Left Ventricular Myocytes Research Articles

Developmental nicotine exposure alters cardiovascular structure and function in neonatal and juvenile rats.

Here we test the hypothesis that continuous nicotine exposure throughout pre- and postnatal development (developmental nicotine exposure, DNE) alters cardiovascular structure and function in neonatal and juvenile rats. Echocardiography showed that DNE reduced left ventricular mass, left ventricular outflow tract (LVOT) diameter, and posterior wall thickness, but only in females. Both male and female DNE rats had a lower end-systolic volume, higher ejection fraction, and increased fractional shortening, with unchanged stroke volume and cardiac output. Left ventricular single cardiac myocytes from male and female DNE animals exhibited increased calcium-evoked maximal tension with no effect on EC50. Tail-cuff plethysmography in awake rats showed that DNE males had lower systolic blood pressure and higher heart rate than control males. No significant changes in preload, afterload, or the in vitro renal artery response to vasodilators was observed. The results suggest that DNE enhances myocyte tension-generating capacity, possibly compensating for an unknown developmental insult, which may differ in males and females. While this adaptation maintains normal resting cardiac function, it may lead to reduced cardiac reserve, increased energy demand, and elevated oxidative stress, potentially compromising both short-and-long-term cardiovascular health in developing neonates.

Inhibiting glycosphingolipids alleviates cardiac hypertrophy by reducing reactive oxygen species and restoring autophagic homeostasis.

Cardiac hypertrophy is a compensatory stress response produced by a variety of factors, and pathologic hypertrophy can lead to irreversible, severe cardiac disease. Glycosphingolipids (GSLs) are vital constituents of cells, and changes in their content and composition are important factors causing mitochondrial dysfunction in diabetic cardiomyopathy; however, the relationship between GSLs expression and cardiac hypertrophy and specific mechanisms associated with it are not clear. Here, using male C57BL/6 mice, we performed aortic arch reduction surgery to establish an animal model of pressure overload cardiac hypertrophy. In addition, phenylephrine was used in vitro to induce H9c2 cells and neonatal rat left ventricular myocytes (NRVMs) to establish a cellular hypertrophy model. Mass spectrometry revealed that the composition of GSLs was altered in pressure overload-induced hypertrophied mouse hearts and in stimulated hypertrophied cardiomyocyte cell lines. Specifically, in both cases, the proportion of endogenous lactosylceramide (LacCer) was significantly higher than in controls. Inhibition of GSL synthesis with Genz-123346 in NRVMs reduced cell hypertrophy, as well as fibrosis and apoptosis. By Western blotting, we detected decreased intracellular expression of Sirt3 and elevated phosphorylation of JNK after phenylephrine stimulation, but this was reversed in cells pretreated with Genz-123346. Additionally, increased protein expression of FoxO3a and Parkin, along with a decreased LC3-II/I protein ratio in phenylephrine-stimulated cells (compared with unstimulated cells), indicated that the mitochondrial autophagy process was disrupted; again, pretreatment with Genz-123346 reversed that. Our results revealed that changes in GSLs in cardiomyocytes, especially an increase of LacCer, may be a factor causing cellular hypertrophy, which can be alleviated by inhibition of GSLs synthesis. A possible mechanism is that GSLs inhibition increases the expression of Sirt3 protein, scavenges intracellular reactive oxygen species, and restores mitochondrial autophagy homeostasis, thereby lessening cardiomyocyte hypertrophy. In all, these results provide a new perspective for developing drugs for cardiac hypertrophy.

Abstract Mo070: Metabolic Syndrome Alters cAMP Homeostasis and Contractile Function of Cardiomyocytes

Approximately one third of the population in the US has metabolic syndrome (MetS), a condition associated with increased risk for coronary artery disease and myocardial infarction. But whether MetS alters properties of the myocardium before the occurrence of ischemic insults remains to be fully elucidated. For this purpose, MetS was induced in C57Bl/6 female mice with a dietary paradigm recapitulating Western-style alimentary habits in humans. Animals on regular chow were used as control (Ctrl). Cardiac function was assessed by echocardiography and myocytes were studied under field stimulation, following enzymatic dissociation. From 3-12 months on the diet, MetS mice had increased body weight, impaired glucose metabolism, augmented left ventricular (LV) mass, but preserved cardiac function, with respect to Ctrl mice. Using ECGs, heart rate variability was attenuated in MetS mice, suggesting that metabolic disorders alter cardiac sympathovagal balance. At the cellular level, LV myocytes from MetS mice had increased volume (+24%), enhanced fractional cell shortening (+42%), and faster kinetics of relaxation (-27%), with respect to Ctrl myocytes. Because cAMP and protein Kinase A (PKA) modulates myocardial contractility upon activation of G-protein coupled receptors, including beta-adrenergic receptors (B-AR), levels of these molecules were assessed by ELISA assay. Consistent with enhanced cell shortening and faster relaxation, levels of cAMP and PKA activity were, respectively, 1.8-fold and 1.9-fold larger in MetS myocytes, in comparison to Ctrl cells. Interestingly, inhibition of PKA with H-89 reduced cell shortening (-30%) and delayed kinetics of relaxation (+50%) in MetS myocytes but had no major effects on Ctrl cells. To establish the contribution of the cAMP/PKA signaling in the functional behavior of the heart with MetS, mice were treated with the B-AR blocker propranolol. Under this condition, MetS mice had reduced ejection fraction and impaired indices of diastolic function, with respect to Ctrl animals. Interestingly, myocytes obtained from Ctrl and MetS mice treated with the B-AR blocker had comparable cell shortening and kinetics of relaxation. Collectively, these results indicate that metabolic syndrome alters the functional properties of the myocytes and heart by affecting cAMP homeostasis. These properties may increase myocardial oxygen demand predisposing the heart to ischemic insults.

Dysferlin Enables Tubular Membrane Proliferation in Cardiac Hypertrophy.

Cardiac hypertrophy compensates for increased biomechanical stress of the heart induced by prevalent cardiovascular pathologies but can result in heart failure if left untreated. Here, we hypothesized that the membrane fusion and repair protein dysferlin is critical for the integrity of the transverse-axial tubule (TAT) network inside cardiomyocytes and contributes to the proliferation of TAT endomembranes during pressure overload-induced cardiac hypertrophy. Stimulated emission depletion and electron microscopy were used to localize dysferlin in mouse and human cardiomyocytes. Data-independent acquisition mass spectrometry revealed the cardiac dysferlin interactome and proteomic changes of the heart in dysferlin-knockout mice. After transverse aortic constriction, we compared the hypertrophic response of wild-type versus dysferlin-knockout hearts and studied TAT network remodeling mechanisms inside cardiomyocytes by live-cell membrane imaging. We localized dysferlin in a vesicular compartment in nanometric proximity to contact sites of the TAT network with the sarcoplasmic reticulum, a.k.a. junctional complexes for Ca2+-induced Ca2+ release. Interactome analyses demonstrated a novel protein interaction of dysferlin with the membrane-tethering sarcoplasmic reticulum protein juncophilin-2, a putative interactor of L-type Ca2+ channels and ryanodine receptor Ca2+ release channels in junctional complexes. Although the dysferlin-knockout caused a mild progressive phenotype of dilated cardiomyopathy, global proteome analysis revealed changes preceding systolic failure. Following transverse aortic constriction, dysferlin protein expression was significantly increased in hypertrophied wild-type myocardium, while dysferlin-knockout animals presented markedly reduced left-ventricular hypertrophy. Live-cell membrane imaging showed a profound reorganization of the TAT network in wild-type left-ventricular myocytes after transverse aortic constriction with robust proliferation of axial tubules, which critically depended on the increased expression of dysferlin within newly emerging tubule components. Dysferlin represents a new molecular target in cardiac disease that protects the integrity of tubule-sarcoplasmic reticulum junctional complexes for regulated excitation-contraction coupling and controls TAT network reorganization and tubular membrane proliferation in cardiomyocyte hypertrophy induced by pressure overload.

Regional variations in left ventricular myocyte diameter and nuclear length in post mortem population: implications in assessing myocyte hypertrophy

ABSTRACT Cardiac hypertrophy can be appreciated microscopically by assessing for myocyte hypertrophy. Features of myocyte hypertrophy include increased myocyte diameter, together with increase in nuclear hyperchromicity, size and irregularity, and an increase in binucleated myocytes. There is limited literature to demonstrate whether these features are uniform within the myocardium in the post mortem setting. This study compared the myocyte diameter, myocyte nuclear length, and proportion binucleated myocytes between the free wall of the left ventricle and interventricular septum in 21 post mortem cases. It showed regional variation in myocyte hypertrophy parameters between the free walls of the left ventricle and intraventricular septum. Over 75% of cases showed significant differences in myocyte nuclear length and myocyte diameter (ANOVA one-way, p < 0.05), whereas binucleated myocyte nuclei did not show any significant difference. The result of this study has implication in sampling and assessing myocyte hypertrophy. Further studies are recommended to explore the reasons why this variation occurred and whether it is clinically significant.

ERK3 is involved in regulating cardiac fibroblast function.

ERK3/MAPK6 activates MAP kinase-activated protein kinase (MK)-5 in selected cell types. Male MK5 haplodeficient mice show reduced hypertrophy and attenuated increase in Col1a1 mRNA in response to increased cardiac afterload. In addition, MK5 deficiency impairs cardiac fibroblast function. This study determined the effect of reduced ERK3 on cardiac hypertrophy following transverse aortic constriction (TAC) and fibroblast biology in male mice. Three weeks post-surgery, ERK3, but not ERK4 or p38α, co-immunoprecipitated with MK5 from both sham and TAC heart lysates. The increase in left ventricular mass and myocyte diameter was lower in TAC-ERK3+/- than TAC-ERK3+/+ hearts, whereas ERK3 haploinsufficiency did not alter systolic or diastolic function. Furthermore, the TAC-induced increase in Col1a1 mRNA abundance was diminished in ERK3+/- hearts. ERK3 immunoreactivity was detected in atrial and ventricular fibroblasts but not myocytes. In both quiescent fibroblasts and "activated" myofibroblasts isolated from adult mouse heart, siRNA-mediated knockdown of ERK3 reduced the TGF-β-induced increase in Col1a1 mRNA. In addition, intracellular type 1 collagen immunoreactivity was reduced following ERK3 depletion in quiescent fibroblasts but not myofibroblasts. Finally, knocking down ERK3 impaired motility in both atrial and ventricular myofibroblasts. These results suggest that ERK3 plays an important role in multiple aspects of cardiac fibroblast biology.

Cardiac length-dependent activation driven by force-dependent thick-filament dynamics

The length-dependent activation (LDA) of maximum force and calcium sensitivity are established features of cardiac muscle contraction but the dominant underlying mechanisms remain to be fully clarified. Alongside the well-documented regulation of contraction via the thin filaments, experiments have identified an additional force-dependent thick-filament activation, whereby myosin heads parked in a so-called off state become available to generate force. This process produces a feedback effect that may potentially drive LDA. Using biomechanical modeling of a human left-ventricular myocyte, this study investigates the extent to which the off-state dynamics could, by itself, plausibly account for LDA, depending on the specific mathematical formulation of the feedback. We hypothesized four different models of the off-state regulatory feedback based on (A) total force, (B) active force, (C) sarcomere strain, and (D) passive force. We tested if these models could reproduce the isometric steady-state and dynamic LDA features predicted by an earlier published model of a human left-ventricle myocyte featuring purely phenomenological length dependences. The results suggest that only total-force feedback (A) is capable of reproducing the expected behaviors, but that passive tension could provide a length-dependent signal on which to initiate the feedback. Furthermore, by attributing LDA to off-state dynamics, our proposed model also qualitatively reproduces experimentally observed effects of the off-state-stabilizing drug mavacamten. Taken together, these results support off-state dynamics as a plausible primary mechanism underlying LDA.

The entrainment of action potential duration and contraction amplitude by recapitulation of respiratory sinus arrhythmia in isolated cardiac myocytes

Abstract Introduction Respiratory sinus arrhythmia (RSA) is the oscillation of heart rate in phase with ventilation and represents the high frequency component of heart rate variability. RSA is prevalent in the young and the endurance-trained and its loss is considered a prognostic indicator for sudden cardiac death. The recapitulation of RSA improves cardiac function in animal models of heart failure1,2. However, the precise mechanisms by which RSA affects cardiac function are unknown. Hypothesis: We examined the hypothesis that the recapitulation of RSA in isolated ventricular myocytes affects the stimulus interval-dependence of action potential duration (APD) and sarcomere shortening through mechanisms involving sarcoplasmic reticulum (SR) Ca2+ release. Methods Hearts were excised from adult male guinea pigs under general anaesthesia (140 mg/kg bodyweight euthatal, i.p.) and left ventricular myocytes isolated. Isolated myocytes were superfused with Tyrode’s solution for perforated patch whole-cell current clamp recording and action potentials and sarcomere length measured simultaneously. Sample sizes are presented as n/N (cell/animal number) and nested hierarchical analysis applied using a generalized linear mixed model (P&amp;lt;0.05). Results Pacing at a basic cycle length (CL, 350 ms) induced APD, Ca2+ transient amplitude (CaT) and sarcomere shortening alternans. The recapitulation of RSA through oscillatory pacing (OP), in which CL was set to 20% below and 20% above the corresponding basic CL in a 2-short, 2-long stimulus train, reduced the incidence of alternans. Furthermore, OP entrained APD, CaT and sarcomere shortening and this entrainment persisted in voltage clamp experiments with fixed APD. Using a three-step protocol to examine the dependence of APD and sarcomere shortening on both preceding (CLn-1) and anteceding (CLn-2) stimulus intervals, both APD and sarcomere shortening were found to depend directly on the preceding cycle length (CLn-1, P&amp;lt;0.001). In contrast, sarcomere shortening depended inversely on the anteceding CL (CLn-2, P&amp;lt;0.001; n/N = 24/4). Following treatment of the cells with caffeine (50 µM) to sensitise ryanodine receptors, the dependence of APD and sarcomere shortening on CLn-1 was markedly suppressed, and the inverse dependence of sarcomere shortening on CLn-2 was lost (P&amp;lt;0.001, n/N = 6/3). The effects of OP are akin to post-extrasystolic potentiation3 and mathematical simulation4 supports the involvement of changes in the sarcoplasmic reticulum Ca2+ content. Conclusion These data are consistent with an obligatory role for the SR in the entrainment of APD, CaT and sarcomere shortening and the suppression of alternans by the recapitulation of RSA. We hypothesise that recapitulation of RSA would be protective against ventricular tachyarrhythmia and further work is required to test this.

Toxicity Studies of Cardiac-Targeting Peptide Reveal a Robust Safety Profile.

Targeted delivery of therapeutics specifically to cardiomyocytes would open up new frontiers for common conditions like heart failure. Our prior work using a phage display methodology identified a 12-amino-acid-long peptide that selectively targets cardiomyocytes after an intravenous injection in as little as 5 min and was hence termed a cardiac-targeting peptide (CTP: APHLSSQYSRT). CTP has been used to deliver imaging agents, small drug molecules, photosensitizing nanoparticles, exosomes, and even miRNA to cardiomyocytes. As a natural extension to the development of CTP as a clinically viable cardiac vector, we now present toxicity studies performed with the peptide. In vitro viability studies were performed in a human left ventricular myocyte cell line with 10 µM of Cyanine-5.5-labeled CTP (CTP-Cy5.5). In vitro ion channel profiles were completed for CTP followed by extensive studies in stably transfected cell lines for several GPCR-coupled receptors. Positive data for GPCR-coupled receptors were interrogated further with RT-qPCRs performed on mouse heart tissue. In vivo studies consisted of pre- and post-blood pressure monitoring acutely after a single CTP (10 mg/Kg) injection. Further in vivo toxicity studies consisted of injecting CTP (150 µg/Kg) in 60, 6-week-old, wild-type CD1, male/female mice (1:1), with cohorts of mice euthanized on days 0, 1, 2, 7, and 14 with inhalational CO2, followed by blood collection via cardiac puncture, complete blood count analysis, metabolic profiling, and finally, liver, renal, and thyroid studies. Lastly, mouse cardiac MRI was performed immediately before and after CTP (150 µg/Kg) injection to assess changes in cardiac size or function. Human left ventricular cardiomyocytes showed no decrease in viability after a 30 min incubation with CTP-Cy5.5. No significant activation or inhibition of any of seventy-eight protein channels was observed other than OPRM1 and COX2 at the highest tested concentration, neither of which were expressed in mouse heart tissue as assessed using RT-qPCR. CTP (10 mg/Kg) injections led to no change in blood pressure. Blood counts and chemistries showed no evidence of significant hematological, hepatic, or renal toxicities. Lastly, there was no difference in cardiac function, size, or mass acutely in response to CTP injections. Our studies with CTP showed no activation or inhibition of GPCR-associated receptors in vitro. We found no signals indicative of toxicity in vivo. Most importantly, cardiac functions remained unchanged acutely in response to CTP uptake. Further studies using good laboratory practices are needed with prolonged, chronic administration of CTP conjugated to a specific cargo of choice before human studies can be contemplated.

From cells to circulating biomarker: BMP10 is a myocyte-secreted peptide with potential to detect atrial fibrillation

Abstract Background Early onset detection and recurrence prediction of atrial fibrillation (AF) may improve therapeutic decision-making, prognosis and long-term outcome. Plasma proteome analyses and mechanistic studies identified bone morphogenetic protein 10 (BMP10) as an atrial-specific secreted biomolecule for AF associated with reduced paired-like homeodomain transcription factor 2 (PITX2) expression. Purpose Our aim is 1) to confirm BMP10 synthesis and secretion by human atrial isolated myocytes, 2) to compare BMP10 secreted levels between right atrial, left atrial and left ventricular myocytes, and 3) to assess if BMP10 secretion is indeed associated with AF in human myocytes. Methods Right atrial (RA), left atrial (LA) and left ventricular (LV) myocytes were isolated from 16 tissues samples of patients undergoing open heart surgery which were currently in sinus rhythm (SR) or AF. Freshly isolated myocytes were resuspended in minimal essential medium (MEM, M1) containing fetal bovine serum (FBS) and plated in laminin coated dishes. M1 was collected after 3 hours of culture and substituted by FBS-free medium (M2), which was collected after 48 hours of culture. Cardiomyocyte-released BMP10 levels were measured in M1 and M2 by ELISA. Results Cardiomyocytes from twelve patients (68±2.7 years, 52±2.1 % ejection fraction, 17 % women, 50 % with AF, 50 % valvular surgery and 75 % bypass) were cultured and BMP10 secreted levels analyzed. BMP10 concentrations were high in medium from atrial cardiomyocytes (2.76±1.8 ng/µl) and negligible in medium from ventricular cardiomyocytes (0.01±0.005 ng/µl). Left atrial cardiomyocytes released less BMP10 than right atrial cardiomyocytes (LA 0.02±0.007 ng/µl; RA 3.50±2.3 ng/µl). In addition, BMP10 concentrations were higher in right atrial cardiomyocytes from patients with AF (3.76±3.6 ng/µl) than in the right atrial cardiomyocytes from patients in SR (0.97±0.9 ng/µl). Conclusion Our results provide a direct proof that BMP10 is secreted by human atrial myocytes, mainly right atrial myocytes. Furthermore, atrial myocytes from patients with AF secreted significantly more BMP10 than myocytes from patients in sinus rhythm. These observations validate the atrial cardiomyocyte origin of circulating BMP10, making BMP10 a potentially attractive atrial-specific cardiac biomarker.

Synchrotron-based X-ray phase contrast imaging of transmural cardiac tissue in patients treated for advanced heart failure

Abstract Background Cardiac imaging is essential for identifying structural changes in advanced heart failure (HF) and understanding underlying pathophysiology. Synchrotron radiation-based X-ray phase contrast imaging (X-PCI) is a non-destructive imaging modality that can provide high resolution three-dimensional (3D) visualisation of cardiac tissue on the microstructural level. Thus far, such analyses including the quantification of myocyte aggregates ranging from the epicardium to the endocardium, have been confined to animal models and human fetal tissue. We aimed to explore the feasibility of using X-PCI to quantify and compare structural organisation and orientation of myocyte aggregates in the myocardium across different advanced HF aetiologies. Methods and design Four adult patients were included - two receiving a left ventricular assist device (LVAD), and two undergoing heart transplantation (HTx). Aetiology of advanced HF in both the LVAD and HTx group was ischaemic heart disease and dilated cardiomyopathy (DCM), respectively. Transmural tissue samples were obtained by left ventricular apical coring (LVAD) and from the explanted hearts (HTx). The tissue specimens were imaged by X-PCI at the TOMCAT beamline using an effective pixel size of 5.8 µm. Orientation of aggregates of cardiomyocytes was assessed using a structure tensor-based method and morphological parameters were derived - the helical angle (HA), or the angle between the circumferential left ventricular plane and myocyte aggregates, and fractional anisotropy (FA), the degree of anisotropy or disorganisation of the local myocardium. Results The general patient characteristics are shown in Table 1. X-PCI enabled 3D visualisation of transmural myocardial tissue samples showing differences in organisation of myocardial structure in different HF aetiologies (Figure 1). The epicardial adipose layer was thicker in DCM, paired with a thinner epicardial myocyte layer (yellow arrows). Furthermore, the epi-to-endocardial HA transition was clearly visualized in ICM (double arrows), whereas there was clear disruption of the HA gradient in DCM samples – a finding further quantified via FA (green arrows). Conclusion X-PCI allows non-destructive advanced imaging of ex-vivo tissue. By harnessing the power of digital 3D imaging on a cellular level, it provides a novel insight to differential information on tissue organisation in different aetiologies of advanced heart failure.Table 1Figure 1

Increased CaMKII activation and contrast changes of cardiac β1-and β3-Adrenergic signaling pathways in a humanized angiotensinogen model of hypertension

AimsUpregulation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) contributes to the pathogenesis of cardiovascular disease, including hypertension. Transgenic rats expressing the human angiotensinogen gene [TGR (hAGT)L1623] are a new novel humanized model of hypertension that associates with declines in cardiac contractile function and β-adrenergic receptor (AR) reserve. The molecular mechanisms are unclear. We tested the hypothesis that in TGR (hAGT)L1623 rats, left ventricular (LV) myocyte CaMKIIδ and β3-AR are upregulated, but β1-AR is down-regulated, which are important causes of cardiac dysfunction and β-AR desensitization. Main methodsWe compared LV myocyte CaMKIIδ, CaMKIIδ phosphorylation (at Thr287) (pCaMKIIδ), and β1-and β3-AR expressions and determined myocyte functional and [Ca2+]I transient ([Ca2+]iT) responses to β-AR stimulation with and without pretreatment of myocytes using an inhibitor of CaMKII, KN-93 (10−6 M, 30 min) in male Sprague Dawley (SD; N = 10) control and TGR (hAGT)L1623 (N = 10) adult rats. Key findingsHypertension in TGR (hAGT)L1623 rats was accompanied by significantly increased LV myocyte β3-AR protein levels and reduced β1-AR protein levels. CaMKIIδ phosphorylation (at Thr287), pCaMKIIδ was significantly increased by 35%. These changes were followed by significantly reduced basal cell contraction (dL/dtmax), relaxation (dR/dtmax), and [Ca2+]iT. Isoproterenol (10−8 M) produced significantly smaller increases in dL/dtmax, dR/dtmax, and [Ca2+]iT. Moreover, only in TGR (hAGT)L1623 rats, pretreatment of LV myocytes with KN-93 (10−6 M, 30 min) fully restored normal basal and isoproterenol-stimulated myocyte contraction, relaxation, and [Ca2+]iT. SignificanceLV myocyte CaMKIIδ overactivation with associated contrast changes in β3-AR and β1-AR may be the key molecular mechanism for the abnormal contractile phenotype and β-AR desensitization in this humanized model of hypertension.

Altered cardiac repolarization associated with cellular electrophysiological remodeling following chronic testosterone undecanoate administration in a canine model

Abstract Funding Acknowledgements Type of funding sources: None. Background The influence of testosterone on ventricular ion channels is widely investigated. Despite the legally stipulated availability of different androgen anabolic steroids (AAS), these agents are very popular among young adults, however, the direct effects of supra-physiological testosterone level are still unclear. Supposedly, the chronic use of AASs may result in structural and functional remodeling of the heart. Purpose The aim of our study was to investigate the potential electrophysiological effects of chronic administration of testosterone-undecanoate in a canine model in in vivo and in vitro studies. Methods Eight male beagle dogs were randomized into control (’Cont’) and treated (’Tr’) groups (n = 4; n = 4). The latter group received 15 mg/kg of long-lasting testosterone-undecanoate intramuscular injections weekly for 3 months. Blood samples were taken for monitoring testosterone levels. To investigate the altered repolarization in conscious dogs electrocardiography studies were performed. Ventricular myocytes were enzymatically dissociated via retrograde perfusion. The transmembrane ionic currents were recorded using the whole-cell configuration of the patch-clamp technique and the action potential duration (APD) was measured by the perforated patch-clamp technique. Results Testosterone level was significantly higher in the ‘Tr’ group compared to the ‘Cont’ group (47.02 nm/L vs. 15.23 nm/L; p=0.0002). The chronic treatment did not affect the heart rate between the examined groups (93.7±23.5 vs.108.8±21.4 beats/min), however, it resulted in significantly shortened QT (226±49 vs. 244.9±26.6 ms; p&amp;lt;0.05) and QTc (26.06±2.5 vs. 29.6±3.01 ms, p&amp;lt;0.05) intervals. ECG recordings also presented prolonged PQ (112.1±15.5 vs. 61.65±12.7ms; p&amp;lt;0.05), QRS (72.37±15.4 vs. 61.65±12.7 ms; p&amp;lt;0.05), and Tp-Te (32.95±7.45 vs. 53.46±16.6 ms; p&amp;lt;0.05) intervals in the ‘Tr’ group. Additionally, the APD of isolated left ventricular myocytes significantly shortened in the ‘Tr’ group compared to the ‘Con’ group (235.2±26.7 vs. 283.6±28.5 ms; p&amp;lt;0.05). Patch-clamp experiments revealed increased magnitude of transient outward potassium current (Ito), the inward rectifier potassium current (Ik1), and the slowed delayed rectifier potassium current (Iks) in the ‘Tr’ group. Conclusion The repolarization of the canine ventricular myocardium was significantly modified by constantly high level of testosterone. Many beneficial effects are attributed to physiological testosterone levels; however, the supra-physiological testosterone level may lead to potentially harmful alternations in the cardiac repolarization that may promote arrhythmogenesis, especially in the presence of various heart diseases. Presumably, the constantly high testosterone level induced electrophysiological changes may contribute to the development of life-threatening arrhythmias under certain circumstances.

PO-01-124 THE CALCIUM HANDLING MACHINERY IS REMODELED IN FRIEDREICH’S ATAXIA

Friedreich's ataxia (FA) is an inherited neurodegenerative disorder that causes progressive nervous system damage. FA is caused by a deficiency in the frataxin protein due to an expansion of GAA repeats in the first intron of the frataxin gene. Reduced frataxin protein expression is thought to negatively affect mitochondrial function, leading to increased oxidative damage. Although FA is considered a neurodegenerative disorder, it is not completely understood why most patients present with left ventricular systolic dysfunction and die from cardiac events. The objective of our study is to determine whether the abnormal calcium handling machinery is a molecular mechanism that perpetuates cardiac dysfunction in FA. We used the frataxin knock-out (KO) mouse model of FA as well as human heart samples from donors with FA and from unaffected donors. The expression of calcium handling machinery proteins was assessed with proteomics and western blot. In left ventricular myocytes from KO and WT mice, the IonOptix system was used for calcium imaging, the seahorse assay was utilized to measure oxygen consumption rate (OCR) and confocal imaging was used to quantify the mitochondrial membrane potential (Δψm), and reactive oxygen species (ROS). ECG and echocardiography were used to assess cardiac function in the mice. In the proteomic analysis and western blot, SERCA2, Ryr2 and CaMKII were significantly downregulated, and the mitochondrial calcium handling proteins voltage-dependent anion channel and mitochondrial calcium uniporter were significantly upregulated in left ventricular tissue from humans with FA and from KO mice. On the ECG, the RR, PR, QRS, and QTc intervals were significantly longer in the KO mice. Echocardiography showed that the ejection fraction and fractional shortening were significantly decreased and left ventricular wall thickness and diameter were significantly increased in the KO mice. Moreover, the calcium transient decay was significantly prolonged in ventricular myocytes from KO mice compared to WT. Additionally, the Δψm was significantly depolarized, the OCR was significantly decreased, and ROS levels were significantly elevated in left ventricular myocytes from KO compared to WT. The development of left ventricular contractile dysfunction in FA is associated with reduced expression of calcium handling proteins, abnormal calcium cycling, and mitochondrial dysfunction.

Rapid Pacing Decreases L-type Ca2+ Current and Alters Cacna1c Isogene Expression in Primary Cultured Rat Left Ventricular Myocytes

The L-type calcium current (ICaL) is the first step in cardiac excitation–contraction-coupling and plays an important role in regulating contractility, but also in electrical and mechanical remodeling. Primary culture of cardiomyocytes, a widely used tool in cardiac ion channel research, is associated with substantial morphological, functional and electrical changes some of which may be prevented by electrical pacing. We therefore investigated ICaL directly after cell isolation and after 24 h of primary culture with and without regular pacing at 1 and 3 Hz in rat left ventricular myocytes. Moreover, we analyzed total mRNA expression of the pore forming subunit of the L-type Ca2+ channel (cacna1c) as well as the expression of splice variants of its exon 1 that contribute to specificity of ICaL in different tissue such as cardiac myocytes or smooth muscle. 24 h incubation without pacing decreased ICaL density by ~ 10% only. Consistent with this decrease we observed a decrease in the expression of total cacna1c and of exon 1a, the dominant variant of cardiomyocytes, while expression of exon 1b and 1c increased. Pacing for 24 h at 1 and 3 Hz led to a substantial decrease in ICaL density by 30%, mildly slowed ICaL inactivation and shifted steady-state inactivation to more negative potentials. Total cacna1c mRNA expression was substantially decreased by pacing, as was the expression of exon 1b and 1c. Taken together, electrical silence introduces fewer alterations in ICaL density and cacna1c mRNA expression than pacing for 24 h and should therefore be the preferred approach for primary culture of cardiomyocytes.Graphical

Systemic Hypoxemia Induces Cardiomyocyte Hypertrophy and Right Ventricular Specific Induction of Proliferation.

A recent study suggests that systemic hypoxemia in adult male mice can induce cardiac myocytes to proliferate. The goal of the present experiments was to confirm these results, provide new insights on the mechanisms that induce adult cardiomyocyte cell cycle reentry, and to determine if hypoxemia also induces cardiomyocyte proliferation in female mice. EdU-containing mini pumps were implanted in 3-month-old, male and female C57BL/6 mice. Mice were placed in a hypoxia chamber, and the oxygen was lowered by 1% every day for 14 days to reach 7% oxygen. The animals remained in 7% oxygen for 2 weeks before terminal studies. Myocyte proliferation was also studied with a mosaic analysis with double markers mouse model. Hypoxia induced cardiac hypertrophy in both left ventricular (LV) and right ventricular (RV) myocytes, with LV myocytes lengthening and RV myocytes widening and lengthening. Hypoxia induced an increase (0.01±0.01% in normoxia to 0.11±0.09% in hypoxia) in the number of EdU+ RV cardiomyocytes, with no effect on LV myocytes in male C57BL/6 mice. Similar results were observed in female mice. Furthermore, in mosaic analysis with double markers mice, hypoxia induced a significant increase in RV myocyte proliferation (0.03±0.03% in normoxia to 0.32±0.15% in hypoxia of RFP+ myocytes), with no significant change in LV myocyte proliferation. RNA sequencing showed upregulation of mitotic cell cycle genes and a downregulation of Cullin genes, which promote the G1 to S phase transition in hypoxic mice. There was significant proliferation of nonmyocytes and mild cardiac fibrosis in hypoxic mice that did not disrupt cardiac function. Male and female mice exhibited similar gene expression following hypoxia. Systemic hypoxia induces a global hypertrophic stress response that was associated with increased RV proliferation, and while LV myocytes did not show increased proliferation, our results minimally confirm previous reports that hypoxia can induce cardiomyocyte cell cycle activity in vivo.

A small erythropoietin derived non-hematopoietic peptide reduces cardiac inflammation, attenuates age associated declines in heart function and prolongs healthspan.

Aging is associated with increased levels of reactive oxygen species and inflammation that disrupt proteostasis and mitochondrial function and leads to organism-wide frailty later in life. ARA290 (cibinetide), an 11-aa non-hematopoietic peptide sequence within the cardioprotective domain of erythropoietin, mediates tissue protection by reducing inflammation and fibrosis. Age-associated cardiac inflammation is linked to structural and functional changes in the heart, including mitochondrial dysfunction, impaired proteostasis, hypertrophic cardiac remodeling, and contractile dysfunction. Can ARA290 ameliorate these age-associated cardiac changes and the severity of frailty in advanced age? We conducted an integrated longitudinal (n = 48) and cross-sectional (n = 144) 15 months randomized controlled trial in which 18-month-old Fischer 344 x Brown Norway rats were randomly assigned to either receive chronic ARA290 treatment or saline. Serial echocardiography, tail blood pressure and body weight were evaluated repeatedly at 4-month intervals. A frailty index was calculated at the final timepoint (33 months of age). Tissues were harvested at 4-month intervals to define inflammatory markers and left ventricular tissue remodeling. Mitochondrial and myocardial cell health was assessed in isolated left ventricular myocytes. Kaplan-Meier survival curves were established. Mixed ANOVA tests and linear mixed regression analysis were employed to determine the effects of age, treatment, and age-treatment interactions. Chronic ARA290 treatment mitigated age-related increases in the cardiac non-myocyte to myocyte ratio, infiltrating leukocytes and monocytes, pro-inflammatory cytokines, total NF-κB, and p-NF-κB. Additionally, ARA290 treatment enhanced cardiomyocyte autophagy flux and reduced cellular accumulation of lipofuscin. The cardiomyocyte mitochondrial permeability transition pore response to oxidant stress was desensitized following chronic ARA290 treatment. Concurrently, ARA290 significantly blunted the age-associated elevation in blood pressure and preserved the LV ejection fraction. Finally, ARA290 preserved body weight and significantly reduced other markers of organism-wide frailty at the end of life. Administration of ARA290 reduces cell and tissue inflammation, mitigates structural and functional changes within the cardiovascular system leading to amelioration of frailty and preserved healthspan.

Alternating Early Afterdepolarizations Underlying Bradycardia-Dependent Macroscopic T Wave and Discordant Mechanical Alternans.

Macroscopic T wave alternans (macro-TWA) often heralds the onset of Torsades de Pointes in patients with QT prolongation. However, the mechanisms underlying macro-TWA remain unclear. We examined the cellular and ionic basis for macro-TWA in rabbits with left ventricular hypertrophy (LVH). The renovascular hypertension model was used to induce LVH in rabbits. Action potentials were simultaneously recorded from epicardium and endocardium together with a transmural ECG and isometric contractility in arterially perfused left ventricular wedges. Late sodium current (INa-L) was recorded in single-isolated left ventricular myocytes with the whole cell patch-clamp technique. Macro-TWA and accompanied mechanical alternans occurred spontaneously in 8 of 33 LVH rabbits (P<0.05, versus 0/15 in controls) and were induced by an INa-L enhancer ATX-II at 1 to 3 nM in additional 7. Macro-TWA and mechanical alternans occurred discordantly, that is, that longer QT interval and larger T wave were associated with weaker isometric contvractility. Alternating early afterdepolarizations in the endocardium caused macro-TWA in 12 of 15 LVH rabbits and, therefore, early afterdepolarization-dependent R-from-T extrasystoles and Torsades de Pointes always originated from the beats with longer QT and larger T wave during macro-TWA. INa-L density was significantly larger in LVH myocytes than that of control myocytes. Macro-TWA, mechanical alternans, R-from-T extrasystoles, and Torsades de Pointes were all abolished by INa-L blocker ranolazine or mexiletine. LVH enhances INa-L density and promotes alternating early afterdepolarizations in the left ventricular endocardium that manifest as macro-TWA with discordant mechanical alternans. INa-L blockade abolishes macro-TWA, mechanical alternans, early afterdepolarization-dependent R-from-T extrasystoles, and Torsades de Pointes.

Abstract 14952: Lysosomal Regulation of Calcium and Iron Homeostasis and Ferroptosis in Mouse Cardiomyocytes

Background: Lysosomes are specific organelles that contain various digestive enzymes and also store both iron (Fe) and calcium (Ca). However, it is not known whether cytosolic and mitochondrial Fe and Ca handling and ferroptosis are modulated by lysosomal dysfunction. We aimed to evaluate the interaction between lysosomal, mitochondrial and cytosolic Ca and Fe dynamics and its relevance to cardiomyocyte function and ferroptosis. Methods: Left ventricular myocytes were enzymatically isolated from mouse hearts. Cytosolic Ca i was imaged using fluo-4-AM. Cytosolic Fe was determined with Phen green while mitochondrial Fe was measured using rhodamine B-[(1,10-phenanthroline-5-yl)-aminocarbonyl]benzyl ester (RPA). Mitochondrial membrane potential (Δ Ψ m ) was monitored by TMRM. Ferroptosis was analyzed using the Live/Dead cell viability assay. Results: Treatment with nicotinic acid dinucleotide phosphate (NAADP-AM 50 nM), an agonist of the two pore channel in the lysosomal membrane, increased the aptitude of the Ca i transient (1.94 ± 0.09 to 2.41 ± 0.14, n = 12, p &lt; 0.05) and induced spontaneous Ca i waves. Diastolic Ca i became elevated (1.00 to 1.30 ± 0.10, n = 12, p &lt; 0.05) in intact myocytes. Elevation of basal Ca i level and an increase in spontaneous Ca i wave amplitude and frequency were confirmed in saponin-permeabilized myocytes. NAADP also increased the level of cytosolic Fe. These results suggest both Ca and Fe are released from lysosomes into the cytosol by NAADP. Our previous studies have shown that mitochondrial Fe uptake is essential for Fe overload-induced ferroptosis in cardiac myocytes. Therefore, we further assessed how lysosome function affects mitochondrial Fe loading and induction of ferroptosis. We found that NAAPD treatment resulted in an elevation of mitochondrial Fe levels (by 28.4 ± 3.5 %, n=23, p &lt;0.05), depolarization of Δ Ψ m as well as resulted in an increased sensitivity to Fe overload-induced ferroptosis. Similar results were observed using GPN that causes selective disruption of lysosomes. Conclusions: Lysosomal dysfunction may contribute to mishandling of Ca and Fe in myocytes and promote the occurrence of ferroptosis. A lysosomal storage disorder mouse model ( i.e. RagA/B KO) will be assessed in future studies.

Reversible and Irreversible Effects of Electroporation on Contractility and Calcium Homeostasis in Isolated Cardiac Ventricular Myocytes.

Irreversible electroporation is an energy form utilizing high-voltage pulsed electric field, leading to cellular homeostasis disruption and cell death. Recently, irreversible electroporation has shown promising results for the treatment of cardiac arrhythmias. However, reversible and irreversible effects of pulsed electric field on cardiac myocytes remain poorly understood. Here, we evaluated the influence of a monophasic single electric pulse (EP) on the contractility, Ca2+ homeostasis and recovery of cardiac myocytes. Isolated rat left ventricular myocytes were electroporated using single monophasic EP of different durations and voltages. Sarcomere length and intracellular Ca2+ were simultaneously monitored for up to 20 minutes after EP application in Fura-2 loaded left ventricular myocytes. Lethal voltage thresholds were determined using 100 µs and 10 ms pulses and by discriminating cell orientation with respect to the electric field. Electroporation led to an immediate increase in intracellular Ca2+ which was dependent upon the voltage delivered to the cell. Intermediate-voltage EP (140 V, 100 µs) increased sarcomere shortening, Ca2+ transient amplitude, and diastolic Ca2+ level measured 1 minute post-EP. Although sarcomere shortening returned to pre-EP level within 5 minutes, Ca2+ transient amplitude decreased further below pre-EP level and diastolic Ca2+ level remained elevated within 20 minutes post-EP. Spontaneous contractions were observed after sublethal EP application but their frequency decreased progressively within 20 minutes. Lethal EP voltage threshold was lower in myocytes oriented perpendicular than parallel to the electric field using 100 µs pulses while an opposite effect was found using 10 ms pulses. Sublethal EP affected rat left ventricular myocytes contractility and disrupted Ca2+ homeostasis as a function of the EP voltage. Moreover, EP-induced lethality was preceded by a large increase in intracellular Ca2+ and was dependent upon the EP duration, amplitude and left ventricular myocytes orientation with respect to the electric field. These findings provide new insights into the effect of pulsed electric field on cardiac myocytes.