Cellular and Molecular Mechanisms of Atrial Arrhythmogenesis in Patients With Paroxysmal Atrial Fibrillation

Abstract

Background—: Electrical, structural, and Ca 2+ -handling remodeling contribute to the perpetuation/progression of atrial fibrillation (AF). Recent evidence has suggested a role for spontaneous sarcoplasmic reticulum Ca 2+ -release events in long-standing persistent AF, but the occurrence and mechanisms of sarcoplasmic reticulum Ca 2+ -release events in paroxysmal AF (pAF) are unknown. Method and Results—: Right-atrial appendages from control sinus rhythm patients or patients with pAF (last episode a median of 10–20 days preoperatively) were analyzed with simultaneous measurements of [Ca 2+ ] i (fluo-3-acetoxymethyl ester) and membrane currents/action potentials (patch-clamp) in isolated atrial cardiomyocytes, and Western blot. Action potential duration, L-type Ca 2+ current, and Na + /Ca 2+ -exchange current were unaltered in pAF, indicating the absence of AF-induced electrical remodeling. In contrast, there were increases in SR Ca 2+ leak and incidence of delayed after-depolarizations in pAF. Ca 2+ -transient amplitude and sarcoplasmic reticulum Ca 2+ load (caffeine-induced Ca 2+ -transient amplitude, integrated Na + /Ca 2+ -exchange current) were larger in pAF. Ca 2+ -transient decay was faster in pAF, but the decay of caffeine-induced Ca 2+ transients was unaltered, suggesting increased SERCA2a function. In agreement, phosphorylation (inactivation) of the SERCA2a-inhibitor protein phospholamban was increased in pAF. Ryanodine receptor fractional phosphorylation was unaltered in pAF, whereas ryanodine receptor expression and single-channel open probability were increased. A novel computational model of the human atrial cardiomyocyte indicated that both ryanodine receptor dysregulation and enhanced SERCA2a activity promote increased sarcoplasmic reticulum Ca 2+ leak and sarcoplasmic reticulum Ca 2+ -release events, causing delayed after-depolarizations/triggered activity in pAF. Conclusions—: Increased diastolic sarcoplasmic reticulum Ca 2+ leak and related delayed after-depolarizations/triggered activity promote cellular arrhythmogenesis in pAF patients. Biochemical, functional, and modeling studies point to a combination of increased sarcoplasmic reticulum Ca 2+ load related to phospholamban hyperphosphorylation and ryanodine receptor dysregulation as underlying mechanisms.