Mechanism of L- and T-type Ca2+ channel blockade by flunarizine in ventricular myocytes of the guinea-pig

l-DOPA receptor GPR143 inhibits neurite outgrowth via L-type calcium channels in PC12 cells

The gene product of ocular albinism 1 (OA1)/G-protein-coupled receptor (GPR)143 is a receptor for L-3,4-dihydroxyphenylanine (l-DOPA), the most effective agent for Parkinson's disease. When overexpressed, human wild-type GPR143, but not its mutants, inhibits neurite outgrowth in PC12 cells. We investigated the downstream signaling pathway for GPR143-induced inhibition of neurite outgrowth. Nifedipine restored GPR143-induced neurite outgrowth inhibition to the level of control transfectant but did not affect outgrowth in GPR143-knockdown cells. Cilnidipine and flunarizine also suppressed the GPR143-induced inhibition, but their effects at higher concentrations still occurred even in GPR143-knockdown cells. These results suggest that GPR143 regulates neurite outgrowth via L-type calcium channel(s).

Betulinic acid analogs inhibit N- and T-type voltage-gated calcium channels to attenuate nerve-injury associated neuropathic and formalin models of pain

Over the past three decades, there has been a significant growth in the use of natural products, with approximately 80% of individuals using them for some aspect of primary healthcare. Our laboratories have identified and studied natural compounds with analgesic effects from dry land plants or their associated fungus during the past ten years. Here, we isolated and characterized thirteen betulin analogs and fifteen betulinic acid analogs for their capacity to prevent calcium influx brought on by depolarization in sensory neurons. The in vitro inhibition of voltage-gated calcium channels by the top drugs was then assessed using whole cell patch clamp electrophysiology. In vivo experiments, conducted at two sites, evaluated the best compound in acute and tonic, neuropathic, inflammatory, post-operative and visceral models of pain. We found that the betulinic acid analog 8 inhibited calcium influx in rat dorsal root ganglion neurons by inhibiting N- (CaV2.2) and T- (CaV3) type voltage-gated calcium channels. Moreover, intrathecal delivery of analog 8 had analgesic activity in both spared nerve injury model of neuropathic pain and acute and tonic pain induced by formalin. The results presented herein highlight the potential antinociceptive properties of betulinic acid analog 8 and set the stage for the development of novel non-opioid pain therapeutics based on the triterpenoid scaffold of betulinic acid.

T-Type Calcium Channels: A Mixed Blessing

The role of T-type calcium channels is well established in excitable cells, where they preside over action potential generation, automaticity, and firing. They also contribute to intracellular calcium signaling, cell cycle progression, and cell fate; and, in this sense, they emerge as key regulators also in non-excitable cells. In particular, their expression may be considered a prognostic factor in cancer. Almost all cancer cells express T-type calcium channels to the point that it has been considered a pharmacological target; but, as the drugs used to reduce their expression are not completely selective, several complications develop, especially within the heart. T-type calcium channels are also involved in a specific side effect of several anticancer agents, that act on microtubule transport, increase the expression of the channel, and, thus, the excitability of sensory neurons, and make the patient more sensitive to pain. This review puts into context the relevance of T-type calcium channels in cancer and in chemotherapy side effects, considering also the cardiotoxicity induced by new classes of antineoplastic molecules.

Linked biaromatic compounds as cardioprotective agents

Cardiovascular diseases (CVDs) are widespread in the modern world, and their number is constantly growing. For a long time, CVDs have been the leading cause of morbidity and mortality worldwide. Drugs for the treatment of CVD have been developed almost since the beginning of the 20th century, and a large number of effective cardioprotective agents of various classes have been created. Nevertheless, the need for the design and development of new safe drugs for the treatment of CVD remains. Literature data indicate that a huge number of cardioprotective agents of various generations and mechanisms correspond to a single generalized pharmacophore model containing two aromatic nuclei linked by a linear linker. In this regard, we put forward a concept for the design of a new generation of cardioprotective agents with a multitarget mechanism of action within the indicated pharmacophore model. This review is devoted to a generalization of the currently known compounds with cardioprotective properties and corresponding to the pharmacophore model of biaromatic compounds linked by a linear linker. Particular attention is paid to the history of the creation of these drugs, approaches to their design, and analysis of the structure-action relationship within each class.

Cyclic Peptides as T-Type Calcium Channel Blockers: Characterization and Molecular Mapping of the Binding Site

T-type calcium (Ca_V3) channels play a crucial role in the generation and propagation of action potentials in excitable cells and are considered potential drug targets for the treatment of neurological and cardiovascular diseases. Given the limited pharmacological repertoire for these channels, there is a great need for novel potent and selective Ca_V3 channel inhibitors. In this study, we used Xenopus oocytes to heterologously express Ca_V3.1 channels and characterized the interaction with a small cyclic peptide, PnCS1. Using molecular modeling, PnCS1 was docked into the cryo-electron microscopy structure of the human Ca_V3.1 channel and molecular dynamics were performed on the resultant complex. The binding site of the peptide was mapped with the involvement of critical amino acids located in the pore region and fenestrations of the channel. More specifically, we found that PnCS1 reclines in the central cavity of the pore domain of the Ca_V3.1 channel and resides stably between the selectivity filter and the intracellular gate, blocking the conduction pathway of the channel. Using Multiple Attribute Positional Scanning approaches, we developed a series of PnCS1 analogues. These analogues had a reduced level of inhibition, confirming the importance of specific residues and corroborating our modeling. In summary, functional studies of PnCS1 on the Ca_V3.1 channel combined with molecular dynamics results provide the basis for understanding the molecular interactions of PnCS1 with Ca_V3.1 and are fundamental to structure-based drug discovery for treating Ca_V3 channelopathies.

Mibefradil and Flunarizine, Two T-Type Calcium Channel Inhibitors, Protect Mice against Lipopolysaccharide-Induced Acute Lung Injury

Recent studies have illuminated that blocking Ca²⁺ influx into effector cells is an attractive therapeutic strategy for lung injury. We hypothesize that T-type calcium channel may be a potential therapeutic target for acute lung injury (ALI). In this study, the pharmacological activity of mibefradil (a classical T-type calcium channel inhibitor) was assessed in a mouse model of lipopolysaccharide- (LPS-) induced ALI. In LPS challenged mice, mibefradil (20 and 40 mg/kg) dramatically decreased the total cell number, as well as the productions of TNF-α and IL-6 in bronchoalveolar lavage fluid (BALF). Mibefradil also suppressed total protein concentration in BALF, attenuated Evans blue extravasation, MPO activity, and NF-κB activation in lung tissue. Furthermore, flunarizine, a widely prescripted antimigraine agent with potent inhibition on T-type channel, was also found to protect mice against lung injury. These data demonstrated that T-type calcium channel inhibitors may be beneficial for treating acute lung injury. The important role of T-type calcium channel in the acute lung injury is encouraged to be further investigated.

The Discovery and Characterization of ML218: A Novel, Centrally Active T-Type Calcium Channel Inhibitor with Robust Effects in STN Neurons and in a Rodent Model of Parkinson's Disease

T-type Ca(2+) channel inhibitors hold tremendous therapeutic potential for the treatment of pain, epilepsy, sleep disorders, essential tremor and other neurological disorders; however, a lack of truly selective tools has hindered basic research, and selective tools from the pharmaceutical industry are potentially burdened with intellectual property (IP) constraints. Thus, an MLPCN high-throughput screen (HTS) was conducted to identify novel T-type Ca(2+) channel inhibitors free from IP constraints, and freely available through the MLPCN, for use by the biomedical community to study T-type Ca(2+) channels. While the HTS provided numerous hits, these compounds could not be optimized to the required level of potency to be appropriate tool compounds. Therefore, a scaffold hopping approach, guided by SurflexSim, ultimately afforded ML218 (CID 45115620) a selective T-Type Ca(2+) (Ca(v)3.1, Ca(v)3.2, Ca(v)3.3) inhibitor (Ca(v)3.2, IC(50) = 150 nM in Ca(2+) flux; Ca(v)3.2 IC(50) = 310 nM and Ca(v)3.3 IC(50) = 270 nM, respectively in patch clamp electrophysiology) with good DMPK properties, acceptable in vivo rat PK and excellent brain levels. Electrophysiology studies in subthalamic nucleus (STN) neurons demonstrated robust effects of ML218 on the inhibition of T-Type calcium current, inhibition of low threshold spike and rebound burst activity. Based on the basal ganglia circuitry in Parkinson's disease (PD), the effects of ML218 in STN neurons suggest a therapeutic role for T-type Ca(2+) channel inhibitors, and ML218 was found to be orally efficacious in haloperidol-induced catalepsy, a preclinical PD model, with comparable efficacy to an A(2A) antagonist, a clinically validated PD target. ML218 proves to be a powerful new probe to study T-Type Ca(2+) function in vitro and in vivo, and freely available.

Robust anti-arrhythmic efficacy of verapamil and flunarizine against dofetilide-induced TdP arrhythmias is based upon a shared and a different mode of action

BACKGROUND AND PURPOSE

The high predisposition to Torsade de Pointes (TdP) in dogs with chronic AV-block (CAVB) is well documented. The anti-arrhythmic efficacy and mode of action of Ca(2+) channel antagonists, flunarizine and verapamil against TdP were investigated.

EXPERIMENTAL APPROACH

Mongrel dogs with CAVB were selected based on the inducibility of TdP with dofetilide. The effects of flunarizine and verapamil were assessed after TdP and in different experiments to prevent dofetilide-induced TdP. Electrocardiogram and ventricular monophasic action potentials were recorded. Electrophysiological parameters and short-term variability of repolarization (STV) were determined. In vitro, flunarizine and verapamil were added to determine their effect on (i) dofetilide-induced early after depolarizations (EADs) in canine ventricular myocytes (VM); (ii) diastolic Ca(2+) sparks in RyR2(R4496+/+) mouse myocytes; and (iii) peak and late I(Na) in SCN5A-HEK 293 cells.

KEY RESULTS

Dofetilide increased STV prior to TdP and in VM prior to EADs. Both flunarizine and verapamil completely suppressed TdP and reversed STV to baseline values. Complete prevention of TdP was achieved with both drugs, accompanied by the prevention of an increase in STV. Suppression of EADs was confirmed after flunarizine. Only flunarizine blocked late I(Na). Ca(2+) sparks were reduced with verapamil.

CONCLUSIONS AND IMPLICATIONS

Robust anti-arrhythmic efficacy was seen with both Ca(2+) channel antagonists. Their divergent electrophysiological actions may be related to different additional effects of the two drugs.

Use-dependent block of voltage-gated Cav2.1 Ca2+ channels by petasins and eudesmol isomers

Migraine is a frequent and often disabling disease. Treatment is unsatisfactory in many patients. A disturbed dynamic balance between excitatory and inhibitory signal processing with enhanced cortical activity probably underlies common forms of migraine. Presynaptic voltage-gated Ca(2+) channels are critical determinants of neurotransmitter release and also contribute to trigeminovascular signal transduction. Because clinical evidence exists for migraine-prophylactic actions of Petasites hybridus extracts, we investigated whether petasins comprising the main constituents of the extract inhibit currents through presynaptic Ca(v)2.1 channels expressed in Xenopus laevis oocytes. P. hybridus extract (0.02 mg/ml), petasin, neopetasin, isopetasin, S-petasin, and iso-S-petasin (50 microM) were weak tonic blockers of Ca(v)2.1-mediated barium currents (I(Ba)) during infrequent depolarizations (0.1 Hz), but their inhibitory potency increased at higher stimulation rates (1 Hz), indicating preferential block of open and/or inactivated channels. Sulfur-containing compounds (S-petasin, Iso-S-petasin) were the most potent significantly promoting the accumulation of Ca(v)2.1 channel in inactivated states during pulse trains (I(Ba) decrease during 1-Hz pulse trains: control, 45%, S-petasin, 79%; iso-S-petasin, 80%). For the Eucalyptus williamsiania sesquiterpenes alpha- and gamma-eudesmol, a comparable use-dependent inhibition was found in addition to a tonic block component. Alpha-eudesmol and petasins accelerated the voltage-dependent inactivation of Ca(v)2.1 channels during depolarizations. We demonstrate that S-petasin, iso-S-petasin, and eudesmol are Ca(v)2.1 channel inhibitors preferentially acting as use-dependent channel blockers and with the sulfur-containing substituent in position 3 of the petasins serving as important functional feature. The Ca(v)2.1-inhibitory properties of these petasins may contribute to migraine-prophylactic properties described for P. hybridus extracts.

Termination of a tachyarrhythmia by flunarizine is not a specific marker for a triggered mechanism

BACKGROUND

Prior studies have indicated that tachyarrhythmia termination by flunarizine demonstrates a triggered mechanism. This concept was not confirmed in atrial tachyarrhythmias.

OBJECTIVE

The purpose of this study was to test the hypothesis that flunarizine will not terminate reentrant atrial flutter (AFL).

METHODS

We administered flunarizine (2 mg/kg intravenously over 2 minutes) in 11 episodes of reproducibly inducible, sustained AFL in eight canines with sterile pericarditis. If flunarizine terminated AFL, we studied AFL reinducibility. We also studied pacing thresholds, refractoriness, and intra-atrial conduction time during closed-chest studies and pacing at selected cycle lengths (CLs) from selected sites before and after flunarizine administration. Atrial mapping (510 electrodes) assessed the epicardial activation sequence during AFL and its termination in six episodes. Four AFL episodes were studied in the closed-chest state.

RESULTS

Flunarizine increased AFL CL in all episodes (mean 21 ms; range 7-49 ms), which is explained by slowing conduction in the AFL reentrant circuit, principally in the area of slow conduction. AFL was terminated in 10/11 episodes after drug initiation (mean 3.7 minutes; range 0.5-6.5 minutes) by block in the area of slow conduction. AFL was then not immediately reinducible until >20 minutes after drug administration. Flunarizine had no meaningful effect on atrial pacing thresholds for capture or refractoriness and only affected conduction time in the area of slow conduction in the reentrant circuit.

CONCLUSIONS

Flunarizine (1) causes progressive slowing and block in the area of slow conduction of the AFL reentrant circuit in the canine sterile pericarditis model and (2) is effective in terminating reentrant AFL and so is not a specific marker for a triggered mechanism.

Flunarizine is a Highly Potent Inhibitor of Cardiac hERG Potassium Current

Flunarizine has been widely used for the management of a variety of disorders such as peripheral vascular diseases, migraine, and epilepsy. The majority of its beneficial effects have been attributed to its ability to inhibit voltage-gated Ca2+ channels in the low micromolar range, albeit non-selectively, as flunarizine has been shown to inhibit a variety of ion channels. We examined the effects of flunarizine on potassium currents through cardiac channels encoded by the human ether-a-go-go related gene (hERG) stably expressed in CHO cells. In this study, we have characterized the effect of flunarizine on biophysical properties of hERG potassium currents with standard whole-cell voltage-clamp techniques. Notably, flunarizine is a highly potent inhibitor of hERG current with an IC50 value of 5.7 nM. The effect of flunarizine on hERG potassium current is concentration and time dependent, and displays voltage dependence over the voltage range between -40 and 0 mV. At concentrations near or above the IC50, flunarizine causes a negative shift in the voltage dependence of hERG current activation and accelerates tail current deactivation. Flunarizine preferentially blocks the activated state of the channel and displays weak frequency dependence of inhibition. Flunarizine also inhibits KCNQ1/KCNE1 channel current with an IC50 of 0.76 microM.

Clostridium perfringens alpha-toxin-induced hemolysis of horse erythrocytes is dependent on Ca2+ uptake

Clostridium perfringens alpha-toxin is able to lyse various erythrocytes. Exposure of horse erythrocytes to alpha-toxin simultaneously induced hot-cold hemolysis and stimulated production of diacylglycerol and phosphorylcholine. When A23187-treated erythrocytes were treated with the toxin, these events were dependent on the concentration of extracellular Ca2+ . Incubation with the toxin of BAPTA-AM-treated horse erythrocytes caused no hemolysis or production of phosphorylcholine, but that of the BAPTA-treated erythrocytes did. When Quin 2-AM-treated erythrocytes were incubated with the toxin in the presence of 45Ca2+, the cells accumulated 45Ca2+ in a dose- and a time-dependent manner. These results suggest that the toxin-induced hemolysis and hydrolysis of phosphatidylcholine are closely related to the presence of Ca2+ in the cells. Flunarizine, a T-type Ca2+ channel blocker, and tetrandrine, an L- and T-type Ca2+ channel blocker, inhibited the toxin-induced hemolysis and Ca2+ uptake. However, L-type Ca2+ channel blockers, nifedipine, verpamil and diltiazem, an N-type blocker, omega-conotoxin SVIB, P-type blockers, omega-agatoxin TK and omega-agatoxin IVA, and a Q-type blocker, omega-conotoxin MVII C, had no such inhibitory effect. The observation suggests that Ca2+ taken up through T-type Ca2+ channels activated by the toxin plays an important role in hemolysis induced by the toxin.

Selectivity of different calcium antagonists on T- and L-type calcium currents in guinea-pig ventricular myocytes

Both L- and T-type calcium channels are present in the heart. In cardiac myocytes L-type calcium channels are blocked by the classical calcium channel blockers, while T-type calcium channels are thought to be insensitive to these drugs and to be selectively blocked by mibefradil. We aimed to compare the T/L calcium channel blocking selectivity of several calcium channel blockers by evaluating their effects on both components evoked in the same cell from a holding potential corresponding to the normal physiological value (-90mV). Currents were recorded in single patch-clamped guinea-pig ventricular myocytes, superfused with a Na(+)- and K(+)-free solution to abolish overlapping currents. Two dihydropyridines (amlodipine and lacidipine), verapamil diltiazem and mibefradil were tested; for each compound concentrations equieffective on L-type Ca(2+) current were used. All calcium channel blockers, at concentrations blocking less than 30% of L-type Ca(2+) current, inhibited a significant amount of T-type Ca(2+) current, varying from 0.8% (diltiazem) to 28% (mibefradil). We calculated for each compound the T/L ratio. As expected, mibefradil showed the highest T selectivity; lacidipine and diltiazem resulted to be L selective. Verapamil and amlodipine were not selective. Thus, the calcium channel blockers can be differentiated on the basis of their T/L selectivity.

Differential inhibition of T-type calcium channels by neuroleptics

T-type calcium channels play critical roles in cellular excitability and have been implicated in the pathogenesis of a variety of neurological disorders including epilepsy. Although there have been reports that certain neuroleptics that primarily target D2 dopamine receptors and are used to treat psychoses may also interact with T-type Ca channels, there has been no systematic examination of this phenomenon. In the present paper we provide a detailed analysis of the effects of several widely used neuroleptic agents on a family of exogenously expressed neuronal T-type Ca channels (alpha1G, alpha1H, and alpha1I subtypes). Among the neuroleptics tested, the diphenylbutylpiperidines pimozide and penfluridol were the most potent T-type channel blockers with Kd values (approximately 30-50 nm and approximately 70-100 nm, respectively), in the range of their antagonism of the D2 dopamine receptor. In contrast, the butyrophenone haloperidol was approximately 12- to 20-fold less potent at blocking the various T-type Ca channels. The diphenyldiperazine flunarizine was also less potent compared with the diphenylbutylpiperadines and preferentially blocked alpha1G and alpha1I T-type channels compared with alpha1H. The various neuroleptics did not significantly affect T-type channel activation or kinetic properties, although they shifted steady-state inactivation profiles to more negative values, indicating that these agents preferentially bind to channel inactivated states. Overall, our findings indicate that T-type Ca channels are potently blocked by a subset of neuroleptic agents and suggest that the action of these drugs on T-type Ca channels may significantly contribute to their therapeutic efficacy.

Phosphatidylinositol 3-kinase-dependent extracellular calcium influx is essential for CX(3)CR1-mediated activation of the mitogen-activated protein kinase cascade

Fractalkine, the first member of the CX(3)C chemokine family, induces leukocyte chemotaxis through activation of its high affinity receptor, CX(3)CR1. Like other chemokine receptors, CX(3)CR1 is coupled to a pertussis toxin-sensitive heterotrimeric G(i) protein, which is necessary for rapid rise in the concentration of intracellular calcium. Using a Chinese hamster ovary cell line stably transfected with the CX(3)CR1 receptor, we show that the source of calcium mobilized by fractalkine stimulation is the extracellular pool. Calcium influx is blocked by extracellular calcium chelators, as well as by divalent heavy metals such as Ni(2+), Co(2+), and Cd(2+) without affecting the integrity of intracellular stores. Remarkably, selective phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin and LY294002, abolish the wave extracellular calcium, suggesting that an active PI3K is necessary for this event. The influx of extracellular calcium is in turn required to trigger the activation of the p42/44 mitogen-activated protein/extracellular signal-regulated kinase pathway, but is not necessary for other signals downstream to PI3K, such as phosphorylation of Akt. The potential role of this signaling cascade in fractalkine-mediated chemotaxis is discussed.

Molecular pharmacology of T-type Ca2+ channels

Over the past few years increasing attention has been focused on T-type calcium channels and their possible physiological and pathophysiological roles. Efforts toward elucidating the exact role(s) of these calcium channels have been hampered by the lack of T-type specific antagonists, resulting in the subsequent use of less selective calcium channel antagonists. In addition, the activity of these blockers often varies with cell or tissue type, as well as recording conditions. This review summarizes a variety of compounds that exhibit varying degrees of blocking activity towards T-type Ca2+ channels. It is designed as an aid for researchers in need of antagonists to study the biophysical and pathological nature of T-type channels, as well as a starting point for those attempting to develop potent and selective antagonists of the channel.