Channels, Inwardly Rectifying
  • Receptor, Muscarinic M1
  • Acetylcholine