Dissociation between ionic remodeling and ability to sustain atrial fibrillation during recovery from experimental congestive heart failure.

Abstract

Congestive heart failure (CHF) downregulates atrial transient outward (I(to)), slow delayed rectifier (I(Ks)), and L-type Ca(2+) (I(Ca,L)) currents and upregulates Na(+)-Ca(2+) exchange current (I(NCX)) (ionic remodeling) and causes atrial fibrosis (structural remodeling). The relative importance of ionic versus structural remodeling in CHF-related atrial fibrillation (AF) is controversial. We measured hemodynamic and echocardiographic parameters, mean duration of burst pacing-induced AF (DAF), and atrial-myocyte ionic currents in dogs with CHF induced by 2-week ventricular tachypacing (240 bpm), CHF dogs allowed to recover without pacing for 4 weeks (REC), and unpaced controls. Left ventricular ejection fraction averaged 58.6+/-1.2% (control), 36.2+/-2.3% (CHF, P<0.01), and 57.9+/-1.6% (REC), indicating full hemodynamic recovery. Similarly, left atrial pressures were 2.2+/-0.3 (control), 13.1+/-1.5 (CHF), and 2.4+/-0.4 (REC) mm Hg. CHF reduced I(to) density by approximately 65% (P<0.01), decreased I(Ca,L) density by approximately 50% (P<0.01), and diminished I(Ks) density by approximately 40% (P<0.01) while increasing I(NCX) density by approximately 110% (P<0.05). In REC, all ionic current densities returned to control values. DAF increased in CHF (1132+/-207 versus 14.3+/-8.8 seconds, control) and remained increased with REC (1014+/-252 seconds). Atrial fibrous tissue content also increased in CHF (2.1+/-0.2% for control versus 10.2+/-0.7% for CHF, P<0.01), with no recovery observed in REC (9.4+/-0.8%, P<0.01 versus control, P=NS versus CHF). With reversal of CHF, there is complete recovery of ionic remodeling, but the prolonged-AF substrate and structural remodeling remain. This suggests that structural, not ionic, remodeling is the primary contributor to AF maintenance in experimental CHF.