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

Abstract

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. 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. 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. 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.