Store-operated Ca2+ entry modulates sarcoplasmic reticulum Ca2+ loading in neonatal rabbit cardiac ventricular myocytes

The SOCE Machinery: An Unbalanced Knowledge between Left and Right Ventricular Pathophysiology

Right ventricular failure (RVF) is the most important prognostic factor for morbidity and mortality in pulmonary arterial hypertension (PAH) or pulmonary hypertension (PH) caused by left heart diseases. However, right ventricle (RV) remodeling is understudied and not targeted by specific therapies. This can be partly explained by the lack of basic knowledge of RV remodeling. Since the physiology and hemodynamic function of the RV differ from those of the left ventricle (LV), the mechanisms of LV dysfunction cannot be generalized to that of the RV, albeit a knowledge of these being helpful to understanding RV remodeling and dysfunction. Store-operated Ca²⁺ entry (SOCE) has recently emerged to participate in the LV cardiomyocyte Ca²⁺ homeostasis and as a critical player in Ca²⁺ mishandling in a pathological context. In this paper, we highlight the current knowledge on the SOCE contribution to the LV and RV dysfunctions, as SOCE molecules are present in both compartments. he relative lack of studies on RV dysfunction indicates the necessity of further investigations, a significant challenge over the coming years.

Blood flow modeling reveals improved collateral artery performance during the regenerative period in mammalian hearts

Collateral arteries bridge opposing artery branches, forming a natural bypass that can deliver blood flow downstream of an occlusion. Inducing coronary collateral arteries could treat cardiac ischemia, but more knowledge on their developmental mechanisms and functional capabilities is required. Here we used whole-organ imaging and three-dimensional computational fluid dynamics modeling to define spatial architecture and predict blood flow through collaterals in neonate and adult mouse hearts. Neonate collaterals were more numerous, larger in diameter and more effective at restoring blood flow. Decreased blood flow restoration in adults arose because during postnatal growth coronary arteries expanded by adding branches rather than increasing diameters, altering pressure distributions. In humans, adult hearts with total coronary occlusions averaged 2 large collaterals, with predicted moderate function, while normal fetal hearts showed over 40 collaterals, likely too small to be functionally relevant. Thus, we quantify the functional impact of collateral arteries during heart regeneration and repair-a critical step toward realizing their therapeutic potential.

STIM1-Orai1 interaction mediated calcium influx activation contributes to cardiac contractility of insulin-resistant rats

PURPOSE

Metabolic syndrome (MetS) became a tremendous public health burden in the last decades. Store-operated calcium entry (SOCE) is a unique mechanism that causes a calcium influx, which is triggered by calcium store depletion. MetS-induced alterations in cardiac calcium signaling, especially in SOCE are still unclear. Therefore, we aim to examine the possible role of SOCE and its components (STIM1 and Orai1) in the MetS-induced cardiac remodeling.

METHODS

We used male, adult (12 weeks) Wistar albino rats (n = 20). Animals were randomly divided into two groups which were: control (C) and MetS. We gave 33% sucrose solution to animals instead of water for 24 weeks to establish MetS model. In the end, papillary muscle function was evaluated, and various electrophysiological analyses were made in isolated cardiomyocytes. Additionally, STIM1 and Orai1 protein and mRNA expressions were analyzed.

RESULTS

We observed a deterioration in contractility in MetS animals and demonstrated the contribution of SOCE by applying a SOCE inhibitor (BTP2). Calcium spark frequency was increased while its amplitude was decreasing in MetS hearts, which was reversed after SOCE inhibition. The amplitude of transient calcium changes in the MetS group was decreased, and it decreased further BTP2 application. Both protein and mRNA levels of STIM1 and Orai1 were increased significantly in MetS hearts.

CONCLUSION

Current data indicate the significant contribution of SOCE to cardiac calcium handling in the MetS model. We think MetS-induced SOCE activation is a compensation mechanism that is required for the continuum of proper cardiac functioning, although the activation can also cause cardiac hypertrophy.

Pathophysiological Mechanisms of Hypertension Development Induced by Fructose Consumption

During the past several decades, there has been a dramatic increase in fructose consumption worldwide in parallel with epidemics of metabolic diseases. Accumulating evidence has suggested that excessive fructose consumption...

The interplay between mitochondria and store-operated Ca2+ entry: Emerging insights into cardiac diseases

Store‐operated Ca²⁺ entry (SOCE) machinery, including Orai channels, TRPCs, and STIM1, is key to cellular calcium homeostasis. The following characteristics of mitochondria are involved in the physiological and pathological regulation of cells: mitochondria mediate calcium uptake through calcium uniporters; mitochondria are regulated by mitochondrial dynamic related proteins (OPA1, MFN1/2, and DRP1) and form mitochondrial networks through continuous fission and fusion; mitochondria supply NADH to the electron transport chain through the Krebs cycle to produce ATP; under stress, mitochondria will produce excessive reactive oxygen species to regulate mitochondria‐endoplasmic reticulum interactions and the related signalling pathways. Both SOCE and mitochondria play critical roles in mediating cardiac hypertrophy, diabetic cardiomyopathy, and cardiac ischaemia‐reperfusion injury. All the mitochondrial characteristics mentioned above are determinants of SOCE activity, and vice versa. Ca²⁺ signalling dictates the reciprocal regulation between mitochondria and SOCE under the specific pathological conditions of cardiomyocytes. The coupling of mitochondria and SOCE is essential for various pathophysiological processes in the heart. Herein, we review the research focussing on the reciprocal regulation between mitochondria and SOCE and provide potential interplay patterns in cardiac diseases.

IP3R-Mediated Compensatory Mechanism for Calcium Handling in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes With Cardiac Ryanodine Receptor Deficiency

In adult cardiomyocytes (CMs), the type 2 ryanodine receptor (RYR2) is an indispensable Ca²⁺ release channel that ensures the integrity of excitation-contraction coupling, which is fundamental for every heartbeat. However, the role and importance of RYR2 during human embryonic cardiac development are still poorly understood. Here, we generated two human induced pluripotent stem cell (iPSC)-based RYR2 knockout (RYR2^–/–) lines using the CRISPR/Cas9 gene editing technology. We found that RYR2^–/–-iPSCs could differentiate into CMs with the efficiency similar to control-iPSCs (Ctrl-iPSCs); however, the survival of iPSC-CMs was markedly affected by the lack of functional RYR2. While Ctrl-iPSC-CMs exhibited regular Ca²⁺ handling, we observed significantly reduced frequency and intense abnormalities of Ca²⁺ transients in RYR2^–/–-iPSC-CMs. Ctrl-iPSC-CMs displayed sensitivity to extracellular Ca²⁺ ([Ca²⁺ ]_o) and caffeine in a concentration-dependent manner, while RYR2^–/–-iPSC-CMs showed inconsistent reactions to [Ca²⁺ ]_o and were insensitive to caffeine, indicating there is no RYR2-mediated Ca²⁺ release from the sarcoplasmic reticulum (SR). Instead, compensatory mechanism for calcium handling in RYR2^–/–-iPSC-CMs is partially mediated by the inositol 1,4,5-trisphosphate receptor (IP3R). Similar to Ctrl-iPSC-CMs, SR Ca²⁺ refilling in RYR2^–/–-iPSC-CMs is mediated by SERCA. Additionally, RYR2^–/–-iPSC-CMs showed a decreased beating rate and a reduced peak amplitude of L-type Ca²⁺ current. These findings demonstrate that RYR2 is not required for CM lineage commitment but is important for CM survival and contractile function. IP3R-mediated Ca²⁺ release is one of the major compensatory mechanisms for Ca²⁺ cycling in human CMs with the RYR2 deficiency.

Specific Upregulation of TRPC1 and TRPC5 Channels by Mineralocorticoid Pathway in Adult Rat Ventricular Cardiomyocytes

Whereas cardiac TRPC (transient receptor potential canonical) channels and the associated store-operated Ca²⁺ entry (SOCE) are abnormally elevated during cardiac hypertrophy and heart failure, the mechanism of this upregulation is not fully elucidated but might be related to the activation of the mineralocorticoid pathway. Using a combination of biochemical, Ca²⁺ imaging, and electrophysiological techniques, we determined the effect of 24-h aldosterone treatment on the TRPCs/Orai-dependent SOCE in adult rat ventricular cardiomyocytes (ARVMs). The 24-h aldosterone treatment (from 100 nM to 1 µM) enhanced depletion-induced Ca²⁺ entry in ARVMs, as assessed by a faster reduction of Fura-2 fluorescence decay upon the addition of Mn²⁺ and increased Fluo-4/AM fluorescence following Ca²⁺ store depletion. These effects were prevented by co-treatment with a specific mineralocorticoid receptor (MR) antagonist, RU-28318, and they are associated with the enhanced depletion-induced N-[4-[3,5-Bis(trifluoromethyl)-1H-pyrazol-1-yl]phenyl]-4-methyl-1,2,3-thiadiazole-5-carboxamide (BTP2)-sensitive macroscopic current recorded by patch-clamp experiments. Molecular screening by qRT-PCR and Western blot showed a specific upregulation of TRPC1, TRPC5, and STIM1 expression at the messenger RNA (mRNA) and protein levels upon 24-h aldosterone treatment of ARVMs, corroborated by immunostaining. Our study provides evidence that the mineralocorticoid pathway specifically promotes TRPC1/TRPC5-mediated SOCE in adult rat cardiomyocytes.

Potential Arrhythmogenic Role of TRPC Channels and Store-Operated Calcium Entry Mechanism in Mouse Ventricular Myocytes

Background and Purpose: Store-operated calcium entry (SOCE) is an important physiological phenomenon that extensively mediates intracellular calcium ion (Ca²⁺) load. It has been previously found in myocytes isolated from neonatal or diseased hearts. We aimed to determine its existence, molecular nature in undiseased hearts and its potential arrhythmogenic implications under hyperactive conditions.

Experimental Approach: Ventricular myocytes isolated from adult FVB mice were studied by using Ca²⁺ imaging and whole-cell perforated patch-clamp recording. In addition, lead II ECGs were recorded in isolated Langendorff-perfused mice hearts. Functional TRPC channel antibodies and inhibitors, and TRPC6 activator hyperforin were used.

Key Results: In this study, we demonstrate the existence and contribution of SOCE in normal adult mouse cardiac myocytes. For an apparent SOCE activation, complete depletion of sarcoplasmic reticulum (SR) Ca²⁺ by employing both caffeine (10 mM) and thapsigargin (1 μM) or cyclopiazonic acid (10 μM) was required. Consistent with the notion that SOCE may be mediated by heteromultimeric TRPC channels, SOCEs observed from those myocytes were significantly reduced by the pretreatment with anti-TRPC1, 3, and 6 antibodies as well as by gadolinium, a non-selective TRPC channel blocker. In addition, we showed that SOCE may regulate spontaneous SR Ca²⁺ release, Ca²⁺ waves, and triggered activities which may manifest cardiac arrhythmias. Since the spontaneous depolarization in membrane potential preceded the elevation of intracellular Ca²⁺, an inward membrane current presumably via TRPC channels was considered as the predominant cause of cellular arrhythmias. The selective TRPC6 activator hyperforin (0.1–10 μM) significantly facilitated the SOCE, SOCE-mediated inward current, and calcium load in the ventricular myocytes. ECG recording further demonstrated the proarrhythmic effects of hyperforin in ex vivo mouse hearts.

Conclusion and Implications: We suggest that SOCE, which is at least partially mediated by TRPC channels, exists in adult mouse ventricular myocytes. TRPC channels and SOCE mechanism may be involved in cardiac arrhythmogenesis via promotion of spontaneous Ca²⁺ waves and triggered activities under hyperactivated conditions.

ATP triggers a robust intracellular [Ca2+ ]-mediated signalling pathway in human synovial fibroblasts

NEW FINDINGS

What is the central question of this study? What are the main [Ca²⁺ ]_i signalling pathways activated by ATP in human synovial fibroblasts? What is the main finding and its importance? In human synovial fibroblasts ATP acts through a linked G-protein (G_q ) and phospholipase C signalling mechanism to produce IP₃ , which then markedly enhances release of Ca²⁺ from the endoplasmic reticulum. These results provide new information for the detection of early pathophysiology of arthritis.

ABSTRACT

In human articular joints, synovial fibroblasts (HSFs) have essential physiological functions that include synthesis and secretion of components of the extracellular matrix and essential articular joint lubricants, as well as release of paracrine substances such as ATP. Although the molecular and cellular processes that lead to a rheumatoid arthritis (RA) phenotype are not fully understood, HSF cells exhibit significant changes during this disease progression. The effects of ATP on HSFs were studied by monitoring changes in intracellular Ca²⁺ ([Ca²⁺ ]_i ), and measuring electrophysiological properties. ATP application to HSF cell populations that had been enzymatically released from 2-D cell culture revealed that ATP (10-100 μm), or its analogues UTP or ADP, consistently produced a large transient increase in [Ca²⁺ ]_i . These changes (i) were initiated by activation of the P₂ Y purinergic receptor family, (ii) required G_q -mediated signal transduction, (iii) did not involve a transmembrane Ca²⁺ influx, but instead (iv) arose almost entirely from activation of endoplasmic reticulum (ER)-localized inositol 1,4,5-trisphosphate (IP₃ ) receptors that triggered Ca²⁺ release from the ER. Corresponding single cell electrophysiological studies revealed that these ATP effects (i) were insensitive to [Ca²⁺ ]_o removal, (ii) involved an IP₃ -mediated intracellular Ca²⁺ release process, and (iii) strongly turned on Ca²⁺ -activated K⁺ current(s) that significantly hyperpolarized these cells. Application of histamine produced very similar effects in these HSF cells. Since ATP is a known paracrine agonist and histamine is released early in the inflammatory response, these findings may contribute to identification of early steps/defects in the initiation and progression of RA.

Cardiovascular and Hemostatic Disorders: SOCE in Cardiovascular Cells: Emerging Targets for Therapeutic Intervention

The discovery of the store-operated Ca²⁺ entry (SOCE) phenomenon is tightly associated with its recognition as a pathway of high (patho)physiological significance in the cardiovascular system. Early on, SOCE has been investigated primarily in non-excitable cell types, and the vascular endothelium received particular attention, while a role of SOCE in excitable cells, specifically cardiac myocytes and pacemakers, was initially ignored and remains largely enigmatic even to date. With the recent gain in knowledge on the molecular components of SOCE as well as their cellular organization within nanodomains, potential tissue/cell type-dependent heterogeneity of the SOCE machinery along with high specificity of linkage to downstream signaling pathways emerged for cardiovascular cells. The basis of precise decoding of cellular Ca²⁺ signals was recently uncovered to involve correct spatiotemporal organization of signaling components, and even minor disturbances in these assemblies trigger cardiovascular pathologies. With this chapter, we wish to provide an overview on current concepts of cellular organization of SOCE signaling complexes in cardiovascular cells with particular focus on the spatiotemporal aspects of coupling to downstream signaling and the potential disturbance of these mechanisms by pathogenic factors. The significance of these mechanistic concepts for the development of novel therapeutic strategies will be discussed.

Tissue Specificity: Store-Operated Ca2+ Entry in Cardiac Myocytes

Calcium (Ca²⁺) is a key regulator of cardiomyocyte contraction. The Ca²⁺ channels, pumps, and exchangers responsible for the cyclical cytosolic Ca²⁺ signals that underlie contraction are well known. In addition to those Ca²⁺ signaling components responsible for contraction, it has been proposed that cardiomyocytes express channels that promote the influx of Ca²⁺ from the extracellular milieu to the cytosol in response to depletion of intracellular Ca²⁺ stores. With non-excitable cells, this store-operated Ca²⁺ entry (SOCE) is usually easily demonstrated and is essential for prolonging cellular Ca²⁺ signaling and for refilling depleted Ca²⁺ stores. The role of SOCE in cardiomyocytes, however, is rather more elusive. While there is published evidence for increased Ca²⁺ influx into cardiomyocytes following Ca²⁺ store depletion, it has not been universally observed. Moreover, SOCE appears to be prominent in embryonic cardiomyocytes but declines with postnatal development. In contrast, there is overwhelming evidence that the molecular components of SOCE (e.g., STIM, Orai, and TRPC proteins) are expressed in cardiomyocytes from embryo to adult. Moreover, these proteins have been shown to contribute to disease conditions such as pathological hypertrophy, and reducing their expression can attenuate hypertrophic growth. It is plausible that SOCE might underlie Ca²⁺ influx into cardiomyocytes and may have important signaling functions perhaps by activating local Ca²⁺-sensitive processes. However, the STIM, Orai, and TRPC proteins appear to cooperate with multiple protein partners in signaling complexes. It is therefore possible that some of their signaling activities are not mediated by Ca²⁺ influx signals, but by protein-protein interactions.

Transient Receptor Potential Canonical (TRPC)/Orai1-dependent Store-operated Ca2+ Channels: NEW TARGETS OF ALDOSTERONE IN CARDIOMYOCYTES

Store-operated Ca(2+) entry (SOCE) has emerged as an important mechanism in cardiac pathology. However, the signals that up-regulate SOCE in the heart remain unexplored. Clinical trials have emphasized the beneficial role of mineralocorticoid receptor (MR) signaling blockade in heart failure and associated arrhythmias. Accumulated evidence suggests that the mineralocorticoid hormone aldosterone, through activation of its receptor, MR, might be a key regulator of Ca(2+) influx in cardiomyocytes. We thus assessed whether and how SOCE involving transient receptor potential canonical (TRPC) and Orai1 channels are regulated by aldosterone/MR in neonatal rat ventricular cardiomyocytes. Molecular screening using qRT-PCR and Western blotting demonstrated that aldosterone treatment for 24 h specifically increased the mRNA and/or protein levels of Orai1, TRPC1, -C4, -C5, and stromal interaction molecule 1 through MR activation. These effects were correlated with a specific enhancement of SOCE activities sensitive to store-operated channel inhibitors (SKF-96365 and BTP2) and to a potent Orai1 blocker (S66) and were prevented by TRPC1, -C4, and Orai1 dominant negative mutants or TRPC5 siRNA. A mechanistic approach showed that up-regulation of serum- and glucocorticoid-regulated kinase 1 mRNA expression by aldosterone is involved in enhanced SOCE. Functionally, 24-h aldosterone-enhanced SOCE is associated with increased diastolic [Ca(2+)]i, which is blunted by store-operated channel inhibitors. Our study provides the first evidence that aldosterone promotes TRPC1-, -C4-, -C5-, and Orai1-mediated SOCE in cardiomyocytes through an MR and serum- and glucocorticoid-regulated kinase 1 pathway.

The Sarcoplasmic Reticulum and the Evolution of the Vertebrate Heart

The sarcoplasmic reticulum (SR) is crucial for contraction and relaxation of the mammalian cardiomyocyte, but its role in other vertebrate classes is equivocal. Recent evidence suggests differences in SR function across species may have an underlying structural basis. Here, we discuss how SR recruitment relates to the structural organization of the cardiomyocyte to provide new insight into the evolution of cardiac design and function in vertebrates.

Emergence of Orai3 activity during cardiac hypertrophy

AIMS

Stromal interaction molecule 1 (STIM1) has been shown to control a calcium (Ca(2+)) influx pathway that emerges during the hypertrophic remodelling of cardiomyocytes. Our aim was to determine the interaction of Orai1 and Orai3 with STIM1 and their role in the constitutive store-independent and the store-operated, STIM1-dependent, Ca(2+) influx in cardiomyocytes.

METHODS AND RESULTS

We characterized the expression profile of Orai proteins and their interaction with STIM1 in both normal and hypertrophied adult rat ventricular cardiomyocytes. Orai1 and 3 protein levels were unaltered during the hypertrophic process and both proteins co-immunoprecipitated with STIM1. The level of STIM1 and Orai1 were significantly greater in the macromolecular complex precipitated by the Orai3 antibody in hypertrophied cardiomyocytes. We then used a non-viral method to deliver Cy3-tagged siRNAs in vivo to adult ventricular cardiomyocytes and silence Orai channel candidates. Cardiomyocytes were subsequently isolated then the voltage-independent, i.e. store-independent and store-operated Ca(2+) entries were measured on Fura-2 AM loaded Cy3-labelled and control isolated cardiomyocytes. The whole cell patch-clamp technique was used to measure Orai-mediated currents. Specific Orai1 and Orai3 knockdown established Orai3, but not Orai1, as the critical partner of STIM1 carrying these voltage-independent Ca(2+) entries in the adult hypertrophied cardiomyocytes. Orai3 also drove an arachidonic acid-activated inward current.

CONCLUSION

Cardiac Orai3 is the essential partner of STIM1 and drives voltage-independent Ca(2+) entries in adult cardiomyocytes. Arachidonic acid-activated currents, which are supported by Orai3, are present in adult cardiomyocytes and increased during hypertrophy.

Calcium homeostasis in myogenic differentiation factor 1 (MyoD)-transformed, virally-transduced, skin-derived equine myotubes

Dysfunctional skeletal muscle calcium homeostasis plays a central role in the pathophysiology of several human and animal skeletal muscle disorders, in particular, genetic disorders associated with ryanodine receptor 1 (RYR1) mutations, such as malignant hyperthermia, central core disease, multiminicore disease and certain centronuclear myopathies. In addition, aberrant skeletal muscle calcium handling is believed to play a pivotal role in the highly prevalent disorder of Thoroughbred racehorses, known as Recurrent Exertional Rhabdomyolysis. Traditionally, such defects were studied in human and equine subjects by examining the contractile responses of biopsied muscle strips exposed to caffeine, a potent RYR1 agonist. However, this test is not widely available and, due to its invasive nature, is potentially less suitable for valuable animals in training or in the human paediatric setting. Furthermore, increasingly, RYR1 gene polymorphisms (of unknown pathogenicity and significance) are being identified through next generation sequencing projects. Consequently, we have investigated a less invasive test that can be used to study calcium homeostasis in cultured, skin-derived fibroblasts that are converted to the muscle lineage by viral transduction with a MyoD (myogenic differentiation 1) transgene. Similar models have been utilised to examine calcium homeostasis in human patient cells, however, to date, there has been no detailed assessment of the cells’ calcium homeostasis, and in particular, the responses to agonists and antagonists of RYR1. Here we describe experiments conducted to assess calcium handling of the cells and examine responses to treatment with dantrolene, a drug commonly used for prophylaxis of recurrent exertional rhabdomyolysis in horses and malignant hyperthermia in humans.

Zinc inhibits osteoclast differentiation by suppression of Ca2+-Calcineurin-NFATc1 signaling pathway

BACKGROUND

Zinc, an essential trace element, inhibits osteoclast differentiation in vitro and in vivo. The molecular mechanism for the inhibitory effect of zinc, however, is poorly understood. The purpose of this study was to investigate the effect of zinc and determine its molecular mechanism on receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis in mouse bone marrow-derived monocyte cells (BMMs) and RAW264.7 cells.

RESULTS

In BMMs, zinc treatment during osteoclast differentiation decreased RANKL-induced osteoclast formation in a dose-dependent manner. We show that zinc suppressed the mRNA levels of nuclear factor of activated T-cells, cytoplasmic 1 (Nfatc1). Zinc also accumulated phospho-Nfatc1 (p-Nfatc1) in the cytosol in a dose-dependent manner and inhibited the translocation of Nfatc1 to the nucleus in RAW264.7 cells. Zinc suppressed the activities of Nfatc1 in the nucleus without changing the activities of NF-κB in RAW264.7 cells. In contrast, calcineurin activity decreased in response to zinc but its protein level was unchanged. RANKL-induced Ca2+ oscillations were inhibited by zinc treatment, but phospho-phospholipase Cγ1 (p-PLCγ1), the upstream signaling molecule of Ca2+ oscillations, was unaffected. Moreover, a constitutively active form of Nfatc1 obviously rescued suppression of osteoclastogenesis by zinc.

CONCLUSIONS

Taken together, these results demonstrate for the first time that the inhibitory effect of zinc during osteoclastogesis is caused by suppressing the Ca2+-Calcineurin-NFATc1 signaling pathway. Thus, zinc may be a useful therapeutic candidate for the prevention of bone loss caused by NFATc1 activation in osteoclasts.

STIM1/Orai1-mediated SOCE: current perspectives and potential roles in cardiac function and pathology

Store-operated Ca²⁺ entry (SOCE) is critical for Ca²⁺ signaling in nonexcitable cells; however, its role in the regulation of cardiomyocyte Ca²⁺ homeostasis has only recently been investigated. The increased understanding of the role of stromal interaction molecule 1 (STIM1) in regulating SOCE combined with recent studies demonstrating the presence of STIM1 in cardiomyocytes provides support that this pathway co-exists in the heart with the more widely recognized Ca²⁺ handling pathways associated with excitation-contraction coupling. There is now substantial evidence that STIM1-mediated SOCE plays a key role in mediating cardiomyocyte hypertrophy, both in vitro and in vivo, and there is growing support for the contribution of SOCE to Ca²⁺ overload associated with ischemia/reperfusion injury. Here, we provide an overview of our current understanding of the molecular regulation of SOCE and discuss the evidence supporting the role of STIM1/Orai1-mediated SOCE in regulating cardiomyocyte function.

Functional transient receptor potential canonical type 1 channels in human atrial myocytes

Transient receptor potential (TRP) channels are not well understood in human atrium, and the present study was therefore designed to investigate whether TRPC channels would mediate the nonselective cation current reported previously and are involved in the formation of store-operated Ca(2+) entry (SOCE) channels in human atrial myocytes using approaches of whole-cell patch voltage-clamp, RT-PCR, Western blotting, co-immunoprecipitation, and confocal scanning approaches, etc. We found that a nonselective cation current was recorded under K(+)-free conditions in human atrial myocytes, and the current was inhibited by the TRP channel blocker La(3+). Thapsigargin enhanced the current, and its effect was suppressed by La(3+) and prevented by pipette inclusion of anti-TRPC1 antibody. Endothlin-1 and angiotensin II enhanced the current that could be inhibited by La(3+). Gene and protein expression of TRPC1 channels were abundant in human atria. In addition, mRNA and protein of STIM1 and Orai1, components of SOCE channels, were abundantly expressed in human atria. Co-immunoprecipitation analysis demonstrated an interaction of TRPC1 with STIM1 and/or Orai1. Ca(2+) signaling mediated by SOCE channels was detected by a confocal microscopy technique. These results demonstrate the novel evidence that TRPC1 channels not only mediate the nonselective cation current, but also form SOCE channels in human atria as a component. TRPC1 channels can be activated by endothelin-1 or angiotensin II, which may be involved in the atrial electrical remodeling in patients with atrial fibrillation.

Modification of Ca(2+)-handling in cardiomyocytes by redox sensitive mechanisms in response to ouabain

We examined the role of redox-sensitive signal transduction mechanisms in modifying the changes in [Ca(2+)](i) produced by ouabain upon incubating adult rat cardiomyocytes with antioxidants or inhibitors of different protein kinases and monitoring alterations in fura-2 fluorescence. Ouabain increased basal [Ca(2+)](i), augmented the KCl-induced increase in [Ca(2+)](i), and promoted oxyradical production in cardiomyocytes. These actions of ouabain were attenuated by an oxyradical scavenging mixture (superoxide dismutase plus catalase), and the antioxidants (N-acetyl-L-cysteine and N-(2-mercaptoproprionyl)glycine). An inhibitor of MAP kinase (PD98059) depressed the ouabain-induced increase in [Ca(2+)], whereas inhibitors of tyrosine kinase (tyrphostin and genistein) and PI3 kinase (Wortmannin and LV294002) enhanced the ouabain-induced increase in [Ca(2+)](i). Inhibitors of protein kinase C (calphostin and bisindolylmalaimide) augmented the ouabain-induced increase in [Ca(2+)](i), whereas stimulation of protein kinase C by a phorbol ester (phorbol 12-myristate 13-acetate) depressed the action of ouabain. These results suggest that ouabain-induced inhibition of Na (+)-K(+) ATPase may alter the redox status of cardiomyocytes through the production of oxyradicals, and increase the activities of various protein kinases. Thus, these redox-sensitive signal transduction mechanisms involving different protein kinases may modify Ca(2+)-handling sites in cardiomyocytes and determine the magnitude of net increase in [Ca(2+)](i) in response to ouabain.

Calcium flux balance in the heart

This article reviews the consequences of the need for the cardiac cell to be in calcium flux balance in the steady state. We first discuss how this steady state condition affects the control of resting [Ca(2+)]i. The next section considers how sarcoplasmic reticulum (SR) Ca content is controlled by a feedback mechanism whereby changes of SR Ca affect the amplitude of the Ca transient and this, in turn, controls sarcolemmal Ca fluxes. Subsequent sections review the effects of altering the activity of individual Ca handling proteins. Increasing the activity of the SR Ca-ATPase (SERCA) increases both the amplitude and rate constant of decay of the systolic Ca transient. The Ca flux balance condition requires that this must be achieved with no change of Ca efflux placing constraints on the magnitude of change of amplitude and decay rate. We analyze the quantitative dependence of Ca transient amplitude and SR content on SERCA activity. Increasing the open probability of the RyR during systole is predicted to have no steady state effect on the amplitude of the systolic Ca transient. We discuss the effects of changing the amplitude of the L-type Ca current in the context of both triggering Ca release from the SR and loading the cell with calcium. These manoeuvres are considered in the context of the effects of β-adrenergic stimulation. Finally, we review calcium flux balance in the presence of Ca waves.

The temperature challenges on cardiac performance in winter-quiescent and migration-stage eels Anguilla anguilla

The present study was undertaken to examine cardiac responses to some of the temperature challenges that eels encounter in their natural environment. The contractile properties of ventricular muscle was studied on electrically paced tissue strips after long term acclimation at 0 °C, 10 °C, or 20 °C, and following acute ± 10 °C temperature changes. The time-course of contraction, and thus maximal attainable heart rates, was greatly influenced by working temperature, but was independent of acclimation history. The absolute force of contraction and power production (i.e. the product of force and stimulation frequency) was significantly influenced by acute temperature decrease from 20 °C to 10 °C. The role of adrenaline as a modulator of contraction force, power production, rates of contraction and relaxation, and minimum time in contraction was assessed. Increased adrenergic tonus elicited a positive inotropic, temperature-dependent response, but did not influence twitch duration. This suggests that adrenaline acts as an agent in maintaining an adequate contractile force following temperature challenges. A significant increased relative ventricular mass was observed in 0 °C and 10 °C-acclimated eels compared to 20 °C-acclimated, which suggests that at low temperatures, eels secure cardiac output by heart enlargement. Inhibition of specific sarcolemmal Ca(2+) channels by selective drug treatment revealed that, depending on temperature, L-type channels is the major entry site, but also that reverse-mode Na(+)/Ca(2+)-exchange and store operated calcium entry contribute to the pool of activator Ca(2+).

Calcium oscillations and pacemaking

Calcium plays important role in biological systems where it is involved in diverse mechanisms such as signaling, muscle contraction and neuromodulation. Action potentials are generated by dynamic interaction of ionic channels located on the plasma-membrane and these drive the rhythmic activity of biological systems such as the smooth muscle and the heart. However, ionic channels are not the only pacemakers; an intimate interaction between intracellular Ca(2+) stores and ionic channels underlie rhythmic activity. In this review we will focus on the role of Ca(2+) stores in regulation of rhythmical behavior.

Store-operated calcium entry is present in HL-1 cardiomyocytes and contributes to resting calcium

Store-operated Ca(2+) entry (SOCE) has recently been shown to be of physiological and pathological importance in the heart, particularly during cardiac hypertrophy. However, measuring changes in intracellular Ca(2+) during SOCE is very difficult to study in adult primary cardiomyocytes. As a result there is a need for a stable and reliable in vitro model of SOCE which can be used to test cardiac drugs and investigate the role of SOCE in cardiac pathology. HL-1 cells are the only immortal cardiomyocyte cell line available that continuously divides and spontaneously contracts while maintaining phenotypic characteristics of the adult cardiomyocyte. To date the role of SOCE has not yet been investigated in the HL-1 cardiac cell line. We report for the first time that these cells expressed stromal interaction molecule 1 (STIM1) and the Ca(2+) release-activated Ca(2+) (CRAC) channel Orai1, which are essential components of the SOCE machinery. In addition, SOCE was tightly coupled to sarcoplasmic reticulum (SR)-Ca(2+) release in HL-1 cells, and such response was not impaired in the presence of voltage dependent Ca(2+) channels (L-type and T-type channels) or reverse mode Na(+)/Ca(2+) exchanger (NCX) inhibitors. We were able to abolish the SOCE response with known SOCE inhibitors (BTP-2 and SKF-96365) and by targeted knockdown of Orai1 with RNAi. In addition, knockdown of Orai1 resulted in lower baseline Ca(2+) and an attenuated response to thapsigargin (TG) and caffeine, indicating that SOCE may play a role in Ca(2+) homeostasis during unstressed conditions in cardiomyocytes. Currently, there is little knowledge about SOCE in cardiomyocytes, and the present results suggest that HL-1 cells will be of great utility in investigating the role of SOCE in the heart.

Detection, properties, and frequency of local calcium release from the sarcoplasmic reticulum in teleost cardiomyocytes

Calcium release from the sarcoplasmic reticulum (SR) plays a central role in the regulation of cardiac contraction and rhythm in mammals and humans but its role is controversial in teleosts. Since the zebrafish is an emerging model for studies of cardiovascular function and regeneration we here sought to determine if basic features of SR calcium release are phylogenetically conserved. Confocal calcium imaging was used to detect spontaneous calcium release (calcium sparks and waves) from the SR. Calcium sparks were detected in 16 of 38 trout atrial myocytes and 6 of 15 ventricular cells. The spark amplitude was 1.45±0.03 times the baseline fluorescence and the time to half maximal decay of sparks was 27±3 ms. Spark frequency was 0.88 sparks µm(-1) min(-1) while calcium waves were 8.5 times less frequent. Inhibition of SR calcium uptake reduced the calcium transient (F/F(0)) from 1.77±0.17 to 1.12±0.18 (p = 0.002) and abolished calcium sparks and waves. Moreover, elevation of extracellular calcium from 2 to 10 mM promoted early and delayed afterdepolarizations (from 0.6±0.3 min(-1) to 8.1±2.0 min(-1), p = 0.001), demonstrating the ability of SR calcium release to induce afterdepolarizations in the trout heart. Calcium sparks of similar width and duration were also observed in zebrafish ventricular myocytes. In conclusion, this is the first study to consistently report calcium sparks in teleosts and demonstrate that the basic features of calcium release through the ryanodine receptor are conserved, suggesting that teleost cardiac myocytes is a relevant model to study the functional impact of abnormal SR function.

A key role of TRPC channels in the regulation of electromechanical activity of the developing heart

AIMS

It is well established that dysfunction of voltage-dependent ion channels results in arrhythmias and conduction disturbances in the foetal and adult heart. However, the involvement of voltage-insensitive cationic TRPC (transient receptor potential canonical) channels remains unclear. We assessed the hypothesis that TRPC channels play a crucial role in the spontaneous activity of the developing heart.

METHODS AND RESULTS

TRPC isoforms were investigated in isolated hearts obtained from 4-day-old chick embryos. Using RT-PCR, western blotting and co-immunoprecipitation, we report for the first time that TRPC1, 3, 4, 5, 6, and 7 isoforms are expressed at the mRNA and protein levels and that they can form a macromolecular complex with the α1C subunit of the L-type voltage-gated calcium channel (Cav1.2) in atria and ventricle. Using ex vivo electrocardiograms, electrograms of isolated atria and ventricle and ventricular mechanograms, we found that inhibition of TRPC channels by SKF-96365 leads to negative chrono-, dromo-, and inotropic effects, prolongs the QT interval, and provokes first- and second-degree atrioventricular blocks. Pyr3, a specific antagonist of TRPC3, affected essentially atrioventricular conduction. On the other hand, specific blockade of the L-type calcium channel with nifedipine rapidly stopped ventricular contractile activity without affecting rhythmic electrical activity.

CONCLUSIONS

These results give new insights into the key role that TRPC channels, via interaction with the Cav1.2 channel, play in regulation of cardiac pacemaking, conduction, ventricular activity, and contractility during cardiogenesis.

Ca2+ paradox injury mediated through TRPC channels in mouse ventricular myocytes

BACKGROUND AND PURPOSE The Ca(2+) paradox is an important phenomenon associated with Ca(2+) overload-mediated cellular injury in myocardium. The present study was undertaken to elucidate molecular and cellular mechanisms for the development of the Ca(2+) paradox. EXPERIMENTAL APPROACH Fluorescence imaging was performed on fluo-3 loaded quiescent mouse ventricular myocytes using confocal laser scanning microscope. KEY RESULTS The Ca(2+) paradox was readily evoked by restoration of the extracellular Ca(2+) following 10-20 min of nominally Ca(2+)-free superfusion. The Ca(2+) paradox was significantly reduced by blockers of transient receptor potential canonical (TRPC) channels (2-aminoethoxydiphenyl borate, Gd(3+), La(3+)) and anti-TRPC1 antibody. The sarcoplasmic reticulum (SR) Ca(2+) content, assessed by caffeine application, gradually declined during Ca(2+)-free superfusion, which was further accelerated by metabolic inhibition. Block of SR Ca(2+) leak by tetracaine prevented Ca(2+) paradox. The Na(+) /Ca(2+) exchange (NCX) blocker KB-R7943 significantly inhibited Ca(2+) paradox when applied throughout superfusion period, but had little effect when added for a period of 3 min before and during Ca(2+) restoration. The SR Ca(2+) content was better preserved during Ca(2+) depletion by KB-R7943. Immunocytochemistry confirmed the expression of TRPC1, in addition to TRPC3 and TRPC4, in mouse ventricular myocytes. CONCLUSIONS AND IMPLICATIONS These results provide evidence that (i) the Ca(2+) paradox is primarily mediated by Ca(2+) entry through TRPC (probably TRPC1) channels that are presumably activated by SR Ca(2+) depletion; and (ii) reverse mode NCX contributes little to the Ca(2+) paradox, whereas inhibition of NCX during Ca(2+) depletion improves SR Ca(2+) loading, and is associated with reduced incidence of Ca(2+) paradox in mouse ventricular myocytes.

Stress-induced opening of the permeability transition pore in the dystrophin-deficient heart is attenuated by acute treatment with sildenafil

Susceptibility of cardiomyocytes to stress-induced damage has been implicated in the development of cardiomyopathy in Duchenne muscular dystrophy, a disease caused by the lack of the cytoskeletal protein dystrophin in which heart failure is frequent. However, the factors underlying the disease progression are unclear and treatments are limited. Here, we tested the hypothesis of a greater susceptibility to the opening of the mitochondrial permeability transition pore (PTP) in hearts from young dystrophic ( mdx) mice (before the development of overt cardiomyopathy) when subjected to a stress protocol and determined whether the prevention of a PTP opening is involved in the cardioprotective effect of sildenafil, which we have previously reported in mdx mice. Using the 2-deoxy-[3H]glucose method to quantify the PTP opening in ex vivo perfused hearts, we demonstrate that when compared with those of controls, the hearts from young mdx mice subjected to ischemia-reperfusion (I/R) display an excessive PTP opening as well as enhanced activation of cell death signaling, mitochondrial oxidative stress, cardiomyocyte damage, and poorer recovery of contractile function. Functional analyses in permeabilized cardiac fibers from nonischemic hearts revealed that in vitro mitochondria from mdx hearts display normal respiratory function and reactive oxygen species handling, but enhanced Ca2+ uptake velocity and premature opening of the PTP, which may predispose to I/R-induced injury. The administration of a single dose of sildenafil to mdx mice before I/R prevented excessive PTP opening and its downstream consequences and reduced tissue Ca2+ levels. Furthermore, mitochondrial Ca2+ uptake velocity was reduced following sildenafil treatment. In conclusion, beyond our documentation that an increased susceptibility to the opening of the mitochondrial PTP in the mdx heart occurs well before clinical signs of overt cardiomyopathy, our results demonstrate that sildenafil, which is already administered in other pediatric populations and is reported safe and well tolerated, provides efficient protection against this deleterious event, likely by reducing cellular Ca2+ loading and mitochondrial Ca2+ uptake.

Colocalization of voltage-gated Na+ channels with the Na+/Ca2+ exchanger in rabbit cardiomyocytes during development

Reverse-mode activity of the Na+/Ca2+ exchanger (NCX) has been previously shown to play a prominent role in excitation-contraction coupling in the neonatal rabbit heart, where we have proposed that a restricted subsarcolemmal domain allows a Na+ current to cause an elevation in the Na+ concentration sufficiently large to bring Ca2+ into the myocyte through reverse-mode NCX. In the present study, we tested the hypothesis that there is an overlapping expression and distribution of voltage-gated Na+ (Nav) channel isoforms and the NCX in the neonatal heart. For this purpose, Western blot analysis, immunocytochemistry, confocal microscopy, and image analyses were used. Here, we report the robust expression of skeletal Nav1.4 and cardiac Nav1.5 in neonatal myocytes. Both isoforms colocalized with the NCX, and Nav1.5-NCX colocalization was not statistically different from Nav1.4-NCX colocalization in the neonatal group. Western blot analysis also showed that Nav1.4 expression decreased by sixfold in the adult ( P < 0.01) and Nav1.1 expression decreased by ninefold ( P < 0.01), whereas Nav1.5 expression did not change. Although Nav1.4 underwent large changes in expression levels, the Nav1.4-NCX colocalization relationship did not change with age. In contrast, Nav1.5-NCX colocalization decreased ∼50% with development. Distance analysis indicated that the decrease in Nav1.5-NCX colocalization occurs due to a statistically significant increase in separation distances between Nav1.5 and NCX objects. Taken together, the robust expression of both Nav1.4 and Nav1.5 isoforms and their colocalization with the NCX in the neonatal heart provides structural support for Na+ current-induced Ca2+ entry through reverse-mode NCX. In contrast, this mechanism is likely less efficient in the adult heart because the expression of Nav1.4 and NCX is lower and the separation distance between Nav1.5 and NCX is larger.

Calcium handling in zebrafish ventricular myocytes

The zebrafish is an important model for the study of vertebrate cardiac development with a rich array of genetic mutations and biological reagents for functional interrogation. The similarity of the zebrafish (Danio rerio) cardiac action potential with that of humans further enhances the relevance of this model. In spite of this, little is known about excitation-contraction coupling in the zebrafish heart. To address this issue, adult zebrafish cardiomyocytes were isolated by enzymatic perfusion of the cannulated ventricle and were subjected to amphotericin-perforated patch-clamp technique, confocal calcium imaging, and/or measurements of cell shortening. Simultaneous recordings of the voltage dependence of the L-type calcium current (I(Ca,L)) amplitude and cell shortening showed a typical bell-shaped current-voltage (I-V) relationship for I(Ca,L) with a maximum at +10 mV, whereas calcium transients and cell shortening showed a monophasic increase with membrane depolarization that reached a plateau at membrane potentials above +20 mV. Values of I(Ca,L) were 53, 100, and 17% of maximum at -20, +10, and +40 mV, while the corresponding calcium transient amplitudes were 64, 92, and 98% and cell shortening values were 62, 95, and 96% of maximum, respectively, suggesting that I(Ca,L) is the major contributor to the activation of contraction at voltages below +10 mV, whereas the contribution of reverse-mode Na/Ca exchange becomes increasingly more important at membrane potentials above +10 mV. Comparison of the recovery of I(Ca,L) from acute and steady-state inactivation showed that reduction of I(Ca,L) upon elevation of the stimulation frequency is primarily due to calcium-dependent I(Ca,L) inactivation. In conclusion, we demonstrate that a large yield of healthy atrial and ventricular myocytes can be obtained by enzymatic perfusion of the cannulated zebrafish heart. Moreover, zebrafish ventricular myocytes differed from that of large mammals by having larger I(Ca,L) density and a monophasically increasing contraction-voltage relationship, suggesting that caution should be taken upon extrapolation of the functional impact of mutations on calcium handling and contraction in zebrafish cardiomyocytes.

Molecular mechanisms underlying Ca2+-mediated motility of human pancreatic duct cells

We recently reported that transforming growth factor-β (TGF-β) induces an increase in cytosolic Ca(2+) ([Ca(2+)](cyt)) in pancreatic cancer cells, but the mechanisms by which TGF-β mediates [Ca(2+)](cyt) homeostasis in these cells are currently unknown. Transient receptor potential (TRP) channels and Na(+)/Ca(2+) exchangers (NCX) are plasma membrane proteins that play prominent roles in controlling [Ca(2+)](cyt) homeostasis in normal mammalian cells, but little is known regarding their roles in the regulation of [Ca(2+)](cyt) in pancreatic cancer cells and pancreatic cancer development. Expression and function of NCX1 and TRPC1 proteins were characterized in BxPc3 pancreatic cancer cells. TGF-β induced both intracellular Ca(2+) release and extracellular Ca(2+) entry in these cells; however, 2-aminoethoxydiphenyl borate [2-APB; a blocker for both inositol 1,4,5-trisphosphate (IP(3)) receptor and TRPC], LaCl(3) (a selective TRPC blocker), or KB-R7943 (a selective inhibitor for the Ca(2+) entry mode of NCX) markedly inhibited the TGF-β-induced increase in [Ca(2+)](cyt). 2-APB or KB-R7943 treatment was able to dose-dependently reverse membrane translocation of PKCα induced by TGF-β. Transfection with small interfering RNA (siRNA) against NCX1 almost completely abolished NCX1 expression in BxPc3 cells and also inhibited PKCα serine phosphorylation induced by TGF-β. Knockdown of NCX1 or TRPC1 by specific siRNA transfection reversed TGF-β-induced pancreatic cancer cell motility. Therefore, TGF-β induces Ca(2+) entry likely via TRPC1 and NCX1 and raises [Ca(2+)](cyt) in pancreatic cancer cells, which is essential for PKCα activation and subsequent tumor cell invasion. Our data suggest that TRPC1 and NCX1 may be among the potential therapeutic targets for pancreatic cancer.

Perivascular fluid cuffs decrease lung compliance by increasing tissue resistance

OBJECTIVE

Lung inflammation causes perivascular fluid cuffs to form around extra-alveolar blood vessels; however, the physiologic consequences of such cuffs remain poorly understood. Herein, we tested the hypothesis that perivascular fluid cuffs, without concomitant alveolar edema, are sufficient to decrease lung compliance.

DESIGN

Prospective, randomized, controlled study.

SETTING

Research laboratory.

SUBJECTS

One hundred twenty male CD40 rats.

INTERVENTIONS

To test this hypothesis, the plant alkaloid thapsigargin was used to activate store-operated calcium entry and increase cytosolic calcium in endothelium. Thapsigargin was infused into a central venous catheter of intact, sedated, and mechanically ventilated rats.

MEASUREMENTS

Static and dynamic lung mechanics and hemodynamics were measured continuously.

MAIN RESULTS

Thapsigargin produced perivascular fluid cuffs along extra-alveolar vessels but did not cause alveolar flooding or blood gas abnormalities. Lung compliance dose-dependently decreased after thapsigargin infusion, attributable to an increase in tissue resistance that was attributed to increased tissue damping and tissue elastance. Airway resistance was not changed. Neither central venous pressure nor left ventricular end diastolic pressure was altered by thapsigargin. Heart rate did not change, although thapsigargin decreased left ventricular systolic function sufficient to reduce cardiac output by 50%. Infusion of the type 4 phosphodiesterase inhibitor, rolipram, prevented thapsigargin from inducing perivascular cuffs and decreasing lung compliance. Rolipram also normalized pressure over time and corrected the deficit in cardiac output.

CONCLUSIONS

Our findings resolve for the first time that perivascular cuff formation negatively impacts mechanical coupling between the bronchovascular bundle and the lung parenchyma, decreasing lung compliance without impacting central venous pressure.

Calcium entry is regulated by Zn2+ in relation to extracellular ionic environment in human airway epithelial cells

The extracellular pH, sodium and divalent cation concentrations influence the ATP-induced changes in cytosolic Ca(2+) concentration ([Ca(2+)](i)). This elevation of [Ca(2+)](i) and activation of Ca(2+)-dependent Cl(-) channels represent a possible therapeutic approach in cystic fibrosis (CF). We investigated the changes of [Ca(2+)](i) in different external ionic environment, and P2X purinergic receptors (P2XRs) expression in the control and CF airway epithelial cells. The parallel removal of Na(+) and alkalinization of the extracellular solution increased the amplitude of sustained ATP-induced Ca(2+) signals independent of wild-type or mutant CFTR expression. The ATP-induced Ca(2+) entry was either inhibited or stimulated by Zn(2+) depending on the extracellular Na(+) concentration. In Na(+)-free environment, Zn(2+) and other divalent cations elicited a biphasic Ca(2+) signal. Immunohistochemical data suggest that, multiple subtypes of P2XRs are expressed in these airway epithelial cells. In conclusion, Ca(2+) entry is finely regulated by external ionic environment. Therefore, we speculate that properly compiled aerosols could influence efficacy of zinc-based therapy in CF.

Alterations in mitochondrial function as a harbinger of cardiomyopathy: lessons from the dystrophic heart

While compelling evidence supports the central role of mitochondrial dysfunction in the pathogenesis of heart failure, there is comparatively less information available on mitochondrial alterations that occur prior to failure. Building on our recent work with the dystrophin-deficient mdx mouse heart, this review focuses on how early changes in mitochondrial functional phenotype occur prior to overt cardiomyopathy and may be a determinant for the development of adverse cardiac remodelling leading to failure. These include alterations in energy substrate utilization and signalling of cell death through increased permeability of mitochondrial membranes, which may result from abnormal calcium handling, and production of reactive oxygen species. Furthermore, we will discuss evidence supporting the notion that these alterations in the dystrophin-deficient heart may represent an early "subclinical" signature of a defective nitric oxide/cGMP signalling pathway, as well as the potential benefit of mitochondria-targeted therapies. While the mdx mouse is an animal model of Duchenne muscular dystrophy (DMD), changes in the structural integrity of dystrophin, the mutated cytoskeletal protein responsible for DMD, have also recently been implicated as a common mechanism for contractile dysfunction in heart failure. In fact, altogether our findings support a critical role for dystrophin in maintaining optimal coupling between metabolism and contraction in the heart.

Energetics of Ca2+ homeostasis during ischemia–reperfusion on neonatal rat hearts under high-[K+] cardioplegia

The mechanocalorimetric consequences and mechanisms involved in Ca2+ homeostasis during ischemia–reperfusion (I/R) as well as the protective role of cardioplegic pretreatment with high [K+] (25 mmol/L) and low or near-normal [Ca2+] (0.5 or 2 mmol/L) were evaluated in a model of neonatal rat heart. Beating hearts from 10–12-day-old rats were perfused with Krebs solution (2 mmol/L Ca2+) under both isotonic and isometric conditions. During pretreatment, hearts were exposed for 20 min to either Krebs (control) or cardioplegia (CPG) before 15 min ischemia and 45 min reperfusion while being continuously measured for either contractility or total heat rate (Ht) in a flow calorimeter. Contractile recovery after reperfusion in hearts exposed to ischemia only (control) was higher in the isometric hearts under optimal length (87.9% ± 8.1%) than in the isotonic hearts (57.3% ± 10.6%). This same behavior was found in hearts pretreated with CPG-0.5 mmol/L Ca2+. Ht in controls was reduced from 11.5 ± 0.8 mW/g in the initial beating condition to 1.11 ± 0.33 mW/g during ischemia and was increased to 13.02 ± 0.93 mW/g (113.8% ± 5.0% of preischemic) after reperfusion. Hearts pretreated with CPG-0.5 mmol/L Ca2+ showed the same behavior. However, when extracellular calcium ([Ca]o) was increased to 2 mmol/L under CPG, isotonic hearts, but not isometric hearts, significantly increased the contractile recovery to a maximum of 88.7% ± 10.8% of preischemic levels. Ht was recovered to 92.1% ± 4.3% of preischemic, suggesting that contractile recovery was less energetically expensive after CPG-2 mmol/L Ca2+ than it was in postischemic hearts exposed to control or CPG-0.5 mmol/L Ca2+. The role of the sarcoplasmic reticulum store was evaluated by pretreating hearts with 10 mmol/L caffeine, which reduced contractile recovery only under isometric conditions or after increasing [Ca]o in CPG under isotonic conditions, suggesting that the contribution of the sarcoplasmic reticulum was dependent on the fibre length or the [Ca]o. The inhibition of the reverse mode of the sarcolemmal Na/Ca exchanger (NCX) and the mitochondrial Ca uniporter (CaU) by KB-R7943 (KBR) at 5 µmol/L in CPG-0.5 mmol/L Ca2+ improved contractile recovery of isotonic hearts, whereas it decreased Ht at the start of reperfusion, suggesting that mitochondria could uptake Ca2+ vía the mitochondrial CaU. Neither the positive inotropism nor Ht were changed by inhibiting the mitochondrial NCX with 10 µmol/L clonazepam in CPG-0.5 mmol/L Ca2+ + 5 µmol/L KBR, which suggests that the mitochondrial NCX does not have a role. Finally, the role of the forward mode of the sarcolemmal NCX was evidenced by the fall in contractile recovery with increased Ht when KBR was increased to 20 µmol/L and added to CPG-2 mmol/L Ca2+ + 10 mmol/L caffeine before I/R. Thus the sarcolemmal NCX was essential for removing the diastolic Ca2+ during the periods of CPG and I/R. In summary, Ca2+ homeostasis during I/R of neonatal rat hearts is different from that of adult rats. High-[K+] CPG protected neonatal hearts only under isotonic conditions, at a near-normal [Ca]o, or by exposure to KBR. Mitochondria were able to uptake Ca2+ via the mitochondrial CaU and reduce the Ca2+ available for contractile recovery. Nevertheless, after increasing [Ca]o in CPG, the sarcoplasmic reticulum had a main role in restoring contractility during reperfusion, as it does in adults. Thus, the degree of maturation of the heart must be taken into account to evaluate the effects of CPG and drugs on I/R.

Drugs that interact with cardiac electro-mechanics: old and new targets for treatment

The concept of mechano-electrical feedback was derived from the observation that a short stretch applied to the beating heart can invoke an electrical response in the form of an afterdepolarization or a premature ventricular beat. More recent work has identified stretch-activated channels whose specific inhibition might help to treat atrial fibrillation in the near future. But the interaction between electrical and mechanical function of the heart is a continuum from short-term (within milliseconds) to long-term (within weeks or months) effects. The long-term effects of pressure overload have been well-described on the molecular and cellular level, and substances that interact with these processes are used in clinical routine in the care of patients with cardiac hypertrophy and heart failure. These treatments help to prevent lethal arrhythmias (sudden death) and potentially atrial fibrillation. The intermediate interaction between mechanical and electrical function of the heart is less well-understood. Several recently identified regulatory mechanisms may provide novel antiarrhythmic targets associated with the "intermediate" response of the myocardium to stretch.

Ontogeny of Ca2+-induced Ca2+ release in rabbit ventricular myocytes

It is commonly accepted that L-type Ca(2+) channel-mediated Ca(2+)-induced Ca(2+) release (CICR) is the dominant mode of excitation-contraction (E-C) coupling in the adult mammalian heart and that there is no appreciable CICR in neonates. However, we have observed that cell contraction in the neonatal heart was significantly decreased after sarcoplasmic reticulum (SR) Ca(2+) depletion with caffeine. Therefore, the present study investigated the developmental changes of CICR in rabbit ventricular myocytes at 3, 10, 20, and 56 days of age. We found that the inhibitory effect of the L-type Ca(2+) current (I(Ca)) inhibitor nifedipine (Nif; 15 microM) caused an increasingly larger reduction of Ca(2+) transients on depolarization in older age groups [from approximately 15% in 3-day-old (3d) myocytes to approximately 90% in 56-day-old (56d) myocytes]. The remaining Ca(2+) transient in the presence of Nif in younger age groups was eliminated by the inhibition of Na(+)/Ca(2+) exchanger (NCX) with the subsequent addition of 10 microM KB-R7943 (KB-R). Furthermore, Ca(2+) transients were significantly reduced in magnitude after the depletion of SR Ca(2+) with caffeine in all age groups, although the effect was significantly greater in the older age groups (from approximately 40% in 3d myocytes up to approximately 70% in 56d myocytes). This SR Ca(2+)-sensitive Ca(2+) transient in the earliest developmental stage was insensitive to Nif but was sensitive to the subsequent addition of KB-R, indicating the presence of NCX-mediated CICR that decreased significantly with age (from approximately 37% in 3d myocytes to approximately 0.5% in 56d myocytes). In contrast, the I(Ca)-mediated CICR increased significantly with age (from approximately 10% in 3d myocytes to approximately 70% in 56d myocytes). The CICR gain as estimated by the integral of the CICR Ca(2+) transient divided by the integral of its Ca(2+) transient trigger was smaller when mediated by NCX ( approximately 1.0 for 3d myocytes) than when mediated by I(Ca) ( approximately 3.0 for 56d myocytes). We conclude that the lower-efficiency NCX-mediated CICR is a predominant mode of CICR in the earliest developmental stages that gradually decreases as the more efficient L-type Ca(2+) channel-mediated CICR increases in prominence with ontogeny.

SR Ca2+refilling upon depletion and SR Ca2+uptake rates during development in rabbit ventricular myocytes

While it has been reported that a sparse sarcoplasmic reticulum (SR) and a low SR Ca2+pump density exist at birth, we and others have recently shown that significant amounts of Ca2+are stored in the neonatal rabbit heart SR. Here we try to determine developmental changes in SR Ca2+loading mechanisms and Ca2+pump efficacy in rabbit ventricular myocytes. SR Ca2+loading (loadSR) and k0.5(Ca2+concentration at half-maximal SR Ca2+uptake) were higher and lower, respectively, in younger age groups. Inhibition of the L-type calcium current ( ICa) with 15 μM nifedipine dramatically reduced loadSRin older but not in younger age groups. In contrast, subsequent inhibition of the Na+/Ca2+exchanger (NCX) with 10 μM KB-R7943 strongly reduced loadSRin the younger but not the older age groups. Accordingly, the time integral of the inward NCX current (tail INCX) elicited on repolarization was highly sensitive to nifedipine in the older groups and sensitive to KB-R7943 in the younger groups. Interestingly, slow SR loading took place in the presence of both nifedipine and KB-R7943 in all age groups, although it was less prominent in the older groups. We conclude that the SR loading capacity at the earliest postnatal stages is at least as large as that of adult myocytes. However, reverse-mode NCX plays a prominent role in SR Ca2+loading at early postnatal stages while ICais the main source of SR Ca2+loading at late postnatal and adult stages.

Three-dimensional distribution of cardiac Na+-Ca2+ exchanger and ryanodine receptor during development

Mechanisms of cardiac excitation-contraction coupling in neonates are still not clearly defined. Previous work in neonates shows reverse-mode Na(+)-Ca(2+) exchange to be the primary route of Ca(2+) entry during systole and the neonatal sarcoplasmic reticulum to have similar capability as that of adult in storing and releasing Ca(2+). We investigated Na(+)-Ca(2+) exchanger (NCX) and ryanodine receptor (RyR) distribution in developing ventricular myocytes using immunofluorescence, confocal microscopy, and digital image analysis. In neonates, both NCX and RyR clusters on the surface of the cell displayed a short longitudinal periodicity of approximately 0.7 microm. However, by adulthood, both proteins were also found in the interior. In the adult, clusters of NCX on the surface of the cell retained the approximately 0.7-microm periodicity whereas clusters of RyR adopted a longer longitudinal periodicity of approximately 2.0 microm. This suggests that neonatal myocytes also have a peri-M-line RyR distribution that is absent in adult myocytes. NCX and RyR colocalized voxel density was maximal in neonates and declined significantly with ontogeny. We conclude in newborns, Ca(2+) influx via NCX could potentially activate the dense network of peripheral Ca(2+) stores via peripheral couplings, evoking Ca(2+)-induced Ca(2+) release.

Store-operated Ca2+ entry and TRPC expression; possible roles in cardiac pacemaker tissue

Store-operated Ca(2+) channels (SOCCs) were first identified in non-excitable cells by the observation that depletion of Ca(2+) stores caused increased influx of extracellular Ca(2+). Recent studies have suggested that SOCCs might be related to the transient receptor potential (TRPC) gene family. The mechanism of cardiac pacemaking involves voltage-dependent pacemaker current; in addition there is growing evidence that intracellular sarcoplasmic reticulum (SR) Ca(2+) release plays an important role. In the present short review we assess preliminary evidence for Ca(2+) entry related to SR store depletion and expression of TRPCs in pacemaker tissue. These newer findings suggest that Ca(2+) entry and inward current triggered by store depletion might also contribute to the pacemaker current. Many hormones, drugs and interventions such as ischaemia and stretch, which alter Ca(2+) handling, will also modulate pacemaker firing thought their effect on SOCCs.

Sarcolemmal cation channels and exchangers modify the increase in intracellular calcium in cardiomyocytes on inhibiting Na+-K+-ATPase

Although inhibition of the sarcolemmal (SL) Na+-K+-ATPase is known to cause an increase in the intracellular concentration of Ca2+([Ca2+]i) by stimulating the SL Na+/Ca2+exchanger (NCX), the involvement of other SL sites in inducing this increase in [Ca2+]iis not fully understood. Isolated rat cardiomyocytes were treated with or without different agents that modify Ca2+movements by affecting various SL sites and were then exposed to ouabain. Ouabain was observed to increase the basal levels of both [Ca2+]iand intracellular Na+concentration ([Na+]i) as well as to augment the KCl-induced increases in both [Ca2+]iand [Na+]iin a concentration-dependent manner. The ouabain-induced changes in [Na+]iand [Ca2+]iwere attenuated by treatment with inhibitors of SL Na+/H+exchanger and SL Na+channels. Both the ouabain-induced increase in basal [Ca2+]iand augmentation of the KCl response were markedly decreased when cardiomyocytes were exposed to 0–10 mM Na+. Inhibitors of SL NCX depressed but decreasing extracellular Na+from 105–35 mM augmented the ouabain-induced increase in basal [Ca2+]iand the KCl response. Not only was the increase in [Ca2+]iby ouabain dependent on the extracellular Ca2+concentration, but it was also attenuated by inhibitors of SL L-type Ca2+channels and store-operated Ca2+channels (SOC). Unlike the SL L-type Ca2+-channel blocker, the blockers of SL Na+channel and SL SOC, when used in combination with SL NCX inhibitor, showed additive effects in reducing the ouabain-induced increase in basal [Ca2+]i. These results support the view that in addition to SL NCX, SL L-type Ca2+channels and SL SOC may be involved in raising [Ca2+]ion inhibition of the SL Na+-K+-ATPase by ouabain. Furthermore, both SL Na+/H+exchanger and Na+channels play a critical role in the ouabain-induced Ca2+increase in cardiomyocytes.

Na:Ca Stoichiometry and Cytosolic Ca-Dependent Activation of NCX in Intact Cardiomyocytes

We are studying both Na:Ca exchange stoichiometry and cytosolic [Ca] ([Ca]i)-dependent regulation of Na-Ca exchange (NCX) in intact rabbit ventricular myocytes. Analysis of NCX fluxes in subcellular systems strongly supports a dominant 3Na:1Ca exchange, and our measurements in intact cells confirm this. However, in intact native cells, local ion gradients and other factors complicate the process of inferring stoichiometry. From a functional viewpoint, NCX stoichiometry is near 3:1 but is affected by ion accumulation/depletion as well as non-NCX fluxes. We and others have viewed [Ca]i-dependent NCX regulation as a static process (dependent on instantaneous local [Ca]i). However, evidence from subcellular and expression systems shows the process to be dynamic, and our observations confirm this to be the case in intact cardiac cells as well.

Calcineurin-dependent cardiomyopathy is activated by TRPC in the adult mouse heart

The manner in which Ca2+-sensitive signaling proteins are activated in contracting cardiomyocytes is an intriguing theoretical problem given that the cytoplasm is continually bathed with systolic Ca2+ concentrations that should maximally activate most Ca2+-sensitive signaling kinases and phosphatases. Store-operated Ca2+ entry, partially attributed to transient receptor potential (TRP) proteins, can mediate activation of the Ca2+-sensitive phosphatase calcineurin in nonexcitable cells. Here we investigated the gain-of-function phenotype associated with TRPC3 expression in the mouse heart using transgenesis to examine the potential role of store-operated Ca2+ entry in regulating cardiac calcineurin activation and ensuing hypertrophy/myopathy. Adult myocytes isolated from TRPC3 transgenic mice showed abundant store-operated Ca2+ entry that was inhibited with SKF96365 but not verapamil or KB-R7943. Associated with this induction in store-operated Ca2+ entry, TRPC3 transgenic mice showed increased calcineurin-nuclear factor of activated T cells (NFAT) activation in vivo, cardiomyopathy, and increased hypertrophy after neuroendocrine agonist or pressure overload stimulation. The cardiomyopathic phenotype and increased hypertrophy after pressure overload stimulation were blocked by targeted disruption of the calcineurin Abeta gene. Thus, enhanced store-operated Ca2+ entry in the heart can regulate calcineurin-NFAT signaling in vivo, which could secondarily impact the hypertrophic response and cardiomyopathy.