title: Enhanced sarcoplasmic reticulum Ca2+ leak and increased Na+-Ca2+ exchanger function underlie delayed afterdepolarizations in patients with chronic atrial fibrillation.
abstract: BACKGROUND
  Delayed afterdepolarizations (DADs) carried by Na(+)-Ca(2+)-exchange current (I(NCX)) in response to sarcoplasmic reticulum (SR) Ca(2+) leak can promote atrial fibrillation (AF). The mechanisms leading to delayed afterdepolarizations in AF patients have not been defined.
  
  
  RESULTS
  Protein levels (Western blot), membrane currents and action potentials (patch clamp), and [Ca(2+)](i) (Fluo-3) were measured in right atrial samples from 76 sinus rhythm (control) and 72 chronic AF (cAF) patients. Diastolic [Ca(2+)](i) and SR Ca(2+) content (integrated I(NCX) during caffeine-induced Ca(2+) transient) were unchanged, whereas diastolic SR Ca(2+) leak, estimated by blocking ryanodine receptors (RyR2) with tetracaine, was ≈50% higher in cAF versus control. Single-channel recordings from atrial RyR2 reconstituted into lipid bilayers revealed enhanced open probability in cAF samples, providing a molecular basis for increased SR Ca(2+) leak. Calmodulin expression (60%), Ca(2+)/calmodulin-dependent protein kinase-II (CaMKII) autophosphorylation at Thr287 (87%), and RyR2 phosphorylation at Ser2808 (protein kinase A/CaMKII site, 236%) and Ser2814 (CaMKII site, 77%) were increased in cAF. The selective CaMKII blocker KN-93 decreased SR Ca(2+) leak, the frequency of spontaneous Ca(2+) release events, and RyR2 open probability in cAF, whereas protein kinase A inhibition with H-89 was ineffective. Knock-in mice with constitutively phosphorylated RyR2 at Ser2814 showed a higher incidence of Ca(2+) sparks and increased susceptibility to pacing-induced AF compared with controls. The relationship between [Ca(2+)](i) and I(NCX) density revealed I(NCX) upregulation in cAF. Spontaneous Ca(2+) release events accompanied by inward I(NCX) currents and delayed afterdepolarizations/triggered activity occurred more often and the sensitivity of resting membrane voltage to elevated [Ca(2+)](i) (diastolic [Ca(2+)](i)-voltage coupling gain) was higher in cAF compared with control.
  
  
  CONCLUSIONS
  Enhanced SR Ca(2+) leak through CaMKII-hyperphosphorylated RyR2, in combination with larger I(NCX) for a given SR Ca(2+) release and increased diastolic [Ca(2+)](i)-voltage coupling gain, causes AF-promoting atrial delayed afterdepolarizations/triggered activity in cAF patients.
authors.affiliation: Division of Experimental Cardiology, Medical Faculty Mannheim, University of Heidelberg, Germany.
authors.forename: Niels
authors.initials: N
authors.lastname: Voigt
authors.forename: Na
authors.initials: N
authors.lastname: Li
authors.forename: Qiongling
authors.initials: Q
authors.lastname: Wang
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Trafford
authors.forename: Issam
authors.initials: I
authors.lastname: Abu-Taha
authors.forename: Qiang
authors.initials: Q
authors.lastname: Sun
authors.forename: Thomas
authors.initials: T
authors.lastname: Wieland
authors.forename: Ursula
authors.initials: U
authors.lastname: Ravens
authors.forename: Stanley
authors.initials: S
authors.lastname: Nattel
authors.forename: Xander H T
authors.initials: XH
authors.lastname: Wehrens
authors.forename: Dobromir
authors.initials: D
authors.lastname: Dobrev
mesh_terms: D000200:Action Potentials; D000368:Aged; D000818:Animals; D001281:Atrial Fibrillation / Q000378:metabolism / Q000503:physiopathology*; D001596:Benzylamines / Q000494:pharmacology; D001693:Biological Transport, Active; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2 / Q000378:metabolism; D002147:Calmodulin / Q000378:metabolism; D002908:Chronic Disease; D005260:Female; D055879:Gene Knock-In Techniques; D006801:Humans; D008051:Lipid Bilayers; D008297:Male; D008564:Membrane Potentials; D051379:Mice; D032383:Myocytes, Cardiac / Q000502:physiology; D018408:Patch-Clamp Techniques; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D019837:Ryanodine Receptor Calcium Release Channel / Q000235:genetics / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000502:physiology*; D013449:Sulfonamides / Q000494:pharmacology; D000096926:Benzenesulfonamides
chemical_list: D001596:Benzylamines; D002110:Caffeine; D002118:Calcium; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D002147:Calmodulin; D008051:Lipid Bilayers; D011993:Recombinant Fusion Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019831:Sodium-Calcium Exchanger; D013449:Sulfonamides; C110374:sodium-calcium exchanger 1; C072105:KN 93; D000096926:Benzenesulfonamides