Molecular determinants of diltiazem block in domains IIIS6 and IVS6 of L-type Ca(2+) channels

Voltage gated sodium and calcium channels: Discovery, structure, function, and Pharmacology

Voltage-gated sodium channels initiate action potentials in nerve and muscle, and voltage-gated calcium channels couple depolarization of the plasma membrane to intracellular events such as secretion, contraction, synaptic transmission, and gene expression. In this Review and Perspective article, I summarize early work that led to identification, purification, functional reconstitution, and determination of the amino acid sequence of the protein subunits of sodium and calcium channels and showed that their pore-forming subunits are closely related. Decades of study by antibody mapping, site-directed mutagenesis, and electrophysiological recording led to detailed two-dimensional structure-function maps of the amino acid residues involved in voltage-dependent activation and inactivation, ion permeation and selectivity, and pharmacological modulation. Most recently, high-resolution three-dimensional structure determination by X-ray crystallography and cryogenic electron microscopy has revealed the structural basis for sodium and calcium channel function and pharmacological modulation at the atomic level. These studies now define the chemical basis for electrical signaling and provide templates for future development of new therapeutic agents for a range of neurological and cardiovascular diseases.

Voltage-gated calcium channels in genetic epilepsies

Voltage-gated calcium channels (VGCC) are abundant in the central nervous system and serve a broad spectrum of functions, either directly in cellular excitability or indirectly to regulate Ca2+ homeostasis. Ca2+ ions act as one of the main connections in excitation-transcription coupling, muscle contraction and excitation-exocytosis coupling, including synaptic transmission. In recent years, many genes encoding VGCCs main α or additional auxiliary subunits have been associated with epilepsy. This review sums up the current state of knowledge on disease mechanisms and provides guidance on disease-specific therapies where applicable.

Rate-dependent effects of state-specific sodium channel blockers in cardiac tissue: Insights from idealized models

A common side effect of pharmaceutical drugs is an increased propensity for cardiac arrhythmias. Many drugs bind to cardiac ion-channels in a state-specific manner, which alters the ionic conductances in complicated ways, making it difficult to identify the mechanisms underlying pro-arrhythmic drug effects. To better understand the fundamental mechanisms underlying the diverse effects of state-dependent sodium (Na+) channel blockers on cellular excitability, we consider two canonical motifs of drug-ion-channel interactions and compare the effects of Na+ channel blockers on the rate-dependence of peak upstroke velocity, conduction velocity, and vulnerable window size. In the literature, both motifs are referred to as "guarded receptor," but here we distinguish between state-specific binding that does not alter channel gating (referred to here as "guarded receptor") and state-specific binding that blocks certain gating transitions ("gate immobilization"). For each drug binding motif, we consider drugs that bind to the inactivated state and drugs that bind to the non-inactivated state of the Na+ channel. Exploiting the idealized nature of the canonical binding motifs, we identify the fundamental mechanisms underlying the effects on excitability of the various binding interactions. Specifically, we derive the voltage-dependence of the drug binding time constants and the equilibrium fractions of channels bound to drug, and we then derive a formula that incorporates these time constants and equilibrium fractions to elucidate the fundamental mechanisms. In the case of charged drug, we find that drugs that bind to inactivated channels exhibit greater rate-dependence than drugs that bind to non-inactivated channels. For neutral drugs, the effects of guarded receptor interactions are rate-independent, and we describe a novel mechanism for reverse rate-dependence resulting from neutral drug binding to non-inactivated channels via the gate immobilization motif.

Glucocorticoid effects on working memory impairment require l-type calcium channel activity within prefrontal cortex

Previous findings have indicated that glucocorticoid hormones impair working memory via an interaction with the β-adrenoceptor-cAMP signaling cascade to rapidly increase cAMP-dependent protein kinase (PKA) activity within the prefrontal cortex (PFC). However, it remains elusive how such activation of PKA can affect downstream cellular mechanisms in regulating PFC cognitive function. PKA is known to activate l-type voltage-gated Ca²⁺ channels (LTCCs) which regulate a broad range of cellular processes, including neuronal excitability and neurotransmitter release. The present experiments examined whether LTCC activity within the PFC is required in mediating glucocorticoid and PKA effects on spatial working memory. Male Sprague Dawley rats received bilateral administration of the LTCC inhibitor diltiazem together with either the glucocorticoid receptor agonist RU 28362 or PKA activator Sp-cAMPS into the PFC before testing on a delayed alternation task in a T-maze. Both RU 28362 and Sp-cAMPS impaired working memory, whereas the LTCC inhibitor diltiazem fully blocked the working memory impairment induced by either RU 28362 or Sp-cAMPS. Conversely, bilateral administration of the LTCC agonist Bay K8644 into the PFC was sufficient to impair working memory. Thus, these findings provide support for the view that glucocorticoids, via an interaction with the β-adrenergic signaling cascade and enhanced PKA activity levels, impair working memory by increasing LTCC activity in the PFC.

Identification of a critical binding site for local anaesthetics in the side pockets of Kv 1 channels

BACKGROUND AND PURPOSE

Local anaesthetics block sodium and a variety of potassium channels. Although previous studies identified a residue in the pore signature sequence together with three residues in the S6 segment as a putative binding site, the precise molecular basis of inhibition of K_v channels by local anaesthetics remained unknown. Crystal structures of K_v channels predict that some of these residues point away from the central cavity and face into a drug binding site called side pockets. Thus, the question arises whether the binding site of local anaesthetics is exclusively located in the central cavity or also involves the side pockets.

EXPERIMENTAL APPROACH

A systematic functional alanine mutagenesis approach, scanning 58 mutants, together with in silico docking experiments and molecular dynamics simulations was utilized to elucidate the binding site of bupivacaine and ropivacaine.

KEY RESULTS

Inhibition of K_v 1.5 channels by local anaesthetics requires binding to the central cavity and the side pockets, and the latter requires interactions with residues of the S5 and the back of the S6 segments. Mutations in the side pockets remove stereoselectivity of inhibition of K_v 1.5 channels by bupivacaine. Although binding to the side pockets is conserved for different local anaesthetics, the binding mode in the central cavity and the side pockets shows considerable variations.

CONCLUSION AND IMPLICATIONS

Local anaesthetics bind to the central cavity and the side pockets, which provide a crucial key to the molecular understanding of their K_v channel affinity and stereoselectivity, as well as their spectrum of side effects.

T1143 essential for CaV1.2 inhibition by diltiazem

Careful analysis of previously published reports and some new insights into the structure activity studies revealed an important role of Threonine 1143 in drug binding. Substituting T1143 by alanine and other residues significantly reduced channel inhibition by qDil and Dil. Mutation T1143A did not affect channel activation or inactivation while almost completely diminishing channel block by Dil or qDil. These findings support the view that T1143 serves as drug binding determinant. Other mutations in this position than T1143A (T1143L/Y/S/N/C/V/E) diminished channel inhibition by qDil but additionally affected channel activation and inactivation and may therefore affect channel block allosterically. Collectively, our data suggest that T1143 is an essential diltiazem binding determinant.

Structure and Pharmacology of Voltage-Gated Sodium and Calcium Channels

Voltage-gated sodium and calcium channels are evolutionarily related transmembrane signaling proteins that initiate action potentials, neurotransmission, excitation-contraction coupling, and other physiological processes. Genetic or acquired dysfunction of these proteins causes numerous diseases, termed channelopathies, and sodium and calcium channels are the molecular targets for several major classes of drugs. Recent advances in the structural biology of these proteins using X-ray crystallography and cryo-electron microscopy have given new insights into the molecular basis for their function and pharmacology. Here we review this recent literature and integrate findings on sodium and calcium channels to reveal the structural basis for their voltage-dependent activation, fast and slow inactivation, ion conductance and selectivity, and complex pharmacology at the atomic level. We conclude with the theme that new understanding of the diseases and therapeutics of these channels will be derived from application of the emerging structural principles from these recent structural analyses.

Structural Basis for Diltiazem Block of a Voltage-Gated Ca2+ Channel

Diltiazem is a widely prescribed Ca²⁺ antagonist drug for cardiac arrhythmia, hypertension, and angina pectoris. Using the ancestral Ca_V channel construct Ca_VAb as a molecular model for X-ray crystallographic analysis, we show here that diltiazem targets the central cavity of the voltage-gated Ca²⁺ channel underneath its selectivity filter and physically blocks ion conduction. The diltiazem-binding site overlaps with the receptor site for phenylalkylamine Ca²⁺ antagonist drugs such as verapamil. The dihydropyridine Ca²⁺ channel blocker amlodipine binds at a distinct site and allosterically modulates the binding sites for diltiazem and Ca²⁺ Our studies resolve two distinct binding poses for diltiazem in the absence and presence of amlodipine. The binding pose in the presence of amlodipine may mimic a high-affinity binding configuration induced by voltage-dependent inactivation, which is favored by dihydropyridine binding. In this binding pose, the tertiary amino group of diltiazem projects upward into the inner end of the ion selectivity filter, interacts with ion coordination Site 3 formed by the backbone carbonyls of T175, and alters binding of Ca²⁺ to ion coordination Sites 1 and 2. Altogether, our results define the receptor site for diltiazem and elucidate the mechanisms for pore block and allosteric modulation by other Ca²⁺ channel-blocking drugs at the atomic level. SIGNIFICANCE STATEMENT: Calcium antagonist drugs that block voltage-gated calcium channels in heart and vascular smooth muscle are widely used in the treatment of cardiovascular diseases. Our results reveal the chemical details of diltiazem binding in a blocking position in the pore of a model calcium channel and show that binding of another calcium antagonist drug alters binding of diltiazem and calcium. This structural information defines the mechanism of drug action at the atomic level and provides a molecular template for future drug discovery.

How CaV1.2-bound verapamil blocks Ca2+ influx into cardiomyocyte: Atomic level views

The first clinically used antiarrhythmic, antianginal and anti-hypertensive phenylalkylamine, verapamil's cardiovascular activity is inextricably linked to its ability to antagonize Ca²⁺ overload via blocking Ca_V1.2, a cardiac L-type Ca²⁺ channel of undisputed physiological and pharmacological importance in cardiovascular disorders such as myocardial ischemia-reperfusion injury. From a structural point of view, however, the action mechanism of verapamil is still elusive. Therefore, incorporating previous findings for verapamil and Ca_V1.2, this review article puts forward two experimental data-derived and -supported 3D structure models for Ca_V1.2's α₁ subunit and its verapamil-bound form. Furthermore, this article suggests three biophysical mechanisms, namely competitive binding, steric hindrance and electrostatic repulsion, towards an atomic level understanding of how verapamil blocks the L-type Ca²⁺ current mediated by Ca_V1.2 in reality, which can be useful for the design and development of next-generation Ca²⁺ antagonists to provide safer and more effective treatment of cardiovascular diseases.

Molecular Determinants of the Differential Modulation of Cav1.2 and Cav1.3 by Nifedipine and FPL 64176

Nifedipine and FPL 64176 (FPL), which block and potentiate L-type voltage-gated Ca2+ channels, respectively, modulate Cav1.2 more potently than Cav1.3. To identify potential strategies for developing subtype-selective inhibitors, we investigated the role of divergent amino acid residues in transmembrane domains IIIS5 and the extracellular IIIS5-3P loop region in modulation of these channels by nifedipine and FPL. Insertion of the extracellular IIIS5-3P loop from Cav1.2 into Cav1.3 (Cav1.3+) reduced the IC50 of nifedipine from 289 to 101 nM, and substitution of S1100 with an A residue, as in Cav1.2, accounted for this difference. Substituting M1030 in IIIS5 to V in Cav1.3+ (Cav1.3+V) further reduced the IC50 of nifedipine to 42 nM. FPL increased current amplitude with an EC50 of 854 nM in Cav1.3, 103 nM in Cav1.2, and 99 nM in Cav1.3+V. In contrast to nifedipine block, substitution of M1030 to V in Cav1.3 had no effect on potency of FPL potentiation of current amplitude, but slowed deactivation in the presence and absence of 10 μM FPL. FPL had no effect on deactivation of Cav1.3/dihydropyridine-insensitive (DHPi), a channel with very low sensitivity to nifedipine block (IC50 ∼93 μM), but did shift the voltage-dependence of activation by ∼-10 mV. We conclude that the M/V variation in IIIS5 and the S/A variation in the IIIS5-3P loop of Cav1.2 and Cav1.3 largely determine the difference in nifedipine potency between these two channels, but the difference in FPL potency is determined by divergent amino acids in the IIIS5-3P loop.

Optical control of L-type Ca2+ channels using a diltiazem photoswitch

L-type Ca²⁺ channels (LTCCs) play a crucial role in excitation-contraction coupling and release of hormones from secretory cells. They are targets of antihypertensive and antiarrhythmic drugs such as diltiazem. Here, we present a photoswitchable diltiazem, FHU-779, which can be used to reversibly block endogenous LTCCs by light. FHU-779 is as potent as diltiazem and can be used to place pancreatic β-cell function and cardiac activity under optical control.

The Contribution of L-Type Cav1.3 Channels to Retinal Light Responses

L-type voltage-gated calcium channels (LTCCs) regulate tonic neurotransmitter release from sensory neurons including retinal photoreceptors. There are three types of LTCCs (Ca_v1.2, Ca_v1.3, and Ca_v1.4) expressed in the retina. While Ca_v1.2 is expressed in all retinal cells including the Müller glia and neurons, Ca_v1.3 and Ca_v1.4 are expressed in the retinal neurons with Ca_v1.4 exclusively expressed in the photoreceptor synaptic terminals. Mutations in the gene encoding Ca_v1.4 cause incomplete X-linked congenital stationary night blindness in humans. Even though Ca_v1.3 is present in the photoreceptor inner segments and the synaptic terminals in various vertebrate species, its role in vision is unclear, since genetic alterations in Ca_v1.3 are not associated with severe vision impairment in humans or in Ca_v1.3-null (Ca_v1.3^-/-) mice. However, a failure to regulate Ca_v1.3 was found in a mouse model of Usher syndrome, the most common cause of combined deafness and blindness in humans, indicating that Ca_v1.3 may contribute to retinal function. In this report, we combined physiological and morphological data to demonstrate the role of Ca_v1.3 in retinal physiology and function that has been undervalued thus far. Through ex vivo and in vivo electroretinogram (ERG) recordings and immunohistochemical staining, we found that Ca_v1.3 plays a role in retinal light responses and synaptic plasticity. Pharmacological inhibition of Ca_v1.3 decreased ex vivo ERG a- and b-wave amplitudes. In Ca_v1.3^-/- mice, their dark-adapted ERG a-, b-wave, and oscillatory potential amplitudes were significantly dampened, and implicit times were delayed compared to the wild type (WT). Furthermore, the density of ribbon synapses was reduced in the outer plexiform layer of Ca_v1.3^-/- mice retinas. Hence, Ca_v1.3 plays a more prominent role in retinal physiology and function than previously reported.

Coupling of excitation to Ca2+ release is modulated by dysferlin

KEY POINTS

Dysferlin, the protein missing in limb girdle muscular dystrophy 2B and Miyoshi myopathy, concentrates in transverse tubules of skeletal muscle, where it stabilizes voltage-induced Ca²⁺ transients against loss after osmotic shock injury (OSI). Local expression of dysferlin in dysferlin-null myofibres increases transient amplitude to control levels and protects them from loss after OSI. Inhibitors of ryanodine receptors (RyR1) and L-type Ca²⁺ channels protect voltage-induced Ca²⁺ transients from loss; thus both proteins play a role in injury in dysferlin's absence. Effects of Ca²⁺ -free medium and S107, which inhibits SR Ca²⁺ leak, suggest the SR as the primary source of Ca²⁺ responsible for the loss of the Ca²⁺ transient upon injury. Ca²⁺ waves were induced by OSI and suppressed by exogenous dysferlin. We conclude that dysferlin prevents injury-induced SR Ca²⁺ leak.

ABSTRACT

Dysferlin concentrates in the transverse tubules of skeletal muscle and stabilizes Ca²⁺ transients when muscle fibres are subjected to osmotic shock injury (OSI). We show here that voltage-induced Ca²⁺ transients elicited in dysferlin-null A/J myofibres were smaller than control A/WySnJ fibres. Regional expression of Venus-dysferlin chimeras in A/J fibres restored the full amplitude of the Ca²⁺ transients and protected against OSI. We also show that drugs that target ryanodine receptors (RyR1: dantrolene, tetracaine, S107) and L-type Ca²⁺ channels (LTCCs: nifedipine, verapamil, diltiazem) prevented the decrease in Ca²⁺ transients in A/J fibres following OSI. Diltiazem specifically increased transients by ∼20% in uninjured A/J fibres, restoring them to control values. The fact that both RyR1s and LTCCs were involved in OSI-induced damage suggests that damage is mediated by increased Ca²⁺ leak from the sarcoplasmic reticulum (SR) through the RyR1. Congruent with this, injured A/J fibres produced Ca²⁺ sparks and Ca²⁺ waves. S107 (a stabilizer of RyR1-FK506 binding protein coupling that reduces Ca²⁺ leak) or local expression of Venus-dysferlin prevented OSI-induced Ca²⁺ waves. Our data suggest that dysferlin modulates SR Ca²⁺ release in skeletal muscle, and that in its absence OSI causes increased RyR1-mediated Ca²⁺ leak from the SR into the cytoplasm.

The L-type Ca(2+) Channel Blocker Nifedipine Inhibits Mycelial Growth, Sporulation, and Virulence of Phytophthora capsici

The oomycete vegetable pathogen Phytophthora capsici causes significant losses of important vegetable crops worldwide. Calcium and other plant nutrients have been used in disease management of oomycete pathogens. Calcium homeostasis and signaling is essential for numerous biological processes, and Ca(2+) channel blockers prevent excessive Ca(2+) influx into the fungal cell. However, it is not known whether voltage-gated Ca(2+) channel blockers improve control over oomycete pathogens. In the present study, we compared the inhibitory effects of CaCl2 and the extracellular Ca(2+) chelator EDTA on mycelial growth and found that calcium assimilation plays a key role in P. capsici mycelial growth. Next, we involved the voltage-gated Ca(2+) channel blockers verapamil (VP) and nifedipine (NFD) to analyze the effect of Ca(2+) channel blockers on mycelial growth and sporulation; the results suggested that NFD, but not VP, caused significant inhibition. Ion rescue in an NFD-induced inhibition assay suggested that NFD-induced inhibition is calcium-dependent. In addition, NFD increased P. capsici sensitivity to H2O2 in a calcium-dependent manner, and extracellular calcium rescued it. Furthermore, NFD inhibited the virulence and gene expression related to its pathogenicity. These results suggest that NFD inhibits mycelial growth, sporulation, and virulence of P. capsici.

A whole-genome RNAi screen uncovers a novel role for human potassium channels in cell killing by the parasite Entamoeba histolytica

The parasite Entamoeba histolytica kills human cells resulting in ulceration, inflammation and invasion of the colonic epithelium. We used the cytotoxic properties of ameba to select a genome-wide RNAi library to reveal novel host factors that control susceptibility to amebic killing. We identified 281 candidate susceptibility genes and bioinformatics analyses revealed that ion transporters were significantly enriched among susceptibility genes. Potassium (K⁺) channels were the most common transporter identified. Their importance was further supported by colon biopsy of humans with amebiasis that demonstrated suppressed K⁺ channel expression. Inhibition of human K⁺ channels by genetic silencing, pharmacologic inhibitors and with excess K⁺ protected diverse cell types from E. histolytica-induced death. Contact with E. histolytica parasites triggered K⁺ channel activation and K⁺ efflux by intestinal epithelial cells, which preceded cell killing. Specific inhibition of Ca²⁺-dependent K⁺ channels was highly effective in preventing amebic cytotoxicity in intestinal epithelial cells and macrophages. Blockade of K⁺ efflux also inhibited caspase-1 activation, IL-1β secretion and pyroptotic death in THP-1 macrophages. We concluded that K⁺ channels are host mediators of amebic cytotoxicity in multiple cells types and of inflammasome activation in macrophages.

Structural Basis for Pharmacology of Voltage-Gated Sodium and Calcium Channels

Voltage-gated sodium channels initiate action potentials in nerve, muscle, and other electrically excitable cells. Voltage-gated calcium channels are activated by depolarization during action potentials, and calcium influx through them is the key second messenger of electrical signaling, initiating secretion, contraction, neurotransmission, gene transcription, and many other intracellular processes. Drugs that block sodium channels are used in local anesthesia and the treatment of epilepsy, bipolar disorder, chronic pain, and cardiac arrhythmia. Drugs that block calcium channels are used in the treatment of epilepsy, chronic pain, and cardiovascular disorders, including hypertension, angina pectoris, and cardiac arrhythmia. The principal pore-forming subunits of voltage-gated sodium and calcium channels are structurally related and likely to have evolved from ancestral voltage-gated sodium channels that are widely expressed in prokaryotes. Determination of the structure of a bacterial ancestor of voltage-gated sodium and calcium channels at high resolution now provides a three-dimensional view of the binding sites for drugs acting on sodium and calcium channels. In this minireview, we outline the different classes of sodium and calcium channel drugs, review studies that have identified amino acid residues that are required for their binding and therapeutic actions, and illustrate how the analogs of those key amino acid residues may form drug-binding sites in three-dimensional models derived from bacterial channels.

Pharmacological Inhibition of Voltage-gated Ca(2+) Channels for Chronic Pain Relief

Chronic pain is a major therapeutic problem as the current treatment options are unsatisfactory with low efficacy and deleterious side effects. Voltage-gated Ca2+ channels (VGCCs), which are multi-complex proteins consisting of α1, β, γ, and α2δ subunits, play an important role in pain signaling. These channels are involved in neurogenic inflammation, excitability, and neurotransmitter release in nociceptors. It has been previously shown that N-type VGCCs (Cav2.2) are a major pain target. U.S. FDA approval of three Cav2.2 antagonists, gabapentin, pregabalin, and ziconotide, for chronic pain underlies the importance of this channel subtype. Also, there has been increasing evidence that L-type (Cav1.2) or T-type (Cav3.2) VGCCs may be involved in pain signaling and chronic pain. In order to develop novel pain therapeutics and to understand the role of VGCC subtypes, discovering subtype selective VGCC inhibitors or methods that selectively target the inhibitor into nociceptors would be essential. This review describes the various VGCC subtype inhibitors and the potential of utilizing VGCC subtypes as targets of chronic pain. Development of VGCC subtype inhibitors and targeting them into nociceptors will contribute to a better understanding of the roles of VGCC subtypes in pain at a spinal level as well as development of a novel class of analgesics for chronic pain.

PKC independent inhibition of voltage gated calcium channels by volatile anesthetics in freshly isolated vascular myocytes from the aorta

In this study we used barium currents through voltage gated L-type calcium channels (recorded in freshly isolated cells with a conventional patch-clamp technique) to elucidate the cellular action mechanism for volatile anesthetics. It was found that halothane and isoflurane inhibited (dose-dependently and voltage independently) Ba2+ currents through voltage gated Ca2+ channels. Half maximal inhibitions occurred at 0.64 ± 0.07 mM and 0.86 ± 0.1 mM. The Hill slope value was 2 for both volatile anesthetics, suggesting the presence of more than one interaction site. Current inhibition by volatile anesthetics was prominent over the whole voltage range without changes in the peak of the current voltage relationship. Intracellular infusion of the GDPβS (100 μM) together with staurosporine (200 nM) did not prevent the inhibitory effect of volatile anesthetics. Unlike pharmacological Ca2+ channel blockers, volatile anesthetics blocked Ca2+ channel currents at resting membrane potentials. In other words, halothane and isoflurane induced an 'initial block'. After the first 4-7 control pulses, the cells were left unstimulated and anesthetics were applied. The first depolarization after the pause evoked a Ca2+ channel current whose amplitude was reduced to 41 ± 3.4% and to 57 ± 4.2% of control values. In an analysis of the steady-state inactivation curve for voltage dependence, volatile anesthetics induced a negative shift of the 50% inactivation of the calcium channels. By contrast, the steepness factor characterizing the voltage sensitivity of the channels was unaffected. Unitary L-type Ca2+ channels blockade occurred under cell-attached configuration, suggesting a possible action of volatile anesthetics from within the intracellular space or from the part of the channel inside the lipid bilayer.

Side pockets provide the basis for a new mechanism of Kv channel-specific inhibition

Most known small-molecule inhibitors of voltage-gated ion channels have poor subtype specificity because they interact with a highly conserved binding site in the central cavity. Using alanine-scanning mutagenesis, electrophysiological recordings and molecular modeling, we have identified a new drug-binding site in Kv1.x channels. We report that Psora-4 can discriminate between related Kv channel subtypes because, in addition to binding the central pore cavity, it binds a second, less conserved site located in side pockets formed by the backsides of S5 and S6, the S4-S5 linker, part of the voltage sensor and the pore helix. Simultaneous drug occupation of both binding sites results in an extremely stable nonconducting state that confers high affinity, cooperativity, use-dependence and selectivity to Psora-4 inhibition of Kv1.x channels. This new mechanism of inhibition represents a molecular basis for the development of a new class of allosteric and selective voltage-gated channel inhibitors.

P/Q-type calcium channel modulators

P/Q-type calcium channels are high-voltage-gated calcium channels contributing to vesicle release at synaptic terminals. A number of neurological diseases have been attributed to malfunctioning of P/Q channels, including ataxia, migraine and Alzheimer's disease. To date, only two specific P/Q-type blockers are known: both are peptides deriving from the spider venom of Agelenopsis aperta, ω-agatoxins. Other peptidic calcium channel blockers with activity at P/Q channels are available, albeit with less selectivity. A number of low molecular weight compounds modulate P/Q-type currents with different characteristics, and some exhibit a peculiar bidirectional pattern of modulation. Interestingly, there are a number of therapeutics in clinical use, which also show P/Q channel activity. Because selectivity as well as the exact mode of action is different between all P/Q-type channel modulators, the interpretation of clinical and experimental data is complicated and needs a comprehensive understanding of their target profile. The situation is further complicated by the fact that information on potency varies vastly in the literature, which may be the result of different experimental systems, conditions or the splice variants of the P/Q channel. This review attempts to provide a comprehensive overview of the compounds available that affect the P/Q-type channel and should help with the interpretation of results of in vitro experiments and animal models. It also aims to explain some clinical observations by implementing current knowledge about P/Q channel modulation of therapeutically used non-selective drugs. Chances and challenges of the development of P/Q channel-selective molecules are discussed.

Ca²⁺ channels and praziquantel: a view from the free world

Targeting the cellular Ca(2+) channels and pumps that underpin parasite Ca(2+) homeostasis may realize novel antihelmintic agents. Indeed, the antischistosomal drug praziquantel (PZQ) is a key clinical agent that has been proposed to work in this manner. Heterologous expression data has implicated an action of PZQ on voltage-operated Ca(2+) channels, although the relevant in vivo target of this drug has remained undefined over three decades of clinical use. The purpose of this review is to bring new perspective to this issue by discussing the potential utility of free-living planarian flatworms for providing new insight into the mechanism of PZQ action. First, we discuss in vivo functional genetic data from the planarian system that broadly supports the molecular data collected in heterologous systems and the 'Ca(2+) hypothesis' of PZQ action. On the basis of these similarities we highlight our current knowledge of platyhelminth voltage operated Ca(2+) channels, their unique molecular pharmacology and the downstream functional PZQ interactome engaged by dysregulation of Ca(2+) influx that has potential to yield novel antischistosomal targets. Overall the broad dataset underscores a common theme of PZQ-evoked disruptions of Ca(2+) homeostasis in trematodes, cestodes and turbellarians, and showcases the utility of the planarian model for deriving insight into drug action and targets in parasitic flatworms.

Decrease in calcium concentration triggers neuronal retinoic acid synthesis during homeostatic synaptic plasticity

Blockade of synaptic activity induces homeostatic plasticity, in part by stimulating synthesis of all-trans retinoic acid (RA), which in turn increases AMPA receptor synthesis. However, the synaptic signal that triggers RA synthesis remained unknown. Using multiple activity-blockade protocols that induce homeostatic synaptic plasticity, here we show that RA synthesis is activated whenever postsynaptic Ca(2+) entry is significantly decreased and that RA is required for upregulation of synaptic strength under these homeostatic plasticity conditions, suggesting that Ca(2+) plays an inhibitory role in RA synthesis. Consistent with this notion, we demonstrate that both transient Ca(2+) depletion by membrane-permeable Ca(2+) chelators and chronic blockage of L-type Ca(2+)-channels induces RA synthesis. Moreover, the source of dendritic Ca(2+) entry that regulates RA synthesis is not specific because mild depolarization with KCl is sufficient to reverse synaptic scaling induced by L-type Ca(2+)-channel blocker. By expression of a dihydropyridine-insensitive L-type Ca(2+) channel, we further show that RA acts cell autonomously to modulate synaptic transmission. Our findings suggest that, in synaptically active neurons, modest "basal" levels of postsynaptic Ca(2+) physiologically suppress RA synthesis, whereas in synaptically inactive neurons, decreases in the resting Ca(2+) levels induce homeostatic plasticity by stimulating synthesis of RA that then acts in a cell-autonomous manner to increase AMPA receptor function.

Distinct properties of amlodipine and nicardipine block of the voltage-dependent Ca2+ channels Cav1.2 and Cav2.1 and the mutant channels Cav1.2/dihydropyridine insensitive and Cav2.1/dihydropyridine sensitive

The binding site within the L-type Ca(2+) channel Ca(v)1.2 for neutral dihydropyridines is well characterized. However, the contributions of the alkylamino side chains of charged dihydropyridines such as amlodipine and nicardipine to channel block are not clear. We tested the hypothesis that the distinct locations of the charged side chains on amlodipine and nicardipine would confer distinct properties of channel block by these two drugs. Using whole-cell voltage clamp, we investigated block of wild type Ca(v) 2.1, wild type Ca(v)1.2, and Ca(v)1.2/Dihydropyridine insensitive, a mutant channel insensitive to neutral DHPs, by amlodipine and nicardipine. The potency of nicardipine and amlodipine for block of closed (stimulation frequency of 0.05 Hz) Ca(v)1.2 channels was not different (IC(50) values of 60 nM and 57 nM, respectively), but only nicardipine block was enhanced by increasing the stimulation frequency to 1 Hz. The frequency-dependent block of Ca(v)1.2 by nicardipine is the result of a strong interaction of nicardipine with the inactivated state of Ca(v)1.2. However, nicardipine block of Ca(v)1.2/Dihydropyridine insensitive was much more potent than block by amlodipine (IC(50) values of 2.0 μM and 26 μM, respectively). A mutant Ca(v)2.1 channel containing the neutral DHP binding site (Ca(v)2.1/Dihydropyridine sensitive) was more potently blocked by amlodipine (IC(50)=41 nM) and nicardipine (IC(50)=175 nM) than the parent Ca(v)2.1 channel. These data suggest that the alkylamino group of nicardipine and amlodipine project into distinct regions of Ca(v)1.2 such that the side chain of nicardipine, but not amlodipine, contributes to the potency of closed-channel block, and confers frequency-dependent block.

Interaction of diltiazem with an intracellularly accessible binding site on Ca(V)1.2

BACKGROUND AND PURPOSE

Diltiazem inhibits Ca_V1.2 channels and is widely used in clinical practice to treat cardiovascular diseases. Binding determinants for diltiazem are located on segments IIIS6, IVS6 and the selectivity filter of the pore forming α₁ subunit of Ca_V1.2. The aim of the present study was to clarify the location of the diltiazem binding site making use of its membrane-impermeable quaternary derivative d-cis-diltiazem (qDil) and mutant α₁ subunits.

EXPERIMENTAL APPROACH

Ca_V1.2 composed of α1, α2-δ and β2a subunits were expressed in tsA-201 cells and barium currents through Ca_V1.2 channels were recorded using the patch clamp method in the whole cell configuration. qDil was synthesized and applied to the intracellular side (via the patch pipette) or to the extracellular side of the membrane (by bath perfusion).

KEY RESULTS

Quaternary derivative d-cis-diltiazem inhibited Ca_V1.2 when applied to the intracellular side of the membrane in a use-dependent manner (59 ± 4% at 300 µM) and induced only a low level of tonic (non-use-dependent) block (16 ± 2% at 300 µM) when applied to the extracellular side of the membrane. Mutations in IIIS6 and IVS6 that have previously been shown to reduce the sensitivity of Ca_V1.2 to tertiary diltiazem also had reduced sensitivity to intracellularly applied qDil.

CONCLUSION AND IMPLICATIONS

The data show that use-dependent block of in Ca_V1.2 by diltiazem occurs by interaction with a binding site accessible via a hydrophilic route from the intracellular side of the membrane.

Inhibition of recombinant L-type voltage-gated calcium channels by positive allosteric modulators of GABAA receptors

Benzodiazepines (BDZs) depress neuronal excitability via positive allosteric modulation of inhibitory GABA(A) receptors (GABA(A)R). BDZs and other positive GABA(A)R modulators, including barbiturates, ethanol, and neurosteroids, can also inhibit L-type voltage-gated calcium channels (L-VGCCs), which could contribute to reduced neuronal excitability. Because neuronal L-VGCC function is up-regulated after long-term GABA(A)R modulator exposure, an interaction with L-VGCCs may also play a role in physical dependence. The current studies assessed the effects of BDZs (diazepam, flurazepam, and desalkylflurazepam), allopregnanolone, pentobarbital, and ethanol on whole-cell Ba(2+) currents through recombinant neuronal Ca(v)1.2 and Ca(v)1.3 L-VGCCs expressed with β(3) and α(2)δ-1 in HEK293T cells. Allopregnanolone was the most potent inhibitor (IC(50), ∼10 μM), followed by BDZs (IC(50), ∼50 μM), pentobarbital (IC(50), 0.3-1 mM), and ethanol (IC(50), ∼300 mM). Ca(v)1.3 channels were less sensitive to pentobarbital inhibition than Ca(v)1.2 channels, similar to dihydropyridine (DHP) L-VGCC antagonists. All GABA(A)R modulators induced a negative shift in the steady-state inactivation curve of Ca(v)1.3 channels, but only BDZs and pentobarbital induced a negative shift in Ca(v)1.2 channel inactivation. Mutation of the high-affinity DHP binding site (T1039Y and Q1043M) in Ca(v)1.2 channels reduced pentobarbital potency. Despite the structural similarity between benzothiazepines and BDZs, mutation of an amino acid important for diltiazem potency (I1150A) did not affect diazepam potency. Although L-VGCC inhibition by BDZs occurred at concentrations that are possibly too high to be clinically relevant and is not likely to play a role in the up-regulation of L-VGCCs during long-term treatment, pentobarbital and ethanol inhibited L-VGCCs at clinically relevant concentrations.

Alternative splicing modulates diltiazem sensitivity of cardiac and vascular smooth muscle Ca(v)1.2 calcium channels

BACKGROUND AND PURPOSE

As a calcium channel blocker, diltiazem acts mainly on the voltage-gated calcium channels, Ca(v)1.2, for its beneficial effects in cardiovascular diseases such as hypertension, angina and/or supraventricular arrhythmias. However, the effects of diltiazem on different isoforms of Ca(v)1.2 channels expressed in heart and vascular smooth muscles remain to be investigated. Here, we characterized the effects of diltiazem on the splice variants of Ca(v)1.2 channels, predominant in cardiac and vascular smooth muscles.

EXPERIMENTAL APPROACH

Cardiac and smooth muscle isoforms of Ca(v)1.2 channels were expressed in human embryonic kidney cells and their electrophysiological properties were characterized using whole-cell patch-clamp techniques.

KEY RESULTS

Under closed-channel and use-dependent block (0.03 Hz), cardiac splice variant Ca(v)1.2CM was less sensitive to diltiazem than two major smooth muscle splice variants, Ca(v)1.2SM and Ca(v)1.2b. Ca(v)1.2CM has a more positive half-inactivation potential than the smooth muscle channels, and diltiazem shifted it less to negative potential. Additionally, the current decay was slower in Ca(v)1.2CM channels. When we modified alternatively spliced exons of cardiac Ca(v)1.2CM channels into smooth muscle exons, we found that all three loci contribute to the different diltiazem sensitivity between cardiac and smooth muscle splice isoforms.

CONCLUSIONS AND IMPLICATIONS

Alternative splicing of Ca(v)1.2 channels modifies diltiazem sensitivity in the heart and blood vessels. Gating properties altered by diltiazem are different in the three channels.

Cav1.2 and Cav1.3 are differentially coupled to glucagon-like peptide-1 potentiation of glucose-stimulated insulin secretion in the pancreatic beta-cell line INS-1

The incretin peptides, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), potentiate glucose-stimulated insulin secretion (GSIS) and beta-cell proliferation and differentiation. Ca(2+) influx via voltage-gated L-type Ca(2+) channels is required for GLP-1 and GIP potentiation of GSIS. We investigated the role of the L-type Ca(2+) channels Ca(v)1.2 and Ca(v)1.3 in mediating GLP-1- and GIP-stimulated events in INS-1 cells and INS-1 cell lines expressing dihydropyridine-insensitive (DHPi) mutants of either Ca(v)1.2 or Ca(v)1.3. Ca(v)1.3/DHPi channels supported full potentiation of GSIS by GLP-1 (50 nM) compared with untransfected INS-1 cells. However, GLP-1-potentiated GSIS mediated by Ca(v)1.2/DHPi channels was markedly reduced compared with untransfected INS-1 cells. In contrast, GIP (10 nM) potentiation of GSIS mediated by both Ca(v)1.2/DHPi and Ca(v)1.3/DHPi channels was similar to that observed in untransfected INS-1 cells. Disruption of intracellular Ca(2+) release with thapsigargin, ryanodine, or 2-aminoethyldiphenylborate and inhibition of protein kinase A (PKA) or protein kinase C (PKC) significantly reduced GLP-1 potentiation of GSIS by Ca(v)1.3/DHPi channels and by endogenous L-type channels in INS-1 cells, but not by Ca(v)1.2/DHPi channels. Inhibition of glucose-stimulated phospholipase C activity with 1-(6-((17b-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122) did not inhibit potentiation of GSIS by GLP-1 in INS-1 cells. In contrast, wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and 2'-amino-3'-methoxyflavone (PD98059), an inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase, both markedly inhibited GLP-1 potentiation of GSIS by endogenous channels in INS-1 cells and Ca(v)1.3/DHPi channels, but not by Ca(v)1.2/DHPi channels. Thus, Ca(v)1.3 is preferentially coupled to GLP-1 potentiation of GSIS in INS-1 cells via a mechanism that requires intact intracellular Ca(2+) stores, PKA and PKC activity, and activation of ERK1/2.

L-type voltage-dependent calcium channel antagonists impair perirhinal long-term recognition memory and plasticity processes

The perirhinal cortex of the temporal lobe is essential for the familiarity discrimination component of recognition memory. In view of the importance of changes in calcium ion concentration for synaptic plasticity, the present study examined the effects of L-type voltage-dependent calcium channel (VDCC) antagonism on rat perirhinal-based familiarity discrimination processes and plasticity including long-term depression (LTD), long-term potentiation (LTP), and depotentiation. Single doses of three different types of L-type VDCC antagonists, verapamil, diltiazem, and nifedipine, administered systemically, or verapamil administered locally into the perirhinal cortex, impaired acquisition of long-term (24 h) but not shorter-term (20 min) recognition memory. L-type VDCC antagonism also disrupted memory retrieval after 24 h but not 20 min. Differential neuronal activation produced by viewing novel or familiar visual stimuli was measured by Fos expression. L-type VDCC antagonism by verapamil in perirhinal cortex during memory acquisition disrupted the normal pattern of differential Fos expression, so paralleling the antagonist-induced memory impairment. In slices of perirhinal cortex maintained in vitro, verapamil was without effect on baseline excitability or LTP but blocked LTD and depotentiation. The consistency of effects across the behavioral and cellular levels of analysis provides strong evidence for the involvement of perirhinal L-type VDCCs in long-term recognition memory processes.

Effects of phenylalkylamines and benzothiazepines on Cav1.3-mediated Ca2+ currents in neonatal mouse inner hair cells

Calcium currents (I(Ca)) in inner hair cells (IHCs) are carried by the Ca(v)1.3 subtype of L-type calcium channels. They play an important role in synaptic transmission of sound-evoked mechanical stimuli. L-type calcium channels are targets of the organic blocker classes dihydropyridines, phenylalkylamines and benzothiazepines. Previously a low sensitivity of the Ca(v)1.3 subtype towards dihydropyridines has been demonstrated. Therefore, this study evaluates the effect of two phenylalkylamines (verapamil and gallopamil) and the benzothiazepine diltiazem on I(Ca) through Ca(v)1.3 channels in mouse IHCs. Whole-cell I(Ca) was measured using the patch-clamp technique in mouse IHCs aged postnatal day 3-7 with 5 mM calcium as a charge carrier. The phenylalkylamines verapamil and gallopamil and the benzothiazepine diltiazem inhibited I(Ca) in IHCs in a concentration-dependent manner. This block was largely reversible. Dose-response curves revealed IC(50) values of 199+/-19 microM for verapamil, 466+/-151 microM for gallopamil and 326+/-67 microM for diltiazem. The inhibition of peak I(Ca) by phenylalkylamines and benzothiazepines was voltage-independent. Verapamil (300 microM) enhanced current inactivation from -20 to +20 mV while diltiazem (300 microM) did so only at very depolarised potentials (+20 mV). In conclusion, the concentrations of phenylalkylamines and benzothiazepine necessary to inhibit 50% of I(Ca) in IHCs were one order larger compared to concentrations which inhibited I(Ca) through Ca(v)1.2 channels in native cells or expression systems. However, inhibitory concentrations were in the same range as those required for block of I(Ca) in turtle hair cells.

Adenoviral-mediated expression of dihydropyridine-insensitive L-type calcium channels in cardiac ventricular myocytes and fibroblasts

Cardiac voltage-gated Ca2+ channels regulate the intracellular Ca2+ concentration and are therefore essential for muscle contraction, second messenger activation, gene expression and electrical signaling. As a first step in accessing the structural versus functional properties of the L-type Ca2+ channel in the heart, we have expressed a dihydropyridine (DHP)-insensitive CaV1.2 channel in rat ventricular myocytes and fibroblasts. Following isolation and culture, cells were infected with adenovirus expressing either LacZ or a mutant CaV1.2 channel (CaV1.2DHPi) containing the double mutation (T1039Y & Q1043M). This mutation renders the channel insensitive to neutral DHP compounds such as nisoldipine. The whole-cell, L-type Ca2+ current (ICa) measured in control myocytes was inhibited in a concentration-dependent manner by nisoldipine with an IC50 of 66 nM and complete block at 250 nM. In contrast, ICa in cells infected with AdCaV1.2DHPi was inhibited by only 35% by 500 nM nisoldipine but completely blocked by 50 microM diltiazem. In order to study CaV1.2DHPi in isolation, myocytes infected with AdCaV1.2DHPi were incubated with nisoldipine. Under this condition the cells expressed a large ICa (12 pA/pF) and displayed Ca2+ transients during field stimulation. Furthermore, addition of 2 microM forskolin and 100 microM 3-isobutyl-1-methylxanthine (IBMX), to stimulate protein kinase A, strongly increased IBa in the AdCaV1.2DHPi-infected cells. A Cd2+-sensitive IBa was also recorded in cardiac fibroblasts infected with AdCaV1.2DHPi. Thus, expression of CaV1.2DHPi will provide an important tool in studies of cardiac myocyte and fibroblast function.

Calcium-channel modulators for cardiovascular disease

It is generally accepted that hypertension doubles the risk of cardiovascular disease, of which coronary heart disease is the most common and lethal. Hypertension is a predisposing factor for the development of stroke, peripheral arterial disease, heart failure and end-state renal disease. Atherosclerosis-causing coronary heart disease is related to the severity of hypertension. Inhibition of calcium entry reduces the active tone of vascular smooth muscle and produces vasodilatation. This pharmacological action has been the basis for the use of calcium-channel blockers (CCBs) for the management of hypertension. Other drug families may achieve this: diuretics, beta-blockers, angiotensin-converting enzyme inhibitors, angiotensin-receptor antagonists. Cardiovascular hypertrophy and atherosclerosis are major complications related to high blood pressure. Cardiac hypertrophy is considered as an independent risk factor associated with abnormalities of diastolic function and can result in heart failure. Atherosclerosis is associated with activation of innate immunity. Atherosclerosis is expressing itself not only as coronary heart disease, but as a cerebrovascular and peripheral arterial disease. By impairing physiological vasomotor function, atherosclerosis includes ultimately necrosis of myocardium. CCBs reduce blood pressure. Do they prevent the progress of the main complications of hypertension? This major question is the matter of the present paper.

Effects of voltage-gated calcium channel subunit genes on calcium influx in cultured C. elegans mechanosensory neurons

Voltage-gated calcium channels (VGCCs) serve as a critical link between electrical signaling and diverse cellular processes in neurons. We have exploited recent advances in genetically encoded calcium sensors and in culture techniques to investigate how the VGCC alpha1 subunit EGL-19 and alpha2/delta subunit UNC-36 affect the functional properties of C. elegans mechanosensory neurons. Using the protein-based optical indicator cameleon, we recorded calcium transients from cultured mechanosensory neurons in response to transient depolarization. We observed that in these cultured cells, calcium transients induced by extracellular potassium were significantly reduced by a reduction-of-function mutation in egl-19 and significantly reduced by L-type calcium channel inhibitors; thus, a main source of touch neuron calcium transients appeared to be influx of extracellular calcium through L-type channels. Transients did not depend directly on intracellular calcium stores, although a store-independent 2-APB and gadolinium-sensitive calcium flux was detected. The transients were also significantly reduced by mutations in unc-36, which encodes the main neuronal alpha2/delta subunit in C. elegans. Interestingly, while egl-19 mutations resulted in similar reductions in calcium influx at all stimulus strengths, unc-36 mutations preferentially affected responses to smaller depolarizations. These experiments suggest a central role for EGL-19 and UNC-36 in excitability and functional activity of the mechanosensory neurons.

Differential modulation of Cav1.2 and Cav1.3-mediated glucose-stimulated insulin secretion by cAMP in INS-1 cells: distinct roles for exchange protein directly activated by cAMP 2 (Epac2) and protein kinase A

Using insulin-secreting cell line (INS)-1 cells stably expressing dihydropyridine-insensitive mutants of either Cav1.2 or Cav1.3, we previously demonstrated that Cav1.3 is preferentially coupled to insulin secretion and [Ca2+]i oscillations stimulated by 11.2 mM glucose. Using the same system, we found that insulin secretion in 7.5 mM glucose plus 1 mM 8-bromo-cAMP (8-Br-cAMP) is mediated by both Cav1.2 and Cav1.3. Treatment of INS-1 cells or INS-1 cells stably expressing Cav1.2/dihydropyridine-insensitive (DHPi) channels in the presence of 10 microM nifedipine, with effector-specific cAMP analogs 8-(4-chlorophenylthio)-2'-O-methyladenosine-cAMP [8-pCPT-2'-O-Me-cAMP; 100 microM; Exchange Protein directly Activated by cAMP 2 (Epac2)-selective] or N6-benzoyl-cAMP [50 microM; Protein Kinase A (PKA)-selective] partially increased insulin secretion. Secretion stimulated by a combination of the two cAMP analogs was additive and comparable with that stimulated by 1 mM 8-Br-cAMP. In INS-1 cells stably expressing Cav1.3/DHPi in the presence of 10 microM nifedipine, N6-benzoyl-cAMP, but not 8-pCPT-2'-O-Me-cAMP, significantly increased glucose-stimulated insulin secretion. However, the combination of N6-benzoyl-cAMP and 8-pCPT-2'-O-Me-cAMP significantly increased glucose-stimulated secretion compared with N6-benzoyl-cAMP alone. In INS-1 cells, 8-Br-cAMP potentiation of insulin secretion in 7.5 mM glucose is blocked by thapsigargin (1 microM) and ryanodine (0.5 microM). In contrast, ryanodine has no effect on insulin secretion or [Ca2+]i oscillations stimulated by 11.2 mM glucose in INS-1 cells. Our data suggest that both Cav1.2 and Cav1.3 mediate insulin secretion stimulated by 7.5 mM glucose and cAMP via a mechanism that requires internal stores of Ca2+. Furthermore, cAMP modulation of secretion mediated by Cav1.2 seems to involve both Epac2 and PKA independently. In contrast, cAMP modulation of Cav1.3-mediated secretion depends upon PKA activation, whereas the contribution of Epac2 is dependent upon PKA activation.

Towards selective antagonists of T-type calcium channels: design, characterization and potential applications of NNC 55-0396

NNC 55-0396 is a structural analog of mibefradil (Ro 40-5967) that inhibits both T-type and high-voltage-activated (HVA) Ca2+ channels with a higher selectivity for T-type Ca2+ channels. The inhibitory effect of mibefradil on HVA Ca2+ channels can be attributed to a hydrolyzed metabolite of the drug: the methoxy acetate side chain of mibefradil is removed by intracellular enzymes, thus it forms (1S,2S)-2-(2-(N-[(3-benzoimidazol-2-yl)propyl]-N-methylamino)ethyl)-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-naphtyl hydroxy dihydrochloride (dm-mibefradil), which causes potent inhibition of HVA Ca2+ currents. By replacing the methoxy acetate chain of mibefradil with cyclopropanecarboxylate, a more stable analog was developed (NNC 55-0396). The acute IC50 of NNC 55-0396 to block recombinant Cav3.1 T-type channels expressed in HEK293 cells is approximately 7 muM, whereas 100 microM NNC 55-0396 has no detectable effect on high voltage-activated currents in INS-1 cells. Block of T-type Ca2+ current was partially reduced by membrane hyperpolarization and was enhanced at high stimulus frequency. Washing NNC 55-0396 out of the recording chamber did not reverse the T-type Ca2+ current activity, suggesting that the compound dissolves in or passes through the plasma membrane to exert its effect; however, intracellular perfusion of the compound did not block T-type Ca2+ currents, arguing against a cytoplasmic route of action. We conclude that NNC 55-0396, by virtue of its modified structure, does not produce the metabolite that causes inhibition of L-type Ca2+ channel channels, thus rendering it more selective to T-type Ca2+ channels.

A Cell-Sparing Electric Field Stimulation Technique for High-Throughput Screening of Voltage-Gated Ion Channels

The Trans Cell Layer Electrical Field Stimulation (TCL-EFS) system has been developed for high-throughput screening (HTS) of voltage-gated ion channels in microplate format on a Voltage-Ion Probe Reader (VIPR) platform. In this design, a wire electrode is placed above the cell layer of each filter well, and a whole plate perimeter electrode resides beneath the filter layer. This configuration allows the electrodes to be placed away from the cell layer to minimize the near electrode field effects on cell function and dye bleaching observed with other existing designs. Mathematical simulation indicates that the electric field at the cell layer becomes uniform as the top electrode is raised to a position near the surface of the solution in the well. Using the TCL-EFS system and membrane potential fluorescence resonance energy transfer (FRET) dyes, the sensitivity of voltage-gated sodium channels to tetrodotoxin and other channel inhibitors was found to be similar to those determined by established electrophysiological and more conventional VIPR techniques. A good correlation was also observed with the TCL-EFS system for inhibition of Cav2.2 by omega-conotoxin-GVIA and for block of Cav1.2 by known small molecule inhibitors. Thus, the TCLEFS system is suitable for both quantitative analysis and HTS of voltage-gated sodium and calcium channels, without the liabilities of previously reported EFS methodologies.

Molecular regulation of voltage-gated Ca2+ channels

Voltage-gated Ca2+ (Ca(v)) channels are found in all excitable cells and many nonexcitable cells, in which they govern Ca2+ influx, thereby contributing to determine a host of important physiological processes including gene transcription, muscle contraction, hormone secretion, and neurotransmitter release. The past years have seen some significant advances in our understanding of the functional, pharmacological, and molecular properties of Ca(v) channels. Molecular studies have revealed that several of these channels are oligomeric complexes consisting of an ion-conducting alpha1 subunit and auxiliary alpha2delta, beta, and gamma subunits. In addition, cloning of multiple Ca(v) channel alpha1 subunits has offered the opportunity to investigate the regulation of these proteins at the molecular level. The regulation of Ca(v) channels by intracellular second messengers constitutes a key mechanism for controlling Ca2+ influx. This review summarizes recent advances that have provided important clues to the underlying molecular mechanisms involved in the regulation of Ca(v) channels by protein phosphorylation, G-protein activation, and interactions with Ca(2+)-binding and SNARE proteins.

Differential binding properties of [3H]dextrorphan and [3H]MK-801 in heterologously expressed NMDA receptors

The N-methyl-D-aspartate receptor (NMDAR) antagonists: MK-801, phencyclidine and ketamine are open-channel blockers with limited clinical value due to psychotomimetic effects. Similarly, the psychotomimetic effects of the dextrorotatory opioids, dextromethorphan and its metabolite dextrorphan, derive from their NMDAR antagonist actions. Differences in the use dependency of blockade, however, suggest that the binding sites for MK-801 and dextrorphan are distinct. In the absence of exogenous glutamate and glycine, the rate of association of [3H]MK-801 with wild-type NR1-1a/NR2A receptors was considerably slower than that for [3H]dextrorphan. Glutamate individually, and in the presence of the co-agonist glycine, had substantial effects on the specific binding of [3H]MK-801, while the binding of [3H]dextrorphan was not affected. Mutation of residues N616 and A627 in the NR1 subunit had a profound effect on [3H]MK-801 binding affinity, while that of [3H]dextrorphan was unaltered. In contrast, NR1 residues, W611 and N812, were critical for specific binding of [3H]dextrorphan to NR1-1a/NR2A complexes with no corresponding influence on that of [3H]MK-801. Thus, [3H]dextrorphan and [3H]MK-801 have distinct molecular determinants for high-affinity binding. The ability of [3H]dextrorphan to bind to a closed channel, moreover, indicates that its recognition site is shallower in the ion channel domain than that of MK-801 and may be associated with the extracellular vestibule of the NMDAR.

Molecular determinants of frequency dependence and Ca2+ potentiation of verapamil block in the pore region of Cav1.2

Verapamil block of Ca(v)1.2 is frequency-dependent and potentiated by Ca(2+). We examined the molecular determinants of these characteristics using mutations that effect Ca(2+) interactions with Ca(v)1.2. Mutant and wild-type Ca(v)1.2 channels were transiently expressed in tsA 201 cells with beta(1b) and alpha(2)delta subunits. The four conserved glutamates that compose the Ca(2+) selectivity filter in Ca(v)1.2 were mutated to Gln (E363Q, E709Q, E1118Q, E1419Q) and the adjacent conserved threonine in each domain was mutated to Ala (T361A, T707A, T1116A, T1417A). The L-type-specific residues in the domain III pore region (F1117G) and the C-terminal tail (I1627A) were also mutated and assayed for block by verapamil using whole-cell voltage-clamp recordings in 10 mM Ba(2+) or 10 mM Ca(2+). In Ba(2+), none of the pore-region mutations reduced the fraction of current blocked by 30 microM verapamil at 0.05 Hz stimulation. However, all of the pore-region mutations abolished Ca(2+) potentiation of verapamil block at 0.05 Hz. The T1116A, F1117G, E1118Q, and E1419Q mutations all significantly reduced frequency-dependent verapamil block (1-Hz stimulation) in both Ba(2+) and Ca(2+). The I1627A mutation, which disrupts Ca(2+)-dependent inactivation, increased the fraction of closed channels blocked by 30 microM verapamil in Ba(2+) but did not affect frequency-dependent block in Ba(2+) or Ca(2+). Our data suggest that the pore region of domain III may contribute to a high affinity verapamil binding site accessed during 1-Hz stimulation and that Ca(2+) binding to multiple sites may be required for potentiation of verapamil block of closed channels.

L-type Ca2+ channel antagonists block voltage-dependent Ca2+ channels in identified leech neurons

We investigated the effect of L-type Ca2+ channel antagonists on the Ca2+ influx through voltage-gated Ca2+ channels in leech Retzius, Leydig, AP, AE, P, and N neurons. The efficacy of the antagonists was quantified by monitoring their effect on the increase in the intracellular free Ca2+ concentration ([Ca2+]i; measured by Fura-2) that was induced by depolarizing the cell membrane by raising the extracellular K+ concentration. This K+-induced [Ca2+]i increase was blocked by the phenylalkylamines verapamil, gallopamil, and devapamil, the benzothiazepine diltiazem, as well as by the 1,4-dihydropyridine nifedipine. The blocking effect of the three phenylalkylamines was similar, being most pronounced in P and N neurons and smaller in Leydig, Retzius, AP, and AE neurons. Contrastingly, diltiazem and nifedipine were similarly effective in the neurons investigated, whereby their efficacy was like that of the phenylalkylamines in Retzius, Leydig, AP, and AE neurons. Depending on cell type and blocking agent, the concentrations necessary to suppress the K+-induced [Ca2+]i increase by 50% were estimated to vary between 5 and 190 microM. At high concentrations, the phenylalkylamines and diltiazem by themselves caused a marked [Ca2+]i increase in Leydig, P, and N neurons, which is probably due to activation of the caffeine-sensitive ion channels present in the plasma membrane of these cells. Together with previous observations, the results indicate a distant relationship of the voltage-gated Ca2+ channels present in many if not all leech neurons to vertebrate L-type Ca2+ channels.

Molecular determinants of Ca2+ potentiation of diltiazem block and Ca2+-dependent inactivation in the pore region of cav1.2

Diltiazem block of Cav1.2 is frequency-dependent and potentiated by Ca2+. We examined the molecular determinants of these characteristics using mutations that affect Ca2+ interactions with Cav1.2. Mutant and wild-type (WT) Cav1.2 channels were transiently expressed in tsA 201 cells with beta1b and alpha2delta subunits. The four conserved glutamates that compose the Ca2+ selectivity filter in Cav1.2 were mutated to Gln (E363Q, E709Q, E1118Q, E1419Q), and each single mutant was assayed for block by diltiazem using whole-cell voltage-clamp recordings in either 10 mM Ba2+ or 10 mM Ca2+. In Ba2+, none of the mutations affected the potency of diltiazem block of closed channels (0.05 Hz stimulation). However, frequency-dependent block (1Hz stimulation) was eliminated in the mutant E1419Q (domain IV), which recovered more rapidly than WT channels from inactivated channel block. Potentiation of diltiazem block of closed Cav1.2 channels in Ca2+ was abolished in the E1118Q, F1117G (domain III), and E1419Q mutants. Frequency-dependent block in Ca2+ was reduced compared with WT Cav1.2 in the F1117G, E1118Q, and E1419Q mutants. The C-terminal tail IQ domain mutation I1627A, which disrupts Ca2+ dependent inactivation, enhanced diltiazem block of closed channels in Ba2+. We conclude that, in Ba2+, E1419 slows recovery from diltiazem block of depolarized Cav1.2 channels, but in Ca2+, E1118, E1419, and F1117 form a Ca2+ binding site that mediates the potentiation of diltiazem block of both closed and inactivated Cav1.2 channels. Furthermore, Ca2+-dependent inactivation, which is impaired in E709Q, E1118Q, E1419Q, and I1627A, is not required for Ca2+ potentiation of diltiazem block.

Inhibitory action of diltiazem on voltage-gated calcium channels in cone photoreceptors

The benzothiazepine, diltiazem, is commonly used as an inhibitor of vascular L-type Ca channels, and is a clinically important anti-anginal and antihypertensive medication. In the retina, diltiazem also inhibits cyclic-nucleotide gated (CNG) channels, including the cGMP-gated channels in photoreceptors, and has been suggested to be a neuroprotectant in an animal model of retinitis pigmentosa, a degenerative disease of photoreceptors. In contrast to CNG channels, the actions of diltiazem on photoreceptor Ca channels have not been studied. We show that D-cis-diltiazem can block Ca channels in cone photoreceptors and that the potency and efficacy of cone photoreceptor Ca channel inhibition by this drug is unconventional. Over the concentration range of 5-500 microM diltiazem, the dose response curve was biphasic with a high affinity saturation level of approximately 30% block in the 20-50 microM range (IC(50)=4.9 microM) and a low affinity saturation block (near 100%) with concentrations up to 500 microM (IC(50)=100.4 microM). The degree of block was found to be equivalent when Bay K 8644 was used to increase Ca channel current, indicating that the levels of block do not result from multiple Ca channel subtypes having differing sensitivities to diltiazem. Calcium imaging showed that the relatively low efficacy of the high-affinity Ca channel block was not due to the species of charge-carrying divalent cation nor that it was associated with dialysis of cellular contents. These data contribute to an emerging perspective that the photoreceptor Ca channel has properties unique from other L-type channels, an important consideration should these channels become a target for testing putative neuroprotective therapies.

Ca v 1.3 is preferentially coupled to glucose-stimulated insulin secretion in the pancreatic beta-cell line INS-1

L-Type Ca(2+) channel blockers inhibit glucose and KCl-stimulated insulin secretion by pancreatic beta cells. However, the role of the two distinct L-type channels expressed by beta cells, Ca(v)1.2 and Ca(v)1.3, in this process is not clear. Therefore, we stably transfected INS-1 cells with two mutant channel constructs, Ca(v)1.2DHPi or Ca(v)1.3 DHPi. Whole-cell patch-clamp recordings demonstrated that both mutant channels are insensitive to dihydropyridines (DHPs), but are blocked by diltiazem. INS-1 cells expressing Ca(v)1.3/DHPi maintained glucose- and KCl-stimulated insulin secretion in the presence of DHPs, whereas cells expressing Ca(v)1.2/DHPi demonstrated DHP resistance to only KCl-induced secretion. INS-1 cells were also stably transfected with cDNAs encoding the intracellular loop between domains II and III of either Ca(v)1.2 or Ca(v)1.3 (Ca(v)1.2/II-III or Ca(v)1.3/II-III). Glucose- and KCl-stimulated insulin secretion in Ca(v)1.2/II-III cells were not different from untransfected INS-1 cells. However, glucose-stimulated insulin secretion was completely inhibited and KCl-stimulated secretion was substantially resistant to inhibition by DHPs, but sensitive to omega-agatoxin IVA in Ca(v)1.3/II-III cells. Moreover, the L-type channel agonist FPL 64176 markedly enhanced KCl-stimulated secretion by Ca(v)1.3/II-III cells. Together, our results suggest that Ca(2+) influx via Ca(v)1.3 is preferentially coupled to glucose-stimulated insulin secretion in INS-1 cells.

Pharmacological modulators of voltage-gated calcium channels and their therapeutical application

Calcium channels (CCs) play an important role in the transduction of action potential to the cytosol. An influx of Ca(2+) is essential for muscle contraction, neurotransmitter, and hormonal release. Level of cytosolic Ca(2+) controls activities of many enzymes and regulatory proteins. Voltage-gated calcium channels (VGCCs) serve as sensors for membrane depolarization. Blood pressure reduction is due to relaxation of actomyosine filaments in vascular smooth muscles. Calcium channel blockers (CCBs) are traditionally used for treatment of cardiovascular diseases. Neurotransmitter release from presynaptic neurons is triggered by Ca(2+) influx. Blockers of neuronal CCs may be applied for pain treatment. Overload of neurons by Ca(2+) is toxic. CCBs may be applied for prevention of some neurodegenerative disorders.