Late INa increases diastolic SR-Ca2+-leak in atrial myocardium by activating PKA and CaMKII.

Aims: Enhanced cardiac late Na-current (late I_Na) and increased sarcoplasmic reticulum (SR) Ca²⁺-leak are both highly arrhythmogenic. This study seeks to identify signalling pathways interconnecting late I_Na and SR-Ca²⁺-leak in atrial cardiomyocytes (CMs). Methods and Results: In murine atrial CMs SR-Ca²⁺-leak was increased by the late I_Na enhancer ATX-II. An inhibition of Ca²⁺/calmodulin-dependent protein-kinase II (AIP), Protein kinase A (H89), or late I_Na (Ranolazine or Tetrodotoxin) all prevented ATX-II-dependent SR-Ca²⁺-leak. The SR-Ca²⁺-leak induction by ATX-II was neither detected when the Na+/Ca²⁺-exchanger was inhibited (KBR) nor in CaMKIIdc-knockout mice. FRET-measurements revealed increased cAMP-levels upon ATX-II stimulation, which could be prevented by inhibition of adenylylcyclase 5 and 6 (NKY 80) but not by inhibition of phosphodiesterases (IBMX), suggesting PKA-activation via an adenylylcyclase-dependent increase of cAMP-levels. Western blots showed late I_Na-dependent hyperphosphorylation of CaMKII- as well as PKA target-sites at RyR2 (-S2815, -S2809) and PLB (-Thr17, -S16). Enhancement of late I_Na did not alter Ca²⁺-transient amplitude or SR-Ca²⁺-load. However, upon late I_Na activation and simultaneous CaMKII inhibition, Ca²⁺-transient amplitude and SR-Ca²⁺-load were increased whereas PKA-inhibition reduced Ca²⁺ transient amplitude and load and slowed Ca²⁺-elimination. In atrial CMs from patients with atrial fibrillation, inhibition of late I_Na, CaMKII, or PKA reduced the SR-Ca²⁺-leak. Conclusion: Late I_Na exerts distinct effects on Ca²⁺-homeostasis in atrial myocardium through activation of CaMKII and PKA. Inhibition of late I_Na represents a potential approach to attenuate CaMKII-activation and decrease SR-Ca²⁺-leak in atrial rhythm disorders. The interconnection with the cAMP/PKA-system further increases the antiarrhythmic potential of late I_Na-inhibition. [ABSTRACT FROM AUTHOR]