Enhanced late INa induces proarrhythmogenic SR Ca leak in a CaMKII-dependent manner

Abstract

Objective: Enhanced late Na current (late I-Na) induces Na-dependent Ca overload as well as proarrhythmogenic events on the cellular level that include spatio-temporally uncoordinated diastolic Ca release from the sarcoplasmic reticulum (SR) and delayed afterdepolarizations (DADs). The Ca/calmodulin-dependent protein kinase II (CaMKII) gets activated upon increases in [Ca](1) and mediates diastolic SR Ca leak as well as DADs. Rationale: We hypothesized that increased late I-Na (in disease-comparable ranges) exerts proarrhythmogenic events in isolated ventricular mouse myocytes in a manner depending on CaMKII-dependent SR Ca leak. We further tested whether inhibition of disease-related late I-Na may reduce proarrhythmogenic SR Ca leak in myocytes from failing human hearts. Methods: Ventricular myocytes were isolated from healthy wildtype (WT), failing CaMKII delta c transgenic (TG) mouse, and failing human hearts. ATX-II (0.25-10 nmol/L) was used to enhance late I-Na. Spontaneous Ca loss from the SR during diastole (Ca sparks), DADs, non-triggered diastolic Ca transients in myocytes and premature beats of isometrically twitching papillary muscles were used as readouts for proarrhythmogenic events. CaMKII autophosphorylation was assessed by immunoblots. Late I-Na was inhibited using ranolazine (Ran, 10 mu mol/L) or 'FIX (2 mu mol/L), and CaMKII by KN-93 (1 mu mol/L) or ALP (1 mu mol/L). Results: In WTmyocytes, sub-nanomolar ATX-II exposure (0.5 nmol/L) enhanced late I-Na by -60%, which resulted in increased diastolic SR Ca loss despite unaltered SR Ca content. In parallel, DADs and non-triggered diastolic Ca transients arose. Inhibition of enhanced late I-Na by RAN or TTX significantly attenuated diastolic SR Ca loss and suppressed DADs as well as mechanical altemans in mouse and diastolic SR Ca loss in failing human myocytes. ATX-II caused Ca-dependent CaMKII-activation without changes in protein expression, which was reversible by Ran or AIP. Conversely, CaMKII-inhibition decreased diastolic SR Ca loss, DADs and non-triggered diastolic Ca transients despite ATX-II-exposure. Finally, failing mouse myocytes with increased CaMKII activity (TG CaMKII delta c) showed an even aggravated diastolic SR Ca loss that was associated with an increased frequency of non-triggered diastolic Ca transients upon enhanced late I-Na. Conclusions: Increased late 'Na (in disease-comparable ranges) induces proarrhythmogenic events during diastole in healthy and failing mouse myocytes, which are mediated via CaMKII-dependent SR Ca loss. Inhibition of late I-Na not only attenuated these cellular arrhythmias in mouse myocytes but also in failing human myocytes indicating some antiarrhythmic potential for an inhibition of the elevated late I-Na/CaMKII signaling pathway in this setting. (c) 2014 Elsevier Ltd. All rights reserved.