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Increased late sodium current (late INa) is an important arrhythmogenic trigger in cardiac disease. It prolongs cardiac action potential and leads to an increased SR Ca2+ leak. This study investigates the contribution of Ca2+/Calmodulin-dependent kinase II (CaMKII), protein kinase A (PKA) and conversely acting protein phosphatases 1 and 2A (PP1, PP2A) to this subcellular crosstalk. Augmentation of late INa (ATX-II) in murine cardiomyocytes led to an increase of diastolic Ca2+ spark frequency and amplitudes of Ca2+ transients but did not affect SR Ca2+ load. Interestingly, inhibition of both, CaMKII and PKA, attenuated the late INa-dependent induction of the SR Ca2+ leak. PKA inhibition additionally reduced the amplitudes of systolic Ca2+ transients. FRET-measurements revealed increased levels of cAMP upon late INa augmentation, which could be prevented by simultaneous inhibition of Na+/Ca2+-exchanger (NCX) suggesting that PKA is activated by Ca2+-dependent cAMP-production. Whereas inhibition of PP2A showed no effect on late INa-dependent alterations of Ca2+ cycling, additional inhibition of PP1 further increased the SR Ca2+ leak. In line with this, selective activation of PP1 yielded a strong reduction of the late INa-induced SR Ca2+ leak and did not affect systolic Ca2+ release. This study indicates that phosphatase/kinase-balance is perturbed upon increased Na+ influx leading to disruption of ventricular Ca2+ cycling via CaMKII- and PKA-dependent pathways. Importantly, an activation of PP1 at RyR2 may represent a promising new toehold to counteract pathologically increased kinase activity. [ABSTRACT FROM AUTHOR]