Alterations in ion channel expression and function known as "electrical remodeling" contribute to the development of hypertrophy and to the emergence of arrhythmias and sudden cardiac death. However, comparing current density values - an electrophysiological parameter commonly utilized to assess ion channel function - between normal and hypertrophied cells may be flawed when current amplitude does not scale with cell size. Even more, common routines to study equally sized cells or to discard measurements when large currents do not allow proper voltage-clamp control may introduce a selection bias and thereby confound direct comparison. To test a possible dependence of current density on cell size and shape, we employed whole-cell patch-clamp recording of voltage-gated sodium and calcium currents in Langendorff-isolated ventricular cardiomyocytes and Purkinje myocytes, as well as in cardiomyocytes derived from trans-aortic constriction operated mice. Here, we describe a distinct inverse relationship between voltage-gated sodium and calcium current densities and cell capacitance both in normal and hypertrophied cells. This inverse relationship was well fit by an exponential function and may be due to physiological adaptations that do not scale proportionally with cell size or may be explained by a selection bias. Our study emphasizes the need to consider cell size bias when comparing current densities in cardiomyocytes of different sizes, particularly in hypertrophic cells. Conventional comparisons based solely on mean current density may be inadequate for groups with unequal cell size or non-proportional current amplitude and cell size scaling.

Background: There is currently no therapy targeting cancer-related cardiomyopathy and treatment of heart failure in the oncological setting is nonspecific. Moreover, cancer is often assocaited with cachexia and the pathophysiology of cancer-cachexia induced cardiac (dys)functions are not fully known. Methods: Colon-26 adenocarcinoma (C26; n=30) or shIL-6 (C26 shIL-6; n=30) cells were inoculated subcutaneously into the flank of syngeneic adult male BALB/cmice, meanwhile control mice were injected with PBS (n=25). Twenty days after the cells injection, cardiac function was assessed using transthoracic echocardiography, ex vivo isolated working heart methods. In addition, intracellular Ca 2+ transient and force-calcium relationships were assessed in isolated single ventricular cardiomyocytes (CMs). Cardiac inflammation, metabolism and fibrosis were also assessed. Results: Despite that tumor size was comparable between the cancer groups, C26 group showed a loss of subcutaneous fat and skeletal muscle confirming a cachectic phenotype in association with an elevation of serum IL-6 levels. Tumor-bearing mice groups show a tendecy towards to both LV systolic and diastolic dysfunction. Sarcomere dysfunction, including significantly reduced maximum calcium-activated tension (Tmax) and increased calcium sensitivity (decreased EC 50 ) was found in skinned cardiomyocyte preparation from both tumor-bearing mice in compared to controls (p<0.05, respectively). Intracellular Ca 2+ transient was exclusively increased in CM isolated from cachectic mice, suggesting the SERCA2 upregulation. Infiltration of macrophage or T-cells, nor interstital fibrosis were difference. β-myosin heavy chain expression is upregulated in a cell autonomous fashion in C26 mice. Comprehensive energetic and metabolims analysis showed shift to a profound glucose metabolsim and reduction in fatty acid oxidation in LV samples. This was futher proven in h9c2 cells were incubated with C26 or shIL6 cell culture medium. Discussion: Our results suggest that LV dysfunction in cancer mice is associated with sarcomere dysfunction while additional abnormal intracellular Ca 2+ handling is solely present in mice with cachectic phenotype. These functional alterations are independent from changes in myocardial fibrosis and inflammation but may rely on the cardiac metabolism alterations. These data provide new insights into how cancer and cancer-cachexia impacts the cardiac performance even prior to cancer therapy treatments.

Dystrophin deficiency in cardiomyocytes leads to abnormally reduced Na + currents. These can be rescued by long-term empagliflozin treatment.

The muscular dystrophies caused by dystrophin deficiency, the so-called dystrophinopathies, are associated with impaired cardiac contractility and arrhythmias, which considerably contribute to disease morbidity and mortality. Impaired Ca handling in ventricular cardiomyocytes has been identified as a causative factor for complications in the dystrophic heart, and restoration of normal Ca handling in myocytes has emerged as a promising new therapeutic strategy. In the present study, we explored the hypothesis that ivabradine, a drug clinically approved for the treatment of heart failure and stable angina pectoris, improves Ca handling in dystrophic cardiomyocytes and thereby enhances contractile performance in the dystrophic heart. Therefore, ventricular cardiomyocytes were isolated from the hearts of adult dystrophin-deficient DMDmdx rats, and the effects of acutely applied ivabradine on intracellular Ca transients were tested. In addition, the drug's acute impact on cardiac function in DMDmdx rats was assessed by transthoracic echocardiography. We found that administration of ivabradine to DMDmdx rats significantly improved cardiac function. Moreover, the amplitude of electrically induced intracellular Ca transients in ventricular cardiomyocytes isolated from DMDmdx rats was increased by the drug. We conclude that ivabradine enhances Ca release from the sarcoplasmic reticulum in dystrophic cardiomyocytes and thereby improves contractile performance in the dystrophic heart.

Alterations in cardiac impulse conduction may exert both beneficial and detrimental effects. The assessment of ventricular conduction properties is of paramount importance both in clinical and in experimental settings. Currently the duration of the QRS complex is regarded as hallmark of in-vivo assessment of global ventricular conduction time. In addition, the amplitude of the QRS complex has been suggested to reflect ventricular conduction time in man and in rats. Here, for the first time, we systematically investigated the relationship between QRS duration ("QRS") and QRS amplitude ("RS-height"; RSh) in the murine ECG obtained during anesthesia. In mice harbouring a homozygous knockout of the transmembrane protein podoplanin (PDPN-/-; n = 10) we found both a shorter QRS and a greater RSh than in wild-type animals (n = 13). In both genotypes cumulative i.p. administration of 5 mg/kg and 10 mg/kg of the Na channel blocker flecainide resulted in dose-dependent QRS increase and RSh decrease, whereby the drug-induced changes in RSh were greater than in QRS. In both genotypes the flecainide-induced changes in QRS and in RSh were significantly correlated with each other (R = -0.56, P = 0.004). Whereas dispersion of QRS and RSh was similar between genotypes, dispersion of the ratio QRS/RSh was significantly smaller in PDPN-/- than in wild-types. We conclude that in the murine ECG QRS is inversely related to RSh. We suggest that both parameters should be considered in the analysis of ventricular conduction time in the murine ECG.

IntroductionTransverse-aortic constriction (TAC) operation is a widely used animal model to induce hypertrophy and heart failure through left-ventricular pressure overload. In mice, the cardiac response to TAC exhibits considerable variability influenced by factors such as strain, sub-strain, age, sex and vendor.MethodsTo investigate the impact of suture material (silk versus prolene) and size (6-0 versus 7-0) on the TAC-induced phenotype, we performed surgeries on male C57BL6/N mice at 9 weeks of age defining the aortic constriction by a 27G needle, thereby employing most frequently used methodological settings. The mice were randomly assigned into four separate groups, 6-0 silk, 7-0 silk, 6-0 prolene and 7-0 prolene (10 mice per group). Echocardiography was conducted before TAC and every 4 weeks thereafter to monitor the development of heart failure. Repeated measures correlation analysis was employed to compare disease progression among the different groups.ResultsOur findings reveal a significant influence of the chosen suture material on TAC outcomes. Mice operated with prolene showed increased mortality, slower body weight gain, faster left-ventricular mass increase, and a faster decline in left-ventricular ejection fraction, fractional shortening and aortic pressure gradient compared to silk-operated mice. Moreover, despite non significant, using thinner suture threads (7-0) tended to result in a more severe phenotype compared to thicker threads (6-0) across all tested parameters.DiscussionCollectively, our results highlight the importance of suture material selection in determining the cardiac phenotype induced by TAC and emphasize the need to consider this factor when comparing data across different research laboratories.

T-type Ca channels are strongly expressed and important in the developing heart. In the adult heart, these channels play a significant role in pacemaker tissues, but there is uncertainty about their presence and physiological relevance in the working myocardium. Here, we show that the T-type Ca channel isoforms Cav3.1 and Cav3.2 are expressed at a protein level in ventricular cardiomyocytes from healthy adult C57/BL6 mice. Myocytes isolated from adult wild-type and Cav3.2 KO mice showed considerable whole cell T-type Ca currents under beta-adrenergic stimulation with isoprenaline. We further show that the detectability of basal T-type Ca currents in murine wild-type cardiomyocytes depends on the applied experimental conditions. Together, these findings reveal the presence of functional T-type Ca channels in the membrane of ventricular myocytes. In addition, electrically evoked Ca release from the sarcoplasmic reticulum was significantly impaired in Cav3.2 KO compared to wild-type cardiomyocytes. Our work implies a physiological role of T-type Ca channels in the healthy adult murine ventricular working myocardium.

Psilocybin, a hallucinogen contained in "magic" mushrooms, holds great promise for the treatment of various psychiatric disorders, and early clinical trials are encouraging. Adverse cardiac events after intake of high doses of psilocybin and a trial reporting QT interval prolongation in the electrocardiogram attributed to the drug's main metabolite, psilocin, gave rise to safety concerns. Here we show that clinical concentrations of psilocin do not cause significant human ether-a-go-go-related gene (hERG) potassium channel inhibition, a major risk factor for adverse cardiac events. We conclude that hERG channel blockage by psilocin is not liable for psilocybin- associated cardiotoxic effects.

Background and purpose: Ivabradine is clinically administered to lower the heart rate, proposedly by inhibiting hyperpolarization-activated cyclic nucleotide-gated cation channels in the sinoatrial node. Recent evidence suggests that voltage-gated sodium channels (VGSC) are inhibited within the same concentration range. VGSCs are expressed within the sinoatrial node and throughout the conduction system of the heart. A block of these channels thus likely contributes to the established and newly raised clinical indications of ivabradine. We, therefore, investigated the pharmacological action of ivabradine on VGSCs in sufficient detail in order to gain a better understanding of the pro- and anti-arrhythmic effects associated with the administration of this drug.Experimental Approach: Ivabradine was tested on VGSCs in native cardiomyocytes isolated from mouse ventricles and the His-Purkinje system and on human Nav1.5 in a heterologous expression system. We investigated the mechanism of channel inhibition by determining its voltage-, frequency-, state-, and temperature-dependence, complemented by a molecular drug docking to the recent Nav1.5 cryoEM structure. Automated patch-clamp experiments were used to investigate ivabradine-mediated changes in Nav1.5 inactivation parameters and inhibition of different VGSC isoforms.Key results: Ivabradine inhibited VGSCs in a voltage- and frequency-dependent manner, but did not alter voltage-dependence of activation and fast inactivation, nor recovery from fast inactivation. Cardiac (Nav1.5), neuronal (Nav1.2), and skeletal muscle (Nav1.4) VGSC isoforms were inhibited by ivabradine within the same concentration range, as were sodium currents in native cardiomyocytes isolated from the ventricles and the His-Purkinje system. Molecular drug docking suggested an interaction of ivabradine with the classical local anesthetic binding site.Conclusion and Implications: Ivabradine acts as an atypical inhibitor of VGSCs. Inhibition of VGSCs likely contributes to the heart rate lowering effect of ivabradine, in particular at higher stimulation frequencies and depolarized membrane potentials, and to the observed slowing of intra-cardiac conduction. Inhibition of VGSCs in native cardiomyocytes and across channel isoforms may provide a potential basis for the anti-arrhythmic potential as observed upon administration of ivabradine.