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PubMed1984

Ca2+-induced activation and irreversible inactivation of chloride channels in the perfused plasmalemma of Nitellopsis obtusa.

Experiments were carried out on the algal cells with removed tonoplast using both continuous intracellular perfusion and voltage clamp on plasmalemma. The transient plasmalemma current induced by depolarization disappeared upon perfusion with the Ca2+-chelating agent, EGTA, since the voltage-dependent calcium channels lost their ability to activate. Subsequent replacement of the perfusion medium containing EGTA by another with Ca2+ for clamped plasmalemma (-100 mV) induced an inward C1- current which showed both activation and inactivation. The maximal amplitude of the current at [C1-]in = 15 mmol/l (which is similar to that in native cells) was approximately twice that in electrically excited cell in vivo. The inactivation of C1 channels in the presence of internal Ca2+ was irreversible and had a time constant of 1-3 min. This supports our earlier suggestion (Lunevsky et al. 1983) that the inactivation of C1 channels in an intact cell (with a time constant of 1-3 s) is due to a decrease in Ca2+ concentration rather than to the activity of their own inactivation mechanism. The C1 channel selectivity sequence was following: C1- much greater than CH3SO-4 approximately equal to K+ much greater than SO2-4 (PK/PSO4 approximately 10). Activation of one half the channels occurs at a Ca2+ concentration of 2 X 10(-5) mol/l. Sr2+ also (though to a lesser extent) activated C1 channels but had to be present in a much more higher concentration than Ca2+. Mg2+ and Ba2+ appeared ineffective. Ca2+ activation did not, apparently, require participation of water-soluble intermediator including ATP. Thus, C1 channel functioning is controlled by Ca2+-, Sr2+-sensitive elements of the subplasmalemma cytoskeleton.

Calcium↗
PubMed1991

Novel site-directed affinity ligands for GABA-gated chloride channels: synthesis, characterization, and molecular modeling of 1-(isothiocyanatophenyl)-4-tert-butyl-2,6,7-trioxabicyclo[2.2.2]octanes .

p-, m-, and o-isothiocyanate derivatives (1-3, respectively) of tert-butylbicycloorthobenzoate (TBOB) were synthesized from 3-tert-butyloxetane-3-methanol (4) as the starting material. While 2 was readily obtained in four steps via catalytic hydrogenation of the m-nitro-tert-butylbicycloorthobenzoate (9) intermediate, 1 and 3 could not be obtained this way. 1 and 3 were instead synthesized by an alternative four-step approach while made use of the stability of the isothiocyanate moiety to strong Lewis acids such as boron trifluoride etherate, conditions that would isomerize isothiocyanato oxetane ester intermediates to their corresponding orthoesters. The p-isothiocyanate derivative of TBOB, compound 1, inhibited [35S]-tert-butylbicyclophosphorothionate (TBPS) binding to rat cortical membranes with a potency (IC50 62 nM) comparable to the parent compound while 2 and 3 were approximately 10-fold less potent (IC50 values 570 and 609 nM, respectively). Preincubating tissue with radioligand further reduced the potencies of 2 and 3 by approximately 1 order of magnitude (IC50 values 5400 and 7500 nM, respectively) while the potency of 1 (IC50 90 nM) was only marginally affected by this procedure. Pretreatment of membranes with 1 and 2 followed by extensive washing resulted in a concentration-dependent inhibition of [35S]TBPS binding. In contrast, preincubating tissues with up to 2.4 microM of 3 did not elicit an apparent acylation of [35S]TBPS binding sites. Molecular modeling of the effective diameters of 1-3 in their thermodynamically most stable conformations indicates a relationship between these diameters and their relative efficacies as site-directed acylators; the smaller the diameter, the more potent the acylator. This hypothesis explains both the relative potencies of these compounds and their differential abilities to acylate the TBPS binding site.

Animals↗
PubMed1987

Voltage-dependent chloride channels with several substates in excised patches from mouse neuroblastoma cells.

Single channels in mouse neuroblastoma cells with a high conductance of about 400 pS were described using the patch-clamp technique in the inside-out configuration. The channels were selective for Cl- as compared to cations and exhibited a linear I-V relationship between +40 and -40 mV. These Cl- channels were voltage-dependent and were activated by both depolarizing and hyperpolarizing potential steps from 0 mV to 10-40 mV. They closed, becoming inactivated, in tens of milliseconds (for depolarization) up to tens of seconds (for hyperpolarization) after each potential step. The typical feature of Cl- channels described was the dissipation of their conductance into several substates during the course of individual recordings.

Animals↗
PubMed1987

Caesium ions activate chloride channels in rat cultured spinal cord neurones.

1. Caesium ions (Cs+), applied extracellularly, caused a decrease in the input resistance of cultured spinal cord (s.c.) neurones and depolarized the neurones when they contained 140 mM-CsCl. 2. The reversal potential for Cs+-activated currents shifted 56 mV on average for a 10-fold reduction in the intracellular chloride ion (Cl-) activity, indicating that the Cs+-activated currents were specific to Cl-. 3. The activation of Cl- currents by Cs+ was not due to the depolarization-evoked release of neurotransmitter from presynaptic terminals. We therefore suggest that Cs+ were acting directly on the extracellular surface of the s.c. neurones to activate Cl- currents. 4. Cs+-activated currents showed desensitization in the presence of 140 mM-Cs+. 5. The log-log plot of the dose-response data could be fitted with a straight line with a slope of 1.7 +/- 0.4 (n = 6), indicating that at least 2 Cs+ were needed to activate a single Cl- channel. The KD of the Cs+-induced response was greater than 69 mM. 6. In outside-out patches Cs+ activated single Cl- channels. These channels were not activated by sodium or potassium ions. 7. The Cs+-activated channels displayed a total of five distinct conductance states which had mean conductances of 20, 30, 43, 66 and 92 pS. The 30 and 43 pS states were the most frequently occurring states. 8. The conductance states of the Cs+-activated channel have the same conductances as those reported for gamma-aminobutyric acid (GABA)- and glycine-activated channels in rat s.c. neurones. We therefore conclude that Cs+ activate the same type of Cl- channel as GABA and glycine through an unidentified receptor.

Action Potentials↗
PubMed2002

Modulation of the GABA(A)-gated chloride channel by reactive oxygen species.

The accumulation of reactive oxygen species during cellular injury leads to oxidative stress. This can have profound effects on ionic homeostasis and neuronal transmission. Gamma-aminobutyric acid (GABA) neurotransmission is sensitive to reactive oxygen species, but most studies have indicated that this is due to alterations in GABA release. Here, we determined whether reactive oxygen species can alter GABA(A) receptor-gated Cl- channels in the adult hippocampus. First, we measured the effects of hydrogen peroxide on intracellular Cl- using UV laser scanning confocal microscopy and the Cl(-)-sensitive probe, 6-methoxy-N-ethylquinolium iodide (MEQ). Superfusion of adult rat hippocampal slices with hydrogen peroxide for 10 min decreased MEQ fluorescence (elevation in [Cl-]i) significantly in area CA1 pyramidal cell soma. Alterations in [Cl-]i were prevented by the vitamin E analog Trolox, an antioxidant that scavenges free radicals. After exposure of slices to hydrogen peroxide, the ability of the GABA agonist muscimol to increase [Cl-]i was attenuated. To determine if GABA(A) receptors were sensitive to oxidative insults, the effect of hydrogen peroxide on the binding of [35S]t-butylbicyclophosphorothionate (TBPS) to GABA-gated Cl- channels was measured using receptor autoradiography and homogenate binding assays. Hydrogen peroxide inhibited [35S]TBPS binding in a regionally selective manner, with the greatest inhibition in cerebral cortex, hippocampus and striatum, areas vulnerable to oxidative stress. Similarly, xanthine and xanthine oxidase, which generate superoxide radicals, reduced [35S]TBPS binding in these regions. The effect of hydrogen peroxide on [35S]TBPS binding was non-competitive and was prevented by Trolox and the iron chelator, deferoxamine. We conclude that reactive oxygen species may compromise GABA(A)-mediated neuronal inhibition via interaction with pre and postsynaptic sites. A reduction in GABA(A)-gated Cl- channel function during periods of oxidative stress may contribute to the development of neuronal damage.

Animals↗
PubMed2004

4-Chloro-benzo[F]isoquinoline (CBIQ) activates CFTR chloride channels and KCNN4 potassium channels in Calu-3 human airway epithelial cells.

1 Calu-3 cells have been used to investigate the actions of 4-chloro-benzo[F]isoquinoline (CBIQ) on short-circuit current (SCC) in monolayers, whole-cell recording from single cells and by patch clamping. 2 CBIQ caused a sustained, reversible and repeatable increase in SCC in Calu-3 monolayers with an EC50 of 4.0 microm. Simultaneous measurements of SCC and isotopic fluxes of 36Cl- showed that CBIQ caused electrogenic chloride secretion. 3 Apical membrane permeabilisation to allow recording of basolateral membrane conductance in the presence of a K+ gradient suggested that CBIQ activated the intermediate-conductance calcium-sensitive K(+)-channel (KCNN4). Permeabilisation of the basolateral membranes of epithelial monolayers in the presence of a Cl- gradient suggested that CBIQ activated the Cl(-)-channel CFTR in the apical membrane. 4 Whole-cell recording in the absence of ATP/GTP of Calu-3 cells showed that CBIQ generated an inwardly rectifying current sensitive to clotrimazole. In the presence of the nucleotides, a more complex I/V relation was found that was partially sensitive to glibenclamide. The data are consistent with the presence of both KCNN4 and CFTR in Calu-3. 5 Isolated inside-out patches from Calu-3 cells revealed clotrimazole-sensitive channels with a conductance of 12 pS at positive potentials after activation with CBIQ and demonstrating inwardly rectifying properties, consistent with the known properties of KCNN4. Cell-attached patches showed single channel events with a conductance of 7 pS and a linear I/V relation that were further activated by CBIQ by an increase in open state probability, consistent with known properties of CFTR. It is concluded that CBIQ activates CFTR and KCNN4 ion channels in Calu-3 cells.

Cell Line↗
PubMed1989

Characterization of chloride channels in membrane vesicles from the kidney outer medulla.

The basolateral membrane of the thick ascending loop of Henle (TALH) of the mammalian kidney is highly enriched in Na+/K+ ATPase and has been shown by electrophysiological methods to be highly conductive to Cl-. In order to study the Cl- conductive pathways, membrane vesicles were isolated from the TALH-containing region of the porcine kidney, the red outer medulla, and Cl- channel activity was determined by a 36Cl uptake assay where the uptake of the radioactive tracer is driven by the membrane potential (positive inside) generated by an outward Cl- gradient. The accumulation of 36Cl- inside the vesicles was found to be dependent on the intravesicular Cl- concentration and was abolished by clamping the membrane potential with valinomycin. The latter finding indicated the involvement of conductive pathways. Cl- channel activity was also observed using a fluorescent potential-sensitive carbocyanine dye, which detected a diffusion potential induced by an imposed inward Cl- gradient. The anion selectivity of the channels was Cl- greater than NO3- = I- much greater than gluconate. Among the Cl- transport inhibitors tested, 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPAB), 4,4'-diisothiocyano-stilbene-2,2'-disulfonate (DIDS), and diphenylamine-2-carboxylate (DPC) showed IC50 of 110, 200 and 550 microM, respectively. Inhibition of 36Cl uptake by NPPAB and two other structural analogues was fully reversible, whereas that by DIDS was not. The nonreactive analogue of DIDS, 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS), was considerably less inhibitory than DIDS (25% inhibition at 200 microM). The irreversible inhibition by DIDS was prevented by NPPAB, whereas DPC was ineffective, consistent with its low inhibitory potency.(ABSTRACT TRUNCATED AT 250 WORDS)

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed2006

Interactions between impermeant blocking ions in the cystic fibrosis transmembrane conductance regulator chloride channel pore: evidence for anion-induced conformational changes.

It is well known that extracellular Cl(-) ions can weaken the inhibitory effects of intracellular open channel blockers in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel pore. This effect is frequently attributed to repulsive ion-ion interactions inside the pore. However, since Cl(-) ions are permeant in CFTR, it is also possible that extracellular Cl(-) ions are directly competing with intracellular blocking ions for a common binding site; thus, this does not provide direct evidence for multiple, independent anion binding sites in the pore. To test for the possible through-space nature of ion-ion interactions inside the CFTR pore, we investigated the interaction between impermeant anions applied to either end of the pore. We found that inclusion of low concentrations of impermeant Pt(NO(2))(4) (2-) ions in the extracellular solution weaken the blocking effects of three different intracellular blockers [Pt(NO(2))(4) (2-), glibenclamide and 5-nitro-2-(3-phenylpropylamino)benzoic acid] without affecting their apparent voltage dependence. However, the effects of extracellular Pt(NO(2))(4) (2-) ions are too strong to be accounted for by simple competitive models of ion binding inside the pore. In addition, extracellular Fe(CN)(6) (3-) ions, which do not appear to enter the pore, also weaken the blocking effects of intracellular Pt(NO(2))(4) (2-) ions. In contrast to previous models that invoked interactions between anions bound concurrently inside the pore, we propose that Pt(NO(2))(4) (2-) and Fe(CN)(6) (3-) binding to an extracellularly accessible site outside of the channel permeation pathway alters the structure of an intracellular anion binding site, leading to weakened binding of intracellular blocking ions.

Animals↗
PubMed1988

Apical membrane chloride channels in a colonic cell line activated by secretory agonists.

We characterized the anion channel responsible for the increase in apical membrane Cl secretion using a model salt-secreting epithelium, the T84 colonic cell line. The adenosine 3',5'-cyclic monophosphate (cAMP)-mediated secretagogues, prostaglandin E2, forskolin, and 8-bromo-cAMP, evoked activity of an outwardly rectifying Cl channel in previously quiet cell-attached membrane patches. The channel remained active in excised, inside-out membranes, where its single-channel conductance was 40-45 pS at 0 mV with 160 mM NaCl in pipette and bath. Selectivities were PCl/PNa = 50 and for halides I(1.8)/Br(1.4)/Cl(1.0)/F(0.4). This halide sequence illustrates that the ability of various anions to undergo transepithelial secretion is determined by the selectivity of the basolateral membrane Cl entry step rather than by the apical Cl channel. Open-channel probability increased with depolarization, an effect that would adjust the rate of Cl exit across secretory cell apical membranes with agonist-induced changes in apical membrane potential. Comparison with the properties of Cl channels detected in other cell types suggests that this cAMP-stimulated Cl channel is uniquely present in the apical membranes of salt-secreting epithelial cells.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1988

Inhibition by chloride channel blockers of anion secretion in cultured epididymal epithelium and intact epididymis of rats.

1. Anion secretion by primary monolayer cultures of rat epididymal cells was studied by the short circuit current technique. 2. Monolayers had a transepithelial potential difference of 1.34 mV, apical side negative and a short circuit current of 2.45 microA cm-2. The transepithelial resistance was 504 omega cm2. 3. Addition of anthracene-9-carboxylate (9-AC) to the apical side caused a biphasic response, a decrease followed by an increase in the short circuit current (SCC) which then returned to the basal level. Addition of 9-AC to the basolateral side also caused a biphasic response but the increase in current was sustained. 4. Addition of diphenylamine-2-carboxylate (DPC) and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) caused an inhibition of the SCC when added to the apical or basolateral side. 5. When the epithelium was stimulated with adrenaline (0.23 microM, basolaterally), the SCC rose to a peak value of 10.36 microA cm-2 and then stabilized at 3.82 microA cm-2 after 15 min. Addition of 9-AC to the apical side caused a triphasic response: a decrease, reversal to the original level followed by a slow inhibition which was sustained. The inhibition achieved at the steady state was concentration-dependent with an apparent IC50 value of 2.51 mM. Addition of 9-AC to the basolateral side produced a similar response but a time lag of 20 s was observed. 6. DPC and NPPB also caused the SCC to decrease when added to the apical side of the monolayers stimulated with adrenaline. The IC50 values 0.148 mM and 0.049 mM for DPC and NPPB, respectively. Basolateral application also caused an inhibition but higher concentrations were required to inhibit 50% of the adrenaline stimulated SCC. 7. Serosal to mucosal flux of chloride was studied in intact epididymis luminally perfused in vivo. 9-AC added to the luminal perfusion solution inhibited the C1 flux in a dose-dependent manner with an IC50 value similar to that observed in cultured epithelium in vitro. 8. It is concluded that anion secretion by the epididymal cells may conform to models proposed for other Cl secreting epithelia and may involve anion diffusion across the apical channels which are blocked by Cl channel blockers.

Animals↗
PubMed2005

Mechanism of chloride permeation in the cystic fibrosis transmembrane conductance regulator chloride channel.

The cystic fibrosis transmembrane conductance regulator (CFTR) functions as a Cl- channel important in transepithelial salt and water transport. While there is a paucity of direct structural information on CFTR, much has been learned about the molecular determinants of the CFTR Cl- channel pore region and the mechanism of Cl- permeation through the pore from indirect structure-function studies. The first and sixth transmembrane regions of the CFTR protein play major roles in forming the channel pore and determining its functional properties by interacting with permeating Cl- ions. Positively charged amino acid side-chains are involved in attracting negatively charged Cl- ions into the pore region, where they interact briefly with a number of discrete sites on the pore walls. The pore appears able to accommodate more than one Cl- ion at a time, and Cl- ions bound inside the pore are probably sensitive to one another's presence. Repulsive interactions between Cl- ions bound concurrently within the pore may be important in ensuring rapid movement of Cl- ions through the pore. Chloride ion binding sites also interact with larger anions that can occlude the pore and block Cl- permeation, thus inhibiting CFTR function. Other ions besides Cl- are capable of passing through the pore, and specific amino acid residues that may be important in allowing the channel to discriminate between different anions have been identified. This brief review summarizes these mechanistic insights and tries to incorporate them into a simple cartoon model depicting the interactions between the channel and Cl- ions that are important for ion translocation.

Animals↗
PubMed1991

Chloride channel inhibition by the venom of the scorpion Leiurus quinquestriatus.

The venom of the scorpion Leiurus quinquestriatus produced a significant, reversible inhibition of reconstituted Cl- channels of the small conductance type found in rat colonic epithelial cells. The kinetics of single-channel block by this venom were consistent with a first-order binding reaction in which the binding of one ligand molecule is sufficient to induce channel block. Single-channel mean block times were c.6 sec at -20 mV, and a KI in the submicromolar range is predicted. The active component has a mol. wt of roughly 5000 as judged by molecular sieve chromatography.

Animals↗
PubMed1988

Two types of cage convulsant action at the GABA-gated chloride channel.

Cage convulsants appear to fall into two groups based on correlations between their potency for inhibiting [35S]t-butylbicyclophosphorothionate binding or gamma-aminobutyric acid-stimulated 36chloride uptake in brain membrane preparations and their toxicity to mice. Polychlorocycloalkanes, picrotoxinin and 2,6,7-trioxabicyclo[2.2.2]octanes with large 1-substituents exhibit high potency in these in vitro brain membrane systems relative to their toxicity (type A action). In contrast, trioxabicyclooctanes with small 1-substituents and tetramethylenedisulfotetramine show much lower in vitro potency relative to their toxicity (type B action). Compounds with type A action are generally more potent insecticides than those with type b action.

Animals↗
PubMed1987

Antagonist-type interaction of glucocorticoids with the GABA receptor-coupled chloride channel.

The interaction of glucocorticoids, corticosterone, cortisol and cortisone, and their reduced metabolites with t-[35S]-butyl bicyclo-phosphorothionate (TBPS) binding sites was examined in vitro. At physiologically relevant concentrations glucocorticoids alter TBPS binding in a biphasic manner. At low nanomolar concentrations glucocorticoids potentiate TBPS binding (20-50% above control) and at high nanomolar and micromolar concentrations they slightly reduce it. The enhancement of TBPS binding by glucocorticoids is due to an increase in the apparent affinity and density of TBPS recognition sites and qualitatively resembles the action of bicuculline methiodide, to potentiate TBPS binding (150% above control). These findings suggest that glucocorticoids may play an important role in modulating neuronal excitability via interactions with the GABA receptor/chloride ionophore complex.

Animals↗
PubMed1984

[Chloride channel].

Cl- current in GABA-sensitive neurons of the frog dorsal root ganglia was separated from other Na+, Ca2+ and K+ currents using the suction pipette technique which allows internal perfusion and current clamp. Adequacy of the internal perfusion technique was assessed from the reversal potential for GABA-induced Cl- current (EGABA) at various intracellular Cl- concentrations. EGABA was equal to the Cl- equilibrium potential (ECl) and behaved as a simple Cl- electrode following changes of external and internal Cl- concentrations [(Cl]0 and [Cl]i. EGABA changed by 58 mV for a tenfold change in either [Cl]0 or [Cl]i at constant [Cl]0 or [Cl]i, respectively. GABA-induced Cl- conductance increased dose-dependently. In addition, single Cl- channel current (icl) and conductance (gamma Cl) were estimated from the Cl- current fluctuation by using both power spectrum and amplitude histogram methods, and the theoretical values were compared with those measured directly by a conventional inside-out patch clamp technique.

Animals↗
PubMed1990

Chloride channels in human platelets: evidence for activation by internal calcium.

Whole-cell patch-clamp recordings were made from freshly isolated human platelets. The pipette contained a high concentration of divalent cations, which permitted easy disruption of cell-attached membrane patches by suction. Single-channel currents were measured when the pipette contained isotonic BaCl2 or MgCl2 saline; over 30 sec -5 min an increasing number of channels appeared until conductance steps through individual channels could no longer be distinguished. The current-voltage relationship was curvilinear; chord conductance at -35 mV was 25 pS increasing to 45 to 52 pS at +45 mV. Ion substitution experiments showed the current to be primarily carried by Cl-. Erev was shifted 30 mV/10-fold change in external Cl- (replaced by gluconate), was similar with BaCl2 or MgCl2 in the pipette and was not significantly shifted by replacing external Na+ with K+. Addition of 1 mM BAPTA to the MgCl2 pipette saline prevented activation of Cl- currents; with isotonic CaCl2 internal saline, current appeared immediately upon patch rupture, suggesting that the Cl- channels are dependent on internal Ca2+. 5-nitro-2-(3-phenylpropylamino)-benzoate, reported to block a Cl- conductance in studies of rat epithelial cells, caused a potent flickery block and may be a useful tool with which to investigate the physiological role of Cl- currents in human platelets.

Blood Platelets↗
PubMed1995

Intracellular loop between transmembrane segments IV and V of cystic fibrosis transmembrane conductance regulator is involved in regulation of chloride channel conductance state.

The cystic fibrosis transmembrane conductance regulator (CFTR) contains two membrane-spanning domains; each consists of six transmembrane segments joined by three extracellular and two intracellular loops of different length. To examine the role of intracellular loops in CFTR channel function, we studied a deletion mutant of CFTR (delta 19 CFTR) in which 19 amino acids were removed from the intracellular loop joining transmembrane segments IV and V. This mutant protein was expressed in a human embryonic kidney cell line (293 HEK). Fully mature glycosylated CFTR (approximately 170 kDa) was immunoprecipitated from cells transfected with wild-type CFTR cDNA, while cells transfected with the mutant gene expressed only a core-glycosylated form (approximately 140 kDa). The chloride efflux rate (measured by 6-methoxyl-N-(3-sulfopropyl) quinolinium SPQ fluorescence) from cells expressing wild-type CFTR increased 600% in response to forskolin. In contrast, delta 19 CFTR-expressing cells had no significant response to forskolin. Western blotting performed on subcellular membrane fractions showed that delta 19 CFTR was located in the same fractions as delta F508 CFTR, a processing mutant of CFTR. These results suggest that delta 19 CFTR is located in the intracellular membranes, without reaching the cell surface. Upon reconstitution into lipid bilayer membranes, delta 19 CFTR formed a functional Cl- channel with gating properties nearly identical to those of the wild-type CFTR channel. However, delta 19 CFTR channels exhibited frequent transitions to a 6-picosiemens subconductance state, whereas wild-type CFTR channels rarely exist in this subconductance state. These data suggest that the intracellular loop is involved in stabilizing the full conductance state of the CFTR Cl- channel.

Amino Acid Sequence↗