Enhanced Sarcoplasmic Reticulum Ca ²⁺ Leak and Increased Na ⁺ -Ca ²⁺ Exchanger Function Underlie Delayed Afterdepolarizations in Patients With Chronic Atrial Fibrillation

Abstract

Background Delayed afterdepolarizations (DADs) carried by Na+-Ca2+-exchange current (INCX) in response to sarcoplasmic reticulum (SR) Ca2+-leak can promote atrial fibrillation (AF). The mechanisms leading to DADs in AF-patients have not been defined. Methods and Results Protein levels (Western-blot), membrane-currents and action-potentials (patch-clamp), and [Ca2+]i (Fluo-3) were measured in right-atrial samples from 77 sinus-rhythm (Ctl) and 69 chronic-AF (cAF) patients. Diastolic [Ca2+]i and SR-Ca2+-content (integrated INCX during caffeine-induced-Ca2+-transient [cCaT]) were unchanged, whereas diastolic SR Ca2+-leak, estimated by blocking RyR2 with tetracaine, was ~50% higher in cAF vs. Ctl. Single-channel recordings from atrial RyR2 reconstituted into lipid-bilayers revealed enhanced open-probability in cAF-samples, providing a molecular basis for increased SR Ca2+-leak. Calmodulin-expression (+60%), CaMKII-autophosphorylation at Thr287 (+40%) and RyR2-phosphorylation at Ser2808 (PKA/CaMKII-site, +236%) and Ser2814 (CaMKII-site, +77%) were increased in cAF. The selective CaMKII-blocker KN-93 decreased SR Ca2+-leak, the frequency of spontaneous Ca2+-release events and RyR2 open-probability in cAF, whereas PKA-inhibition with H-89 was ineffective. Knock-in mice with constitutively-phosphorylated RyR2 at Ser2814 showed a higher incidence of Ca2+-sparks and increased susceptibility to pacing-induced AF vs. controls. The relationship between [Ca2+]i and INCX-density revealed INCX-upregulation in cAF. Spontaneous Ca2+-release events accompanied by inward INCX-currents and DADs/triggered-activity occurred more often and the sensitivity of resting membrane voltage to elevated [Ca2+]i (diastolic [Ca2+]i–voltage coupling gain) was higher in cAF vs. Ctl. Conclusions Enhanced SR Ca2+-leak through CaMKII-hyperphosphorylated RyR2, in combination with larger INCX for a given SR Ca2+-release and increased diastolic [Ca2+]i–voltage coupling gain, cause AF-promoting atrial DADs/triggered-activity in cAF patients.

Arrhythmias, elicited by catecholamines and serotonin, vanish in human chronic atrial fibrillation

“…S7B). Increased phosphorylation of RyR2 channels at Ser-2808, catalyzed by PKA (11,12), and Ser-2814, catalyzed by CaMKII, has been implicated in the arrhythmogenic release of Ca 2+ through leaky RyR2 channels (13,20); however, in contrast to previous reports, neither basal phosphorylation (SI Appendix, Figs. S6 and S7) nor phosphorylation stimulated by agonists of RyR2 at Ser-2808 ( Fig.…”

2016 ESC Guidelines for the management of atrial fibrillation developed in collaboration with EACTS

“…Natomiast strukturalna choroba serca powoduje na ogół wydłużenie okresu refrakcji w przedsionkach, co ilustruje niejednorodny charakter mechanizmów wywołujących AF u różnych pacjentów [96]. Hiperfosforylacja różnych białek uczestniczących w gospodarce jonów wapniowych może się przyczyniać do zwiększenia incydentów samoistnego uwalniania jonów wapnia oraz wyzwalanej aktywności [97,98], wywołując w ten sposób ektopię i sprzyjając AF. Mimo że koncepcję niestabilności gospodarki jonów wapniowych ostatnio zakwestionowano [106,107], to czynnik ten może leżeć u podłoża AF w przedsionkach, które uległy przebudowie strukturalnej, i może tłumaczyć, w jaki sposób zmiany napięcia układu autonomicznego mogą generować AF [80,105].…”

2016 ESC Guidelines for the management of atrial fibrillation developed in collaboration with EACTS

“…94,95 Structural heart disease, in contrast, tends to prolong the atrial refractory period, illustrating the heterogeneous nature of mechanisms that cause AF in different patients. 96 Hyperphosphorylation of various Ca 2+ -handling proteins may contribute to enhanced spontaneous Ca 2+ release events and triggered activity, 97,98 thus causing ectopy and promoting AF. Although the concept of Ca 2+ -handling instability has been challenged recently, 106,107 it may mediate AF in structurally remodelled atria and explain how altered autonomic tone can generate AF.…”