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

Sex-specific mechanisms of cerebral microvascular BKCa dysfunction in a mouse model of Alzheimer's disease

INTRODUCTION

Cerebrovascular dysfunction occurs in Alzheimer's disease (AD), impairing hemodynamic regulation. Large conductance Ca2+-activated K+ channels (BKCa) regulate cerebrovascular reactivity and are impaired in AD. BKCa activity depends on intracellular Ca2+ (Ca2+ sparks) and nitro-oxidative post-translational modifications. However, whether these mechanisms underlie BKCa impairment in AD remains unknown.

METHODS

Cerebral arteries from 5x-FAD and wild-type (WT) littermates were used for molecular biology, electrophysiology, ex vivo, and in vivo experiments.

RESULTS

Arterial BKCa activity is reduced in 5x-FAD via sex-dependent mechanisms: in males, there is lower BKα subunit expression and less Ca2+ sparks. In females, we observed reversible nitro-oxidative modification of BKCa. Further, BKCa is involved in hemodynamic regulation in WT mice, and its dysfunction is associated with vascular deficits in 5x-FAD.

DISCUSSION

Our data highlight the central role played by BKCa in cerebral hemodynamic regulation and that molecular mechanisms of its impairment diverge based on sex in 5x-FAD.

HIGHLIGHTS

Cerebral microvascular BKCa dysfunction occurs in both female and male 5x-FAD. Reduction in BKα subunit protein and Ca2+ sparks drive the dysfunction in males. Nitro-oxidative stress is present in females, but not males, 5x-FAD. Reversible nitro-oxidation of BKα underlies BKCa dysfunction in female 5x-FAD.

Redox Regulation of K+ Channel: Role of Thioredoxin

Significance: Potassium channels regulate the influx and efflux of K+ ions in various cell types that generate and propagate action potential associated with excitation, contraction, and relaxation of various cell types. Although redox active cysteines are critically important for channel activity, the redox regulation of K+ channels by thioredoxin (Trx) has not been systematically reviewed. Recent Advances: Redox regulation of K+ channel is now increasingly recognized as drug targets in the pathological condition of several cardiovascular disease processes. The role of Trx in regulation of these channels and its implication in pathological conditions have not been adequately reviewed. This review specifically focuses on the redox-regulatory role of Trx on K+ channel structure and function in physiological and pathophysiological conditions. Critical Issues: Ion channels, including K+ channel, have been implicated in the functioning of cardiomyocyte excitation-contraction coupling, vascular hyperpolarization, cellular proliferation, and neuronal stimulation in physiological and pathophysiological conditions. Although oxidation-reduction of ion channels is critically important in their function, the role of Trx, redox regulatory protein in regulation of these channels, and its implication in pathological conditions need to be studied to gain further insight into channel function. Future Directions: Future studies need to map all redox regulatory pathways in channel structure and function using novel mouse models and redox proteomic and signal transduction studies, which modulate various currents and altered excitability of relevant cells implicated in a pathological condition. We are yet at infancy of studies related to redox control of various K+ channels and structured and focused studies with novel animal models. Antioxid. Redox Signal. 41, 818-844.

Sex-specific mechanisms of cerebral microvascular BK Ca dysfunction in a mouse model of Alzheimer's disease

BACKGROUND

Cerebral microvascular dysfunction and nitro-oxidative stress are present in patients with Alzheimer's disease (AD) and may contribute to disease progression and severity. Large conductance Ca 2+ -activated K + channels (BK Ca ) play an essential role in vasodilatory responses and maintenance of myogenic tone in resistance arteries. BK Ca impairment can lead to microvascular dysfunction and hemodynamic deficits in the brain. We hypothesized that reduced BK Ca function in cerebral arteries mediates microvascular and neurovascular responses in the 5x-FAD model of AD.

METHODS

BK Ca activity in the cerebral microcirculation was assessed by patch clamp electrophysiology and pressure myography, in situ Ca 2+ sparks by spinning disk confocal microscopy, hemodynamics by laser speckle contrast imaging. Molecular and biochemical analyses were conducted by affinity-purification assays, qPCR, Western blots and immunofluorescence.

RESULTS

We observed that pial arteries from 5-6 months-old male and female 5x-FAD mice exhibited a hyper-contractile phenotype than wild-type (WT) littermates, which was linked to lower vascular BK Ca activity and reduced open probability. In males, BK Ca dysfunction is likely a consequence of an observed lower expression of the pore-forming subunit BKα and blunted frequency of Ca 2+ sparks, which are required for BK Ca activity. However, in females, impaired BK Ca function is, in part, a consequence of reversible nitro-oxidative changes in the BK α subunit, which reduces its open probability and regulation of vascular tone. We further show that BK Ca function is involved in neurovascular coupling in mice, and its dysfunction is linked to neurovascular dysfunction in the model.

CONCLUSION

These data highlight the central role played by BK Ca in cerebral microvascular and neurovascular regulation, as well as sex-dependent mechanisms underlying its dysfunction in a mouse model of AD.

Redox Regulation of Mitochondrial Potassium Channels Activity

Redox reactions exert a profound influence on numerous cellular functions with mitochondria playing a central role in orchestrating these processes. This pivotal involvement arises from three primary factors: (1) the synthesis of reactive oxygen species (ROS) by mitochondria, (2) the presence of a substantial array of redox enzymes such as respiratory chain, and (3) the responsiveness of mitochondria to the cellular redox state. Within the inner mitochondrial membrane, a group of potassium channels, including ATP-regulated, large conductance calcium-activated, and voltage-regulated channels, is present. These channels play a crucial role in conditions such as cytoprotection, ischemia/reperfusion injury, and inflammation. Notably, the activity of mitochondrial potassium channels is intricately governed by redox reactions. Furthermore, the regulatory influence extends to other proteins, such as kinases, which undergo redox modifications. This review aims to offer a comprehensive exploration of the modulation of mitochondrial potassium channels through diverse redox reactions with a specific focus on the involvement of ROS.

The Potential of a Protein Model Synthesized Absent of Methionine

Methionine is an amino acid long thought to be essential, but only in the case of protein synthesis initiation. In more recent years, methionine has been found to play an important role in antioxidant defense, stability, and modulation of cell and protein activity. Though these findings have expanded the previously held sentiment of methionine having a singular purpose within cells and proteins, the essential nature of methionine can still be challenged. Many of the features that give methionine its newfound functions are shared by the other sulfur-containing amino acid: cysteine. While the antioxidant, stabilizing, and cell/protein modulatory functions of cysteine have already been well established, recent findings have shown a similar hydrophobicity to methionine which suggests cysteine may be able to replace methionine in all functions outside of protein synthesis initiation with little effect on cell and protein function. Furthermore, a number of novel mechanisms for alternative initiation of protein synthesis have been identified that suggest a potential to bypass the traditional methionine-dependent initiation during times of stress. In this review, these findings are discussed with a number of examples that demonstrate a potential model for synthesizing a protein in the absence of methionine.

Potential role of hydrogen sulfide in central nervous system tumors: a narrative review

Central nervous system tumors are classified as diseases of special clinical significance with high disability and high mortality. In addition to cerebrovascular diseases and craniocerebral injuries, tumors are the most common diseases of the central nervous system. Hydrogen sulfide, the third endogenous gas signaling molecule discovered in humans besides nitric oxide and carbon monoxide, plays an important role in the pathophysiology of human diseases. It is reported that hydrogen sulfide not only exerts a wide range of biological effects, but also develops a certain relationship with tumor development and neovascularization. A variety of studies have shown that hydrogen sulfide acts as a vasodilator and angiogenetic factor to facilitate growth, proliferation, migration and invasion of cancer cells. In this review, the pathological mechanisms and the effect of hydrogen sulfide on the central nervous system tumors are introduced.

Role of Airway Smooth Muscle in Inflammation Related to Asthma and COPD

Airway smooth muscle contributes to both contractility and inflammation in the pathophysiology of asthma and COPD. Airway smooth muscle cells can change the degree of a variety of functions, including contraction, proliferation, migration, and the secretion of inflammatory mediators (phenotype plasticity). Airflow limitation, airway hyperresponsiveness, β₂-adrenergic desensitization, and airway remodeling, which are fundamental characteristic features of these diseases, are caused by phenotype changes in airway smooth muscle cells. Alterations between contractile and hyper-contractile, synthetic/proliferative phenotypes result from Ca²⁺ dynamics and Ca²⁺ sensitization. Modulation of Ca²⁺ dynamics through the large-conductance Ca²⁺-activated K⁺ channel/L-type voltage-dependent Ca²⁺ channel linkage and of Ca²⁺ sensitization through the RhoA/Rho-kinase pathway contributes not only to alterations in the contractile phenotype involved in airflow limitation, airway hyperresponsiveness, and β₂-adrenergic desensitization but also to alteration of the synthetic/proliferative phenotype involved in airway remodeling. These Ca²⁺ signal pathways are also associated with synergistic effects due to allosteric modulation between β₂-adrenergic agonists and muscarinic antagonists. Therefore, airway smooth muscle may be a target tissue in the therapy for these diseases. Moreover, the phenotype changing in airway smooth muscle cells with focuses on Ca²⁺ signaling may provide novel strategies for research and development of effective remedies against both bronchoconstriction and inflammation.

BK channel openers NS1619 and NS11021 reverse hydrogen peroxide-induced membrane potential changes in skeletal muscle

Large conductance calcium-activated potassium (BK) channels play a crucial role in the repolarization and after-hyperpolarization phases of the cell membrane. The channel openers are also used in treatment of some diseases, including hypo/hyperkalemic periodic paralysis. However, little is known about the effects of BK channels and the channel activators on membrane potentials in skeletal muscle. In addition, the effects of reactive oxygen species (ROS) on BK channels in skeletal muscle are also unknown. Therefore, the aim of this study was to determine the effects of BK channel openers and ROS on membrane potentials in skeletal muscle fibers. For this purpose, resting membrane potentials and action potentials (AP) of frog gastrocnemius muscles were recorded in the presence of commonly used BK channel openers NS1619 and NS11021, H₂O₂ (a type of ROS), and both using intracellular microelectrode technique. The channel activators significantly and dose-dependently decreased amplitude and increased rise time of AP but did not impact repolarization. The presence of H₂O₂ plus NS1619 or NS11021 resulted in significant change because the channel openers completely reversed the deleterious effects of hydrogen peroxide on the repolarization phase of AP in skeletal muscle fibers. In the present study, the contributions of BK channel activation and the modulatory role of H₂O₂ on membrane potentials was demonstrated in skeletal muscle fibers, for the first time. Moreover, it should be noted that BK channel openers should be used in the treatment of reactive oxygen species-induced skeletal muscle diseases.

Protein Promiscuity in H2O2 Signaling

SIGNIFICANCE

Decrypting the cellular response to oxidative stress relies on a comprehensive understanding of the redox signaling pathways stimulated under oxidizing conditions. Redox signaling events can be divided into upstream sensing of oxidants, midstream redox signaling of protein function, and downstream transcriptional redox regulation. Recent Advances: A more and more accepted theory of hydrogen peroxide (H₂O₂) signaling is that of a thiol peroxidase redox relay, whereby protein thiols with low reactivity toward H₂O₂ are instead oxidized through an oxidative relay with thiol peroxidases.

CRITICAL ISSUES

These ultrareactive thiol peroxidases are the upstream redox sensors, which form the first cellular port of call for H₂O₂. Not all redox-regulated interactions between thiol peroxidases and cellular proteins involve a transfer of oxidative equivalents, and the nature of redox signaling is further complicated through promiscuous functions of redox-regulated "moonlighting" proteins, of which the precise cellular role under oxidative stress can frequently be obscured by "polygamous" interactions. An ultimate goal of redox signaling is to initiate a rapid response, and in contrast to prokaryotic oxidant-responsive transcription factors, mammalian systems have developed redox signaling pathways, which intersect both with kinase-dependent activation of transcription factors, as well as direct oxidative regulation of transcription factors through peroxiredoxin (Prx) redox relays.

FUTURE DIRECTIONS

We highlight that both transcriptional regulation and cell fate can be modulated either through oxidative regulation of kinase pathways, or through distinct redox-dependent associations involving either Prxs or redox-responsive moonlighting proteins with functional promiscuity. These protein associations form systems of crossregulatory networks with multiple nodes of potential oxidative regulation for H₂O₂-mediated signaling.

Endogenous and Agonist-induced Opening of Mitochondrial Big Versus Small Ca2+-sensitive K+ Channels on Cardiac Cell and Mitochondrial Protection

Both big (BKCa) and small (SKCa) conductance Ca-sensitive K channels are present in mammalian cardiac cell mitochondria (m). We used pharmacological agonists and antagonists of BKCa and SKCa channels to examine the importance of endogenous opening of these channels and the relative contribution of either or both of these channels to protect against contractile dysfunction and reduce infarct size after ischemia reperfusion (IR) injury through a mitochondrial protective mechanism. After global cardiac IR injury of ex vivo perfused Guinea pig hearts, we found the following: both agonists NS1619 (for BKCa) and DCEB (for SKCa) improved contractility; BKCa antagonist paxilline (PAX) alone or with SKCa antagonist NS8593 worsened contractility and enhanced infarct size; both antagonists PAX and NS8593 obliterated protection by their respective agonists; BKCa and SKCa antagonists did not block protection afforded by SKCa and BKCa agonists, respectively; and all protective effects by the agonists were blocked by scavenging superoxide anions (O2) with Mn(III) tetrakis (4-benzoic acid) porphyrin (TBAP). Contractile function was inversely associated with global infarct size. In in vivo rats, infusion of NS8593, PAX, or both antagonists enhanced regional infarct size while infusion of either NS1619 or DCEB reduced infarct size. In cardiac mitochondria isolated from ex vivo hearts after IR, combined SKCa and BKCa agonists improved respiratory control index and Ca retention capacity compared with IR alone, whereas the combined antagonists did not alter respiratory control index but worsened Ca retention capacity. Although the differential protective bioenergetics effects of endogenous or exogenous BKCa and SKCa channel opening remain unclear, each channel likely responds to different sensing Ca concentrations and voltage gradients over time during oxidative stress-induced injury to individually or together protect cardiac mitochondria and myocytes.

What do we not know about mitochondrial potassium channels?

In this review, we summarize our knowledge about mitochondrial potassium channels, with a special focus on unanswered questions in this field. The following potassium channels have been well described in the inner mitochondrial membrane: ATP-regulated potassium channel, Ca(2+)-activated potassium channel, the voltage-gated Kv1.3 potassium channel, and the two-pore domain TASK-3 potassium channel. The primary functional roles of these channels include regulation of mitochondrial respiration and the alteration of membrane potential. Additionally, they modulate the mitochondrial matrix volume and the synthesis of reactive oxygen species by mitochondria. Mitochondrial potassium channels are believed to contribute to cytoprotection and cell death. In this paper, we discuss fundamental issues concerning mitochondrial potassium channels: their molecular identity, channel pharmacology and functional properties. Attention will be given to the current problems present in our understanding of the nature of mitochondrial potassium channels. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Oxidative Stress and Maxi Calcium-Activated Potassium (BK) Channels

All cells contain ion channels in their outer (plasma) and inner (organelle) membranes. Ion channels, similar to other proteins, are targets of oxidative impact, which modulates ion fluxes across membranes. Subsequently, these ion currents affect electrical excitability, such as action potential discharge (in neurons, muscle, and receptor cells), alteration of the membrane resting potential, synaptic transmission, hormone secretion, muscle contraction or coordination of the cell cycle. In this chapter we summarize effects of oxidative stress and redox mechanisms on some ion channels, in particular on maxi calcium-activated potassium (BK) channels which play an outstanding role in a plethora of physiological and pathophysiological functions in almost all cells and tissues. We first elaborate on some general features of ion channel structure and function and then summarize effects of oxidative alterations of ion channels and their functional consequences.

Role of in vivo vascular redox in resistance arteries

Vascular thiol redox state has been shown to modulate vasodilator functions in large conductance Ca2+ -activated K+ channels and other related channels. However, the role of vascular redox in small resistance arteries is unknown. To determine how in vivo modulation of thiol redox state affects small resistance arteries relaxation, we generated a transgenic mouse strain that overexpresses thioredoxin, a small redox protein (Trx-Tg), and another strain that is thioredoxin-deficient (dnTrx-Tg). The redox state of the mesenteric arteries (MAs) in Trx-Tg mice is found to be predominantly in reduced state; in contrast, MAs from dnTrx-Tg mice remain in oxidized state. Thus, we created an in vivo redox system of mice and isolated the second-order branches of the main superior MAs from wild-type, Trx-Tg, or dnTrx-Tg mice to assess endothelium-dependent relaxing responses in a wire myograph. In MAs isolated from Trx-Tg mice, we observed an enhanced intermediate-conductance Ca2+ -activated potassium channel contribution resulting in a larger endothelium-dependent hyperpolarizing (EDH) relaxation in response to indirect (acetylcholine) and direct (NS309) opening of endothelial calcium-activated potassium channels. MAs derived from dnTrx-Tg mice showed both blunted nitric oxide-mediated and EDH-mediated relaxation compared with Trx-Tg mice. In a control study, diamide decreased EDH relaxations in MAs of wild-type mice, whereas dithiothreitol improved EDH relaxations and was able to restore the diamide-induced impairment in EDH response. Furthermore, the basal or angiotensin II-mediated systolic blood pressure remained significantly lower in Trx-Tg mice compared with wild-type or dnTrx-Tg mice, thus directly establishing redox-mediated EDH in blood pressure control.

Putative Structural and Functional Coupling of the Mitochondrial BKCa Channel to the Respiratory Chain

Potassium channels have been found in the inner mitochondrial membranes of various cells. These channels regulate the mitochondrial membrane potential, the matrix volume and respiration. The activation of these channels is cytoprotective. In our study, the single-channel activity of a large-conductance Ca(2+)-regulated potassium channel (mitoBKCa channel) was measured by patch-clamping mitoplasts isolated from the human astrocytoma (glioblastoma) U-87 MG cell line. A potassium-selective current was recorded with a mean conductance of 290 pS in symmetrical 150 mM KCl solution. The channel was activated by Ca(2+) at micromolar concentrations and by the potassium channel opener NS1619. The channel was inhibited by paxilline and iberiotoxin, known inhibitors of BKCa channels. Western blot analysis, immuno-gold electron microscopy, high-resolution immunofluorescence assays and polymerase chain reaction demonstrated the presence of the BKCa channel β4 subunit in the inner mitochondrial membrane of the human astrocytoma cells. We showed that substrates of the respiratory chain, such as NADH, succinate, and glutamate/malate, decrease the activity of the channel at positive voltages. This effect was abolished by rotenone, antimycin and cyanide, inhibitors of the respiratory chain. The putative interaction of the β4 subunit of mitoBKCa with cytochrome c oxidase was demonstrated using blue native electrophoresis. Our findings indicate possible structural and functional coupling of the mitoBKCa channel with the mitochondrial respiratory chain in human astrocytoma U-87 MG cells.

Inhibition of ATP-induced calcium influx by homocysteine in human umbilical vein endothelial cells

Mechanisms involved in the association between hyperhomocysteinemia and vascular occlusive diseases remain unclear. Homocysteine (Hcy) may disturb calcium (Ca(2+) ) cytosolic regulation in endothelial cells, a process that can directly affect the synthesis of vasoactive substances, such as nitric oxide (NO). We have investigated the effect of acute and chronic incubation with high concentrations of Hcy (100 and 500 μmol/L) on the changes in the intracellular Ca(2+) concentration ([Ca(2+) ]i ) induced by ATP, using primary cultures of human umbilical vein endothelial cells (HUVEC). The changes in [Ca(2+) ]i , expressed as ΔFt /Fb , were measured using the microspectrofluorimetric technique with Fluo-3 as Ca(2+) indicator. HUVEC acutely exposed to Hcy did not produce significant effects on any of the parameters studied. However, chronic exposition (24 h) caused a significant decrease in the speed of store-mediated Ca(2+) entry, expressed as (ΔFt /Fb )/t (s(-1) ). Exposure of HUVEC to 100 and 500 µmol/L Hcy gave significantly lower values (0.019 ± 0.002 s(-1) , n = 5 and 0.021 ± 0.004 s(-1) , n = 6, respectively) compared to the controls (0.046 ± 0.004 s(-1) , n = 8, P < 0.003). This was detected only when the sustained phase of the ATP-induced [Ca(+2) ]i increase was isolated. These results demonstrate that high concentrations of Hcy can affect the mechanisms involved in [Ca(2+) ]i regulation of HUVEC, and that alteration occurs specifically in the sustained phase, which has been directly associated with NO synthesis.

Distinct activity of BK channel β1-subunit in cerebral and pulmonary artery smooth muscle cells

This study was designed to test a hypothesis that the functional activity of big-conductance, Ca(2+)-activated K(+) (BK) channels is different in cerebral and pulmonary artery smooth muscle cells (CASMCs and PASMCs). Using patch-clamp recordings, we found that the activity of whole cell and single BK channels were significantly higher in CASMCs than in PASMCs. The voltage and Ca(2+) sensitivity of BK channels were greater in CASMCs than in PASMCs. Targeted gene knockout of β(1)-subunits significantly reduced BK currents in CASMCs but had no effect in PASMCs. Western blotting experiments revealed that BK channel α-subunit protein expression level was comparable in CASMCs and PASMCs; however, β(1)-subunit protein expression level was higher in CASMCs than in PASMCs. Inhibition of BK channels by the specific blocker iberiotoxin enhanced norepinephrine-induced increase in intracellular calcium concentration in CASMCs but not in PASMCs. Systemic artery blood pressure was elevated in β(1)(-/-) mice. In contrast, pulmonary artery blood pressure was normal in β(1)(-/-) mice. These findings provide the first evidence that the activity of BK channels is higher in cerebral than in PASMCs. This heterogeneity is primarily determined by the differential β(1)-subunit function and contributes to diverse cellular responses in these two distinct types of cells.

Protection against cardiac injury by small Ca(2+)-sensitive K(+) channels identified in guinea pig cardiac inner mitochondrial membrane

We tested if small conductance, Ca(2+)-sensitive K(+) channels (SK(Ca)) precondition hearts against ischemia reperfusion (IR) injury by improving mitochondrial (m) bioenergetics, if O(2)-derived free radicals are required to initiate protection via SK(Ca) channels, and, importantly, if SK(Ca) channels are present in cardiac cell inner mitochondrial membrane (IMM). NADH and FAD, superoxide (O(2)(-)), and m[Ca(2+)] were measured in guinea pig isolated hearts by fluorescence spectrophotometry. SK(Ca) and IK(Ca) channel opener DCEBIO (DCEB) was given for 10 min and ended 20 min before IR. Either TBAP, a dismutator of O(2)()(-), NS8593, an antagonist of SK(Ca) isoforms, or other K(Ca) and K(ATP) channel antagonists, were given before DCEB and before ischemia. DCEB treatment resulted in a 2-fold increase in LV pressure on reperfusion and a 2.5 fold decrease in infarct size vs. non-treated hearts associated with reduced O(2)(-) and m[Ca(2+)], and more normalized NADH and FAD during IR. Only NS8593 and TBAP antagonized protection by DCEB. Localization of SK(Ca) channels to mitochondria and IMM was evidenced by a) identification of purified mSK(Ca) protein by Western blotting, immuno-histochemical staining, confocal microscopy, and immuno-gold electron microscopy, b) 2-D gel electrophoresis and mass spectroscopy of IMM protein, c) [Ca(2+)]-dependence of mSK(Ca) channels in planar lipid bilayers, and d) matrix K(+) influx induced by DCEB and blocked by SK(Ca) antagonist UCL1684. This study shows that 1) SK(Ca) channels are located and functional in IMM, 2) mSK(Ca) channel opening by DCEB leads to protection that is O(2)(-) dependent, and 3) protection by DCEB is evident beginning during ischemia.

Hydrogen sulfide preconditioning or neutrophil depletion attenuates ischemia-reperfusion-induced mitochondrial dysfunction in rat small intestine

The objectives of this study were to determine whether neutrophil depletion with anti-neutrophil serum (ANS) or preconditioning with the hydrogen sulfide (H(2)S) donor NaHS (NaHS-PC) 24 h prior to ischemia-reperfusion (I/R) would prevent postischemic mitochondrial dysfunction in rat intestinal mucosa and, if so, whether calcium-activated, large conductance potassium (BK(Ca)) channels were involved in this protective effect. I/R was induced by 45-min occlusion of the superior mesenteric artery followed by 60-min reperfusion in rats preconditioned with NaHS (NaHS-PC) or a BK(Ca) channel activator (NS-1619-PC) 24 h earlier or treated with ANS. Mitochondrial function was assessed by measuring mitochondrial membrane potential, mitochondrial dehydrogenase function, and cytochrome c release. Mucosal myeloperoxidase (MPO) and TNF-α levels were also determined, as measures of postischemic inflammation. BK(Ca) expression in intestinal mucosa was detected by immunohistochemistry and Western blotting. I/R induced mitochondrial dysfunction and increased tissue MPO and TNF-α levels. Although mitochondrial dysfunction was attenuated by NaHS-PC or NS-1619-PC, the postischemic increases in mucosal MPO and TNF-α levels were not. The protective effect of NaHS-PC or NS-1619-PC on postischemic mitochondrial function was abolished by coincident treatment with BK(Ca) channel inhibitors. ANS prevented the I/R-induced increase in tissue MPO levels and reversed mitochondrial dysfunction. These data indicate that neutrophils play an essential role in I/R-induced mucosal mitochondrial dysfunction. In addition, NaHS-PC prevents postischemic mitochondrial dysfunction (but not inflammation) by a BK(Ca) channel-dependent mechanism.

Electrochemical strategy to scout 1,4-naphthoquinones effect on voltage gated potassium channels

Naphthoquinone (NQ) was tested on voltage-gated ion channels expressed in Xenopus laevis oocytes. The activity of potassium Shaker channel with Inactivation domain Removed (ShIR) was not affected; in contrast, NQ diminished Kv1.3 currents. A current decrease was barely observed with the oxidant H(2)O(2). These findings suggested that redox properties were involved in the naphthoquinone-Kv1.3 channel interaction. NQ and some derivatives (NQs) were characterized in DMSO and physiological (ND-96) media by cyclic voltammetry. A typical two-stage mono-electronic reduction mechanism was observed in DMSO, while a one-stage bi-electronic reduction process was found in ND-96 medium. NQs with the lowest and the highest redox potential values were tested on both channels. Voltage-clamp recordings showed that inhibition of Kv1.3 was dependent on NQs redox potential. Results demonstrated that structural features (aromaticity and substituents prone to hydrogen bonds formation) of NQs were also important. This effect could be explained by interactions of some channel residues with NQs that contribute to favor their reduction process in the protein surroundings. The electrochemical strategy presented to simulate the cellular environments (aqueous and non-aqueous) that NQs may face, is an important contribution to pre-select (in a fine and simple way) the best redox compounds for electrophysiological testing.

Low resistance, large dimension entrance to the inner cavity of BK channels determined by changing side-chain volume

Large-conductance Ca²⁺- and voltage-activated K⁺ (BK) channels have the largest conductance (250–300 pS) of all K⁺-selective channels. Yet, the contributions of the various parts of the ion conduction pathway to the conductance are not known. Here, we examine the contribution of the entrance to the inner cavity to the large conductance. Residues at E321/E324 on each of the four α subunits encircle the entrance to the inner cavity. To determine if 321/324 is accessible from the inner conduction pathway, we measured single-channel current amplitudes before and after exposure and wash of thiol reagents to the intracellular side of E321C and E324C channels. MPA⁻ increased currents and MTSET⁺ decreased currents, with no difference between positions 321 and 324, indicating that side chains at 321/324 are accessible from the inner conduction pathway and have equivalent effects on conductance. For neutral amino acids, decreasing the size of the entrance to the inner cavity by substituting large side-chain amino acids at 321/324 decreased outward single-channel conductance, whereas increasing the size of the entrance with smaller side-chain substitutions had little effect. Reductions in outward conductance were negated by high [K⁺]_i. Substitutions had little effect on inward conductance. Fitting plots of conductance versus side-chain volume with a model consisting of one variable and one fixed resistor in series indicated an effective diameter and length of the entrance to the inner cavity for wild-type channels of 17.7 and 5.6 Å, respectively, with the resistance of the entrance ∼7% of the total resistance of the conduction pathway. The estimated dimensions are consistent with the structure of MthK, an archaeal homologue to BK channels. Our observations suggest that BK channels have a low resistance, large entrance to the inner cavity, with the entrance being as large as necessary to not limit current, but not much larger.

Biphasic effects of H(2)O(2) on BK(Ca) channels

The inhibitory or activating effect of H(2)O(2) on large conductance calcium and voltage-dependent potassium (BK(Ca)) channels has been reported. However, the mechanism by which this occurs is unclear. In this paper, BK(Ca) channels encoded by mouse Slo were expressed in HEK 293 cells and BK(Ca) channel activity was measured by electrophysiology. The results showed that H(2)O(2) inhibited BK(Ca) channel activity in inside-out patches but enhanced BK(Ca) channel activity in cell-attached patches. The inhibition by H(2)O(2) in inside-out patches may be due to oxidative modification of cysteine residues in BK(Ca) channels or other membrane proteins that regulate BK(Ca) channel function. PI3K/AKT signaling modulates the H(2)O(2)-induced BK(Ca) channel activation in cell-attached patches. BK(Ca) channels and PI3K signaling pathway were involved in H(2)O(2)-induced vasodilation and H(2)O(2)-induced vasodilation by PI3K pathway was mainly due to modulation of BK(Ca) channel activity.

Daidzein relaxes rat cerebral basilar artery via activation of large-conductance Ca2+-activated K+ channels in vascular smooth muscle cells

Daidzein, a phytoestrogen, has been reported to produce vasodilation via inhibition of Ca(2+) inflow. However, the involvement of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels in the effect of daidzein is debated. Therefore, the present study was designed to investigate the effect of daidzein on the rat cerebral basilar artery and the underlying molecular mechanisms. Isolated cerebral basilar artery rings and single vascular smooth muscle cells (VSMCs) were used for vascular reactivity and electrophysiology measurements, to investigate the effect of daidzein on BK(Ca) channels in cerebral basilar artery smooth muscle. In addition, the human BK(Ca) channel alpha-subunit gene (hslo) was transfected into HEK293 cells, to directly assess whether daidzein activates BK(Ca) channels. The results showed that daidzein produced a concentration-dependent but endothelium-independent relaxation in rat cerebral basilar arteries. Paxilline, a selective BK(Ca) channel blocker, significantly inhibited the daidzein-induced vasodilation, whereas NS1619, a selective BK(Ca) channel opener, enhanced the vasodilation. In the whole-cell configuration, daidzein increased noisy oscillation currents in cerebral basilar artery VSMCs in a concentration-dependent manner, and washout of daidzein or blockade of BK(Ca) channels with paxilline fully reversed the increase. However, daidzein did not substantially affect hSlo currents in HEK293 cells when applied to the outside of the cell membrane. In conclusion, these results indicate that the activation of BK(Ca) channels in VSMCs at least partly contributes to the daidzein-induced vasodilation of the rat cerebral basilar artery. The beta1-subunit of BK(Ca) channels plays a critical role in the activation of BK(Ca) currents by daidzein.

Mitochondria, oxidative stress, and temporal lobe epilepsy

Mitochondrial oxidative stress and dysfunction are contributing factors to various neurological disorders. Recently, there has been increasing evidence supporting the association between mitochondrial oxidative stress and epilepsy. Although certain inherited epilepsies are associated with mitochondrial dysfunction, little is known about its role in acquired epilepsies such as temporal lobe epilepsy (TLE). Mitochondrial oxidative stress and dysfunction are emerging as key factors that not only result from seizures, but may also contribute to epileptogenesis. The occurrence of epilepsy increases with age, and mitochondrial oxidative stress is a leading mechanism of aging and age-related degenerative disease, suggesting a further involvement of mitochondrial dysfunction in seizure generation. Mitochondria have critical cellular functions that influence neuronal excitability including production of adenosine triphosphate (ATP), fatty acid oxidation, control of apoptosis and necrosis, regulation of amino acid cycling, neurotransmitter biosynthesis, and regulation of cytosolic Ca(2+) homeostasis. Mitochondria are the primary site of reactive oxygen species (ROS) production making them uniquely vulnerable to oxidative stress and damage which can further affect cellular macromolecule function, the ability of the electron transport chain to produce ATP, antioxidant defenses, mitochondrial DNA stability, and synaptic glutamate homeostasis. Oxidative damage to one or more of these cellular targets may affect neuronal excitability and increase seizure susceptibility. The specific targeting of mitochondrial oxidative stress, dysfunction, and bioenergetics with pharmacological and non-pharmacological treatments may be a novel avenue for attenuating epileptogenesis.

Rotenone partially reverses decreased BK Ca currents in cerebral artery smooth muscle cells from streptozotocin-induced diabetic mice

1. Reactive oxygen species (ROS) cause vascular complications and impair vasodilation in diabetes mellitus. Large-conductance Ca(2+)-activated potassium channels (BK(Ca)) modulate vascular tone and play an important negative feedback role in vasoconstriction. In the present study, we tested the hypothesis that ROS regulate the function of BK(Ca) in diabetic cerebral artery smooth muscle cells. 2. Diabetes was induced in male BALB/c mice by injection of streptozotocin (STZ; 180 mg/kg, i.p., dissolved in sterile saline). Control and diabetic mice were treated with 12.7 micromol/L rotenone, an inhibitor of the mitochondrial electron transport chain complex I, or placebo every other day for 5 weeks. The whole-cell patch clamp-technique and functional vasomotor methods were used to record BK(Ca) currents and myogenic tone of cerebral artery smooth muscle cells. 3. In the diabetic group, there was a significant decrease in spontaneous transient outward currents in cerebral artery smooth muscle cells compared with control. Although the currents were only moderately increased in rotenone-treated diabetic mice, they remained significantly lower than in the control group. Furthermore, the macroscopic BK(Ca) currents that were decreased in diabetic mice were partially recovered in rotenone-treated diabetic mice (P < 0.05 vs untreated diabetic group). 4. The posterior cerebral artery from diabetic mice had a significantly higher myogenic tone than the control group, but this impaired contraction was partially reversed in the rotenone-treated diabetic group (P < 0.05 vs untreated diabetic group). 5. The H(2)O(2) concentration was significantly increased in cerebral arteries from diabetic mice compared with control. This increase in H(2)O(2) was significantly blunted by rotenone treatment. 6. In conclusion, rotenone partially reverses the decreased macroscopic BK(Ca) currents in STZ-induced Type 1 diabetic mice and this reversal of BK(Ca) currents may be related to the inhibitory effects of rotenone on H(2)O(2) production. Reactive oxygen species, particularly H(2)O(2), are important regulators of BK(Ca) channels and myogenic tone in diabetic cerebral artery.

Modulation of BKCa channel gating by endogenous signaling molecules

Large-conductance Ca(2+)- and voltage-activated K(+) (BK(Ca), MaxiK, or Slo1) channels are expressed in almost every tissue in our body and participate in many critical functions such as neuronal excitability, vascular tone regulation, and neurotransmitter release. The functional versatility of BK(Ca) channels owes in part to the availability of a spectacularly wide array of biological modulators of the channel function. In this review, we focus on modulation of BK(Ca) channels by small endogenous molecules, emphasizing their molecular mechanisms. The mechanistic information available from studies on the small naturally occurring modulators is expected to contribute to our understanding of the physiological and pathophysiological roles of BK(Ca) channels.

Functional and molecular evidence for impairment of calcium-activated potassium channels in type-1 diabetic cerebral artery smooth muscle cells

Cerebral vascular dysfunction and associated diseases often occur in type-1 diabetes, but the underlying mechanisms are largely unknown. In this study, we sought to determine whether big-conductance, Ca(2+)-activated K(+) (BK) channels were impaired in vascular (cerebral artery) smooth muscle cells (CASMCs) from streptozotocin-induced type-1 diabetic mice using patch clamp, molecular biologic, and genetic approaches. Our data indicate that the frequency and amplitude of spontaneous transient outward currents (STOCs) are significantly decreased, whereas the activity of spontaneous Ca(2+) sparks is increased, in diabetic CASMCs. The sensitivity of BK channels to voltage, Ca(2+), and the specific inhibitor iberiotoxin are all reduced in diabetic myocytes. Diabetic mice show increased myogenic tone and decreased contraction in response to iberiotoxin in cerebral arteries and elevated blood pressure. The expression of the BK channel beta1, but not alpha-subunit protein, is markedly decreased in diabetic cerebral arteries. Diabetic impairment of BK channel activity is lost in CASMCs from BK channel beta1-subunit gene deletion mice. In conclusion, the BK channel beta1-subunit is impaired in type-1 diabetic vascular SMCs, resulting in increased vasoconstriction and elevated blood pressure, thereby contributing to vascular diseases in type-1 diabetes.

Anoxia-induced changes in reactive oxygen species and cyclic nucleotides in the painted turtle

The Western painted turtle survives months without oxygen. A key adaptation is a coordinated reduction of cellular ATP production and utilization that may be signaled by changes in the concentrations of reactive oxygen species (ROS) and cyclic nucleotides (cAMP and cGMP). Little is known about the involvement of cyclic nucleotides in the turtle's metabolic arrest and ROS have not been previously measured in any facultative anaerobes. The present study was designed to measure changes in these second messengers in the anoxic turtle. ROS were measured in isolated turtle brain sheets during a 40-min normoxic to anoxic transition. Changes in cAMP and cGMP were determined in turtle brain, pectoralis muscle, heart and liver throughout 4 h of forced submergence at 20-22 degrees C. Turtle brain ROS production decreased 25% within 10 min of cyanide or N(2)-induced anoxia and returned to control levels upon reoxygenation. Inhibition of electron transfer from ubiquinol to complex III caused a smaller decrease in [ROS]. Conversely, inhibition of complex I increased [ROS] 15% above controls. In brain [cAMP] decreased 63%. In liver [cAMP] doubled after 2 h of anoxia before returning to control levels with prolonged anoxia. Conversely, skeletal muscle and heart [cAMP] remained unchanged; however, skeletal muscle [cGMP] became elevated sixfold after 4 h of submergence. In liver and heart [cGMP] rose 41 and 127%, respectively, after 2 h of anoxia. Brain [cGMP] did not change significantly during 4 h of submergence. We conclude that turtle brain ROS production occurs primarily between mitochondrial complexes I and III and decreases during anoxia. Also, cyclic nucleotide concentrations change in a manner suggestive of a role in metabolic suppression in the brain and a role in increasing liver glycogenolysis.

H2O2 activates redox- and 4-aminopyridine-sensitive Kv channels in coronary vascular smooth muscle

Previously, we demonstrated that coronary vasodilation in response to hydrogen peroxide (H(2)O(2)) is attenuated by 4-aminopyridine (4-AP), an inhibitor of voltage-gated K(+) (K(V)) channels. Using whole cell patch-clamp techniques, we tested the hypothesis that H(2)O(2) increases K(+) current in coronary artery smooth muscle cells. H(2)O(2) increased K(+) current in a concentration-dependent manner (increases of 14 +/- 3 and 43 +/- 4% at 0 mV with 1 and 10 mM H(2)O(2), respectively). H(2)O(2) increased a conductance that was half-activated at -18 +/- 1 mV and half-inactivated at -36 +/- 2 mV. H(2)O(2) increased current amplitude; however, the voltages of half activation and inactivation were not altered. Dithiothreitol, a thiol reductant, reversed the effect of H(2)O(2) on K(+) current and significantly shifted the voltage of half-activation to -10 +/- 1 mV. N-ethylmaleimide, a thiol-alkylating agent, blocked the effect of H(2)O(2) to increase K(+) current. Neither tetraethylammonium (1 mM) nor iberiotoxin (100 nM), antagonists of Ca(2+)-activated K(+) channels, blocked the effect of H(2)O(2) to increase K(+) current. In contrast, 3 mM 4-AP completely blocked the effect of H(2)O(2) to increase K(+) current. These findings lead us to conclude that H(2)O(2) increases the activity of 4-AP-sensitive K(V) channels. Furthermore, our data support the idea that 4-AP-sensitive K(V) channels are redox sensitive and contribute to H(2)O(2)-induced coronary vasodilation.

Modulation by homocysteine of the iberiotoxin-sensitive, Ca2+ -activated K+ channels of porcine coronary artery smooth muscle cells

We evaluated the acute effect of homocysteine on the iberiotoxin-sensitive, Ca(2+)-activated K(+) (BK(Ca)) channels of the porcine coronary artery smooth muscle cells. NS 1619 (1 to 30 microM) caused a concentration-dependent enhancement of the BK(Ca) amplitude (recorded using the whole-cell, membrane-rupture configuration) only with an elevated [Ca(2+)](i) of approximately 444 nM, but not with [Ca(2+)](i) of approximately 100 nM. Homocysteine (30 microM) caused a small inhibition ( approximately 16%) of the BK(Ca) amplitude ([Ca(2+)](i)= approximately 444 nM), and a greater inhibition ( approximately 77%) was observed with 100 microM NADH present in the pipette solution. The inhibition persisted after washing. With NADPH (100 microM), a smaller magnitude of inhibition ( approximately 34%) of the BK(Ca) amplitude was recorded. The NS 1619-mediated enhancement of the BK(Ca) amplitude (with elevated [Ca(2+)](i) plus NADH in the pipette) was attenuated by homocysteine. The homocysteine-mediated inhibition of the BK(Ca) amplitude was suppressed by Tiron (10 mM) or diphenylene iodonium (30 nM), applied alone, but not by superoxide dismutase (500 U/ml) and catalase (500 U/ml). Generation of superoxide (O(2)(-)) of the smooth muscle cells (with NADH presence), measured using the lucigenin-enhanced chemiluminescence, was markedly increased by angiotensin II (100 nM) and homocysteine (30 microM). The chemiluminescence signal was sensitive to apocynin (300 microM) or Tiron, applied alone, but not to superoxide dismutase and catalase. In conclusion, our results demonstrate that acute homocysteine application inhibits the iberiotoxin-sensitive BK(Ca) channels (with elevated [Ca(2+)](i) and NADH present) which is probably caused by the NADH oxidase activation and the concomitant generation of intracellular superoxide.

Cysteine oxidation and rundown of large-conductance Ca2+-dependent K+ channels

Gating of Slo1 calcium- and voltage-gated potassium (BK) channels involves allosteric interactions among the channel pore, voltage sensors, and Ca(2+)-binding domains. The allosteric activation of the Slo1 channel is in turn modulated by a variety of regulatory processes, including oxidation. Cysteine oxidation alters functional properties of Slo1 channels and has been suggested to contribute to the decrease in the channel activity following patch excision often referred to as rundown. This study examined the biophysical mechanism of rundown and whether oxidation of cysteine residues located in the C-terminus of the human Slo1 channel (C430 and C911) plays a role. Comparison of the changes in activation properties in different concentrations of Ca(2+) among the wild-type, C430A, and C911A channels during rundown and by treatment with the oxidant hydrogen peroxide showed that oxidation of C430 and C911 markedly contributes to the rundown process.

Dual effects of nitric oxide on the large conductance calcium-activated potassium channels of rat brain

Previously, we have shown that nitric oxide (NO) directly activates the Maxi-K channels. In the present study, we have investigated whether NO has prolonged effects on the Maxi-K channels reconstituted in lipid bilayer. Application of S-nitroso-N-acetyl-D, L-penicillamine (SNAP), a NO donor, induced an immediate increase of open probability (Po) of Maxi-K channel in a dose-dependent manner. When SNAP was removed from the cytosolic solution, the Po did not simply returned to, but irreversibly decreased to a level lower than that of the control Po. At 0.2 mM, (Z)-[N-(3-Ammoniopropyl)-N-(n-propyl)amino] diazen-1-ium-1,2-diolate (PAPA-NO), another NO donor, produced a similar increase of Po and decrease of Po upon washout. The increasing effects of SNAP on Po were not blocked by either 50 U/ml superoxide dismutase (SOD) or 2 mM Nethylmaleimide (NEM) pre-treatments. However, NEM appears to be ineffective when applied after SNAP. These results suggest that NO can modulate Maxi-K channel via direct interaction and chemical modification, such as Snitrosylation in the brain.

An antiviral mechanism investigated with ribavirin as an RNA virus mutagen for foot-and-mouth disease virus

To prove whether error catastrophe/lethal mutagenesis is the primary antiviral mechanism of action of ribavirin against foot-and-mouth disease virus (FMDV). Ribavirin passage experiments were performed and supernatants of Rp1 to Rp5 were harvested. Morphological alterations as well as the levels of viral RNAs, proteins, and infectious particles in the BHK-21 cells infected using the supernatants of Rp1 to Rp5 and control were measured by microscope, real-time RT-PCR, western-blotting and plaque assays, respectively. The mutation frequency was measured by sequencing the complete P1- and 3D-encoding region of FMDV after a single round of virus infection from ribavirin-treated or untreated FMDV-infected cells. Ribavirin treatment for FMDV caused dramatically inhibition of multiplication in cell cultures. The levels of viral RNAs, proteins, and infectious particles in the BHK-21 cells infected were more greatly reduced along with the passage from Rp1 to Rp5, moreover, nucleocapsid protein could not be detected and no recovery of infectious virus in the supernatant or detection of intracellular viral RNA was observed at the Rp5-infected cells. A high mutation rate, giving rise to an 8-and 11-fold increase in mutagenesis and resulting in some amino acid substitutions, was found in viral RNA synthesized at a single round of virus infection in the presence of ribavirin of 1000 microM and caused a 99.7% loss in viral infectivity in contrast with parallel untreated control virus. These results suggest that the antiviral molecular mechanism of ribavirin is based on the lethal mutagenesis/error catastrophe, that is, the ribavirin is not merely an antiviral reagent but also an effective mutagen.

The Effect of Superoxide Anion on Autoregulation of Cerebral Blood Flow

Background and Purpose— Recent studies have suggested that autoregulation of cerebral blood flow (CBF) is impaired after traumatic and ischemic brain injury. Given that the levels of superoxide anion (O 2 · − ) are increased in these conditions, we postulate that O 2 · − contributes to the impairment of CBF autoregulation.

Methods— CBF was monitored with laser Doppler flowmetry during increases in blood pressure.

Results— During the control period, CBF was well autoregulated after the increase in mean arterial pressure (MAP) from 98±3 to 140±6 mm Hg. The autoregulation index (AI; ΔCBF/ΔMAP) averaged 0.25±0.02 (n=6). O 2 · − in the brain was then increased by subdural perfusion of xanthine/xanthine oxidase (different concentrations) and catalase. Low concentrations of O 2 · − decreased basal CBF by 10±1.6% but had no effect on autoregulation (AI, 0.19±0.02; n=6). Higher concentrations of O 2 · − (0.2 mmol/L xanthine and either 3 or 20 mU xanthine oxidase) increased basal CBF by 30±2% and 42±4%, respectively, and impaired autoregulation of CBF (AI, 0.55±0.03 and 0.76±0.02; n=6). Inclusion of superoxide dismutase in the O 2 · − -generating system restored autoregulation (AI, 0.28±0.05; n=6). Neither inhibition of NO synthase nor the addition of deferioxamine had any effect on the ability of higher concentrations of O 2 · − to impair autoregulation of CBF (AI, 0.65±0.07 and 0.72±0.05 respectively; n=6). O 2 · − also increased the activity of K Ca channels in cerebral vascular smooth muscle cells (VSMCs; n=8).

Conclusion— These results suggest that O 2 · − increases basal CBF and impairs autoregulation of CBF, likely through the activation of K Ca channels in cerebral VSMCs.

Homocysteine inhibits store-mediated calcium entry in human endothelial cells: evidence for involvement of membrane potential and actin cytoskeleton

The role of homocysteine for store-operated calcium influx was investigated in human umbilical cord endothelial cell line. Homocysteine significantly decreased thapsigargin-evoked Ca2+ entry, membrane hyperpolarization and actin polymerization. GSH and DTT prevented homocysteine-induced inhibition of thapsigargin-evoked Ca2+ entry, membrane hyperpolarization and actin polymerization; while GSSG had the opposite effect. Homocysteine blocked large conductance Ca2+-activated K+ (BK(Ca)) channels in a concentration-dependent manner and related to the redox status of the endothelial cells. BK(Ca) channels opener NS1619 reversed thapsigargin-evoked Ca2+ entry, membrane hyperpolarization and actin polymerization; BK(Ca) channels inhibitor iberiotoxin had the opposite effect. The findings suggest that homocysteine is involved in store-regulated Ca2+ entry through membrane potential-dependent and actin cytoskeleton-dependent mechanisms, redox status of homocysteine and BK(Ca) channels may play a regulatory role in it.

Cardiac mitochondrial preconditioning by Big Ca2+-sensitive K+ channel opening requires superoxide radical generation

ATP-sensitive K+ channel opening in inner mitochondrial membranes protects hearts from ischemia-reperfusion (I/R) injury. Opening of the Big conductance Ca2+-sensitive K+ channel (BK(Ca)) is now also known to elicit cardiac preconditioning. We investigated the role of the pharmacological opening of the BK(Ca) channel on inducing mitochondrial preconditioning during I/R and the role of O2-derived free radicals in modulating protection by putative mitochondrial (m)BK(Ca) channel opening. Left ventricular (LV) pressure (LVP) was measured with a balloon and transducer in guinea pig hearts isolated and perfused at constant pressure. NADH, reactive oxygen species (ROS), principally superoxide (O2(-*)), and m[Ca2+] were measured spectrophotofluorometrically at the LV free wall using autofluorescence and fluorescent dyes dihydroethidium and indo 1, respectively. BK(Ca) channel opener 1-(2'-hydroxy-5'-trifluoromethylphenyl)-5-trifluoromethyl-2(3H)benzimid-axolone (NS; NS-1619) was given for 15 min, ending 25 min before 30 min of global I/R. Either Mn(III)tetrakis(4-benzoic acid)porphyrin (TB; MnTBAP), a synthetic dismutator of O2(-*), or an antagonist of the BK(Ca) channel paxilline (PX) was given alone or for 5 min before, during, and 5 min after NS. NS pretreatment resulted in a 2.5-fold increase in developed LVP and a 2.5-fold decrease in infarct size. This was accompanied by less O2(-*) generation, decreased m[Ca2+], and more normalized NADH during early ischemia and throughout reperfusion. Both TB and PX antagonized each preconditioning effect. This indicates that 1) NS induces a mitochondrial-preconditioned state, evident during early ischemia, presumably on mBK(Ca) channels; 2) NS effects are blocked by BK(Ca) antagonist PX; and 3) NS-induced preconditioning is dependent on the production of ROS. Thus NS may induce mitochondrial ROS release to initiate preconditioning.

Mitochondria-derived reactive oxygen species dilate cerebral arteries by activating Ca2+ sparks

Mitochondria regulate intracellular calcium (Ca2+) signals in smooth muscle cells, but mechanisms mediating these effects, and the functional relevance, are poorly understood. Similarly, antihypertensive ATP-sensitive potassium (KATP) channel openers (KCOs) activate plasma membrane KATP channels and depolarize mitochondria in several cell types, but the contribution of each of these mechanisms to vasodilation is unclear. Here, we show that cerebral artery smooth muscle cell mitochondria are most effectively depolarized by diazoxide (-15%, tetramethylrhodamine [TMRM]), less so by levcromakalim, and not depolarized by pinacidil. KCO-induced mitochondrial depolarization increased the generation of mitochondria-derived reactive oxygen species (ROS) that stimulated Ca2+ sparks and large-conductance Ca2+-activated potassium (KCa) channels, leading to transient KCa current activation. KCO-induced mitochondrial depolarization and transient KCa current activation were attenuated by 5-HD and glibenclamide, KATP channel blockers. MnTMPyP, an antioxidant, and Ca2+ spark and KCa channel blockers reduced diazoxide-induced vasodilations by >60%, but did not alter dilations induced by pinacidil, which did not elevate ROS. Data suggest diazoxide drives ROS generation by inducing a small mitochondrial depolarization, because nanomolar CCCP, a protonophore, similarly depolarized mitochondria, elevated ROS, and activated transient KCa currents. In contrast, micromolar CCCP, or rotenone, an electron transport chain blocker, induced a large mitochondrial depolarization (-84%, TMRM), reduced ROS, and inhibited transient KCa currents. In summary, data demonstrate that mitochondria-derived ROS dilate cerebral arteries by activating Ca2+ sparks, that some antihypertensive KCOs dilate by stimulating this pathway, and that small and large mitochondrial depolarizations lead to differential regulation of ROS and Ca2+ sparks.

Heme oxygenase-2 products activate IKCa: role of CO and iron in guinea pig portal vein smooth muscle cells

Hemin (10 microM) and carbon monoxide (CO) increased iberiotoxin-blockable IKCa in portal vein smooth muscle cells. CO-induced IKCa activation was abolished by 10 microM ODQ, 10 microM cyclopiazonic acid and 1 microM KT5823. The hemin-induced effect on IKCa was abolished by pretreatment with Sn-protoporphyrin IX, a heme oxygenase inhibitor and Fe2+ chelator but was insensitive to inhibitors of soluble guanylate cyclase (GC) and cGMP-dependent protein kinase (PKG). There was no effect of hemin on IKCa in the presence of 3 microM dithiotreitol into the bath or 3 mM glutathione into the pipette solution. Superoxide dismutase (1000 U/ml) or catalase (3000 U/ml) added into the pipette solution also abolished the effect of hemin on IKCa in this tissue. Additionally, 10 microM hemin could not influence IKCa in Ca2+-free external solution or in the presence of 30 microM SKF 95356. It was concluded that CO increases IKCa via its "conventional" signaling pathway, which involves soluble GC and PKG activation and subsequent stimulation of sarcoplasmic reticulum Ca2+ pump activity resulting in Ca2+-dependent activation of IKCa due to the accumulation of Ca2+ into the space near the plasma membrane. On the other hand, internally produced CO could not yield the same IKCa increase, while Fe2+ derived from heme oxygenase 2-dependent degradation of hemin in portal vein smooth muscle cells gives rise to reactive oxygen species namely hydroxyl and superoxide radicals. Both radicals are responsible for the SKF 95356-sensitive non-selective cation channel activation, the Ca2+ influx and the subsequent increase of Ca2+ concentration near the plasma membrane that augments the KCa channel activity.

Redox Modulation of Vascular Tone

Opening of potassium channels on vascular smooth muscle cells with resultant hyperpolarization plays a central role in several mechanisms of vasodilation. For example, in the arteriolar circulation where tissue perfusion is regulated, there is an endothelial derived hyperpolarizing factor that opens vascular smooth muscle calcium-activated potassium channels, eliciting dilation. Metabolic vasodilation involves the opening of sarcolemmal ATP-sensitive potassium channels. Adrenergic dilation as well as basal vasomotor tone in several vascular beds depend upon voltage-dependent potassium channels in smooth muscle. Thus hyperpolarization through potassium channel opening is a fundamental mechanism for vasodilation. Disease states such as coronary atherosclerosis and its risk factors are associated with elevated levels of reactive oxygen (ROS) and nitrogen species that have well-defined inhibitory effects on nitric oxide-mediated vasodilation. Effects of ROS on hyperpolarization mechanisms of dilation involving opening of potassium channels are less well understood but are very important because hyperpolarization-mediated dilation often compensates for loss of other dilator mechanisms. We review the effect of ROS on potassium channel function in the vasculature. Depending on the oxidative species, ROS can activate, inhibit, or leave unaltered potassium channel function in blood vessels. Therefore, discerning the activity of enzymes regulating production or degradation of ROS is important when assessing tissue perfusion in health and disease.

The beta1 subunit enhances oxidative regulation of large-conductance calcium-activated K+ channels

Oxidative stress may alter the functions of many proteins including the Slo1 large conductance calcium-activated potassium channel (BK_Ca). Previous results demonstrated that in the virtual absence of Ca²⁺, the oxidant chloramine-T (Ch-T), without the involvement of cysteine oxidation, increases the open probability and slows the deactivation of BK_Ca channels formed by human Slo1 (hSlo1) α subunits alone. Because native BK_Ca channel complexes may include the auxiliary subunit β1, we investigated whether β1 influences the oxidative regulation of hSlo1. Oxidation by Ch-T with β1 present shifted the half-activation voltage much further in the hyperpolarizing direction (−75 mV) as compared with that with α alone (−30 mV). This shift was eliminated in the presence of high [Ca²⁺]_i, but the increase in open probability in the virtual absence of Ca²⁺ remained significant at physiologically relevant voltages. Furthermore, the slowing of channel deactivation after oxidation was even more dramatic in the presence of β1. Oxidation of cysteine and methionine residues within β1 was not involved in these potentiated effects because expression of mutant β1 subunits lacking cysteine or methionine residues produced results similar to those with wild-type β1. Unlike the results with α alone, oxidation by Ch-T caused a significant acceleration of channel activation only when β1 was present. The β1 M177 mutation disrupted normal channel activation and prevented the Ch-T–induced acceleration of activation. Overall, the functional effects of oxidation of the hSlo1 pore-forming α subunit are greatly amplified by the presence of β1, which leads to the additional increase in channel open probability and the slowing of deactivation. Furthermore, M177 within β1 is a critical structural determinant of channel activation and oxidative sensitivity. Together, the oxidized BK_Ca channel complex with β1 has a considerable chance of being open within the physiological voltage range even at low [Ca²⁺]_i.

Inhibitors arrest myofibrillogenesis in skeletal muscle cells at early stages of assembly

A three-step model for myofibrillogenesis has been proposed for the formation of myofibrils [Rhee et al., 1994: Cell Motil. Cytoskeleton 28:1-24; Sanger et al., 2002: Adv. Exp. Med. 481:89-105]: premyofibril to nascent myofibril to mature myofibril. We have found two chemically related inhibitors that will arrest development at both the first and second step. Cultured quail embryonic skeletal myoblasts were treated with ethyl methane sulfonate (EMS) or 2-aminoethyl-methanesulfonate (MTSEA+). When the myoblasts fused in the presence of either of these compounds, myosheets rather than myotubes formed. Treated cells were fixed and immunostained against multiple proteins commonly found in muscle cells. Protein expression and localization throughout the myosheet were similar to that of developing myotube tips. Cells treated with high concentrations of EMS (10 mM) stained for non-muscle myosin II, sarcomeric alpha-actinin, and tropomyosin. No zeugmatin (Z-band region of titin) or muscle myosin II antibody staining was detected in fibers in this treatment group. These fibers are comparable to premyofibrils in control myotubes. At lower concentrations of EMS (7.5 to 5 mM), fibers that formed stained for muscle myosin II and titin as well as for non-muscle myosin IIB, sarcomeric alpha-actinin, and tropomyosin. Muscle myosin II was in an unbanded pattern. These fibers are comparable to nascent myofibrils observed during normal myofibrillogenesis. Similar effects to those obtained by treating cells with EMS were obtained when we treated cultured cells with MTSEA+ (5 mM) and stained them with sarcomeric alpha-actinin. MTSEA+ is chemically related to EMS, and is a well-known inhibitor of ryanodine receptors in skeletal muscle cells. Some abnormalities such as nemaline-like rods and other protein aggregates also appear within the myosheet during EMS and MTSEA+ treatment. Removal of these two inhibitors of myofibrillogenesis allows the premyofibrils and nascent myofibrils to form mature myofibrils.

Rosuvastatin treatment reverses impaired coronary artery vasodilation in fructose-fed, insulin-resistant rats

Insulin resistance (IR) impairs vascular responses in coronary arteries, but mechanisms of dysfunction and approaches to treatment remain unclear. We examined the ability of a new 3-hydroxy-methylglutaryl coenzyme A reductase inhibitor, rosuvastatin, to reverse reduced dilator responses in rats made IR by feeding a fructose-rich diet (FF). Sprague-Dawley rats were randomized to control (normal rat diet) or FF. After 1 wk, rats received rosuvastatin (2 mg/kg) or placebo (saline) subcutaneously for 5 wk. Biochemical measurements and in vitro functional studies of small coronary arteries were performed. Fasting insulin and triglyceride (TG) levels were markedly increased in FF-placebo rats compared with other groups. Rosuvastatin treatment of FF rats normalized TG and modestly decreased insulin levels. ACh-induced dilator responses were depressed in arteries from FF-placebo rats. This impairment was due to decreased responses via calcium-dependent K channels (KCa). Rosuvastatin treatment of FF rats completely reversed the response to ACh to normal levels. Moreover, this recovery in function was due to an improvement in vasodilation via KCa. Thus rosuvastatin treatment of IR rats normalizes coronary vascular dilator responses by improving the KCa function.

Hydroxyl radical activation of a Ca(2+)-sensitive nonselective cation channel involved in epithelial cell necrosis

In a previous work the involvement of a fenamate-sensitive Ca(2+)-activated nonselective cation channel (NSCC) in free radical-induced rat liver cell necrosis was demonstrated (5). Therefore, we studied the effect of radical oxygen species and oxidizing agents on the gating behavior of a NSCC in a liver-derived epithelial cell line (HTC). Single-channel currents were recorded in HTC cells by the excised inside-out configuration of the patch-clamp technique. In this cell line, we characterize a 19-pS Ca(2+)-activated, ATP- and fenamate-sensitive NSCC nearly equally permeable to monovalent cations. In the presence of Fe(2+), exposure of the intracellular side of NSCC to H(2)O(2) increased their open probability (P(o)) by approximately 40% without affecting the unitary conductance. Desferrioxamine as well as the hydroxyl radical (.OH) scavenger MCI-186 inhibited the effect of H(2)O(2), indicating that the increase in P(o) was mediated by.OH. Exposure of the patch membrane to the oxidizing agent 5,5'-dithio-bis-2-nitrobenzoic acid (DTNB) had a similar effect to.OH. The increase in P(o) induced by.OH or DTNB was not reverted by preventing formation or by DTNB washout, respectively. However, the reducing agent dithiothreitol completely reversed the effects on P(o) of both.OH and DTNB. A similar increase in P(o) was observed by applying the physiological oxidizing molecule GSSG. Moreover, GSSG-oxidized channels showed enhanced sensitivity to Ca(2+). The effect of GSSG was fully reversed by GSH. These results suggest an intracellular site(s) of action of oxidizing agents on cysteine targets on the fenamate-sensitive NSCC protein implicated in epithelial cell necrosis.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Reactive oxygen species impair Slo1 BK channel function by altering cysteine-mediated calcium sensing

Vascular dysfunction is a hallmark of many diseases, including coronary heart disease, stroke and diabetes. The underlying mechanisms of these disorders, which are intimately associated with inflammation and oxidative stress caused by excess reactive oxygen species (ROS), have remained elusive. Here we report that ROS are powerful inhibitors of vascular smooth muscle calcium-dependent Slo1 BK or Maxi-K potassium channels, an important physiological determinant of vascular tone. By targeting a cysteine residue near the Ca(2+) bowl of the BK alpha subunit, H(2)O(2) virtually eliminates physiological activation of the channel, with an inhibitory potency comparable to a knockout of the auxiliary subunit BK beta 1. These results reveal a molecular structural basis for the vascular dysfunction involving oxidative stress and provide a solid rationale for a potential use of BK openers in the prevention and treatment of cardiovascular disorders.

Differential modulation of voltage-dependent K+ currents in colonic smooth muscle by oxidants

The effect of oxidants on voltage-dependent K+ currents was examined in mouse colonic smooth muscle cells. Exposure to either chloramine-T (Ch-T), an agent known to oxidize both cysteine and methionine residues, or the colon-specific oxidant monochloramine (NH2Cl) completely suppressed the transient outward K+ current (Ito) while simultaneously enhancing the sustained delayed rectifier K+ current (Idr). In contrast, the cysteine-specific oxidants hydrogen peroxide (H2O2) and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) exhibited partial and slow suppression of Ito by inducing a shift in channel availability of -18 mV without affecting Idr. After enhancement by NH2Cl or Ch-T, Idr was sensitive to 10 mM tetraethylammonium but not to other K+ channel blockers, suggesting that it represented activation of the resting Idr and not a separate K+ conductance. Extracellular dithiothreitol (DTT) partially reversed the effect of H2O2 and DTNB on Ito but not the actions of NH2Cl and Ch-T on either Idr or Ito. Dialysis of myocytes with GSH (5 mM) or DTT (5 mM) prevented suppression of Ito by H2O2 and DTNB but did not alter the effects of NH2Cl or Ch-T on either Idr or Ito. Ch-T and NH2Cl completely blocked Ito generated by murine K(v)4.1, 4.2, and 4.3 in Xenopus oocytes, an effect not reversible by intracellular DTT. In contrast, intracellular DTT reversed the effect of H2O2 and DTNB on the cloned channels. These results suggest that I(to) is suppressed via modification of both methionine and cysteine residues, whereas enhancement of Idr likely results from methionine oxidation alone.

Sodium (2-sulfonatoethyl) methanethiosulfonate prevents S-nitroso-L-cysteine activation of Ca2+-activated K+ (BKCa) channels in myocytes of the guinea-pig taenia caeca

1. The modulation of large conductance Ca(2+)-activated K(+) (BK(Ca)) channels by the nitric oxide (NO(.)) donor, S-nitroso-L-cysteine (NOCys) and three sulfhydryl-specific methanethiosulfonate (MTS) reagents, positively charged 2-aminoethyl MTS hydrobromide (MTSEA C(3)H(9)NO(2)S(2)HBr) and [2-(trimethylammonium) ethyl MTS bromide (MTSET C(6)H(16)NO(2)S(2)Br), and negatively charged sodium (2-sulfonatoethyl) MTS (MTSES C(3)H(7)O(5)S(3)Na) were compared in excised inside-out membrane patches of the guinea-pig taenia caeca. 2. In membrane patches bathed in a low Ca(2+) (15 nM) high K(+) physiological salt solution, 1-3 BK(Ca) channels opened with a low probability (N.P(o)) of 0.019+/-0.011 at 0 mV. N.P(o) readily increased with membrane depolarization, raised Ca(2+) concentration or upon the addition of NOCys (10 micro M for 2-5 min) such that 5-15 open BK(Ca) channels were evident. 3. MTSEA (2.5 mM) decreased, while MTSES (2.5 mM) increased N.P(o) (at 0 mV) and the number of open BK(Ca) channels at positive potentials. These changes in channel activity remained after a prolonged washout of these two MTS reagents. 4. MTSET (2.5 mM) increased N.P(o) (at 0 mV) and voltage-dependently decreased BK(Ca) current amplitudes in a manner readily reversed upon washout. 5. Pre-exposure of excised membrane patches to MTSES or N-ethylmaleimide (NEM 1 mM), a specific alkylating agent of cysteine sulfhydryls, but not MTSEA or MTSET, prevented the excitatory actions of NOCys (10 micro M). 6. It was concluded that NOCys-evoked increases in BK(Ca) channel activity occur via the S-nitrosylation of cysteine residues within basic regions of the channel alpha subunit that have an accessible water interface.

Hydrogen peroxide modulates K+ ion currents in cultured Aplysia sensory neurons

Hydrogen peroxide (H(2)O(2)) causes oxidative stress and is considered a mediator of cell death in various organisms. Our previous studies showed that prolonged (>6 h) treatment of Aplysia sensory neurons with 1 mM H(2)O(2) produced hyperpolarization of the resting membrane potential, followed by apoptotic morphological changes. In this study, we examined the effect of H(2)O(2) on the membrane conductance of Aplysia sensory neurons. Hyperpolarization was induced by 10 mM H(2)O(2) within 1 h, and this was attributed to increased membrane conductance. In addition, treatment with 10 mM H(2)O(2) for 3 min produced immediate depolarization, which was due to decreased membrane conductance. The H(2)O(2)-induced hyperpolarization and depolarization were completely blocked by dithiothreitol, a disulfide-reducing agent. The later increase of membrane conductance induced by H(2)O(2) was completely blocked by 100 mM TEA, a K(+) channel blocker, suggesting that H(2)O(2)-induced hyperpolarization is due to the activation of K(+) conductance. However, the inhibition of K(+) efflux by TEA did not protect against H(2)O(2)-induced cell death in cultured Aplysia sensory neurons, which indicates that the signal pathway leading to H(2)O(2)-induced cell death is more complicated than expected.