Amiodarone inhibits arrhythmias in hypertensive rats by improving myocardial biomechanical properties

The prevalence of arrhythmia in patients with hypertension has gradually attracted widespread attention. However, the relationship between hypertension and arrhythmia still lacks more attention. Herein, we explore the biomechanical mechanism of arrhythmia in hypertensive rats and the effect of amiodarone on biomechanical properties. We applied micro-mechanics and amiodarone to stimulate single ventricular myocytes to compare changes of mechanical parameters and the mechanism was investigated in biomechanics. Then we verified the expression changes of genes and long non-coding RNAs (lncRNAs) related to myocardial mechanics to explore the effect of amiodarone on biomechanical properties. The results found that the stiffness of ventricular myocytes and calcium ion levels in hypertensive rats were significantly increased and amiodarone could alleviate the intracellular calcium response and biomechanical stimulation. In addition, experiments showed spontaneously hypertensive rats were more likely to induce arrhythmia and preoperative amiodarone intervention significantly reduced the occurrence of arrhythmias. Meanwhile, high-throughput sequencing showed the genes and lncRNAs related to myocardial mechanics changed significantly in the spontaneously hypertensive rats that amiodarone was injected. These results strengthen the evidence that hypertension rats are prone to arrhythmia with abnormal myocardial biomechanical properties. Amiodarone effectively inhibit arrhythmia by improving the myocardial biomechanical properties and weakening the sensitivity of mechanical stretch stimulation.

Abstract 14930: Involvement of Chloride Channel (clic) in Myocardial Fibrosis of Pressure-overload Model Mice and Dilated Cardiomyopathy Patients

Introduction: Pressure overload induces cardiac hypertrophy, electrical remodeling due to changes in various channels, and eventually leads to fibrosis, hardening of the heart, left ventricular diastolic dysfunction, and heart failure. Chloride channels (chloride intracellular channels, Clic) are localized in both plasma membranes and intracellular organelles of various cells, and have recently been reported to be associated with atrial fibrosis. In this study, we examined single-cell RNA-seq analysis of cardiomyocytes for changes in Clic using a pressure overload model due to transverse aortic coarctation (TAC). In addition, a similar analysis was performed on ventricular myocytes in dilated cardiomyopathy. Methods: Single ventricular myocytes were collected from the free wall of the left ventricle after TAC by collagenase treatment in the cardiac hypertrophy phase (3 days after TAC, 1 or 2 weeks (W)) and the heart failure phase (4,8W). The expression of various genes in ventricular myocytes was analyzed for single cells by RNA-seq and compared with sham mice. In addition, a similar study was performed from single ventricular myocytes obtained from dilated cardiomyopathy patients and donors. Results: The expression of myosin heavy chain β (Myh7), a fetal gene, was increased by pressure overload, but the expression of BNP and ANP genes was increased along with the expression of adult Myh6. Regarding Clic, Clic1,4 and 5 increased as compared with Sham mice. Clic1, Clic1 and 4 increased from 3d, and at 8W, Clic1, 4, and 5 also increased . The expression level of these Clic genes is associated with the genes associated with fibrosis (Col4a1, 4a2, 6a2, connective tissue growth factor (CTGF), transforming growth factor-beta 2 (TGFβ2). The KEGG pathway analysis using DAVID for genes with significant associations with Clic1,4, and 5 revealed a strong association with activation of Focal adhesion pathway. Single cell RNA-seq analysis of single ventricular myocytes from patients with dilated cardiomyopathy also showed a increase in CLIC4 and 5 genes compared to healthy donors. Conclusions: Single cell RNA of ventricular myocytes -seq analysis revealed the involvement of chloride channel Clic in myocardial fibrosis and structural remodeling in failing hearts.

TRPA1 channels are a source of calcium-driven cardiac mechano-arrhythmogenicity

SUMMARY PARAGRAPHPhysiological systems require feedback to maintain normal function. In the heart, electrical excitation causes mechanical contraction1, with feedback of mechanics to electrics occurring through ‘mechano-electric coupling’ processes2. In diseases that affect cardiac mechanics, this feedback can result in deadly mechanically-induced arrythmias (‘mechano-arrhythmogenicity’)3. However, the molecular identity of the specific factor(s) driving mechano-arrhythmogenicity are unknown4. Here we show that mechano-sensitive5–10 transient receptor potential kinase ankyrin 1 (TRPA1) channels11 are a source of cardiac mechano-arrhythmogenicity through a calcium (Ca2+)-driven mechanism. Using a cell-level approach involving stretch of single ventricular myocytes combined with simultaneous voltage-Ca2+ imaging, we found that activation of TRPA1 channels resulted in an increase in diastolic Ca2+ load and the appearance of stretch-induced arrhythmias, which were driven by trans-sarcolemmal fluxes and intracellular oscillations of Ca2+, and prevented by pharmacological TRPA1 channel block or Ca2+ buffering. Our results demonstrate that TRPA1 channels act as a trigger for stretch-induced excitation (via Ca2+-influx) and create a substrate for complex arrhythmic activity (via Ca2+-overload), and thus may represent a novel anti-arrhythmic target in cardiac diseases in which TRPA1 channel expression and activity are augmented12–16.

Mepivacaine reduces calcium transients in isolated murine ventricular cardiomyocytes

Background The potential mechanism of mepivacaine’s myocardial depressant effect observed in papillary muscle has not yet been investigated at cellular level. Therefore, we evaluated mepivacaine’s effects on Ca2+ transient in isolated adult mouse cardiomyocytes. Methods Single ventricular myocytes were enzymatically isolated from wild-type C57Bl/6 mice and loaded with 10 μM fluorescent Ca2+ indicator Fluo-4-AM to record intracellular Ca2+ transients upon electrical stimulation. The mepivacaine effects at half-maximal inhibitory concentration (IC50) was determined on calibrated cardiomyocytes’ Ca2+ transients by non-parametric statistical analyses on biophysical parameters. Combination of mepivacaine with NCX blockers ORM-10103 or NiCl2 were used to test a possible mechanism to explain mepivacaine-induced Ca2+ transients’ reduction. Results A significant inhibition at mepivacaine’s IC50 (50 μM) on Ca2+ transients was measured in biophysical parameters such as peak (control: 528.6 ± 73.61 nM vs mepivacaine: 130.9 ± 15.63 nM; p < 0.05), peak area (control: 401.7 ± 63.09 nM*s vs mepivacaine: 72.14 ± 10.46 nM*s; p < 0.05), slope (control: 7699 ± 1110 nM/s vs mepivacaine: 1686 ± 226.6 nM/s; p < 0.05), time to peak (control: 107.9 ± 8.967 ms vs mepivacaine: 83.61 ± 7.650 ms; p < 0.05) and D50 (control: 457.1 ± 47.16 ms vs mepivacaine: 284.5 ± 22.71 ms; p < 0.05). Combination of mepivacaine with NCX blockers ORM-10103 or NiCl2 showed a significant increase in the baseline of [Ca2+] and arrhythmic activity upon electrical stimulation. Conclusion At cellular level, mepivacaine blocks Na+ channels, enhancing the reverse mode activity of NCX, leading to a significant reduction of Ca2+ transients. These results suggest a new mechanism for the mepivacaine-reduction contractility effect.

Diastolic calcium is elevated in metabolic recovery of cardiomyocytes expressing elevated levels of the Na+/H+ exchanger

In the myocardium, the Na+/H+ exchanger isoform 1 (NHE1) plays a pivotal role in mediating ischemia–reperfusion (I/R) injury by causing intracellular Na+ accumulation that results in a subsequent increase in intracellular calcium (Ca2+ overload). One of the major clinical correlates of I/R injury is contractile dysfunction, in which Ca2+ overload via increased Na+/Ca2+ exchange is a major contributor. To better understand the cellular role of NHE1 during I/R injury, contractile function and calcium transients were measured during metabolic inhibition and recovery in single ventricular myocytes from transgenic mice with elevated NHE1 expression. During normoxic conditions, no differences were seen between NHE1-overexpressing cardiomyocytes and wild-type (WT) cardiomyocytes with respect to fractional cell shortening (FCS), rate of shortening (+dL/dt), and rate of relaxation (–dL/dt). When metabolic recovery followed metabolic inhibition, NHE1-overexpressing ventricular myocytes exhibited a significant increase in FCS (130.2% ± 11.77% baseline) and ±dL/dt (146.93% ± 12.27% baseline). This correlated with a significant increase in the concentration of diastolic intracellular calcium, which was attenuated by the NHE1 inhibitor HOE694. These results indicate that in normoxic conditions, elevated NHE1 expression does not alter contractile function. During metabolic recovery, however, elevated NHE1 expression increased diastolic Ca2+ loading that led to augmented cell contractility.

Photoperiod-dependent modulation of cardiac excitation contraction coupling in the Siberian hamster

In mammals, changes in photoperiod regulate a diverse array of physiological and behavioral processes, an example of which in the Siberian hamster ( Phodopus sungorus) is the expression of bouts of daily torpor following prolonged exposure to a short photoperiod. During torpor, body temperature drops dramatically; however, unlike in nonhibernating or nontorpid species, the myocardium retains the ability to contract and is resistant to the development of arrhythmias. In the present study, we sought to determine whether exposure to a short photoperiod results in alterations to cardiac excitation-contraction coupling, thus potentially enabling the heart to survive periods of low temperature during torpor. Experiments were performed on single ventricular myocytes freshly isolated from the hearts of Siberian hamsters that had been exposed to either 12 wk of short-day lengths (SD) or 12 wk of long-day lengths (LD). In SD-acclimated animals, the amplitude of the systolic Ca2+ transient was increased (e.g., from 142 ± 17 nmol/l in LD to 229 ± 31 nmol/l in SD at 4 Hz; P < 0.001). The increased Ca2+ transient amplitude in the SD-acclimated animals was not associated with any change in the shape or duration of the action potential. However, sarcoplasmic reticulum Ca2+ content measured after current-clamp stimulation was increased in the SD-acclimated animals (at 4 Hz, 110 ± 5 vs. 141 ± 15 μmol/l, P < 0.05). We propose that short photoperiods reprogram the function of the cardiac sarcoplasmic reticulum, resulting in an increased Ca2+ content, and that this may be a necessary precursor for maintenance of cardiac function during winter torpor.

Novel method for measuring junctional proton permeation in isolated ventricular myocyte cell pairs

Partial exposure of single ventricular myocytes to membrane-permeant weak acids or bases, using a dual-microperfusion technique, generates large and stable intracellular pH (pHi) gradients. In this study, we have investigated the feasibility of using the technique to estimate junctional proton permeability. This was done by recording the pHi gradient developed across the junctional region of a pair of conjoined ventricular myocytes, isolated enzymically from a guinea pig heart when one of the cells was partially exposed to acetate or ammonium. We show that under HEPES-buffered conditions, the junctional discontinuity in the pHi profile can be used to derive an apparent proton permeability coefficient ( PHapp). The mean PHapp obtained was 4.45 ± 0.21·10−4 cm/s ( n = 43) at an average junctional pHi of 7.04 ± 0.02. In the presence of the junctional inhibitor α-glycyrrhetinic acid, exposure of the proximal cell to weak acid or base produced no pHi change in the distal cell, confirming that distal changes were normally caused by acid-base flux through connexons assembled into junctional channels. The validity of the dual-microperfusion method was tested further by using a diffusion-permeation-reaction model for intracellular protons, designed to highlight possible errors in the estimates of PHapp. Our technique for measuring PHapp provides a useful alternative to the previous, more invasive technique of locally loading acid through a cell-attached patch pipette. The technique may provide a simple method for investigating the factors regulating cell-to-cell proton transmission.