Abstract 1534: Structural and Functional Remodeling of the Coupling between Sarcolemmal Ca2+ Channels and the Ryanodine Receptor in Atrial Fibrillation
Background: Permanent atrial fibrillation (AF) in humans has been associated with structural and electrical remodelling. Data on contractile remodelling and control of sarcoplasmic reticulum (SR) Ca2+ release via the ryanodine receptor (RyR) in permanent AF are however limited.
Methods: Ewes were atrially paced at 600 bmp for a median of 23 weeks resulting in permanent AF (mean of 89.6±6 days, N=13) and compared to matched, non-instrumented control animals (CTRL, N=17). Atrial myocytes were isolated and cell shortening (field stimulation), membrane currents (whole cell voltage clamp) and [Ca2+]i (Fluo-3) were measured. Protein expression was analyzed by immunoblotting. T-tubule density was quantified from confocal Z-stack images of cells stained with di-8-ANEPPS. Data are shown as mean±SEM of at least 17 cells and 5 animals.
Results: Myocyte shortening and underlying [Ca2+]i transients were profoundly reduced in AF (by 54.8% and 62%, p<0.01). This reduced cell shortening could be corrected by increasing [Ca2+]i during caffeine-induced Ca2+ release from the SR (L/L0 9.7±0.5% in AF vs. 10.8±0.5% in CTRL). SR Ca2+ content (integrated Na+/Ca2+ exchange current during caffeine application) was not different (CTRL 1.9±0.3 pC/pF vs. AF 2±0.4 pC/pF), but calculated fractional SR Ca2+ release during a depolarizing step to +10 mV was reduced in AF (by 20.6%, p<0.05). Peak Ca2+ current density was modestly decreased (at <10 mV by 23.9%, p<0.01). T-tubules were present in the CTRL atrial myocytes though their density was much lower than in ventricular cells (11.4% vs. 20.6%, p<0.05). T-tubule density was robustly reduced in AF vs. CTRL (by 45%, p<0.01) with a reduction of myocyte surface:volume ratio (by 26%, p<0.01). The organization of RyR was apparently unchanged but protein expression was reduced (by 18.6%, p<0.05).
Conclusion: In permanent AF, reduced SR Ca2+ release is a major factor in the reduced cell contraction. Loss of T-tubules contributes to uncoupling of sarcolemmal Ca2+ channels to RyR and reduced fractional SR Ca2+ release despite preserved SR Ca2+ content.