Calcium Handling In Myocytes Research Articles

Assessing sex-based differences in postsurgical clinical outcomes after use of del Nido cardioplegia.

del Nido cardioplegia is used to pharmacologically arrest the heart during cardiac surgery and decrease reperfusion- and ischaemia-related myocardial injury. Studies have demonstrated the physiological differences between male and female hearts, potentially related to cardiac size or myocyte calcium handling; we aimed to assess for between-sex differences in clinical outcomes after receipt of del Nido cardioplegia. Patients who underwent coronary artery bypass or coronary artery bypass graft/valve surgery at our institution using del Nido cardioplegia (January 2014 to December 2019) were included (n = 2118). Clinical data were collected retrospectively. After the creation of a propensity-matched cohort (n = 1252), multivariable logistic regression was used to analyse binary postoperative outcomes, and a Gamma model was used for a continuous postoperative outcome. Our primary end-point was a composite end-point comprised of 30-day mortality and/or need for a post-bypass mechanical support device. The final cohort included 459 females and 793 males (matched up to 1:2, all standardized mean differences <0.1). Multivariable logistic regression showed that biological sex was not associated with the composite primary end-point (odds ratio = 0.898, P = 0.779). A Gamma model indicated that there were no sex-related differences in vasoactive-inotropic scores reflecting vasopressor and inotrope usage at the time of patient operating room exit (exp[est] = 1.394, P = 0.189). Our findings showed no significant between-sex differences in clinical outcomes after receiving del Nido cardioplegia, suggesting adequate myocardial protection as currently administered. Further research is warranted to elicit if there are sex-based differences between cardioplegic solutions. 26 May 2021 (AAAR8359).

Diabetes disturbs functional adaptation of the remote myocardium after ischemia/reperfusion

Diabetes mellitus type 2 is associated with adverse clinical outcome after myocardial infarction. To better understand the underlying causes we here investigated sarcomere protein function and its calcium-dependent regulation in the non-ischemic remote myocardium (RM) of diabetic mice (db/db) after transient occlusion of the left anterior descending coronary artery. Before and 24 h after surgery db/db and non-diabetic db/+ underwent magnetic resonance imaging followed by histological and biochemical analyses of heart tissue. Intracellular calcium transients and sarcomere function were measured in isolated cardiomyocytes. Active and passive force generation was assessed in skinned fibers and papillary muscle preparations. Before ischemia and reperfusion (I/R), beat-to-beat calcium cycling was depressed in diabetic cardiomyocytes. Nevertheless, contractile function was preserved owing to increased myofilament calcium sensitivity and higher responsiveness of myocardial force production to β-adrenergic stimulation in db/db compared to db/+. In addition, protein kinase C activity was elevated in db/db hearts leading to strong phosphorylation of the titin PEVK region and increased titin-based tension of myofilaments. I/R impaired the function of whole hearts and RM sarcomeres in db/db to a larger extent than in non-diabetic db/+, and we identified several reasons. First, the amplitude and the kinetics of cardiomyocyte calcium transients were further reduced in the RM of db/db. Underlying causes involved altered expression of calcium regulatory proteins. Diabetes and I/R additively reduced phospholamban S16-phosphorylation by 80% (P < 000.1) leading to strong inhibition of the calcium ATPase SERCA2a. Second, titin stiffening was only observed in the RM of db/+, but not in the RM of db/db. Finally, db/db myofilament calcium sensitivity and force generation upon β-adrenergic stimulation were no longer enhanced over db/+ in the RM. The findings demonstrate that impaired cardiomyocyte calcium cycling of db/db hearts is compensated by increased myofilament calcium sensitivity and increased titin-based stiffness prior to I/R. In contrast, sarcomere function of the RM 24 h after I/R is poor because both these compensatory mechanisms fail and myocyte calcium handling is further depressed.

Muscarinic-dependent phosphorylation of the cardiac ryanodine receptor by protein kinase G is mediated by PI3K–AKT–nNOS signaling

Post-translational modifications of proteins involved in calcium handling in myocytes, such as the cardiac ryanodine receptor (RyR2), critically regulate cardiac contractility. Recent studies have suggested that phosphorylation of RyR2 by protein kinase G (PKG) might contribute to the cardioprotective effects of cholinergic stimulation. However, the specific mechanisms underlying these effects remain unclear. Here, using murine ventricular myocytes, immunoblotting, proximity ligation as-says, and nitric oxide imaging, we report that phosphorylation of Ser-2808 in RyR2 induced by the muscarinic receptor agonist carbachol is mediated by a signaling axis comprising phosphoinositide 3-phosphate kinase, Akt Ser/Thr kinase, nitric oxide synthase 1, nitric oxide, soluble guanylate cyclase, cyclic GMP (cGMP), and PKG. We found that this signaling pathway is compartmentalized in myocytes, as it was distinct from atrial natriuretic peptide receptor-cGMP-PKG-RyR2 Ser-2808 signaling and independent of muscarinic-induced phosphorylation of Ser-239 in vasodilator-stimulated phosphoprotein. These results provide detailed insights into muscarinic-induced PKG signaling and the mediators that regulate cardiac RyR2 phosphorylation critical for cardiovascular function.

4087Functional assessment of the myocardium in non-ischaemic cardiomyopathy using manganese-enhanced T1 mapping

Abstract Background Delayed-enhancement magnetic resonance imaging (DEMRI) with gadolinium detects macroscopic myocardial fibrosis, but does not directly assess myocardial function. Manganese, a paramagnetic calcium-analogue, enables direct dynamic assessment myocyte calcium handling and is a potential early marker of myocardial dysfunction. We have previously demonstrated manganese-enhanced MRI (MEMRI) can detect altered calcium-handling in remote remodelling myocardium in a rodent infarct model. Purpose To evaluate MEMRI as a marker of altered myocardial calcium handling in patients with non-ischaemic cardiomyopathy (NCT03607669). Methods Healthy volunteers (HV, n=20, 13 male, 42±11 years) and patients with hypertrophic (HCM, n=11, 5 male, 57±15 years) or dilated (DCM, n=5, 3 male, 56±14 years) cardiomyopathies underwent both DEMRI and MEMRI. Myocardial T1 was measured for 40 min after intravenous manganese dipyridoxyl diphosphate (Mn-DPDP, 5 μmol/kg) administration. Images were acquired at 3T with T1 imaging performed using Shortened Modified Look-Locker Inversion recovery [Figure A] (WIP #1048 Siemens Healthcare Ltd). Results No adverse effects were observed. In HV, the myocardialT1 profile following Mn-DPDP administration demonstrated a rapid initial descent (perfusion phase) followed by a slower, more gradual decrease which continued throughout the 40 min imaging period (mean reduction 25.2%, 283±9ms). In patients with HCM, T1 values followed a different pattern following Mn-DPDP administration. In areas of focal myocardial fibrosis (defined by DEMRI) there was again a sharp initial shortening of T1 but this was followed by a steady increase towards baseline, which eventually recovered to &gt;40% of its initial value at 40 minutes. In areas of diffuse myocardial fibrosis, the initial shortening in the perfusion phase was followed by a more gradual recovery which then plateaued, remaining stable at 21% of native T1 by 40 min [Figure B]. Hypertrophied non-fibrosed myocardium had higher native T1 than HV (1178±45 vs 1133±36 ms P=0.005) and the rate of T1 reduction following the perfusion phase was slower (P=0.0005), leading to less overall T1 shortening at 40 min (880±40 vs 841±28 ms, P=0.004, [Figure C]). No patient with DCM had fibrosis by DEMRI. Native T1 was higher than HV (1234±26 vs 1133±36 ms, P&lt;0.0001) and the rate of T1 reduction following the perfusion phase was lower in DCM myocardium (P&lt;0.0001) resulting in less overall T1 shortening at 40 min (914±60 vs 841±28 ms, P=0.0006, [Figure D]). MEMRI in non-ischaemic cardiomyopathy Conclusion MEMRI identifies altered calcium-handling in pathological myocardium of non-ischaemic cardiomyopathy, irrespective of fibrosis by DEMRI. This feasible, safe and novel method of functional myocardial tissue characterisation has exciting potential for future diagnostic, functional and clinical applications. Acknowledgement/Funding British Heart Foundation

Impact of dietary nitrate on age-related diastolic dysfunction.

Diastolic dysfunction is highly prevalent, and ageing is the main contributor due to impairments in active cardiac relaxation, ventriculo-vascular stiffening, and endothelial dysfunction. Nitric oxide (NO) affects cardiovascular functions, and NO bioavailability is critically reduced with ageing. Whether replenishment of NO deficiency with dietary inorganic nitrate would offer a novel approach to reverse age-related cardiovascular alterations was not known. A dietary nitrate supplementation was applied to young (6 month) and old (20 month) wild-type mice for 8 weeks and compared with controls. High-resolution ultrasound, pressure-volume catheter techniques, and isolated heart measurements were applied to assess cardiac diastolic and vascular functions. Cardiac manganese-enhanced magnetic resonance imaging was performed to study the effects of dietary nitrate on myocyte calcium handling. In aged mice with preserved systolic function, dietary nitrate supplementation improved LV diastolic function, arterial compliance, and coronary flow reserve. Mechanistically, improved cardiovascular functions were associated with an accelerated cardiomyocyte calcium handling and augmented NO/cyclic guanosine monophosphate/protein kinase G signalling, while enhanced nitrate reduction was related to age-related differences in the oral microbiome. Dietary inorganic nitrate reverses age-related LV diastolic dysfunction and improves vascular functions. Our results highlight the potential of a dietary approach in the therapy of age-related cardiovascular alterations.

Properties of Atrial Myocyte Calcium Handling in Canine Model of Chronic Heart Failure

Weakened atrial contractility is a common comorbidity in heart failure (HF) that increases severity of HF symptoms. The role of intracellular calcium (Ca) handling remodeling in pathogenesis of ventricular contractile failure has been well established. However, the relationship between Ca signaling and atrial dysfunction in HF is not well recognized. We studied intracellular Ca handling in atrial myocytes isolated from control dogs and dogs with tachypacing-induced chronic HF. Ca transients and cellular shortening were reduced, while sarcoplasmic reticulum (SR) Ca content was increased in field-stimulated myocytes from HF compared to controls. Both control and HF myocytes have scarce t-tubule density, and Ca transients originated predominantly at the periphery of myocytes in both groups. Diminished Ca transients in HF myocytes were associated with decreased peripheral Ca release and impaired propagation of Ca signal from sarcolemma to the central areas of the myocyte. Peak density of the L-type Ca currents and gain of excitation-contraction coupling were reduced in voltage-clamped HF myocytes compared to control myocytes. Analysis of decay of caffeine- and electrically-induced Ca transients suggests reduced Na/Ca exchanger activity and increased SR Ca ATPase (SERCA) activity in HF myocytes. SERCA inhibition with cyclopiazonic acid did not improve centripetal propagation of Ca signal in HF myocytes. In contrast, L-type Ca channel agonist BayK 8644 increased peripheral Ca release, improved centripetal Ca wave propagation and restored shortening of HF myocytes to control levels.We conclude that defective activation and impaired propagation of the SR Ca release in atrial myocytes contribute to compromised atrial contractility in chronic HF. Our results suggest L-type Ca channels as a therapeutic target for treatment of atrial contractile dysfunction in HF.

Mitochondrial Hyperfusion during Oxidative Stress Is Coupled to a Dysregulation in Calcium Handling within a C2C12 Cell Model

Atrial Fibrillation is the most common sustained cardiac arrhythmia worldwide harming millions of people every year. Atrial Fibrillation (AF) abruptly induces rapid conduction between atrial myocytes which is associated with oxidative stress and abnormal calcium handling. Unfortunately this new equilibrium promotes perpetuation of the arrhythmia. Recently, in addition to being the major source of oxidative stress within cells, mitochondria have been observed to fuse, forming mitochondrial networks and attach to intracellular calcium stores in response to cellular stress. We sought to identify a potential role for rapid stimulation, oxidative stress and mitochondrial hyperfusion in acute changes to myocyte calcium handling. In addition we hoped to link altered calcium handling to increased sarcoplasmic reticulum (SR)-mitochondrial contacts, the so-called mitochondrial associated membrane (MAM). We selected the C2C12 murine myotube model as it has previously been successfully used to investigate mitochondrial dynamics and has a myofibrillar system similar to atrial myocytes. We observed that rapid stimulation of C2C12 cells resulted in mitochondrial hyperfusion and increased mitochondrial colocalisation with calcium stores. Inhibition of mitochondrial fission by transfection of mutant DRP1K38E resulted in similar effects on mitochondrial fusion, SR colocalisation and altered calcium handling. Interestingly the effects of ‘forced fusion’ were reversed by co-incubation with the reducing agent N-Acetyl cysteine (NAC). Subsequently we demonstrated that oxidative stress resulted in similar reversible increases in mitochondrial fusion, SR-colocalisation and altered calcium handling. Finally, we believe we have identified that myocyte calcium handling is reliant on baseline levels of reactive oxygen species as co-incubation with NAC both reversed and retarded myocyte response to caffeine induced calcium release and re-uptake. Based on these results we conclude that the coordinate regulation of mitochondrial fusion and MAM contacts may form a point source for stress-induced arrhythmogenesis. We believe that the MAM merits further investigation as a therapeutic target in AF-induced remodelling.

Low-dose bisphenol A and estrogen increase ventricular arrhythmias following ischemia–reperfusion in female rat hearts

Bisphenol A (BPA) is an environmental estrogenic endocrine disruptor that may have adverse health impacts on a range of tissue/systems. In previous studies, we reported that BPA rapidly promoted arrhythmias in female rodent hearts through alteration of myocyte calcium handling. In the present study we investigated the acute effects of BPA on ventricular arrhythmias and infarction following ischemia-reperfusion in rat hearts. Rat hearts were subjected to 20 min of global ischemia followed by reperfusion. In female, but not male hearts, acute exposure to 1 nM BPA, either alone or combined with 1 nM 17β-estradiol (E2), during reperfusion resulted in a marked increase in the duration of sustained ventricular arrhythmias. BPA plus E2 increased the duration ventricular fibrillation, and the duration of VF as a fraction of total duration of sustained ventricular arrhythmia. The pro-arrhythmic effects of estrogens were abolished by MPP combined with PHTPP, suggesting the involvements of both ERα and ERβ signaling. In contrast to their pro-arrhythmic effects, BPA and E2 reduced infarction size, agreeing with previously described protective effect of estrogen against cardiac infarction. In conclusion, rapid exposure to low dose BPA, particularly when combined with E2, exacerbates ventricular arrhythmia following IR injury in female rat hearts.

Abstract 9575: Myocyte-Specific Catalase Overexpression Prevents Age-Related Left Ventricular Diastolic Dysfunction: Association with Reduction of Oxidation of SERCA at Cysteine 674

Background: Age-related myocardial diastolic dysfunction is associated with impaired calcium handling and reduced sarcoplasmic reticulum calcium ATPase (SERCA) activity. Oxidative sulfonylation of SERCA at cysteine 674 inhibits SERCA activity. We tested the hypothesis that a) aging is associated with sulfonylation of SRECA at cysteine 674, and b) cardiac myocyte-specific over-expression of catalase would decrease SERCA sulfonylation, restore SERCA activity, improve myocyte function and correct diastolic dysfunction. Methods and Results: Young (Y, 5-month) and senescent (S, 21-month) wild-type (WT) or CAT mice were compared (n=8-13). In S-WT there was markedly impaired diastolic function as reflected by a) decreased peak early-to-late diastolic transmitral velocity ratio (E/A, 1.36±0.06 vs. 1.96±0.10 in Y-WT or 1.89±0.06 in Y-CAT, P&lt;0.01), b) decreased early myocardial tissue velocity (E m , mm/s, 20.1±0.6 vs. 26.6±0.1 in Y-WT or 24.9±0.7 in Y-CAT, P&lt;0.01), and c) leftward/upward shift in the end-diastolic pressure volume relationship by Langendorff perfusion. Diastolic dysfunction in S-WT was associated with decreased SERCA activity (nmol/mg protein/min, 3.37±0.45 vs. 5.50±0.55 in Y-WT, P&lt;0.01) and increased oxidation of SERCA at cysteine 674 (Scores, 2.4±0.3 vs. 1.1±0.2 in Y-WT or 1.1±0.1 in Y-CAT, P&lt;0.02). Likewise, in isolated myocytes from S-WT there was slowed relaxation (+dL/dt, μ m/s, 1.35±0.07 vs. 2.68±0.17 in Y-WT, P&lt;0.01) and prolongation of Ca 2+ reuptake (Tau, ms, 46.7±0.15 vs. 35.7±0.16 in Y-WT, P&lt;0.01). In senescent mice with CAT overexpression (S-CAT) there was complete normalization of LV diastolic function (E/A, 1.97±0.12; E m , 24.9±1.4), SERCA activity (6.10±0.36 nmol/mg/min), SERCA oxidation (Score, 1.23±0.08), myocyte relaxation (+dL/dt, 2.34±0.14 μ m/s) and myocyte calcium handling (Tau, 38.3±0.08 ms). Conclusion: Age-related impairment of SERCA activity is associated with sulfonylation of cysteine 674. Myocyte-specific overexpression of CAT, which prevents aging-induced LV diastolic dysfunction and myocyte calcium dysregulation, prevents SERCA sulfonylation at cysteine 674 and restores SERCA activity, suggesting that SERCA sulfonylation plays a central role in the age-related myocyte dysfunction.

Abstract 8142: By Virtue of Its Interaction with the β 2 -adrenergic Receptor, CXCR4 Plays a Role in the Regulation of β 2 -adrenergic Receptor Signaling Pathway Affecting Calcium and Contractility in Cardiac Myocytes

Chemokines are small secreted proteins with chemoattractant properties that play a key role in inflammation, metastasis, and embryonic development. We previously demonstrated a nonchemotactic role for one such chemokine pair, stromal cell-derived factor-1α (SDF-1α) and its G-protein coupled receptor, CXCR4. SDF-1 α /CXCR4 are expressed on cardiac myocytes and have direct consequences on cardiac myocyte physiology by inhibiting contractility in response to the nonselective β-adrenergic receptor (βAR) agonist, isoproterenol. As a result of the importance of β-adrenergic signaling in heart failure pathophysiology, we investigated the underlying mechanism involved in CXCR4 modulation of βAR signaling. We hypothesis that β 2 adrenergic receptor in the heart is negatively modulated by interactions with CXCR4 chemokine receptor. For this we have used biophysical techniques such as bioluminescence resonance energy transfer assay and biochemical techniques such as immunoprecipitation and signaling assays to identify the CXCR4-β2AR interaction and determine the physiological significance of this interaction. Our studies demonstrate activation of CXCR4 by SDF-1 leads to a decrease in βAR-induced PKA activity as assessed by cAMP accumulation and PKA-dependent phosphorylation of phospholamban, an inhibitor of SERCA2a. We determined CXCR4 regulation of βAR downstream targets is β2AR-dependent. In conclusion, we demonstrated a physical interaction between CXCR4 and β2AR as determined by coimmunoprecipitation, confocal microscopy, and BRET techniques. The CXCR4-β2AR interaction leads to G-protein signal switching from Gαs to Gαi-mediated pathway and suggests the interaction is a novel mechanism for regulating cardiac myocyte calcium handling and subsequent contractility. Chemokines are physiologically and developmentally relevant to myocardial biology and represent a novel receptor class of cardiac modulators. The CXCR4- β2AR complex could represent a hitherto unknown target for therapeutic intervention.

Effects of dietary omega–3 fatty acids on ventricular function in dogs with healed myocardial infarctions: in vivo and in vitro studies

Since omega-3 polyunsaturated fatty acids (n-3 PUFAs) can alter ventricular myocyte calcium handling, these fatty acids could adversely affect cardiac contractile function, particularly following myocardial infarction. Therefore, 4 wk after myocardial infarction, dogs were randomly assigned to either placebo (corn oil, 1 g/day, n = 16) or n-3 PUFAs supplement [docosahexaenoic acid (DHA) + eicosapentaenoic acid (EPA) ethyl esters; 1, 2, or 4 g/day; n = 7, 8, and 12, respectively] groups. In vivo, ventricular function was evaluated by echocardiography before and after 3 mo of treatment. At the end of the 3-mo period, hearts were removed and in vitro function was evaluated using right ventricular trabeculae and isolated left ventricular myocytes. The treatment elicited significant (P < 0.0001) dose-dependent increases (16.4-fold increase with 4 g/day) in left ventricular tissue and red blood cell n-3 PUFA levels (EPA + DHA, placebo, 0.42 +/- 0.04; 1 g/day, 3.02 +/- 0.23; 2 g/day, 3.63 +/- 0.17; and 4 g/day, 6.97 +/- 0.33%). Regardless of the dose, n-3 PUFA treatment did not alter ventricular function in the intact animal (e.g., 4 g/day, fractional shortening: pre, 42.9 +/- 1.6 vs. post, 40.1 +/- 1.7%; placebo: pre, 39.2 +/- 1.3 vs. post, 38.4 +/- 1.6%). The developed force per cross-sectional area, changes in length- and frequency-dependent behavior in contractile force, and the inotropic response to beta-adrenoceptor activation were also similar for trabeculae obtained from placebo- or n-3 PUFA-treated dogs. Finally, calcium currents and calcium transients were the same in myocytes from n-3 PUFA- and placebo-treated dogs. Thus dietary n-3 PUFAs did not adversely alter either in vitro or in vivo ventricular contractile function in dogs with healed infarctions.

Catecholaminergic Polymorphic Ventricular Tachycardia from Bedside to Bench and Beyond

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a primary electrical myocardial disease characterized by exercise- and stress-related ventricular tachycardia manifested as syncope and sudden death. The disease has a heterogeneous genetic basis, with mutations in the cardiac Ryanodine Receptor channel (RyR2) gene accounting for an autosomal-dominant form (CPVT1) in approximately 50% and mutations in the cardiac calsequestrin gene (CASQ2) accounting for an autosomal-recessive form (CPVT2) in up to 2% of CPVT cases. Both RyR2 and calsequestrin are important participants in the cardiac cellular calcium homeostasis. We review the physiology of the cardiac calcium homeostasis, including the cardiac excitation contraction coupling and myocyte calcium cycling. The pathophysiology of cardiac arrhythmias related to myocyte calcium handling and the effects of different modulators are discussed. The putative derangements in myocyte calcium homeostasis responsible for CPVT, as well as the clinical manifestations and therapeutic options available, are described.

Abnormalities of Doppler Measures of Diastolic Function in the Healthy Elderly Are Not Related to Alterations of Left Atrial Pressure

Normal healthy aging results in changes in Doppler indexes of diastolic function. One widely accepted explanation for these alterations that is based on animal data is an age-associated impairment of myocardial relaxation resulting from abnormal myocyte calcium handling. However, an alternative theory based on altered left atrial compliance with aging has been proposed in which early left atrial pressure could be low in the aged heart but rise rapidly during atrial diastole, resulting in a higher late atrial pressure and thus a normal mean pressure. We sought to explore this issue directly by performing a detailed analysis of the pulmonary capillary wedge pressure waveform obtained by right-heart catheterization during 5 different loading conditions. Twelve healthy elderly sedentary subjects (mean age, 69.8+/-3 years) were recruited for the seniors group. An additional 12 young sedentary subjects (mean age, 35+/-8 years) made up the young group. All subjects were rigorously screened for comorbidities. All subjects underwent transthoracic echocardiography to determine Doppler variables of early and late transmitral filling (E and A velocities) and isovolumetric relaxation time. Each subject also underwent pulmonary artery catheterization with measurement of pulmonary capillary wedge pressure waveform during 5 different loading conditions: baseline, lower-body negative pressures of -15 and -30 mm Hg, and rapid saline infusions of 10 to 15 and 20 to 30 mL/kg. Pressure was measured at 6 points of the waveform: point 1, peak of the atrial contraction (a wave); point 2, the left atrial pressure during the start of ventricular systole; point 3, peak of atrial filling (v wave); point 4, earliest left atrial pressure during ventricular filling; and the line between points 5 and 6, pressure during diastasis. Aging resulted in a decrease in the E/A ratio (P<0.001) and a prolongation of the isovolumetric relaxation time (P<0.001) as assessed by echocardiography, but there was no effect of age on pulmonary capillary wedge pressure at any point throughout the cardiac cycle (P=0.290). Specifically, at no measured point at any level of cardiac filling volume was the pulmonary capillary wedge pressure of the seniors lower than that of the young subjects. We conclude that the age-related echocardiographic change of decreasing E/A ratio is not the result of a lowering of early diastolic left atrial pressure.

Clinical potential of sodium-calcium exchanger inhibitors as antiarrhythmic agents.

The Na(+)/Ca(2+) exchanger (NaCaX) plays an important role in calcium handling in myocytes, but in the setting of calcium overload NaCaX can also contribute to the activation of an arrhythmogenic transient inward current (I(ti)). Therefore, approaches to inhibit NaCaX could have potential antiarrhythmic effects in pathophysiological states such as heart failure (HF) or myocardial ischaemia and reperfusion. NaCaX typically functions in a forward (Ca(2+) extrusion) mode but can also function in a reverse (Ca(2+) influx) mode. The determining factors for the directionality of NaCaX ion movement are the electrochemical gradients of calcium and sodium, and membrane potential (E(m)). In HF, upregulated NaCaX plays a dual role: it decreases sarcoplasmic reticulum (SR) calcium load, which leads to contractile dysfunction, and it underlies the I(ti) responsible for delayed after-depolarisations (DADs) and ventricular arrhythmias. In myocardial ischaemia and reperfusion, increases in [Na(+)](i) (as a result of acidosis and activation of the Na(+)/H(+) exchanger [NHE]) lead to calcium overload via the NaCaX and arrhythmogenesis is probably mediated by I(ti) activation due to NaCaX. As such, inhibition of NaCaX could provide a novel therapeutic approach to the prevention and treatment of arrhythmias. Unfortunately, it is difficult to assess the efficacy of such an approach since there are no specific NaCaX inhibitors. Currently available agents are hampered by their nonspecific effects on other ion channels and carriers. The potential utility of specific inhibition of forward or reverse mode NaCaX as an antiarrhythmic approach in the settings of HF and ischaemia/ reperfusion is discussed within the context of current knowledge of myocyte calcium and sodium handling. NaCaX is a challenging and complex therapeutic target because of the delicate balance of SR calcium load (too little contributes to contractile dysfunction and too much leads to calcium overload and arrhythmogenesis). Further understanding of NaCaX function, [Na(+)](i) and [Ca(2+)](i) in HF and ischaemia/reperfusion, combined with the development and assessment of specific NaCaX inhibitors, will ultimately define the potential role of NaCaX inhibition in the prevention and treatment of ventricular arrhythmias.

Thyroid hormone regulation of calcium cycling proteins.

Alterations in thyroid hormone levels have a profound impact on myocardial contractility, speed of relaxation, cardiac output, and heart rate. The mechanisms for these changes include altered expression of several key proteins, involved in the regulation of intracellular calcium homeostasis. Most notably, increases in thyroid hormone and the coordinated increases in cardiac contractile parameters are marked by increases in the levels of the sarcoplasmic reticulum (SR) Ca2+-adenosine triphosphatase (ATPase) and decreases in its inhibitor, phospholamban. These changes at the protein level result in enhanced SR calcium transport and myocyte calcium cycling, leading to increases in the force and rates of contraction as well as relaxation rates at the organ level. However, decreases in thyroid hormone levels are associated with opposite alterations in these two proteins, leading to reduced myocyte calcium handling capacity and lower cardiac contractility. Furthermore, changes in the relative ratio of phospholamban/Ca2+-ATPase correlate with changes in the affinity of the SR Ca2+-transport system and relaxation rates in beating hearts. These findings suggest that thyroid hormone directly regulates SR protein levels and thus, cardiac function.