T-type calcium currents in rat cardiomyocytes during postnatal development: contribution to hormone secretion

Ionic currents underlying different patterns of electrical activity in working cardiac myocytes of mammals and non-mammalian vertebrates

The orderly contraction of the vertebrate heart is determined by generation and propagation of cardiac action potentials (APs). APs are generated by the integrated activity of time- and voltage-dependent ionic channels which carry inward Na⁺ and Ca²⁺ currents, and outward K⁺ currents. This review compares atrial and ventricular APs and underlying ion currents between different taxa of vertebrates. We have collected literature data and attempted to find common electrophysiological features for two or more vertebrate groups, show differences between taxa and cardiac chambers, and indicate gaps in the existing data. Although electrical excitability of the heart in all vertebrates is based on the same superfamily of channels, there is a vast variability of AP waveforms between atrial and ventricular myocytes, between different species of the same vertebrate class and between endothermic and ectothermic animals. The wide variability of AP shapes is related to species-specific differences in animal size, heart rate, stage of ontogenetic development, excitation-contraction coupling, temperature and oxygen availability. Some of the differences between taxa are related to evolutionary development of genomes, which appear e.g. in the expression of different Na⁺ and K⁺ channel orthologues in cardiomyocytes of vertebrates. There is a wonderful variability of AP shapes and underlying ion currents with which electrical excitability of vertebrate heart can be generated depending on the intrinsic and extrinsic conditions of animal body. This multitude of ionic mechanisms provides excellent material for studying how the function of the vertebrate heart can adapt or acclimate to prevailing physiological and environmental conditions.

Regulation of calcium dynamics and propagation velocity by tissue microstructure in engineered strands of cardiac tissue

Disruptions to cardiac tissue microstructure are common in diseased or injured myocardium and are known substrates for arrhythmias. However, we have a relatively coarse understanding of the relationships between myocardial tissue microstructure, propagation velocity and calcium cycling, due largely to the limitations of conventional experimental tools. To address this, we used microcontact printing to engineer strands of cardiac tissue with eight different widths, quantified several structural and functional parameters and established correlation coefficients. As strand width increased, actin alignment, nuclei density, sarcomere index and cell aspect ratio decreased with unique trends. The propagation velocity of calcium waves decreased and the rise time of calcium transients increased with increasing strand width. The decay time constant of calcium transients decreased and then slightly increased with increasing strand width. Based on correlation coefficients, actin alignment was the strongest predictor of propagation velocity and calcium transient rise time. Sarcomere index and cell aspect ratio were also strongly correlated with propagation velocity. Actin alignment, sarcomere index and cell aspect ratio were all weak predictors of the calcium transient decay time constant. We also measured the expression of several genes relevant to propagation and calcium cycling and found higher expression of the genes that encode for connexin 43 (Cx43) and a subunit of L-type calcium channels in thin strands compared to isotropic tissues. Together, these results suggest that thinner strands have higher values of propagation velocity and calcium transient rise time due to a combination of favorable tissue microstructure and enhanced expression of genes for Cx43 and L-type calcium channels. These data are important for defining how microstructural features regulate intercellular and intracellular calcium handling, which is needed to understand mechanisms of propagation in physiological situations and arrhythmogenesis in pathological situations.

Non-Mendelian inheritance during inbreeding of Cav3.2 and Cav2.3 deficient mice

The mating of 77 heterozygous pairs (Ca_v3.2[+|-] x Ca_v3.2[+|-]) revealed a significant deviation of genotype distribution from Mendelian inheritance in weaned pups. The mating of 14 pairs (Ca_v3.2[-|-] female x Ca_v3.2[+|-] male) and 8 pairs (Ca_v3.2[+|-] female x Ca_v3.2[-|-] male) confirmed the significant reduction of deficient homozygous Ca_v3.2[-|-] pups, leading to the conclusion that prenatal lethality may occur, when one or both alleles, encoding the Ca_v3.2T-type Ca²⁺ channel, are missing. Also, the mating of 63 heterozygous pairs (Ca_v2.3[+|-] x Ca_v2.3[+|-]) revealed a significant deviation of genotype distribution from Mendelian inheritance in weaned pups, but only for heterozygous male mice, leading to the conclusion that compensation may only occur for Ca_v2.3[-|-] male mice lacking both alleles of the R-type Ca²⁺ channel. During the mating of heterozygous parents, the number of female mice within the weaned population does not deviate from the expected Mendelian inheritance. During prenatal development, both, T- and R-type Ca²⁺ currents are higher expressed in some tissues than postnatally. It will be discussed that the function of voltage-gated Ca²⁺ channels during prenatal development must be investigated in more detail, not least to understand devastative diseases like developmental epileptic encephalopathies (DEE).

A low voltage activated Ca2+ current found in a subset of human ventricular myocytes

Low voltage activated (I_Ca-LVA) calcium currents including Cav1.3 and T-type calcium current (I_Ca-T) have not been reported in adult human left ventricular myocytes (HLVMs). We tried to examine their existence and possible correlation with etiology and patient characteristics in a big number of human LVMs isolated from explanted terminally failing (F) hearts, failing hearts with left ventricular assist device (F-LVAD) and nonfailing (NF) human hearts. LVA (I_Ca-LVA) was determined by subtracting L-type Ca²⁺ current (I_Ca-L) recorded with the holding potential of −50 mV from total Ca²⁺ current recorded with the holding potential of −90 mV or −70 mV. I_{Ca- LVA} was further tested with its sensitivity to 100 µM CdCl₂ and tetrodotoxin. Three HLVMs (3 of 137 FHLVMs) from 2 (N = 30 hearts) failing human hearts, of which one was idiopathic and the other was due to primary pulmonary hypertension, were found with I_Ca-LVA. I_Ca-LVA in one FHLVM was not sensitive to 100 µM CdCl₂ while I_Ca-LVA in another two FHLVMs was not sensitive to tetrodotoxin. It peaked at the voltage of −40~-20 mV and had a time-dependent decay faster than I_Ca-L but slower than sodium current (I_Na). I_Ca-LVA was not found in any HLVMs from NF (75 HLVMs from 17 hearts) or F-LVAD hearts (82 HLVMs from 18 hearts) but a statistically significant correlation could not be established. In conclusion, I_Ca-LVA was detected in some HLVMs of a small portion of human hearts that happened to be nonischemic failing hearts.

Low voltage-activated channels in rat pulmonary vein cardiomyocytes: coexistence of a non-selective cationic channel and of T-type Ca channels

In rat pulmonary vein (PV) cardiomyocytes (CM), norepinephrine (NE) induces an automatic activity consisting of bursts of slow action potentials which depend on Ca²⁺ (upstroke) and Na⁺ (inter-burst) channels. Our objective was to characterize low voltage-activated (LVA) currents in rat PVCM susceptible to trigger this activity. Whole-cell I_Ca (5 mM Ca²⁺) was recorded from - 100 mV with classical Na⁺- and K⁺-free solutions. A fast LVA I_Ca (FLVA-I_Ca), present in ≈ 56% of PVCM between ~ - 50 to - 20 mV, was blocked by 10 μM TTX and markedly increased by addition of NaCl (1 or 3 mM) or KCl (5 or 10 mM). Permeability ratios P'_Ca/P_Na and P'_Ca/P_K calculated for bi-ionic conditions were respectively 2.25 ± 0.51 and 1.88 ± 0.25, and not different from a value of 2. FLVA-I_Ca was increased by 10 μM NE and 300 nM BayK8644, decreased by 5 μM nifedipine but not blocked by ranolazine (10 μM). NiCl₂ (40 μM) and TTA-A2 (10 or 100 nM) increased FLVA-I_Ca. Similar results were obtained in left atrial (LA) CM. Neither Ba²⁺ nor Sr²⁺ alone could permeate the FLVA channel or block Ca²⁺ influx but revealed a large slower activating and inactivating LVA Ca²⁺ current (SLVA-I_Ca), present in 10 out of 80 PVCM, absent in LACM, and partially inhibited by 100 nM TTA-A2. Therefore, the ionic channel underlying FLVA-I_Ca is likely a fast voltage-gated non-selective channel with a dihydropyridine binding site. SLVA-I_Ca might correspond to Ca²⁺ influx through Ca_v3.x channels and contribute to triggering NE-induced automatic activity in the PV myocardial sleeve.

The extracellular matrix and Ca(2+)signaling mechanisms

The extracellular matrix (ECM) consists of proteins, glycosaminoglycans and glycoproteins, that support the dynamic interactions between cells, including intercellular communication, cell attachment, cell differentiation, cell growth and migration. As such, the ECM represents an essential and very sensitive system within the tissue microenvironment that is involved in processes such as tissue regeneration and carcinogenesis. The aim of the present review is to evaluate its diversity through Ca(2+) signaling and its role in muscle cell function. Here, we discuss some methodological approaches dissecting Ca(2+) handling mechanisms in myogenic and non-myogenic cells, e.g. the importance of Ca(2+) and calpains in muscle dystrophy. We also consider the reconstruction of skeletal muscle by colonization of decellularized ECM with muscle-derived cells isolated from skeletal muscle. Therefore, it is necessary to establish new methodological procedures based on Ca(2+) signaling in skeletal muscle cells and their effect on ECM homeostasis, allowing the monitoring of skeletal muscle reconstruction and organ repair.

Dexamethasone-induced upregulation of CaV3.2 T-type Ca2+ channels in rat cardiac myocytes

Glucocorticoids are widely used to treat acute and chronic diseases. Unfortunately, their therapeutic use is associated with severe side effects. Glucocorticoids are known to regulate several ion channels in cardiac myocytes, including voltage-dependent Ca²⁺ channels. Low-voltage-activated T-type Ca²⁺ channels are expressed in ventricular myocytes during the fetal and perinatal period, but are practically absent in the adult. However, these channels can be re-expressed in adult cardiomyocytes under some pathological conditions. We have investigated the glucocorticoid regulation of T-type Ca²⁺ channels in rat cardiomyocytes. Molecular studies revealed that dexamethasone induces the upregulation of Ca_V3.2 mRNA in neonatal rat ventricular myocytes, whereas Ca_V3.1 mRNA is only slightly affected. Patch-clamp recordings confirmed that T-type Ca²⁺ channel currents were upregulated in dexamethasone treated cardiomyocytes, and the addition of 50 μmol/L NiCl₂ demonstrated that the Ca_V3.2 channel is responsible for this upregulation. The effect of dexamethasone on Ca_V3.2 is mediated by the activation and translocation to the cell nucleus of the glucocorticoid receptor (GR). We have isolated the upstream promoter of the Cacna1h gene and tested its activity in transfected ventricular myocytes. The initial in silico analysis of Cacna1h promoter revealed putative glucocorticoid response elements (GREs). Transcriptional activity assays combined with deletion analyses and chromatin immunoprecipitation assays demonstrated that GR binds to a region a GRE located in -1006/-985 bp of Cacna1h promoter. Importantly, upregulation of the Ca_V3.2 channel is also observed in vitro in adult rat ventricular myocytes, and in vivo in a rat model of excess of glucocorticoids.

Cardiomyocytes Sense Matrix Rigidity through a Combination of Muscle and Non-muscle Myosin Contractions

Mechanical properties are cues for many biological processes in health or disease. In the heart, changes to the extracellular matrix composition and cross-linking result in stiffening of the cellular microenvironment during development. Moreover, myocardial infarction and cardiomyopathies lead to fibrosis and a stiffer environment, affecting cardiomyocyte behavior. Here, we identify that single cardiomyocyte adhesions sense simultaneous (fast oscillating) cardiac and (slow) non-muscle myosin contractions. Together, these lead to oscillating tension on the mechanosensitive adaptor protein talin on substrates with a stiffness of healthy adult heart tissue, compared with no tension on embryonic heart stiffness and continuous stretching on fibrotic stiffness. Moreover, we show that activation of PKC leads to the induction of cardiomyocyte hypertrophy in a stiffness-dependent way, through activation of non-muscle myosin. Finally, PKC and non-muscle myosin are upregulated at the costameres in heart disease, indicating aberrant mechanosensing as a contributing factor to long-term remodeling and heart failure.

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Talin in cardiomyocytes is unstretched, cyclically stretched, or continuously stretched

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Talin stretching depends on stiffness, myofibrillar tension, and non-myofibrillar tension

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Non-myofibrillar contractility requires PKC, Src, FHOD1, and non-muscle myosin

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PKC and non-muscle myosin activity are enhanced in cardiac disease

Ursodeoxycholic acid prevents ventricular conduction slowing and arrhythmia by restoring T-type calcium current in fetuses during cholestasis

Background

Increased maternal serum bile acid concentrations in intrahepatic cholestasis of pregnancy (ICP) are associated with fetal cardiac arrhythmias. Ursodeoxycholic acid (UDCA) has been shown to demonstrate anti-arrhythmic properties via preventing ICP-associated cardiac conduction slowing and development of reentrant arrhythmias, although the cellular mechanism is still being elucidated.

Methods

High-resolution fluorescent optical mapping of electrical activity and electrocardiogram measurements were used to characterize effects of UDCA on one-day-old neonatal and adult female Langendorff-perfused rat hearts. ICP was modelled by perfusion of taurocholic acid (TC, 400μM). Whole-cell calcium currents were recorded from neonatal rat and human fetal cardiomyocytes.

Results

TC significantly prolonged the PR interval by 11.0±3.5% (P<0.05) and slowed ventricular conduction velocity (CV) by 38.9±5.1% (P<0.05) exclusively in neonatal and not in maternal hearts. A similar CV decline was observed with the selective T-type calcium current (I_Ca,T) blocker mibefradil 1μM (23.0±6.2%, P<0.05), but not with the L-type calcium current (I_Ca,L) blocker nifedipine 1μM (6.9±6.6%, NS). The sodium channel blocker lidocaine (30μM) reduced CV by 60.4±4.5% (P<0.05). UDCA co-treatment was protective against CV slowing induced by TC and mibefradil, but not against lidocaine. UDCA prevented the TC-induced reduction in the I_Ca,T density in both isolated human fetal (−10.2±1.5 versus −5.5±0.9 pA/pF, P<0.05) and neonatal rat ventricular myocytes (−22.3±1.1 versus −9.6±0.8 pA/pF, P<0.0001), whereas UDCA had limited efficacy on the I_Ca,L.

Conclusion

Our findings demonstrate that I_Ca,T plays a significant role in ICP-associated fetal cardiac conduction slowing and arrhythmogenesis, and is an important component of the fetus-specific anti-arrhythmic activity of UDCA.

Tetrodotoxin-sensitive Ca2+ Currents, but No T-type Currents in Normal, Hypertrophied, and Failing Mouse Cardiomyocytes

AIMS

To obtain functional evidence that ICa,T is involved in the pathogenesis of cardiac hypertrophy and heart failure. We unexpectedly identified ICa(TTX) rather than ICa,T, therefore, we adjusted our aim to encompass these findings.

METHODS AND RESULTS

We investigated (1) Cav3.1 (α1G) transgenic (Tg) mice compared with nontransgenic (tTA-Ntg); (2) Cav3.1-deficient mice (Cav3.1) compared with wild type (Wt) after chemically and surgically induced cardiac remodeling; and (3) spontaneous hypertensive rats and thoracic aortic constriction (TAC) rats. Whole-cell patch-clamp technique was used to measure ICa in ventricular myocytes. Cav3.1-Tg expressed ICa,T (-18.35 ± 1.02 pA/pF at -40 mV) without signs of compromised cardiac function. While we failed to detect ICa,T after hypertrophic stimuli, instead we demonstrated that both Wt and Cav3.1 mouse exhibit ICa(TTX). Using TAC rats, only 2 of 24 VMs showed ICa,T under our experimental conditions. Without TTX, ICa(TTX) occurred in VMs from Wt, spontaneous hypertensive rats, and TAC rats also.

CONCLUSIONS

These findings demonstrate for the first time that mouse VMs express ICa(TTX). We suggest that future studies should take into consideration the measuring conditions when interpreting ICa,T reappearance in ventricular myocytes in response to hypertrophic stress. Contamination with ICa(TTX) could possibly confuse the relevance of the data.

Microarray analysis reveals a potential role of LncRNAs expression in cardiac cell proliferation

Background

Long non-coding RNAs (LncRNAs) have been identified to play important roles in epigenetic processes that underpin organogenesis. However, the role of LncRNAs in the regulation of transition from fetal to adult life of human heart has not been evaluated.

Methods

Immunofiuorescent staining was used to determine the extent of cardiac cell proliferation. Human LncRNA microarrays were applied to define gene expression signatures of the fetal (13–17 weeks of gestation, n = 4) and adult hearts (30–40 years old, n = 4). Pathway analysis was performed to predict the function of differentially expressed mRNAs (DEM). DEM related to cell proliferation were selected to construct a lncRNA-mRNA co-expression network. Eight lncRNAs were confirmed by quantificational real-time polymerase chain reaction (n = 6).

Results

Cardiac cell proliferation was significant in the fetal heart. Two thousand six hundred six lncRNAs and 3079 mRNAs were found to be differentially expressed. Cell cycle was the most enriched pathway in down-regulated genes in the adult heart. Eight lncRNAs (RP11-119 F7.5, AX747860, HBBP1, LINC00304, TPTE2P6, AC034193.5, XLOC_006934 and AL833346) were predicted to play a central role in cardiac cell proliferation.

Conclusions

We discovered a profile of lncRNAs differentially expressed between the human fetal and adult heart. Several meaningful lncRNAs involved in cardiac cell proliferation were disclosed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-016-0139-4) contains supplementary material, which is available to authorized users.

Hypoxic induction of T-type Ca(2+) channels in rat cardiac myocytes: role of HIF-1α and RhoA/ROCK signalling

T-type Ca(2+) channels are expressed in the ventricular myocytes of the fetal and perinatal heart, but are normally downregulated as development progresses. Interestingly, however, these channels are re-expressed in adult cardiomyocytes under pathological conditions. We investigated low voltage-activated T-type Ca(2+) channel regulation in hypoxia in rat cardiomyocytes. Molecular studies revealed that hypoxia induces the upregulation of Cav 3.2 mRNA, whereas Cav 3.1 mRNA is not significantly altered. The effect of hypoxia on Cav 3.2 mRNA was time- and dose-dependent, and required hypoxia inducible factor-1α (HIF-1α) stabilization. Patch-clamp recordings confirmed that T-type Ca(2+) channel currents were upregulated in hypoxic conditions, and the addition of 50 μm NiCl2 (a T-type channel blocker) demonstrated that the Cav 3.2 channel is responsible for this upregulation. This increase in current density was not accompanied by significant changes in the Cav 3.2 channel electrophysiological properties. The small monomeric G-protein RhoA and its effector Rho-associated kinase I (ROCKI), which are known to play important roles in cardiovascular physiology, were also upregulated in neonatal rat ventricular myocytes subjected to hypoxia. Pharmacological experiments indicated that both proteins were involved in the observed upregulation of the Cav 3.2 channel and the stabilization of HIF-1α that occurred in response to hypoxia. These results suggest a possible role for Cav 3.2 channels in the increased probability of developing arrhythmias observed in ischaemic situations, and in the pathogenesis of diseases associated with hypoxic Ca(2+) overload.

Voltage-gated calcium channel blockers deregulate macroautophagy in cardiomyocytes

Voltage-gated calcium channel blockers are widely used for the management of cardiovascular diseases, however little is known about their effects on cardiac cells in vitro. We challenged neonatal ventricular cardiomyocytes (CMs) with therapeutic L-type and T-type Ca(2+) channel blockers (nifedipine and mibefradil, respectively), and measured their effects on cell stress and survival, using fluorescent microscopy, Q-PCR and Western blot. Both nifedipine and mibefradil induced a low-level and partially transient up-regulation of three key mediators of the Unfolded Protein Response (UPR), indicative of endoplasmic (ER) reticulum stress. Furthermore, nifedipine triggered the activation of macroautophagy, as evidenced by increased lipidation of microtubule-associated protein 1 light chain 3 (LC3), decreased levels of polyubiquitin-binding protein p62/SQSTM1 and ubiquitinated protein aggregates, that was followed by cell death. In contrast, mibefradil inhibited CMs constitutive macroautophagy and did not promote cell death. The siRNA-mediated gene silencing approach confirmed the pharmacological findings for T-type channels. We conclude that L-type and T-type Ca(2+) channel blockers induce ER stress, which is divergently transduced into macroautophagy induction and inhibition, respectively, with relevance for cell viability. Our work identifies VGCCs as novel regulators of autophagy in the heart muscle and provides new insights into the effects of VGCC blockers on CMs homeostasis, that may underlie both noxious and cardioprotective effects.

Calcium signalling in developing cardiomyocytes: implications for model systems and disease

Adult cardiomyocytes exhibit complex Ca(2+) homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca(2+) sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca(2+) homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as cultured neonatal cells and stem cells.

Electrical excitability of the heart in a Chondrostei fish, the Siberian sturgeon (Acipenser baerii)

Sturgeon (family Acipenseridae) are regarded as living fossils due to their ancient origin and exceptionally slow evolution. To extend our knowledge of fish cardiac excitability to a Chondrostei fish, we examined electrophysiological phenotype of the Siberian sturgeon ( Acipenser baerii) heart with recordings of epicardial ECG, intracellular action potentials (APs), and sarcolemmal ion currents. Epicardial ECG of A. baerii had the typical waveform of the vertebrate ECG with Q-T interval (average duration of ventricular AP) of 650 ± 30 ms and an intrinsic heart rate of 45.5 ± 5 beats min−1 at 20°C. Similar to other fish species, atrial AP was shorter in duration (402 ± 33 ms) than ventricular AP (585 ± 40) ( P < 0.05) at 20°C. Densities of atrial and ventricular Na+ currents were similar (−47.6 ± 4.5 and −53.2 ± 5.1 pA/pF, respectively) and close to the typical values of teleost hearts. Two major K+ currents, the inward rectifier K+ current ( IK1), and the delayed rectifier K+ current ( IKr) were found under basal conditions in sturgeon cardiomyocytes. The atrial IKr (3.3 ± 0.2 pA/pF) was about twice as large as the ventricular IKr (1.3 ± 0.4 pA/pF) ( P < 0.05) conforming to the typical pattern of teleost cardiac IKr. Divergent from other fishes, the ventricular IK1 was remarkably small (−2.5 ± 0.07 pA/pF) and not different from that of the atrial myocytes (−1.9 ± 0.06 pA/pF) ( P > 0.05). Two ligand-gated K+ currents were also found: ACh-activated inward rectifier ( IKACh) was present only in atrial cells, while ATP-sensitive K+ current ( IKATP) was activated by a mitochondrial blocker, CCCP, in both atrial and ventricular cells. The most striking difference to other fishes appeared in Ca2+ currents ( ICa). In atrial myocytes, ICa was predominated by nickel-sensitive and nifedipine-resistant T-type ICa, while ventricular myocytes had mainly nifedipine-sensitive and nickel-resistant L-type ICa. ICaT/ ICaL ratio of the sturgeon atrial myocytes (2.42) is the highest value ever measured for a vertebrate species. In ventricular myocytes, ICaT/ ICaL ratio was 0.09. With the exception of the large atrial ICaT and small ventricular IK1, electrical excitability of A. baerii heart is similar to that of teleost hearts.

Ca2+ channel inhibitor NNC 55-0396 inhibits voltage-dependent K+ channels in rabbit coronary arterial smooth muscle cells

We demonstrated the inhibitory effect of NNC 55-0396, a T-type Ca(2+) channel inhibitor, on voltage-dependent K(+) (K(V)) channels in freshly isolated rabbit coronary arterial smooth muscle cells. NNC 55-0396 decreased the amplitude of K(V) currents in a concentration-dependent manner, with an IC(50) of 0.080 μM and a Hill coefficient of 0.76.NNC 55-0396 did not affect steady-state activation and inactivation curves, indicating that the compound does not affect the voltage sensitivity of K(V) channel gating. Both the K(V) currents and the inhibitory effect of NNC 55-0396 on K(V) channels were not altered by depletion of extracellular Ca(2+) or intracellular ATP, suggesting that the inhibitory effect of NNC 55-0396 is independent of Ca(2+)-channel activity and phosphorylation-dependent signaling cascades. From these results, we concluded that NNC 55-0396 dosedependently inhibits K(V) currents, independently of Ca(2+)-channel activity and intracellular signaling cascades.

Lessons from the heart: mirroring electrophysiological characteristics during cardiac development to in vitro differentiation of stem cell derived cardiomyocytes

The ability of human pluripotent stem cells (hPSCs) to differentiate into any cell type of the three germ layers makes them a very promising cell source for multiple purposes, including regenerative medicine, drug discovery, and as a model to study disease mechanisms and progression. One of the first specialized cell types to be generated from hPSC was cardiomyocytes (CM), and differentiation protocols have evolved over the years and now allow for robust and large-scale production of hPSC-CM. Still, scientists are struggling to achieve the same, mainly ventricular, phenotype of the hPSC-CM in vitro as their adult counterpart in vivo. In vitro generated cardiomyocytes are generally described as fetal-like rather than adult. In this review, we compare the in vivo development of cardiomyocytes to the in vitro differentiation of hPSC into CM with focus on electrophysiology, structure and contractility. Furthermore, known epigenetic changes underlying the differences between adult human CM and CM differentiated from pluripotent stem cells are described. This should provide the reader with an extensive overview of the current status of human stem cell-derived cardiomyocyte phenotype and function. Additionally, the reader will gain insight into the underlying signaling pathways and mechanisms responsible for cardiomyocyte development.

Pharmacoresistant Cav 2·3 (E-type/R-type) voltage-gated calcium channels influence heart rate dynamics and may contribute to cardiac impulse conduction

Voltage-gated Ca(2+) channels regulate cardiac automaticity, rhythmicity and excitation-contraction coupling. Whereas L-type (Cav 1·2, Cav 1·3) and T-type (Cav 3·1, Cav 3·2) channels are widely accepted for their functional relevance in the heart, the role of Cav 2·3 Ca(2+) channels expressing R-type currents remains to be elucidated. We have investigated heart rate dynamics in control and Cav 2·3-deficient mice using implantable electrocardiogram radiotelemetry and pharmacological injection experiments. Autonomic block revealed that the intrinsic heart rate does not differ between both genotypes. Systemic administration of isoproterenol resulted in a significant reduction in interbeat interval in both genotypes. It remained unaffected after administering propranolol in Cav 2·3(-|-) mice. Heart rate from isolated hearts as well as atrioventricular conduction for both genotypes differed significantly. Additionally, we identified and analysed the developmental expression of two splice variants, i.e. Cav 2·3c and Cav 2·3e. Using patch clamp technology, R-type currents could be detected in isolated prenatal cardiomyocytes and be related to R-type Ca(2+) channels. Our results indicate that on the systemic level, the pharmacologically inducible heart rate range and heart rate reserve are impaired in Cav 2·3 (-|-) mice. In addition, experiments on Langendorff perfused hearts elucidate differences in basic properties between both genotypes. Thus, Cav 2·3 does not only contribute to the cardiac autonomous nervous system but also to intrinsic rhythm propagation.

Transcriptional regulation of T-type calcium channel CaV3.2: bi-directionality by early growth response 1 (Egr1) and repressor element 1 (RE-1) protein-silencing transcription factor (REST)

The pore-forming Ca(2+) channel subunit Ca(V)3.2 mediates a low voltage-activated (T-type) Ca(2+) current (I(CaT)) that contributes pivotally to neuronal and cardiac pacemaker activity. Despite the importance of tightly regulated Ca(V)3.2 levels, the mechanisms regulating its transcriptional dynamics are not well understood. Here, we have identified two key factors that up- and down-regulate the expression of the gene encoding Ca(V)3.2 (Cacna1h). First, we determined the promoter region and observed several stimulatory and inhibitory clusters. Furthermore, we found binding sites for the transcription factor early growth response 1 (Egr1/Zif268/Krox-24) to be highly overrepresented within the Ca(V)3.2 promoter region. mRNA expression analyses and dual-luciferase promoter assays revealed that the Ca(V)3.2 promoter was strongly activated by Egr1 overexpression in vitro and in vivo. Subsequent chromatin immunoprecipitation assays in NG108-15 cells and mouse hippocampi confirmed specific Egr1 binding to the Ca(V)3.2 promoter. Congruently, whole-cell I(CaT) values were significantly larger after Egr1 overexpression. Intriguingly, Egr1-induced activation of the Ca(V)3.2 promoter was effectively counteracted by the repressor element 1-silencing transcription factor (REST). Thus, Egr1 and REST can bi-directionally regulate Ca(V)3.2 promoter activity and mRNA expression and, hence, the size of I(CaT). This mechanism has critical implications for the regulation of neuronal and cardiac Ca(2+) homeostasis under physiological conditions and in episodic disorders such as arrhythmias and epilepsy.

Difference of sodium currents between pediatric and adult human atrial myocytes: evidence for developmental changes of sodium channels

Voltage-gated calcium currents and potassium currents were shown to undergo developmental changes in postnatal human and animal cardiomocytes. However, so far, there is no evidence whether sodium currents also presented the developmental changes in postnatal human atrial cells. The aim of this study was to observe age-related changes of sodium currents between pediatric and adult atrial myocytes. Human atrial myocytes were acutely isolated and the whole-cell patch clamp technique was used to record sodium currents isolated from pediatric and adult atrial cardiomocytes. The peak amplitude of sodium currents recorded in adult atrial cells was significantly larger than that in pediatric atrial myocytes. However, there was no significant difference of the activation voltage for peak sodium currents between two kinds of atrial myocytes. The time constants for the activation and inactivation of sodium currents were smaller in adult atria than pediatric atria. The further study revealed that the voltage-dependent inactivation of sodium currents were more slow in adult atrial cardiomyocytes than pediatric atrial cells. A significant difference was also observed in the recovery process of sodium channel from inactivation. In summary, a few significant differences were demonstrated in sodium currents characteristics between pediatric and adult atrial myocytes, which indicates that sodium currents in human atria also undergo developmental changes.

Splice-variant changes of the Ca(V)3.2 T-type calcium channel mediate voltage-dependent facilitation and associate with cardiac hypertrophy and development

Low voltage-activated T-type calcium (Ca) channels contribute to the normal development of the heart and are also implicated in pathophysiological states such as cardiac hypertrophy. Functionally distinct T-type Ca channel isoforms can be generated by alternative splicing from each of three different T-type genes (Ca(V)3.1, Ca(V)3.2,Ca(V)3.3), although it remains to be described whether specific splice variants are associated with developmental states and pathological conditions. We aimed to identify and functionally characterize Ca(V)3.2 T-type Ca channel alternatively spliced variants from newborn animals and to compare with adult normotensive and spontaneously hypertensive rats (SHR). DNA sequence analysis of full-length Ca(V)3.2 cDNA generated from newborn heart tissue identified ten major regions of alternative splicing, the more common variants of which were analyzed by quantitative real-time PCR (qRT-PCR) and also subject to functional examination by whole-cell patch clamp. The main findings are that: (1) cardiac Ca(V)3.2 T-type Ca channels are subject to considerable alternative splicing, (2) there is preferential expression of Ca(V)3.2(-25) splice variant channels in newborn rat heart with a developmental shift in adult heart that results in approximately equal levels of expression of both (+25) and (-25) exon variants, (3) in the adult stage of hypertensive rats there is a both an increase in overall Ca(V)3.2 expression and a shift towards expression of Ca(V)3.2(+25) containing channels as the predominant form, and (4) alternative splicing confers a variant-specific voltage-dependent facilitation of Ca(V)3.2 channels. We conclude that Ca(V)3.2 alternative splicing generates significant T-type Ca channel structural and functional diversity with potential implications relevant to cardiac developmental and pathophysiological states.

Contribution of mineralocorticoid and glucocorticoid receptors to the chronotropic and hypertrophic actions of aldosterone in neonatal rat ventricular myocytes

Mineralocorticoids and glucocorticoids have been involved in the genesis of ventricular arrhythmias associated with pathological heart hypertrophy. We previously observed, using isolated neonate rat ventricular cardiomyocytes, that both aldosterone (Aldo) and corticosterone induced in vitro a marked acceleration of the spontaneous contractions of these cells, a phenomenon dependent on the expression of the low threshold T-type calcium channels. Because both mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) mediated the chronotropic response to corticosteroids, we characterized the role of each receptor using spironolactone and mifepristone (RU-486) as specific antagonists. We first observed that GR antagonism, but not MR antagonism, completely disrupted the significant correlation existing between the level of T channel mRNA and the beating frequency; this difference could not be explained by a specific regulation of channel expression or activity by one of the receptors. Moreover, the chronotropic action of Aldo was additive to that of forskolin, a direct activator of the cAMP pathway. This additive response was selectively abolished upon GR inhibition. Finally, myocyte hypertrophy induced in vitro by Aldo was completely prevented by GR antagonism, whereas spironolactone had only a marginal effect. These results suggest that, in isolated rat ventricular cardiomyocytes, the activation of both MR and GR is necessary for a complete electrical remodeling and a maximal chronotropic response to corticosteroids. However, GR alone appears involved in the sensitization of the cells to the chronotropic regulation through the cAMP pathway and in the hypertrophic response to steroids. These observations have therapeutic implications given the fact that MR becomes a major target of pharmacological drugs in the clinical practice for preventing cardiac function decompensation and evolution toward heart failure and lethal arrhythmias.

T-type calcium channels are regulated by hypoxia/reoxygenation in ventricular myocytes

Low-voltage-activated calcium channels are reexpressed in ventricular myocytes in pathological conditions associated with hypoxic episodes, but a direct relation between oxidative stress and T-type channel function and regulation in cardiomyocytes has not been established. We aimed to investigate low-voltage-activated channel regulation under oxidative stress in neonatal rat ventricular myocytes. RT-PCR measurements of voltage-gated Ca(2+) (Ca(v))3.1 and Ca(v)3.2 mRNA levels in oxidative stress were compared with whole cell patch-clamp recordings of T-type calcium current. The results indicate that hypoxia reduces T-type current density at -30 mV (the hallmark of this channel) based on the shift of the voltage dependence of activation to more depolarized values and downregulation of Ca(v)3.1 at the mRNA level. Upon reoxygenation, both Ca(v)3.1 mRNA levels and the voltage dependence of total T-type current are restored, although differently for activation and inactivation. Using Ni(2+), we distinguished different effects of hypoxia/reoxygenation on the two current components. Long-term incubation in the presence of 100 microM CoCl(2) reproduced the effects of hypoxia on T-type current activation and inactivation, indicating that the chemically induced oxidative state is sufficient to alter T-type calcium current activity, and that hypoxia-inducible factor-1alpha is involved in Ca(v)3.1 downregulation. Our results demonstrate that Ca(v)3.1 and Ca(v)3.2 T-type calcium channels are differentially regulated by hypoxia/reoxygenation injury, and, therefore, they may serve different functions in the myocyte in response to hypoxic injury.

Development of newer calcium channel antagonists: therapeutic potential of efonidipine in preventing electrical remodelling during atrial fibrillation

Calcium channel antagonists are most frequently prescribed for the treatment of hypertension and the majority specifically inhibit the L-type Ca2+ channel. In order to prevent reflex sympathetic over activity caused by L-type calcium channel antagonists (calcium channel blockers [CCBs]), increasing attention has focused on the blockade of the T-type Ca2+ channel. The T-type Ca2+ channel is found in the kidney and can also appear in the ventricle of the heart when in failure. Therefore, the T-type Ca2+ channel is a possible new target for the treatment of nephropathy and heart failure. In clinical trials, the efficacy and safety of T-type CCBs in hypertension and chronic renal disease have been reported. It is well known that the T-type Ca2+ channel is present in the adult atrium and plays a role in the cardiac pacemaker, but recent experimental studies suggest that this current also promotes electrical remodelling of the atrium. Using efonidipine, a dual L- and T-type CCB, it has been demonstrated that atrial electrical remodelling can be diminished in dogs. Furthermore, the T-type Ca2+ channel has recently been found in the pulmonary veins, contributing to the pulmonary vein pacemaker activity and triggered activity. A variety of drugs having T-type CCB effects have been shown to be effective in the management of atrial fibrillation, suggesting that this channel may be a novel therapeutic target.

Molecular and electrical remodeling of L- and T-type Ca(2+) channels in rat right atrium with monocrotaline-induced pulmonary hypertension

BACKGROUND

Atrial arrhythmia is often encountered in chronic pulmonary disease with pulmonary hypertension (PH), but few studies have investigated the electrical remodeling of atrial Ca(2+) channels under PH.

METHODS AND RESULTS

Wistar rats were injected with monocrotaline (MCT), resulting in PH with right atrial and ventricular hypertrophy. The L-type Ca(2+) channel current density was significantly decreased in right atrial cells of MCT-treated rats, accompanied by a significant reduction in mRNA expression of the CaV1.2 (alpha(1C)) subunit and accessory beta(2) subunit. Conversely, the low voltage-activated Ca(2+) current was more marked in the right atrial cells of MCT-treated rats than in those of control rats. The current-voltage relationship and the time course of inactivation closely resembled those of T-type Ca(2+) channels, although the current was only slightly inhibited by 10-100 micromol/L Ni(2+). No significant differences were observed in the mRNA expression levels of CaV3.1 (alpha(1G)) and CaV3.2 (alpha(1H)) or the protein level of the CaV3.1 subunit. In left atrial cells, the electrophysiological molecular properties of Ca(2+) channels were unaffected by MCT treatment.

CONCLUSIONS

PH causes right atrial hypertrophy, associated with alteration of the electrophysiological molecular properties of Ca(2+) channels.

Regulation of T-type Cav3.1 channels expression by synthetic glucocorticoid dexamethasone in neonatal cardiac myocytes

The effect of the dexamethasone (Dex) on the regulation of the T-type Ca(2+) channel expressions was investigated in primary cultures of neonatal rat ventricular myocytes. We found that Dex (1 microM) increases the T-type Ca(2+) current (I(CaT)) associated with an increase in Ca(v)3.1 mRNA amount. We isolated the upstream region from Ca(v)3.1 encoding gene and tested the activity of the promoter in transfected ventricular myocytes. We found a minimal Dex-responsive region that displayed putative glucocorticoid receptor (GR) and nuclear factor kappa-B (NFkappaB) targets. The GR selective antagonist, RU38486 (10 microM), nearly turned off the transcriptional activity of Ca(v)3.1 encoding gene, and an NFkappaB inhibitor, pyrrolodine dithiocarbonate (10 microM), completely abolished the Dex-induced mRNA increase. However, Dex-induced GR and NFkappaB synthesis and nuclear translocation were not timely related to Ca(v)3.1 mRNA increase. These results indicate that both GR and NFkappaB were necessary, but not sufficient, to trigger the increase in Ca(v)3.1 mRNA amount. This study showed the relationship between glucocorticoid and T-type channels up-regulation that may be involved in cardiac development and pathology.

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.

Mathematical model of the neonatal mouse ventricular action potential

Therapies for heart disease are based largely on our understanding of the adult myocardium. The dramatic differences in action potential (AP) shape between neonatal and adult cardiac myocytes, however, indicate that a different set of molecular interactions in neonatal myocytes necessitates different treatment for newborns. Computational modeling is useful for synthesizing data to determine how interactions between components lead to systems-level behavior, but this technique has not been used extensively to study neonatal heart cell function. We created a mathematical model of the neonatal (day 1) mouse myocyte by modifying, on the basis of experimental data, the densities and/or formulations of ion transport mechanisms in an adult cell model. The new model reproduces the characteristic AP shape of neonatal cells, with a brief plateau phase and longer duration than the adult (action potential duration at 80% repolarization = 60.1 vs. 12.6 ms). The simulation results are consistent with experimental data, including 1) decreased density and altered inactivation of transient outward K+ currents, 2) increased delayed rectifier K+ currents, 3) Ca2+ entry through T-type as well as L-type Ca2+ channels, 4) increased Ca2+ influx through Na+/Ca2+ exchange, and 5) Ca2+ transients resulting from transmembrane Ca2+ entry rather than release from the sarcoplasmic reticulum (SR). Simulations performed with the model generated novel predictions, including increased SR Ca2+ leak and elevated intracellular Na+ concentration in neonatal compared with adult myocytes. This new model can therefore be used for testing hypotheses and obtaining a better quantitative understanding of differences between neonatal and adult physiology.

Making the T-type even tinier: corticotropin-releasing factor-mediated inhibition of low-voltage-activated calcium channel activity

T-type calcium channels are important for a variety of physiological processes such as brain and heart function. Proper regulation of these channels by second messengers is fundamental; however, our knowledge of the molecular pathways that regulate T-type calcium channels is limited in comparison with other voltage-dependent calcium channels. In this issue of Molecular Pharmacology, Tao et al. (p. 1596) demonstrate that Cav3.2 is regulated by activation of the corticotropin-releasing factor receptor 1 in a Gbetagamma-dependent manner.

Subunit-specific modulation of T-type calcium channels by zinc

Zinc (Zn2+) functions as a signalling molecule in the nervous system and modulates many ionic channels. In this study, we have explored the effects of Zn2+ on recombinant T-type calcium channels (CaV3.1, CaV3.2 and CaV3.3). Using tsA-201 cells, we demonstrate that CaV3.2 current (IC50, 0.8 microm) is significantly more sensitive to Zn2+ than are CaV3.1 and CaV3.3 currents (IC50, 80 microm and approximately 160 microm, respectively). This inhibition of CaV3 currents is associated with a shift to more negative membrane potentials of both steady-state inactivation for CaV3.1, CaV3.2 and CaV3.3 and steady-state activation for CaV3.1 and CaV3.3 currents. We also document changes in kinetics, especially a significant slowing of the inactivation kinetics for CaV3.1 and CaV3.3, but not for CaV3.2 currents. Notably, deactivation kinetics are significantly slowed for CaV3.3 current (approximately 100-fold), but not for CaV3.1 and CaV3.2 currents. Consequently, application of Zn2+ results in a significant increase in CaV3.3 current in action potential clamp experiments, while CaV3.1 and CaV3.2 currents are significantly reduced. In neuroblastoma NG 108-15 cells, the duration of CaV3.3-mediated action potentials is increased upon Zn2+ application, indicating further that Zn2+ behaves as a CaV3.3 channel opener. These results demonstrate that Zn2+ exhibits differential modulatory effects on T-type calcium channels, which may partly explain the complex features of Zn2+ modulation of the neuronal excitability in normal and disease states.

Long-term and immediate effect of testosterone on single T-type calcium channel in neonatal rat cardiomyocytes

In the cardiovascular system, T-type calcium channels play an important role for the intracellular calcium homeostasis and spontaneous pacemaker activity and are involved in the progression of structural heart diseases. Androgens influence the cardiovascular physiology and pathophysiology. However, their effect on native T-type calcium currents (I(Ca,T)) remains unclear. To test the chronic effect of testosterone on the cardiac I(Ca,T), cultured neonatal rat ventricular cardiomyocytes were treated with testosterone (1 nM-10 microM) for 24-30 h. Current measurements were performed after testosterone washout to exclude any acute testosterone effects. Testosterone (100 nm) pretreatment significantly increased whole-cell I(Ca,T) density from 1.26 +/- 0.48 pA/pF (n = 8) to 5.06 +/- 1.75 pA/pF (n = 7; P < 0.05) and accelerated beating rate. This was attributed to both increased expression levels of the pore-forming subunits Ca(v)3.1 and Ca(v)3.2 and increased T-type single-channel activity. On single-channel level, the increase of the ensemble average current by testosterone vs. time-matched controls was due to an increased availability (58.1 +/- 4.2 vs. 21.5 +/- 4.0%, P < 0.01) and open probability (2.78 +/- 0.29 vs. 0.85 +/- 0.23%, P < 0.01). Cotreatment with the selective testosterone receptor antagonist flutamide (10 mum) prevented these chronic testosterone-induced effects. Conversely, acute application of testosterone (10 microM) decreased T-type single-channel activity in testosterone pretreated cells by reducing the open probability (0.78 +/- 0.13 vs. 2.91 +/- 0.38%, P < 0.01), availability (23.6 +/- 3.3 vs. 57.6 +/- 4.5%, P < 0.01), and peak current (-20 +/- 4 vs. -58 +/- 4 fA, P < 0.01). Flutamide (10 microM) did not abolish the testosterone-induced acute block of T-type calcium channels. Our results indicate that long-term testosterone treatment increases, whereas acute testosterone decreases neonatal rat T-type calcium currents. These effects seem to be mediated by a genomic chronic stimulation and a nongenomic acute inhibitory action.

T-type channels-secretion coupling: evidence for a fast low-threshold exocytosis

T-type channels are transient low-voltage-activated (LVA) Ca(2+) channels that control Ca(2+) entry in excitable cells during small depolarizations around resting potential. Studies in the past 20 years focused on the biophysical, physiological, and molecular characterization of T-type channels in most tissues. This led to a well-defined picture of the functional role of LVA channels in controlling low-threshold spikes, oscillatory cell activity, muscle contraction, hormone release, cell growth and differentiation. So far, little attention has been devoted to the role of T-type channels in transmitter release, which mainly involves channel types belonging to the high-voltage-activated (HVA) Ca(2+) channel family. However, evidence is accumulating in favor of a unique participation of T-type channels in fast transmitter release. Clear data are now reported in reciprocal synapses of the retina and olfactory bulb, synaptic contacts between primary afferent and second order nociceptive neurons, rhythmic inhibitory interneurons of invertebrates and clonal cell lines transfected with recombinant alpha(1) channel subunits. T-type channels also regulate the large dense-core vesicle release of neuroendocrine cells where Ca(2+) dependence, rate of vesicle release, and size of readily releasable pool appear comparable to those associated to HVA channels. This suggests that when sufficiently expressed and properly located near the release zones, T-type channels can trigger fast low-threshold secretion. In this study, we will review the main findings that assign a specific task to T-type channels in fast exocytosis, discussing their possible involvement in the control of the Ca(2+)-dependent processes regulating exocytosis like vesicle depletion and vesicle recycling.

Low-threshold exocytosis induced by cAMP-recruited CaV3.2 (alpha1H) channels in rat chromaffin cells

We have studied the functional role of CaV3 channels in triggering fast exocytosis in rat chromaffin cells (RCCs). CaV3 T-type channels were selectively recruited by chronic exposures to cAMP (3 days) via an exchange protein directly activated by cAMP (Epac)-mediated pathway. Here we show that cAMP-treated cells had increased secretory responses, which could be evoked even at very low depolarizations (-50, -40 mV). Potentiation of exocytosis in cAMP-treated cells did not occur in the presence of 50 microM Ni2+, which selectively blocks T-type currents in RCCs. This suggests that the "low-threshold exocytosis" induced by cAMP is due to increased Ca2+ influx through cAMP-recruited T-type channels, rather than to an enhanced secretion downstream of Ca2+ entry, as previously reported for short-term cAMP treatments (20 min). Newly recruited T-type channels increase the fast secretory response at low voltages without altering the size of the immediately releasable pool. They also preserve the Ca2+ dependence of exocytosis, the initial speed of vesicle depletion, and the mean quantal size of single secretory events. All this indicates that cAMP-recruited CaV3 channels enhance the secretory activity of RCCs at low voltages by coupling to the secretory apparatus with a Ca2+ efficacy similar to that of already existing high-threshold Ca2+ channels. Finally, using RT-PCRs we found that the fast inactivating low-threshold Ca2+ current component recruited by cAMP is selectively associated to the alpha1H (CaV3.2) channel isoform.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

The ablation of the Cav2.3/E-type voltage-gated Ca2+ channel causes a mild phenotype despite an altered glucose induced glucagon response in isolated islets of Langerhans

Glucagon release upon hypoglycemia is an important homeostatic mechanism utilized by vertebrates to restore blood glucose to normal. Glucagon secretion itself is triggered by Ca2+ influx through voltage-gated ion channels, and the gene inactivation of R-type Ca2+ channels, with Ca(v)2.3 as the ion conducting subunit, has been shown to disturb glucose homeostasis. To understand how glucagon release may be affected in Ca(v)2.3-deficient mice, carbachol, insulin and glucose induced glucagon response was investigated. While the rise of insulin and glucose induced by carbachol is normal, mutant mice show an impaired glucagon-response. Further, the effect of insulin injection on glucagon levels was altered by the loss of the Ca(v)2.3 subunit. Ca(v)2.3-deficient mice are characterized by an impaired glucose suppression of glucagon release. This was most obvious at the level of isolated islets suggesting that Ca(v)2.3 containing R-type voltage-gated Ca2+ channels are involved in the glucose-mediated signalling to glucagon release in mice.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Pathophysiological Significance of T-type Ca2+ Channels: Transcriptional Regulation of T-type Ca2+ Channel — Regulation of CACNA1H by Neuron-Restrictive Silencer Factor

Expression of T-type Ca(2+) current in the ventricle varies during development and in cardiac diseases. The alteration in quantity of two isoforms of T-type Ca(2+) channel genes in the heart, CACNA1G and CACNA1H, contributes to the changes of T-type Ca(2+) channel activity. However, the precise mechanisms governing the transcription of T-type Ca(2+) channel genes remain largely unknown. In this review, we briefly describe our recent finding that a transcriptional repressor named neuron-restrictive silencer factor is a potent regulator of T-type Ca(2+) channel gene expression.

Pathophysiological Significance of T-type Ca2+ Channels: T-type Ca2+ Channels and Drug Development

T-type Ca(2+) channels are present in cardiovascular, neuronal, and endocrine systems; and they are now receiving attention as novel therapeutic targets. Many drugs and compounds non-specificaly block T-type Ca(2+) channels. Certain dihydropyridine compounds, such as efonidipine, have blocking activity on both L-type and T-type Ca(2+) channels which possibly underlies their excellent clinical profiles such as minimum reflex tachycardia and renal protection. Selective inhibitors of T-type Ca(2+) channels, such as non-hydrolyzable mibefradil and R(-)-efonidipine, are powerful pharmacological tools for further studies and may lead to the development of novel therapeutic strategies.

Ablation of Cav2.3 / E?type voltage?gated calcium channel results in cardiac arrhythmia and altered autonomic control within the murine cardiovascular system

Voltage-gated calcium channels are key components in cardiac electrophysiology. We demonstrate that Ca(v)2.3 is expressed in mouse and human heart and that mice lacking the Ca(v)2.3 voltage-gated calcium channel exhibit severe alterations in cardiac function. Amplified cDNA fragments from murine heart and single cardiomyocytes reveal the expression of three different Ca(v)2.3 splice variants. The ablation of Ca(v)2.3 was found to be accompanied by a compensatory upregulation of the Ca(v)3.1 T-type calcium channel, while other voltage-gated calcium channels remained unaffected. Telemetric ECG recordings from Ca(v)2.3 deficient mice displayed subsidiary escape rhythm, altered atrial activation patterns, atrioventricular conduction disturbances and alteration in QRS-morphology. Furthermore, time domain analysis of heart rate variability (HRV) in Ca(v)2.3(-/-) mice exhibited a significant increase in heart rate as well as in the coefficient of variance (CV) compared to control mice. Administration of atropin/propranolol revealed that increased heart rate was due to enhanced sympathetic tonus and that partial decrease of CV in Ca(v)2.3(-/-) mice after autonomic block was in accordance with a complete abolishment of 2(nd) degree atrioventricular block. However, escape rhythms, atrial activation disturbances and QRS-dysmorphology remained unaffected, indicating that these are intrinsic cardiac features in Ca(v)2.3(-/-) mice. We conclude that the expression of Ca(v)2.3 is essential for normal impulse generation and conduction in murine heart.

Disruption of growth hormone secretion alters Ca2+ current density and expression of Ca2+ channel and insulin-like growth factor genes in rat atria

The influence of the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis on expression of low-voltage-activated (LVA) Ca2+ current in atrial tissue was investigated using spontaneous dwarf (SpDwf) rats, a mutant strain that lacks GH. Atrial myocytes from SpDwf rats express LVA and high-voltage-activated (HVA) Ca2+ currents and the Ca2+ channel alpha1-subunit genes CaV1.2, CaV2.3, CaV3.1, and CaV3.2. LVA current density decreases significantly beginning at, or shortly after, birth in normal animals; however, its density is maintained in SpDwf rats at 1 pA/pF for > or =12 wk after birth. The abundance of mRNAs encoding CaV2.3 and CaV3.2 declines with advancing age in normal atrial development, yet expression of CaV2.3 mRNA remains significantly elevated in older SpDwf animals. Quantitation of local transcript levels for mRNAs encoding IGF-I and IGF-I receptor (IGF-IR) also reveals significant differences in expression of these transcripts in atrial tissue of SpDwf animals compared with controls. In SpDwf rats, the abundance of IGF-IR mRNA remains elevated at many postnatal ages, whereas mRNA encoding IGF-I is maintained only in older animals. Physiological concentrations of IGF-I cause two- to threefold increases in LVA current density in primary cultures of atrial myocytes, and this effect is blocked by an antisense oligonucleotide targeting the IGF-IR. Thus disruption of GH production in SpDwf animals alters expression of atrial LVA Ca2+ channel and IGF genes as well as postnatal regulation of LVA Ca2+ current density, most likely acting through compensatory mechanisms via the local IGF-IR.

Cav3.2 subunit underlies the functional T-type Ca2+ channel in murine hearts during the embryonic period

T-type Ca2+ channels are implicated in cardiac automaticity, cell growth, and cardiovascular remodeling. Two voltage-gated Ca2+ subtypes (Ca(v)3.1 and Ca(v)3.2) have been cloned for the pore-forming alpha(1)-subunit of the T-type Ca2+ channel in cardiac muscle, but their differential roles remain to be clarified. The aim of this study was to elucidate the relative contribution of the two subtypes in the normal development of mouse hearts. A whole cell patch clamp was used to record ionic currents from ventricular myocytes isolated from mice of early (E9.5) and late embryonic days (E18) and from adult 10-wk-old mice. Large T-type Ca2+ current (I(Ca,T)) was observed at both E9.5 and E18, displaying similar voltage-dependence and kinetics of activation and inactivation. The current was inhibited by Ni2+ at relatively low concentrations (IC(50) 26-31 microM). I(Ca,T) was undetectable in adult myocytes. Quantitative PCR analysis revealed that Ca(v)3.2 mRNA is the predominant subtype encoding T-type Ca2+ channels at both E9.5 and E18. Ca(v)3.1 mRNA increased from E9.5 to E18, but remained low compared with Ca(v)3.2 mRNA during the whole embryonic period. In the adulthood, in contrast, Ca(v)3.1 mRNA is greater than Ca(v)3.2 mRNA. These results indicate that Ca(v)3.2 underlies the functional T-type Ca2+ channels in the embryonic murine heart, and there is a subtype switching of transcripts from Ca(v)3.2 to Ca(v)3.1 in the perinatal period.

Induced automaticity in isolated rat atrial cells by incorporation of a stretch-activated conductance

Stretch of the atrium and sympathetic activity have been implicated as substrates for atrial fibrillation. We investigate how a model of stretch in combination with sympathetic stimulation can induce automaticity in atrial cells. We adapted our coupling clamp circuit so that a model ionic current that represents stretch-activated channels (SACs) was injected into an isolated rat atrial cell in real time. This current was calculated as ISAC= GSAC (Vm-ESAC), where GSAC and ESAC are the conductance and reversal potential of SACs and Vm is the cell's membrane potential. Repetitive automaticity was induced by a sufficiently large GSAC and this critical value of GSAC was decreased by exposure to isoproterenol. The critical value of GSAC decreased from 0.63+/-0.05 nS (mean+/-SE) in control to 0.40+/-0.07 nS in isoproterenol (P<0.05). Additionally, after exposure to isoproterenol, automaticity continued after GSAC was no longer applied and was accompanied by delayed after-depolarizations. In three cells, repetitive automaticity could not be induced at any value of GSAC. Exposure to 10 nM isoproterenol converted these cells to cells with repetitive automaticity in response to GSAC. We conclude that automaticity can be induced in isolated rat atrial cells by application of a model of SACs. Exposure to isoproterenol enhances this effect.

Angiotensin II Signaling Pathways Mediate Expression of Cardiac T-Type Calcium Channels

Recent studies indicate that cardiac T-type Ca 2+ current ( I CaT ) reappears in hypertrophied ventricular cells. The aim of this study was to investigate the role of angiotensin II (Ang II), a major inducer of cardiac hypertrophy, in the reexpression of T-type channel in left ventricular hypertrophied myocytes. We induced cardiac hypertrophy in rats by abdominal aorta stenosis for 12 weeks and thereafter animals were treated for 2 weeks with losartan (12 mg/kg per day), an antagonist of type 1 Ang II receptors (AT 1 ). In hypertrophied myocytes, we showed that the reexpressed I CaT is generated by the Ca V 3.1 and Ca V 3.2 subunits. After losartan treatment, I CaT density decreased from 0.40±0.05 pA/pF (n=26) to 0.20±0.03 pA/pF (n=27, P <0.01), affecting Ca V 3.1- and Ca V 3.2-related currents. The amount of Ca V 3.1 mRNA increased during hypertrophy and retrieved its nonhypertrophic level after losartan treatment, whereas the amount of Ca V 3.2 mRNA was unaffected by stenosis. In cultured newborn ventricular cells, chronic Ang II application (0.1 μmol/L) also increased I CaT density and Ca V 3.1 mRNA amount. UO126, a mitogen-activated protein kinase kinase-1/2 (MEK1/2) inhibitor, reduced Ang II–increased I CaT density and Ca V 3.1 mRNA amount. Bosentan, an endothelin (ET) receptor antagonist, reduced Ang II–increased I CaT density without affecting the amount of Ca V 3.1 mRNA. Finally, cotreatment with bosentan and UO126 abolished the Ang II–increased I CaT density. Our results show that AT 1 -activated MEK pathway and autocrine ET-activated independent MEK pathway upregulate T-type channel expression. Ang II–increased of I CaT density observed in hypertrophied myocytes may play a role in the pathogenesis of Ca 2+ overload and arrhythmias seen in cardiac pathology.

Characterization and regulation of T-type Ca2+ channels in embryonic stem cell-derived cardiomyocytes

T-type Ca2+ channels may play a role in cardiac development. We studied the developmental regulation of the T-type currents (ICa,T) in cardiomyocytes (CMs) derived from mouse embryonic stem cells (ESCs). ICa,T was studied in isolated CMs by whole cell patch clamp. Subsequently, CMs were identified by the myosin light chain 2v-driven green fluorescent protein expression, and laser capture microdissection was used to isolate total RNA from groups of cells at various developmental time points. ICa,T showed characteristics of Cav3.1, such as resistance to Ni2+ block, and a transient increase during development, correlating with measures of spontaneous electrical activity. Real-time RT-PCR showed that Cav3.1 mRNA abundance correlated (r2 = 0.81) with ICa,T. The mRNA copy number was low at 7+4 days (2 copies/cell), increased significantly by 7+10 days (27/cell; P < 0.01), peaked at 7+16 days (174/cell), and declined significantly at 7+27 days (25/cell). These data suggest that ICa,T is developmentally regulated at the level of mRNA abundance and that this regulation parallels measures of pacemaker activity, suggesting that ICa,T might play a role in the spontaneous contractions during CM development.

Developmental changes of intracellular Ca2+ transients in beating rat hearts

Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca(2+)-induced Ca(2+) release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca(2+) that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca(2+) entry and the Ca(2+) released from the SR to the Ca(2+) transient during the first 3 wk of postnatal development. To this end, intracellular Ca(2+) transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca(2+) flux to the global intracellular Ca(2+) transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca(2+) release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca(2+) current, Ni(2+) for the current resulting from the reverse-mode Na(+)/Ca(2+) exchange, and mibefradil for the T-type Ca(2+) current. Our results showed that the relative contribution of each Ca(2+) flux changes considerably during the first 3 wk of postnatal development. Early after birth (1-5 days), the sarcolemmal Ca(2+) flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.