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

Background: Basal and acetylcholine-gated inward-rectifier K+-currents (I-K1 and I-K,I-ACh, respectively) are altered in atrial fibrillation (AF). G(i)-protein-coupled muscarinic (M) receptors type-2 are considered the predominant receptors activating I-K,I-ACh. Although a role for G(q)-coupled non-M-2-receptor subtypes has been suggested, the precise regulation of I-K,I-ACh by multiple M-receptor subtypes in the human atrium is unknown. Here, we investigated M-1-receptor-mediated I-K,I-ACh regulation and its remodeling in chronic AF (cAF). Methods and results: M-1-receptor mRNA and protein abundance were increased in atrial cardiomyocyte fractions and atrial homogenates from cAF patients, whereas M-2-receptor levels were unchanged. The regulation of I-K,I-ACh by M-1-receptors was investigated in right-atrial cardiomyocytes using two applications of the M-receptor agonist carbachol (CCh, 2 mu M), with pharmacological interventions during the second application. CCh application produced a rapid current increase (Peak-I-K,I-ACh), which declined to a quasi-steady-state level (Qss-I-K,I-ACh). In sinus rhythm (Ctl) the selective M-1-receptor antagonists pirenzepine (10 nM) and muscarinic toxin-7 (MT-7, 10 nM) significantly inhibited CCh-activated Peak-IK, ACh, whereas in cAF they significantly reduced both Peak- and Qss-I-K,I-ACh, with no effects on basal inward-rectifier currents in either group. Conversely, the selective M-1-receptor agonist McN-A-343 (100 mu M) induced a current similar to the CCh-activated current in Ctl atrial cardiomyocytes pretreated with pertussis toxin to inhibit M-2-receptor-mediated G(i)-protein signaling, which was abolished by MT-7. Computational modeling indicated that M-1- and M-2-receptors redundantly activate I-K,I-ACh to abbreviate APD, albeit with predominant effects of M-2-receptors. Conclusion: Our data suggest that G(q)-coupledM(1)-receptors also regulate human atrial I-K,I-ACh and that their relative contribution to I-K,I-ACh activation is increased in cAF patients. We provide novel insights about the role of non-M-2-receptors in human atrial cardiomyocytes, which may have important implications for understanding AF pathophysiology. (c) 2017 Elsevier B.V. All rights reserved.