Stilbene disulfonates block ATP-sensitive K+ channels in guinea pig ventricular myocytes

4,4'-Diisothiocyanatostilbene-2,2'-disulfonate modulates voltage-gated K+ current and influences cell cycle arrest in androgen sensitive and insensitive human prostate cancer cell lines

The stilbene derivative, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), an anion channel blocker is used in the present study to evaluate its modulatory effect on voltage-gated K⁺ current (I_K) in human prostate cancer cell lines (LNCaP and PC-3). Voltage-gated K⁺ (K_V) channels in the plasma membrane are critically involved in the proliferation of tumor cells. Therefore, K_V channels are considered as a novel potential target for cancer treatment. The results of the present study show that the external perfusion of DIDS activates I_K in a concentration-dependent manner, although the known K⁺ channel blocker TEA failed to block the DIDS activated I_K in PC-3 cells. Whereas, in LNCaP cells, the higher concentration of DIDS blocked I_K, though this effect was not completely recovered after washout. The difference in function of DIDS might be due to the expression of different Kv channel isoforms in LNCaP and PC-3 cells. Further, the anticancer studies show that treatment of DIDS significantly induced G2/M phase cell cycle arrest and induced moderate and low level of cell death in LNCaP and PC-3 cells respectively. This finding reveals that DIDS modulates I_K and exerts cell cycle arrest and cell death in LNCaP and PC-3 cells.

Human SLC4A11-C functions as a DIDS-stimulatable H⁺(OH⁻) permeation pathway: partial correction of R109H mutant transport

The SLC4A11 gene mutations cause a variety of genetic corneal diseases, including congenital hereditary endothelial dystrophy 2 (CHED2), Harboyan syndrome, some cases of Fuchs' endothelial dystrophy (FECD), and possibly familial keratoconus. Three NH2-terminal variants of the human SLC4A11 gene, named SLC4A11-A, -B, and -C are known. The SLC4A11-B variant has been the focus of previous studies. Both the expression of the SLC4A11-C variant in the cornea and its functional properties have not been characterized, and therefore its potential pathophysiological role in corneal diseases remains to be explored. In the present study, we demonstrate that SLC4A11-C is the predominant SLC4A11 variant expressed in human corneal endothelial mRNA and that the transporter functions as an electrogenic H(+)(OH(-)) permeation pathway. Disulfonic stilbenes, including 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS), 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonate (H2DIDS), and 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonate (SITS), which are known to bind covalently, increased SLC4A11-C-mediated H(+)(OH(-)) flux by 150-200% without having a significant effect in mock-transfected cells. Noncovalently interacting 4,4'-diaminostilbene-2,2'-disulfonate (DADS) was without effect. We tested the efficacy of DIDS on the functionally impaired R109H mutant (SLC4A11-C numbering) that causes CHED2. DIDS (1 mM) increased H(+)(OH(-)) flux through the mutant transporter by ∼40-90%. These studies provide a basis for future testing of more specific chemically modified dilsulfonic stilbenes as potential therapeutic agents to improve the functional impairment of specific SLC4A11 mutant transporters.

Activation of the K(ATP) channel by Mg-nucleotide interaction with SUR1

The mechanism of adenosine triphosphate (ATP)-sensitive potassium (K_ATP) channel activation by Mg-nucleotides was studied using a mutation (G334D) in the Kir6.2 subunit of the channel that renders K_ATP channels insensitive to nucleotide inhibition and has no apparent effect on their gating. K_ATP channels carrying this mutation (Kir6.2-G334D/SUR1 channels) were activated by MgATP and MgADP with an EC₅₀ of 112 and 8 µM, respectively. This activation was largely suppressed by mutation of the Walker A lysines in the nucleotide-binding domains of SUR1: the remaining small (∼10%), slowly developing component of MgATP activation was fully inhibited by the lipid kinase inhibitor LY294002. The EC₅₀ for activation of Kir6.2-G334D/SUR1 currents by MgADP was lower than that for MgATP, and the time course of activation was faster. The poorly hydrolyzable analogue MgATPγS also activated Kir6.2-G334D/SUR1. AMPPCP both failed to activate Kir6.2-G334D/SUR1 and to prevent its activation by MgATP. Maximal stimulatory concentrations of MgATP (10 mM) and MgADP (1 mM) exerted identical effects on the single-channel kinetics: they dramatically elevated the open probability (P_O > 0.8), increased the mean open time and the mean burst duration, reduced the frequency and number of interburst closed states, and eliminated the short burst states. By comparing our results with those obtained for wild-type K_ATP channels, we conclude that the MgADP sensitivity of the wild-type K_ATP channel can be described quantitatively by a combination of inhibition at Kir6.2 (measured for wild-type channels in the absence of Mg²⁺) and activation via SUR1 (determined for Kir6.2-G334D/SUR1 channels). However, this is not the case for the effects of MgATP.

Na+/K+-ATPase inhibitor palytoxin enhances vulnerability of cultured cerebellar neurons to domoic acid via sodium-dependent mechanisms

Dysfunction or deficiency of the Na(+)/K(+)-ATPase appears to be a common event in a variety of pathological conditions in the central nervous system. Studies on neurotoxicity associated to impaired Na(+)/K(+)-ATPase activity have focused on NMDA receptors, while the involvement of non-NMDA receptors has been much less explored. We show that mild, non-toxic, exposures to the Na(+)/K(+)-ATPase inhibitor palytoxin (PTX) synergistically sensitized the vulnerability of neurons to normally non-toxic concentrations of domoic acid, leaving NMDA receptor-mediated excitotoxic response unaltered. Enhancement of excitotoxicity required at least 1 h pre-exposure to PTX, was not observed after longer exposures to PTX, and did not require RNA synthesis. PTX caused a voltage-sensitive Na(+) channel-independent increase in intracellular Na(+). Both intracellular Na(+) increase and potentiation of excitotoxicity depended upon the external concentrations of Na(+) and Cl(-), and were suppressed by the anion exchanger blocker 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid in a dose-dependent manner. Other stilbene derivatives, chloride channel antagonists or Na(+) cotransporter inhibitors proved ineffective. Our results demonstrate a crucial role for Na(+)/K(+)-ATPase activity in determining neuronal vulnerability to domoic acid-mediated excitotoxicity. They also raise reasonable concern about possible risks for human health associated to the ingestion of low amounts of phycotoxins PTX and domoic acid in food.

Discovery of potent CLC chloride channel inhibitors

Anion-transport proteins are central to all of physiology, for processes ranging from regulating bone-density, muscle excitability, and blood pressure, to facilitating extreme-acid survival of pathogenic bacteria. 4,4-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) has been used as an anion-transport inhibitor for decades. In this study, we demonstrate that polythiourea products derived from DIDS hydrolysis inhibit three different CLC chloride-transport proteins, ClC-ec1, ClC-0, and ClC-Ka, more effectively than DIDS itself. The structures of the five major products were determined by NMR spectroscopy, mass spectrometry, and chemical synthesis. These compounds bind directly to the CLC proteins, as evidenced by the fact that inhibition of ClC-0 occurs only from the intracellular side and inhibition of ClC-Ka is prevented by the point mutation N68D. These polythioureas are the highest affinity inhibitors known for the CLCs and provide a new class of chemical probes for dissecting the molecular mechanisms of chloride transport.

Role of transporters and ion channels in neuronal injury under hypoxia

The aims of the current study were to 1) examine the effects of hypoxia and acidosis on cultured cortical neurons and 2) explore the role of transporters and ion channels in hypoxic injury. Cell injury was measured in cultured neurons or hippocampal slices following hypoxia (1% O(2)) or acidosis (medium pH 6.8) treatment. Inhibitors of transporters and ion channels were employed to investigate their roles in hypoxic injury. Our results showed that 1) neuronal damage was apparent at 5-7 days of hypoxia exposure, i.e., 36-41% of total lactate dehydrogenase was released to medium and 2) acidosis alone did not lead to significant injury compared with nonacidic, normoxic controls. Pharmacological studies revealed 1) no significant difference in neuronal injury between controls (no inhibitor) and inhibition of Na(+)-K(+)-ATP pump, voltage-gated Na(+) channel, ATP-sensitive K(+) channel, or reverse mode of Na(+)/Ca(2+) exchanger under hypoxia; however, 2) inhibition of NBCs with 500 microM DIDS did not cause hypoxic death in either cultured cortical neurons or hippocampal slices; 3) in contrast, inhibition of Na(+)/H(+) exchanger isoform 1 (NHE1) with either 10 microM HOE-642 or 2 microM T-162559 resulted in dramatic hypoxic injury (+95% for HOE-642 and +100% for T-162559 relative to normoxic control, P < 0.001) on treatment day 3, when no death occurred for hypoxic controls (no inhibitor). No further damage was observed by NHE1 inhibition on treatment day 5. We conclude that inhibition of NHE1 accelerates hypoxia-induced neuronal damage. In contrast, DIDS rescues neuronal death under hypoxia. Hence, DIDS-sensitive mechanism may be a potential therapeutic target.

IADS, a decomposition product of DIDS activates a cation conductance in Xenopus oocytes and human erythrocytes: new compound for the diagnosis of cystic fibrosis

DIDS (4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid) is a commonly used blocker of plasma membrane anion channels and transporters. We observed that DIDS undergoes decomposition while stored in DMSO (dimethyl sulfoxide) forming a biologically active compound. One decomposition product, called IADS, was identified and synthesized. Voltage-clamp and patch clamp experiments on Xenopus laevis oocytes and human erythrocytes revealed that IADS is able to activate a plasma membrane cation conductance in both cell types. Furthermore, we found that IADS induces hemolysis in red blood cells of healthy donors but fails to hemolyze erythrocytes of donors with cystic fibrosis. Thus, IADS stimulated activation of a cation conductance could form the basis for a novel diagnostic test of cystic fibrosis.

Resveratrol binds to the sulfonylurea receptor (SUR) and induces apoptosis in a SUR subtype-specific manner

Sulfonylurea receptors (SURs) constitute the regulatory subunits of ATP-sensitive K+ channels (K(ATP) channels). SUR binds nucleotides and synthetic K(ATP) channel modulators, e.g. the antidiabetic sulfonylurea glibenclamide, which acts as a channel blocker. However, knowledge about naturally occurring ligands of SUR is very limited. In this study, we show that the plant phenolic compound trans-resveratrol can bind to SUR and displace binding of glibenclamide. Electrophysiological measurements revealed that resveratrol is a blocker of pancreatic SUR1/K(IR)6.2 K(ATP) channels. We further demonstrate that, like glibenclamide, resveratrol induces enhanced apoptosis. This was shown by analyzing different apoptotic parameters (cell detachment, nuclear condensation and fragmentation, and activities of different caspase enzymes). The observed apoptotic effect was specific to cells expressing the SUR1 isoform and was not mediated by the electrical activity of K(ATP) channels, as it was observed in human embryonic kidney 293 cells expressing SUR1 alone. Enhanced susceptibility to resveratrol was not observed in pancreatic beta-cells from SUR1 knock-out mice or in cells expressing the isoform SUR2A or SUR2B or the mutant SUR1(M1289T). Resveratrol was much more potent than glibenclamide in inducing SUR1-specific apoptosis. Treatment with etoposide, a classical inducer of apoptosis, did not result in SUR isoform-specific apoptosis. In conclusion, resveratrol is a natural SUR ligand that can induce apoptosis in a SUR isoform-specific manner. Considering the tissue-specific expression patterns of SUR isoforms and the possible effects of SUR mutations on susceptibility to apoptosis, these observations could be important for diabetes and/or cancer research.

Mitochondrial potassium transport: the K(+) cycle

Potassium transport plays three distinct roles in mitochondria. Volume homeostasis to prevent excess matrix swelling is a housekeeping function that is essential for maintaining the structural integrity of the organelle. This function is mediated by the K(+)/H(+) antiporter and was first proposed by Peter Mitchell. Volume homeostasis to prevent excess matrix contraction is a recently discovered function that maintains a fully expanded matrix when diffusive K(+) influx declines due to membrane depolarization caused by high rates of electron transport. Maintaining matrix volume under these conditions is important because matrix contraction inhibits electron transport and also perturbs the structure-function of the intermembrane space (IMS). This volume regulation is mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). Cell signaling functions to protect the cell from ischemia-reperfusion injury and also to trigger transcription of genes required for cell growth. This function depends on the ability of mitoK(ATP) opening to trigger increased mitochondrial production of reactive oxygen species (ROS). This review discusses the properties of the mitochondrial K(+) cycle that help to understand the basis of these diverse effects.

Protection of cortical neurons against oxygen-glucose deprivation and N-methyl-D-aspartate by DIDS and SITS

The Cl(-) channel blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) or 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) dose-dependently protected against oxygen-glucose deprivation in cultured rat cortical neurons. DIDS or SITS attenuated oxygen-glucose deprivation-induced increases in extracellular glutamate concentrations and intracellular Ca(2+). DIDS or SITS provided moderate protection against N-methyl-D-aspartate (NMDA) toxicity and decreased NMDA receptor-mediated increases in intracellular Ca(2+). Whole-cell NMDA receptor currents were attenuated 39+/-2% and 21+/-3% by 1 mM DIDS and SITS, respectively. Other Cl(-) channel blockers as equipotent as DIDS and SITS did not decrease oxygen-glucose deprivation- or NMDA-mediated neuronal Ca(2+) influx or toxicity. Neurotoxicity by exogenous glutamate was not prevented by SITS and was exacerbated by DIDS. Reductions in oxygen-glucose deprivation-induced increases in intracellular Ca(2+) levels underlie neuroprotection by DIDS and SITS. This was a reflection of lower extracellular [glutamate], direct inhibition of Ca(2+) influx through postsynaptic NMDA receptors, and possibly through other protective properties associated with DIDS and SITS.

Role for chloride but not potassium channels in apoptosis in primary rat cortical cultures

Recent evidence suggests a predominant role for potassium (K) efflux in apoptotic cell death yet there exists controversy as to the exact nature of this involvement of K. In the present study we tested the anti-apoptotic efficacy of K channel blockers, tetraethylammonium Cl (TEA), and high extracellular K, the sodium (Na) channel blocker, tetrodotoxin (TTX) and the Cl channel blocker, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, (SITS) against staurosporine-induced apoptosis in primary rat cortical cultures. Surprisingly, we failed to observe anti-apoptotic effects with TEA, high K or TTX. We did, however, observe significant dose-dependent inhibition of apoptosis with SITS. In conclusion we demonstrate no role for K or Na in neuronal apoptosis, but rather an important role for a SITS-sensitive mechanism such as Cl.

Mg2+ sensitizes KATP channels to inhibition by DIDS: dependence on the sulphonylurea receptor subunit

1. ATP-sensitive potassium channels (K(ATP) channels) consist of pore-forming Kir6.x subunits and of sulphonylurea receptors (SURs). In the absence of Mg(2+), the stilbene disulphonate, DIDS, irreversibly inhibits K(ATP) channels by binding to the Kir subunit. Here, the effects of Mg(2+) on the interaction of DIDS with recombinant K(ATP) channels were studied in electrophysiological and [(3)H]-glibenclamide binding experiments. 2. In inside-out macropatches, Mg(2+) (0.7 mM) increased the sensitivity of K(ATP) channels towards DIDS up to 70 fold (IC(50)=2.7 micro M for Kir6.2/SUR2B). Inhibition of current at DIDS concentrations > or =10 micro M was irreversible. 3. Mg(2+) sensitized the truncated Kir6.2Delta26 channel towards inhibition by DIDS only upon coexpression with a SUR subunit (SUR2B). The effect of Mg(2+) did not require the presence of nucleotides. 4. [(3)H]-glibenclamide binding to SUR2B(Y1206S), a mutant with improved affinity for glibenclamide, was inhibited by DIDS. The potency of inhibition was increased by Mg(2+) and by coexpression with Kir6.2. 5. In the presence of Mg(2+), DIDS inhibited binding of [(3)H]-glibenclamide to Kir6.2/SUR2B(Y1206S) with IC(50)=7.9 micro M by a non-competitive mechanism. Inhibition was fully reversible. 6. It is concluded that the binding site of DIDS on SUR that is sensed by glibenclamide does not mediate channel inhibition. Instead, Mg(2+) binding to SUR may allosterically increase the accessibility and/or reactivity of the DIDS site on Kir6.2. The fact that the Mg(2+) effect does not require the presence of nucleotides underlines the importance of this ion in modulating the properties of the K(ATP) channel.

Interaction of stilbene disulphonates with cloned K(ATP) channels

In this study, we tested the effects of the stilbene disulphonates DIDS and SITS on three different types of cloned K(ATP) channel (Kir6.2/SUR1, Kir6.2/SUR2A and Kir6.2DeltaC) heterologously expressed in Xenopus oocytes, with the aim of identifying the part of the channel which is involved in mediating disulphonate inhibition. We found that the inhibitory site(s) for these drugs lies within the Kir6.2 subunit of the channel, although its properties are further modulated by the sulphonylurea (SUR) subunit. In particular, SUR2A reduces both the rate and extent of block, by impairing the ability of DIDS binding to produce channel closure. The disulphonate-binding site interacts with the ATP inhibitory site on Kir6.2 because ATP is able to protect against irreversible channel inhibition by disulphonates. This effect is not mimicked by tolbutamide (at a concentration that interacts with Kir6.2) and is abolished by mutations that render the channel ATP insensitive. A number of point mutations in both the N and C termini of Kir6.2 reduced the extent and reversibility of channel inhibition by SITS. The results are consistent with the idea that residue C42 of Kir6.2 is likely to be involved in covalently linking of SITS to the channel. Other types of Kir channel (Kir1.1, Kir2.1 and Kir4.1) were also irreversibly blocked by DIDS, suggesting that these channels may share common binding sites for these stilbene disulphonates.

The N-terminus of KIR6.2 limits spontaneous bursting and modulates the ATP-inhibition of KATP channels

KATP channels are heteromultimers of a sulfonylurea receptor SUR and KIR6.2 with the inward rectifier forming the pore which is regulated by SUR. We have examined the contributions of the cytoplasmic domains of KIR6.2 to control of spontaneous bursting and ATP-inhibition in human SUR1/KIR6.2 KATP channels. Truncations of the N-terminus of KIR6.2 nearly eliminate transitions to interburst closed states without affecting the open or intraburst closed states, thus producing SUR1/DeltaNKIR6.2 channels with an extremely high open probability in the absence of nucleotides. These channels have a decrease apparent ATP-sensitivity which is consistent with the involvement of the N-terminus in a transition to an interburst closed state that preferentially binds inhibitory ATP. Mutations in both the N- and proximal C-termini of KIR6.2 can synergistically attenuate the ATP-inhibition. The results identify the N-terminus of KIR6.2 as a determinant of the interburst kinetics of KATP channels and suggest that the two cytoplasmic domains of KIR6.2 participate in ATP-inhibitory gating through distinct mechanisms.

Pharmacology of CFTR chloride channel activity

Pharmacology of CFTR Chloride Channel Activity. Physiol. Rev. 79, Suppl.: S109-S144, 1999. - The pharmacology of cystic fibrosis transmembrane conductance regulator (CFTR) is at an early stage of development. Here we attempt to review the status of those compounds that modulate the Cl- channel activity of CFTR. Three classes of compounds, the sulfonylureas, the disulfonic stilbenes, and the arylaminobenzoates, have been shown to directly interact with CFTR to cause channel blockade. Kinetic analysis has revealed the sulfonylureas and arylaminobenzoates interact with the open state of CFTR to cause blockade. Suggestive evidence indicates the disulfonic stilbenes act by a similar mechanism but only from the intracellular side of CFTR. Site-directed mutagenesis studies indicate the involvement of specific amino acid residues in the proposed transmembrane segment 6 for disulfonic stilbene blockade and segments 6 and 12 for arylaminobenzoate blockade. Unfortunately, these compounds (sulfonylureas, disulfonic stilbenes, arylaminobenzoate) also act at a number of other cellular sites that can indirectly alter the activity of CFTR or the transepithelial secretion of Cl-. The nonspecificity of these compounds has complicated the interpretation of results from cellular-based experiments. Compounds that increase the activity of CFTR include the alkylxanthines, phosphodiesterase inhibitors, phosphatase inhibitors, isoflavones and flavones, benzimidazolones, and psoralens. Channel activation can arise from the stimulation of the cAMP signal transduction cascade, the inhibition of inactivating enzymes (phosphodiesterases, phosphatases), as well as the direct binding to CFTR. However, in contrast to the compounds that block CFTR, a detailed understanding of how the above compounds increase the activity of CFTR has not yet emerged.

Inhibition of sodium current by chloride channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) in guinea pig cardiac ventricular cells

The effects of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), a potent anion transport blocker, on transmembrane action potentials (APs) and the sodium current (I[Na]) of guinea pig ventricular myocytes were examined by using conventional microelectrode and whole-cell patch-clamp recording techniques. In papillary muscle preparations, DIDS (> or =0.1 mM) suppressed the maximal upstroke velocity (.v[max]) of the AP without significant changes in other AP parameters. Extracellular application of DIDS on single cardiomyocytes isolated from the guinea pig ventricle markedly reduced the peak amplitude of the tetrodotoxin (TTX)-sensitive and voltage-activated sodium current. The concentration-dependent block of DIDS could be expressed by the Hill equation with a Hill coefficient of 0.97 and a dissociation constant of 0.15 mM at a holding potential of (VH) -120 mV. DIDS (0.1 mM) shifted the steady-state inactivation curve for I(Na) toward more negative potentials by 6.0 +/- 0.5 mV and the activation curve to more positive potentials by 5.0 +/- 1.0 mV, although the slope factors were unaffected. With repetitive depolarizing pulses from -120 mV, DIDS produced a use-dependent block on the I(Na). Recovery of I(Na) from inactivation was slowed (time constant = 245 ms, compared with 10 ms of control) in the presence of 0.1 mM DIDS. In the two-pulse experiments, DIDS produced two distinct phases of development of I(Na) block, the rapid phase (tau = 5 ms) caused by an open channel block, and the slower phase (tau = 382 ms) induced by an inactivated channel block. These results suggest that the Cl- transport blocker DIDS has a direct inhibitory effect on the cardiac sodium channel. DIDS-induced use dependence of I(Na) block may result from the interaction of the drug with sodium channels in both the open and inactivated channel states.

Ligand-insensitive state of cardiac ATP-sensitive K+ channels. Basis for channel opening

The mechanism by which ATP-sensitive K+ (KATP) channels open in the presence of inhibitory concentrations of ATP remains unknown. Herein, using a four-state kinetic model, we found that the nucleotide diphosphate UDP directed cardiac KATP channels to operate within intraburst transitions. These transitions are not targeted by ATP, nor the structurally unrelated sulfonylurea glyburide, which inhibit channel opening by acting on interburst transitions. Therefore, the channel remained insensitive to ATP and glyburide in the presence of UDP. "Rundown" of channel activity decreased the efficacy with which UDP could direct and maintain the channel to operate within intraburst transitions. Under this condition, the channel was sensitive to inhibition by ATP and glyburide despite the presence of UDP. This behavior of the KATP channel could be accounted for by an allosteric model of ligand-channel interaction. Thus, the response of cardiac KATP channels towards inhibitory ligands is determined by the relative lifetime the channel spends in a ligand-sensitive versus -insensitive state. Interconversion between these two conformational states represents a novel basis for KATP channel opening in the presence of inhibitory concentrations of ATP in a cardiac cell.

Macroscopic and single-channel chloride currents in neuropile glial cells of the leech central nervous system

In patch-clamp experiments we characterized four Cl- channels (42 pS, 70 pS, 80 pS and 229 pS) underlying the large Cl- conductance of leech neuropile glial cells. They differed with respect to their gating, their rectification and their activity in the cell-attached configuration, showed the selectivity sequence I->Cl->/=Br->F- and were impermeable to SO42-. The four channels were blocked by NPPB, DPC, niflumic acid and DIDS and exhibited either three or four sublevel states. The outward rectifying 42 pS, 70 pS and 80 pS Cl- channels were classified as intermediate conductance Cl- channels and they could contribute to the high Cl- conductance of the glial membrane, which stabilizes the glial membrane potential. The inward rectifying 229 pS Cl- channel is very similar to vertebrate high conductance Cl- channels, which are assumed to be part of an emergency system that is activated under pathophysiological conditions. In voltage-clamp experiments we calculated that the Cl- conductance amounts to one-third of the total membrane conductance. Reduction of this Cl- conductance by Cl- channel inhibitors markedly depolarized the glial cell membrane. These prominent depolarizations depended on Na+ influx and in most cases the glial cells failed to regulate their membrane potential following wash-out of the inhibitors.

Inhibition by P1075 and pinacidil of a calcium-independent chloride conductance in conditionally-immortal renal glomerular mesangial cells

1. Depolarization of mesangial cells has been shown to occur following an outward movement of chloride ions from the cell. We have shown previously that mesangial cells from the H-2Kb-tsA58 transgenic mouse possess a significant whole-cell chloride conductance and consequently are a suitable preparation for the study of potential chloride channel inhibitors. 2. The effects on the whole-cell chloride conductance of the chloride channel inhibitor, 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) and the potassium channel openers, (KCOs) P1075 and pinacidil were investigated in mesangial cells from the H-2Kb-tsA58 transgenic mouse cultured in permissive conditions (at 33 degrees C in the presence of 50 u ml-1 murine gamma-interferon). 3. In symmetrical solutions of 140 mM tetramethylammonium chloride (TMAC1) the whole-cell chloride conductance was 1.08 +/- 0.05 nS (n = 63) and this could be reversibly inhibited by 5 x 10(-5) M NPPB. 4. Both P1075 and pinacidil inhibited the whole-cell chloride conductance. This inhibition was not reversible after drug washout and was demonstrated only when drugs were applied to the extracellular surface of the cells. Very low concentrations of the drugs were found to reduce the chloride conductance after 16 h incubation but under no circumstances studied was the conductance totally inhibited, leaving a mean residual current of 0.33 +/- 0.03 nS (n = 12). 5. The effects of different peptide calcium concentrations on the magnitude of the residual current in the presence of the drugs were investigated. The residual current was reduced with 10(-8) M calcium in the pipette and increased with 10(-3) M pipette calcium. Therefore, these data suggest that P1075 and pinacidil selectively inhibit a calcium-independent chloride conductance in mesangial cells from the H-2Kb-tsA58 transgenic mouse.

The mitochondrial inner membrane anion channel is inhibited by DIDS

The mitochondrial inner membrane anion channel (IMAC) is a channel, identified by flux studies in intact mitochondria, which has a broad anion selectivity and is maintained closed or inactive by matrix Mg2+ and H+. We now present evidence that this channel, like many other chloride/anion channels, is reversibly blocked/inhibited by stilbene-2,2'-disulfonates. Inhibition of malonate transport approaches 100% with IC50 values of 26, 44, and 88 mu M for DIDS, H2-DIDS, and SITS respectively and Hill coefficients < or = 1. In contrast, inhibition of Cl- transport is incomplete, reaching a maximum of about 30% at pH 7.4 and 65% at pH 8.4 with an IC50 which is severalfold higher than that for malonate. The IC50 for malonate transport is decreased about 50% by pretreatment of the mitochondria with N-ethylmaleimide. Raising the assay pH from 7.4 to 8.4 increases the IC50 by about 50%, but under conditions where only the matrix pH is made alkaline the IC50 is decreased slightly. These properties and competition studies suggest that DIDS inhibits by binding to the same site as Cibacron blue 3GA. In contrast, DIDS does not appear to compete with the fluorescein derivative Erythrosin B for inhibition. These findings not only provide further evidence that IMAC may be more closely related to other "Cl-" channels than previously thought, but also suggest that other Cl- channels may be sensitive to some of the many regulators of IMAC which have been identified.

Effects of chloride ion substitutes and chloride channel blockers on the transient outward current in rat ventricular myocytes

The Cai(2+)-insensitive transient outward current, ilo was studied at 20-24 degrees C in rat ventricular myocytes with the whole cell recording patch-clamp technique. The current was recorded before and after replacement of chloride by methanesulfonate or aspartate or in the absence and the presence of chloride channel blockers, SITS or 9-anthracene carboxylic acid. In control conditions (in the presence of external divalent cations, Ca2+ and Cd2+, Cd2+ being used to suppress Ca2+ current), ilo inactivation was composed of a fast and a slow component. When methanesulfonate was substituted for external Cl-, the peak current decreased to a variable extent, but the inactivation of the remaining current was still composed of a fast and a slow component. In contrast, the inactivation of the difference current was well fitted by a single exponential. The time to peak of the difference current was shorter than that of the current recorded either in the absence or the presence of methanesulfonate. Both activation- and steady-state inactivation-voltage curves were either unchanged (n = 4) or shifted by a few mV (5.5 mV, n = 14) towards positive potentials when methanesulfonate was substituted for Cl-. The current remaining in methanesulfonate reversed at potentials closed to EK. The difference current was composed of a peak and a steady-state component. The peak was suppressed by 4-aminopyridine whereas the steady-state component was not. The peak was also suppressed when pipette solution contained Cs+ instead of K+ but was still present when the Hepes concentration in both external and pipette media was increased 5-fold (50 mM vs. 10 mM). When aspartate was substituted for Cl- or when 2 mM SITS was added to the external solution (in the absence of Ca2+ and Cd2+ because aspartate is known to chelate Ca2+ ions and possibly other divalent cations), ilo was reduced to a similar extent in the two cases and the difference current was composed of a peak (inactivation fitted by a single exponential) and a steady-state component. The SITS-sensitive transient current reversed at a potential close to ECl. When 5 mM 9-anthracene carboxylic acid was added to external solution (in the presence of Ca2+ and Cd2+), the peak of the difference current was similar to that observed when Cl- was substituted by methanesulfonate. The difference current resulting from the substitution of methanesulfonate for chloride was not changed when the pipette solution contained either 50 mM EGTA (instead of 5 mM) or 10 mM EGTA and 10 mM BAPTA. The nature of Cs(+)- and 4-aminopyridine-sensitive transient outward current suppressed by chloride ion substitutes or chloride channel blockers is discussed.