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

Transient Receptor Potential Canonical 5 (TRPC5): Regulation of Heart Rate and Protection against Pathological Cardiac Hypertrophy

TRPC5 is a non-selective cation channel that is expressed in cardiomyocytes, but there is a lack of knowledge of its (patho)physiological role in vivo. Here, we examine the role of TRPC5 on cardiac function under basal conditions and during cardiac hypertrophy. Cardiovascular parameters were assessed in wild-type (WT) and global TRPC5 knockout (KO) mice. Despite no difference in blood pressure or activity, heart rate was significantly reduced in TRPC5 KO mice. Echocardiography imaging revealed an increase in stroke volume, but cardiac contractility was unaffected. The reduced heart rate persisted in isolated TRPC5 KO hearts, suggesting changes in basal cardiac pacing. Heart rate was further investigated by evaluating the reflex change following drug-induced pressure changes. The reflex bradycardic response following phenylephrine was greater in TRPC5 KO mice but the tachycardic response to SNP was unchanged, indicating an enhancement in the parasympathetic control of the heart rate. Moreover, the reduction in heart rate to carbachol was greater in isolated TRPC5 KO hearts. To evaluate the role of TRPC5 in cardiac pathology, mice were subjected to abdominal aortic banding (AAB). An exaggerated cardiac hypertrophy response to AAB was observed in TRPC5 KO mice, with an increased expression of hypertrophy markers, fibrosis, reactive oxygen species, and angiogenesis. This study provides novel evidence for a direct effect of TRPC5 on cardiac function. We propose that (1) TRPC5 is required for maintaining heart rate by regulating basal cardiac pacing and in response to pressure lowering, and (2) TRPC5 protects against pathological cardiac hypertrophy.

Regulation of cardiac function by cAMP nanodomains

Cyclic adenosine monophosphate (cAMP) is a diffusible intracellular second messenger that plays a key role in the regulation of cardiac function. In response to the release of catecholamines from sympathetic terminals, cAMP modulates heart rate and the strength of contraction and ease of relaxation of each heartbeat. At the same time, cAMP is involved in the response to a multitude of other hormones and neurotransmitters. A sophisticated network of regulatory mechanisms controls the temporal and spatial propagation of cAMP, resulting in the generation of signaling nanodomains that enable the second messenger to match each extracellular stimulus with the appropriate cellular response. Multiple proteins contribute to this spatiotemporal regulation, including the cAMP-hydrolyzing phosphodiesterases (PDEs). By breaking down cAMP to a different extent at different locations, these enzymes generate subcellular cAMP gradients. As a result, only a subset of the downstream effectors is activated and a specific response is executed. Dysregulation of cAMP compartmentalization has been observed in cardiovascular diseases, highlighting the importance of appropriate control of local cAMP signaling. Current research is unveiling the molecular organization underpinning cAMP compartmentalization, providing original insight into the physiology of cardiac myocytes and the alteration associated with disease, with the potential to uncover novel therapeutic targets. Here, we present an overview of the mechanisms that are currently understood to be involved in generating cAMP nanodomains and we highlight the questions that remain to be answered.

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.

Pharmacology of TRPC Channels and Its Potential in Cardiovascular and Metabolic Medicine

Transient receptor potential canonical (TRPC) proteins assemble to form homo- or heterotetrameric, nonselective cation channels permeable to K⁺, Na⁺, and Ca²⁺. TRPC channels are thought to act as complex integrators of physical and chemical environmental stimuli. Although the understanding of essential physiological roles of TRPC channels is incomplete, their implication in various pathological mechanisms and conditions of the nervous system, kidneys, and cardiovascular system in combination with the lack of major adverse effects of TRPC knockout or TRPC channel inhibition is driving the search of TRPC channel modulators as potential therapeutics. Here, we review the most promising small-molecule TRPC channel modulators, the understanding of their mode of action, and their potential in the study and treatment of cardiovascular and metabolic disease.

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.

Calcium and Heart Failure: How Did We Get Here and Where Are We Going?

The occurrence and prevalence of heart failure remain high in the United States as well as globally. One person dies every 30 s from heart disease. Recognizing the importance of heart failure, clinicians and scientists have sought better therapeutic strategies and even cures for end-stage heart failure. This exploration has resulted in many failed clinical trials testing novel classes of pharmaceutical drugs and even gene therapy. As a result, along the way, there have been paradigm shifts toward and away from differing therapeutic approaches. The continued prevalence of death from heart failure, however, clearly demonstrates that the heart is not simply a pump and instead forces us to consider the complexity of simplicity in the pathophysiology of heart failure and reinforces the need to discover new therapeutic approaches.

TRPC1/5-Ca V 3 Complex Mediates Leptin-Induced Excitability in Hypothalamic Neurons

Leptin regulates hypothalamic POMC⁺ (pro-opiomelanocortin) neurons by inducing TRPC (Transient Receptor Potential Cation) channel-mediate membrane depolarization. The role of TRPC channels in POMC neuron excitability is clearly established; however, it remains unknown whether their activity alone is sufficient to trigger excitability. Here we show that the right-shift voltage induced by the leptin-induced TRPC channel-mediated depolarization of the resting membrane potential brings T-type channels into the active window current range, resulting in an increase of the steady state T-type calcium current from 40 to 70% resulting in increased intrinsic excitability of POMC neurons. We assessed the role and timing of T-type channels on excitability and leptin-induced depolarization in vitro in cultured mouse POMC neurons. The involvement of TRPC channels in the leptin-induced excitability of POMC neurons was corroborated by using the TRPC channel inhibitor 2APB, which precluded the effect of leptin. We demonstrate T-type currents are indispensable for both processes, as treatment with NNC-55-0396 prevented the membrane depolarization and rheobase changes induced by leptin. Furthermore, co-immunoprecipitation experiments suggest that TRPC1/5 channels and Ca_V3.1 and Ca_V3.2 channels co-exist in complex. The functional relevance of this complex was corroborated using intracellular Ca²⁺ chelators; intracellular BAPTA (but not EGTA) application was sufficient to preclude POMC neuron excitability. However, leptin-induced depolarization still occurred in the presence of either BAPTA or EGTA suggesting that the calcium entry necessary to self-activate the TRPC1/5 complex is not blocked by the presence of BAPTA in hypothalamic neurons. Our study establishes T-type channels as integral part of the signaling cascade induced by leptin, modulating POMC neuron excitability. Leptin activation of TRPC channels existing in a macromolecular complex with T-type channels recruits the latter by locally induced membrane depolarization, further depolarizing POMC neurons, triggering action potentials and excitability.

Activation of TRPC (Transient Receptor Potential Canonical) Channel Currents in Iron Overloaded Cardiac Myocytes

BACKGROUND

Arrhythmias and heart failure are common cardiac complications leading to substantial morbidity and mortality in patients with hemochromatosis, yet mechanistic insights remain incomplete. We investigated the effects of iron (Fe) on electrophysiological properties and intracellular Ca²⁺ (Ca²⁺_i) handling in mouse left ventricular cardiomyocytes.

METHODS

Cardiomyocytes were isolated from the left ventricle of mouse hearts and were superfused with Fe³⁺/8-hydroxyquinoline complex (5-100 μM). Membrane potential and ionic currents including TRPC (transient receptor potential canonical) were recorded using the patch-clamp technique. Ca²⁺_i was evaluated by using Fluo-4. Cell contraction was measured with a video-based edge detection system. The role of TRPCs in the genesis of arrhythmias was also investigated by using a mathematical model of a mouse ventricular myocyte with the incorporation of the TRPC component.

RESULTS

We observed prolongation of the action potential duration and induction of early and delayed afterdepolarizations in myocytes superfused with 15 µmol/L Fe³⁺/8-hydroxyquinoline complex. Iron treatment decreased the peak amplitude of the L-type Ca²⁺ current and total K⁺ current, altered Ca²⁺_i dynamics, and decreased cell contractility. During the final phase of Fe treatment, sustained Ca²⁺_i waves and repolarization failure occurred and ventricular cells became unexcitable. Gadolinium abolished Ca²⁺_i waves and restored the resting membrane potential to the normal range. The involvement of TRPC activation was confirmed by TRPC channel current recordings in the absence or presence of functional TRPC channel antibodies. Computer modeling captured the same action potential and Ca²⁺_i dynamics and provided additional mechanistic insights.

CONCLUSIONS

We conclude that iron overload induces cardiac dysfunction that is associated with TRPC channel activation and alterations in membrane potential and Ca²⁺_i dynamics.

Isoproterenol-induced hypertrophy of neonatal cardiac myocytes and H9c2 cell is dependent on TRPC3-regulated CaV1.2 expression

Ca_V1.2 and transient receptor potential canonical channel 3 (TRPC3) are two proteins known to have important roles in pathological cardiac hypertrophy; however, such roles still remain unclear. A better understanding of these roles is important for furthering the clinical understanding of heart failure. We previously reported that Trpc3-knockout (KO) mice are resistant to pathologic hypertrophy and that their Ca_V1.2 protein expression is reduced. In this study, we aimed to examine the relationship between these two proteins and characterize their role in neonatal cardiomyocytes. We measured Ca_V1.2 expression in the hearts of wild-type (WT) and Trpc3^-/- mice, and examined the effects of Trpc3 knockdown and overexpression in the rat cell line H9c2. We also compared the hypertrophic responses of neonatal cardiomyocytes cultured from Trpc3^-/- mice to a representative hypertrophy-causing drug, isoproterenol (ISO), and measured the activity of nuclear factor of activated T cells 3 (NFAT3) in neonatal cardiomyocytes (NCMCs). We inhibited the L-type current with nifedipine, and measured the intracellular calcium concentration using Fura-2 with 1-oleoyl-2-acetyl-sn-glycerol (OAG)-induced Ba²⁺ influx. When using the Trpc3-mediated Ca²⁺ influx, both intracellular calcium concentration and calcium influx were reduced in Trpc3-KO myocytes. Not only was the expression of Ca_V1.2 greatly reduced in Trpc3-KO cardiac lysate, but the size of the Ca_V1.2 currents in NCMCs was also greatly reduced. When NCMCs were treated with Trpc3 siRNA, it was confirmed that the expression of Ca_V1.2 and the intracellular nuclear transfer activity of NFAT decreased. In H9c2 cells, the ISO activated- and verapamil inhibited- Ca²⁺ influxes were dramatically attenuated by Trpc3 siRNA treatment. In addition, it was confirmed that both the expression of Ca_V1.2 and the size of H9c2 cells were regulated according to the expression and activation level of TRPC3. We found that after stimulation with ISO, cell hypertrophy occurred in WT myocytes, while the increase in size of Trpc3-KO myocytes was greatly reduced. These results suggest that not only the cell hypertrophy process in neonatal cardiac myocytes and H9c2 cells were regulated according to the expression level of Ca_V1.2, but also that the expression level of Ca_V1.2 was regulated by TRPC3 through the activation of NFAT.

Mechanisms and significance of Ca2+ entry through TRPC channels

The transient receptor potential (TRP) superfamily of plasma membrane cation channels has been recognized as a signaling hub in highly diverse settings of human physiopathology. In the past three decades of TRP research, attention was focused mainly on the channels Ca2+ signaling function, albeit additional cellular functions, aside of providing a Ca2+ entry pathway, have been identified. Our understanding of Ca2+ signaling by TRP proteins has recently been advanced by a gain in high-resolution structure information on these pore complexes, and by the development of novel tools to investigate their role in spatiotemporal Ca2+ handling. This review summarizes recent discoveries as well as remaining, unresolved aspects of the canonical subfamily of transient receptor potential channels (TRPC) research. We aim at a concise overview on current mechanistic concepts of Ca2+ entry through- and Ca2+ signaling by TRPC channels.

Dynamic Properties of Heart Fragments from Different Regions and Their Synchronization

The dynamic properties of the heart differ based on the regions that effectively circulate blood throughout the body with each heartbeat. These properties, including the inter-beat interval (IBI) of autonomous beat activity, are retained even in in vitro tissue fragments. However, details of beat dynamics have not been well analyzed, particularly at the sub-mm scale, although such dynamics of size are important for regenerative medicine and computational studies of the heart. We analyzed the beat dynamics in sub-mm tissue fragments from atria and ventricles of hearts obtained from chick embryos over a period of 40 h. The IBI and contraction speed differed by region and atrial fragments retained their values for a longer time. The major finding of this study is synchronization of these fragment pairs physically attached to each other. The probability of achieving this and the time required differ for regional pairs: atrium-atrium, ventricle-ventricle, or atrium-ventricle. Furthermore, the time required to achieve 1:1 synchronization does not depend on the proximity of initial IBI of paired fragments. Various interesting phenomena, such as 1:n synchronization and a reentrant-like beat sequence, are revealed during synchronization. Finally, our observation of fragment dynamics indicates that mechanical motion itself contributes to the synchronization of atria.

Role of Transient Receptor Potential Canonical Channels in Heart Physiology and Pathophysiology

Transient receptor potential canonical (TRPC) channels are involved in the regulation of cardiac function under (patho)physiological conditions and are closely associated with the pathogenesis of cardiac hypertrophy, arrhythmias, and myocardial infarction. Understanding the molecular mechanisms and the regulatory pathway/locus of TRPC channels in related heart diseases will provide potential new concepts for designing novel drugs targeting TRPC channels. We will present the properties and regulation of TRPC channels and their roles in the development of various forms of heart disease. This article provides a brief review on the role of TRPC channels in the regulation of myocardial function as well as how TRPC channels may serve as a therapeutic target in heart failure and cardiac arrhythmias including atrial fibrillation.

TRPC Channels in Cardiac Plasticity

The heart flexibly changes its structure in response to changing environments and oxygen/nutrition demands of the body. Increased and decreased mechanical loading induces hypertrophy and atrophy of cardiomyocytes, respectively. In physiological conditions, these structural changes of the heart are reversible. However, chronic stresses such as hypertension or cancer cachexia cause irreversible remodeling of the heart, leading to heart failure. Accumulating evidence indicates that calcium dyshomeostasis and aberrant reactive oxygen species production cause pathological heart remodeling. Canonical transient receptor potential (TRPC) is a nonselective cation channel subfamily whose multimodal activation or modulation of channel activity play important roles in a plethora of cellular physiology. Roles of TRPC channels in cardiac physiology have been reported in pathological cardiac remodeling. In this review, we summarize recent findings regarding the importance of TRPC channels in flexible cardiac remodeling (i.e., cardiac plasticity) in response to environmental stresses and discuss questions that should be addressed in the near future.

Transient receptor potential channels in cardiac health and disease

Transient receptor potential (TRP) channels are nonselective cationic channels that are generally Ca²⁺ permeable and have a heterogeneous expression in the heart. In the myocardium, TRP channels participate in several physiological functions, such as modulation of action potential waveform, pacemaking, conduction, inotropy, lusitropy, Ca²⁺ and Mg²⁺ handling, store-operated Ca²⁺ entry, embryonic development, mitochondrial function and adaptive remodelling. Moreover, TRP channels are also involved in various pathological mechanisms, such as arrhythmias, ischaemia-reperfusion injuries, Ca²⁺-handling defects, fibrosis, maladaptive remodelling, inherited cardiopathies and cell death. In this Review, we present the current knowledge of the roles of TRP channels in different cardiac regions (sinus node, atria, ventricles and Purkinje fibres) and cells types (cardiomyocytes and fibroblasts) and discuss their contribution to pathophysiological mechanisms, which will help to identify the best candidates for new therapeutic targets among the cardiac TRP family.

TRPC Channels: Dysregulation and Ca2+ Mishandling in Ischemic Heart Disease

Transient receptor potential canonical (TRPC) channels are ubiquitously expressed in excitable and non-excitable cardiac cells where they sense and respond to a wide variety of physical and chemical stimuli. As other TRP channels, TRPC channels may form homo or heterotetrameric ion channels, and they can associate with other membrane receptors and ion channels to regulate intracellular calcium concentration. Dysfunctions of TRPC channels are involved in many types of cardiovascular diseases. Significant increase in the expression of different TRPC isoforms was observed in different animal models of heart infarcts and in vitro experimental models of ischemia and reperfusion. TRPC channel-mediated increase of the intracellular Ca²⁺ concentration seems to be required for the activation of the signaling pathway that plays minor roles in the healthy heart, but they are more relevant for cardiac responses to ischemia, such as the activation of different factors of transcription and cardiac hypertrophy, fibrosis, and angiogenesis. In this review, we highlight the current knowledge regarding TRPC implication in different cellular processes related to ischemia and reperfusion and to heart infarction.

TRP Channels: Current Perspectives in the Adverse Cardiac Remodeling

Calcium is an important second messenger required not only for the excitation-contraction coupling of the heart but also critical for the activation of cell signaling pathways involved in the adverse cardiac remodeling and consequently for the heart failure. Sustained neurohumoral activation, pressure-overload, or myocardial injury can cause pathologic hypertrophic growth of the heart followed by interstitial fibrosis. The consequent heart’s structural and molecular adaptation might elevate the risk of developing heart failure and malignant arrhythmia. Compelling evidences have demonstrated that Ca²⁺ entry through TRP channels might play pivotal roles in cardiac function and pathology. TRP proteins are classified into six subfamilies: TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPML (mucolipin), and TRPP (polycystin), which are activated by numerous physical and/or chemical stimuli. TRP channels participate to the handling of the intracellular Ca²⁺ concentration in cardiac myocytes and are mediators of different cardiovascular alterations. This review provides an overview of the current knowledge of TRP proteins implication in the pathologic process of some frequent cardiac diseases associated with the adverse cardiac remodeling such as cardiac hypertrophy, fibrosis, and conduction alteration.

The Biological Role of Klotho Protein in the Development of Cardiovascular Diseases

Klotho is a membrane-bound or soluble antiaging protein, whose protective activity is essential for a proper function of many organs. In 1997, an accidental insertion of a transgene led to creation of transgenic mice with several age-related disorders. In Klotho-deficient mice, the inherited phenotypes closely resemble human aging, while in an animal model of Klotho overexpression, the lifespan is extended. Klotho protein is detected mainly in the kidneys and brain. It is a coreceptor for fibroblast growth factor and hence is involved in maintaining endocrine system homeostasis. Furthermore, an inhibition of insulin/insulin-like growth factor-1 signaling pathway by Klotho regulates oxidative stress and reduces cell death. The association between serum Klotho and the classic risk factors, as well as the clinical history of cardiovascular disease, was also shown. There are a lot of evidences that Klotho deficiency correlates with the occurrence and development of coronary artery disease, atherosclerosis, myocardial infarction, and left ventricular hypertrophy. Therefore, an involvement of Klotho in the signaling pathways and in regulation of a proper cell metabolism could be a crucial factor in the cardiac and vascular protection. It is also well established that Klotho protein enhances the antioxidative response via augmented production of superoxide dismutase and reduced generation of reactive oxygen species. Recent studies have proven an expression of Klotho in cardiomyocytes and its increased expression in stress-related heart injury. Thus, the antioxidative and antiapoptotic activity of Klotho could be considered as the novel protective factor in cardiovascular disease and heart injury.

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.

The Complex Role of Store Operated Calcium Entry Pathways and Related Proteins in the Function of Cardiac, Skeletal and Vascular Smooth Muscle Cells

Cardiac, skeletal, and smooth muscle cells shared the common feature of contraction in response to different stimuli. Agonist-induced muscle's contraction is triggered by a cytosolic free Ca²⁺ concentration increase due to a rapid Ca²⁺ release from intracellular stores and a transmembrane Ca²⁺ influx, mainly through L-type Ca²⁺ channels. Compelling evidences have demonstrated that Ca²⁺ might also enter through other cationic channels such as Store-Operated Ca²⁺ Channels (SOCCs), involved in several physiological functions and pathological conditions. The opening of SOCCs is regulated by the filling state of the intracellular Ca²⁺ store, the sarcoplasmic reticulum, which communicates to the plasma membrane channels through the Stromal Interaction Molecule 1/2 (STIM1/2) protein. In muscle cells, SOCCs can be mainly non-selective cation channels formed by Orai1 and other members of the Transient Receptor Potential-Canonical (TRPC) channels family, as well as highly selective Ca²⁺ Release-Activated Ca²⁺ (CRAC) channels, formed exclusively by subunits of Orai proteins likely organized in macromolecular complexes. This review summarizes the current knowledge of the complex role of Store Operated Calcium Entry (SOCE) pathways and related proteins in the function of cardiac, skeletal, and vascular smooth muscle cells.

Canonical Transient Receptor Potential Channels and Their Link with Cardio/Cerebro-Vascular Diseases

The canonical transient receptor potential channels (TRPCs) constitute a series of nonselective cation channels with variable degrees of Ca²⁺ selectivity. TRPCs consist of seven mammalian members, TRPC1, TRPC2, TRPC3, TRPC4, TRPC5, TRPC6, and TRPC7, which are further divided into four subtypes, TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7. These channels take charge of various essential cell functions such as contraction, relaxation, proliferation, and dysfunction. This review, organized into seven main sections, will provide an overview of current knowledge about the underlying pathogenesis of TRPCs in cardio/cerebrovascular diseases, including hypertension, pulmonary arterial hypertension, cardiac hypertrophy, atherosclerosis, arrhythmia, and cerebrovascular ischemia reperfusion injury. Collectively, TRPCs could become a group of drug targets with important physiological functions for the therapy of human cardio/cerebro-vascular diseases.

Leukocyte TRP channel gene expressions in patients with non-valvular atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice and is a major cause of morbidity and mortality. The upregulation of TRP channels is believed to mediate the progression of electrical remodelling and the arrhythmogenesis of the diseased heart. However, there is limited data about the contribution of the TRP channels to development of AF. The aim of this study was to investigate leukocyte TRP channels gene expressions in non-valvular atrial fibrillation (NVAF) patients. The study included 47 NVAF patients and 47 sex and age matched controls. mRNA was extracted from blood samples, and real-time polymerase chain reaction was performed for gene expressions by using a dynamic array system. Low levels of TRP channel expressions in the controls were markedly potentiated in NVAF group. We observed marked increases in MCOLN1 (TRPML1), MCOLN2 (TRPML2), MCOLN3 (TRPML3), TRPA1, TRPM1, TRPM2, TRPM3, TRPM4, TRPM5, TRPM6, TRPM7, TRPM8, TRPC1, TRPC2, TRPC3, TRPC4, TRPC5, TRPC6, TRPC7, TRPV1, TRPV2, TRPV3, TRPV4, TRPV5, TRPV6, and PKD2 (TRPP2) gene expressions in NVAF patients (P < 0.05). However, there was no change in PKD1 (TRPP1) gene expression. This is the first study to provide evidence that elevated gene expressions of TRP channels are associated with the pathogenesis of NVAF.

Physiological and pathophysiological role of transient receptor potential canonical channels in cardiac myocytes

Transient receptor potential canonical (TRPC) channels constitute a family of seven Ca²⁺ permeable ion channels, named TRPC1 to 7. These channels are abundantly expressed in the mammalian heart, yet mechanisms underlying activation of TRPC channels and their precise role in cardiac physiology remain poorly understood. In this review, we perused original literature regarding TRPC channels in cardiomyocytes. We first reviewed studies on TRPC channel assembly and sub-cellular localization across multiple species and cell types. Our review indicates that TRPC localization in cardiac cells is still a topic of controversy. We then examined common molecular biology tools used to infer on location and physiological roles of TRPC channels in the heart. We subsequently reviewed pharmacological tools used to modulate TRPC activity in both cardiac and non-cardiac cells. Suggested physiological roles in the heart include modulation of heart rate and sensing of mechanical strain. We examined studies on the contribution of TRPC to cardiac pathophysiology, mainly hypertrophic signaling. Several TRPC channels, particularly TRPC1, 3 and 6 were proposed to play a crucial role in hypertrophic signaling. Finally, we discussed gaps in our understanding of the location and physiological role of TRPC channels in cardiomyocytes. Closing these gaps will be crucial to gain a full understanding of the role of TRPC channels in cardiac pathophysiology and to further explore these channels as targets for treatments for cardiac diseases, in particular, hypertrophy.

Muscling in on TRP channels in vascular smooth muscle cells and cardiomyocytes

The human TRP protein family comprises a family of 27 cation channels with diverse permeation and gating properties. The common theme is that they are very important regulators of intracellular Ca²⁺ signaling in diverse cell types, either by providing a Ca²⁺ influx pathway, or by depolarising the membrane potential, which on one hand triggers the activation of voltage-gated Ca²⁺ channels, and on the other limits the driving force for Ca²⁺ entry. Here we focus on the role of these TRP channels in vascular smooth muscle and cardiac striated muscle. We give an overview of highlights from the recent literature, and highlight the important and diverse roles of TRP channels in the pathophysiology of the cardiovascular system. The discovery of the superfamily of Transient Receptor Potential (TRP) channels has significantly enhanced our knowledge of multiple signal transduction mechanisms in cardiac muscle and vascular smooth muscle cells (VSMC). In recent years, multiple studies have provided evidence for the involvement of these channels, not only in the regulation of contraction, but also in cell proliferation and remodeling in pathological conditions. The mammalian family of TRP cation channels is composed by 28 genes which can be divided into 6 subfamilies groups based on sequence similarity: TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipins), TRPV (Vanilloid), TRPP (Policystin) and TRPA (Ankyrin-rich protein). Functional TRP channels are believed to form four-unit complexes in the plasma, each of them expressed with six transmembrane domain and intracellular N and C termini. Here we review the current knowledge on the expression of TRP channels in both muscle types, and discuss their functional properties and role in physiological and pathophysiological processes.

TRPC1 and TRPC4 channels functionally interact with STIM1L to promote myogenesis and maintain fast repetitive Ca2+ release in human myotubes

STIM1 and Orai1 are essential players of store-operated Ca²⁺ entry (SOCE) in human skeletal muscle cells and are required for adult muscle differentiation. Besides these two proteins, TRPC (transient receptor potential canonical) channels and STIM1L (a longer STIM1 isoform) are also present on muscle cells. In the present study, we assessed the role of TRPC1, TRPC4 and STIM1L in SOCE, in the maintenance of repetitive Ca²⁺ transients and in muscle differentiation. Knockdown of TRPC1 and TRPC4 reduced SOCE by about 50% and significantly delayed the onset of Ca²⁺ entry, both effects similar to STIM1L invalidation. Upon store depletion, TRPC1 and TRPC4 appeared to interact preferentially with STIM1L compared to STIM1. STIM1L invalidation affected myoblast differentiation, with the formation of smaller myotubes, an effect similar to what we reported for TRPC1 and TRPC4 knockdown. On the contrary, the overexpression of STIM1L leads to the formation of larger myotubes. All together, these data strongly suggest that STIM1L and TRPC1/4 are working together in myotubes to ensure efficient store refilling and a proper differentiation program.

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.

Chronic LiCl pretreatment suppresses thrombin-stimulated intracellular calcium mobilization through TRPC3 in astroglioma cells

OBJECTIVES

Transient receptor potential canonical type 3 (TRPC3) channels are activated in B lymphoblast cell lines from patients with bipolar disorder (BD), and its expression is reduced by chronic lithium treatment, implicating TRPC3 in the intracellular calcium (Ca²⁺ ) dyshomeostasis of BD. Thrombin, via a protease-activated receptor, moderates Ca²⁺ signaling and TRPC3 in astrocytes, and also cell proliferation. We examined whether lithium pretreatment attenuates thrombin-stimulated TRPC3 expression and function in astrocytes, and levels of the calcium-binding peptide, S100B, which is expressed mainly in these cells.

METHODS

Human astroglioma, U-87MG, cells were pretreated with 1 mmol L^-1 LiCl for 1 day (acute), 3 days (subacute), and 7 days (chronic). To examine the role of TRPC3, genetically stable knockdown TRPC3 cells (TRPC3^Low cells) were constructed using U-87MG cells. Thrombin (2.0 U/mL)-stimulated Ca²⁺ mobilization was measured by ratiometric fluorimetry. Changes in TRPC3 and S100B expression levels were determined by quantitative reverse transcription-polymerase chain reaction and immunoblotting, respectively. Cell proliferation was also measured using the WST-8 assay.

RESULTS

In this cell model, thrombin-stimulated Ca²⁺ mobilization, and both TRPC3 and S100B expression were suppressed by chronic LiCl pretreatment and the knockdown of TRPC3. Additionally, cell proliferation was attenuated in TRPC3^Low cells, compared with the negative control vector-transfected cell.

CONCLUSIONS

The reduced Ca²⁺ mobilization and S100B expression levels following chronic LiCl pretreatment and in TRPC3^Low cells support the notion that TRPC3 modulates S100B expression and is the target of the LiCl effect. Downregulation of TRPC3 may be an important mechanism by which lithium ameliorates pathophysiological intracellular Ca²⁺ disturbances as observed in BD, accounting, in part, for its mood-stabilizing effects.

Multiple ion channel block by the cation channel inhibitor SKF-96365 in myocytes from the rabbit atrioventricular node

The atrioventricular node (AVN) of the cardiac conduction system coordinates atrial-ventricular excitation and can act as a subsidiary pacemaker. Recent evidence suggests that an inward background sodium current, IB,Na, carried by nonselective cation channels (NSCCs), contributes to AVN cell pacemaking. The study of the physiological contribution of IB,Na has been hampered, however, by a lack of selective pharmacological antagonists. This study investigated effects of the NSCC inhibitor SKF-96365 on spontaneous activity, IB,Na, and other ionic currents in AVN cells isolated from the rabbit. Whole-cell patch-clamp recordings of action potentials (APs) and ionic currents were made at 35-37°C. A concentration of 10 μmol/L SKF-96365 slowed spontaneous action potential rate by 13.9 ± 5.3% (n = 8) and slope of the diastolic depolarization from 158.1 ± 30.5 to 86.8 ± 30.5 mV sec(-1) (P < 0.01; n = 8). Action potential upstroke velocity and maximum diastolic potential were also reduced. Under IB,Na-selective conditions, 10 μmol/L SKF-96365 inhibited IB,Na at -50 mV by 36.1 ± 6.8% (n = 8); however, effects on additional channel currents were also observed. Thus, the peak l-type calcium current (ICa,L) at +10 mV was inhibited by 38.6 ± 8.1% (n = 8), while the rapid delayed rectifier current, IKr, tails at -40 mV following depolarization to +20 mV were inhibited by 55.6 ± 4.6% (n = 8). The hyperpolarization-activated current, If, was unaffected by SKF-96365. Collectively, these results indicate that SKF-96365 exerts a moderate inhibitory effect on IB,Na and slows AVN cell pacemaking. However, additional effects of the compound on ICa,L and IKr confound the use of SKF-96365 to dissect out selectively the physiological role of IB,Na in the AVN.

Store-operated Ca2+ entry: a key component of the insulin secretion machinery

Normal plasma glucose level is ensured by the action of insulin, the major hypoglycemic hormone. Therefore, it is not surprising that insulin release from pancreatic β-cells of the islets of Langerhans is controlled by an array of balanced mechanisms in which glucose plays the leading role. Glucose triggers insulin secretion through the well-described pathway of ATP-driven closure of ATP-sensitive potassium channels (K_ATP), depolarization of the plasma membrane, and opening of the voltage-dependent Ca²⁺ channels (VDCC). The subsequent rapid rise in cytoplasmic free Ca²⁺ concentration triggers insulin exocytosis. However, despite more than 40 years of investigation, certain aspects of the intracellular Ca²⁺ responses to glucose and secretagogues remain unexplained, suggesting the involvement of additional Ca²⁺ channels. Here, we discuss the emerging role of store-operated Ca²⁺ channels carried by Orai1 and transient receptor potential canonical 1 (TRPC1) proteins and regulated by the stromal interaction molecule 1 (STIM1) in the control of glucose-induced insulin secretion. The role of other voltage-independent cation channels formed by other members of the TRP channels family is also addressed.

Requirement of IP3 receptor 3 (IP3R3) in nitric oxide induced cardiomyocyte differentiation of mouse embryonic stem cells

Nitric oxide (NO) markedly induces cardiomyocyte (CM) differentiation of embryonic stem (ES) cells. Here we examined the role of the Ca(2+) signaling in the NO-induced CM differentiation of mouse ES cells. We found that NO induced intracellular Ca(2+) increases in ES cells in a dose-dependent manner, and application of IP3 pathway antagonists not only significantly inhibited this induced Ca(2+) increase but also abolished NO-induced CM differentiation of ES cells. Subsequently, all 3 types of inositol 1, 4, 5-trisphosphate (IP3) receptors (IP3Rs) in mouse ES cells were individually or triply knocked down. Interestingly, only knockdown of type 3 IP3R (IP3R3) or triple-knockdown of three types of IP3Rs significantly inhibited the NO-induced Ca(2+) increases. Consistently, IP3R3 knockdown blocked the NO-induced CM differentiation of ES cells. CMs derived from IP3R3 knockdown ES cells also showed both structural and functional defects. In summary, our results indicate that the IP3R3-Ca(2+) pathway is required for NO-induced CM differentiation of ES cells.

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.

SKF-96365 blocks human ether-à-go-go-related gene potassium channels stably expressed in HEK 293 cells

SKF-96365 is a TRPC channel antagonist commonly used to characterize the potential functions of TRPC channels in different systems, which was recently reported to induce QTc prolongation on ECG by inhibiting TRPC channels. The present study investigates whether the blockade of cardiac repolarization currents would be involved in the increase of QTc interval. Cardiac repolarization currents were recorded in HEK 293 cells stably expressing human ether-à-go-go-related gene potassium (hERG or hKv11.1) channels, hKCNQ1/hKCNE1 channels (IKs) or hKir2.1 channels and cardiac action potentials were recorded in guinea pig ventricular myocytes using a whole-cell patch technique. The potential effect of SKF-96365 on QT interval was evaluated in ex vivo guinea pig hearts. It was found that SKF-96365 inhibited hERG current in a concentration-dependent manner (IC50, 3.4μM). The hERG mutants S631A in the pore helix and F656V of the S6 region reduced the inhibitory sensitivity with IC50s of 27.4μM and 11.0μM, suggesting a channel pore blocker. In addition, this compound inhibited IKs and hKir2.1currents with IC50s of 10.8 and 8.7μM. SKF-96365 (10μM) significantly prolonged ventricular APD90 in guinea pig ventricular myocytes and QTc interval in ex vivo guinea pig hearts. These results indicate that the TRPC channel antagonist SKF-96365 exerts blocking effects on hERG, IKs, and hKir2.1 channels. Prolongation of ventricular APD and QT interval is related to the inhibition of multiple repolarization potassium currents, especially hERG channels.

Store-operated Ca2+ Entry Mediated by Orai1 and TRPC1 Participates to Insulin Secretion in Rat β-Cells

Store-operated Ca(2+) channels (SOCs) are voltage-independent Ca(2+) channels activated upon depletion of the endoplasmic reticulum Ca(2+) stores. Early studies suggest the contribution of such channels to Ca(2+) homeostasis in insulin-secreting pancreatic β-cells. However, their composition and contribution to glucose-stimulated insulin secretion (GSIS) remains unclear. In this study, endoplasmic reticulum Ca(2+) depletion triggered by acetylcholine (ACh) or thapsigargin stimulated the formation of a ternary complex composed of Orai1, TRPC1, and STIM1, the key proteins involved in the formation of SOCs. Ca(2+) imaging further revealed that Orai1 and TRPC1 are required to form functional SOCs and that these channels are activated by STIM1 in response to thapsigargin or ACh. Pharmacological SOCs inhibition or dominant negative blockade of Orai1 or TRPC1 using the specific pore mutants Orai1-E106D and TRPC1-F562A impaired GSIS in rat β-cells and fully blocked the potentiating effect of ACh on secretion. In contrast, pharmacological or dominant negative blockade of TRPC3 had no effect on extracellular Ca(2+) entry and GSIS. Finally, we observed that prolonged exposure to supraphysiological glucose concentration impaired SOCs function without altering the expression levels of STIM1, Orai1, and TRPC1. We conclude that Orai1 and TRPC1, which form SOCs regulated by STIM1, play a key role in the effect of ACh on GSIS, a process that may be impaired in type 2 diabetes.

Evidence of a Role for Fibroblast Transient Receptor Potential Canonical 3 Ca2+ Channel in Renal Fibrosis

Transient receptor potential canonical (TRPC) Ca(2+)-permeant channels, especially TRPC3, are increasingly implicated in cardiorenal diseases. We studied the possible role of fibroblast TRPC3 in the development of renal fibrosis. In vitro, a macromolecular complex formed by TRPC1/TRPC3/TRPC6 existed in isolated cultured rat renal fibroblasts. However, specific blockade of TRPC3 with the pharmacologic inhibitor pyr3 was sufficient to inhibit both angiotensin II- and 1-oleoyl-2-acetyl-sn-glycerol-induced Ca(2+) entry in these cells, which was detected by fura-2 Ca(2+) imaging. TRPC3 blockade or Ca(2+) removal inhibited fibroblast proliferation and myofibroblast differentiation by suppressing the phosphorylation of extracellular signal-regulated kinase (ERK1/2). In addition, pyr3 inhibited fibrosis and inflammation-associated markers in a noncytotoxic manner. Furthermore, TRPC3 knockdown by siRNA confirmed these pharmacologic findings. In adult male Wistar rats or wild-type mice subjected to unilateral ureteral obstruction, TRPC3 expression increased in the fibroblasts of obstructed kidneys and was associated with increased Ca(2+) entry, ERK1/2 phosphorylation, and fibroblast proliferation. Both TRPC3 blockade in rats and TRPC3 knockout in mice inhibited ERK1/2 phosphorylation and fibroblast activation as well as myofibroblast differentiation and extracellular matrix remodeling in obstructed kidneys, thus ameliorating tubulointerstitial damage and renal fibrosis. In conclusion, TRPC3 channels are present in renal fibroblasts and control fibroblast proliferation, differentiation, and activation through Ca(2+)-mediated ERK signaling. TRPC3 channels might constitute important therapeutic targets for improving renal remodeling in kidney disease.

Arrhythmias in the developing heart

Prevalence of cardiac arrhythmias increases gradually with age; however, specific rhythm disturbances can appear even prior to birth and markedly affect foetal development. Relatively little is known about these disorders, chiefly because of their relative rarity and difficulty in diagnosis. In this review, we cover the most common forms found in human pathology, specifically congenital heart block, pre-excitation, extrasystoles and long QT syndrome. In addition, we cover pertinent literature data from prenatal animal models, providing a glimpse into pathogenesis of arrhythmias and possible strategies for treatment.

Role of TRP channels in the cardiovascular system

The transient receptor potential (TRP) superfamily consists of a large number of nonselective cation channels with variable degree of Ca(2+)-permeability. The 28 mammalian TRP channel proteins can be grouped into six subfamilies: canonical, vanilloid, melastatin, ankyrin, polycystic, and mucolipin TRPs. The majority of these TRP channels are expressed in different cell types including both excitable and nonexcitable cells of the cardiovascular system. Unlike voltage-gated ion channels, TRP channels do not have a typical voltage sensor, but instead can sense a variety of other stimuli including pressure, shear stress, mechanical stretch, oxidative stress, lipid environment alterations, hypertrophic signals, and inflammation products. By integrating multiple stimuli and transducing their activity to downstream cellular signal pathways via Ca(2+) entry and/or membrane depolarization, TRP channels play an essential role in regulating fundamental cell functions such as contraction, relaxation, proliferation, differentiation, and cell death. With the use of targeted deletion and transgenic mouse models, recent studies have revealed that TRP channels are involved in numerous cellular functions and play an important role in the pathophysiology of many diseases in the cardiovascular system. Moreover, several TRP channels are involved in inherited diseases of the cardiovascular system. This review presents an overview of current knowledge concerning the physiological functions of TRP channels in the cardiovascular system and their contributions to cardiovascular diseases. Ultimately, TRP channels may become potential therapeutic targets for cardiovascular diseases.

SKF-96365 strongly inhibits voltage-gated sodium current in rat ventricular myocytes

SKF-96365 (1-(beta-[3-(4-methoxy-phenyl) propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride) is a general TRPC channel antagonist commonly used to characterize the potential functions of TRPC channels in cardiovascular system. Recent reports showed that SKF-96365 induced a reduction in cardiac conduction. The present study investigates whether the reduced cardiac conduction caused by SKF-96365 is related to the blockade of voltage-gated sodium current (I Na) in rat ventricular myocytes using the whole-cell patch voltage-clamp technique. It was found that SKF-96365 inhibited I Na in rat ventricular myocytes in a concentration-dependent manner. The compound (1 μM) negatively shifted the potential of I Na availability by 9.5 mV, increased the closed-state inactivation of I Na, and slowed the recovery of I Na from inactivation. The inhibition of cardiac I Na by SKF-96365 was use-dependent and frequency-dependent, and the IC₅₀ was decreased from 1.36 μM at 0.5 Hz to 1.03, 0.81, 0.61, 0.56 μM at 1, 2, 5, 10 Hz, respectively. However, the selective TRPC3 antagonist Pyr3 decreased cardiac I Na by 8.5% at 10 μM with a weak use and frequency dependence. These results demonstrate that the TRPC channel antagonist SKF-96365 strongly blocks cardiac I Na in use-dependent and frequency-dependent manners. Caution should be taken for interpreting the alteration of cardiac electrical activity when SKF-96365 is used in native cells as a TRPC antagonist.

Studying dynamic events in the developing myocardium

Differentiation and conduction properties of the cardiomyocytes are critically dependent on physical conditioning both in vitro and in vivo. Historically, various techniques were introduced to study dynamic events such as electrical currents and changes in ionic concentrations in live cells, multicellular preparations, or entire hearts. Here we review this technological progress demonstrating how each improvement in spatial or temporal resolution provided answers to old and provoked new questions. We further demonstrate how high-speed optical mapping of voltage and calcium can uncover pacemaking potential within the outflow tract myocardium, providing a developmental explanation of ectopic beats originating from this region in the clinical settings.

Inhibitors of arachidonate-regulated calcium channel signaling suppress triggered activity induced by the late sodium current

Disturbances in myocyte calcium homeostasis are hypothesized to be one cause for cardiac arrhythmia. The full development of this hypothesis requires (i) the identification of all sources of arrhythmogenic calcium and (ii) an understanding of the mechanism(s) through which calcium initiates arrhythmia. To these ends we superfused rat left atria with the late sodium current activator type II Anemonia sulcata toxin (ATXII). This toxin prolonged atrial action potentials, induced early afterdepolarization, and provoked triggered activity. The calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 (N-[2-[[[3-(4-chlorophenyl)-2-propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulphon-amide) suppressed ATXII triggered activity but its inactive congener KN-92 (2-[N-(4-methoxy benzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine) did not. Neither drug affected normal atrial contractility. Calcium entry via L-type channels or calcium leakage from sarcoplasmic reticulum stores are not critical for this type of ectopy as neither verapamil ((RS)-2-(3,4-dimethoxyphenyl)-5-{[2-(3,4-dimethoxyphenyl)ethyl]-(methyl)amino}-2-prop-2-ylpentanenitrile) nor ryanodine affected ATXII triggered activity. By contrast, inhibitors of the voltage independent arachidonate-regulated calcium (ARC) channel and the store-operated calcium channel specifically suppressed ATXII triggered activity without normalizing action potentials or affecting atrial contractility. Inhibitors of cytosolic calcium-dependent phospholipase A2 also suppressed triggered activity suggesting that this lipase, which generates free arachidonate, plays a key role in ATXII ectopy. Thus, increased left atrial late sodium current appears to activate atrial Orai-linked ARC and store operated calcium channels, and these voltage-independent channels may be unexpected sources for the arrhythmogenic calcium that underlies triggered activity.

Expression and localization of TRPC proteins in rat ventricular myocytes at various developmental stages

Growing evidence indicates that transient receptor potential canonical (TRPC) channels play important roles in various Ca(2+)-mediated physiological and pathophysiological processes, including development. Many types of TRPC proteins are expressed in the heart. However, limited data are available comparing the expression and localization among TRPC proteins in the ventricular myocyte at various developmental stages. Our purpose is to investigate the expression and localization profile of TRPC proteins in ventricular myocytes of fetal (18.5 days), neonatal (< 24 h after birth) and adult (8 week old) rats. Western blotting, immunofluorescence and confocal laser scanning microscopy were employed. TRPC1/3-6 proteins were expressed in the rat ventricle throughout the three developmental stages. The expression profile of TRPC1/3/4 in the ventricle followed an upward trend from the fetus to the adult. By contrast, TRPC6 in the ventricle was expressed at the highest level in the fetal group and was sharply down-regulated immediately after birth. TRPC5 expression in the ventricle did not change significantly during the three stages. TRPC1/3/5/6 proteins were localized to the T-tubule and TRPC1/3/4/6 to intercalated disks in adult myocytes. The wide spatiotemporal overlap and dynamic regulation of TRPC expression in ventricular myocytes indicates potential complex combinations and redundancy of native TRPC proteins in the heart and gives important clues for further investigations into the exact subunit compositions and functional properties of native TRPC channels in the heart.

Enhanced contractility in pregnancy is associated with augmented TRPC3, L-type, and T-type voltage-dependent calcium channel function in rat uterine radial artery

In pregnancy, α-adrenoceptor-mediated vasoconstriction is augmented in uterine radial arteries and is accompanied by underlying changes in smooth muscle (SM) Ca2+ activity. This study aims to determine the Ca2+ entry channels associated with altered vasoconstriction in pregnancy, with the hypothesis that augmented vasoconstriction involves transient receptor potential canonical type-3 (TRPC3) and L- and T-type voltage-dependent Ca2+ channels. Immunohistochemistry showed TRPC3, L-type Cav1.2 (as the α1C subunit), T-type Cav3.1 (α1G), and Cav3.2 (α1H) localization to the uterine radial artery SM. Fluorescence intensity of TRPC3, Cav1.2, and Cav3.2 was increased, and Cav3.1 decreased in radial artery SM from pregnant rats. Western blot analysis confirmed increased TRPC3 protein expression in the radial artery from pregnant rats. Pressure myography incorporating pharmacological intervention to examine the role of these channels in uterine radial arteries showed an attenuation of phenylephrine (PE)-induced constriction with Pyr3 {1-[4-[(2,3,3-trichloro-1-oxo-2-propen-1-yl)amino]phenyl]-5-(trifluoromethyl)-1 H-pyrazole-4-carboxylic acid}-mediated TRPC3 inhibition or with nifedipine-mediated L-type channel block alone in vessels from pregnant rats; both effects of which were diminished in radial arteries from nonpregnant rats. Combined TRPC3 and L-type inhibition attenuated PE-induced constriction in radial arteries, and the residual vasoconstriction was reduced and abolished with T-type channel block with NNC 55-0396 in arteries from nonpregnant and pregnant rats, respectively. With SM Ca2+ stores depleted and in the presence of PE, nifedipine, and NNC 55-0396, blockade of TRPC3 reversed PE-induced constriction. These data suggest that TRPC3 channels act synergistically with L- and T-type channels to modulate radial artery vasoconstriction, with the mechanism being augmented in pregnancy.

Phylogeny and expression of canonical transient receptor potential (TRPC) genes in developing zebrafish

BACKGROUND

Canonical transient receptor potential (TRPC) channels are nonselective, calcium-permeable cation channels that are expressed in a great variety of organisms, tissues, and cell types. TRPC channels are known to be involved in the transduction of polymodal sensory input. Additionally, they are implicated in a variety of developmental processes. Distinct gating mechanisms have been elucidated so far, but their exact functional role in vertebrate organisms still needs to be resolved.

RESULTS

We now used the teleost Danio rerio to perform a comprehensive expression analysis of the trpc gene subfamily. Based on the sequence homology to the seven described mammalian TRPC channels, we identified 12 trpc genes in the zebrafish genome. All but trpc1 and trpc3 are represented by two paralogs. We further describe the specific expression patterns of trpc transcripts in whole-mounts during the first 5 days of development.

CONCLUSIONS

Consistent with their proposed role in sensory transduction zebrafish trpcs are predominantly expressed in neural structures such as the olfactory, visual, mechanosensitive, and motor systems. Intriguingly, zebrafish paralogs show mainly nonoverlapping expression patterns, suggesting that duplicated genes have either split their functions or have adapted new ones.

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.

Role of TRPC3 channel in human internal mammary artery

BACKGROUND AND AIMS

Intracellular calcium regulation in endothelial cells depends on transient receptor potential channels (TRPs). Canonical TRPs (TRPCs) are now recognized as the most important Ca(2+)-permeable cation channels in vascular endothelium and TRPC3 channel is reported to play a role in vasodilation in animal vessels. However, little is known about the role of TRPCs in human arteries. We therefore tested the hypothesis that TRPCs play a role in human arteries.

METHODS

Cumulative concentration-relaxation curves to acetylcholine (-11 to -4.5 log M) were established in the human internal mammary artery (IMA) rings (n = 42) taken from 28 patients undergoing coronary artery bypass grafting in precontraction induced by U46619 (-8 log M) in the absence or presence of SKF96365 (10 μmol/L) or Pyr3 (3 μmol/L). Protein expressions of TRPC3 were determined by Western blot and immunohistochemistry staining.

RESULTS

The maximal relaxation induced by acetylcholine was significantly attenuated by the nonspecific cation channels inhibitor, SKF96365 (48.2 ± 3.7 vs. 66.0 ± 0.9% in control, p <0.01) or the selective TRPC3 blocker, Pyr3 (58.4 ± 2.3% vs. 67.7 ± 1.1% in control, p <0.01). Protein expression of TRPC3 was detected in human IMA.

CONCLUSIONS

TRPC3 exists and plays a role in the acetylcholine-induced endothelium-dependent relaxation in the human IMA. This study suggests that TRPC3 may be a potential new target in endothelial protection in patients with endothelial dysfunction such as in patients with coronary artery disease in order to improve the long-term patency of the grafting vessels.