Abstract
BACKGROUND: Basal and acetylcholine-gated inward-rectifier K(+)-currents (I(K1) and I(K,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,ACh). Although a role for G(q)-coupled non-M(2)-receptor subtypes has been suggested, the precise regulation of I(K,ACh) by multiple M-receptor subtypes in the human atrium is unknown. Here, we investigated M(1)-receptor-mediated I(K,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,ACh) by M(1)-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-I(K,ACh)), which declined to a quasi-steady-state level (Qss-I(K,ACh)). In sinus rhythm (Ctl) the selective M(1)-receptor antagonists pirenzepine (10nM) and muscarinic toxin-7 (MT-7, 10nM) significantly inhibited CCh-activated Peak-I(K,ACh), whereas in cAF they significantly reduced both Peak- and Qss-I(K,ACh), with no effects on basal inward-rectifier currents in either group. Conversely, the selective M(1)-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 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,ACh) to abbreviate APD, albeit with predominant effects of M(2)-receptors. CONCLUSION: Our data suggest that G(q)-coupled M(1)-receptors also regulate human atrial I(K,ACh) and that their relative contribution to I(K,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.