Redox regulation of large conductance Ca(2+)-activated K+ channels in smooth muscle cells

A role for the redox site in the modulation of the NMDA receptor by light

Light has been shown to modulate NMDA receptor function. In this study, we have performed experiments aimed at elucidating the putative site of action of light within the receptor structure. Whole-cell recordings were performed in Chinese hamster ovary cells expressing various combinations of NMDA receptor subunits. Although there was no apparent difference in the actions of light between wild-type NR1-NR2A and NR1-NR2B subunit configurations, the light enhancement of NMDA-induced currents was either completely abolished or substantially diminished in the redox site mutants NR1a (C744A, C798A)-NR2B and NR1a (C744A, C798A)-NR2A. Further studies demonstrated that chemical reduction of NR1a-NR2B NMDA receptors decreased its sensitivity to light. In addition, sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES), used to irreversibly bind free sulfhydryl groups and inactivate the redox site, abolished the effects of light on wild-type receptors. In contrast, no free sulfhydryls were available for MTSES following light stimulation, suggesting that light itself could not reduce the redox modulatory site. Our results suggest that a functionally intact, oxidized redox site is necessary for light-induced potentiation. Hence, light and redox modulation of the NMDA receptor may share a common intramolecular pathway for altering the function of this ion channel.

Thiol reagents and nitric oxide modulate the gating of BKCa channels from the guinea-pig taenia caeci

1. The site of the direct modulation of the gating of BKCa channels by the nitric oxide donor s-nitroso-l-cysteine (NOCys) was examined in excised membrane patches of the guinea-pig taenia caeci by the use of various thiol (sulphydryl)-specific reagents, including N-ethylmaleimide (NEM) and three charged methanethiosulphonate (MTS) reagents, namely positively charged 2-aminoethyl MTS hydrobromide (MTSEA) and [2-(trimethylammonium)ethyl] MTS bromide (MTSET) and negatively charged sodium (2-sulphonatoethyl) MTS (MTSES), which all specifically convert sulphydryls to a disulphide. 2. At 10 micro mol/L, NOCys transiently increased the probability of opening (N.Po) of the BKCa channels (at 0 mV) after a delay of 1-2 min. 3. Disulphide-reducing agents, such as dithiothreitol (10 micro mol/L), increased N.Po in a manner that was reversed by the sulphide-oxidizing agent thimerosal (10 micro mol/L). Both positively charged MTSET (2.5 mmol/L) and negatively charged MTSES (2.5 mmol/L) rapidly increased N.Po. However, only the MTSES-evoked increase in N.Po remained after a prolonged washout period. 4. The specific alkylating agent of cysteine thiols NEM (1 mmol/L) and the positively charged, but membrane permeable, MTSEA (2.5 mmol/L) decreased N.Po (at 0 mV). 5. Pre-exposure of excised membrane patches to NEM or MTSES prevented the excitatory actions of NOCys (10 micro mol/L). 6. We conclude that MTSES and NOCys must modify thiols on cysteine residues within basic regions of the channel protein that would electrostatically exclude MTSEA and MTSET. A consensus sequence of various mammalian alpha-subunits of the BKCa channel reveals two pairs of cysteine residues surrounded by basic amino acids that could be the site of action for NOCys and MTSES.

Glutathione and other low-molecular-weight thiols relax guinea pig trachea ex vivo: interactions with nitric oxide?

The aim of this study was to determine the effects of glutathione (GSH) on trachea smooth muscle tension in view of previously reported interactions between GSH and nitric oxide (NO) (Gaston B. Biochim Biophys Acta 1411: 323-333, 1999; Kelm M. Biochim Biophys Acta 1411: 273-289, 1999; and Kharitonov VG, Sundquist AR, and Sharma VS. J Biol Chem 270: 28158-28164, 1995) and the high (millimolar) concentrations of GSH in trachea epithelium (Rahman I, Li XY, Donaldson K, Harrison DJ, and MacNee W. Am J Physiol Lung Cell Mol Physiol 269: L285-L292, 1995). GSH and other thiols (1.0-10 mM) dose dependently decreased the tension in isolated guinea pig tracheas. Relaxations by GSH were paralleled with sevenfold increased nitrite levels (P < 0.05) in the tracheal effluent, suggesting an interaction between GSH and NO. However, preincubation with a NO scavenger did not reduce the relaxations by GSH or its NO adduct, S-nitrosoglutathione (GSNO). Inhibition of guanylyl cyclase inhibited the relaxations induced by GSNO, but not by GSH. Blocking potassium channels, however, completely abolished the relaxing effects of GSH (P < 0.05). Preincubation of tracheas with GSH significantly (P < 0.05) suppressed hyperreactivity to histamine as caused by removal of tracheal epithelium. These data indicate that GSH plays a role in maintaining tracheal tone. The mechanism is probably an antioxidative action of GSH itself rather than an action of NO or GSNO.

Interacting effects of N-terminal variation and strex exon splicing on slo potassium channel regulation by calcium, phosphorylation, and oxidation

We have investigated the structural basis for the phenotype of a native rat Slo (rSlo) potassium channel (BK(Ca); KCNMA1) in a rat pituitary cell line, GH(4)C(1). Opposing regulation of these calcium- and voltage-activated potassium channels by cAMP- and cGMP-dependent protein kinases requires an alternatively spliced exon (strex) of 59 amino acids in the cytoplasmic C terminus of the pore-forming alpha subunit encoded by rslo. However, inclusion of this cysteine-rich exon produces a 10-fold increase in the sensitivity of the channels to inhibition by oxidation. Inclusion of the strex exon also increases channel sensitivity to stimulation by calcium, but responses in the physiological ranges of calcium and voltage require coassembly with beta(1) subunits. With strex present, however, beta(1) subunits only stimulated channels assembled from rSlo alpha subunits with a truncated N terminus beginning MDALI-. Thus N-terminal variation and strex exon splicing in rSlo interact to produce BK(Ca) channels with a physiologically relevant phenotype.

Oxidative stress and potassium channel function

1. Modulation of K+ channel activities by cellular oxidative stress has emerged as a significant determinant of vasomotor function in multiple disease states. 2. Evidence from in vitro and in vivo studies suggest that superoxide (O2-) and hydrogen peroxide (H2O2) enhance BKCa channel activity in rat and cat cerebral arterioles; however, activity is decreased by peroxynitrite (ONOO-) in rat cerebral arteries. The mechanisms of changes in BKCa channel properties are not fully understood and may involve oxidation of cysteine residues that are located in the cell membranes. 3. Studies further suggest that O2- increases KATP channel activity in guinea-pig cardiac myocytes, but decreases opening in cerebral vasculature. Both H2O2 and ONOO- enhance KATP channel activity in the myocardium and in coronary, renal, mesenteric and cerebral vascular beds. Alteration of KATP channels by free radicals may be due to oxidation of SH groups or changes in the cytosolic concentration of ATP. 4. It does appear that O2- produced by either reaction of xanthine and xanthine oxidase or elevated levels of glucose reduces Kv channel activity and the impairments can be partially restored by free radical scavengers, superoxide dismutase and catalase. 5. Thus, redox modulation of potassium channel activity is an important mechanism regulating cell vascular smooth muscle membrane potential.

Ca(2+)-activated K+ channel inhibition by reactive oxygen species

We studied the effect of H(2)O(2) on the gating behavior of large-conductance Ca(2+)-sensitive voltage-dependent K(+) (K(V,Ca)) channels. We recorded potassium currents from single skeletal muscle channels incorporated into bilayers or using macropatches of Xenopus laevis oocytes membranes expressing the human Slowpoke (hSlo) alpha-subunit. Exposure of the intracellular side of K(V,Ca) channels to H(2)O(2) (4-23 mM) leads to a time-dependent decrease of the open probability (P(o)) without affecting the unitary conductance. H(2)O(2) did not affect channel activity when added to the extracellular side. These results provide evidence for an intracellular site(s) of H(2)O(2) action. Desferrioxamine (60 microM) and cysteine (1 mM) completely inhibited the effect of H(2)O(2), indicating that the decrease in P(o) was mediated by hydroxyl radicals. The reducing agent dithiothreitol (DTT) could not fully reverse the effect of H(2)O(2). However, DTT did completely reverse the decrease in P(o) induced by the oxidizing agent 5,5'-dithio-bis-(2-nitrobenzoic acid). The incomplete recovery of K(V,Ca) channel activity promoted by DTT suggests that H(2)O(2) treatment must be modifying other amino acid residues, e.g., as methionine or tryptophan, besides cysteine. Noise analysis of macroscopic currents in Xenopus oocytes expressing hSlo channels showed that H(2)O(2) induced a decrease in current mediated by a decrease both in the number of active channels and P(o).

Glutathione potentiates cloned rat brain large conductance Ca(2+)-activated K(+) channels (rSlo)

We have investigated the modulation of a cloned rat brain alpha-subunit of large conductance Ca(2+)-activated K(+) channels (rSlo K(+) channels) by glutathione (GSH), a physiological sulfhydryl-specific reducing reagent. The application of GSH to the intracellular side of excised inside-out macroscopic patches of rslo-transfected HEK293 cells reversibly activated the currents. The activation rate constants of the current were increased while the deactivation rate constants were decreased by GSH at all voltages tested without any change in the voltage dependence of the rate constants. GSH induced a leftward shift of the steady state conductance-voltage relationship curve of the current with no change in the slope of the curve. These results suggest that modulation by GSH may constitute an important regulatory mechanism of neuronal large conductance Ca(2+)-activated K(+) channels.

SLO-1 potassium channels control quantal content of neurotransmitter release at the C. elegans neuromuscular junction

Six mutants of SLO-1, a large-conductance, Ca(2+)-activated K(+) channel of C. elegans, were obtained in a genetic screen for regulators of neurotransmitter release. Mutants were isolated by their ability to suppress lethargy of an unc-64 syntaxin mutant that restricts neurotransmitter release. We measured evoked postsynaptic currents at the neuromuscular junction in both wild-type and mutants and observed that the removal of SLO-1 greatly increased quantal content primarily by increasing duration of release. The selective isolation of slo-1 as the only ion channel mutant derived from a whole genomic screen to detect regulators of neurotransmitter release suggests that SLO-1 plays an important, if not unique, role in regulating neurotransmitter release.

Rapidly inactivating and non-inactivating calcium-activated potassium currents in frog saccular hair cells

1. Using a semi-intact epithelial preparation we examined the Ca(2+)-activated K(+) (K(Ca)) currents of frog (Rana pipiens) saccular hair cells. After blocking voltage-dependent K(+) (K(V)) currents with 4-aminopyridine (4-AP) an outward current containing inactivating (I(transient)) and non-inactivating (I(steady)) components remained. 2. The contribution of each varied greatly from cell to cell, with I(transient) contributing from 14 to 90 % of the total outward current. Inactivation of I(transient) was rapid (tau approximately 2-3 ms) and occurred within the physiological range of membrane potentials (V(1/2) = -63 mV). Recovery from inactivation was also rapid (tau approximately 10 ms). 3. Suppression of both I(transient) and I(steady) by depolarizations that approached the Ca(2+) equilibrium potential and by treatments that blocked Ca(2+) influx (application Ca(2+)-free saline or Cd(2+)), suggest both are Ca(2+) dependent. Both were blocked by iberiotoxin, a specific blocker of large-conductance K(Ca) channels (BK), but not by apamin, a specific blocker of small-conductance K(Ca) channels. 4. Ensemble-variance analysis showed that I(transient) and I(steady) flow through two distinct populations of channels, both of which have a large single-channel conductance (~100 pS in non-symmetrical conditions). Together, these data indicate that both I(transient) and I(steady) are carried through BK channels, one of which undergoes rapid inactivation while the other does not. 5. Inactivation of I(transient) could be removed by extracellular papain and could later be restored by intracellular application of the 'ball' domain of the auxiliary subunit (beta2) thought to mediate BK channel inactivation in rat chromaffin cells. We hypothesize that I(transient) results from the association of a similar beta subunit with some of the BK channels and that papain removes inactivation by cleaving extracellular sites required for this association.

O(2) modulates large-conductance Ca(2+)-dependent K(+) channels of rat chemoreceptor cells by a membrane-restricted and CO-sensitive mechanism

Hypoxic inhibition of large-conductance Ca(2+)-dependent K(+) channels (maxiK) of rat carotid body type I cells is a well-established fact. However, the molecular mechanisms of such inhibition and the role of these channels in the process of hypoxic transduction remain unclear. We have examined the mechanisms of interaction of O(2) with maxiK channels exploring the effect of hypoxia on maxiK currents recorded with the whole-cell and the inside-out configuration of the patch-clamp technique. Hypoxia inhibits channel activity both in whole-cell and in excised membrane patches. This effect is strongly voltage- and Ca(2+)-dependent, being maximal at low [Ca(2+)] and low membrane potential. The analysis of single-channel kinetics reveals a gating scheme comprising three open and five closed states. Hypoxia inhibits channel activity increasing the time the channel spends in the longest closed states, an effect that could be explained by a decrease in the Ca(2+) sensitivity of those closed states. Reducing maxiK channels with dithiothreitol (DTT) increases channel open probability, whereas oxidizing the channels with 2,2'-dithiopyridine (DTDP) has the opposite effect. These results suggest that hypoxic inhibition is not related with a reduction of channel thiol groups. However, CO, a competitive inhibitor of O(2) binding to hemoproteins, fully reverts hypoxic inhibition, both at the whole-cell and the single-channel level. We conclude that O(2) interaction with maxiK channels does not require cytoplasmic mediators. Such interaction could be mediated by a membrane hemoprotein that, as an O(2) sensor, would modulate channel activity.

High glucose impairs voltage-gated K(+) channel current in rat small coronary arteries

Hyperglycemia is associated with impaired endothelium-dependent dilation that is due to quenching of NO by superoxide (O(2)(. -)). In small coronary arteries (CAs), dilation depends more on smooth muscle hyperpolarization, such as that mediated by voltage-gated K(+) (Kv) channels. We determined whether high glucose enhances O(2)(.-) production and reduces microvascular Kv channel current and functional responses. CAs from Sprague-Dawley rats were incubated 24 hours in medium containing either normal glucose (NG, 5.5 mmol/L D-glucose), high glucose (HG, 23 mmol/L D-glucose), or L-glucose (LG, 5.5 mmol/L D-glucose and 17 mmol/L L-glucose). O(2)(.-) production was increased in HG arteries. Whole-cell patch clamping showed a reduction of 4-aminopyridine (4-AP)-sensitive current (Kv current) from smooth muscle cells of HG CAs versus NG CAs or versus LG CAs (peak density was 9.95+/-5.3 pA/pF for HG versus 27.8+/-6.8 pA/pF for NG and 28.5+/-5.2 pA/pF for LG; P<0.05). O(2)(.-) generation (xanthine+xanthine oxidase) decreased K(+) current density, with no further reduction by 4-AP. Partial restoration was observed with superoxide dismutase and catalase. Constriction to 3 mmol/L 4-AP was reduced in vessels exposed to HG (13+/-5%, P<0.05) versus NG (30+/-7%) or LG (34+/-4%). Responses to KCl and nifedipine were not different among groups. Superoxide dismutase and catalase increased contraction to 4-AP in HG CAs. This is the first direct evidence that exposure of CAs to HG impairs Kv channel activity. We speculate that this O(2)(.-)-induced impairment may reduce vasodilator responsiveness in the coronary circulation of subjects with coronary disease or its risk factors.

Gating properties conferred on BK channels by the beta3b auxiliary subunit in the absence of its NH(2)- and COOH termini

Both beta1 and beta2 auxiliary subunits of the BK-type K(+) channel family profoundly regulate the apparent Ca(2)+ sensitivity of BK-type Ca(2)+-activated K(+) channels. Each produces a pronounced leftward shift in the voltage of half-activation (V(0.5)) at a given Ca(2)+ concentration, particularly at Ca(2)+ above 1 microM. In contrast, the rapidly inactivating beta3b auxiliary produces a leftward shift in activation at Ca(2)+ below 1 microM. In the companion work (Lingle, C.J., X.-H. Zeng, J.-P. Ding, and X.-M. Xia. 2001. J. Gen. Physiol. 117:583-605, this issue), we have shown that some of the apparent beta3b-mediated shift in activation at low Ca(2)+ arises from rapid unblocking of inactivated channels, unlike the actions of the beta1 and beta2 subunits. Here, we compare effects of the beta3b subunit that arise from inactivation, per se, versus those that may arise from other functional effects of the subunit. In particular, we examine gating properties of the beta3b subunit and compare it to beta3b constructs lacking either the NH(2)- or COOH terminus or both. The results demonstrate that, although the NH(2) terminus appears to be the primary determinant of the beta3b-mediated shift in V(0.5) at low Ca(2)+, removal of the NH(2) terminus reveals two other interesting aspects of the action of the beta3b subunit. First, the conductance-voltage curves for activation of channels containing the beta3b subunit are best described by a double Boltzmann shape, which is proposed to arise from two independent voltage-dependent activation steps. Second, the presence of the beta3b subunit results in channels that exhibit an anomalous instantaneous outward current rectification that is correlated with a voltage dependence in the time-averaged single-channel current. The two effects appear to be unrelated, but indicative of the variety of ways that interactions between beta and alpha subunits can affect BK channel function. The COOH terminus of the beta3b subunit produces no discernible functional effects.

Regulation of sodium currents through oxidation and reduction of thiol residues

Changes in redox state are involved in several physiological and pathophysiological processes. Previous experiments have demonstrated that nitric oxide can function as a reactive oxygen species, inhibiting neuronal sodium currents by nitrosylation of thiol residues. We hypothesized that nitric oxide and thiol oxidizers similarly modulate voltage-dependent sodium currents. Voltage-dependent sodium currents were studied with the whole-cell patch-clamp technique in NB41A3 neuroblastoma cells. The nitric oxide donor 3-(2-hydroxy-2-nitroso-1-propylhydrazino)-1-propanamine did not affect sodium currents. In contrast, the thiol oxidizers thimerosal and 4,4'-dithiopyridine significantly inhibited sodium currents. The effect of thimerosal persisted after washout, but could be fully reversed by the reducing agent dithiothreitol. Reduced glutathione did not restore the sodium current amplitude when given extracellularly, while intracellular glutathione prevented the inhibitory effect of thimerosal. Pretreatment with the alkylating agent N-ethylmaleimide blocked the inhibitory action of thimerosal. Thiol oxidation caused a shift in the voltage dependence of fast and slow inactivation to more hyperpolarized potentials without concomitant effects on the voltage dependence of activation. Mercaptoethanol and reduced glutathione enhanced sodium currents by shifting the voltage dependence of inactivation to depolarized potentials. These results demonstrate that the oxidation and reduction of thiol residues alters the properties of voltage-sensitive sodium channels and may play an important role in the regulation of membrane excitability.

Oxidative regulation of large conductance calcium-activated potassium channels

Reactive oxygen/nitrogen species are readily generated in vivo, playing roles in many physiological and pathological conditions, such as Alzheimer's disease and Parkinson's disease, by oxidatively modifying various proteins. Previous studies indicate that large conductance Ca(2+)-activated K(+) channels (BK(Ca) or Slo) are subject to redox regulation. However, conflicting results exist whether oxidation increases or decreases the channel activity. We used chloramine-T, which preferentially oxidizes methionine, to examine the functional consequences of methionine oxidation in the cloned human Slo (hSlo) channel expressed in mammalian cells. In the virtual absence of Ca(2+), the oxidant shifted the steady-state macroscopic conductance to a more negative direction and slowed deactivation. The results obtained suggest that oxidation enhances specific voltage-dependent opening transitions and slows the rate-limiting closing transition. Enhancement of the hSlo activity was partially reversed by the enzyme peptide methionine sulfoxide reductase, suggesting that the upregulation is mediated by methionine oxidation. In contrast, hydrogen peroxide and cysteine-specific reagents, DTNB, MTSEA, and PCMB, decreased the channel activity. Chloramine-T was much less effective when concurrently applied with the K(+) channel blocker TEA, which is consistent with the possibility that the target methionine lies within the channel pore. Regulation of the Slo channel by methionine oxidation may represent an important link between cellular electrical excitability and metabolism.

Regulation of cell function by methionine oxidation and reduction

Reactive oxygen species (ROS) are generated during normal cellular activity and may exist in excess in some pathophysiological conditions, such as inflammation or reperfusion injury. These molecules oxidize a variety of cellular constituents, but sulfur-containing amino acid residues are especially susceptible. While reversible cysteine oxidation and reduction is part of well-established signalling systems, the oxidation and the enzymatically catalysed reduction of methionine is just emerging as a novel molecular mechanism for cellular regulation. Here we discuss how the oxidation of methionine to methionine sulfoxide in signalling proteins such as ion channels affects the function of these target proteins. Methionine sulfoxide reductase, which reduces methionine sulfoxide to methionine in a thioredoxin-dependent manner, is therefore not only an enzyme important for the repair of age- or degenerative disease-related protein modifications. It is also a potential missing link in the post-translational modification cycle involved in the specific oxidation and reduction of methionine residues in cellular signalling proteins, which may give rise to activity-dependent plastic changes in cellular excitability.

Modulation of large conductance calcium-activated potassium channels from rat hippocampal neurons by glutathione

We have investigated the modulation of neuronal large conductance Ca2+-activated K+ channels by glutathione. Single channel recordings were made from cultured neonatal rat hippocampal neurons by using excised inside-out patch clamp method. Glutathione, a physiological sulfhydryl specific reducing reagent, increased channel activities in concentration dependent manner with half activation concentration of 710 microM. Conversely, oxidized form of glutathione inhibited channel activities with half inhibition concentration of 520 microM. Our results provide direct evidence that when neuronal large conductance Ca2+-activated K+ channels are exposed to reducing or oxidizing environments, channel activities are increased or decreased in opposite directions due to the redox modification. This may constitute an important regulatory mechanism of neuronal Ca2+-activated K+ channel activities.

Potent inhibition of the aortic smooth muscle maxi-K channel by clinical doses of ethanol

We investigated the effects of clinically relevant ethanol concentrations (5-20 mM) on the single-channel kinetics of bovine aortic smooth muscle maxi-K channels reconstituted in lipid bilayers (1:1 palmitoyl-oleoyl-phosphatidylethanolamine: palmitoyl-oleoyl-phosphatidylcholine). Ethanol at 10 and 20 mM decreased the channel open probability (P(o)) by 75 +/- 20.3% mainly by increasing the mean closed time (+82 to +960%, n = 7). In some instances, ethanol also decreased the mean open time (-40.8 +/- 22. 5%). The P(o)-voltage relation in the presence of 20 mM ethanol exhibited a rightward shift in the midpoint of voltage activation (DeltaV(1/2) congruent with 17 mV), a slightly steeper relationship (change in slope factor, Deltak, congruent with -2.5 mV), and a decreased maximum P(o) (from approximately 0.82 to approximately 0. 47). Interestingly, channels inhibited by ethanol at low Ca(2+) concentrations (2.5 microM) were very resistant to ethanol in the presence of increased Ca(2+) (>/= 20 microM). Alcohol consumption in clinically relevant amounts may alter the contribution of maxi-K channels to the regulation of arterial tone.

Purinergic activation of BK channels in clonal kidney cells (Vero cells)

We have studied the activation of a high-conductance channel in clonal kidney cells from African green monkey (Vero cells) using patch-clamp recordings and microfluorometric (fura-2) measurements of cytosolic Ca2+. The single-channel conductance in excised patches is 170 pS in symmetrical 140 mM KCl. The channel is highly selective for K+ and activated by membrane depolarization and application of Ca2+ to the cytoplasmatic side of the patch. The channel is, thus, a large-conductance Ca2+-activated K+ channel (BK channel). Cell-attached recordings revealed that the channel is inactive in unstimulated cells. Extracellular application of less than 0.1 microM ATP transiently increased the cytosolic Ca2+ concentration ([Ca2+]i) to about 550 nM, and induced membrane hyperpolarization caused by Ca2+-activated K+ currents. ATP stimulation also activated BK channels in cell-attached patches at both the normal-resting potential and during membrane hyperpolarization. The increase in [Ca2+]i was owing to Ca2+ release from internal stores, suggesting that Vero cells express G-protein-coupled purinergic receptors (P2Y) mediating IP3-induced release of Ca2+. The P2Y receptors were sensitive to both uracil triphosphate (UTP) and adenosine diphosphate (ADP), and the rank of agonist potency was ATP >> UTP >/= ADP. This result indicates the presence of both P2Y1 and P2Y2 receptors or a receptor subtype with untypical agonist sensitivity. It has previously been shown that hypotonic challenge activates BK channels in both normal and clonal kidney cells. The subsequent loss of KCl may be an important factor in cellular volume regulation. Our results support the idea of an autocrine role of ATP in this process. A minute release of ATP induced by hypotonically evoked membrane stretch may activate the P2Y receptors, subsequently increasing [Ca2+]i and thus causing K+ efflux through BK channels.

Redox modulation of large conductance calcium-activated potassium channels in CA1 pyramidal neurons from adult rat hippocampus

Redox regulation of BK(Ca) channels was studied in CA1 pyramidal neurons of adult rat hippocampus by using inside-out configuration of patch clamp. Intracellular application of oxidizing agent 5, 5'-dithio-bis(2-nitrobenzoic acid) (DTNB) markedly increased activity of BK(Ca) channels and this stimulating action persisted even after washout. In contrast, the reducing agent dithiothreitol (DTT) had no apparent effects on channel activity but could reverse the pre-exposure of DTNB-induced enhancement. The increase in channel activity produced by DTNB was due to shortened closed time as well as prolonged open time. The effects exerted by another redox couple glutathione disulphide and its reducing form were similar as DTNB and DTT. The present results indicate that BK(Ca) channels in CA1 pyramidal neurons can be modulated by intracellular redox potential, and that augmentation of BK(Ca) channels by oxidative stress might contribute to the postischemic electrophysiological alterations of CA1 pyramidal neurons.

Peroxynitrite reversibly inhibits Ca(2+)-activated K(+) channels in rat cerebral artery smooth muscle cells

Peroxynitrite (ONOO(-)) is a contractile agonist of rat middle cerebral arteries. To determine the mechanism responsible for this component of ONOO(-) bioactivity, the present study examined the effect of ONOO(-) on ionic current and channel activity in rat cerebral arteries. Whole cell recordings of voltage-clamped cells were made under conditions designed to optimize K(+) current. The effects of iberiotoxin, a selective inhibitor of large-conductance Ca(2+)-activated K(+) (BK) channels, and ONOO(-) (10-100 microM) were determined. At a pipette potential of +50 mV, ONOO(-) inhibited 39% of iberiotoxin-sensitive current. ONOO(-) was selective for iberiotoxin-sensitive current, whereas decomposed ONOO(-) had no effect. In excised, inside-out membrane patches, channel activity was recorded using symmetrical K(+) solutions. Unitary currents were sensitive to increases in internal Ca(2+) concentration, consistent with activity due to BK channels. Internal ONOO(-) dose dependently inhibited channel activity by decreasing open probability and mean open times. The inhibitory effect of ONOO(-) could be overcome by reduced glutathione. Glutathione, added after ONOO(-), restored whole cell current amplitude to control levels and reverted single-channel gating to control behavior. The inhibitory effect of ONOO(-) on membrane K(+) current is consistent with its contractile effects in isolated cerebral arteries and single myocytes. Taken together, our data suggest that ONOO(-) has the potential to alter cerebral vascular tone by inhibiting BK channel activity.

Nitric oxide and thiol reagent modulation of Ca2+-activated K+ (BKCa) channels in myocytes of the guinea-pig taenia caeci

The modulation of large conductance Ca2+-activated K+ (BKCa) channels by the nitric oxide (NO) donors S-nitroso-L-cysteine (NOCys) and sodium nitroprusside (SNP) and agents which oxidize or reduce reactive thiol groups were compared in excised inside-out membrane patches of the guinea-pig taenia caeci. When the cytosolic side of excised patches was bathed in a physiological salt solution (PSS) containing 130 mM K+ and 15 nM Ca2+, few BKCa channel openings were recorded at potentials negative to 0 mV. However, the current amplitude and open probability (NPo) of these BKCa channels increased with patch depolarization. A plot of ln(NPo) against the membrane potential (V) fitted with a straight line revealed a voltage at half-maximal activation (V0.5) of 9.4 mV and a slope (K) indicating an e-fold increase in NPo with 12.9 mV depolarization. As the cytosolic Ca2+ was raised to 150 nM, V0.5 shifted 11.5 mV in the negative direction, with little change in K (13.1 mV). NOCys (10 microM) and SNP (100 microM) transiently increased NPo 16- and 3. 7-fold, respectively, after a delay of 2-5 min. This increase in NPo was associated with an increase in the number of BKCa channel openings evoked at positive potentials by ramped depolarizations (between -60 and +60 mV). Moreover, this NOCys-induced increase in NPo was still evident in the presence of 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM), the specific blocker of soluble guanylyl cyclase. The sulfhydryl reducing agents dithiothreitol (DTT; 10 and 100 microM) and reduced glutathione (GSH; 1 mM) also significantly increased NPo (at 0 mV) 7- to 9-fold, as well as increasing the number of BKCa channel openings evoked during ramped depolarizations. Sulfhydryl oxidizing agents thimerosal (10 microM) and 4,4'-dithiodipyridine (4,4DTDP; 10 microM) and the thiol-specific alkylating agent N-ethylmaleimide (NEM; 1 mM) significantly decreased NPo (at 0 mV) to 40-50% of control values after 5-10 min. Ramped depolarizations to +100 mV evoked relatively few BKCa channel openings. The effects of thimerosal on NPo were readily reversed by DTT, while the effects of NOCys were prevented by NEM. It was concluded that both redox modulation and nitrothiosylation of cysteine groups on the cytosolic surface of the alpha subunit of the BKCa channel protein can alter channel gating.

Inhibitory action of thimerosal, a sulfhydryl oxidant, on sodium channels in rat sensory neurons

The effects of thimerosal, a sulfhydryl oxidizing agent, on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium channels in rat dorsal root ganglion neurons were studied using the whole-cell patch clamp technique. Thimerosal blocked the two types of sodium channels in a dose-dependent manner. The inhibitory effect of thimerosal was much more pronounced in TTX-R sodium channels than TTX-S sodium channels. The effect of thimerosal was irreversible upon wash-out with thimerosal-free external solution. However, dithiothreitol, a reducing agent, partially reversed it. Thimerosal shifted the steady-state inactivation curves for both types of sodium channels in the hyperpolarizing direction. The voltage dependence of activation of both types of sodium channels was shifted in the depolarizing direction by thimerosal. The inactivation rate in both types of sodium channels increased after thimerosal treatment. All these effects of thimerosal would add up to cause a depression of sodium channel function leading to a diminished neuronal excitability.

N-Ethylmaleimide modulation of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels in rat dorsal root ganglion neurons

The effects of N-ethylmaleimide (NEM), an alkylating reagent to protein sulfhydryl groups, on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium channels in rat dorsal root ganglion (DRG) neurons were studied using the whole cell configuration of patch-clamp technique. When currents were evoked by step depolarizations to 0 mV from a holding potential of -80 mV NEM decreased the amplitude of TTX-S sodium current, but exerted little or no effect on that of TTX-R sodium current. The inhibitory effect of NEM on TTX-S sodium channel was mainly due to the shift of the steady-state inactivation curve in the hyperpolarizing direction. NEM did not affect the voltage-dependence of the activation of TTX-S sodium channel. The steady-state inactivation curve for TTX-R sodium channel was shifted by NEM in the hyperpolarizing direction as that for TTX-S sodium channel. NEM caused a change in the voltage-dependence of the activation of TTX-R sodium channel unlike TTX-S sodium channel. After NEM treatment, the amplitudes of TTX-R sodium currents at test voltages below -10 mV were increased, but those at more positive voltages were not affected. This was explained by the shift in the conductance-voltage curve for TTX-R sodium channels in the hyperpolarizing direction after NEM treatment.

Monochloramine directly modulates Ca(2+)-activated K(+) channels in rabbit colonic muscularis mucosae

BACKGROUND & AIMS

Mesenteric ischemia, infection, and inflammatory bowel disease may eventuate in severe colitis, complicated by toxic megacolon with impending intestinal perforation. Monochloramine (NH(2)Cl) is a membrane-permeant oxidant generated during colitis by the large amount of ambient luminal NH(3) in the colon. Reactive oxygen metabolites can modulate smooth muscle ion channels and thereby affect colonic motility, which is markedly impaired in colitis.

METHODS

Effects of NH(2)Cl on ionic currents in the innermost smooth muscle layer of the colon, the tunica muscularis mucosae, were examined using the patch clamp technique. Membrane potential in whole tissue strips was measured using high-resistance microelectrodes.

RESULTS

Whole cell voltage clamp experiments showed that NH(2)Cl (3-30 micromol/L) enhanced outward currents in a dose-dependent manner, increasing currents more than 8-fold at a test potential of +30 mV. Tail current analysis showed that the currents enhanced by NH(2)Cl were K(+) currents. Inhibition by tetraethylammonium and iberiotoxin suggested that these currents represented activation of large-conductance, Ca(2+)-activated K(+) channels. The membrane-impermeant oxidant taurine monochloramine, however, had no effect on whole cell currents. Single-channel studies in inside-out patches showed that NH(2)Cl increased open probability of a 257-pS channel in symmetrical (140 mmol/L) K(+). In the presence of NH(2)Cl, the steady-state voltage dependence of activation was shifted by -22 mV to the left with no change in the single-channel amplitude. The sulfhydryl alkylating agent N-ethylmaleimide prevented NH(2)Cl-induced channel activation. NH(2)Cl also hyperpolarized intact muscle strips, an effect blocked by iberiotoxin.

CONCLUSIONS

NH(2)Cl, at concentrations expected to be found during colitis, may contribute to smooth muscle dysfunction by a direct oxidant effect on maxi K(+) channels.

O(2) deprivation inhibits Ca(2+)-activated K(+) channels via cytosolic factors in mice neocortical neurons

O(2) deprivation induces membrane depolarization in mammalian central neurons. It is possible that this anoxia-induced depolarization is partly mediated by an inhibition of K(+) channels. We therefore performed experiments using patch-clamp techniques and dissociated neurons from mice neocortex. Three types of K(+) channels were observed in both cell-attached and inside-out configurations, but only one of them was sensitive to lack of O(2). This O(2)-sensitive K(+) channel was identified as a large-conductance Ca(2+)-activated K(+) channel (BK(Ca)), as it exhibited a large conductance of 210 pS under symmetrical K(+) (140 mM) conditions, a strong voltage-dependence of activation, and a marked sensitivity to Ca(2+). A low-O(2) medium (PO(2) = 10-20 mmHg) markedly inhibited this BK(Ca) channel open probability in a voltage-dependent manner in cell-attached patches, but not in inside-out patches, indicating that the effect of O(2) deprivation on BK(Ca) channels of mice neocortical neurons was mediated via cytosol-dependent processes. Lowering intracellular pH (pH(i)), or cytosolic addition of the catalytic subunit of a cAMP-dependent protein kinase A in the presence of Mg-ATP, caused a decrease in BK(Ca) channel activity by reducing the sensitivity of this channel to Ca(2+). In contrast, the reducing agents glutathione and DTT increased single BK(Ca) channel open probability without affecting unitary conductance. We suggest that in neocortical neurons, (a) BK(Ca) is modulated by O(2) deprivation via cytosolic factors and cytosol-dependent processes, and (b) the reduction in channel activity during hypoxia is likely due to reduced Ca(2+) sensitivity resulting from cytosolic alternations such as in pH(i) and phosphorylation. Because of their large conductance and prevalence in the neocortex, BK(Ca) channels may be considered as a target for pharmacological intervention in conditions of acute anoxia or ischemia.

Redox agents regulate ion channel activity in vacuoles from higher plant cells

The ability of redox agents to modulate certain characteristics of voltage- and calcium-activated channels has been recently investigated in a variety of animal cells. We report here the first evidence that redox agents regulate the activation of ion channels in the tonoplast of higher plants. Using the patch-clamp technique, we have demonstrated that, in tonoplasts from the leaves of the marine seagrass Posidonia oceanica and the root of the sugar beet, a variety of sulphydryl reducing agents, added at the cytoplasmic side of the vacuole, reversibly favoured the activation of the voltage-dependent slow vacuolar (SV) channel. Antioxidants, like dithiothreitol (DTT) and the reduced form of glutathione, gave a reversible increase of the voltage-activated current and faster kinetics of channel activation. Other reducing agents, such as ascorbic acid, also increased the SV currents, although to a lesser extent in comparison with DTT and glutathione, while the oxidising agent chloramine-T irreversibly abolished the activity of the channel. Single channel experiments demonstrated that DTT reversibly increased the open probability of the channel, leaving the conductance unaltered. The regulation of channel activation by glutathione may correlate ion transport with other crucial mechanisms that in plants control turgor regulation, response to oxidative stresses, detoxification and resistance to heavy metals.

Direct activation of K(Ca) channel in airway smooth muscle by nitric oxide: involvement of a nitrothiosylation mechanism?

Clinically, nitric oxide (NO*) is widely used as a pulmonary vaso- and bronchodilator agent. However, the precise molecular mechanisms by which NO. induces smooth muscle relaxation are not well established. It has been suggested that NO. relaxes airway smooth muscle (ASM) via a 3',5'-cyclic guanosine monophosphate (cGMP)-dependent pathway, and our previous work has shown that Ca2+-activated K+ (KCa) channels are susceptible to cGMP-dependent protein kinase (PKG)-dependent phosphorylation (A. Alioua, J. P. Huggins, and E. Rousseau. Am. J. Physiol. 1995;268:L1057-L1063). To assess whether KCa channels are also directly activated by NO. or one of its derivatives such as peroxynitrite, the activity of these channels was measured upon fusion of sarcolemmal vesicles derived from bovine tracheal smooth muscle cells into planar lipid bilayers (PLB). It was found that in the absence of adenosine triphosphate (ATP), cGMP, and cGMP-dependent protein kinase, NO* donors such as 1-propanamine-3-(2-hydroxy-2-nitroso-1-propylhydrazine) (PAPA NONOate) or 3-morpholinosydnonimine hydrochloride (SIN-1) in the presence of superoxide dismutase (SOD), added on either side of the bilayer, caused a concentration- dependent increase in the open probability (Po) of KCa channels without altering their unitary conductance. Release of NO*, which was measured by chemiluminescence analysis in parallel experiments, affected the gating behavior of KCa channels in the presence of SOD and ethyleneglycol-bis-(beta-aminoethyl ether)- N,N'-tetraacetic acid (EGTA) by reducing the mean closed times and increasing the number and duration of short open events. PAPA NONOate, a true NO. donor, had similar effects in the presence of ethylenediaminetetraacetic acid (EDTA), a heavy-metal chelator, and K-urate, a peroxynitrite scavenger. Addition of either 5 mM dithiothreitol (DTT) or 5 mM reduced glutathione (GSH), as well as 5 mM N-ethylmaleimide (NEM)-an alkylating agent-to the trans (intracellular) side of an experimental chamber slightly increased channel Po but prevented further channel activation by NO* donors. However, neither DTT nor GSH was able to reverse the effect of NO*. In contrast to SIN-1, DTT had no effect when added to the cis (extracellular) side of the chamber. This suggests that the effect of NO* is most likely due to a chemical modification (nitrothiosylation) of intracellular sulfhydryl group(s). Neither PAPA NONOate (NO*), nor SIN-1 had any effect on sarcolemmal Cl- channels reconstituted from the same membrane preparations. Pharmacomechanical measurements made on epithelium-denuded rat bronchus showed that 100 nM charybdotoxin decreased the sensitivity of bronchial smooth muscle to SIN-1-induced relaxations. Altogether, our data suggest that NO-induced bronchorelaxation occurs partly via a direct activation of KCa channels, possibly through a covalent interaction with the cytoplasmic side of their alpha subunit.

pH modulation of Ca2+ responses and a Ca2+-dependent K+ channel in cultured rat hippocampal neurones

1. The effects of changes in extra- and intracellular pH (pHo and pHi, respectively) on depolarization-evoked rises in intracellular free Ca2+ concentration ([Ca2+]i) and the activity of a Ca2+-dependent K+ channel were investigated in cultured fetal rat hippocampal neurones. 2. In neurones loaded with 2', 7'-bis-(2-carboxyethyl)-5-(and -6)-carboxyfluorescein (BCECF), changes in pHo evoked changes in pHi. At room temperature, the ratio DeltapHi : DeltapHo (the slope of the regression line relating pHi to pHo) was 0.37 under HCO3-/CO2-buffered conditions and 0.45 under Hepes-buffered conditions; corresponding values at 37 C were 0.71 and 0.79, respectively. The measurements of changes in pHi evoked by changes in pHo were employed in subsequent experiments to correct for the effects of changes in pHi on the Kd of fura-2 for Ca2+. 3. In fura-2-loaded neurones, rises in [Ca2+]i evoked by transient exposure to 50 mM K+ were reduced and enhanced during perfusion with acidic and alkaline media, respectively, compared with control responses at pHo 7.3. Fifty percent inhibition of high-[K+]o-evoked rises in [Ca2+]i corresponded to pHo 7.23. In the presence of 10 microM nifedipine, 50 % inhibition of high-[K+]o-evoked responses corresponded to pHo 7.20, compared with a pHo of 7.31 for 50% inhibition of [Ca2+]i transients evoked by N-methyl-D-aspartate. 4. Changes in pHi at a constant pHo were evoked by exposing neurones to weak acids or bases and quantified in BCECF-loaded cells. Following pH-dependent corrections for the Kd of fura-2 for Ca2+, rises in [Ca2+]i evoked by high-[K+]o in fura-2-loaded cells were found to be affected only marginally by changes in pHi. When changes in pHi similar to those observed during the application of weak acids or bases were elicited by changing pHo, reductions in pH inhibited rises in [Ca2+]i evoked by 50 mM K+ whereas increases in pH enhanced them. 5. The effects of changes in pH on the kinetic properties of a BK-type Ca2+-dependent K+ channel were investigated. In inside-out patches excised from neurones in sister cultures to those used in the microspectrofluorimetric studies, with internal [Ca2+] at 20 microM, channel openings at an internal pH of 6.7 were generally absent whereas at pH 7.3 (or 7.8) the open probability was high. In contrast, channel activity in outside-out patches was not affected by reducing the pH of the bath (external) solution from 7.3 to 6.7. In inside-out patches with internal [Ca2+] at 0.7 microM, a separate protocol was applied to generate transient activation of the channel at a potential of 0 mV following a step from a holding level of -80 mV. In this case open probabilities were 0.81 (at pH 7.8), 0.57 (pH 7.3), 0.19 (pH 7.0) and 0.04 (pH 6.7). Channel conductance was not affected by changes in internal pH. 6. The results indicate that, in fetal rat hippocampal neurones, depolarization-evoked rises in [Ca2+]i mediated by the influx of Ca2+ ions through dihydropyridine-sensitive and -resistant voltage-activated Ca2+ channels are modulated by changes in pHo. The effects of pHo cannot be accounted for by changes in pHi consequent upon changes in pHo. However, changes in pHi affect the unitary properties of a Ca2+-dependent K+ channel. The results support the notion that pHo and/or pHi transients may serve a modulatory role in neuronal function.

Calcium-activated potassium channels

Calcium-activated potassium channels are fundamental regulators of neuronal excitability, participating in interspike interval and spike-frequency adaptation. For large-conductance calcium-activated potassium (BK) channels, recent experiments have illuminated the fundamental biophysical mechanisms of gating, demonstrating that BK channels are voltage gated and calcium modulated. Structurally, BK channels have been shown to possess an extracellular amino-terminal domain, different from other potassium channels. Domains and residues involved in calcium-gating, and perhaps calcium binding itself, have been identified. For small- and intermediate-conductance calcium-activated potassium channels, SK and IK channels, clones have only recently become available, and they show that SK channels are a distinct subfamily of potassium channels. The biophysical properties of SK channels demonstrate that kinetic differences between apamin-sensitive and apamin-insensitive slow afterhyperpolarizations are not attributable to intrinsic gating differences between the two subtypes. Interestingly, SK and IK channels may prove effective drug targets for diseases such as myotonic muscular dystrophy and sickle cell anemia.

Distribution of Ca2+-activated K+ channel isoforms along the tonotopic gradient of the chicken's cochlea

In some cochleae, the number and kinetic properties of Ca2+-activated K+ (KCa) channels partly determine the characteristic frequency of each hair cell and thus help establish a tonotopic map. In the chicken's basilar papilla, we found numerous isoforms of KCa channels generated by alternative mRNA splicing at seven sites in a single gene, cSlo. In situ polymerase chain reactions demonstrated cSlo expression in hair cells and revealed differential distributions of KCa channel isoforms along the basilar papilla. Analysis of single hair cells by the reverse transcription polymerase chain reaction confirmed the differential expression of channel variants. Heterologously expressed cSlo variants differed in their sensitivities to Ca2+ and voltage, suggesting that the distinct spatial distributions of cSlo variants help determine the tonotopic map.