Ion-dependent inactivation of barium current through L-type calcium channels

Investigating postsynaptic effects of a Drosophila neuropeptide on muscle contraction

The Drosophila neuropeptide, DPKQDFMRFamide, was previously shown to enhance excitatory junctional potentials (EJPs) and muscle contraction by both presynaptic and postsynaptic actions. Since the peptide acts on both sides of the synaptic cleft, it has been difficult to examine postsynaptic modulatory mechanisms, particularly when contractions are elicited by nerve stimulation. Here, postsynaptic actions are examined in 3rd instar larvae by applying peptide and the excitatory neurotransmitter, l-glutamate, in the bathing solution to elicit contractions after silencing motor output by removing the central nervous system (CNS). DPKQDFMRFamide enhanced glutamate-evoked contractions at low concentrations (EC50 1.3 nM), consistent with its role as a neurohormone, and the combined effect of both substances was supra-additive. Glutamate-evoked contractions were also enhanced when transmitter release was blocked in temperature-sensitive (Shibire) mutants, confirming the peptide's postsynaptic action. The peptide increased membrane depolarization in muscle when co-applied with glutamate, and its effects were blocked by nifedipine, an L-type channel blocker, indicating effects at the plasma membrane involving calcium influx. DPKQDFMRFamide also enhanced contractions induced by caffeine in the absence of extracellular calcium, suggesting increased calcium release from the sarcoplasmic reticulum (SR) or effects downstream of calcium release from the SR. The peptide's effects do not appear to involve calcium/calmodulin-dependent protein kinase II (CaMKII), previously shown to mediate presynaptic effects. The approach used here might be useful for examining postsynaptic effects of neurohormones and cotransmitters in other systems.NEW & NOTEWORTHY Distinguishing presynaptic and postsynaptic effects of neurohormones is a long-standing challenge in many model organisms. Here, postsynaptic actions of DPKQDFMRFamide are demonstrated by assessing its ability to potentiate contractions elicited by direct application of the neurotransmitter, glutamate, when axons are silent and when transmitter release is blocked. The peptide acts at multiple sites to increase contraction, increasing glutamate-induced depolarization at the cell membrane, acting on L-type channels, and acting downstream of calcium release from the sarcoplasmic reticulum.

Nutrigenomics of inward rectifier potassium channels

Inwardly rectifying potassium (Kir) channels play a key role in maintaining the resting membrane potential and supporting potassium homeostasis. There are many variants of Kir channels, which are usually tetramers in which the main subunit has two trans-membrane helices attached to two N- and C-terminal cytoplasmic tails with a pore-forming loop in between that contains the selectivity filter. These channels have domains that are strongly modulated by molecules present in nutrients found in different diets, such as phosphoinositols, polyamines and Mg²⁺. These molecules can impact these channels directly or indirectly, either allosterically by modulation of enzymes or via the regulation of channel expression. A particular type of these channels is coupled to cell metabolism and inhibited by ATP (KATP channels, essential for insulin release and for the pathogenesis of metabolic diseases like diabetes mellitus). Genomic changes in Kir channels have a significant impact on metabolism, such as conditioning the nutrients and electrolytes that an individual can take. Thus, the nutrigenomics of ion channels is an important emerging field in which we are attempting to understand how nutrients and diets can affect the activity and expression of ion channels and how genomic changes in such channels may be the basis for pathological conditions that limit nutrition and electrolyte intake. In this contribution we briefly review Kir channels, discuss their nutrigenomics, characterize how different components in the diet affect their function and expression, and suggest how their genomic changes lead to pathological phenotypes that affect diet and electrolyte intake.

Direct Effects of Toxic Divalent Cations on Contractile Proteins with Implications for the Heart: Unraveling Mechanisms of Dysfunction

The binding of calcium and magnesium ions to proteins is crucial for regulating heart contraction. However, other divalent cations, including xenobiotics, can accumulate in the myocardium and enter cardiomyocytes, where they can bind to proteins. In this article, we summarized the impact of these cations on myosin ATPase activity and EF-hand proteins, with special attention given to toxic cations. Optimal binding to EF-hand proteins occurs at an ionic radius close to that of Mg²⁺ and Ca²⁺. In skeletal Troponin C, Cd²⁺, Sr²⁺, Pb²⁺, Mn²⁺, Co²⁺, Ni²⁺, Ba²⁺, Mg²⁺, Zn²⁺, and trivalent lanthanides can substitute for Ca²⁺. As myosin ATPase is not a specific MgATPase, Ca²⁺, Fe²⁺, Mn²⁺, Ni²⁺, and Sr²⁺ could support myosin ATPase activity. On the other hand, Zn²⁺ and Cu² significantly inhibit ATPase activity. The affinity to various divalent cations depends on certain proteins or their isoforms and can alter with amino acid substitution and post-translational modification. Cardiac EF-hand proteins and the myosin ATP-binding pocket are potential molecular targets for toxic cations, which could significantly alter the mechanical characteristics of the heart muscle at the molecular level.

Novel CACNA1C R511Q mutation, located in domain Ⅰ-Ⅱ linker, causes non-syndromic type-8 long QT syndrome

Background

Gain-of-function mutations in CACNA1C encoding Cav1.2 cause syndromic or non-syndromic type-8 long QT syndrome (LQTS) (sLQT8 or nsLQT8). The cytoplasmic domain (D)Ⅰ-Ⅱ linker in Cav1.2 plays a pivotal role in calcium channel inactivation, and mutations in this site have been associated with sLQT8 (such as Timothy syndrome) but not nsLQT8.

Objective

Since we identified a novel CACNA1C mutation, located in the DⅠ-Ⅱ linker, associated with nsLQTS, we sought to reveal its biophysical defects.

Methods

Target panel sequencing was employed in 24 genotype-negative nsLQTS probands (after Sanger sequencing) and three family members. Wild-type (WT) or R511Q Cav1.2 was transiently expressed in tsA201 cells, then whole-cell Ca²⁺ or Ba²⁺ currents (I_Ca or I_Ba) were recorded using whole-cell patch-clamp techniques.

Results

We identified two CACNA1C mutations, a previously reported R858H mutation and a novel R511Q mutation located in the DⅠ-Ⅱ linker. Four members of one nsLQTS family harbored the CACNA1C R511Q mutation. The current density and steady-state activation were comparable to those of WT-I_Ca. However, persistent currents in R511Q-I_Ca were significantly larger than those of WT-I_Ca (WT at +20 mV: 3.3±0.3%, R511Q: 10.8±0.8%, P<0.01). The steady-state inactivation of R511Q-I_Ca was weak in comparison to that of WT-I_Ca at higher prepulse potentials, resulting in increased window currents in R511Q-I_Ca. Slow component of inactivation of R511Q-I_Ca was significantly delayed compared to that of WT-I_Ca (WT-tau at +20 mV: 81.3±3.3 ms, R511Q-tau: 125.1±5.0 ms, P<0.01). Inactivation of R511Q-I_Ba was still slower than that of WT-I_Ba, indicating that voltage-dependent inactivation (VDI) of R511Q-I_Ca was predominantly delayed.

Conclusions

Delayed VDI, increased persistent currents, and increased window currents of R511Q-I_Ca cause nsLQT8. Our data provide novel insights into the structure-function relationships of Cav1.2 and the pathophysiological roles of the DⅠ-Ⅱ linker in phenotypic manifestations.

Hypocalcemia-Induced QT Interval Prolongation

An 87-year-old man with a history of transcatheter aortic valve replacement, pulmonary hypertension, diastolic dysfunction with preserved systolic function, and myelofibrosis had a 12-lead ECG showed a prolonged QT interval of 508 ms with heart-rate correction placing it in the 99th percentile of the population. Reduction in the dose of furosemide and calcium supplementation increased serum calcium and shortened the QT interval. This case provides an opportunity to examine newer concepts for the understanding of the mechanisms by which hypocalcemia might induce QT prolongation. Hypocalcemia likely produces corrected QT interval prolongation primarily through a calcium-dependent inactivation (CDI) mechanism on the L-type calcium channel (LTCC). Lower extracellular calcium leads to a decreased ICaL, subsequently causing intracellular calcium to take longer to reach the critical threshold to induce CDI of the LTCC. The resulting prolonged repolarization of the ventricular myocyte can lead to early after-depolarizations and ensuing life-threatening ventricular arrhythmias. Genetic polymorphisms in Ca2+-binding protein calmodulin which can prolong QT, underscore the role for disturbances of intracellular myocardial calcium handling in arrhythmogenesis. Hypocalcemia is an under-recognized cause of QT prolongation and should be taken into careful consideration in patients presenting with incidental findings of a prolonged QT interval.

Which proteinase-activated receptor-1 antagonist is better?: Evaluation of vorapaxar and parmodulin-2 effects on human left internal mammary artery endothelial function

OBJECTIVE

Endothelial dysfunction occurs as an early event in cardiovascular disease. Previously, vorapaxar, a proteinase-activated receptor-1 antagonist, was shown to cause endothelial damage in a cell culture study. Therefore, our study aimed to compare the effects of vorapaxar and parmodulin-2, proteinase-activated receptor-1 biased agonist, on human left internal mammary artery endothelial function in vitro.

METHOD

Isolated arteries were hung in the organ baths. Acetylcholine responses (10^-11-10^-6 M) were obtained in endothelium-intact tissues the following incubation with vorapaxar/parmodulin-2 (10^-6 M) to determine the effects of these molecules on the endothelium-dependent relaxation. Subsequently, endothelium-dependent relaxation responses of tissues were investigated in the presence of L-NAME (10^-4 M), L-arginine (10^-5 M), indomethacin (10^-5 M), and charybdotoxin-apamin (10^-7 M) in addition to vorapaxar/parmodulin-2 incubation. Besides, the effect of these molecules on endothelium-independent relaxation response was evaluated with sodium nitroprusside (10^-11-10^-6 M). Finally, the sections of human arteries were imaged using a transmission electron microscope, and the integrity of the endothelial layer was evaluated.

RESULTS

We found that vorapaxar caused significant endothelial dysfunction by disrupting nitric oxide and endothelium-derived hyperpolarizing factor-dependent relaxation mechanisms. Parmodulin-2 did not cause endothelial damage. Neither vorapaxar nor parmodulin-2 disrupted endothelium-independent relaxation responses. The effect of vorapaxar on the endothelial layer was supported by the transmission electron microscope images.

CONCLUSION

Parmodulin-2 may be a better option than vorapaxar in treating cardiovascular diseases since it can inhibit PAR-1 without caused endothelial dysfunction.

Functional consequences of lead and mercury exposomes in the heart

Lead and mercury are heavy metals that are highly toxic to life forms. There are no known physiological processes that require them, and they do not have a particular threshold concentration to produce biologic damage. They are non-biodegradable, and they slowly accumulate in the environment in a dynamic equilibrium between air, water, soil, food, and living organisms. Their accumulation in the environment has been increasing over time, because they were not banned from use in anthropogenic industrial production. In their +2 cationic state they are powerful oxidizing agents with the ability to interfere significantly with processes that require specific divalent cations. Acute or chronic exposure to lead and mercury can produce multisystemic damage, especially in the developing nervous systems of children and fetuses, resulting in variety of neurological consequences. They can also affect the cardiovascular system and especially the heart, either directly through their action on cardiomyocytes or indirectly through their effects on innervation, humoral responses or blood vessel alterations. For example, heart function modified by these heavy metals are heart rate, contraction, excitability, and rhythm. Some cardiac molecular targets have been identified and characterized. The direct mechanisms of damage of these heavy metals on heart function are discussed. We conclude that exposome to these heavy metals, should be considered as a major relevant risk factor for cardiac diseases.

Wenxin Keli for the Treatment of Arrhythmia-Systems Pharmacology and In Vivo Pharmacological Assessment

This study employed a systems pharmacology approach to identify the active compounds and action mechanisms of Wenxin Keli for arrhythmia treatment. Sixty-eight components identified in vivo and in vitro by UPLC/Q-TOF-MS were considered the potential active components of Wenxin Keli. Network pharmacology further revealed 33 key targets and 75 KEGG pathways as possible pathways and targets involved in WK-mediated treatment, with the CaMKII/CNCA1C/Ca²⁺ pathway being the most significantly affected. This finding was validated using an AC-induced rat arrhythmias model. Pretreatment with Wenxin Keli reduced the malignant arrhythmias and shortened RR, PR, and the QT interval. Wenxin Keli exerted some antiarrhythmic effects by inhibiting p-CaMKII and intracellular Ca²⁺ transients and overexpressing CNCA1C. Thus, suppressing SR Ca²⁺ release and maintaining intracellular Ca²⁺ balance may be the primary mechanism of Wenxin Keli against arrhythmia. In view of the significance of CaMKII and NCX identified in this experiment, we suggest that CaMKII and NCX are essential targets for treating arrhythmias.

Cardiac potassium inward rectifier Kir2: Review of structure, regulation, pharmacology, and arrhythmogenesis

Potassium inward rectifier channel Kir2 is an important component of terminal cardiac repolarization and resting membrane stability. This functionality is part of balanced cardiac excitability and is a defining feature of excitable cardiac membranes. “Gain-of-function” or “loss-of-function” mutations in KCNJ2, the gene encoding Kir2.1, cause genetic sudden cardiac death syndromes, and loss of the Kir2 current I_K1 is a major contributing factor to arrhythmogenesis in failing human hearts. Here we provide a contemporary review of the functional structure, physiology, and pharmacology of Kir2 channels. Beyond the structure and functional relationships, we will focus on the elements of clinically used drugs that block the channel and the implications for treatment of atrial fibrillation with I_K1-blocking agents. We will also review the clinical disease entities associated with KCNJ2 mutations and the growing area of research into associated arrhythmia mechanisms. Lastly, the presence of Kir2 channels has become a tipping point for electrical maturity in induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) and highlights the significance of understanding why Kir2 in iPS-CMs is important to consider for Comprehensive In Vitro Proarrhythmia Assay and drug safety testing.

Advances in L-Type Calcium Channel Structures, Functions and Molecular Modeling

L-type Calcium Channels (LTCCs), also termed as Ca_v1, belong to voltage-gated calcium channels (VGCCs/Ca_vs), which play a critical role in a wide spectrum of physiological processes, including neurotransmission, cell cycle, muscular contraction, cardiac action potential and gene expression. Aberrant regulation of calcium channels is involved in neurological, cardiovascular, muscular and psychiatric disorders. Accordingly, LTCCs have been regarded as important drug targets, and a number of LTCC drugs are in clinical use. In this review, the recent development of structures and biological functions of LTCCs are introduced. Moreover, the representative modulators and ligand binding sites of LTCCs are discussed. Finally, molecular modeling and Computer-aided Drug Design (CADD) methods for understanding structure-function relations of LTCCs are summarized.

Utilising Automated Electrophysiological Platforms in Epilepsy Research

Genetic mutations have long been implicated in epilepsy, particularly in genes that encode ion channels and neurotransmitter receptors. Among some of those identified are voltage-gated sodium, potassium and calcium channels, and ligand-gated gamma-aminobutyric acid (GABA), neuronal nicotinic acetylcholine (CHRN), and glutamate receptors, making them key therapeutic targets. In this chapter we discuss the use of automated electrophysiological technologies to examine the impact of gene defects in two potassium channels associated with different epilepsy syndromes. The hKCNC1 gene encodes the voltage-gated potassium channel hK_V3.1, and mutations in this gene cause progressive myoclonus epilepsy (PME) and ataxia due to a potassium channel mutation (MEAK). The hKCNT1 gene encodes the weakly voltage-dependent sodium-activated potassium channel hKCNT1, and mutations in this gene cause a wide spectrum of seizure disorders, including severe autosomal dominant sleep-related hypermotor epilepsy (ADSHE) and epilepsy of infancy with migrating focal seizures (EIMFS), both conditions associated with drug-resistance. Importantly, both of these potassium channels play vital roles in regulating neuronal excitability. Since its discovery in the late nineteen seventies, the patch-clamp technique has been regarded as the bench-mark technology for exploring ion channel characteristics. In more recent times, innovations in automated patch-clamp technologies, of which there are many, are enabling the study of ion channels with much greater productivity that manual systems are capable of. Here we describe aspects of Nanion NPC-16 Patchliner, examining the effects of temperature on stably and transiently transfected mammalian cells, the latter of which for most automated systems on the market is quite challenging. Remarkable breakthroughs in the development of other automated electrophysiological technologies, such as multielectrode arrays that support extracellular signal recordings, provide additional features to examine network activity in the area of ion channel research, particularly epilepsy. Both of these automated technologies enable the acquisition of consistent, robust, and reproducible data. Numerous systems have been developed with very similar capabilities, however, not all the systems on the market are adapted to work with primary cells, particularly neurons that can be problematic. This chapter also showcases methods that demonstrate the versatility of Nanion NPC-16 Patchliner and the Multi Channel Systems (MCS) multielectrode array (MEA) assay for acutely dissociated murine primary cortical neurons, enabling the study of potassium channel mutations implicated in severe refractory epilepsies.

Automated Planar Patch-Clamp Recording of P2X Receptors

P2X receptors are a structurally and functionally distinctive family of ligand-gated ion channels that play important roles in mediating extracellular adenosine 5'-triphosphate (ATP) signaling in diverse physiological and pathophysiological processes. For several decades, the "manual" patch-clamp technique was regarded as the gold standard assay for investigating ion channel properties. More recently, breakthroughs in the development of automated patch-clamp technologies are enabling the study of ion channels, with much greater throughput capacities. These automated platforms, of which there are many, generate consistent, reliable, high-fidelity data. This chapter demonstrates the versatility of one of these technologies for ligand-gated ion channels, with a particular emphasis on protocols that address some of the issues of receptor desensitization that are commonly associated with P2X receptor-mediated currents.

How Ca2+ influx is attenuated in the heart during a "fight or flight" response

Bazmi and Escobar highlight a recent investigation of the mechanisms that regulate Ca²⁺ influx during sympathetic stimulation.

Lead poisoning: acute exposure of the heart to lead ions promotes changes in cardiac function and Cav1.2 ion channels

Lead ions (Pb²⁺) possess characteristics similar to Ca²⁺. Because of this and its redox capabilities, lead causes different toxic effects. The neurotoxic effects have been well documented; however, the toxic effects on cardiac tissues remain allusive. We utilized isolated guinea pig hearts and measured the effects of Pb²⁺ on their contractility and excitability. Acute exposure to extracellular Pb²⁺ had a negative inotropic effect and increased diastolic tension. The speed of contraction and relaxation were affected, though the effects were more dramatic on the speed of contraction. Excitability was also altered. Heart beat frequency increased and later diminished after lead ion exposure. Pro-arrhytmic events, such as early after-depolarization and a reduction of the action potential plateau, were also observed. In isolated cardiomyocytes and tsA 201 cells, extracellular lead blocked currents through Cav1.2 channels, diminished their activation, and enhanced their fast inactivation, negatively affecting their gating currents. Thus, Pb²⁺ was cardiotoxic and reduced cardiac contractility, making the heart prone to arrhythmias. This was due, in part, to Pb²⁺ effects on the Cav1.2 channels; however, other channels, transporters or pathways may also be involved. Acute cardiotoxic effects were observed at Pb²⁺ concentrations achievable during acute lead poisoning. The results suggest how Cav1.2 gating can be affected by divalent cations, such as Pb², and also suggest a more thorough evaluation of heart function in individuals affected by lead poisoning.

Mechanisms of the cyclic nucleotide cross-talk signaling network in cardiac L-type calcium channel regulation

Regulation of L-type Calcium (Ca2+) Channel (LCC) gating is critical to shaping the cardiac action potential (AP) and triggering the initiation of excitation-contraction (EC) coupling in cardiac myocytes. The cyclic nucleotide (cN) cross-talk signaling network, which encompasses the β-adrenergic and the Nitric Oxide (NO)/cGMP/Protein Kinase G (PKG) pathways and their interaction (cross-talk) through distinctively-regulated phosphodiesterase isoenzymes (PDEs), regulates LCC current via Protein Kinase A- (PKA) and PKG-mediated phosphorylation. Due to the tightly-coupled and intertwined biochemical reactions involved, it remains to be clarified how LCC gating is regulated by the signaling network from receptor to end target. In addition, the large number of EC coupling-related phosphorylation targets of PKA and PKG makes it difficult to quantify and isolate changes in L-type Ca2+ current (ICaL) responses regulated by the signaling network. We have developed a multi-scale, biophysically-detailed computational model of LCC regulation by the cN signaling network that is supported by experimental data. LCCs are modeled with functionally distinct PKA- and PKG-phosphorylation dependent gating modes. The model exhibits experimentally observed single channel characteristics, as well as whole-cell LCC currents upon activation of the cross-talk signaling network. Simulations show 1) redistribution of LCC gating modes explains changes in whole-cell current under various stimulation scenarios of the cN cross-talk network; 2) NO regulation occurs via potentiation of a gating mode characterized by prolonged closed times; and 3) due to compensatory actions of cross-talk and antagonizing functions of PKA- and PKG-mediated phosphorylation of LCCs, the effects of individual inhibitions of PDEs 2, 3, and 4 on ICaL are most pronounced at low levels of β-adrenergic stimulation. Simulations also delineate the contribution of the following two mechanisms to overall LCC regulation, which have otherwise been challenging to distinguish: 1) regulation of PKA and PKG activation via cN cross-talk (Mechanism 1); and 2) LCC interaction with activated PKA and PKG (Mechanism 2). These results provide insights into how cN signals transduced via the cN cross-talk signaling network are integrated via LCC regulation in the heart.

The Rotavirus NSP4 Viroporin Domain is a Calcium-conducting Ion Channel

Viroporins are small virus-encoded ion channel proteins. Most viroporins are monovalent selective cation channels, with few showing the ability to conduct divalent cations, like calcium (Ca2+). Nevertheless, some viroporins are known to disrupt host cell Ca2+ homeostasis, which is critical for virus replication and pathogenesis. Rotavirus nonstructural protein 4 (NSP4) is an endoplasmic reticulum transmembrane glycoprotein that has a viroporin domain (VPD), and NSP4 viroporin activity elevates cytosolic Ca2+ in mammalian cells. The goal of this study was to demonstrate that the NSP4 VPD forms an ion channel and determine whether the channel can conduct Ca2+. Using planar lipid bilayer and liposome patch clamp electrophysiology, we show that a synthetic peptide of the NSP4 VPD has ion channel activity. The NSP4 VPD was selective for cations over anions and channel activity was observed to have both well-defined "square top" openings as well as fast current fluctuations, similar to other viroporins. Importantly, the NSP4 VPD showed similar conductance of divalent cations (Ca2+ and Ba2+) as monovalent cations (K+), but a viroporin defective mutant lacked Ca2+ conductivity. These data demonstrate that the NSP4 VPD is a Ca2+-conducting viroporin and establish the mechanism by which NSP4 disturbs host cell Ca2+ homeostasis.

Potassium channels in the Cx43 gap junction perinexus modulate ephaptic coupling: an experimental and modeling study

It was recently demonstrated that cardiac sodium channels (Nav1.5) localized at the perinexus, an intercalated disc (ID) nanodomain associated with gap junctions (GJ), may contribute to electrical coupling between cardiac myocytes via an ephaptic mechanism. Impairment of ephaptic coupling by acute interstitial edema (AIE)-induced swelling of the perinexus was associated with arrhythmogenic, anisotropic conduction slowing. Given that Kir2.1 has also recently been reported to localize at intercalated discs, we hypothesized that Kir2.1 channels may reside within the perinexus and that inhibiting them may mitigate arrhythmogenic conduction slowing observed during AIE. Using gated stimulated emission depletion (gSTED) and stochastic optical reconstruction microscopy (STORM) super-resolution microscopy, we indeed find that a significant proportion of Kir2.1 channels resides within the perinexus. Moreover, whereas Nav1.5 inhibition during AIE exacerbated arrhythmogenic conduction slowing, inhibiting Kir2.1 channels during AIE preferentially increased transverse conduction velocity-decreasing anisotropy and ameliorating arrhythmia risk compared to AIE alone. Comparison of our results with a nanodomain computer model identified enrichment of both Nav1.5 and Kir2.1 at intercalated discs as key factors underlying the experimental observations. We demonstrate that Kir2.1 channels are localized within the perinexus alongside Nav1.5 channels. Further, targeting Kir2.1 modulates intercellular coupling between cardiac myocytes, anisotropy of conduction, and arrhythmia propensity in a manner consistent with a role for ephaptic coupling in cardiac conduction. For over half a century, electrical excitation in the heart has been thought to occur exclusively via gap junction-mediated ionic current flow between cells. Further, excitation was thought to depend almost exclusively on sodium channels with potassium channels being involved mainly in returning the cell to rest. Here, we demonstrate that sodium and potassium channels co-reside within nanoscale domains at cell-to-cell contact sites. Experimental and computer modeling results suggest a role for these channels in electrical coupling between cardiac muscle cells via an ephaptic mechanism working in tandem with gap junctions. This new insight into the mechanism of cardiac electrical excitation could pave the way for novel therapies against cardiac rhythm disturbances.

A human ventricular myocyte model with a refined representation of excitation-contraction coupling

Cardiac Ca(2+)-induced Ca(2+) release (CICR) occurs by a regenerative activation of ryanodine receptors (RyRs) within each Ca(2+)-releasing unit, triggered by the activation of L-type Ca(2+) channels (LCCs). CICR is then terminated, most probably by depletion of Ca(2+) in the junctional sarcoplasmic reticulum (SR). Hinch et al. previously developed a tightly coupled LCC-RyR mathematical model, known as the Hinch model, that enables simulations to deal with a variety of functional states of whole-cell populations of a Ca(2+)-releasing unit using a personal computer. In this study, we developed a membrane excitation-contraction model of the human ventricular myocyte, which we call the human ventricular cell (HuVEC) model. This model is a hybrid of the most recent HuVEC models and the Hinch model. We modified the Hinch model to reproduce the regenerative activation and termination of CICR. In particular, we removed the inactivated RyR state and separated the single step of RyR activation by LCCs into triggering and regenerative steps. More importantly, we included the experimental measurement of a transient rise in Ca(2+) concentrations ([Ca(2+)], 10-15 μM) during CICR in the vicinity of Ca(2+)-releasing sites, and thereby calculated the effects of the local Ca(2+) gradient on CICR as well as membrane excitation. This HuVEC model successfully reconstructed both membrane excitation and key properties of CICR. The time course of CICR evoked by an action potential was accounted for by autonomous changes in an instantaneous equilibrium open probability of couplons. This autonomous time course was driven by a core feedback loop including the pivotal local [Ca(2+)], influenced by a time-dependent decay in the SR Ca(2+) content during CICR.

Impaired calcium-calmodulin-dependent inactivation of Cav1.2 contributes to loss of sarcoplasmic reticulum calcium release refractoriness in mice lacking calsequestrin 2

AIMS

In cardiac muscle, Ca(2+) release from sarcoplasmic reticulum (SR) is reduced with successively shorter coupling intervals of premature stimuli, a phenomenon known as SR Ca(2+) release refractoriness. We recently reported that the SR luminal Ca(2+) binding protein calsequestrin 2 (Casq2) contributes to release refractoriness in intact mouse hearts, but the underlying mechanisms remain unclear. Here, we further investigate the mechanisms responsible for physiological release refractoriness.

METHODS AND RESULTS

Gene-targeted ablation of Casq2 (Casq2 KO) abolished SR Ca(2+) release refractoriness in isolated mouse ventricular myocytes. Surprisingly, impaired Ca(2+)-dependent inactivation of L-type Ca(2+) current (ICa), which is responsible for triggering SR Ca(2+) release, significantly contributed to loss of Ca(2+) release refractoriness in Casq2 KO myocytes. Recovery from Ca(2+)-dependent inactivation of ICa was significantly accelerated in Casq2 KO compared to wild-type (WT) myocytes. In contrast, voltage-dependent inactivation measured by using Ba(2+) as charge carrier was not significantly different between WT and Casq2 KO myocytes. Ca(2+)-dependent inactivation of ICa was normalized by intracellular dialysis of excess apo-CaM (20 μM), which also partially restored physiological Ca(2+) release refractoriness in Casq2 KO myocytes.

CONCLUSIONS

Our findings reveal that the intra-SR protein Casq2 is largely responsible for the phenomenon of SR Ca(2+) release refractoriness in murine ventricular myocytes. We also report a novel mechanism of impaired Ca(2+)-CaM-dependent inactivation of Cav1.2, which contributes to the loss of SR Ca(2+) release refractoriness in the Casq2 KO mouse model and, therefore, may further increase risk for ventricular arrhythmia in vivo.

Gain-of-function mutations in the calcium channel CACNA1C (Cav1.2) cause non-syndromic long-QT but not Timothy syndrome

Gain-of-function mutations in CACNA1C, encoding the L-type Ca(2+) channel Cav1.2, cause Timothy syndrome (TS), a multi-systemic disorder with dysmorphic features, long-QT syndrome (LQTS) and autism spectrum disorders. TS patients have heterozygous mutations (G402S and G406R) located in the alternatively spliced exon 8, causing a gain-of-function by reduced voltage-dependence of inactivation. Screening 540 unrelated patients with non-syndromic forms of LQTS, we identified six functional relevant CACNA1C mutations in different regions of the channel. All these mutations caused a gain-of-function combining different mechanisms, including changes in current amplitude, rate of inactivation and voltage-dependence of activation or inactivation, similar as in TS. Computer simulations support the theory that the novel CACNA1C mutations prolong action potential duration. We conclude that genotype-negative LQTS patients should be investigated for mutations in CACNA1C, as a gain-of-function in Cav1.2 is likely to cause LQTS and only specific and rare mutations, i.e. in exon 8, cause the multi-systemic TS.

Characterization of Cav1.4 complexes (α11.4, β2, and α2δ4) in HEK293T cells and in the retina

In photoreceptor synaptic terminals, voltage-gated Cav1.4 channels mediate Ca(2+) signals required for transmission of visual stimuli. Like other high voltage-activated Cav channels, Cav1.4 channels are composed of a main pore-forming Cav1.4 α1 subunit and auxiliary β and α2δ subunits. Of the four distinct classes of β and α2δ, β2 and α2δ4 are thought to co-assemble with Cav1.4 α1 subunits in photoreceptors. However, an understanding of the functional properties of this combination of Cav subunits is lacking. Here, we provide evidence that Cav1.4 α1, β2, and α2δ4 contribute to Cav1.4 channel complexes in the retina and describe their properties in electrophysiological recordings. In addition, we identified a variant of β2, named here β2X13, which, along with β2a, is present in photoreceptor terminals. Cav1.4 α1, β2, and α2δ4 were coimmunoprecipitated from lysates of transfected HEK293 cells and mouse retina and were found to interact in the outer plexiform layer of the retina containing the photoreceptor synaptic terminals, by proximity ligation assays. In whole-cell patch clamp recordings of transfected HEK293T cells, channels (Cav1.4 α1 + β2X13) containing α2δ4 exhibited weaker voltage-dependent activation than those with α2δ1. Moreover, compared with channels (Cav1.4 α1 + α2δ4) with β2a, β2X13-containing channels exhibited greater voltage-dependent inactivation. The latter effect was specific to Cav1.4 because it was not seen for Cav1.2 channels. Our results provide the first detailed functional analysis of the Cav1.4 subunits that form native photoreceptor Cav1.4 channels and indicate potential heterogeneity in these channels conferred by β2a and β2X13 variants.

Ca(2+) current facilitation is CaMKII-dependent and has arrhythmogenic consequences

The cardiac voltage gated Ca²⁺ current (I_Ca) is critical to the electrophysiological properties, excitation-contraction coupling, mitochondrial energetics, and transcriptional regulation in heart. Thus, it is not surprising that cardiac I_Ca is regulated by numerous pathways. This review will focus on changes in I_Ca that occur during the cardiac action potential (AP), with particular attention to Ca²⁺-dependent inactivation (CDI), Ca²⁺-dependent facilitation (CDF) and how calmodulin (CaM) and Ca²⁺-CaM dependent protein kinase (CaMKII) participate in the regulation of Ca²⁺ current during the cardiac AP. CDI depends on CaM pre-bound to the C-terminal of the L-type Ca²⁺ channel, such that Ca²⁺ influx and Ca²⁺ released from the sarcoplasmic reticulum bind to that CaM and cause CDI. In cardiac myocytes CDI normally pre-dominates over voltage-dependent inactivation. The decrease in I_Ca via CDI provides direct negative feedback on the overall Ca²⁺ influx during a single beat, when myocyte Ca²⁺ loading is high. CDF builds up over several beats, depends on CaMKII-dependent Ca²⁺ channel phosphorylation, and results in a staircase of increasing I_Ca peak, with progressively slower inactivation. CDF and CDI co-exist and in combination may fine-tune the I_Ca waveform during the cardiac AP. CDF may partially compensate for the tendency for Ca²⁺ channel availability to decrease at higher heart rates because of accumulating inactivation. CDF may also allow some reactivation of I_Ca during long duration cardiac APs, and contribute to early afterdepolarizations, a form of triggered arrhythmias.

L-type CaV1.2 calcium channels: from in vitro findings to in vivo function

The L-type Cav1.2 calcium channel is present throughout the animal kingdom and is essential for some aspects of CNS function, cardiac and smooth muscle contractility, neuroendocrine regulation, and multiple other processes. The L-type CaV1.2 channel is built by up to four subunits; all subunits exist in various splice variants that potentially affect the biophysical and biological functions of the channel. Many of the CaV1.2 channel properties have been analyzed in heterologous expression systems including regulation of the L-type CaV1.2 channel by Ca(2+) itself and protein kinases. However, targeted mutations of the calcium channel genes confirmed only some of these in vitro findings. Substitution of the respective serines by alanine showed that β-adrenergic upregulation of the cardiac CaV1.2 channel did not depend on the phosphorylation of the in vitro specified amino acids. Moreover, well-established in vitro phosphorylation sites of the CaVβ2 subunit of the cardiac L-type CaV1.2 channel were found to be irrelevant for the in vivo regulation of the channel. However, the molecular basis of some kinetic properties, such as Ca(2+)-dependent inactivation and facilitation, has been approved by in vivo mutagenesis of the CaV1.2α1 gene. This article summarizes recent findings on the in vivo relevance of well-established in vitro results.

Automated planar patch-clamp

Ion channels are integral membrane proteins that regulate the flow of ions across the plasma membrane and the membranes of intracellular organelles of both excitable and non-excitable cells. Ion channels are vital to a wide variety of biological processes and are prominent components of the nervous system and cardiovascular system, as well as controlling many metabolic functions. Furthermore, ion channels are known to be involved in many disease states and as such have become popular therapeutic targets. For many years now manual patch-clamping has been regarded as one of the best approaches for assaying ion channel function, through direct measurement of ion flow across these membrane proteins. Over the last decade there have been many remarkable breakthroughs in the development of technologies enabling the study of ion channels. One of these breakthroughs is the development of automated planar patch-clamp technology. Automated platforms have demonstrated the ability to generate high-quality data with high throughput capabilities, at great efficiency and reliability. Additional features such as simultaneous intracellular and extracellular perfusion of the cell membrane, current clamp operation, fast compound application, an increasing rate of parallelization, and more recently temperature control have been introduced. Furthermore, in addition to the well-established studies of over-expressed ion channel proteins in cell lines, new generations of planar patch-clamp systems have enabled successful studies of native and primary mammalian cells. This technology is becoming increasingly popular and extensively used both within areas of drug discovery as well as academic research. Many platforms have been developed including NPC-16 Patchliner(®) and SyncroPatch(®) 96 (Nanion Technologies GmbH, Munich), CytoPatch™ (Cytocentrics AG, Rostock), PatchXpress(®) 7000A, IonWorks(®) Quattro and IonWorks Barracuda™, (Molecular Devices, LLC); Dynaflow(®) HT (Cellectricon AB, Mölndal), QPatch HT (Sophion A/S, Copenhagen), IonFlux HT (Fluxion Bioscience Inc, USA), which have demonstrated the capability to generate recordings similar in quality to that of conventional patch clamping. Here we describe features of Nanion's NPC-16 Patchliner(®) and processes and protocols suited for this particularly flexible and successful high-throughput automated platform, which is based on planar patch-clamp technology. However, many of the protocols and notes given in this chapter can be applied to other automated patch-clamp platforms, similarly.

Coordinated regulation of murine cardiomyocyte contractility by nanomolar (-)-epigallocatechin-3-gallate, the major green tea catechin

Green tea polyphenolic catechins exhibit biological activity in a wide variety of cell types. Although reports in the lay and scientific literature suggest therapeutic potential for improving cardiovascular health, the underlying molecular mechanisms of action remain unclear. Previous studies have implicated a wide range of molecular targets in cardiac muscle for the major green tea catechin, (-)-epigallocatechin-3-gallate (EGCG), but effects were observed only at micromolar concentrations of unclear clinical relevance. Here, we report that nanomolar concentrations of EGCG significantly enhance contractility of intact murine myocytes by increasing electrically evoked Ca(2+) transients, sarcoplasmic reticulum (SR) Ca(2+) content, and ryanodine receptor type 2 (RyR2) channel open probability. Voltage-clamp experiments demonstrate that 10 nM EGCG significantly inhibits the Na(+)-Ca(2+) exchanger. Of importance, other Na(+) and Ca(2+) handling proteins such as Ca(2+)-ATPase, Na(+)-H(+) exchanger, and Na(+)-K(+)-ATPase were not affected by EGCG ≤ 1 μM. Thus, nanomolar EGCG increases contractility in intact myocytes by coordinately modulating SR Ca(2+) loading, RyR2-mediated Ca(2+) release, and Na(+)-Ca(2+) exchange. Inhibition of Na(+)-K(+)-ATPase activity probably contributes to the positive inotropic effects observed at EGCG concentrations >1 μM. These newly recognized actions of nanomolar and micromolar EGCG should be considered when the therapeutic and toxicological potential of green tea supplementation is evaluated and may provide a novel therapeutic strategy for improving contractile function in heart failure.

Theoretical study of L-type Ca(2+) current inactivation kinetics during action potential repolarization and early afterdepolarizations

Sarcoplasmic reticulum (SR) Ca(2+) release mediates excitation–contraction coupling (ECC) in cardiac myocytes. It is triggered upon membrane depolarization by entry of Ca(2+) via L-type Ca(2+) channels (LTCCs), which undergo both voltage- and Ca(2+)-dependent inactivation (VDI and CDI, respectively). We developed improved models of L-type Ca(2+) current and SR Ca(2+) release within the framework of the Shannon-Bers rabbit ventricular action potential (AP) model. The formulation of SR Ca(2+) release was modified to reproduce high ECC gain at negative membrane voltages. An existing LTCC model was extended to reflect more faithfully contributions of CDI and VDI to total inactivation. Ba(2+) current inactivation included an ion-dependent component (albeit small compared with CDI), in addition to pure VDI. Under physiological conditions (during an AP) LTCC inactivates predominantly via CDI, which is controlled mostly by SR Ca(2+) release during the initial AP phase, but by Ca(2+) through LTCCs for the remaining part. Simulations of decreased CDI or K(+) channel block predicted the occurrence of early and delayed after depolarizations. Our model accurately describes ECC and allows dissection of the relative contributions of different Ca(2+) sources to total CDI, and the relative roles of CDI and VDI, during normal and abnormal repolarization.

Influence of pH on Ca²⁺ current and its control of electrical and Ca²⁺ signaling in ventricular myocytes

Modulation of L-type Ca(2+) current (I(Ca,L)) by H(+) ions in cardiac myocytes is controversial, with widely discrepant responses reported. The pH sensitivity of I(Ca,L) was investigated (whole cell voltage clamp) while measuring intracellular Ca(2+) (Ca(2+)(i)) or pH(i) (epifluorescence microscopy) in rabbit and guinea pig ventricular myocytes. Selectively reducing extracellular or intracellular pH (pH(o) 6.5 and pH(i) 6.7) had opposite effects on I(Ca,L) gating, shifting the steady-state activation and inactivation curves to the right and left, respectively, along the voltage axis. At low pH(o), this decreased I(Ca,L), whereas at low pH(i), it increased I(Ca,L) at clamp potentials negative to 0 mV, although the current decreased at more positive potentials. When Ca(2+)(i) was buffered with BAPTA, the stimulatory effect of low pH(i) was even more marked, with essentially no inhibition. We conclude that extracellular H(+) ions inhibit whereas intracellular H(+) ions can stimulate I(Ca,L). Low pH(i) and pH(o) effects on I(Ca,L) were additive, tending to cancel when appropriately combined. They persisted after inhibition of calmodulin kinase II (with KN-93). Effects are consistent with H(+) ion screening of fixed negative charge at the sarcolemma, with additional channel block by H(+)(o) and Ca(2+)(i). Action potential duration (APD) was also strongly H(+) sensitive, being shortened by low pH(o), but lengthened by low pH(i), caused mainly by H(+)-induced changes in late Ca(2+) entry through the L-type Ca(2+) channel. Kinetic analyses of pH-sensitive channel gating, when combined with whole cell modeling, successfully predicted the APD changes, plus many of the accompanying changes in Ca(2+) signaling. We conclude that the pH(i)-versus-pH(o) control of I(Ca,L) will exert a major influence on electrical and Ca(2+)-dependent signaling during acid-base disturbances in the heart.

Simulation of the undiseased human cardiac ventricular action potential: model formulation and experimental validation

Cellular electrophysiology experiments, important for understanding cardiac arrhythmia mechanisms, are usually performed with channels expressed in non myocytes, or with non-human myocytes. Differences between cell types and species affect results. Thus, an accurate model for the undiseased human ventricular action potential (AP) which reproduces a broad range of physiological behaviors is needed. Such a model requires extensive experimental data, but essential elements have been unavailable. Here, we develop a human ventricular AP model using new undiseased human ventricular data: Ca(2+) versus voltage dependent inactivation of L-type Ca(2+) current (I(CaL)); kinetics for the transient outward, rapid delayed rectifier (I(Kr)), Na(+)/Ca(2+) exchange (I(NaCa)), and inward rectifier currents; AP recordings at all physiological cycle lengths; and rate dependence and restitution of AP duration (APD) with and without a variety of specific channel blockers. Simulated APs reproduced the experimental AP morphology, APD rate dependence, and restitution. Using undiseased human mRNA and protein data, models for different transmural cell types were developed. Experiments for rate dependence of Ca(2+) (including peak and decay) and intracellular sodium ([Na(+)](i)) in undiseased human myocytes were quantitatively reproduced by the model. Early afterdepolarizations were induced by I(Kr) block during slow pacing, and AP and Ca(2+) alternans appeared at rates >200 bpm, as observed in the nonfailing human ventricle. Ca(2+)/calmodulin-dependent protein kinase II (CaMK) modulated rate dependence of Ca(2+) cycling. I(NaCa) linked Ca(2+) alternation to AP alternans. CaMK suppression or SERCA upregulation eliminated alternans. Steady state APD rate dependence was caused primarily by changes in [Na(+)](i), via its modulation of the electrogenic Na(+)/K(+) ATPase current. At fast pacing rates, late Na(+) current and I(CaL) were also contributors. APD shortening during restitution was primarily dependent on reduced late Na(+) and I(CaL) currents due to inactivation at short diastolic intervals, with additional contribution from elevated I(Kr) due to incomplete deactivation.

Minor contribution of cytosolic Ca2+ transients to the pacemaker rhythm in guinea pig sinoatrial node cells

The question of the extent to which cytosolic Ca(2+) affects sinoatrial node pacemaker activity has been discussed for decades. We examined this issue by analyzing two mathematical pacemaker models, based on the "Ca(2+) clock" (C) and "membrane clock" (M) hypotheses, together with patch-clamp experiments in isolated guinea pig sinoatrial node cells. By applying lead potential analysis to the models, the C mechanism, which is dependent on potentiation of Na(+)/Ca(2+) exchange current via spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) during diastole, was found to overlap M mechanisms in the C model. Rapid suppression of pacemaker rhythm was observed in the C model by chelating intracellular Ca(2+), whereas the M model was unaffected. Experimental rupturing of the perforated-patch membrane to allow rapid equilibration of the cytosol with 10 mM BAPTA pipette solution, however, failed to decrease the rate of spontaneous action potential within ∼30 s, whereas contraction ceased within ∼3 s. The spontaneous rhythm also remained intact within a few minutes when SR Ca(2+) dynamics were acutely disrupted using high doses of SR blockers. These experimental results suggested that rapid disruption of normal Ca(2+) dynamics would not markedly affect spontaneous activity. Experimental prolongation of the action potentials, as well as slowing of the Ca(2+)-mediated inactivation of the L-type Ca(2+) currents induced by BAPTA, were well explained by assuming Ca(2+) chelation, even in the proximity of the channel pore in addition to the bulk cytosol in the M model. Taken together, the experimental and model findings strongly suggest that the C mechanism explicitly described by the C model can hardly be applied to guinea pig sinoatrial node cells. The possible involvement of L-type Ca(2+) current rundown induced secondarily through inhibition of Ca(2+)/calmodulin kinase II and/or Ca(2+)-stimulated adenylyl cyclase was discussed as underlying the disruption of spontaneous activity after prolonged intracellular Ca(2+) concentration reduction for >5 min.

Interplay of voltage and Ca-dependent inactivation of L-type Ca current

Inactivation of L-type Ca channels (LTCC) is regulated by both Ca and voltage-dependent processes (CDI and VDI). To differentiate VDI and CDI, several experimental and theoretical studies have considered the inactivation of Ba current through LTCC (I(Ba)) as a measure of VDI. However, there is evidence that Ba can weakly mimic Ca, such that I(Ba) inactivation is still a mixture of CDI and VDI. To avoid this complication, some have used the monovalent cation current through LTCC (I(NS)), which can be measured when divalent cation concentrations are very low. Notably, I(NS) inactivation rate does not depend on current amplitude, and hence may reflect purely VDI. However, based on analysis of existent and new data, and modeling, we find that I(NS) can inactivate more rapidly and completely than I(Ba), especially at physiological temperature. Thus VDI that occurs during I(Ba) (or I(Ca)) must differ intrinsically from VDI during I(NS). To account for this, we have extended a previously published LTCC mathematical model of VDI and CDI into an excitation-contraction coupling model, and assessed whether and how experimental I(Ba) inactivation results (traditionally used in VDI experiments and models) could be recapitulated by modifying CDI to account for Ba-dependent inactivation. Thus, the view of a slow and incomplete I(NS) inactivation should be revised, and I(NS) inactivation is a poor measure of VDI during I(Ca) or I(Ba). This complicates VDI analysis experimentally, but raises intriguing new questions about how the molecular mechanisms of VDI differ for divalent and monovalent currents through LTCCs.

Ca(2+)-dependent components of inactivation of unitary cardiac L-type Ca(2+) channels

A Ca(2+) ion-dependent inactivation (CDI) of L-type Ca(2+) channels (LCC) is vital in limiting and shaping local Ca(2+) ion signalling in a variety of excitable cell types. However, under physiological conditions the unitary LCC properties that underlie macroscopic inactivation are unclear. Towards this end, we have probed the gating kinetics of individual cardiac LCCs recorded with a physiological Ca(2+) ion concentration (2 mM) permeating the channel, and in the absence of channel agonists. Upon depolarization the ensemble-averaged LCC current decayed with a fast and a slow exponential component. We analysed the unitary behaviour responsible for this biphasic decay by means of a novel kinetic dissection of LCC gating parameters. We found that inactivation was caused by a rapid decrease in the frequency of LCC reopening, and a slower decline in mean open time of the LCC. In contrast, with barium ions permeating the channel ensemble-averaged currents displayed only a single, slow exponential decay and little time dependence of the LCC open time. Our results demonstrate that the fast and slow phases of macroscopic inactivation reflect the distinct time courses for the decline in the frequency of LCC reopening and the open dwell time, both of which are modulated by Ca(2+) influx. Analysis of the evolution of CDI in individual LCC episodes was employed to examine the stochastic nature of the underlying molecular switch, and revealed that influx on the order of a thousand Ca(2+) ions may be sufficient to trigger CDI. This is the first study to characterize both the unitary kinetics and the stoichiometry of CDI of LCCs with a physiological Ca(2+) concentration. These novel findings may provide a basis for understanding the mechanisms regulating unitary LCC gating, which is a pivotal element in the local control of Ca(2+)-dependent signalling processes.

Cooperative regulation of Ca(v)1.2 channels by intracellular Mg(2+), the proximal C-terminal EF-hand, and the distal C-terminal domain

L-type Ca(2+) currents conducted by Ca(v)1.2 channels initiate excitation-contraction coupling in cardiac myocytes. Intracellular Mg(2+) (Mg(i)) inhibits the ionic current of Ca(v)1.2 channels. Because Mg(i) is altered in ischemia and heart failure, its regulation of Ca(v)1.2 channels is important in understanding cardiac pathophysiology. Here, we studied the effects of Mg(i) on voltage-dependent inactivation (VDI) of Ca(v)1.2 channels using Na(+) as permeant ion to eliminate the effects of permeant divalent cations that engage the Ca(2+)-dependent inactivation process. We confirmed that increased Mg(i) reduces peak ionic currents and increases VDI of Ca(v)1.2 channels in ventricular myocytes and in transfected cells when measured with Na(+) as permeant ion. The increased rate and extent of VDI caused by increased Mg(i) were substantially reduced by mutations of a cation-binding residue in the proximal C-terminal EF-hand, consistent with the conclusion that both reduction of peak currents and enhancement of VDI result from the binding of Mg(i) to the EF-hand (K(D) approximately 0.9 mM) near the resting level of Mg(i) in ventricular myocytes. VDI was more rapid for L-type Ca(2+) currents in ventricular myocytes than for Ca(v)1.2 channels in transfected cells. Coexpression of Ca(v)beta(2b) subunits and formation of an autoinhibitory complex of truncated Ca(v)1.2 channels with noncovalently bound distal C-terminal domain (DCT) both increased VDI in transfected cells, indicating that the subunit structure of the Ca(v)1.2 channel greatly influences its VDI. The effects of noncovalently bound DCT on peak current amplitude and VDI required Mg(i) binding to the proximal C-terminal EF-hand and were prevented by mutations of a key divalent cation-binding amino acid residue. Our results demonstrate cooperative regulation of peak current amplitude and VDI of Ca(v)1.2 channels by Mg(i), the proximal C-terminal EF-hand, and the DCT, and suggest that conformational changes that regulate VDI are propagated from the DCT through the proximal C-terminal EF-hand to the channel-gating mechanism.

Disruption of the IS6-AID linker affects voltage-gated calcium channel inactivation and facilitation

Two processes dominate voltage-gated calcium channel (Ca_V) inactivation: voltage-dependent inactivation (VDI) and calcium-dependent inactivation (CDI). The Ca_Vβ/Ca_Vα₁-I-II loop and Ca²⁺/calmodulin (CaM)/Ca_Vα₁–C-terminal tail complexes have been shown to modulate each, respectively. Nevertheless, how each complex couples to the pore and whether each affects inactivation independently have remained unresolved. Here, we demonstrate that the IS6–α-interaction domain (AID) linker provides a rigid connection between the pore and Ca_Vβ/I-II loop complex by showing that IS6-AID linker polyglycine mutations accelerate Ca_V1.2 (L-type) and Ca_V2.1 (P/Q-type) VDI. Remarkably, mutations that either break the rigid IS6-AID linker connection or disrupt Ca_Vβ/I-II association sharply decelerate CDI and reduce a second Ca²⁺/CaM/Ca_Vα₁–C-terminal–mediated process known as calcium-dependent facilitation. Collectively, the data strongly suggest that components traditionally associated solely with VDI, Ca_Vβ and the IS6-AID linker, are essential for calcium-dependent modulation, and that both Ca_Vβ-dependent and CaM-dependent components couple to the pore by a common mechanism requiring Ca_Vβ and an intact IS6-AID linker.

Physiological modulation of voltage-dependent inactivation in the cardiac muscle L-type calcium channel: A modelling study

The inactivation of the L-type Ca2+ current is composed of voltage-dependent and calcium-dependent mechanisms. The relative contribution of these processes is still under dispute and the idea that the voltage-dependent inactivation could be subject to further modulation by other physiological processes had been ignored. This study sought to model physiological modulation of inactivation of the current in cardiac ventricular myocytes, based upon the recent detailed experimental data that separated total and voltage-dependent inactivation (VDI) by replacing extracellular Ca2+ with Mg2+ and monitoring L-type Ca2+ channel behaviour by outward K+ current flowing through the channel in the absence of inward current flow. Calcium-dependent inactivation (CDI) was based upon Ca2+ influx and formulated from data that was recorded during beta-adrenergic stimulation of the myocytes. Ca2+ influx and its competition with non-selective monovalent cation permeation were also incorporated into channel permeation in the model. The constructed model could closely reproduce the experimental Ba2+ and Ca2+ current results under basal condition where no beta-stimulation was added after a slight reduction of the development of fast voltage-dependent inactivation with depolarization. The model also predicted that under beta-adrenergic stimulation voltage-dependent inactivation is lost and calcium-dependent inactivation largely compensates it. The developed model thus will be useful to estimate the respective roles of VDI and CDI of L-type Ca2+ channels in various physiological and pathological conditions of the heart which would otherwise be difficult to show experimentally.

Ca2+/CaM-dependent inactivation of the skeletal muscle L-type Ca2+ channel (Cav1.1)

Ca2+-dependent modulation via calmodulin (CaM) has been documented for most high-voltage-activated Ca2+ channels, but whether the skeletal muscle L-type channel (Cav1.1) exhibits this property has been unknown. In this paper, whole-cell current and fluorescent resonance energy transfer (FRET) recordings were obtained from cultured mouse myotubes to test for potential involvement of CaM in function of Cav1.1. When prolonged depolarization (800 ms) was used to evoke Cav1.1 currents in normal myotubes, the fraction of current remaining at the end of the pulse displayed classic signs of Ca2+-dependent inactivation (CDI), including U-shaped voltage dependence, maximal inactivation (approximately 30%) at potentials eliciting maximal inward current, and virtual elimination of inactivation when Ba2+ replaced external Ca2+ or when 10 mM BAPTA was included in the pipette solution. Furthermore, CDI was virtually eliminated (from 30 to 8%) in normal myotubes overexpressing mutant CaM (CaM1234) that does not bind Ca2+, whereas CDI was unaltered in myotubes overexpressing wild-type CaM (CaMwt). In addition, a significant FRET signal (E=4.06%) was detected between fluorescently tagged Cav1.1 and CaMwt coexpressed in dysgenic myotubes, demonstrating for the first time that these two proteins associate in vivo. These findings show that CaM associates with and modulates Cav1.1.

Ca2+-dependent inactivation of CaV1.2 channels prevents Gd3+ block: does Ca2+ block the pore of inactivated channels?

Lanthanide gadolinium (Gd(3+)) blocks Ca(V)1.2 channels at the selectivity filter. Here we investigated whether Gd(3+) block interferes with Ca(2+)-dependent inactivation, which requires Ca(2+) entry through the same site. Using brief pulses to 200 mV that relieve Gd(3+) block but not inactivation, we monitored how the proportions of open and open-blocked channels change during inactivation. We found that blocked channels inactivate much less. This is expected for Gd(3+) block of the Ca(2+) influx that enhances inactivation. However, we also found that the extent of Gd(3+) block did not change when inactivation was reduced by abolition of Ca(2+)/calmodulin interaction, showing that Gd(3+) does not block the inactivated channel. Thus, Gd(3+) block and inactivation are mutually exclusive, suggesting action at a common site. These observations suggest that inactivation causes a change at the selectivity filter that either hides the Gd(3+) site or reduces its affinity, or that Ca(2+) occupies the binding site at the selectivity filter in inactivated channels. The latter possibility is supported by previous findings that the EEQE mutation of the selectivity EEEE locus is void of Ca(2+)-dependent inactivation (Zong Z.Q., J.Y. Zhou, and T. Tanabe. 1994. Biochem. Biophys. Res. Commun. 201:1117-11123), and that Ca(2+)-inactivated channels conduct Na(+) when Ca(2+) is removed from the extracellular medium (Babich O., D. Isaev, and R. Shirokov. 2005. J. Physiol. 565:709-717). Based on these results, we propose that inactivation increases affinity of the selectivity filter for Ca(2+) so that Ca(2+) ion blocks the pore. A minimal model, in which the inactivation "gate" is an increase in affinity of the selectivity filter for permeating ions, successfully simulates the characteristic U-shaped voltage dependence of inactivation in Ca(2+).

Block of CaV1.2 channels by Gd3+ reveals preopening transitions in the selectivity filter

Using the lanthanide gadolinium (Gd³⁺) as a Ca²⁺ replacing probe, we investigated the voltage dependence of pore blockage of Ca_V1.2 channels. Gd⁺³ reduces peak currents (tonic block) and accelerates decay of ionic current during depolarization (use-dependent block). Because diffusion of Gd³⁺ at concentrations used (<1 μM) is much slower than activation of the channel, the tonic effect is likely to be due to the blockage that occurred in closed channels before depolarization. We found that the dose–response curves for the two blocking effects of Gd³⁺ shifted in parallel for Ba²⁺, Sr²⁺, and Ca²⁺ currents through the wild-type channel, and for Ca²⁺ currents through the selectivity filter mutation EEQE that lowers the blocking potency of Gd³⁺. The correlation indicates that Gd³⁺ binding to the same site causes both tonic and use-dependent blocking effects. The apparent on-rate for the tonic block increases with the prepulse voltage in the range −60 to −45 mV, where significant gating current but no ionic current occurs. When plotted together against voltage, the on-rates of tonic block (−100 to −45 mV) and of use-dependent block (−40 to 40 mV) fall on a single sigmoid that parallels the voltage dependence of the gating charge. The on-rate of tonic block by Gd³⁺ decreases with concentration of Ba²⁺, indicating that the apparent affinity of the site to permeant ions is about 1 mM in closed channels. Therefore, we propose that at submicromolar concentrations, Gd³⁺ binds at the entry to the selectivity locus and that the affinity of the site for permeant ions decreases during preopening transitions of the channel.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Caldendrin, a Neuron-specific Modulator of Cav/1.2 (L-type) Ca2+ Channels

EF-hand Ca2+-binding proteins such as calmodulin and CaBP1 have emerged as important regulatory subunits of voltage-gated Ca2+ channels. Here, we show that caldendrin, a variant of CaBP1 enriched in the brain, interacts with and distinctly modulates Cav1.2 (L-type) voltage-gated Ca2+ channels relative to other Ca2+-binding proteins. Caldendrin binds to the C-terminal IQ-domain of the pore-forming alpha1-subunit of Cav1.2 (alpha(1)1.2) and competitively displaces calmodulin and CaBP1 from this site. Compared with CaBP1, caldendrin causes a more modest suppression of Ca2+-dependent inactivation of Cav1.2 through a different subset of molecular determinants. Caldendrin does not bind to the N-terminal domain of alpha11.2, a site that is critical for functional interactions of the channel with CaBP1. Deletion of the N-terminal domain inhibits CaBP1, but spares caldendrin modulation of Cav1.2 inactivation. In contrast, mutations of the IQ-domain abolish physical and functional interactions of caldendrin and Cav1.2, but do not prevent channel modulation by CaBP1. Using antibodies specific for caldendrin and Cav1.2, we show that caldendrin coimmunoprecipitates with Cav1.2 from the brain and colocalizes with Cav1.2 in somatodendritic puncta of cortical neurons in culture. Our findings reveal functional diversity within related Ca2+-binding proteins, which may enhance the specificity of Ca2+ signaling by Cav1.2 channels in different cellular contexts.

Heterogeneous ventricular chamber response to hypokalemia and inward rectifier potassium channel blockade underlies bifurcated T wave in guinea pig

It was previously demonstrated that transmural electrophysiological heterogeneities can inscribe the ECG T wave. However, the bifurcated T wave caused by loss of inward rectifier potassium current (I(K1)) function is not fully explained by transmural heterogeneities. Since right ventricular (RV) guinea pig myocytes have significantly lower I(K1) than left ventricular (LV) myocytes, we hypothesized that the complex ECG can be inscribed by heterogeneous chamber-specific responses to hypokalemia and partial I(K1) blockade. Ratiometric optical action potentials were recorded from the epicardial surface of the RV and LV. BaCl(2) (10 micromol/l) was perfused to partially block I(K1) in isolated guinea pig whole heart preparations. BaCl(2) or hypokalemia alone significantly increased RV basal (RV(B)) action potential duration (APD) by approximately 30% above control compared with LV apical (LV(A)) APD (14%, P<0.05). In the presence of BaCl(2), 2 mmol/l extracellular potassium (hypokalemia) further increased RV(B) APD to a greater extent (31%) than LV(A) APD (19%, P<0.05) compared with BaCl(2) perfusion alone. Maximal dispersion between RV(B) and LV(A) APD increased by 105% (P<0.05), and the QT interval prolonged by 55% (P<0.05) during hypokalemia and BaCl(2). Hypokalemia and BaCl(2) produced an ECG with a double repolarization wave. The first wave (QT1) corresponded to selective depression of apical LV plateau potentials, while the second wave (QT2) corresponded to the latest repolarizing RV(B) myocytes. These data suggest that final repolarization is more sensitive to extracellular potassium changes in regions with reduced I(K1), particularly when I(K1) availability is reduced. Furthermore, underlying I(K1) heterogeneities can potentially contribute to the complex ECG during I(K1) loss of function and hypokalemia.

Kinetic properties of the cardiac L-type Ca2+ channel and its role in myocyte electrophysiology: a theoretical investigation

The L-type Ca(2+) channel (Ca(V)1.2) plays an important role in action potential (AP) generation, morphology, and duration (APD) and is the primary source of triggering Ca(2+) for the initiation of Ca(2+)-induced Ca(2+)-release in cardiac myocytes. In this article we present: 1), a detailed kinetic model of Ca(V)1.2, which is incorporated into a model of the ventricular mycoyte where it interacts with a kinetic model of the ryanodine receptor in a restricted subcellular space; 2), evaluation of the contribution of voltage-dependent inactivation (VDI) and Ca(2+)-dependent inactivation (CDI) to total inactivation of Ca(V)1.2; and 3), description of dynamic Ca(V)1.2 and ryanodine receptor channel-state occupancy during the AP. Results are: 1), the Ca(V)1.2 model reproduces experimental single-channel and macroscopic-current data; 2), the model reproduces rate dependence of APD, [Na(+)](i), and the Ca(2+)-transient (CaT), and restitution of APD and CaT during premature stimuli; 3), CDI of Ca(V)1.2 is sensitive to Ca(2+) that enters the subspace through the channel and from SR release. The relative contributions of these Ca(2+) sources to total CDI during the AP vary with time after depolarization, switching from early SR dominance to late Ca(V)1.2 dominance. 4), The relative contribution of CDI to total inactivation of Ca(V)1.2 is greater at negative potentials, when VDI is weak; and 5), loss of VDI due to the Ca(V)1.2 mutation G406R (linked to the Timothy syndrome) results in APD prolongation and increased CaT.

Ca2+ enhances U-type inactivation of N-type (CaV2.2) calcium current in rat sympathetic neurons

Ca2+ -dependent inactivation (CDI) has recently been shown in heterologously expressed N-type calcium channels (CaV2.2), but CDI has been inconsistently observed in native N-current. We examined the effect of Ca2+ on N-channel inactivation in rat sympathetic neurons to determine the role of CDI on mammalian N-channels. N-current inactivated with fast (tau approximately 150 ms) and slow (tau approximately 3 s) components in Ba2+. Ca2+ differentially affected these components by accelerating the slow component (slow inactivation) and enhancing the amplitude of the fast component (fast inactivation). Lowering intracellular BAPTA concentration from 20 to 0.1 mM accelerated slow inactivation, but only in Ca2+ as expected from CDI. However, low BAPTA accelerated fast inactivation in either Ca2+ or Ba2+, which was unexpected. Fast inactivation was abolished with monovalent cations as the charge carrier, but slow inactivation was similar to that in Ba2+. Increased Ca2+, but not Ba2+, concentration (5-30 mM) enhanced the amplitude of fast inactivation and accelerated slow inactivation. However, the enhancement of fast inactivation was independent of Ca2+ influx, which indicates the relevant site is exposed to the extracellular solution and is inconsistent with CDI. Fast inactivation showed U-shaped voltage dependence in both Ba2+ and Ca2+, which appears to result from preferential inactivation from intermediate closed states (U-type inactivation). Taken together, the data support a role for extracellular divalent cations in modulating U-type inactivation. CDI appears to play a role in N-channel inactivation, but on a slower (sec) time scale.

Cellular basis for electrocardiographic and arrhythmic manifestations of Andersen-Tawil syndrome (LQT7)

BACKGROUND

Andersen-Tawil syndrome, a skeletal muscle syndrome associated with periodic paralysis and long QT intervals on the ECG, has been linked to defects in KCNJ2, the gene encoding for the inward rectifier potassium channel (I(K1).)

OBJECTIVES

The purpose of this study was to examine the cellular mechanisms underlying the ECG and arrhythmic manifestations of Andersen-Tawil syndrome.

METHODS

To investigate the effects of KCNJ2 loss-of-function mutations responsible for Andersen-Tawil syndrome, we used barium chloride (BaCl(2)) to inhibit I(K1) in arterially perfused wedge preparation. Transmembrane action potentials (APs) were simultaneously recorded from endocardial, midmyocardial, and epicardial cells, together with a transmural ECG.

RESULTS

BaCl(2) (1 to 30 microM) produced a concentration-dependent prolongation of the QT interval, secondary to a homogeneous prolongation of AP duration of the three cell types. QT interval was prolonged without an increase in transmural dispersion of repolarization (TDR). Low extracellular potassium (2.0 mM), isoproterenol (20-50 nM), and an abrupt increase in temperature (36 degrees C-39 degrees C) in the presence of 10 microM BaCl(2) did not significantly increase TDR but increased ectopic extrasystolic activity. Early afterdepolarizations were not observed under any condition. Spontaneous torsades de pointes arrhythmias were never observed, nor could they be induced with programmed electrical stimulation under any of the conditions studied.

CONCLUSION

Our results provide an understanding of why QT prolongation associated with Andersen-Tawil syndrome is relatively benign in the clinic and provide further support for the hypothesis that the increase in TDR, rather than QT interval, is responsible for development of torsades de pointes.