Aims The EMPA‐REG OUTCOME study showed reduced mortality and hospitalization due to heart failure (HF) in diabetic patients treated with empagliflozin. Overexpression and Ca²⁺‐dependent activation of Ca²⁺/calmodulin‐dependent kinase II (CaMKII) are hallmarks of HF, leading to contractile dysfunction and arrhythmias. We tested whether empagliflozin reduces CaMKII‐ activity and improves Ca²⁺‐handling in human and murine ventricular myocytes. Methods and results Myocytes from wild‐type mice, mice with transverse aortic constriction (TAC) as a model of HF, and human failing ventricular myocytes were exposed to empagliflozin (1 μmol/L) or vehicle. CaMKII activity was assessed by CaMKII–histone deacetylase pulldown assay. Ca²⁺ spark frequency (CaSpF) as a measure of sarcoplasmic reticulum (SR) Ca²⁺ leak was investigated by confocal microscopy. [Na⁺]_i was measured using Na⁺/Ca²⁺‐exchanger (NCX) currents (whole‐cell patch clamp). Compared with vehicle, 24 h empagliflozin exposure of murine myocytes reduced CaMKII activity (1.6 ± 0.7 vs. 4.2 ± 0.9, P < 0.05, n = 10 mice), and also CaMKII‐dependent ryanodine receptor phosphorylation (0.8 ± 0.1 vs. 1.0 ± 0.1, P < 0.05, n = 11 mice), with similar results upon TAC. In murine myocytes, empagliflozin reduced CaSpF (TAC: 1.7 ± 0.3 vs. 2.5 ± 0.4 1/100 μm⁻¹ s⁻¹, P < 0.05, n = 4 mice) but increased SR Ca²⁺ load and Ca²⁺ transient amplitude. Importantly, empagliflozin also significantly reduced CaSpF in human failing ventricular myocytes (1 ± 0.2 vs. 3.3 ± 0.9, P < 0.05, n = 4 patients), while Ca²⁺ transient amplitude was increased (F/F₀: 0.53 ± 0.05 vs. 0.36 ± 0.02, P < 0.05, n = 3 patients). In contrast, 30 min exposure with empagliflozin did not affect CaMKII activity nor Ca²⁺‐handling but significantly reduced [Na⁺]_i. Conclusions We show for the first time that empagliflozin reduces CaMKII activity and CaMKII‐dependent SR Ca²⁺ leak. Reduced Ca²⁺ leak and improved Ca²⁺ transients may contribute to the beneficial effects of empagliflozin in HF.

Aims Recent clinical trials have proven gliflozins to be cardioprotective in diabetic and non‐diabetic patients. However, the underlying mechanisms are incompletely understood. A potential inhibition of cardiac Na⁺/H⁺ exchanger 1 (NHE1) has been suggested in animal models. We investigated the effect of empagliflozin on NHE1 activity in human atrial cardiomyocytes. Methods and results Expression of NHE1 was assessed in human atrial and ventricular tissue via western blotting. NHE activity was measured as the maximal slope of pH recovery after NH₄⁺ pulse in isolated carboxy‐seminaphtarhodafluor 1 (SNARF1)‐acetoxymethylester‐loaded murine ventricular and human atrial cardiomyocytes. NHE1 is abundantly expressed in human atrial and ventricular tissue. Interestingly, compared with patients without heart failure (HF), atrial NHE1 expression was significantly increased in patients with HF with preserved ejection fraction and atrial fibrillation. The largest increase in atrial and ventricular NHE1 expression, however, was observed in patients with end‐stage HF undergoing heart transplantation. Importantly, acute exposure to empagliflozin (1 μmol/L, 10 min) significantly inhibited NHE activity to a similar extent in human atrial myocytes and mouse ventricular myocytes. This inhibition was also achieved by incubation with the well‐described selective NHE inhibitor cariporide (10 μmol/L, 10 min). Conclusions This is the first study systematically analysing NHE1 expression in human atrial and ventricular myocardium of HF patients. We show that empagliflozin inhibits NHE in human cardiomyocytes. The extent of NHE inhibition was comparable with cariporide and may potentially contribute to the improved outcome of patients in clinical trials.

Sodium-glucose co-transporter 2 inhibitors (SGLT2i) are emerging as a new treatment strategy for heart failure with reduced ejection fraction (HFrEF) and—depending on the wistfully awaited results of two clinical trials (DELIVER and EMPEROR-Preserved)—may be the first drug class to improve cardiovascular outcomes in patients suffering from heart failure with preserved ejection fraction (HFpEF). Proposed mechanisms of action of this class of drugs are diverse and include metabolic and hemodynamic effects as well as effects on inflammation, neurohumoral activation, and intracellular ion homeostasis. In this review we focus on the growing body of evidence for SGLT2i-mediated effects on cardiac intracellular Na⁺ as an upstream mechanism. Therefore, we will first give a short overview of physiological cardiomyocyte Na⁺ handling and its deterioration in heart failure. On this basis we discuss the salutary effects of SGLT2i on Na⁺ homeostasis by influencing NHE1 activity, late I_Na as well as CaMKII activity. Finally, we highlight the potential relevance of these effects for systolic and diastolic dysfunction as well as arrhythmogenesis.

Rationale: Sleep-disordered breathing (SDB) is frequently associated with atrial arrhythmias. Increased CaMKII (Ca/calmodulin-dependent protein kinase II) activity has been previously implicated in atrial arrhythmogenesis. Objective: We hypothesized that CaMKII-dependent dysregulation of Na current (I _Na ) may contribute to atrial proarrhythmic activity in patients with SDB. Methods and Results: We prospectively enrolled 113 patients undergoing elective coronary artery bypass grafting for cross-sectional study and collected right atrial appendage biopsies. The presence of SDB (defined as apnea-hypopnea index ≥15/h) was assessed with a portable SDB monitor the night before surgery. Compared with 56 patients without SDB, patients with SDB (57) showed a significantly increased level of activated CaMKII. Patch clamp was used to measure I _Na . There was a significantly enhanced late I _Na , but reduced peak I _Na due to enhanced steady-state inactivation in atrial myocytes of patients with SDB consistent with significantly increased CaMKII-dependent cardiac Na channel phosphorylation (Na _V 1.5, at serine 571, Western blotting). These gating changes could be fully reversed by acute CaMKII inhibition (AIP [autocamtide-2 related inhibitory peptide]). As a consequence, we observed significantly more cellular afterdepolarizations and more severe premature atrial contractions in atrial trabeculae of patients with SDB, which could be blocked by either AIP or KN93 (N-[2-[[[(E)-3-(4-chlorophenyl)prop-2-enyl]-methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide). In multivariable linear regression models incorporating age, sex, body mass index, existing atrial fibrillation, existing heart failure, diabetes mellitus, and creatinine levels, apnea-hypopnea index was independently associated with increased CaMKII activity, enhanced late I _Na and correlated with premature atrial contraction severity. Conclusions: In atrial myocardium of patients with SDB, increased CaMKII-dependent phosphorylation of Na _V 1.5 results in dysregulation of I _Na with proarrhythmic activity that was independent from preexisting comorbidities. Inhibition of CaMKII may be useful for prevention or treatment of arrhythmias in SDB. Clinical Trial Registration: URL: http://www.clinicaltrials.gov . Unique identifier: NCT02877745. Visual Overview: An online visual overview is available for this article.

Rationale: CaMKII (Ca ²⁺ -Calmodulin dependent protein kinase) δC activation is implicated in pathological progression of heart failure (HF) and CaMKIIδC transgenic mice rapidly develop HF and arrhythmias. However, little is known about early spatio-temporal Ca ²⁺ handling and CaMKII activation in hypertrophy and HF. Objective: To measure time- and location-dependent activation of CaMKIIδC signaling in adult ventricular cardiomyocytes, during transaortic constriction (TAC) and in CaMKIIδC transgenic mice. Methods and Results: We used human tissue from nonfailing and HF hearts, 4 mouse lines: wild-type, KO (CaMKIIδ-knockout), CaMKIIδC transgenic in wild-type (TG), or KO background, and wild-type mice exposed to TAC. Confocal imaging and biochemistry revealed disproportional CaMKIIδC activation and accumulation in nuclear and perinuclear versus cytosolic regions at 5 days post-TAC. This CaMKIIδ activation caused a compensatory increase in sarcoplasmic reticulum Ca ²⁺ content, Ca ²⁺ transient amplitude, and [Ca ²⁺ ] decline rates, with reduced phospholamban expression, all of which were most prominent near and in the nucleus. These early adaptive effects in TAC were entirely mimicked in young CaMKIIδ TG mice (6–8 weeks) where no overt cardiac dysfunction was present. The (peri)nuclear CaMKII accumulation also correlated with enhanced HDAC4 (histone deacetylase) nuclear export, creating a microdomain for transcriptional regulation. At longer times both TAC and TG mice progressed to overt HF (at 45 days and 11–13 weeks, respectively), during which time the compensatory Ca ²⁺ transient effects reversed, but further increases in nuclear and time-averaged [Ca ²⁺ ] and CaMKII activation occurred. CaMKIIδ TG mice lacking δB exhibited more severe HF, eccentric myocyte growth, and nuclear changes. Patient HF samples also showed greatly increased CaMKIIδ expression, especially for CaMKIIδC in nuclear fractions. Conclusions: We conclude that in early TAC perinuclear CaMKIIδC activation promotes adaptive increases in myocyte Ca ²⁺ transients and nuclear transcriptional responses but that chronic progression of this nuclear Ca ²⁺ -CaMKIIδC axis contributes to eccentric hypertrophy and HF.