Leishmania sp.: growth and survival are impaired by ion channel blockers

Repurposing the Antidiabetic Drugs Glyburide, Gliquidone, and Glipizide in Combination with Benznidazole for Trypanosoma cruzi Infection

Infection with the protozoan parasite Trypanosoma cruzi causes human Chagas disease. Benznidazole (BNZ) and nifurtimox are the current drugs for the treatment; however, they induce severe adverse side effects in patients; therefore, there is a need to improve the treatment effectiveness and efficiency of these drugs for its safer use. Background/Objective: Glyburide, glipizide, and gliquidone, hypoglycemic drugs for diabetes treatment, were previously predicted to bind to dihydrofolate reductase-thymidylate synthase from T. cruzi by in silico docking analysis; they also showed antiproliferative effects against T. cruzi epimastigotes, the stage of the insect vector. In the present study, the potential parasiticidal effect of these antidiabetic drugs was tested in monotherapy and bi-therapy with BNZ in human cells in vitro and in animals. Methods: Evaluation was performed in (a) a model of in vitro infection of T. cruzi trypomastigotes using human fibroblasts as host cells and (b) in mice infected with T. cruzi. Results: The antidiabetic drugs in monotherapy showed antiparasitic effects in preventing infection progression (trypomastigotes release), with an IC50 of 8.4-14.3 µM in comparison to that of BNZ (0.26 µM) in vitro. However, in bi-therapy, the presence of just 0.5 or 1 µM of the antidiabetics decreased the BNZ IC50 by 5-10 times to 0.03-0.05 µM. Remarkably, the antidiabetic drugs in monotherapy decreased the infection in mice by 40-60% in a similar extent to BNZ (80%). In addition, the combination of BNZ plus antidiabetics perturbed the antioxidant metabolites in epimastigotes. Conclusions: These results identified antidiabetics as potential drugs in combination therapy with BNZ to treat T. cruzi infection.

Down the membrane hole: Ion channels in protozoan parasites

Parasitic diseases caused by protozoans are highly prevalent around the world, disproportionally affecting developing countries, where coinfection with other microorganisms is common. Control and treatment of parasitic infections are constrained by the lack of specific and effective drugs, plus the rapid emergence of resistance. Ion channels are main drug targets for numerous diseases, but their potential against protozoan parasites is still untapped. Ion channels are membrane proteins expressed in all types of cells, allowing for the flow of ions between compartments, and regulating cellular functions such as membrane potential, excitability, volume, signaling, and death. Channels and transporters reside at the interface between parasites and their hosts, controlling nutrient uptake, viability, replication, and infectivity. To understand how ion channels control protozoan parasites fate and to evaluate their suitability for therapeutics, we must deepen our knowledge of their structure, function, and modulation. However, methodological approaches commonly used in mammalian cells have proven difficult to apply in protozoans. This review focuses on ion channels described in protozoan parasites of clinical relevance, mainly apicomplexans and trypanosomatids, highlighting proteins for which molecular and functional evidence has been correlated with their physiological functions.

Potential therapeutic targets shared between leishmaniasis and cancer

The association of leishmaniasis and malignancies in human and animal models has been highlighted in recent years. The misdiagnosis of coexistence of leishmaniasis and cancer and the use of common drugs in the treatment of such diseases prompt us to further survey the molecular biology of Leishmania parasites and cancer cells. The information regarding common expressed proteins, as possible therapeutic targets, in Leishmania parasites and cancer cells is scarce. Therefore, the current study reviews proteins, and investigates the regulation and functions of several key proteins in Leishmania parasites and cancer cells. The up- and down-regulations of such proteins were mostly related to survival, development, pathogenicity, metabolic pathways and vital signalling in Leishmania parasites and cancer cells. The presence of common expressed proteins in Leishmania parasites and cancer cells reveals valuable information regarding the possible shared mechanisms of pathogenicity and opportunities for therapeutic targeting in leishmaniasis and cancers in the future.

An immunoproteomic approach to identifying immunoreactive proteins in Leishmania infantum amastigotes using sera of dogs infected with canine visceral leishmaniasis

Visceral leishmaniasis (VL), the most severe form of leishmaniasis, is caused by Leishmania donovani and Leishmania infantum. The infected dogs with canine visceral leishmaniasis (CVL) are important reservoirs for VL in humans, so the diagnosis, treatment and vaccination of the infected dogs will ultimately decrease the rate of human VL. Proteomics and immunoproteomics techniques have facilitated the introduction of novel drug, vaccine and diagnostic targets. Our immunoproteomic study was conducted to identify new immunoreactive proteins in amastigote form of L. infantum. The strain of L. infantum (MCAN/IR/07/Moheb-gh) was obtained from CVL-infected dogs. J774 macrophage cells were infected with the L. infantum promastigotes. The infected macrophages were ruptured, and pure amastigotes were extracted from the macrophages. After protein extraction, two-dimensional gel electrophoresis was employed for protein separation followed by Western blotting. Western blotting was performed, using symptomatic and asymptomatic sera of the infected dogs with CVL. Thirteen repeatable immunoreactive spots were identified by Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Some, including prohibitin, ornithine aminotransferase, annexin A4, and apolipoprotein A-I, have been critically involved in metabolic pathways, survival, and pathogenicity of Leishmania parasites. Further investigations are required to confirm our identified immunoreactive proteins as a biomarker for CVL.

Anthelminthic activity of glibenclamide on secondary cystic echinococcosis in mice

Cystic echinococcosis (CE) is a worldwide parasitic zoonosis caused by the larval stage of Echinococcus granulosus. Current chemotherapy against this disease is based on the administration of benzimidazoles (BZMs). However, BZM treatment has a low cure rate and causes several side effects. Therefore, new treatment options are needed. The antidiabetic drug glibenclamide (Glb) is a second-generation sulfonylurea receptor inhibitor that has been shown to be active against protozoan parasites. Hence, we assessed the in vitro and in vivo pharmacological effects of Glb against the larval stage of E. granulosus. The in vitro activity was concentration dependent on both protoscoleces and metacestodes. Moreover, Glb combined with the minimum effective concentration of albendazole sulfoxide (ABZSO) was demonstrated to have a greater effect on metacestodes in comparison with each drug alone. Likewise, there was a reduction in the cyst weight after oral administration of Glb to infected mice (5 mg/kg of body weight administered daily for a period of 8 weeks). However, in contrast to in vitro assays, no differences in effectiveness were found between Glb + albendazole (ABZ) combined treatment and Glb monotherapy. Our results also revealed mitochondrial membrane depolarization and an increase in intracellular Ca²⁺ levels in Glb-treated protoscoleces. In addition, the intracystic drug accumulation and our bioinformatic analysis using the available E. granulosus genome suggest the presence of genes encoding sulfonylurea transporters in the parasite. Our data clearly demonstrated an anti-echinococcal effect of Glb on E. granulosus larval stage. Further studies are needed in order to thoroughly investigate the mechanism involved in the therapeutic response of the parasite to this sulfonylurea.

In this work we demonstrated the in vitro and in vivo efficacy of Glb against the larval stage of Echinococcus granulosus. At the cellular level, the drug triggered mitochondrial membrane depolarization and increased intracellular Ca²⁺ levels, thus affecting ATP generation in the parasite. In addition, since intracystic Glb concentrations were higher than those used in the external medium, we proposed that the drug might enter the cyst through cell surface transporters. The observed effect of the drug on the growth of hydatid cysts in mice leads to the consideration of a novel role of Glb in CE treatment. Therefore, our further studies will focus on the evaluation of ABZ formulations with enhanced bioavailability to achieve an improved in vivo anti-echinococcal effect using both drugs simultaneously.

Investigation of Calcium Channel Blockers as Antiprotozoal Agents and Their Interference in the Metabolism of Leishmania (L.) infantum

Leishmaniasis and Chagas disease are neglected parasitic diseases endemic in developing countries; efforts to find new therapies remain a priority. Calcium channel blockers (CCBs) are drugs in clinical use for hypertension and other heart pathologies. Based on previous reports about the antileishmanial activity of dihydropyridine-CCBs, this work aimed to investigate whether the in vitro anti-Leishmania infantum and anti-Trypanosoma cruzi activities of this therapeutic class would be shared by other non-dihydropyridine-CCBs. Except for amrinone, our results demonstrated antiprotozoal activity for fendiline, mibefradil, and lidoflazine, with IC50 values in a range between 2 and 16 μM and Selectivity Index between 4 and 10. Fendiline demonstrated depolarization of mitochondrial membrane potential, with increased reactive oxygen species production in amlodipine and fendiline treated Leishmania, but without plasma membrane disruption. Finally, in vitro combinations of amphotericin B, miltefosine, and pentamidine against L. infantum showed in isobolograms an additive interaction when these drugs were combined with fendiline, resulting in overall mean sum of fractional inhibitory concentrations between 0.99 and 1.10. These data demonstrated that non-dihydropyridine-CCBs present antiprotozoal activity and could be useful candidates for future in vivo efficacy studies against Leishmaniasis and Chagas' disease.

Bis-acridines as lead antiparasitic agents: structure-activity analysis of a discrete compound library in vitro

Parasitic diseases are of enormous public health significance in developing countries-a situation compounded by the toxicity of and resistance to many current chemotherapeutics. We investigated a focused library of 18 structurally diverse bis-acridine compounds for in vitro bioactivity against seven protozoan and one helminth parasite species and compared the bioactivities and the cytotoxicities of these compounds toward various mammalian cell lines. Structure-activity relationships demonstrated the influence of both the bis-acridine linker structure and the terminal acridine heterocycle on potency and cytotoxicity. The bioactivity of polyamine-linked acridines required a minimum linker length of approximately 10 A. Increasing linker length resulted in bioactivity against most parasites but also cytotoxicity toward mammalian cells. N alkylation, but less so N acylation, of the polyamine linker ameliorated cytotoxicity while retaining bioactivity with 50% effective concentration (EC(50)) values similar to or better than those measured for standard drugs. Substitution of the polyamine for either an alkyl or a polyether linker maintained bioactivity and further alleviated cytotoxicity. Polyamine-linked compounds in which the terminal acridine heterocycle had been replaced with an aza-acridine also maintained acceptable therapeutic indices. The most potent compounds recorded low- to mid-nanomolar EC(50) values against Plasmodium falciparum and Trypanosoma brucei; otherwise, low-micromolar potencies were measured. Importantly, the bioactivity of the library was independent of P. falciparum resistance to chloroquine. Compound bioactivity was a function of neither the potential to bis-intercalate DNA nor the inhibition of trypanothione reductase, an important drug target in trypanosomatid parasites. Our approach illustrates the usefulness of screening focused compound libraries against multiple parasite targets. Some of the bis-acridines identified here may represent useful starting points for further lead optimization.

Leishmania amazonensis: Anionic currents expressed in oocytes upon microinjection of mRNA from the parasite

Transport mechanisms involved in pH homeostasis are relevant for the survival of Leishmania parasites. The presence of chloride conductive pathways in Leishmania has been anticipated since anion channel inhibitors limit the proton extrusion mediated by the H+ATPase, which is the major regulator of intracellular pH in amastigotes. In this study, we used Xenopus laevis oocytes as a heterologous expression system in which to study the expression of ion channels upon microinjection of polyA mRNA from Leishmania amazonensis. After injection of polyA mRNA into the oocytes, we measured three different types of currents. We discuss the possible origin of each, and propose that Type 3 currents could be the result of the heterologous expression of proteins from Leishmania since they show different pharmacological and biophysical properties as compared to endogenous oocyte currents.

Activities of naphthylisoquinoline alkaloids and synthetic analogs against Leishmania major

The current treatments for leishmaniasis are unsatisfactory due to their toxic side effects, high costs, and increasing problems with drug resistance. Thus, there is an urgent need for alternative drugs against leishmaniasis. Different approaches have been used to identify novel pharmacophores against Leishmania sp. parasites, and one strategy has been the analysis of naturally occurring plant-derived compounds, including naphthylisoquinoline alkaloids. In the present study, we examined the abilities of these alkaloids to inhibit the growth of Leishmania major promastigotes and evaluated their effects on macrophages, dendritic cells, and fibroblasts. Furthermore, we determined the efficacy of selected compounds in decreasing the infection rate of macrophages and regulating their production of cytokines and nitric oxide. Our results demonstrate that the naphthylisoquinoline alkaloids ancistrocladiniums A and B (compounds 10 and 11) and the synthetic isoquinolinium salt (compound 14) were effective against intracellular amastigotes in the low submicromolar range, while toxicity against mammalian cells was observed at concentrations that were significantly higher than those needed to impair parasite replication. The activities of compounds 11 and 14 were mainly directed against the amastigote stage of L. major. This effect was not associated with the stimulation of host macrophages to produce nitric oxide or secrete cytokines relevant for the leishmanicidal function. In conclusion, our data suggest that ancistrocladiniums A and B (compounds 10 and 11) and the synthetically prepared isoquinolinium salt (compound 14) are promising candidates to be considered as lead compounds for leishmanicidal drugs.

Glibenclamide, a blocker of K+(ATP) channels, shows antileishmanial activity in experimental murine cutaneous leishmaniasis

Glibenclamide reduced the rate of lesion growth in BALB/c mice infected with Leishmania (Leishmania) mexicana, the effect was dose dependent, and the highest dose proved more effective than glucantime. Cross-resistance to glucantime was found in animals infected with a glibenclamide-resistant line, but combined therapy reduced lesion progression even in the glibenclamide-resistant strain.

Leishmania amazonensis: Metabolic adaptations induced by resistance to an ABC transporter blocker

We compared growth rate, cell glucose turnover and expression of ATP-binding-cassette (ABC) transporters in Leishmania amazonensis (LTB0016; LTB) versus LTB(160) selected for resistance against the ABC transporter blocker glibenclamide. Additionally, we evaluated the influence of drug-resistance on Leishmania sensitivity against 2-mercaptoacetate and 2-deoxyglucose. Our data demonstrate that (1) LTB(160) and LTB constitutively express ABC transporters for neutral substrates, (2) glibenclamide resistance induces the expression of organic anion ABC transporters, members of the drug resistance associated transporters subfamily, (3) LTB(160) parasites use less glucose as energy substrate and exhibit a slower glucose uptake than LTB cells, and (4) LTB(160) parasites are less sensitive to 2-mercaptoacetate and 2-deoxyglucose than the glibenclamide-sensitive Leishmania LTB. Together these and previous results indicate that the metabolic adaptations expressed in drug-resistant LTB(160) differ from those described for mammalian drug resistant cells and constitute general mechanisms that underlie drug resistance in Leishmania and may be helpful for identifying alternative strategies to circumvent drug resistance in leishmaniasis.

Aziridine-2,3-dicarboxylates, peptidomimetic cysteine protease inhibitors with antileishmanial activity

Chemotherapy of leishmaniasis is mainly based on antimonials. However, they are extremely toxic and cause serious side effects, and there is a worldwide increasing frequency of chemoresistance to antimonials. These issues emphasize the urgent need for affordable alternative drugs against leishmaniasis. Leishmania cysteine proteases are essential for parasite growth, differentiation, pathogenicity, and virulence and are thus attractive targets for combating leishmaniasis. Herein we demonstrate that the cysteine protease inhibitors aziridine-2,3-dicarboxylates 13b and 13e impaired promastigote growth at mid-micromolar concentrations and decreased the infection rate of peritoneal macrophages at concentrations 8- to 13-fold lower than those needed to inhibit parasite replication. Simultaneous treatment of infected cells with compound 13b and gamma interferon resulted in an even further reduction of the concentration needed for a significant decrease in macrophage infection rate. Notably, treatment with the compounds alone modulated the cytokine secretion of infected macrophages, with increased levels of interleukin-12 and tumor necrosis factor alpha. Furthermore, the decreased infection rate in the presence of compound 13b correlated with increased nitric oxide production by macrophages. Importantly, at the concentrations used herein, compounds 13b and 13e were not toxic against fibroblasts, macrophages, or dendritic cells. Together, these results suggest that the aziridine-2,3-dicarboxylates 13b and 13e are potential antileishmanial lead compounds with low toxicity against host cells and selective antiparasitic effects.

In vitro leishmanicidal activity of Tityus discrepans scorpion venom

Leishmania parasites are sensitive to peptides with antimicrobial and ion-channel inhibitory activity. Because scorpion venoms are rich sources of such peptides, the leishmanicidal effect of Tityus discrepans venom was investigated. A negative correlation between cell growth and venom concentration was observed for venom-treated cultures of Leishmania (L.) mexicana mexicana promastigotes; 50% growth inhibition was obtained at 0.4 microg/ml. Light microscopy showed rounded, highly vacuolated L. (L.) m. mexicana cells with impaired flagellar motion after 15 min of incubation at 35 microg/ml. Ultrastructural studies confirmed an intense cytoplasm vacuolation and also enlargement of the flagellar pocket. Survival rates for New World Leishmania promastigotes (75% venom effective concentration, microg/ml) obtained after acute (1 h) venom toxicity tests were: L. (L.) m. mexicana (2.3), Leishmania (V.) braziliensis (11.3), and Leishmania (L.) chagasi (56.2). Heat (90 degrees C) treatment of venom and fraction TdII abolished most of their leishmanicidal effect. Acute toxicity assays performed with Sephadex G-50 fractions indicated that leishmanicidal activity is associated with the venom lowest molecular mass components (2.8-7.4 kDa), as determined by MALDI-TOF mass spectrometry.

Biogenesis of Leishmania major-harboring vacuoles in murine dendritic cells

In mammalian hosts, Leishmania sp. parasites are obligatory intracellular organisms that invade macrophages and dendritic cells (DC), where they reside in endocytic organelles termed parasitophorous vacuoles (PV). Most of the present knowledge of the characteristics of PV harboring Leishmania sp. is derived from studies with infected macrophages. Since DC play a key role in host resistance to leishmaniasis, there is a need to understand the properties and biogenesis of PV in Leishmania sp.-infected DC. Therefore, we determined the acquisition of endosomal and lysosomal molecules by Leishmania major-containing compartments in DC at different maturation stages, using fluorescence labeling and confocal microscopy. The results show that newly formed phagosomes in DC rapidly develop into late endosomal compartments. However, the small GTPase Rab7, which regulates late fusion processes, was found only in PV of mature bone marrow-derived DC (BMDC); it was absent in immature BMDC, suggesting an arrest of their PV biogenesis at the stage of late endosomes. Indeed, fusion assays with endocytic tracers demonstrated that the fusion activity of L. major-harboring PV toward lysosomes is higher in mature BMDC than in immature BMDC. The inhibition of PV-lysosome fusion in DC is dependent upon the viability and life cycle stage of the parasite, because live promastigotes blocked the fusion almost completely, whereas killed organisms and amastigotes induced a considerable level of fusion activity. The differences in the fusion competences of immature and mature DC may be relevant for their distinct functional activities in the uptake, transport, and presentation of parasite antigens.

Characterization of the redox components of transplasma membrane electron transport system from Leishmania donovani promastigotes

An investigation has been made of the points of coupling of four nonpermeable electron acceptors e.g., alpha-lipoic acid (ALA), 5,5'-dithiobis (2-nitroaniline-N-sulphonic acid) (DTNS), 1,2-naphthoquinone-4-sulphonic acid (NQSA) and ferricyanide which are mainly reduced via an interaction with the redox sites present in the plasma membrane of Leishmania donovani promastigotes. ALA, DTNS, NQSA and ferricyanide reduction and part of O2 reduction is shown to take place on the exoplasmic face of the cell, for it is affected by external pH and agents that react with the external surface. Redox enzymes of the transplasma membrane electron transport system orderly transfer electron from one redox carrier to the next with the molecular oxygen as the final electron acceptor. The redox carriers mediate the transfer of electrons from metabolically generated reductant to nonpermeable electron acceptors and oxygen. At a pH of 6.4, respiration of Leishmania cells on glucose substrate shut down almost completely upon addition of an uncoupler FCCP and K+-ionophore valinomycin. The most pronounced effects on O2 uptake were obtained by treatment with antimycin A, 2-heptadecyl-4-hydroxyquinone-N-oxide, paracholoromercuribenzene sulphonic acid and trifluoperazine. Relatively smaller effects were obtained by treatment with potassium cyanide. Inhibition observed with respect to the reduction of the electron acceptors ALA, DTNS, NQSA and ferricyanide was not similar in most cases. The redox chain appears to be branched at several points and it is suggested that this redox chain incorporate iron-sulphur center, b-cytochromes, cyanide insensitive oxygen redox site, Na+ and K+ channel, capsaicin inhibited energy coupling site and trifluoperazine inhibited energy linked P-type ATPase. We analyzed the influence of ionic composition of the medium on reduction of electron acceptors in Leishmania donovani promastigotes. Our data suggest that K+ have some role for ALA reduction and Na+ for ferricyanide reduction. No significant effects were found with DTNS and NQSA reduction when Na+ or K+ was omitted from the medium. Stimulation of ALA, DTNS, NQSA and ferricyanide reduction was obtained by omitting Cl- from the medium. We propose that this redox system may be an energy source for control of membrane function in Leishmania cells.

Substrate preferences and glucose uptake in glibenclamide-resistant Leishmania parasites

Several drug-resistant mammalian cell types exhibit increased glycolytic rates, preferential synthesis of ATP through oxidative phosphorylation, and altered glucose transport. Herein we analyzed the influence of parasite growth phase on energy substrate uptake and use in a Leishmania strain [NR(Gr)] selected for resistance against glibenclamide. Glibenclamide is an ABC-transporter blocker which modulates the function of glucose transporters in some mammalian cells. Our results demonstrate for the first time that compared to glibenclamide-sensitive Leishmania, exponential phase glibenclamide-resistant parasites exhibit decreased use of glucose as energy substrate, decreased glucose uptake and decreased glucose transporter expression. However, compared to glibenclamide-sensitive cells, stationary phase resistant parasites display an increased use of amino acids as energy substrate and an increased activity of the enzymes hexokinase, phosphoglucose isomerase, and especially NAD(+)-linked glutamate dehydrogenase. These results suggest that drug resistance in Leishmania involves a metabolic adaptation that promotes a stage dependent modulation of energy substrate uptake and use as a physiological response to the challenge imposed by drug pressure.

Experimental leishmaniasis: the glibenclamide-triggered decrease in parasite growth correlates with changes in macrophage features

Previous studies from our laboratories revealed the susceptibility of Leishmania sp. to glibenclamide (GLIB), a potassium channel blocker which selectively interacts with adenosine-binding-cassette transporters. In the present work, we analyzed whether the drug sensitivity of intracellular amastigotes correlates with changes in macrophage features that are related to their function as antigen-presenting cells. We provide evidence that in BALB/c murine macrophages, GLIB induced a decrease in the interferon-gamma-stimulated expression of major histocompatibility complex class II molecules and the co-stimulatory molecule CD86 (B7-2). Furthermore, it caused a decrease in the interleukin-1 secretion by macrophages. The data indicate that the treatment with GLIB inhibits the Th2 development and polarizes macrophage functions towards the induction of a protective Th1 response.

Intracellular pH in mammalian stages of Trypanosoma cruzi is K+-dependent and regulated by H+-ATPases

Regulation of intracellular pH (pHi) was investigated in Trypanosoma cruzi amastigotes and trypomastigotes using 2',7'-bis-(carboxyethyl)-5(and-6)-carboxyfluorescein (BCECF). pHi was determined to be 7.33 +/- 0.08 and 7.35 +/- 0.07 in amastigotes and trypomastigotes, respectively, and there were no significant differences in the regulation of pH, between the two stages. Steady-state pHi, recovery of pHi from acidification, and H+-efflux were all decreased markedly by the H+-ATPase inhibitors N,N'-dicyclohexylcarbodi-imide (DCCD), diethylstilbestrol (DES) and N-ethylmaleimide (NEM) supporting a significant role for a plasma membrane H+-ATPase in the regulation of pHi. pHi was maintained at neutrality over a range of external pH (pHe) from 5-8 in parasites suspended in a buffer containing Na+ and K+ (standard buffer) but was acidified at low pHe in the absence of these cations (choline buffer). The pHi of trypomastigotes decreased significantly when they transformed into amastigotes. The rate of recovery of pHi by acidified parasites was similar in Na+-free buffer and standard buffer but was slower in the absence of K+ (K+-free or choline buffer) and parasites suspended in choline buffer were acidic by 0.25 pH units as compared with controls. Ba2+ and Cs+ decreased the pHi of parasites suspended in standard but not choline buffer suggesting the presence of an inward directed K+ channel. The pHi of amastigotes and trypomastigotes suspended in Cl(-)-free buffer was decreased by 0.13 and 0.2 pH units, respectively, supporting the presence of a chloride conductive channel. No evidence of pH regulation via a Na+/H+ or Cl-/HCO3- exchanger was found. These results are consistent with the presence of a plasma membrane H+-ATPase that regulates pHi and is supported by K+ and Cl- channels.