Left-to-right atrial inward rectifier potassium current gradients in patients with paroxysmal versus chronic atrial fibrillation

Abstract

Background: Recent evidence suggests that atrial fibrillation (AF) is maintained by high-frequency reentrant sources with a left-to-right-dominant frequency gradient, particularly in patients with paroxysmal AF (pAF). Unequal left-to-right distribution of inward rectifier K(+) currents has been suggested to underlie this dominant frequency gradient, but this hypothesis has never been tested in humans.

Methods and results: Currents were measured with whole-cell voltage-clamp in cardiomyocytes from right atrial (RA) and left (LA) atrial appendages of patients in sinus rhythm (SR) and patients with AF undergoing cardiac surgery. Western blot was used to quantify protein expression of I(K1) (Kir2.1 and Kir2.3) and I(K,ACh) (Kir3.1 and Kir3.4) subunits. Basal current was ≈2-fold larger in chronic AF (cAF) versus SR patients, without RA-LA differences. In pAF, basal current was ≈2-fold larger in LA versus RA, indicating a left-to-right atrial gradient. In both atria, Kir2.1 expression was ≈2-fold greater in cAF but comparable in pAF versus SR. Kir2.3 levels were unchanged in cAF and RA-pAF but showed a 51% decrease in LA-pAF. In SR, carbachol-activated (2 μmol/L) I(K,ACh) was 70% larger in RA versus LA. This right-to-left atrial gradient was decreased in pAF and cAF caused by reduced I(K,ACh) in RA only. Similarly, in SR, Kir3.1 and Kir3.4 proteins were greater in RA versus LA and decreased in RA of pAF and cAF. Kir3.1 and Kir3.4 expression was unchanged in LA of pAF and cAF.

Conclusion: Our results support the hypothesis that a left-to-right gradient in inward rectifier background current contributes to high-frequency sources in LA that maintain pAF. These findings have potentially important implications for development of atrial-selective therapeutic approaches.

Figure 1

A, Ramp protocol from −100 to +40 mV. B through D, Representative current recordings in RA and LA myocytes of SR, pAF, and cAF. Basal current and CCh-sensitive (2 μmol/L) current increase (I_K,ACh) were analyzed at inward (I_in) and outward (I_out) branches at −100 mV and −10 mV, respectively.

Figure 2

Inward rectifier currents in RA and LA myocytes from SR, pAF, and cAF at −100 mV and −10 mV, respectively. Values are mean±SEM. A, Basal current in absence of CCh (2 μmol/L). B, Total current (basal current+CCh-mediated current increase) in presence of CCh. Numbers indicate myocytes per patient. *P<0.05 and #P<0.05 versus corresponding values in SR and pAF, respectively. §P<0.05 versus corresponding values in RA.

Figure 3

Representative time course recordings of I_K,ACh in RA and LA myocytes from patients with SR (A), pAF (B), and cAF (C). Currents recorded during ramp protocol (Figure 1) were analyzed continuously (0.5 Hz) at −100 mV. I_K,ACh was activated with the muscarinic receptor agonist CCh (2 μmol/L) and was defined as CCh-sensitive current component. Despite the continuous presence of CCh during 2 minutes, the initial increase (Peak-I_K,ACh) faded to a quasi-steady-state level (QSS-I_K,ACh) caused by a process termed “desensitization.”

Figure 4

Carbachol-sensitive (2 μmol/L) current increase (I_K,ACh) in RA and LA myocytes from SR, pAF, and cAF at −100 mV and −10 mV, respectively. Values are mean± SEM of Peak (A) and quasi-steady-state (QSS, B) current levels. Numbers indicate myocytes per patient. *P<0.05 versus corresponding values in SR. §P<0.05 versus corresponding values in right atrium.

Figure 5

Effect of the I_K,ACh channel blocker tertiapin (10 nmol/L) on basal current in LA myocytes. LA basal current at −100 mV before (baseline) and during tertiapin application in SR and cAF. Numbers indicate myocytes per patient. *P<0.05 versus baseline.

Figure 6

Expression of I_K1 channel subunits in RA and LA from SR, pAF, and cAF. Representative immunoblots and densitometric analysis of Kir2.1 (A) and Kir2.3 (B) subunits. Numbers indicate tissue samples. *P<0.05 and #P<0.05 versus corresponding values in SR and pAF, respectively. §P<0.05 versus corresponding values in RA.

Figure 7

Expression of I_K,ACh channel subunits in RA and LA from SR, pAF, and cAF. Format as in Figure 6. *P<0.05 versus corresponding values in SR. §P<0.05 versus corresponding values in RA.