Mibefradil (Ro 40-5967): the first selective T-type Ca2+ channel blocker

Current Drug Development Overview: Targeting Voltage-Gated Calcium Channels for the Treatment of Pain

Voltage-gated calcium channels (VGCCs) are targeted to treat pain conditions. Since the discovery of their relation to pain processing control, they are investigated to find new strategies for better pain control. This review provides an overview of naturally based and synthetic VGCC blockers, highlighting new evidence on the development of drugs focusing on the VGCC subtypes as well as mixed targets with pre-clinical and clinical analgesic effects.

Structure-Function Studies of Sponge-Derived Compounds on the Cardiac CaV3.1 Channel

T-type calcium (Ca_V3) channels are involved in cardiac automaticity, development, and excitation-contraction coupling in normal cardiac myocytes. Their functional role becomes more pronounced in the process of pathological cardiac hypertrophy and heart failure. Currently, no Ca_V3 channel inhibitors are used in clinical settings. To identify novel T-type calcium channel ligands, purpurealidin analogs were electrophysiologically investigated. These compounds are alkaloids produced as secondary metabolites by marine sponges, and they exhibit a broad range of biological activities. In this study, we identified the inhibitory effect of purpurealidin I (1) on the rat Ca_V3.1 channel and conducted structure-activity relationship studies by characterizing the interaction of 119 purpurealidin analogs. Next, the mechanism of action of the four most potent analogs was investigated. Analogs 74, 76, 79, and 99 showed a potent inhibition on the Ca_V3.1 channel with IC₅₀'s at approximately 3 μM. No shift of the activation curve could be observed, suggesting that these compounds act like a pore blocker obstructing the ion flow by binding in the pore region of the Ca_V3.1 channel. A selectivity screening showed that these analogs are also active on hERG channels. Collectively, a new class of Ca_V3 channel inhibitors has been discovered and the structure-function studies provide new insights into the synthetic design of drugs and the mechanism of interaction with T-type Ca_V channels.

Central and peripheral contributions of T-type calcium channels in pain

Chronic pain is a severely debilitating condition that reflects a long-term sensitization of signal transduction in the afferent pain pathway. Among the key players in this pathway are T-type calcium channels, in particular the Cav3.2 isoform. Because of their biophysical characteristics, these channels are ideally suited towards regulating neuronal excitability. Recent evidence suggests that T-type channels contribute to excitability of neurons all along the ascending and descending pain pathways, within primary afferent neurons, spinal dorsal horn neurons, and within pain-processing neurons in the midbrain and cortex. Here we review the contribution of T-type channels to neuronal excitability and function in each of these neuronal populations and how they are dysregulated in chronic pain conditions. Finally, we discuss their molecular pharmacology and the potential role of these channels as therapeutic targets for chronic pain.

T-Type Calcium Channels: A Potential Novel Target in Melanoma

T-type calcium channels (TTCCs) are overexpressed in several cancers. In this review, we summarize the recent advances and new insights into TTCC biology, tumor progression, and prognosis biomarker and therapeutic potential in the melanoma field. We describe a novel correlation between the Cav3.1 isoform and the increased basal autophagy in BRAFV600E-mutant melanomas and after acquired resistance to BRAF inhibitors. Indeed, TTCC blockers reduce melanoma cell viability and migration/invasion in vitro and tumor growth in mice xenografts in both BRAF-inhibitor-sensitive and -resistant scenarios. These studies open a new, promising therapeutic approach for disseminated melanoma and improved treatment in BRAFi relapsed melanomas, but further validation and clinical trials are needed for it to become a real therapeutic option.

Human Olfactory Receptors: Novel Cellular Functions Outside of the Nose

Olfactory receptors (ORs) are not exclusively expressed in the olfactory sensory neurons; they are also observed outside of the olfactory system in all other human tissues tested to date, including the testis, lung, intestine, skin, heart, and blood. Within these tissues, certain ORs have been determined to be exclusively expressed in only one tissue, whereas other ORs are more widely distributed in many different tissues throughout the human body. For most of the ectopically expressed ORs, limited data are available for their functional roles. They have been shown to be involved in the modulation of cell-cell recognition, migration, proliferation, the apoptotic cycle, exocytosis, and pathfinding processes. Additionally, there is a growing body of evidence that they have the potential to serve as diagnostic and therapeutic tools, as ORs are highly expressed in different cancer tissues. Interestingly, in addition to the canonical signaling pathways activated by ORs in olfactory sensory neurons, alternative pathways have been demonstrated in nonolfactory tissues. In this review, the existing data concerning the expression, as well as the physiological and pathophysiological functions, of ORs outside of the nose are highlighted to provide insights into future lines of research.

CRMP2 and voltage-gated ion channels: potential roles in neuropathic pain

Neuropathic pain represents a significant and mounting burden on patients and society at large. Management of neuropathic pain, however, is both intricate and challenging, exacerbated by the limited quantity and quality of clinically available treatments. On this stage, dysfunctional voltage-gated ion channels, especially the presynaptic N-type voltage-gated calcium channel (VGCC) (Cav2.2) and the tetrodotoxin-sensitive voltage-gated sodium channel (VGSC) (Nav1.7), underlie the pathophysiology of neuropathic pain and serve as high profile therapeutic targets. Indirect regulation of these channels holds promise for the treatment of neuropathic pain. In this review, we focus on collapsin response mediator protein 2 (CRMP2), a protein with emergent roles in voltage-gated ion channel trafficking and discuss the therapeutic potential of targetting this protein.

CaV channels and cancer: canonical functions indicate benefits of repurposed drugs as cancer therapeutics

The importance of ion channels in the hallmarks of many cancers is increasingly recognised. This article reviews current knowledge of the expression of members of the voltage-gated calcium channel family (Ca_V) in cancer at the gene and protein level and discusses their potential functional roles. The ten members of the Ca_V channel family are classified according to expression of their pore-forming α-subunit; moreover, co-expression of accessory α2δ, β and γ confers a spectrum of biophysical characteristics including voltage dependence of activation and inactivation, current amplitude and activation/inactivation kinetics. Ca_V channels have traditionally been studied in excitable cells including neurones, smooth muscle, skeletal muscle and cardiac cells, and drugs targeting the channels are used in the treatment of hypertension and epilepsy. There is emerging evidence that several Ca_V channels are differentially expressed in cancer cells compared to their normal counterparts. Interestingly, a number of Ca_V channels also have non-canonical functions and are involved in transcriptional regulation of the expression of other proteins including potassium channels. Pharmacological studies show that Ca_V canonical function contributes to the fundamental biology of proliferation, cell-cycle progression and apoptosis. This raises the intriguing possibility that calcium channel blockers, approved for the treatment of other conditions, could be repurposed to treat particular cancers. Further research will reveal the full extent of both the canonical and non-canonical functions of Ca_V channels in cancer and whether calcium channel blockers are beneficial in cancer treatment.

T-Type Calcium Channel: A Privileged Gate for Calcium Entry and Control of Adrenal Steroidogenesis

Intracellular calcium plays a crucial role in modulating a variety of functions such as muscle contraction, hormone secretion, gene expression, or cell growth. Calcium signaling has been however shown to be more complex than initially thought. Indeed, it is confined within cell microdomains, and different calcium channels are associated with different functions, as shown by various channelopathies. Sporadic mutations on voltage-operated L-type calcium channels in adrenal glomerulosa cells have been shown recently to be the second most prevalent genetic abnormalities present in human aldosterone-producing adenoma. The observed modification of the threshold of activation of the mutated channels not only provides an explanation for this gain of function but also reminds us on the importance of maintaining adequate electrophysiological characteristics to make channels able to exert specific cellular functions. Indeed, the contribution to steroid production of the various calcium channels expressed in adrenocortical cells is not equal, and the reason has been investigated for a long time. Given the very negative resting potential of these cells, and the small membrane depolarization induced by their physiological agonists, low threshold T-type calcium channels are particularly well suited for responding under these conditions and conveying calcium into the cell, at the right place for controlling steroidogenesis. In contrast, high threshold L-type channels are normally activated by much stronger cell depolarizations. The fact that dihydropyridine calcium antagonists, specific for L-type channels, are poorly efficient for reducing aldosterone secretion either in vivo or in vitro, strongly supports the view that these two types of channels differently affect steroid biosynthesis. Whether a similar analysis is transposable to fasciculata cells and cortisol secretion is one of the questions addressed in the present review. No similar mutations on L-type or T-type channels have been described yet to affect cortisol secretion or to be linked to the development of Cushing syndrome, but several evidences suggest that the function of T channels is also crucial in fasciculata cells. Putative molecular mechanisms and cellular structural organization making T channels a privileged entry for the "steroidogenic calcium" are also discussed.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Genetic disruption of voltage-gated calcium channels in psychiatric and neurological disorders

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Voltage-gated calcium channel classification—genes and proteins.

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Genetic analysis of neuropsychiatric syndromes.

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Calcium channel genes identified from GWA studies of psychiatric disorders.

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Rare mutations in calcium channel genes in psychiatric disorders.

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Pathophysiological sequelae of CACNA1C mutations and polymorphisms.

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Monogenic disorders resulting from harmful mutations in other voltage-gated calcium channel genes.

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Changes in calcium channel gene expression in disease.

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Involvement of voltage-gated calcium channels in early brain development.

This review summarises genetic studies in which calcium channel genes have been connected to the spectrum of neuropsychiatric syndromes, from bipolar disorder and schizophrenia to autism spectrum disorders and intellectual impairment. Among many other genes, striking numbers of the calcium channel gene superfamily have been implicated in the aetiology of these diseases by various DNA analysis techniques. We will discuss how these relate to the known monogenic disorders associated with point mutations in calcium channels. We will then examine the functional evidence for a causative link between these mutations or single nucleotide polymorphisms and the disease processes. A major challenge for the future will be to translate the expanding psychiatric genetic findings into altered physiological function, involvement in the wider pathology of the diseases, and what potential that provides for personalised and stratified treatment options for patients.

Upregulation of T-type Ca2+ channels in primary sensory neurons in spinal nerve injury

STUDY DESIGN

Painful behavior testing, whole-cell patch clamp recordings, and PCR analysis were served to test the influence of T-type Ca channels in spinal nerve-injured rats.

OBJECTIVE

To determine the changes of T-type Ca channels in dorsal root ganglion (DRG) neurons of different sizes and the contribution to neuronal firing and painful behavior in neuropathic pain induced by nerve injury.

SUMMARY OF BACKGROUND DATA

T-type and high-voltage-activated Ca channels play an important role in the transmission of nociceptive signals, especially in neuronal hyperexcitability in neuropathic pain. However, little is known about how nerve injury affects T-type Ca channels in DRG neurons of different sizes.

METHODS

The effect of intrathecal administration of mibefradil in nerve-ligated rats was examined by painful behavior testing and current clamp. The changes of T-type Ca channels in DRG neurons caused by spinal nerve ligation were determined by RT-PCR analysis and voltage clamp.

RESULTS

Spinal nerve injury significantly increased current density of T-type Ca channels in small DRG neurons. In addition, nerve injury significantly increased the percentage of T-type Ca channels in medium and large DRG neurons. Nerve injury significantly increased the mRNA levels of Cav3.2 and Cav3.3 in DRGs. Block of T-type Ca channels on mibefradil administration significantly normalized painful behavior and hyperexcitability in neuronal firing in spinal nerve-injured rats.

CONCLUSION

Our study first indicated the upregulation of functional T-type Ca channels in DRG neurons of different sizes and the changes in different subtypes of T-type Ca channels by spinal nerve injury. Considering the effect of blocking T-type Ca channels in painful behavior and abnormal neuronal firing in rats with nerve injury, our results suggest that T-type Ca channels are potential therapeutic targets for the treatment of spinal nerve ligation-induced neuropathic pain.

Novel cognitive enhancer ST101 enhances acetylcholine release in mouse dorsal hippocampus through T-type voltage-gated calcium channel stimulation

We recently developed a novel cognitive enhancer, ST101 (spiro[imidazo[1,2-a]pyridine-3,2-indan]-2(3H)-one), that activates T-type voltage-gated calcium channels (VGCCs). Here, we address whether T-type VGCC activation with ST101 mediates its cognitive effects in vivo and the relevance of T-type VGCC activation to acetylcholine (ACh) release in the hippocampus. Acute intraperitoneal administration of ST101 (1 mg/kg, i.p.) improved memory-related behaviors in both olfactory bulbectomized (OBX) and scopolamine-treated mice. Effects of ST101 administration were abolished by both intraperitoneal and intracerebroventricular pre-administration of the T-type VGCC inhibitor mibefradil. Acute administration of ST101 enhanced basal and nicotine-induced ACh release in the dorsal hippocampus in both OBX and sham-treated mice. Enhanced ACh release was abolished by infusion with mibefradil (10 μM) but not with the L-type VGCC inhibitor nifedipine (10 μM). As expected, significantly reduced CaMKIIα, PKCα, and ERK phosphorylation was restored by acute ST101 administration in the OBX mouse hippocampal CA1 region. Enhancement of CaMKIIα and PKCα but not ERK phosphorylation was inhibited by mibefradil (20 mg/kg, i.p.) preadministration. Increased CaMKIIα and PKCα phosphorylation was confirmed by increased phosphorylation of GluR1, synapsin I, and NR1. Taken together, stimulation of T-type VGCCs is critical for the enhanced hippocampal ACh release and improved cognitive function seen following ST101 administration.

CRMP-2 peptide mediated decrease of high and low voltage-activated calcium channels, attenuation of nociceptor excitability, and anti-nociception in a model of AIDS therapy-induced painful peripheral neuropathy

Background

The ubiquity of protein-protein interactions in biological signaling offers ample opportunities for therapeutic intervention. We previously identified a peptide, designated CBD3, that suppressed inflammatory and neuropathic behavioral hypersensitivity in rodents by inhibiting the ability of collapsin response mediator protein 2 (CRMP-2) to bind to N-type voltage-activated calcium channels (CaV2.2) [Brittain et al. Nature Medicine 17:822–829 (2011)].

Results and discussion

Here, we utilized SPOTScan analysis to identify an optimized variation of the CBD3 peptide (CBD3A6K) that bound with greater affinity to Ca²⁺ channels. Molecular dynamics simulations demonstrated that the CBD3A6K peptide was more stable and less prone to the unfolding observed with the parent CBD3 peptide. This mutant peptide, conjugated to the cell penetrating motif of the HIV transduction domain protein TAT, exhibited greater anti-nociception in a rodent model of AIDS therapy-induced peripheral neuropathy when compared to the parent TAT-CBD3 peptide. Remarkably, intraperitoneal administration of TAT-CBD3A6K produced none of the minor side effects (i.e. tail kinking, body contortion) observed with the parent peptide. Interestingly, excitability of dissociated small diameter sensory neurons isolated from rats was also reduced by TAT-CBD3A6K peptide suggesting that suppression of excitability may be due to inhibition of T- and R-type Ca²⁺ channels. TAT-CBD3A6K had no effect on depolarization-evoked calcitonin gene related peptide (CGRP) release compared to vehicle control.

Conclusions

Collectively, these results establish TAT-CBD3A6K as a peptide therapeutic with greater efficacy in an AIDS therapy-induced model of peripheral neuropathy than its parent peptide, TAT-CBD3. Structural modifications of the CBD3 scaffold peptide may result in peptides with selectivity against a particular subset of voltage-gated calcium channels resulting in a multipharmacology of action on the target.

Impedance-based detection of beating rhythm and proarrhythmic effects of compounds on stem cell-derived cardiomyocytes

The xCELLigence real time cell analyzer Cardio system offers a new system for real-time cell analysis that measures impedance-based signals in a label-free noninvasive manner. The aim of this study was to test whether impedance readings are a useful tool to detect compound effects on beating frequency (beats per minute, bpm) and arrhythmias of human induced pluripotent stem cell- and a mouse embryonic stem cell-derived cardiomyocyte line (hiPSC-CM and mESC-CM, respectively). Baseline values for control wells were 45±3 and 179±6 bpm, respectively (n=6). Correspondingly, isoproterenol increased beating frequency by 77% and 71%, whereas carbachol decreased frequency by 11% and 100% (stopped in 5/6 mESC-CM wells). E-4031 decreased beating rate and caused arrhythmias in both cell types, however, more pronounced in the human iPSC-CMs. Amlodipine inhibited contractions in both models, and T-type calcium channel block strongly reduced beating rate and eventually stopped beating in mESC-CM but caused a smaller effect in hiPSC-CM. The results of this initial study show that, under the right conditions, the beating frequency of a monolayer of cells can be stably recorded over several days. Additionally, the system detects changes in beating frequency and amplitude caused by added reference compounds. This assay system has the potential to enable medium-throughput screening, but for implementation into routine daily work, extended validation, testing of additional batches of cardiomyocytes, and further assay optimization (e.g., frequency of media exchange, growth matrix, seeding density, age of cells after plating, and temperature control) will be needed.

Comparison of mibefradil and derivative NNC 55-0396 effects on behavior, cytochrome P450 activity, and tremor in mouse models of essential tremor

NNC 55-0396 [(1S,2S)-2-(2-(N-[(3-benzimidazol-2-yl)propyl]-N-methylamino)ethyl)-6-fluoro-1,2, 3,4-tetrahydro-1-isopropyl-2-naphtyl cyclopropanecarboxylate dihydrochloride], is a mibefradil derivative that retains potent in vitro T-type calcium channel antagonist efficacy. We compared the two compounds for behavioral toxicity, effects on cytochrome P450 activity, and efficacy against tremor in the γ-aminobutyric acid type A (GABAA) receptor subunit α1-null mouse, and the harmaline tremor model of essential tremor in wild-type mice. NNC 55-0396 was better tolerated than mibefradil in the horizontal wire test of sedation/motor function, with 3/6 failing at 300 and 30mg/kg respectively. To assess for a potential interaction with harmaline, mice were given the drugs, followed by harmaline or vehicle, and tested 30min later in the inverted wire grid test. Mibefradil exacerbated, whereas NNC 55-0396 ameliorated harmaline-induced test deficits. In mouse liver microsomes, NNC 55-0396 was a less potent inhibitor of harmaline O-demethylation than mibefradil (Ki: 0.95 and 0.29μM respectively), and also less potent at inhibiting testosterone 6-β-hydroxylation (Ki: 0.71 and 0.12μM respectively). In the GABAA α1-null model, NNC 55-0396 but not mibefradil, (each at 20mg/kg), suppressed tremor while NNC 55-0396 at 12.5mg/kg suppressed harmaline-induced tremor by half by 20-100min, whereas mibefradil at the same dose did not significantly affect tremor. In contrast to mibefradil, NNC 55-0396 is well tolerated and suppresses tremor, and exerts less cytochrome P450 inhibition. These results suggest potential clinical utility for NNC 55-0396 or similar derivatives as a T-type calcium antagonist.

Transient and big are key features of an invertebrate T-type channel (LCav3) from the central nervous system of Lymnaea stagnalis

Here we describe features of the first non-mammalian T-type calcium channel (LCa(v)3) expressed in vitro. This molluscan channel possesses combined biophysical properties that are reminiscent of all mammalian T-type channels. It exhibits T-type features such as "transient" kinetics, but the "tiny" label, usually associated with Ba(2+) conductance, is hard to reconcile with the "bigness" of this channel in many respects. LCa(v)3 is 25% larger than any voltage-gated ion channel expressed to date. It codes for a massive, 322-kDa protein that conducts large macroscopic currents in vitro. LCa(v)3 is also the most abundant Ca(2+) channel transcript in the snail nervous system. A window current at typical resting potentials appears to be at least as large as that reported for mammalian channels. This distant gene provides a unique perspective to analyze the structural, functional, drug binding, and evolutionary aspects of T-type channels.

Functional regulation of T-type calcium channels by s-nitrosothiols in the rat thalamus

Although T-type Ca(2+) channels in the reticular thalamic nucleus (nRT) have a central function in tuning neuronal excitability and are implicated in sensory processing, sleep, and epilepsy, the mechanisms involved in their regulation are poorly understood. Here we recorded T-type Ca(2+) currents from intact nRT neurons in brain slices from young rats and investigated the mechanisms of T-type channel modulation by S-nitrosothiols (SNOs). We found that extracellular application of S-nitrosoglutathione (GSNO), S-nitrosocysteine (CSNO) and S-nitroso-N-acetyl-penicillamin (SNAP) rapidly and reversibly reduced T-type currents. The effects of SNOs are strongly stereoselective at physiological concentrations: (L)-CSNO was fourfold more effective in inhibiting T-type current than was (D)-CSNO. The effects of GSNO were abolished if cells had been treated with free hemoglobin or N-ethylmaleimide, an irreversible alkylating agent but not by 8-bromoguanosine-3',5'-cyclomonophosphate sodium salt, a membrane-permeant cGMP analogue or 1H-(1,2,4) oxadiazolo (4,3-a) quinoxalin-1-one, a specific soluble guanylyl cyclase inhibitor. In addition, bath applications of GSNO inhibited T-type currents in nucleated outside-out patches and whole cell recordings to a similar extent, with minimal effect on cell-attached recordings, suggesting a direct effect of GSNO on putative extracellular thiol residues on T-type channels. Biophysical studies indicate that GSNO decreased the availability of T-type channels at physiological potentials by modifying gating and stabilizing inactive states of the channels. In current-clamp experiments, GSNO diminished the amplitude of low-threshold calcium spikes and frequency of spike firing with minimal effects on the passive membrane properties. Collectively, the results indicate that SNOs may be a class of endogenous agents that control the functional states of the thalamus.

Redox modulation of peripheral T-type Ca2+ channels in vivo: alteration of nerve injury-induced thermal hyperalgesia

We reported recently that redox agents, including the endogenous amino acid L-cysteine, modulate T-type Ca2+ currents in primary sensory neurons in vitro, and alter mechanical and thermal nociception in peripheral nociceptors in vivo in intact animals [Neuron 31 (2001) 75]. Here, we studied the effects of locally applied redox agents (L-cysteine and 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) on thermal hyperalgesia in animals with neuropathic pain due to chronic constrictive injury (CCI) of the sciatic nerve. We found that, following injection into the peripheral receptive fields, the endogenous reducing agent L-cysteine increased thermal hyperalgesia in a dose-dependent manner in rats with CCI of the sciatic nerve as well as in sham-operated rats. However, the magnitude of the increase was smaller and duration of effect was shorter in rats with CCI of the sciatic nerve compared to sham-operated animals. DTNB, an exogenous oxidizing agent, induced dose-dependent alleviation of thermal hyperalgesia in rats with CCI of the sciatic nerve and caused analgesia in sham-operated rats. DTNB completely blocked L-cysteine-induced thermal hyperalgesia in both animal groups. Mibefradil, a potent and preferential T-type Ca2+ channel blocker, abolished L-cysteine-induced increase in thermal hyperalgesia in both animal groups suggesting the involvement of T-type Ca2+ channels in peripheral nociception. These results indicate for the first time that redox modulation of T-type Ca2+ channels in rat peripheral nociceptors is operational in pain states caused by peripheral axonal injury. Since thermal hyperalgesia is a common symptom of axonal injury, locally applied oxidizing agents could be used as a novel treatment to ameliorate neuropathic pain.

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

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

Mechanical and thermal antinociception in rats following systemic administration of mibefradil, a T-type calcium channel blocker

Voltage-gated Ca(2+) channels play a crucial role in pain perception. We studied the antinociceptive potential of systemically administered mibefradil, a peripherally acting antihypertensive agent and preferential T-type Ca(2+) channel blocker. Systemic injections of mibefradil, in clinically relevant doses, induced mechanical and thermal antinociception in adult rats without compromising their sensorimotor abilities. This suggests that peripherally acting T-type Ca(2+) channel blockers can be useful antinociceptive agents.

Redox modulation of T-type calcium channels in rat peripheral nociceptors

Although T-type calcium channels were first described in sensory neurons, their function in sensory processing remains unclear. In isolated rat sensory neurons, we show that redox agents modulate T currents but not other voltage- and ligand-gated channels thought to mediate pain sensitivity. Similarly, redox agents modulate currents through Ca(v)3.2 recombinant channels. When injected into peripheral receptive fields, reducing agents, including the endogenous amino acid L-cysteine, induce thermal hyperalgesia. This hyperalgesia is blocked by the oxidizing agent 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) and the T channel antagonist mibefradil. DTNB alone and in combination with mibefradil induces thermal analgesia. Likewise, L-cysteine induces mechanical DTNB-sensitive hyperalgesia in peripheral receptive fields. These data strongly suggest a role for T channels in peripheral nociception. Redox sites on T channels in peripheral nociceptors could be important targets for agents that modify pain perception.

Energy-dependent accumulation of calcium antagonists in catecholamine storage vesicles

The calcium antagonists verapamil, nitrendipine, mibefradil, and amlodipine accumulate in chromaffin granule ghosts with apparent equilibrium partition coefficients [(mol/mg membrane lipid)/(mol/mg solvent water)] of 246 +/- 105 (N = 8), 2700 +/- 600 (N = 4), 7400 +/- 2200 (N = 4), and 8100 +/- 1100 (N = 5), respectively. In the presence of 1.2 mM MgATP, the partition coefficients were 854 +/- 206 (N = 10), 2300 +/- 600 (N = 4), 32,700 +/- 8,900 (N = 7), and 20,300 +/- 5,000 (N = 11) for verapamil, nitrendipine, mibefradil, and amlodipine, respectively. Except for nitrendipine, the apparent partition coefficients in the presence of MgATP were significantly different from the control (P < 0.001). For amlodipine and verapamil, the vacuolar H(+)-ATPase inhibitors bafilomycin A1 (30 nM) and N-ethylmaleimide (2 mM) and the protonophore (uncoupler) carbonyl cyanide m-chlorophenylhydrazone (CCCP, 10 microM) completely blocked the increase in partition coefficients in response to MgATP. The extra amlodipine, mibefradil, and verapamil that accumulated in response to MgATP were released into the medium by CCCP (10 microM) by 18% (N = 5), 30% (N = 5), and 88% (N = 5) for amlodipine, mibefradil, and verapamil, respectively. Thus, amlodipine, mibefradil, and verapamil, but not nitrendipine, accumulate in catecholamine storage vesicles in response to delta mu H+ generated by the endogenous V-type H(+)-ATPase, and are partially released by de-energetisation. Hence, these calcium antagonists can reach unexpectedly high concentrations in certain target cells, and give pharmacodynamic properties not shared by nitrendipine.

Voltage-gated T-type Ca2+ channels and heart failure

In the cardiovascular system, two types of voltage-gated Ca2+ channels are present: the L-type and the T-type. Under normal conditions, T-type Ca2+ channels are involved in the maintenance of vascular tone and cardiac automaticity but, since they are not present in contractile myocardial cells, they do not contribute significantly to myocardial contraction. In experimental models of cardiac hypertrophy, myocardial T-type Ca2+ channels are upregulated, which could contribute to the increased incidence of ventricular arrhythmia. In addition, T-type Ca2+ channels participate in the regulation of cell proliferation and neurohormonal secretion; through these pathways, T-type Ca2+ channels might participate in myocardial remodeling. The pathophysiological role of T-type Ca2+ channels in heart failure has been investigated using mibefradil, a Ca2+ antagonist that is 10-50 times more potent at blocking T-type than L-type Ca2+ channels. In contrast with classic L-type Ca2+ channel antagonists, miberfradil appears beneficial in many animal models of heart failure; in particular, it does not exert negative inotropic effects nor does it stimulate the neurohormonal system. Furthermore, in the Pfeffer rat model, blockade of T-type Ca2+ channels with mibefradil is associated with an improved survival rate. In humans, however, major metabolic drug interactions independent of T-type Ca2+ channel blockade made it impossible to determine the efficacy of mibefradil in treating heart failure; indeed, these interactions led to the withdrawal of the drug from the market.

Blockade of Ba2+ current through human alpha1E channels by two steroid analogs, (+)-ACN and (+)-ECN

Previous work suggests that different neuroactive steroids may exhibit some selectivity in their blocking effects on different high-voltage activated (HVA) Ca2+ currents. At least some of these effects appear to involve direct blocking actions on Ca2+ channels. Thus, direct investigation of the effects of various steroids on cloned Ca2+ channel variants may lead to the development of potent and selective small-molecular weight Ca2+ channel blockers. Here we examine the effects of two steroids on a cloned human alpha1E Ca2+ channel both with and without a beta3 subunit, when expressed in HEK293 cells. One compound, (+)-ACN, has been previously shown to block N-, Q-, and R-subtypes of HVA current without affecting L- and P-type current. The second compound, (+)-ECN, weakly blocks total HVA current in hippocampal neurons. (+)-ECN differs from (+)-ACN in lacking effects on GABA receptors, but shares with (+)-ACN an ability to partially inhibit T current in DRG neurons (Todorovic, S.M., Prakriya, M., Nakashima, Y.M. et al., 1998. Enantioselective blockade of T-type Ca2+ current in adult rat sensory neurons by a steroid lacking GABA-mimetic activity. Mol. Pharmacol. 54, 918-927). (+)-ACN can block 100% of Ba2+ current in HEK cells arising either from the alpha1E subunit (IC50 approximate to 10 microM) or the alpha1Ebeta3 combination (IC50 approximate to 5 microM), while (+)-ECN maximally blocks only about 80% of the alpha1E (10 microM) or alpha1Ebeta3 (16 microM) current. Blockade by (+)-ACN exhibits several differences from blockade by (+)-ECN. (+)-ACN increases the apparent rate of onset of inactivation, particularly for the alpha1E variant, slows recovery from inactivation, and more profoundly shifts the voltage-dependence of current availability for both alpha1E and alpha1Ebeta3 variants than does (+)-ECN. Although the complexity of the normal inactivation kinetics of alpha1E variants makes interpretation of the (+)-ACN-induced kinetic alterations difficult, the results suggest that the two steroids are to some extent acting by distinct mechanisms, and perhaps at different sites.

T- and L-type Ca2+-channel antagonists reduce contractility in guinea pig cardiac myocytes

The aim of this study was to investigate the influence of L- and T-type Ca2+-channel blockade on myocardial contractility in guinea pig cardiomyocytes. Left ventricular myocardium from guinea pig contains both L- and T-type Ca2+ channels. The T-type Ca2+ influx was inhibited with mibefradil (1-100 microM), a novel compound with a threefold higher affinity for T- compared with L-type Ca2+ channels. In comparison, L-type Ca2+ influx was reduced by the benzodiazepine diltiazem (1-100 microM). The effect of mibefradil and diltiazem on electrically driven (0.5 Hz) isolated cardiomyocytes (n = 12) was studied in a concentration-dependent manner. The change of the contraction amplitude (percentage of cell shortening) was continuously recorded with an one-dimensional high-speed camera. Both mibefradil and diltiazem concentration-dependently reduced (p < 0.05 vs. control) the contraction amplitude in isolated myocytes from guinea pig. The concentration at which the contraction amplitude of guinea pig cardiomyocytes was reduced by 50% (EC50) was 31.6 microM for diltiazem and 6.3 microM for mibefradil, indicating that the T-type Ca2+-channel blocker mibefradil is more potent in reducing contractility in guinea pig cardiac myocytes in comparison with the L-type Ca2+-channel antagonist diltiazem. Mean values for cell shortening in percentage +/- SEM for mibefradil (0, 1, 10, 100 microM) were 100%, 78 +/- 9.2%, 36 +/- 5.4%, and 24 +/- 3.6%. The corresponding values for diltiazem were 100%, 92 +/- 12.5%, 79 +/- 8.9%, and 35 +/- 2.6%. In contrast, the increase of the extracellular Ca2+ concentration (2-7.5 mM) resulted in a significant increase of the contraction amplitude (+213 +/- 14%). Therefore, blockade of the Ca2+ influx through voltage-dependent T- or L-type Ca2+ channels decreases contraction in isolated cardiac myocytes from guinea pigs containing L- and T-type Ca2+ channels.

Pharmacological properties of T-type Ca2+ current in adult rat sensory neurons: effects of anticonvulsant and anesthetic agents

We have used the whole cell patch-clamp method to study pharmacological properties of low-voltage-activated (LVA) Ca2+ current in freshly dissociated neurons from dorsal root ganglia of adult rats. Inward barium current [in the presence of internal fluoride to reduce L-type high-voltage-activated (HVA) and external 1 microM omega-conotoxin GVIA to block N-type HVA current- was evoked from negative holding potentials of -90 mV to test potentials of -25 mV and showed complete inactivation during 200-ms test pulses. Amiloride blocked approximately 90% of current with half-maximal block (EC50) of 75 microM and a Hill coefficient (n) of 0.99. LVA current was blocked completely by inorganic Ca2+ channel blockers: lanthanum (EC50 = 0. 53 microM) > zinc (EC50 = 11.3 microM) > cadmium (EC50 = 20 microM)> nickel (EC50 = 51 microM). The antiepileptics, ethosuximide (EC50 = 23.7 mM, n = 1.4), phenytoin (EC50 = 7.3 microM, n = 1.3), alpha-methyl-alpha-phenylsuccinimide (EC50 = 170 microM, n = 2.1), and valproic acid (EC50 = 330 microM, n = 1.9) maximally blocked approximately 100, 60, 26, and 17% of T current, respectively. Another antiepileptic, carbamazepine (</=100 microM), and convulsants such as pentylenetetrazole (1 mM) and tert-butyl-bicyclo [2.2.2] phosphorothionate (50 microM) had no effect on T current. Barbiturates completely blocked T current: thiopental (EC50 = 153 microM, n =1.2) > pentobarbital (EC50 = 334 microM, n = 1.2) > methohexital (EC50 = 502 microM, n = 1.3) > phenobarbital (EC50 = 1. 7 mM, n = 1.2). Blockade by thiopental and pentobarbital did not show voltage or use dependence. General anesthetics blocked T current completely and reversibly: propofol (EC50 = 12.9 microM, n = 1.3) > octanol(EC50 = 122 microM, n = 1.2) > etomidate (EC50 = 205 microM, n =1.3) > isoflurane (EC50 = 303 microM, n = 2.3) > halothane (EC50 = 655 microM, n = 2.0) > ketamine (EC50 = 2.5 mM, n = 1.1). Mibefradil, a novel Ca2+ channel blocker, blocked dorsal root ganglion T current in a voltage- and use-dependent fashion with an EC50 of approximately 3 microM (n = 1.3). When compared with results on other T currents, these data indicate that significant differences exist among different T currents in terms of pharmacological sensitivities. Furthermore, differences in pharmacological sensitivity of T currents among peripheral neurons, CNS, and neuroendocrine cells may contribute to the spectrum of effects of particular analgesic, anticonvulsant, and anesthetic drugs.