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Aims: Enhanced cardiac late Na-current (late INa) and increased sarcoplasmic reticulum (SR) Ca2+-leak are both highly arrhythmogenic. This study seeks to identify signalling pathways interconnecting late INa and SR-Ca2+-leak in atrial cardiomyocytes (CMs). Methods and Results: In murine atrial CMs SR-Ca2+-leak was increased by the late INa enhancer ATX-II. An inhibition of Ca2+/calmodulin-dependent protein-kinase II (AIP), Protein kinase A (H89), or late INa (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 INa-dependent hyperphosphorylation of CaMKII- as well as PKA target-sites at RyR2 (-S2815, -S2809) and PLB (-Thr17, -S16). Enhancement of late INa did not alter Ca2+-transient amplitude or SR-Ca2+-load. However, upon late INa 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 INa, CaMKII, or PKA reduced the SR-Ca2+-leak. Conclusion: Late INa exerts distinct effects on Ca2+-homeostasis in atrial myocardium through activation of CaMKII and PKA. Inhibition of late INa 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 INa-inhibition. [ABSTRACT FROM AUTHOR]

Aims: Atrial fibrillation (AF) is a commonly occurring arrhythmia after cardiac surgery (postoperative AF, poAF) and is associated with poorer outcomes. Considering that reduced atrial contractile function is a predictor of poAF and that Ca2+ plays an important role in both excitation-contraction coupling and atrial arrhythmogenesis, this study aims to test whether alterations of intracellular Ca2+ handling contribute to impaired atrial contractility and to the arrhythmogenic substrate predisposing patients to poAF., Methods and Results: Right atrial appendages were obtained from patients in sinus rhythm undergoing open-heart surgery. Cardiomyocytes were investigated by simultaneous measurement of [Ca2+]i and action potentials (APs, patch-clamp). Patients were followed-up for 6 days to identify those with and without poAF. Speckle-tracking analysis of preoperative echocardiography revealed reduced left atrial contraction strain in poAF patients. At the time of surgery, cellular Ca2+ transients (CaTs) and the sarcoplasmic reticulum (SR) Ca2+ content were smaller in the poAF group. CaT decay was slower in poAF, but the decay of caffeine-induced Ca2+ transients was unaltered, suggesting preserved sodium-calcium exchanger function. In agreement, western blots revealed reduced SERCA2a expression in poAF patients but unaltered phospholamban expression/phosphorylation. Computational modelling indicated that reduced SERCA activity promotes occurrence of CaT and AP alternans. Indeed, alternans of CaT and AP occurred more often and at lower stimulation frequencies in atrial myocytes from poAF patients. Resting membrane potential and AP duration were comparable between both groups at various pacing frequencies (0.25-8 Hz)., Conclusions: Biochemical, functional, and modelling data implicate reduced SERCA-mediated Ca2+ reuptake into the SR as a major contributor to impaired preoperative atrial contractile function and to the pre-existing arrhythmogenic substrate in patients developing poAF., (© The Author(s) 2020. Published by Oxford University Press on behalf of the European Society of Cardiology.)