Isolated Mouse Cardiomyocytes Research Articles

T-tubule recovery after detubulation in isolated mouse cardiomyocytes.

Remodeling of cardiac t-tubules in normal and pathophysiological conditions is an important process contributing to the functional performance of the heart. While it is well documented that deterioration of t-tubule network associated with various pathological conditions can be reversed under certain conditions, the mechanistic understanding of the recovery process is essentially lacking. Accordingly, in this study we investigated some aspects of the recovery of t-tubules after experimentally-induced detubulation. T-tubules of isolated mouse ventricular myocytes were first sealed using osmotic shock approach, and their recovery under various experimental conditions was then characterized using electrophysiologic and imaging techniques. The data show that t-tubule recovery is a strongly temperature-dependent process involving reopening of previously collapsed t-tubular segments. T-tubule recovery is slowed by (1) metabolic inhibition of cells, (2) reducing influx of extracellular Ca2+ as well as by (3) both stabilization and disruption of microtubules. Overall, the data show that t-tubule recovery is a highly dynamic process involving several central intracellular structures and processes and lay the basis for more detailed investigations in this area.

Junctophilin-2 allosterically interacts with ryanodine receptor type 2 to regulate calcium release units in mouse cardiomyocytes.

The interaction between junctophilin-2 (JPH2) and ryanodine receptor type 2 (RyR2) regulated Ca2+ signaling in mouse cardiomyocytes. However, their exact interaction remains unclear. This study elucidates the interaction between JPH2 with RyR2 using co-immunoprecipitation of cardiac sarcoplasmic reticulum vesicles. Additionally, a glutathione S-transferase (GST) pull-down analysis was performed to investigate the physical interaction between RyR2 and JPH2 fragments. JPH2 interacted with RyR2 and the C terminus of the JPH2 protein can pull-down RyR2 receptors. Confocal immunofluorescence imaging indicated that the majority of JPH2 and RyR2 proteins were colocalized near Z-lines in isolated mouse cardiomyocytes. Knockdown of JPH2 reduced the amplitude of Ca2+ transients and disrupted its interaction with RyR2. Therefore, the C-terminus domain of JPH2 is required for interactions with RyR2 in mouse cardiomyocytes, which provides a molecular mechanism for seeking Ca2+-related disease prevention strategies.

RPI‐194 is a Novel Troponin Activator that Increases the Calcium Sensitivity of Striated Muscle Contraction

IntroductionSystolic heart failure is a cardiac disease in which the contractile function of heart muscle becomes weakened and unable to pump blood with adequate force. As of yet, no existing positive inotrope is able to lower the mortality rate in systolic heart failure. Cardiac troponin complex (cTn) is a trimeric complex consisting of troponin C (cTnC), troponin I (cTnI) and troponin T (cTnT). cTnC regulates blood supply to the heart by turning muscle contraction on and off in a calcium‐dependent manner. More precisely, the regulatory N‐domain of cTnC (cNTnC) binds the switch region of cTnI to activate contraction. Drug‐like molecules need to bind to this cNTnC‐cTnI switch region interface and modulate the pumping activity of the heart. Our objective was to identify a small molecule that has the ability to stabilize the activated conformation of cTn by binding to this interface and thereby enhance cardiac muscle contraction.HypothesisA cardiac troponin activator could potentially compensate the impaired contractile function of the heart in the treatment of systolic heart failureMethodsA total of 47 new compounds were designed using existing knowledge of calcium sensitizers. The compounds were screened against a recombinantly purified cNTnC‐cTnI chimeric construct, named gChimera, using nuclear magnetic resonance and fluorescence spectroscopy. We identified a novel small molecule cardiac troponin activator, RPI‐194, and measured its binding affinity to recombinantly purified cNTnC and gChimera. We also investigated the activity of RPI‐194 in rat skinned cardiac muscle trabeculae, isolated mouse cardiomyocytes, and isolated working mouse hearts. RPI‐194 activity was also tested in skinned skeletal muscle fibers from rats as both cardiac and slow skeletal muscle share the same isoform of cTnC.ResultsWe demonstrated that a small molecule troponin activator RPI‐194 binds to gChimera with a KD of 12‐24 µM and is able to stabilize activated cTn. It also slowed down the rate of calcium release from reconstituted cTn. RPI‐194 showed increasing calcium sensitivity of isometric contraction in skinned cardiac muscle trabeculae, as well as in slow and fast skeletal muscle fibers, suggesting it to be a calcium sensitizer, with cross‐reactivity within striated muscle. Contrarily, RPI‐194 was unable to slow down calcium release from isolated cNTnC. It also reduced the velocity of unloaded shortening in skeletal muscle fibers, suggesting that it slows the rate of actin‐myosin cross‐bridging, although no effect on myosin ATPase activity was found. RPI‐194 decreased the velocity and amplitude of contractions in isolated cardiomyocytes but contractility was preserved in isolated working hearts.ConclusionsRPI‐194 is a small molecule troponin activator that acts as a calcium sensitizer in striated muscle. Because of the isoform sharing of cTnC in cardiac and slow skeletal muscle, it is nearly impossible to develop a cardiac‐specific troponin activator. Whole animal model studies are needed to determine the impact of RPI‐194 on the different striated muscle types and whether this would be acceptable for therapeutic intervention.

Pharmacological modulation of prostaglandin E2 (PGE2 ) EP receptors improves cardiomyocyte function under hyperglycemic conditions.

Type 2 diabetes (T2D) affects >30 million Americans and nearly 70% of individuals with T2D will die from cardiovascular disease (CVD). Circulating levels of the inflammatory signaling lipid, prostaglandin E2 (PGE2), are elevated in the setting of obesity and T2D and are associated with decreased cardiac function. The EP3 and EP4 PGE2 receptors have opposing actions in several tissues, including the heart: overexpression of EP3 in cardiomyocytes impairs function, while EP4 overexpression improves function. Here we performed complementary studies in vitro with isolated cardiomyocytes and in vivo using db/db mice, a model of T2D, to analyze the effects of EP3 inhibition or EP4 activation on cardiac function. Using echocardiography, we found that 2 weeks of systemic treatment of db/db mice with 20 mg/kg of EP3 antagonist, beginning at 6 weeks of age, improves ejection fraction and fractional shortening (with no effect on heart rate). We further show that either EP3 blockade or EP4 activation enhances contractility and calcium cycling in isolated mouse cardiomyocytes cultured in both normal and high glucose. Thus, peak [Ca2+]I transient amplitude was increased, while time to peak [Ca2+]I and [Ca2+]I decay were decreased. These data suggest that modulation of EP3 and EP4 activity has beneficial effects on cardiomyocyte contractility and overall heart function.

Abstract 10689: C-src Contributes to Mitochondrial Ca 2+ Uniporter Increase and Arrhythmic Risk in Ischemic Cardiomyopathy

Introduction: We have described previously an arrhythmic mechanism in nonischemic heart failure (HF) involving an increased mitochondrial Ca 2+ uptake that results in QT prolongation and lethal arrhythmias during cardiomyopathy. This arrhythmic mechanism is associated with mitochondrial Ca 2+ uniporter (MCU) tyrosine phosphorylation (p-Tyr). Hypothesis: Here, we determined which kinase was responsible for MCU phosphorylation in ischemic cardiomyopathy. Methods: Ischemic HF was induced in wild type FVB/NJ mice for three weeks by ligated left anterior descending coronary artery. Western blot, immunoprecipitation, fluorescence resonance energy transfer (FRET) and patch-clamp techniques were employed in isolated mouse cardiomyocytes. Results: When compared to sham hearts, the protein level of p-Src increased significantly in ischemic HF mouse hearts. In a heterologous expression system, c-Src could bind MCU and tyrosine phosphorylate MCU. This phosphorylation was blocked by a c-Src inhibitor, PP1. Overexpression of constitutive active c-Src (Src-Y527F) significantly increased MCU inward current at -160 mV. Downregulation of C-terminal Src kinase (CSK), a regulatory kinase inversely related to c-Src activity, increased p-Src, enhanced mitochondrial and SR contact, and facilitated mitochondrial Ca 2+ uptake in ischemic HF mouse cardiomyocytes. Protein expression of CaMKII, PyK2, and p-PyK2 did not change during ischemic HF. MI activated c-Src by S-nitrosylation of caveolin-1 with subsequent release of the kinase. c-Src activation was associated with APD prolongation and arrhythmia following myocardial infarction in mice. Human heart tissue showed that ischemic HF patients had significantly increased p-Src associated with increased QTc prolongation and arrhythmia. Conclusions: During ischemic HF, c-Src activation directly increased MCU current that contributed to QT prolongation and arrhythmic risk in ischemic cardiomyopathy.

Cardiomyocyte Protection by Hibernating Brown Bear Serum: Toward the Identification of New Protective Molecules Against Myocardial Infarction.

Ischemic heart disease remains one of the leading causes of death worldwide. Despite intensive research on the treatment of acute myocardial infarction, no effective therapy has shown clinical success. Therefore, novel therapeutic strategies are required to protect the heart from reperfusion injury. Interestingly, despite physical inactivity during hibernation, brown bears (Ursus arctos) cope with cardiovascular physiological conditions that would be detrimental to humans. We hypothesized that bear serum might contain circulating factors that could provide protection against cell injury. In this study, we sought to determine whether addition of bear serum might improve cardiomyocyte survival following hypoxia–reoxygenation. Isolated mouse cardiomyocytes underwent 45 min of hypoxia followed by reoxygenation. At the onset of reoxygenation, cells received fetal bovine serum (FBS; positive control), summer (SBS) or winter bear serum (WBS), or adult serums of other species, as indicated. After 2 h of reoxygenation, propidium iodide staining was used to evaluate cell viability by flow cytometry. Whereas, 0.5% SBS tended to decrease reperfusion injury, 0.5% WBS significantly reduced cell death, averaging 74.04 ± 7.06% vs. 79.20 ± 6.53% in the FBS group. This cardioprotective effect was lost at 0.1%, became toxic above 5%, and was specific to the bear. Our results showed that bear serum exerts a therapeutic effect with an efficacy threshold, an optimal dose, and a toxic effect on cardiomyocyte viability after hypoxia–reoxygenation. Therefore, the bear serum may be a potential source for identifying new therapeutic molecules to fight against myocardial reperfusion injury and cell death in general.

Functional Calsequestrin-1 Is Expressed in the Heart and Its Deficiency Is Causally Related to Malignant Hyperthermia-Like Arrhythmia.

Calsequestrins (Casqs), comprising the Casq1 and Casq2 isoforms, buffer Ca2+ and regulate its release in the sarcoplasmic reticulum of skeletal and cardiac muscle, respectively. Human inherited diseases associated with mutations in CASQ1 or CASQ2 include malignant hyperthermia/environmental heat stroke (MH/EHS) and catecholaminergic polymorphic ventricular tachycardia. However, patients with an MH/EHS event often experience arrhythmia for which the underlying mechanism remains unknown. Working hearts from conventional (Casq1-KO) and cardiac-specific (Casq1-CKO) Casq1 knockout mice were monitored in vivo and ex vivo by ECG and electric mapping, respectively. MH was induced by 2% isoflurane and treated intraperitoneally with dantrolene. Time-lapse imaging was used to monitor intracellular Ca2+ activity in isolated mouse cardiomyocytes or neonatal rat ventricular myocytes with knockdown, overexpression, or truncation of the Casq1 gene. Conformational change in both Casqs was determined by cross-linking Western blot analysis. Like patients with MH/EHS, Casq1-KO and Casq1-CKO mice had faster basal heart rate and ventricular tachycardia on exposure to 2% isoflurane, which could be relieved by dantrolene. Basal sinus tachycardia and ventricular ectopic electric triggering also occurred in Casq1-KO hearts ex vivo. Accordingly, the ventricular cardiomyocytes from Casq1-CKO mice displayed dantrolene-sensitive increased Ca2+ waves and diastole premature Ca2+ transients/oscillations on isoflurane. Neonatal rat ventricular myocytes with Casq1-knockdown had enhanced spontaneous Ca2+ sparks/transients on isoflurane, whereas cells overexpressing Casq1 exhibited decreased Ca2+ sparks/transients that were absent in cells with truncation of 9 amino acids at the C terminus of Casq1. Structural evaluation showed that most of the Casq1 protein was present as a polymer and physically interacted with ryanodine receptor-2 in the ventricular sarcoplasmic reticulum. The Casq1 isoform was also expressed in human myocardium. Mechanistically, exposure to 2% isoflurane or heating at 41 °C induced Casq1 oligomerization in mouse ventricular and skeletal muscle tissues, leading to a reduced Casq1/ryanodine receptor-2 interaction and increased ryanodine receptor-2 activity in the ventricle. Casq1 is expressed in the heart, where it regulates sarcoplasmic reticulum Ca2+ release and heart rate. Casq1 deficiency independently causes MH/EHS-like ventricular arrhythmia by trigger-induced Casq1 oligomerization and a relief of its inhibitory effect on ryanodine receptor-2-mediated Ca2+ release, thus revealing a new inherited arrhythmia and a novel mechanism for MH/EHS arrhythmogenesis.

A small molecule inhibitor of Nox2 and Nox4 improves contractile function after ischemia\u2013reperfusion in the mouse heart

The NADPH oxidase enzymes Nox2 and 4, are important generators of Reactive oxygen species (ROS). These enzymes are abundantly expressed in cardiomyocytes and have been implicated in ischemia–reperfusion injury. Previous attempts with full inhibition of their activity using genetically modified animals have shown variable results, suggesting that a selective and graded inhibition could be a more relevant approach. We have, using chemical library screening, identified a new compound (GLX481304) which inhibits Nox 2 and 4 (with IC50 values of 1.25 µM) without general antioxidant effects or inhibitory effects on Nox 1. The compound inhibits ROS production in isolated mouse cardiomyocytes and improves cardiomyocyte contractility and contraction of whole retrogradely (Langendorff) perfused hearts after a global ischemia period. We conclude that a pharmacological and partial inhibition of ROS production by inhibition of Nox 2 and 4 is beneficial for recovery after ischemia reperfusion and might be a promising venue for treatment of ischemic injury to the heart.

Abstract 13033: C-src Phosphorylates the Mitochondrial Ca 2+ Uniporter in Nonischemic Cardiomyopathy

Introduction: We have described previously an arrhythmic mechanism in nonischemic heart failure (NI-HF) involving an increased mitochondrial Ca 2+ uptake that results in QT prolongation and lethal arrhythmias. This arrhythmic mechanism is associated with mitochondrial Ca 2+ uniporter (MCU) tyrosine phosphorylation (p-Tyr). Hypothesis: Here, we determined which kinase was responsible for MCU phosphorylation in cardiomyopathy. Methods: NI-HF was induced by hypertension in Wt C57BL6 mice by unilateral nephrectomy, deoxycorticosterone acetate (DOCA) treatment and substituting drinking water with 1% saline for six weeks. Western blot, immunoprecipitation, fluorescence resonance energy transfer (FRET) and patch-clamp techniques were employed in isolated mouse cardiomyocytes. Results: When compared to control hearts, the protein level of p-Src S17 increased significantly accompanied by significant enhancement of tyrosine but not serine/threonine phosphorylation of MCU in NI-HF mouse hearts. In a heterologous expression system, c-Src could bind MCU and tyrosine phosphorylate MCU. This phosphorylation was blocked by a c-Src inhibitor, PP1. Overexpression of constitutive active c-Src (Src-Y527F) significantly increased MCU inward current at -160 mV. It suggested that c-Src activation was directly related to mitochondrial-mediated arrhythmic risk. Downregulation of C-terminal Src kinase (CSK), a regulatory kinase inversely related to c-Src activity, increased p-Src, enhanced mitochondrial and SR contact, and facilitated mitochondrial Ca 2+ uptake in NI-HF mouse cardiomyocytes. Proline-rich tyrosine kinase 2 (Pyk2) did not contribute more phosphorylated MCU during cardiomyopathy as the protein expression of PyK2, p-PyK2 Y402 and p-PyK2 Y579/580 did not change during NI-HF. Correlation using human heart tissue showed that NI-HF patients had significantly increased p-Src S17 associated with an increased incidence of QTc elongation and arrhythmia. Conclusions: During cardiomyopathy, c-Src activation directly tyrosine phosphorylated MCU, increased MCU current, and increased mitochondrial/SR contact, suggesting this kinase is responsible for the increase in mitochondria-mediated arrhythmic risk seen in NI-HF.

Simultaneous Recording of Subcellular Ca2+ Signals from the Cytosol and Sarco/Endoplasmic Reticulum: Compartmentalized Dye Loading, Imaging, and Analysis.

An increase in the cytosolic Ca2+ concentration triggers the contraction in cardiomyocytes. In these cells sarcoplasmic reticulum (SR) is the major source of Ca2+, and the release from this store is mediated by the ryanodine receptors (RyRs). These receptors are regulated by cytosolic and intra-SR [Ca2+]. The cytosolic Ca2+ regulation is well established, but there are some limitations to determine indirectly the intra-SR Ca2+ concentration and understand its role in the RyRs regulation. Therefore, the interest to directly measure the free intra-SR Ca2+ concentration ([Ca2+]SR) has led to the application of a low-affinity Ca2+ indicator (Fluo-5NAM) to follow changes of [Ca2+]SR in cardiomyocytes. However the loading of this AM-ester dye into the SR has remained a challenge in freshly isolated mouse cardiomyocytes. Here, we describe an optimized protocol to measure changes of [Ca2+]SR in mouse cardiomyocytes using fluorescent Ca2+ indicators and confocal microscopy. The application of this protocol allows to evaluate directly intra-SR Ca2+ in real time in various mouse models of cardiac disease, including transgenic animals harboring mutants of RyRs or other Ca2+ signaling proteins.

Calcitriol, the Bioactive Metabolite of Vitamin D, Increases Ventricular K+ Currents in Isolated Mouse Cardiomyocytes.

Calcitriol, the bioactive metabolite of vitamin D, interacts with the ubiquitously expressed nuclear vitamin D receptor (VDR) to induce genomic effects, but it can also elicit rapid responses via membrane-associated VDR through mechanisms that are poorly understood. The down-regulation of K+ currents is the main origin of electrophysiological remodeling in pathological hypertrophy and heart failure (HF), which can contribute to action potential prolongation and subsequently increase the risk of triggered arrhythmias. Adult mouse ventricular myocytes were isolated and treated with 10 nM calcitriol or vehicle for 15–30 min. In some experiments, cardiomyocytes were pretreated with the Akt inhibitor triciribine. In the adult mouse ventricle, outward K+ currents involved in cardiac repolarization are comprised of three components: the fast transient outward current (Itof), the ultrarapid delayed rectifier K+ current (Ikur), and the non-inactivating steady-state outward current (Iss). K+ currents were investigated using the whole-cell or the perforated patch-clamp technique and normalized to cell capacitance to obtain current densities. Calcitriol treatment of cardiomyocytes induced an increase in the density of Itof and Ikur, which was lost in myocytes isolated from VDR-knockout mice. In addition, calcitriol activated Akt in cardiomyocytes and pretreatment with triciribine prevented the calcitriol-induced increase of outward K+ currents. In conclusion, we demonstrate that calcitriol via VDR and Akt increases both Itof and Ikur densities in mouse ventricular cardiomyocytes. Our findings may provide new mechanistics clues for the cardioprotective role of this hormone in the heart.

Abstract 410: Prostaglandin E2 Reduces Carnitine Palmitoyltransferase 2 in Adult Mouse Cardiomyocytes

Prostaglandin E2 (PGE2) signals differently through its 4 receptor subtypes (EP1-EP4) to elicit diverse physiologic/pathologic effects. We previously reported that PGE2 via its EP3 receptor reduces cardiac contractility and male mice with cardiomyocyte-specific deletion of the EP4 receptor develop dilated cardiomyopathy. To determine pathway(s) responsible for the phenotype, we performed gene array on left ventricles from these mice followed by Ingenuity Pathway Analysis (IPA) which demonstrated that genes differentiating WT mice and EP4 KO mice were significantly overrepresented in mitochondrial (p=2.51x10 -28 ) and oxidative phosphorylation (p=3.16 x10 -30 ) pathways. Importantly, carnitine palmitoyltransferase 2 (cpt 2), an enzyme that transports fatty acids across the inner mitochondrial membrane was down-regulated in EP4 KO mice. We therefore hypothesized that PGE2 via EP3 decreases cpt 2 to impair mitochondrial function. To test this hypothesis, we used isolated mouse cardiomyocytes (AVM) from 16-18 week old male C57Bl/6 mice and treated them with either 1 μM of the EP3 agonist sulprostone (sulp), or PGE2. Treatment of AVM with sulp for 24 hrs. decreased cpt 2 levels from a control level of 1.0 to 0.57 ± 0.15, p < 0.05 and treatment with PGE2 decreased cpt 2 levels to 0.74 ± 0.27. Since cpt 2 is regulated by the transcription factor NR4A2 in other cell types, we investigated whether NR4A2 was regulated by PGE2 or sulp. NR4A2 levels were reduced after 4 hr. treatment with either PGE2 or by treatment with sulp; from a control value of 1.0 to 0.70 ± 0.16 and to 0.86 ± 0.01 respectively, p< 0.05 for sulp, n = 3. To associate these changes with mitochondrial function, we measured complex I activity with a spectrophotometric assay. Complex I activity was reduced by 50% (p < 0.05) by 4 hr. treatment with PGE2, from 1.32 ± 0.36 to 0.66 ± 0.08 mOD/min, and this was associated with reduced expression of multiple genes from mitochondrial pathways including sub units of mitochondrial NADH dehydrogenase ubiquinone flavoprotein (Nduf), a component of complex I. In conclusion, these results suggest that increased PGE2 and its EP3 receptor in heart disease may contribute to impaired mitochondrial function and provide yet another link between inflammation and cardiac dysfunction.

Humoral transfer and intramyocardial signal transduction of protection by remote ischemic perconditioning in pigs, rats, and mice

Remote ischemic perconditioning (RPER) during ongoing myocardial ischemia reduces infarct size. The signal transduction of RPER's cardioprotection is still largely unknown. Anesthetized pigs were therefore subjected to RPER by 4 × 5 min/5 min of hindlimb ischemia-reperfusion during 60 min of coronary occlusion before 3 h of reperfusion. Pigs without RPER served as placebo (PLA). The phosphorylation of Akt and ERK [reperfusion injury salvage kinase (RISK) pathway] and STAT3 [survivor activating factor enhancement (SAFE) pathway] in the area at risk was determined by Western blot analysis. Wortmannin/U0126 or AG490 was used for pharmacological RISK or SAFE blockade, respectively. Pig plasma/plasma dialysate sampled after RPER or PLA, respectively, was transferred to isolated rat and mouse hearts subjected to 30 min/120 min of global ischemia-reperfusion. Mitochondria were isolated from rat hearts at early reperfusion. Isolated mouse cardiomyocytes were subjected to 1 h of hypoxia/5 min of reoxygenation without and with prior plasma dialysate incubation. RPER reduced infarct size in pigs to 21 ± 15% versus 44 ± 9% in PLA (percentage of the area at risk, mean ± SD, P < 0.05) and increased STAT3 phosphorylation at early reperfusion. AG490 but not RISK blockade abolished the protection. RPER plasma/plasma dialysate reduced infarct size in rat (22 ± 3% of ventricular mass vs. 40 ± 11% with PLA plasma, P < 0.05) and mouse (29 ± 4% vs. 63 ± 8% with PLA plasma dialysate, P < 0.05) hearts and improved mitochondrial function (e.g., increased respiration, ATP formation, and calcium retention capacity and decreased reactive oxygen species formation). RPER dialysate also improved the viability of mouse cardiomyocytes after hypoxia/reoxygenation. RISK or SAFE blockade each abrogated these beneficial effects. NEW & NOTEWORTHY Remote ischemic perconditioning salvages the myocardium in patients with acute infarction. We identified a signal transduction with humoral transfer and STAT3 activation in pigs and an involvement of reperfusion injury salvage kinases and STAT3 in rat and mouse hearts, along with better cardiomyocyte viability and mitochondrial function.

Abstract P352: Prostaglandin E2 Reduces Mitochondrial Function in Adult Mouse Cardiomyocytes

Hypertension is a leading cause of heart failure and both conditions are characterized by increased prostaglandin E2 (PGE2) which signals through 4 receptor subtypes (EP1-EP4) to elicit diverse physiologic effects. We previously reported that cardiomyocyte-specific deletion of the EP4 receptor results in a phenotype of dilated cardiomyopathy in male mice that is characterized by reduced ejection fraction. Subsequent gene array on left ventricles from these mice, coupled with Ingenuity Pathway Analysis (IPA) demonstrated that genes differentiating WT mice and EP4 KO mice with low ejection fraction were significantly overrepresented in mitochondrial (p=2.51x10 -28 ) and oxidative phosphorylation (p=3.16 x10 -30 ) pathways. We therefore hypothesized that PGE2 could reduce mitochondrial function. To test this hypothesis, we used isolated mouse cardiomyocytes (AVM) from 16-18 week old male C57Bl/6 mice and treated them with 1 μM PGE2 for various times. Mitochondrial gene expression was examined using a RT-profiler kit for mitochondrial energy metabolism, complex I activity with a spectrophotometric assay, ATP levels with a bioluminescence assay and mitochondrial membrane potential using JC-1 staining. Treatment of AVM with PGE2 for 4 hrs reduced expression of multiple genes from mitochondrial pathways including sub units of mitochondrial NADH dehydrogenase ubiquinone flavoprotein (Nduf), a component of complex I. In accord with the mRNA data, Complex I activity was reduced by 50% (p &lt; 0.05) by 4 hr treatment with PGE2, from 1.32 ± 0.36 to 0.66 ± 0.08 mOD/min. Cytochrome c oxidase subunit 8 (Cox8c) mRNA was also reduced from a control value of 1.00 to -1.75 ± 0.20 (p &lt; 0.005) after PGE2 treatment. Immuno-fluorescence showed that JC-1 aggregates were reduced after 1 or 3 hr treatment with either 1 μM PGE2 or the EP3 agonist, sulprostone, suggesting reduced mitochondrial membrane potential. Subsequent experiments also showed that ATP levels were reduced 16% from 11.18 ± 0.71 nmol to 9.39 ± 0.83 nmol after treatment with sulprostone for only 1 hr. Taken together, these results suggest that increased PGE2 in hypertension may contribute to impaired mitochondrial function and provide yet another link between inflammation and cardiac dysfunction.

Role of protein kinase C in metabolic regulation of the cardiac Na+ channel

The reduced form of nicotinamide adenine dinucleotide (NADH) increases in cardiomyopathy, activates protein kinase C (PKC), up-regulates mitochondrial reactive oxygen species (mitoROS), and down-regulates the cardiac Na+ channel (NaV1.5). The purpose of this study was to determine how NADH signals down-regulation of NaV1.5. Isolated mouse cardiomyocytes were used for patch-clamp recording and for monitoring mitoROS with MitoSOX Red. HEK293 cells were used for transient transfections. HEK293 cells stably expressing human NaV1.5 were used for single channel recording, whole-cell patch-clamp recording, activity measurements of phospholipase C and phospholipase D (PLD), channel protein purification, and co-immunoprecipitation with PKC isoforms. HL-1 cells were used for mitochondria isolation. NADH enhanced PLD activity (1.6- ± 0.1-fold, P <.01) and activated PKCδ. Activated PKCδ translocated to mitochondria and up-regulated mitoROS (2.8- ± 0.3-fold, P <.01) by enhancing the activities of mitochondrial complexes I, II, and IV (1.1- to 1.5-fold, P <.01). PKCδ also interacted with NaV1.5 to down-regulate Na+ current (INa). Reduction in INa by activated PKCδ was prevented by antioxidants and by mutating the known PKC phosphorylation site S1503. At the single channel level, the mechanism of current reduction by PKC and recovery by protein kinase A was a change in single channel conductance. NADH activated PKCδ by enhancing PLD activity. PKCδ modulated both mitoROS and NaV1.5. PKCδ elevated mitoROS by enhancing mitochondrial oxidative phosphorylation complex activities. PKCδ-mediated channel phosphorylation and mitoROS were both required to down-regulate NaV1.5 and alter single channel conductance.

Nitric Oxide Synthase 1 Modulates Basal and β-Adrenergic-Stimulated Contractility by Rapid and Reversible Redox-Dependent S-Nitrosylation of the Heart.

S-nitrosylation of several Ca2+ regulating proteins in response to β-adrenergic stimulation was recently described in the heart; however the specific nitric oxide synthase (NOS) isoform and signaling pathways responsible for this modification have not been elucidated. NOS-1 activity increases inotropism, therefore, we tested whether β-adrenergic stimulation induces NOS-1-dependent S-nitrosylation of total proteins, the ryanodine receptor (RyR2), SERCA2 and the L-Type Ca2+ channel (LTCC). In the isolated rat heart, isoproterenol (10 nM, 3-min) increased S-nitrosylation of total cardiac proteins (+46±14%) and RyR2 (+146±77%), without affecting S-nitrosylation of SERCA2 and LTCC. Selective NOS-1 blockade with S-methyl-L-thiocitrulline (SMTC) and Nω-propyl-l-arginine decreased basal contractility and relaxation (−25–30%) and basal S-nitrosylation of total proteins (−25–60%), RyR2, SERCA2 and LTCC (−60–75%). NOS-1 inhibition reduced (−25–40%) the inotropic response and protein S-nitrosylation induced by isoproterenol, particularly that of RyR2 (−85±7%). Tempol, a superoxide scavenger, mimicked the effects of NOS-1 inhibition on inotropism and protein S-nitrosylation; whereas selective NOS-3 inhibitor L-N5-(1-Iminoethyl)ornithine had no effect. Inhibition of NOS-1 did not affect phospholamban phosphorylation, but reduced its oligomerization. Attenuation of contractility was abolished by PKA blockade and unaffected by guanylate cyclase inhibition. Additionally, in isolated mouse cardiomyocytes, NOS-1 inhibition or removal reduced the Ca2+-transient amplitude and sarcomere shortening induced by isoproterenol or by direct PKA activation. We conclude that 1) normal cardiac performance requires basal NOS-1 activity and S-nitrosylation of the calcium-cycling machinery; 2) β-adrenergic stimulation induces rapid and reversible NOS-1 dependent, PKA and ROS-dependent, S-nitrosylation of RyR2 and other proteins, accounting for about one third of its inotropic effect.

Real-time intra-store confocal Ca2+ imaging in isolated mouse cardiomyocytes

To initiate the contraction of cardiomyocytes, Ca2+ is released from the SR to the cytosol via ryanodine receptors (RyRs), which are activated by the Ca2+–induced Ca2+ release mechanism (CICR). The activity of RyRs is regulated by both, cytosolic and SR luminal Ca2+. Deregulation of the CICR, by dysfunctional SR Ca2+ release or uptake, is frequently associated with cardiac pathologies (e.g. arrhythmias, CPVT, heart failure). Recently, the interest to directly measure changes of the free Ca2+ concentration within the SR ([Ca2+]SR) has led to the application of low affinity Ca2+ indicators (mag-fluo-4, Fluo-5N) to follow changes of [Ca2+]SR in cardiomyocytes from some species. However, direct measurement of Ca2+ signals from the SR have not been possible in freshly isolated mouse cardiomyocytes. Here, we show a new protocol optimized to measure changes of [Ca2+]SR in mouse cardiomyocytes using fluorescent Ca2+ indicators and confocal microscopy. The application of this protocol permits the design of experimental studies with direct evaluation of SR Ca2+ in real time in various mouse models of cardiac disease, including transgenic animals harboring mutants of RyRs or other Ca2+ signaling proteins. The technique, in combination with these models, will help to understand how these diseases and mutations affect Ca2+ signals within the SR and the Ca2+ sensitivity of the RyRs for cytosolic and SR luminal Ca2+, thereby contributing to arrhythmias or weak heart beat.

TRPA1 is functionally co-expressed with TRPV1 in cardiac muscle: Co-localization at z-discs, costameres and intercalated discs

ABSTRACTTransient receptor potential channels of the ankyrin subtype-1 (TRPA1) and vanilloid subtype-1 (TRPV1) are structurally related, non-selective cation channels that show a high permeability to calcium. Previous studies indicate that TRP channels play a prominent role in the regulation of cardiovascular dynamics and homeostasis, but also contribute to the pathophysiology of many diseases and disorders within the cardiovascular system. However, no studies to date have identified the functional expression and/or intracellular localization of TRPA1 in primary adult mouse ventricular cardiomyocytes (CMs). Although TRPV1 has been implicated in the regulation of cardiac function, there is a paucity of information regarding functional expression and localization of TRPV1 in adult CMs. Our current studies demonstrate that TRPA1 and TRPV1 ion channels are co-expressed at the protein level in CMs and both channels are expressed throughout the endocardium, myocardium and epicardium. Moreover, immunocytochemical localization demonstrates that both channels predominantly colocalize at the Z-discs, costameres and intercalated discs. Furthermore, specific TRPA1 and TRPV1 agonists elicit dose-dependent, transient rises in intracellular free calcium concentration ([Ca2+]i) that are abolished in CMs obtained from TRPA1−/− and TRPV1−/− mice. Similarly, we observed a dose-dependent attenuation of the TRPA1 and TRPV1 agonist-induced increase in [Ca2+]i when WT CMs were pretreated with increasing concentrations of selective TRPA1 or TRPV1 channel antagonists. In summary, these findings demonstrate functional expression and the precise ultrastructural localization of TRPA1 and TRPV1 ion channels in freshly isolated mouse CMs. Crosstalk between TRPA1 and TRPV1 may be important in mediating cellular signaling events in cardiac muscle.

Methylene blue counteracts H2S toxicity-induced cardiac depression by restoring L-type Ca channel activity.

We have previously reported that methylene blue (MB) can counteract hydrogen sulfide (H2S) intoxication-induced circulatory failure. Because of the multifarious effects of high concentrations of H2S on cardiac function, as well as the numerous properties of MB, the nature of this interaction, if any, remains uncertain. The aim of this study was to clarify 1) the effects of MB on H2S-induced cardiac toxicity and 2) whether L-type Ca(2+) channels, one of the targets of H2S, could transduce some of the counteracting effects of MB. In sedated rats, H2S infused at a rate that would be lethal within 5 min (24 μM·kg(-1)·min(-1)), produced a rapid fall in left ventricle ejection fraction, determined by echocardiography, leading to a pulseless electrical activity. Blood concentrations of gaseous H2S reached 7.09 ± 3.53 μM when cardiac contractility started to decrease. Two to three injections of MB (4 mg/kg) transiently restored cardiac contractility, blood pressure, and V̇o2, allowing the animals to stay alive until the end of H2S infusion. MB also delayed PEA by several minutes following H2S-induced coma and shock in unsedated rats. Applying a solution containing lethal levels of H2S (100 μM) on isolated mouse cardiomyocytes significantly reduced cell contractility, intracellular calcium concentration ([Ca(2+)]i) transient amplitudes, and L-type Ca(2+) currents (ICa) within 3 min of exposure. MB (20 mg/l) restored the cardiomyocyte function, ([Ca(2+)]i) transient, and ICa The present results offer a new approach for counteracting H2S toxicity and potentially other conditions associated with acute inhibition of L-type Ca(2+) channels.

Unique Localization of Phospholamban in Perinuclear Membranes of Cardiomyocytes from Several Species

Phospholamban (PLB) regulates sarcoplasmic reticulum (SR) calcium-ATPase (SERCA2a) in cardiomyocytes (CMs). In addition to their well-known location in SR membranes, only SERCA2a, but not PLB, is shown expressed in the perinuclear/nuclear membranes of CMs. Here we used confocal immunofluorescence microscopy to examine the intracellular distribution of PLB. As expected, in isolated mouse CMs, co-localized PLB and SERCA2a were detected in SR by the monoclonal antibodies 2D12 (against PLB) and 2A7-A1 (against SERCA2a), respectively. Importantly, in addition to 2A7-A1 binding signals, strong 2D12 binding signal was detected around the DAPI staining of the nuclei, indicating the expression of PLB in perinuclear/nuclear membranes. Further, we probed PLB expression in tissue sections from hearts of human, dog, or rabbit with use of a polyclonal (PLB9) and three monoclonal (2D12, 1G7, or 1F1) antibodies, which were produced against different epitope of PLB in a specie-dependent manner. In particular, 2D12 and PLB9 recognized all isoforms of PLB in these species and revealed PLB expression in SR as well as in the perinuclear/nuclear regions. In comparison, 1F1, the dog specie-specific antibody, recognized PLB expression in dog SR and in perinuclear/nuclear membranes, but is completely negative for human, mouse, or rabbit CMs. Moreover, 1G7, which is specie-specific for human, mouse, or rabbit isoform of PLB, recognized similar pattern of PLB expression in these species including in perinuclear membranes, but not in dog CMs. Finally, 2D12 binding signals were completely eliminated by co-incubation of a PLB peptide (residues 2-25). These results showed for the first time that in addition to its expression in SR, PLB is expressed in and around nuclear envelope of CMs.