Apoptosis recruits two-pore domain potassium channels used for homeostatic volume regulation

Downregulation of TASK-3 Channel Induces Senescence in Granulosa Cells of Bovine Cystic Ovarian Follicles

Ovarian cysts are linked to hormone imbalances and altered gene expressions, but the connection between cysts and ion channel expression is understudied. This study explored the role of TWIK-related acid-sensitive K+ (TASK) channels in bovine ovarian cyst formation. The ovarian follicles were split into small (5 to 10 mm in diameter) and large (>25 mm in diameter) groups. Among the measured K+, Na+, and Cl- concentrations in follicular fluid (FF) obtained from small-sized follicles (SFs) and large-sized follicles (LFs), the K+ concentration was significantly lower in LFFF. Quantitative PCR, Western blot, and immunocytochemistry data revealed that TASK-3 expression levels significantly decreased by approximately 50% in LFs and granulosa cells obtained from LFs (LFGCs) compared to the corresponding controls. The TASK-3 protein was localized to the plasma membranes of GCs. The diameters of LFGCs were larger than those of SFGCs. The cell swelling response to exposure to a hypotonic solution (200 mOsm/L) was highly reduced in TASK-3-overexpressing cells compared to vector-transfected cells. TASK-3-knockdown cells showed arrested growth. Senescence markers were detected in LFGCs and TASK-3-knockdown cells. These findings suggest that reduced TASK-3 expression in LFs is associated with the inhibition of GC growth, leading to senescence and cyst formation.

Ions, the Movement of Water and the Apoptotic Volume Decrease

The movement of water across the cell membrane is a natural biological process that occurs during growth, cell division, and cell death. Many cells are known to regulate changes in their cell volume through inherent compensatory regulatory mechanisms. Cells can sense an increase or decrease in their cell volume, and compensate through mechanisms known as a regulatory volume increase (RVI) or decrease (RVD) response, respectively. The transport of sodium, potassium along with other ions and osmolytes allows the movement of water in and out of the cell. These compensatory volume regulatory mechanisms maintain a cell at near constant volume. A hallmark of the physiological cell death process known as apoptosis is the loss of cell volume or cell shrinkage. This loss of cell volume is in stark contrast to what occurs during the accidental cell death process known as necrosis. During necrosis, cells swell or gain water, eventually resulting in cell lysis. Thus, whether a cell gains or loses water after injury is a defining feature of the specific mode of cell death. Cell shrinkage or the loss of cell volume during apoptosis has been termed apoptotic volume decrease or AVD. Over the years, this distinguishing feature of apoptosis has been largely ignored and thought to be a passive occurrence or simply a consequence of the cell death process. However, studies on AVD have defined an underlying movement of ions that result in not only the loss of cell volume, but also the activation and execution of the apoptotic process. This review explores the role ions play in controlling not only the movement of water, but the regulation of apoptosis. We will focus on what is known about specific ion channels and transporters identified to be involved in AVD, and how the movement of ions and water change the intracellular environment leading to stages of cell shrinkage and associated apoptotic characteristics. Finally, we will discuss these concepts as they apply to different cell types such as neurons, cardiomyocytes, and corneal epithelial cells.

Activation of the TRAAK two-pore domain potassium channels in rd1 mice protects photoreceptor cells from apoptosis

AIM

To investigate the expression of TWIK-related arachidonic acid-stimulated K⁺ channel (TRAAK) in retinal degeneration mice (rd1) and further evaluate how TRAAK affect photoreceptor cell apoptosis.

METHODS

The rd1 mice were distributed into blank (no treatment), control (1.4% DMSO, intraperitoneal injection) and riluzole groups (4 mg/kg·d, intraperitoneal injection) from postnatal 7d to 10, 14 and 18d; C57 group (no treatment), as age-matched wild-type control. The thickness of the outer nuclear layer (ONL) of retina was detected by paraffin section hematoxylin and eosin staining. The expression of TRAAK and the apoptosis of the ONL cells were detected by immunostaining, Western blotting, and real-time polymerase chain reaction.

RESULTS

The channel agonist riluzole activated TRAAK and delayed the apoptosis of photoreceptor cells in ONL layer of rd1 mice. Both at mRNA and protein levels, after riluzole treatment, TRAAK expression was significantly upregulated, when compared with the control and blank group. Then we detected a series of apoptosis related mRNA and protein. The anti-apoptotic factor Bcl-2 downregulated and the pro-apoptotic factors Bax and cleaved-caspase-3 upregulated significantly.

CONCLUSION

Riluzole elevates the expression of TRAAK and inhibits the development of apoptosis. Activation of TRAAK may have some potential effects to put off photoreceptor apoptosis.

Neuronal-like differentiated SH-SY5Y cells adaptation to a mild and transient H2 O2 -induced oxidative stress

Preconditioning (PC) is a cell adaptive response to oxidative stress and, with regard to neurons, can be considered as a neuroprotective strategy. The aim of the present study was to verify how neuronal-like differentiated SH-SY5Y cells adapt to a mild and transient H₂ O₂ -induced oxidative stress and, hence, whether may be considered as more sensitive cell model to study PC pathways. A first screening allowed to define H₂ O₂ concentrations for PC (10μM-50μM), applied before damage(100μM H₂ O₂ ). Cell viability measured 24 hours after 100μM H₂ O₂ -induced damage was ameliorated by 24-hour pre-exposure to low-concentration H₂ O₂ (10μM-30μM) with cell size as well restored. Markers for apoptosis (Bcl-2 and Bad), inflammation (iNOS), and redox system (MnSOD) were also determined, showing that, in cells pre-exposed to 10μM H₂ O₂ and then submitted to 100μM H₂ O₂ , Bcl-2 levels were higher, Bad and iNOS levels were lower than those observed in damaged cells, and MnSOD levels were unchanged. Such findings show that (1) neuronal-like differentiated SH-SY5Y cells are a suitable model to investigate PC response and more sensitive to the effect of a mild and transient H₂ O₂ -induced oxidative stress with respect to other neuronal cells; (2) 10μM H₂ O₂ -induced PC is mediated by apoptotic and inflammatory pathways, unlike antioxidant system; (3) such neuroprotective strategy and underlying signals proven in neuronal-like differentiated SH-SY5Y cells may contribute to understand in vivo PC mechanisms and to define a window for pharmacological intervention, namely, related to ischemic brain damage.

SIGNIFICANCE OF THE STUDY

Neuronal-like differentiated SH-SY5Y cells are a suitable model to investigate PC, an endogenous neuroprotective response to a mild and transient H₂ O₂ -induced oxidative stress, elicited by 24-hour exposure to very low H₂ O₂ concentrations and mediated by both apoptotic and inflammatory pathways. This model reflects in vivo PC mechanisms occurring after brain trauma and provides novel information about pathways and time of protection useful for an appropriate pharmacological intervention.

Dasatinib induces lung vascular toxicity and predisposes to pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a life-threatening disease that can be induced by dasatinib, a dual Src and BCR-ABL tyrosine kinase inhibitor that is used to treat chronic myelogenous leukemia (CML). Today, key questions remain regarding the mechanisms involved in the long-term development of dasatinib-induced PAH. Here, we demonstrated that chronic dasatinib therapy causes pulmonary endothelial damage in humans and rodents. We found that dasatinib treatment attenuated hypoxic pulmonary vasoconstriction responses and increased susceptibility to experimental pulmonary hypertension (PH) in rats, but these effects were absent in rats treated with imatinib, another BCR-ABL tyrosine kinase inhibitor. Furthermore, dasatinib treatment induced pulmonary endothelial cell apoptosis in a dose-dependent manner, while imatinib did not. Dasatinib treatment mediated endothelial cell dysfunction via increased production of ROS that was independent of Src family kinases. Consistent with these findings, we observed elevations in markers of endothelial dysfunction and vascular damage in the serum of CML patients who were treated with dasatinib, compared with CML patients treated with imatinib. Taken together, our findings indicate that dasatinib causes pulmonary vascular damage, induction of ER stress, and mitochondrial ROS production, which leads to increased susceptibility to PH development.

Localization and Targeting of GIRK Channels in Mammalian Central Neurons

G protein-gated inwardly rectifying K(+) (GIRK/K(ir)3) channels are critical to brain function. They hyperpolarize neurons in response to activation of different G protein-coupled receptors, reducing cell excitability. Molecular cloning has revealed four distinct mammalian genes (GIRK1-4), which, with the exception of GIRK4, are broadly expressed in the central nervous system (CNS) and have been implicated in a variety of neurological disorders. Although the molecular structure and composition of GIRK channels are key determinants of their biophysical properties, their cellular and subcellular localization patterns and densities on the neuronal surface are just as important to nerve function. Current data obtained with high-resolution quantitative localization techniques reveal complex, subcellular compartment-specific distribution patterns of GIRK channel subunits. Recent efforts have focused on determining the associated proteins that form macromolecular complexes with GIRK channels. Demonstration of the precise subcellular compartmentalization of GIRK channels and their associated proteins represents a crucial step in understanding the contribution of these channels to specific aspects of neuronal function under both physiological and pathological conditions. Here, we present an overview of studies aimed at determining the cellular and subcellular localization of GIRK channel subunits in mammalian brain neurons and discuss implications for neuronal physiology.

Ion channels in the regulation of apoptosis

Apoptosis, a type of genetically controlled cell death, is a fundamental cellular mechanism utilized by multicellular organisms for disposal of cells that are no longer needed or potentially detrimental. Given the crucial role of apoptosis in physiology, deregulation of apoptotic machinery is associated with various diseases as well as abnormalities in development. Acquired resistance to apoptosis represents the common feature of most and perhaps all types of cancer. Therefore, repairing and reactivating apoptosis represents a promising strategy to fight cancer. Accumulated evidence identifies ion channels as essential regulators of apoptosis. However, the contribution of specific ion channels to apoptosis varies greatly depending on cell type, ion channel type and intracellular localization, pathology as well as intracellular signaling pathways involved. Here we discuss the involvement of major types of ion channels in apoptosis regulation. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Interactions of ion transporters and channels with cancer cell metabolism and the tumour microenvironment

Major changes in intra- and extracellular pH homoeostasis are shared features of most solid tumours. These changes stem in large part from the metabolic shift of most cancer cells towards glycolytic metabolism and other processes associated with net acid production. In combination with oncogenic signalling and impact from factors in the tumour microenvironment, this upregulates acid-extruding plasma membrane transport proteins which maintain intracellular pH normal or even more alkaline compared with that of normal cells, while in turn acidifying the external microenvironment. Mounting evidence strongly indicates that this contributes significantly to cancer development by favouring e.g. cancer cell migration, invasion and chemotherapy resistance. Finally, while still under-explored, it seems likely that non-cancer cells in the tumour microenvironment also exhibit altered pH regulation and that this may contribute to their malignant properties. Thus, the physical tumour microenvironment and the cancer and stromal cells within it undergo important reciprocal interactions which modulate the tumour pH profile, in turn severely impacting on the course of cancer progression. Here, we summarize recent knowledge of tumour metabolism and the tumour microenvironment, placing it in the context of tumour pH regulation, and discuss how interfering with these properties may be exploited clinically.

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

K+ efflux through two-pore domain K+ channels is required for mouse embryonic development

Numerous studies have suggested that K+ channels regulate a wide range of physiological processes in mammalian cells. However, little is known about the specific function of K+ channels in germ cells. In this study, mouse zygotes were cultured in a medium containing K+ channel blockers to identify the functional role of K+ channels in mouse embryonic development. Voltage-dependent K+ channel blockers, such as tetraethylammonium and BaCl2, had no effect on embryonic development to the blastocyst stage, whereas K2P channel blockers, such as quinine, selective serotonin reuptake inhibitors (fluoxetine, paroxetine, and citalopram), gadolinium trichloride, anandamide, ruthenium red, and zinc chloride, significantly decreased blastocyst formation (P<0.05). RT-PCR data showed that members of the K2P channel family, specifically KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9, were expressed in mouse oocytes and embryos. In addition, their mRNA expression levels, except Kcnk3, were up-regulated by above ninefold in morula-stage embryos compared with 2-cell stage embryos (2-cells). Immunocytochemical data showed that KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9 channel proteins were expressed in the membrane of oocytes, 2-cells, and blastocysts. Each siRNA injection targeted at Kcnk2, Kcnk10, Kcnk4, Kcnk3, and Kcnk9 significantly decreased blastocyst formation by ∼38% compared with scrambled siRNA injection (P<0.05). The blockade of K2P channels acidified the intracellular pH and depolarized the membrane potential. These results suggest that K2P channels could improve mouse embryonic development through the modulation of gating by activators.

Cell volume homeostatic mechanisms: effectors and signalling pathways

Cell volume homeostasis and its fine-tuning to the specific physiological context at any given moment are processes fundamental to normal cell function. The understanding of cell volume regulation owes much to August Krogh, yet has advanced greatly over the last decades. In this review, we outline the historical context of studies of cell volume regulation, focusing on the lineage started by Krogh, Bodil Schmidt-Nielsen, Hans-Henrik Ussing, and their students. The early work was focused on understanding the functional behaviour, kinetics and thermodynamics of the volume-regulatory ion transport mechanisms. Later work addressed the mechanisms through which cellular signalling pathways regulate the volume regulatory effectors or flux pathways. These studies were facilitated by the molecular identification of most of the relevant channels and transporters, and more recently also by the increased understanding of their structures. Finally, much current research in the field focuses on the most up- and downstream components of these paths: how cells sense changes in cell volume, and how cell volume changes in turn regulate cell function under physiological and pathophysiological conditions.

Expression of TASK-2 and its upregulation by B cell receptor stimulation in WEHI-231 mouse immature B cells

Stimulation of B cell receptors (BCR ligation) induces apoptosis of immature B cells, which is critical to the elimination of self-reactive clones. In the mouse immature B cell line WEHI-231, the authors previously reported two types of background K+ channels with large (∼300 pS, LKbg) and medium (∼100 pS, MKbg) conductance in divalent cation-free conditions. While the authors have recently identified LKbg as TREK-2, the molecular nature of MKbg is unknown yet. In the present study, the authors found that BCR ligation markedly increased the background K+ conductance of WEHI-231. A single-channel study revealed that MKbg activity is increased by BCR ligation and that the biophysical properties (unitary conductance and pH sensitivity) of MKbg are consistent with those of TWIK-related acid-sensitive K+ channel 2 (TASK-2). The expression of TASK-2 and its upregulation by BCR ligation were confirmed by RT-PCR and immunoblot assays in WEHI-231. The BCR ligation-induced increase of K+ current was prevented by calcineurin inhibitors (cyclosporine A or FK506), and also by TASK-2-specific small interfering RNA (siRNA) transfection (si-TASK-2). Furthermore, si-TASK-2 attenuated the apoptosis of WEHI-231 caused by BCR ligation. TASK-2 activity and its mRNA were also confirmed in the primary splenic B cells of mouse. Summarizing, the authors report for the first time the expression of TASK-2 in B cells and surmise that the upregulation of TASK-2 by BCR ligation is associated with the apoptosis of immature B cells.

Windows to cell function and dysfunction: signatures written in the boundary layers

The medium surrounding cells either in culture or in tissues contains a chemical mix varying with cell state. As solutes move in and out of the cytoplasmic compartment they set up characteristic signatures in the cellular boundary layers. These layers are complex physical and chemical environments the profiles of which reflect cell physiology and provide conduits for intercellular messaging. Here we review some of the most relevant characteristics of the extracellular/intercellular space. Our initial focus is primarily on cultured cells but we extend our consideration to the far more complex environment of tissues, and discuss how chemical signatures in the boundary layer can or may affect cell function. Critical to the entire essay are the methods used, or being developed, to monitor chemical profiles in the boundary layers. We review recent developments in ultramicro electrochemical sensors and tailored optical reporters suitable for the task in hand.

Cerebrovascular responses in mice deficient in the potassium channel, TREK-1

We tested the hypothesis that TREK-1, a two-pore domain K channel, is involved with dilations in arteries. Because there are no selective activators or inhibitors of TREK-1, we generated a mouse line deficient in TREK-1. Endothelium-mediated dilations were not different in arteries from wild-type (WT) and TREK-1 knockout (KO) mice. This includes dilations of the middle cerebral artery to ATP, dilations of the basilar artery to ACh, and relaxations of the aorta to carbachol, a cholinergic agonist. The nitric oxide (NO) and endothelium-dependent hyperpolarizing factor components of ATP dilations were identical in the middle cerebral arteries of WT and TREK-1 KO mice. Furthermore, the NO and cyclooxygenase-dependent components were identical in the basilar arteries of the different genotypes. Dilations of the basilar artery to alpha-linolenic acid, an activator of TREK-1, were not affected by the absence of TREK-1. Whole cell currents recorded using patch-clamp techniques were similar in cerebrovascular smooth muscle cells (CVSMCs) from WT and TREK-1 KO mice. alpha-linolenic acid or arachidonic acid increased whole cell currents in CVSMCs from both WT and TREK-1 KO mice. The selective blockers of large-conductance Ca-activated K channels, penitrem A and iberiotoxin, blocked the increased currents elicited by either alpha-linolenic or arachidonic acid. In summary, dilations were similar in arteries from WT and TREK-1 KO mice. There was no sign of TREK-1-like currents in CVSMCs from WT mice, and there were no major differences in currents between the genotypes. We conclude that regulation of arterial diameter is not altered in mice lacking TREK-1.

Organisation of potassium channels on the neuronal surface

Potassium channels are a family of ion channels that govern the intrinsic electrical properties of neurons in the brain. Molecular cloning has revealed over 100 genes encoding the pore-forming alpha subunits of potassium channels in mammals, making them the most diverse subset of ion channels. Multiplicity in this ion channel family is further generated through alternative splicing. The precise location of potassium channels along the dendro-somato-axonic surface of the neurons is an important factor in determining its functional impact. Today, it is widely accepted that potassium channels can be located at any subcellular compartment on the neuronal surface, at synaptic and extrasynaptic sites, from somata to dendritic shafts, dendritic spines, axons or axon terminals. However, they are not evenly distributed on the neuronal surface and depending on the potassium channel subtype, are instead concentrated at different compartments. This selective localization of ion channels to specific neuronal compartments has many different functional implications. One factor necessary to understand the role of potassium channels in neuronal function is to unravel their specialized distribution and subcellular localization within a cell, and this can only be achieved by electron microscopy. In this review, I summarize anatomical findings, describing their distribution in the central nervous system. The distinct regional, cellular and subcellular distribution of potassium channels in the brain will be discussed in view of their possible functional implications.

TNF-alpha modulates the Na+/ K+ ATPase and the Na+K+2Cl- symporter in LLC-PK cells

BACKGROUND

Tumour necrosis factor alpha (TNF-alpha) has been implicated in the development of diabetic nephropathy and the accompanying increase in sodium retention. Inhibition of renal Na(+)/K(+) ATPase was reported to accompany cell death. As TNF is known to induce both apoptosis and cell survival, this work investigated the effect and mechanism of action of TNF-alpha on the Na(+)/K(+) ATPase and the Na(+)K(+)2Cl(-) symporter using LLC-PK(1) cells, a porcine renal proximal tubules cell line.

MATERIALS AND METHODS

Cells were incubated for 2 h with TNF-alpha in presence and absence of pyrrolidinedithiocarbamate, SP600125 and FK009, respective inhibitors of the transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kappaB), c-Jun N-terminal kinase (JNK) and caspases. The activity of the pump was assayed by measuring the ouabain-inhibitable release of inorganic phosphate. Changes in its expression and the expression of the symporter were monitored by western blot analysis.

RESULTS

TNF-alpha up-regulated both transporters. NF-kappaB, JNK and the caspases were all mediators of the cytokine action. TNF up-regulated the Na(+)/K(+) pump by stimulating JNK which in turn, activated NF-kappaB and inhibited the caspases. TNF effect on the cotransporter was also mediated via activation of JNK which however inhibited NF-kappaB and by so doing prevented activation of caspases. As caspases were demonstrated to down-regulate the two transporters, their inhibition by TNF is responsible for the observed up-regulatory effect.

CONCLUSIONS

It was concluded that the Na(+)/K(+) ATPase and Na(+)K(+)2Cl(-) are both targets of TNF-alpha and the effect of the cytokine favours cell survival over cell death.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Expression and localization of two-pore domain K(+) channels in bovine germ cells

Two-pore domain K(+) (K(2P)) channels that help set the resting membrane potential of excitable and nonexcitable cells are expressed in many kinds of cells and tissues. However, the expression of K(2P) channels has not yet been reported in bovine germ cells. In this study, we demonstrate for the first time that K(2P) channels are expressed in the reproductive organs and germ cells of Korean cattle. RT-PCR data showed that members of the K(2P) channel family, specifically KCNK3, KCNK9, KCNK2, KCNK10, and KCNK4, were expressed in the ovary, testis, oocytes, embryo, and sperm. Out of these channels, KCNK2 and KCNK4 mRNAs were abundantly expressed in the mature oocytes, eight-cell stage embryos, and blastocysts compared with immature oocytes. KCNK4 and KCNK3 were significantly increased in eight-cell stage embryos. Immunocytochemical data showed that KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9 channel proteins were expressed at the membrane of oocytes and blastocysts. KCNK10 and KCNK4 were strongly expressed and distributed in oocyte membranes. These channel proteins were also localized to the acrosome sperm cap. In particular, KCNK3 and KCNK4 were strongly localized to the post-acrosomal region of the sperm head and the equatorial band within the sperm head respectively. These results suggest that K(2P) channels might contribute to the background K(+) conductance of germ cells and regulate various physiological processes, such as maturation, fertilization, and development.

Potentiometric platform for the quantification of cellular potassium efflux

Renewed interest in the measurement of cellular K(+) effluxes has been prompted by the observation that potassium plays an active and important role in numerous key cellular events, in particular cell necrosis and apoptosis. Although necrosis and apoptosis follow different pathways, both induce intracellular potassium effluxes. Here, we report the use of potassium-selective microelectrodes located in a microfluidic platform for cell culture to monitor and quantify such effluxes in real time. Using this platform, we observed and measured the early signs of cell lysis induced by a modification of the extracellular osmolarity. Furthermore, we were able to quantify the number of dying cells by evaluating the extracellular potassium concentration. A comparison between the potentiometric measurement with a fluorescent live-dead assay performed under similar conditions revealed the delay between potassium effluxes and cell necrosis. These results suggest that such platforms may be exploited for applications, such as cytotoxicological screening assays or tumor cell proliferation assays, by using extracellular K(+) as cell death marker.

Arachidonic acid-induced activation of large-conductance potassium channels and membrane hyperpolarization in mouse B cells

Lymphocytes express voltage-gated (Kv) and Ca(2+)-activated (IKCa1) K(+) channels. Recently, we found that WEHI-231, an immature B cell line, expresses voltage-independent K(+) channels called large-conductance background K( + ) channels (LK(bg)). Arachidonic acid (AA) has attracted attention because of its potential regulatory roles in the apoptosis of immature B cells. To elucidate the functional targets of AA, we investigated the effects of AA on membrane currents, voltages, and cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) of WEHI-231 and Bal-17 cells that represent immature and mature mouse B cells, respectively. In whole-cell patch clamp, both Kv and IKCa1 were inhibited by AA. On the other hand, AA activated LK(bg) current and non-selective cationic (NSC) current in WEHI-231 while only NSC current in Bal-17. Inside-out patch clamp study showed that AA directly activates LK(bg). AA induced hyperpolarization of WEHI-231 and depolarization of Bal-17 cells, respectively. The selective functional expression of LK(bg) and their activation by AA were also confirmed in the immature B cells (B220(+)/AA4.1(+)) freshly isolated from mouse spleen. In fura-2 spectrofluorimetry, AA induced persistent increase in [Ca(2+)](c) of WEHI-231 cells, which was attenuated by KCl-induced depolarization. In Bal-17 cell, however, AA induced only a transient increase of [Ca(2+)](c). In summary, the novel type of background K(+) channels (LK(bg)) in immature B cells is strongly activated while the other K(+) channels (Kv and IKCa1) commonly expressed in lymphocytes are inhibited by AA. The hyperpolarization and augmentation of Ca(2+) influx by LK(bg) activation might play a role in the response of immature B cells to AA.

Two-pore K+ channels, NO and metabolic inhibition

Ischemic preconditioning is a potent endogenous mechanism protecting many organs from the devastating effects of prolonged ischemia. In the heart, NO is one mediator of this myoprotective response thought to involve activation of the K(ATP) channel. Ischemic preconditioning is known to be induced by metabolic inhibition using sodium cyanide (NaCN) in single cardiomyocytes. In the present study, we show for the first time that the end effector channel activated by NaCN has been incorrectly identified. The channel activated is not K(ATP) but instead belongs to the relatively new family of two-pore domain potassium channels (K2P). Further when activated by metabolic ischemia, the amplitude of K2P current is directly modulated by activators and inhibitors of the NO pathway.

Hypertonicity induced apoptosis in HL-60 cells in the presence of intracellular potassium

Cell shrinkage is a hallmark of apoptosis. Potassium efflux, which is involved in cell shrinkage, has been previously described as an essential event of apoptosis. This study was designed to address the importance of potassium efflux in hypertonicity (450 mOsm and 600 mOsm) induced apoptosis. We initiated apoptosis in HL-60 cells in hypertonic medium consisting of either high concentrations of NaCl, mannitol or KCl. Apoptotic activity was evaluated based on the DNA content of the cells, annexin-V staining and calcium content. Apoptosis was initiated in hypertonic conditions consisting of high intracellular K(+). We demonstrate that apoptosis can occur in the presence of high intracellular potassium contrary to previous predictions.

Cell shrinkage and monovalent cation fluxes: role in apoptosis

The loss of cell volume or cell shrinkage has been a morphological hallmark of the programmed cell death process known as apoptosis. This isotonic loss of cell volume has recently been term apoptotic volume decrease or AVD to distinguish it from inherent volume regulatory responses that occurs in cells under anisotonic conditions. Recent studies examining the intracellular signaling pathways that result in this unique cellular characteristic have determined that a fundamental movement of ions, particularly monovalent ions, underlie the AVD process and plays an important role on controlling the cell death process. An efflux of intracellular potassium was shown to be a critical aspect of the AVD process, as preventing this ion loss could protect cells from apoptosis. However, potassium plays a complex role as a loss of intracellular potassium has also been shown to be beneficial to the health of the cell. Additionally, the mechanisms that a cell employs to achieve this loss of intracellular potassium vary depending on the cell type and stimulus used to induce apoptosis, suggesting multiple ways exist to accomplish the same goal of AVD. Additionally, sodium and chloride have been shown to play a vital role during cell death in both the signaling and control of AVD in various apoptotic model systems. This review examines the relationship between this morphological change and intracellular monovalent ions during apoptosis.

Non-conducting functions of voltage-gated ion channels

Various studies, mostly in the past 5 years, have demonstrated that, in addition to their well-described function in regulating electrical excitability, voltage-dependent ion channels participate in intracellular signalling pathways. Channels can directly activate enzymes linked to cellular signalling pathways, serve as cell adhesion molecules or components of the cytoskeleton, and their activity can alter the expression of specific genes. Here, I review these findings and discuss the extent to which the molecular mechanisms of such signalling are understood.

Stress-induced corneal epithelial apoptosis mediated by K+ channel activation

One of the functional roles of the corneal epithelial layer is to protect the cornea, lens and other underlying ocular structures from damages caused by environmental insults. It is important for corneal epithelial cells to maintain this function by undergoing continuous renewal through a dynamic process of wound healing. Previous studies in corneal epithelial cells have provided substantial evidence showing that environmental insults, such as ultraviolet (UV) irradiation and other biohazards, can induce stress-related cellular responses resulting in apoptosis and thus interrupt the dynamic process of wound healing. We found that UV irradiation-induced apoptotic effects in corneal epithelial cells are started by the hyperactivation of K+ channels in the cell membrane resulting in a fast loss of intracellular K+ ions. Recent studies provide further evidence indicating that these complex responses in corneal epithelial cells are resulted from the activation of stress-related signaling pathways mediated by K+ channel activity. The effect of UV irradiation on corneal epithelial cell fate shares common signaling mechanisms involving the activation of intracellular responses that are often activated by the stimulation of various cytokines. One piece of evidence for making this distinction is that at early times UV irradiation activates a Kv3.4 channel in corneal epithelial cells to elicit activation of c-Jun N-terminal kinase cascades and p53 activation leading to cell cycle arrest and apoptosis. The hypothetic model is that UV-induced potassium channel hyperactivity as an early event initiates fast cell shrinkages due to the loss of intracellular potassium, resulting in the activation of scaffolding protein kinases and cytoskeleton reorganizations. This review article presents important control mechanisms that determine Kv channel activity-mediated cellular responses in corneal epithelial cells, involving activation of stress-induced signaling pathways, arrests of cell cycle progression and/or induction of apoptosis.

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

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

On the mechanism of ionic regulation of apoptosis: would the Na+/K+-ATPase please stand up?

Apoptosis is an active process with distinct features including loss of cell volume, chromatin condensation, internucleosomal DNA fragmentation, and apoptotic body formation. Among the classical characteristics that define apoptosis, the loss of cell volume has become a very important component of the programmed cell death process. Changes in cell volume result from alterations in the homeostasis of ions and in particular the movement of Na+ and K+ ions. Most living cells have a high concentration of intracellular K+ and a low concentration of intracellular Na+. This is in contrast to the outside of the cell, where there is a high concentration of extracellular Na+ and a low concentration of extracellular K+. Thus a concentration gradient exists for the loss and gain of intracellular K+ and Na+, respectively. This gradient is maintained through the activity of various ionic channels and transporters, but predominantly the activity of the Na+/K+-ATPase. During apoptosis, there is compelling evidence indicating an early increase in intracellular Na+ followed by a decrease in both intracellular K+ and Na+ suggesting a regulatory role for these cations during both the initial signalling, and the execution phase of apoptosis. Recent studies have shown that the Na+/K+-ATPase is involved in controlling perturbations of Na+ and K+ homeostasis during apoptosis, and that anti-apoptotic Bcl-2 and Bcl-XL molecules influence these ionic fluxes. Finally, understanding the regulation or deregulation of ionic homeostasis during apoptosis is critical to facilitate the treatment of cardiovascular, neurological, and renal diseases where apoptosis is known to play a major role.

K+ channels in apoptosis

A proper rate of programmed cell death or apoptosis is required to maintain normal tissue homeostasis. In disease states such as cancer and some forms of hypertension, apoptosis is blocked, resulting in hyperplasia. In neurodegenerative diseases, uncontrolled apoptosis leads to loss of brain tissue. The flow of ions in and out of the cell and its intracellular organelles is becoming increasingly linked to the generation of many of these diseased states. This review focuses on the transport of K(+) across the cell membrane and that of the mitochondria via integral K(+)-permeable channels. We describe the different types of K(+) channels that have been identified, and investigate the roles they play in controlling the different phases of apoptosis: early cell shrinkage, cytochrome c release, caspase activation, and DNA fragmentation. Attention is also given to K(+) channels on the inner mitochondrial membrane, whose activity may underlie anti- or pro-apoptotic mechanisms in neurons and cardiomyocytes.

Biphasic behavior of changes in elemental composition during staurosporine-induced apoptosis

Although the identification of events that occur during apoptosis is a fundamental goal of apoptotic cell death research, little is know about the precise sequence of changes in total elemental composition during apoptosis. We evaluated total elemental composition (Na, Mg, P, Cl, S, and K) in relation to molecular and morphological features in human U937 cells induced to undergo apoptosis with staurosporine, an intrinsic pathway activator. To evaluate total elemental content we used electron probe X-ray microanalysis to measure simultaneously all elements from single, individual cells. We observed two phases in the changes in elemental composition (mainly Na, Cl and K). The early phase was characterized by a decrease in intracellular K (P<0.001) and Cl (P<0.001) content concomitant with cell shrinkage, and preceded the increase in proteolytic activity associated with the activation of caspase-3. The later phase started with caspase-3 activation, and was characterized by a decrease in the K/Na ratio (P<0.001) as a consequence of a significant decrease in K and increase in Na content. The inversion of intracellular K and Na content was related with the inhibition of Na+/K+ ATPase. This later phase was also characterized by a significant increase (P<0.001) in intracellular Cl with respect to the early phase. In addition, we found a decrease in S content and an increase in the P/S ratio. These distinctive changes coincided with chromatin condensation and DNA fragmentation. Together, these findings support the concept that changes in total elemental composition take place in two phases related with molecular and morphological features during staurosporine-induced apoptosis.

Characterization of large-conductance Ca2+-dependent and -independent K+ channels in HaCaT keratinocytes

To characterize ion channels expressed in cell membrane of human keratinocytes, patch-clamp recordings were carried out in HaCaT cells. Two types of large-conductance K(+) channels (about 250 pS) were measured. One type was activated by micromolar concentrations of intracellular Ca(2+) ions ([Ca(2+)](i)) and membrane depolarization, the other was [Ca(2+)](i) independent. The channels were neither dependent on intracellular ATP nor Mg(2+) nor on membrane stretch. We conclude that HaCaT keratinocytes express Ca(2+)-dependent maxi K(+) channels and still unknown large Ca(2+)-independent K(+) channels. These K(+) channels may affect the proliferation and differentiation of human keratinocytes by the influence on the resting potential, which may control the Ca(2+) influx across the cell membrane.

Protective effects of TASK-3 (KCNK9) and related 2P K channels during cellular stress

Tandem pore domain (or 2P) K channels form a recently isolated family of channels that are responsible for background K currents in excitable tissues. Previous studies have indicated that 2P K channel activity produces membrane hyperpolarization, which may offer protection from cellular insults. To study the effect of these channels in neuroprotection, we overexpressed pH-sensitive 2P K channels by transfecting the partially transformed C8 cell line with these channels. Tandem pore weak inward rectifier K channel (TWIK)-related acid-sensitive K channel 3 (TASK-3, KCNK9) as well as other pH sensitive 2P K channels (TASK-1 and TASK-2) enhanced cell viability by inhibiting the activation of intracellular apoptosis pathways. To explore the cellular basis for this protection in a more complex cellular environment, we infected cultured hippocampal slices with Sindbis virus constructs containing the coding sequences of these channels. Expression of TASK-3 throughout the hippocampal structure afforded neurons within the dentate and CA1 regions significant protection from an oxygen-glucose deprivation (OGD) injury. Neuroprotection within TASK-3 expressing slices was also enhanced by incubation with isoflurane. These results confirm a protective physiologic capability of TASK-3 and related 2P K channels, and suggest agents that enhance their activity, such as volatile anesthetics may intensify these protective effects.

The effect of a hyposmotic shock and purinergic agonists on K+(Rb+) efflux from cultured human breast cancer cells

The effect of a hyposmotic shock and extracellular ATP on the efflux of K(+)(Rb(+)) from human breast cancer cell lines (MDA-MB-231 and MCF-7) has been examined. A hyposmotic shock increased the fractional efflux of K(+)(Rb(+)) from MDA-MB-231 cells via a pathway which was unaffected by Cl(-) replacement. Apamin, charybdotoxin or removing extracellular Ca(2+) had no effect on volume-activated K(+)(Rb(+)) efflux MDA-MB-231 cells. An osmotic shock also stimulated K(+)(Rb(+)) efflux from MCF-7 cells but to a much lesser extent than found with MDA-MB-231 cells. ATP-stimulated K(+)(Rb(+)) efflux from MDA-MB-231 cells in a dose-dependent fashion but had little effect on K(+)(Rb(+)) release from MCF-7 cells. ATP-stimulated K(+)(Rb(+)) efflux was only inhibited slightly by replacing Cl(-) with NO(3)(-). Removal of external Ca(2+) during treatment with ATP reduced the fractional efflux of K(+)(Rb(+)) in a manner suggesting a role for cellular Ca(2+) stores. Charybdotoxin, but neither apamin nor iberiotoxin, inhibited ATP-stimulated K(+)(Rb(+)) release from MDA-MB-231 cells. Suramin inhibited the ATP-activated efflux of K(+)(Rb(+)). UTP also stimulated K(+)(Rb(+)) efflux from MDA-MB-231 cells whereas ADP, AMP and adenosine were without effect. A combination of an osmotic shock and ATP increased the fractional efflux of K(+)(Rb(+)) to a level greater than the sum of the individual treatments. It appears that the hyposmotically-activated and ATP-stimulated K(+) efflux pathways are separate entities. However, there may be a degree of 'crosstalk' between the two pathways.

High altitude pulmonary hypertension: role of K+ and Ca2+ channels

Global alveolar hypoxia, as experienced at high-altitude living, has a serious impact on vascular physiology, particularly on the pulmonary vasculature. The effects of sustained hypoxia on pulmonary arteries include sustained vasoconstriction and enhanced medial hypertrophy. As the major component of the vascular media, pulmonary artery smooth muscle cells (PASMC) are the main effectors of the physiological response(s) induced during or following hypoxic exposure. Endothelial cells, on the other hand, can sense humoral and hemodynamic changes incurred by hypoxia, triggering their production of vasoactive and mitogenic factors that then alter PASMC function and growth. Transmembrane ion flux through channels in the plasma membrane not only modulates excitation- contraction coupling in PASMC, but also regulates cell volume, apoptosis, and proliferation. In this review, we examine the roles of K+ and Ca2+ channels in the pulmonary vasoconstriction and vascular remodeling observed during chronic hypoxia-induced pulmonary hypertension.

Heat shock protein 70 inhibits shrinkage-induced programmed cell death via mechanisms independent of effects on cell volume-regulatory membrane transport proteins

Cell shrinkage is a ubiquitous feature of programmed cell death (PCD), but whether it is an obligatory signalling event in PCD is unclear. Heat shock protein 70 (Hsp70) potently counteracts PCD in many cells, by mechanisms that are incompletely understood. In the present investigation, we found that severe hypertonic stress greatly diminished the viability of murine fibrosarcoma cells (WEHI-902) and immortalized murine embryonic fibroblasts (iMEFs). This effect was attenuated markedly by Hsp70 over-expression. To determine whether the protective effect of Hsp70 was mediated via an effect on volume regulatory ion transport, we compared regulatory volume decrease (RVD) and increase (RVI) in control WEHI-902 cells and after increasing Hsp70 levels by heat shock or over-expression (WEHI-912). Hsp70 levels affected neither RVD, RVI nor the relative contributions of the Na(+)/H(+)-exchanger (NHE1) and Na(+),K(+),2Cl(-)-cotransporter (NKCC1) to RVI. Hypertonic stress induced caspase-3 activity in WEHI cells and iMEFs, an effect potentiated by Hsp70 in WEHI cells but inhibited by Hsp70 in iMEFs. Osmotic shrinkage-induced PCD was associated with Hsp70-inhibitable cysteine cathepsin release in iMEFs and attenuated by caspase and cathepsin inhibitors in WEHI cells. Treatment with TNF-alpha or the NHE1 inhibitor 5'-(N-ethyl-N-isopropyl)amiloride (EIPA) reduced the viability of WEHI cells further under isotonic and mildly, but not severely, hypertonic conditions. Thus, it is concluded that shrinkage-induced PCD involves both caspase- and cathepsin-dependent death mechanisms and is potently counteracted by Hsp70.

Overexpression of human KCNA5 increases IK V and enhances apoptosis

Apoptotic cell shrinkage, an early hallmark of apoptosis, is regulated by K+ efflux and K+ channel activity. Inhibited apoptosis and downregulated K+ channels in pulmonary artery smooth muscle cells (PASMC) have been implicated in development of pulmonary vascular medial hypertrophy and pulmonary hypertension. The objective of this study was to test the hypothesis that overexpression of KCNA5, which encodes a delayed-rectifier voltage-gated K+ (Kv) channel, increases K+ currents and enhances apoptosis. Transient transfection of KCNA5 caused 25- to 34-fold increase in KCNA5 channel protein level and 24- to 29-fold increase in Kv channel current (I(K(V))) at +60 mV in COS-7 and rat PASMC, respectively. In KCNA5-transfected COS-7 cells, staurosporine (ST)-mediated increases in caspase-3 activity and the percentage of cells undergoing apoptosis were both enhanced, whereas basal apoptosis (without ST stimulation) was unchanged compared with cells transfected with an empty vector. In rat PASMC, however, transfection of KCNA5 alone caused marked increase in basal apoptosis, in addition to enhancing ST-mediated apoptosis. Furthermore, ST-induced apoptotic cell shrinkage was significantly accelerated in COS-7 cells and rat PASMC transfected with KCNA5, and blockade of KCNA5 channels with 4-aminopyridine (4-AP) reduced K+ currents through KCNA5 channels and inhibited ST-induced apoptosis in KCNA5-transfected COS-7 cells. Overexpression of the human KCNA5 gene increases K+ currents (i.e., K+ efflux or loss), accelerates apoptotic volume decrease (AVD), increases caspase-3 activity, and induces apoptosis. Induction of apoptosis in PASMC by KCNA5 gene transfer may serve as an important strategy for preventing the progression of pulmonary vascular wall thickening and for treating patients with idiopathic pulmonary arterial hypertension (IPAH).

Functional expression of TASK-1/TASK-3 heteromers in cerebellar granule cells

TASK-1 and TASK-3 are functional members of the tandem-pore K+ (K2P) channel family, and mRNAs for both channels are expressed together in many brain regions. Although TASK-1 and TASK-3 subunits are able to form heteromers when their complementary RNAs are injected into oocytes, whether functional heteromers are present in the native tissue is not known. Using cultured cerebellar granule (CG) neurones that express mRNAs of both TASK-1 and TASK-3, we studied the presence of heteromers by comparing the sensitivities of cloned and native K+ channels to extracellular pH (pHo) and ruthenium red. The single-channel conductance of TASK-1, TASK-3 and a tandem construct (TASK-1/TASK-3) expressed in COS-7 cells were 14.2 +/- 0.4, 37.8 +/- 0.7 and 38.1 +/- 0.7 pS (-60 mV), respectively. TASK-3 and TASK-1/TASK-3 (and TASK-3/TASK-1) displayed nearly identical single-channel kinetics. TASK-3 and TASK-1/TASK-3 expressed in COS-7 cells were inhibited by 26 +/- 4 and 36 +/- 2 %, respectively, when pHo was changed from 8.3 to 7.3. In outside-out patches from CG neurones, the K+ channel with single channel properties similar to those of TASK-3 was inhibited by 31 +/- 7 % by the same reduction in pHo. TASK-3 and TASK-1/TASK-3 expressed in COS-7 cells were inhibited by 78 +/- 7 and 3 +/- 4 %, respectively, when 5 microm ruthenium red was applied to outside-out patches. In outside-out patches from CG neurones containing a 38 pS channel, two types of responses to ruthenium red were observed. Ruthenium red inhibited the channel activity by 77 +/- 5 % in 42 % of patches (range: 72-82 %) and by 5 +/- 4 % (range: 0-9 %) in 58 % of patches. When patches contained more than three 38 pS channels, the average response to ruthenium red was 47 +/- 6 % inhibition (n= 5). These electrophysiological studies show that native 38 pS K+ channels of the TASK family in cultured CG neurones consist of both homomeric TASK-3 and heteromeric TASK-1/TASK-3.

Membrane-delimited regulation of novel background K+ channels by MgATP in murine immature B cells

In WEHI-231, a representative immature B cell line, Ca(2+) entry is paradoxically augmented by treatment with 2-aminoethoxydiphenyl borate (2-APB), a blocker of inositol 1,4,5-trisphosphate receptor and of nonselective cation channels (Nam, J. H., Yun, S. S., Kim, T. J., Uhm, D.-Y., and Kim, S. J. (2003) FEBS Lett. 535, 113-118). The initial goal of the present study was to elucidate the effects of 2-APB on membrane currents, which revealed the presence of novel K(+) channels in WEHI-231 cells. Under whole-cell patch clamp conditions, 2-APB induced background K(+) current (I(K,bg)) and hyperpolarization in WEHI-231 cells. Lowering of intracellular MgATP also induced the I(K,bg). The I(K,bg) was blocked by micromolar concentrations of quinidine but not by tetraethylammonium. In a single channel study, two types of voltage-independent K(+) channels were found with large (346 picosiemens) and medium conductance (112 picosiemens), named BK(bg) and MK(bg), respectively. The excision of membrane patches (inside-out (i-o) patches) greatly increased the P(o) of BK(bg). In i-o patches, cytoplasmic MgATP (IC(50) = 0.18 mm) decreased the BK(bg) activity, although non-hydrolyzable adenosine 5'-(beta,gamma-imino)triphosphate had no effect. A pretreatment with Al(3+) or wortmannin (50 microm) blocked the inhibitory effects of MgATP. A direct application of phosphoinositide 4,5-bisphosphate (10 microm) inhibited the BK(bg) activity. Meanwhile, the activity of MK(bg) was unaffected by MgATP. In cell-attached conditions, the BK(bg) activity was largely increased by 2-APB. In i-o patches, however, the MgATP-induced inhibition of BK(bg) was weakly reversed by the addition of 2-APB. In summary, WEHI-231 cells express the unique background K(+) channels. The BK(bg)s are inhibited by membrane-delimited elevation of phosphoinositide 4,5-bisphosphate. The activation of BK(bg) would hyperpolarize the membrane, which augments the calcium influx in WEHI-231 cells.