Rationale: Although research suggests that diastolic Ca²⁺ levels might be increased in atrial fibrillation (AF), this hypothesis has never been tested. Diastolic Ca²⁺ leak from the sarcoplasmic reticulum (SR) might increase diastolic Ca²⁺ levels and play a role in triggering or maintaining AF by transient inward currents through Na⁺/Ca²⁺ exchange. In ventricular myocardium, ryanodine receptor type 2 (RyR2) phosphorylation by Ca²⁺/calmodulin-dependent protein kinase (CaMK)II is emerging as an important mechanism for SR Ca²⁺ leak.

Objective: We tested the hypothesis that CaMKII-dependent diastolic SR Ca²⁺ leak and elevated diastolic Ca²⁺ 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 Ca²⁺ levels were elevated during diastole (229±20 versus 164±8 nmol/L, P<0.05). Most likely, this resulted from an increased SR Ca²⁺ leak in AF (P<0.05), which was not attributable to higher SR Ca²⁺ load. Tetracaine experiments confirmed that SR Ca²⁺ leak through RyR2 leads to the elevated diastolic Ca²⁺ level. CaMKII inhibition normalized SR Ca²⁺ leak and cytosolic Ca²⁺ levels without changes in L-type Ca²⁺ current.

Conclusion: Increased CaMKII-dependent phosphorylation of RyR2 leads to increased SR Ca²⁺ leak in human AF, causing elevated cytosolic Ca²⁺ levels, thereby providing a potential arrhythmogenic substrate that could trigger or maintain AF.