Background—Congestive heart failure (CHF) is frequently associated with atrial fibrillation (AF), but little is known about the effects of CHF on atrial cellular electrophysiology.

Methods and Results—We studied action potential (AP) properties and ionic currents in atrial myocytes from dogs with CHF induced by ventricular pacing at 220 to 240 bpm for 5 weeks. Atrial myocytes from CHF dogs were hypertrophied (mean±SEM capacitance, 89±2 pF versus 71±2 pF in control, n=160 cells per group, P<0.001). CHF significantly reduced the density of L-type Ca²⁺ current (I_Ca) by ≈30%, of transient outward K⁺ current (I_to) by ≈50%, and of slow delayed rectifier current (I_Ks) by ≈30% without altering their voltage dependencies or kinetics. The inward rectifier, ultrarapid and rapid delayed rectifier, and T-type Ca²⁺ currents were not altered by CHF. CHF increased transient inward Na⁺/Ca²⁺ exchanger (NCX) current by ≈45%. The AP duration of atrial myocytes was not altered by CHF at slow rates but was increased at faster rates, paralleling in vivo refractory changes. CHF created a substrate for AF, prolonging mean AF duration from 8±4 to 535±82 seconds (P<0.01).

Conclusions—Experimental CHF selectively decreases atrial I_to, I_Ca, and I_Ks, increases NCX current, and leaves other currents unchanged. The cellular electrophysiological remodeling caused by CHF is quite distinct from that caused by atrial tachycardia, highlighting important differences in the cellular milieu characterizing different clinically relevant AF substrates.