Cellular and molecular mechanisms of atrial arrhythmogenesis in patients with paroxysmal atrial fibrillation

Abstract

Background-Electrical, structural, and Ca2+-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 Ca2+-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 [Ca2+]i (fluo-3-acetoxymethyl ester) and membrane currents/action potentials (patch-clamp) in isolated atrial cardiomyocytes, and Western blot. Action potential duration, L-type Ca2+ 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 Ca2+ leak and incidence of delayed afterdepolarizations in pAF. Ca2+-transient amplitude and sarcoplasmic reticulum Ca2+ load (caffeine-induced Ca2+-transient amplitude, integrated Na+/Ca2+-exchange current) were larger in pAF. Ca2+- 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 Ca2+ leak and sarcoplasmic reticulum Ca2+- release events, causing delayed after-depolarizations/triggered activity in pAF. Conclusions-Increased diastolic sarcoplasmic reticulum Ca2+ 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 Ca2+ load related to phospholamban hyperphosphorylation and ryanodine receptor dysregulation as underlying mechanisms. © 2013 American Heart Association, Inc.