Interaction of stilbene disulphonates with cloned K(ATP) channels

Trophic sympathetic influence weakens pro-contractile role of Cl- channels in rat arteries during postnatal maturation

Membrane transporters and their functional contribution in vasculature change during early postnatal development. Here we tested the hypothesis that the contribution of Cl⁻ channels to arterial contraction declines during early postnatal development and this decline is associated with the trophic sympathetic influence. Endothelium‐denuded saphenous arteries from 1- to 2-week-old and 2- to 3-month-old male rats were used. Arterial contraction was assessed in the isometric myograph, in some experiments combined with measurements of membrane potential. mRNA and protein levels were determined by qPCR and Western blot. Sympathectomy was performed by treatment with guanethidine from the first postnatal day until 8–9-week age. Cl⁻ substitution in the solution as well as Cl⁻-channel blockers (MONNA, DIDS) had larger suppressive effect on the methoxamine-induced arterial contraction and methoxamine-induced depolarization of smooth muscle cells in 1- to 2-week-old compared to 2- to 3-month-old rats. Vasculature of younger group demonstrated elevated expression levels of TMEM16A and bestrophin 3. Chronic sympathectomy increased Cl⁻ contribution to arterial contraction in 2-month-old rats that was associated with an increased TMEM16A expression level. Our study demonstrates that contribution of Cl⁻ channels to agonist-induced arterial contraction and depolarization decreases during postnatal development. This postnatal decline is associated with sympathetic nerves development.

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.

Inward rectifier potassium channels in the HL-1 cardiomyocyte-derived cell line

HL-1 is a line of immortalized cells of cardiomyocyte origin that are a useful complement to native cardiomyocytes in studies of cardiac gene regulation. Several types of ion channel have been identified in these cells, but not the physiologically important inward rectifier K(+) channels. Our aim was to identify and characterize inward rectifier K(+) channels in HL-1 cells. External Ba(2+) (100 µM) inhibited 44 ± 0.05% (mean ± s.e.m., n = 11) of inward current in whole-cell patch-clamp recordings. The reversal potential of the Ba(2+)-sensitive current shifted with external [K(+)] as expected for K(+)-selective channels. The slope conductance of the inward Ba(2+)-sensitive current increased with external [K(+)]. The apparent Kd for Ba(2+) was voltage dependent, ranging from 15 µM at -150  mV to 148 µM at -75  mV in 120  mM external K(+). This current was insensitive to 10 µM glybenclamide. A component of whole-cell current was sensitive to 150 µM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), although it did not correspond to the Ba(2+)-sensitive component. The effect of external 1 mM Cs(+) was similar to that of Ba(2+). Polymerase chain reaction using HL-1 cDNA as template and primers specific for the cardiac inward rectifier K(ir)2.1 produced a fragment of the expected size that was confirmed to be K(ir)2.1 by DNA sequencing. In conclusion, HL-1 cells express a current that is characteristic of cardiac inward rectifier K(+) channels, and express K(ir)2.1 mRNA. This cell line may have use as a system for studying inward rectifier gene regulation in a cardiomyocyte phenotype.

Chloride Is essential for capacitation and for the capacitation-associated increase in tyrosine phosphorylation

After epididymal maturation, sperm capacitation, which encompasses a complex series of molecular events, endows the sperm with the ability to fertilize an egg. This process can be mimicked in vitro in defined media, the composition of which is based on the electrolyte concentration of the oviductal fluid. It is well established that capacitation requires Na(+), HCO(3)(-), Ca(2+), and a cholesterol acceptor; however, little is known about the function of Cl(-) during this important process. To determine whether Cl(-), in addition to maintaining osmolarity, actively participates in signaling pathways that regulate capacitation, Cl(-) was replaced by either methanesulfonate or gluconate two nonpermeable anions. The absence of Cl(-) did not affect sperm viability, but capacitation-associated processes such as the increase in tyrosine phosphorylation, the increase in cAMP levels, hyperactivation, the zona pellucidae-induced acrosome reaction, and most importantly, fertilization were abolished or significantly reduced. Interestingly, the addition of cyclic AMP agonists to sperm incubated in Cl(-)-free medium rescued the increase in tyrosine phosphorylation and hyperactivation suggesting that Cl(-) acts upstream of the cAMP/protein kinase A signaling pathway. To investigate Cl(-) transport, sperm incubated in complete capacitation medium were exposed to a battery of anion transport inhibitors. Among them, bumetanide and furosemide, two blockers of Na(+)/K(+)/Cl(-) cotransporters (NKCC), inhibited all capacitation-associated events, suggesting that these transporters may mediate Cl(-) movements in sperm. Consistent with these results, Western blots using anti-NKCC1 antibodies showed the presence of this cotransporter in mature sperm.

Functional analysis of two Kir6.2 (KCNJ11) mutations, K170T and E322K, causing neonatal diabetes

Heterozygous activating mutations in Kir6.2 (KCNJ11), the pore-forming subunit of the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel, are a common cause of neonatal diabetes (ND). We assessed the functional effects of two Kir6.2 mutations associated with ND: K170T and E322K. K(ATP) channels were expressed in Xenopus oocytes, and the heterozygous state was simulated by coexpression of wild-type and mutant Kir6.2 with SUR1 (the beta cell type of sulphonylurea receptor (SUR)). Both mutations reduced the sensitivity of the K(ATP) channel to inhibition by MgATP and enhanced whole-cell K(ATP) currents. In pancreatic beta cells, such an increase in the K(ATP) current is expected to reduce insulin secretion and thereby cause diabetes. The E322K mutation was without effect when Kir6.2 was expressed in the absence of SUR1, suggesting that this residue impairs coupling to SUR1. This is consistent with its predicted location on the outer surface of the tetrameric Kir6.2 pore. The kinetics of K170T channel opening and closing were altered by the mutation, which may contribute to the lower ATP sensitivity. Neither mutation affected the sensitivity of the channel to inhibition by the sulphonylurea tolbutamide, suggesting that patients carrying these mutations may respond to these drugs.

Isothiocyanate Inhibits Restitution and Wound Repair after Injury in the Stomach: Ex Vivo and in Vitro Studies

The role of isothiocyanate (ITC) in blocking epithelial restitution after injury and in the recovery of round wounds was examined in the ex vivo guinea pig stomach and in rat gastric mucosal-1 (RGM1) cells, respectively. For this, recovery of transepithelial electrical resistance and morphology after injury or the closure of round wounds was evaluated in the presence of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) or 4,4-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid (H2DIDS) (two ITC groups), 4-acetamido-4-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) (one ITC group), or 4,4-diinitrostilbene-2,2'-disulfonic acid (DNDS) (no ITC groups). Wounded RGM1 cells were also incubated with bicarbonate-free buffer, ATP, barium, or phloretin to determine the mechanism of ITC inhibition. At 300 microM, DIDS or H2DIDS blocked restitution and wound repair by 100%, SITS blocked wound repair by 50%, and DNDS blocked wound repair by 2%. These results demonstrate the dependence of restitution and wound repair on ITC. ITC-binding purino (ATP) receptors and KATP channels were investigated as potential sites of inhibition, but they were found not to be the target of ITC in wound repair. Phloretin, blocking the monocarboxylate transporter (MCT), dose-dependently inhibited wound repair, and this result was exacerbated when the sodium bicarbonate cotransporter (NBC) was also blocked by bicarbonate-free conditions, resulting in 100% inhibition of wound repair with no reduction in viability when both transporters were blocked simultaneously. ITC potently inhibits both MCT and NBC, which may account for the inhibitory action of DIDS during restitution and wound repair. Reverse transcription-polymerase chain reaction data verified that MCT-1 is expressed in RGM1 cells. In conclusion, our results suggest that bicarbonate and monocarboxylate transport may work cooperatively to facilitate restitution of the gastric mucosa after injury.

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.

Osmoregulation of taurine efflux from cultured human breast cancer cells: comparison with volume activated Cl- efflux and regulation by extracellular ATP

The properties and regulation of volume-activated taurine efflux from MDA-MB-231 and MCF-7 cells have been investigated. Volume-activated taurine release from both cell lines was almost completely inhibited by diidosalicylate. DIDS , was more effective at inhibiting swelling-induced taurine release from MCF-7 than from MDA-MB-231 cells. On the basis of comparing taurine, Cl(-) and I(-) efflux time courses, it appears that volume-activated taurine efflux does not utilize volume-sensitive anion channels in MDA-MB- 231 and MCF-7 cells. Extracellular ATP stimulated volume-activated taurine release from MDA-MB-231 cells but not from MCF-7 cells. The effect of ATP was mimicked by UTP and was dependent upon external calcium and inhibited by suramin. However, suramin inhibited volume-activated taurine efflux from both MDA-MB-231 and MCF-7 cells even in the absence of exogenously added ATP suggesting that it acts directly on the taurine efflux pathway and/or is inhibiting the effect of ATP released from the cells. Volume-activated taurine efflux from MDA-MB-231 cells was stimulated by ionomycin. In contrast, ionomycin had no effect on taurine release from MCF-7 cells. Adenosine also stimulated volume-activated taurine efflux from MDA-MB-231 cells. The results suggest that purines regulate taurine transport in MDA-MB- 231 cells via more than one type of receptor.

Human ether-a-go-go-related (HERG) gene and ATP-sensitive potassium channels as targets for adverse drug effects

Torsades de pointes (TdP) arrhythmia is a potentially fatal form of ventricular arrhythmia that occurs under conditions where cardiac repolarization is delayed (as indicated by prolonged QT intervals from electrocardiographic recordings). A likely mechanism for QT interval prolongation and TdP arrhythmias is blockade of the rapid component of the cardiac delayed rectifier K+ current (IKr), which is encoded by human ether-a-go-go-related gene (HERG). Over 100 non-cardiovascular drugs have the potential to induce QT interval prolongations in the electrocardiogram (ECG) or TdP arrhythmias. The binding site of most HERG channel blockers is located inside the central cavity of the channel. An evaluation of possible effects on HERG channels during the development of novel drugs is recommended by international guidelines. During cardiac ischaemia activation of ATP-sensitive K+ (KATP) channels contributes to action potential (AP) shortening which is either cardiotoxic by inducing re-entrant ventricular arrhythmias or cardioprotective by inducing energy-sparing effects or ischaemic preconditioning (IPC). KATP channels are formed by an inward-rectifier K+ channel (Kir6.0) and a sulfonylurea receptor (SUR) subunit: Kir6.2 and SUR2A in cardiac myocytes, Kir6.2 and SUR1 in pancreatic beta-cells. Sulfonylureas and glinides stimulate insulin secretion via blockade of the pancreatic beta-cell KATP channel. Clinical studies about cardiotoxic effects of sulfonylureas are contradictory. Sulfonylureas and glinides differ in their selectivity for pancreatic over cardiovascular KATP channels, being either selective (tolbutamide, glibenclamide) or non-selective (repaglinide). The possibility exists that non-selective KATP channel inhibitors might have cardiovascular side effects. Blockers of the pore-forming Kir6.2 subunit are insulin secretagogues and might have cardioprotective or cardiotoxic effects during cardiac ischaemia.

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.

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.

Inhibition of recombinant K(ATP) channels by the antidiabetic agents midaglizole, LY397364 and LY389382

Most imidazolines inhibit ATP-sensitive K(+) (K(ATP)) channels. Since these drugs are potentially clinically relevant insulin secretagogues, it is important to know whether extrapancreatic K(ATP) channels are targeted. We examined the effects of three imidazoline-derived antidiabetic drugs on the cloned K(ATP) channel, expressed in Xenopus laevis oocytes, and their specificity for interaction with the pore-forming Kir6.2 or the sulphonylurea receptor (SUR) 1 subunit. Midaglizole, LY397364 and LY389382 blocked Kir6.2deltaC currents with IC(50) of 3.8, 6.1 and 0.7 microM, respectively. The block of Kir6.2/SUR1 currents by LY397364 and LY389382 was best fit by a two-site model, suggesting that these drugs also interact with SUR1. However, since all three drugs interact with the Kir6.2 subunit, and Kir6.2 forms the pore of extrapancreatic K(ATP) channels, these drugs are unlikely to be specific for the beta-cell.

Ammonia-induced extracellular accumulation of taurine in the rat striatum in vivo: role of ionotropic glutamate receptors

Accumulation of taurine (Tau), glutamate (Glu) and glutamine (Gln) was measured in vivo in microdialysates of the rat striatum following a direct application to the microdialysis tube of 60 mM ammonium chloride which renders the final ammonia concentration in the extracellular space to approximately 5 mM. The following compounds were coadministered with ammonia to distinguish between the different mechanisms that may underlie the accumulation of amino acids: ion transport inhibitors, diisothiocyanostilbene-2,2'-disulfonate (DIDS) and furosemide, a Glu transport inhibitor L-trans-pyrrolidine-2,4-dicarboxylate (PDC), an NMDA receptor antagonist dizocilpine (MK-801) and an 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (KA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). Ammonia stimulated Tau accumulation in the microdialysates to approximately 250% of the basal value. Furosemide did not significantly affect the stimulation by ammonia and DIDS only moderately depressed the effect. The ammonia-dependent Tau accumulation was increased by approximately 50% in the presence of PDC and reduced by approximately 35% in the presence dizocilpine and DNQX. In the microdialysates ammonia stimulated Glu and Gln accumulation somewhat less than Tau accumulation. Except for stimulation of Gln accumulation by DNQX, the effects were not modified by any of the cotreatments. The results are consistent with the assumption that ammonia stimulates Tau efflux mainly via activation of ionotropic Glu receptors.