CaMKII-Dependent Diastolic SR Ca 2+ Leak and Elevated Diastolic Ca 2+ Levels in Right Atrial Myocardium of Patients With Atrial Fibrillation

Abstract

Rationale : Although research suggests that diastolic Ca 2+ levels might be increased in atrial fibrillation (AF), this hypothesis has never been tested. Diastolic Ca 2+ leak from the sarcoplasmic reticulum (SR) might increase diastolic Ca 2+ levels and play a role in triggering or maintaining AF by transient inward currents through Na + /Ca 2+ exchange. In ventricular myocardium, ryanodine receptor type 2 (RyR2) phosphorylation by Ca 2+ /calmodulin-dependent protein kinase (CaMK)II is emerging as an important mechanism for SR Ca 2+ leak. Objective : We tested the hypothesis that CaMKII-dependent diastolic SR Ca 2+ leak and elevated diastolic Ca 2+ levels occurs in atrial myocardium of patients with AF. Methods and Results : We used isolated human right atrial myocytes from patients with AF versus sinus rhythm and found CaMKII expression to be increased by 40±14% ( P <0.05), as well as CaMKII phosphorylation by 33±12% ( P <0.05). This was accompanied by a significantly increased RyR2 phosphorylation at the CaMKII site (Ser2814) by 110±53%. Furthermore, cytosolic Ca 2+ levels were elevated during diastole (229±20 versus 164±8 nmol/L, P <0.05). Most likely, this resulted from an increased SR Ca 2+ leak in AF ( P <0.05), which was not attributable to higher SR Ca 2+ load. Tetracaine experiments confirmed that SR Ca 2+ leak through RyR2 leads to the elevated diastolic Ca 2+ level. CaMKII inhibition normalized SR Ca 2+ leak and cytosolic Ca 2+ levels without changes in L-type Ca 2+ current. Conclusion : Increased CaMKII-dependent phosphorylation of RyR2 leads to increased SR Ca 2+ leak in human AF, causing elevated cytosolic Ca 2+ levels, thereby providing a potential arrhythmogenic substrate that could trigger or maintain AF.