Late I(Na) increases diastolic SR-Ca(2+)-leak in atrial myocardium by activating PKA and CaMKII

AIMS: Enhanced cardiac late Na current (late I(Na)) and increased sarcoplasmic reticulum (SR)-Ca(2+)-leak are both highly arrhythmogenic. This study seeks to identify signalling pathways interconnecting late I(Na) and SR-Ca(2+)-leak in atrial cardiomyocytes (CMs). METHODS AND RESULTS: In murine atrial CMs, SR-Ca(2+)-leak was increased by the late I(Na) enhancer Anemonia sulcata toxin II (ATX-II). An inhibition of Ca(2+)/calmodulin-dependent protein kinase II (Autocamide-2-related inhibitory peptide), protein kinase A (H89), or late I(Na) (Ranolazine or Tetrodotoxin) all prevented ATX-II-dependent SR-Ca(2+)-leak. The SR-Ca(2+)-leak induction by ATX-II was not detected when either the Na(+)/Ca(2+) exchanger was inhibited (KBR) or in CaMKIIδc-knockout mice. FRET measurements revealed increased cAMP levels upon ATX-II stimulation, which could be prevented by inhibition of adenylyl cyclases (ACs) 5 and 6 (NKY 80) but not by inhibition of phosphodiesterases (IBMX), suggesting PKA activation via an AC-dependent increase of cAMP levels. Western blots showed late I(Na)-dependent hyperphosphorylation of CaMKII as well as PKA target sites at ryanodine receptor type-2 (-S2814 and -S2808) and phospholamban (-Thr17, -S16). Enhancement of late I(Na) did not alter Ca(2+)-transient amplitude or SR-Ca(2+)-load. However, upon late I(Na) activation and simultaneous CaMKII inhibition, Ca(2+)-transient amplitude and SR-Ca(2+)-load were increased, whereas PKA inhibition reduced Ca(2+)-transient amplitude and load and additionally slowed Ca(2+) elimination. In atrial CMs from patients with atrial fibrillation, inhibition of late I(Na), CaMKII, or PKA reduced the SR-Ca(2+)-leak. CONCLUSION: Late I(Na) exerts distinct effects on Ca(2+) homeostasis in atrial myocardium through activation of CaMKII and PKA. Inhibition of late I(Na) represents a potential approach to attenuate CaMKII activation and decreases SR-Ca(2+)-leak in atrial rhythm disorders. The interconnection with the cAMP/PKA system further increases the antiarrhythmic potential of late I(Na) inhibition.