Change in K+ current of HeLa cells with progression of the cell cycle studied by patch-clamp technique

Exploring the Role of Surface and Mitochondrial ATP-Sensitive Potassium Channels in Cancer: From Cellular Functions to Therapeutic Potentials

ATP-sensitive potassium (KATP) channels are found in plasma membranes and mitochondria. These channels are a type of ion channel that is regulated by the intracellular concentration of adenosine triphosphate (ATP) and other nucleotides. In cell membranes, they play a crucial role in linking metabolic activity to electrical activity, especially in tissues like the heart and pancreas. In mitochondria, KATP channels are involved in protecting cells against ischemic damage and regulating mitochondrial function. This review delves into the role of KATP channels in cancer biology, underscoring their critical function. Notably responsive to changes in cellular metabolism, KATP channels link metabolic states to electrical activity, a feature that becomes particularly significant in cancer cells. These cells, characterized by uncontrolled growth, necessitate unique metabolic and signaling pathways, differing fundamentally from normal cells. Our review explores the intricate roles of KATP channels in influencing the metabolic and ionic balance within cancerous cells, detailing their structural and operational mechanisms. We highlight the channels' impact on cancer cell survival, proliferation, and the potential of KATP channels as therapeutic targets in oncology. This includes the challenges in targeting these channels due to their widespread presence in various tissues and the need for personalized treatment strategies. By integrating molecular biology, physiology, and pharmacology perspectives, the review aims to enhance the understanding of cancer as a complex metabolic disease and to open new research and treatment avenues by focusing on KATP channels. This comprehensive approach provides valuable insights into the potential of KATP channels in developing innovative cancer treatments.

Exploitation of ATP-sensitive potassium ion (KATP) channels by HPV promotes cervical cancer cell proliferation by contributing to MAPK/AP-1 signalling

Persistent infection with high-risk human papillomaviruses (HPVs) is the causal factor in multiple human malignancies, including >99% of cervical cancers and a growing proportion of oropharyngeal cancers. Prolonged expression of the viral oncoproteins E6 and E7 is necessary for transformation to occur. Although some of the mechanisms by which these oncoproteins contribute to carcinogenesis are well-characterised, a comprehensive understanding of the signalling pathways manipulated by HPV is lacking. Here, we present the first evidence to our knowledge that the targeting of a host ion channel by HPV can contribute to cervical carcinogenesis. Through the use of pharmacological activators and inhibitors of ATP-sensitive potassium ion (KATP) channels, we demonstrate that these channels are active in HPV-positive cells and that this activity is required for HPV oncoprotein expression. Further, expression of SUR1, which forms the regulatory subunit of the multimeric channel complex, was found to be upregulated in both HPV+ cervical cancer cells and in samples from patients with cervical disease, in a manner dependent on the E7 oncoprotein. Importantly, knockdown of SUR1 expression or KATP channel inhibition significantly impeded cell proliferation via induction of a G1 cell cycle phase arrest. This was confirmed both in vitro and in in vivo tumourigenicity assays. Mechanistically, we propose that the pro-proliferative effect of KATP channels is mediated via the activation of a MAPK/AP-1 signalling axis. A complete characterisation of the role of KATP channels in HPV-associated cancer is now warranted in order to determine whether the licensed and clinically available inhibitors of these channels could constitute a potential novel therapy in the treatment of HPV-driven cervical cancer.

The role of potassium and host calcium signaling in Toxoplasma gondii egress

Toxoplasma gondii is an obligate intracellular parasite and replicates inside a parasitophorous vacuole (PV) within the host cell. The membrane of the PV (PVM) contains pores that permits for equilibration of ions and small molecules between the host cytosol and the PV lumen. Ca²⁺ signaling is universal and both T. gondii and its mammalian host cell utilize Ca²⁺ signals to stimulate diverse cellular functions. Egress of T. gondii from host cells is an essential step for the infection cycle of T. gondii, and a cytosolic Ca²⁺ increase initiates a Ca²⁺ signaling cascade that culminates in the stimulation of motility and egress. In this work we demonstrate that intracellular T. gondii tachyzoites are able to take up Ca²⁺ from the host cytoplasm during host cell signaling events. Both intracellular and extracellular Ca²⁺ sources are important in reaching a threshold of parasite cytosolic Ca²⁺ needed for successful egress. Two peaks of Ca²⁺ were observed in egressing single parasites with the second peak resulting from Ca²⁺ entry. We patched infected host cells to allow the delivery of precise concentrations of Ca²⁺ for the stimulation of motility and egress. Using this approach of patching infected host cells, allowed us to determine that increasing the host cytosolic Ca²⁺ to a specific concentration can trigger egress, which is further accelerated by diminishing the concentration of potassium (K⁺).

How Dysregulated Ion Channels and Transporters Take a Hand in Esophageal, Liver, and Colorectal Cancer

Over the last two decades, the understanding of how dysregulated ion channels and transporters are involved in carcinogenesis and tumor growth and progression, including invasiveness and metastasis, has been increasing exponentially. The present review specifies virtually all ion channels and transporters whose faulty expression or regulation contributes to esophageal, hepatocellular, and colorectal cancer. The variety reaches from Ca²⁺, K⁺, Na⁺, and Cl^- channels over divalent metal transporters, Na⁺ or Cl^- coupled Ca²⁺, HCO₃^- and H⁺ exchangers to monocarboxylate carriers and organic anion and cation transporters. In several cases, the underlying mechanisms by which these ion channels/transporters are interwoven with malignancies have been fully or at least partially unveiled. Ca²⁺, Akt/NF-κB, and Ca²⁺- or pH-dependent Wnt/β-catenin signaling emerge as cross points through which ion channels/transporters interfere with gene expression, modulate cell proliferation, trigger epithelial-to-mesenchymal transition, and promote cell motility and metastasis. Also miRs, lncRNAs, and DNA methylation represent potential links between the misexpression of genes encoding for ion channels/transporters, their malfunctioning, and cancer. The knowledge of all these molecular interactions has provided the basis for therapeutic strategies and approaches, some of which will be broached in this review.

Targeting potassium channels in cancer

Potassium channels are pore-forming transmembrane proteins that regulate a multitude of biological processes by controlling potassium flow across cell membranes. Aberrant potassium channel functions contribute to diseases such as epilepsy, cardiac arrhythmia, and neuromuscular symptoms collectively known as channelopathies. Increasing evidence suggests that cancer constitutes another category of channelopathies associated with dysregulated channel expression. Indeed, potassium channel–modulating agents have demonstrated antitumor efficacy. Potassium channels regulate cancer cell behaviors such as proliferation and migration through both canonical ion permeation–dependent and noncanonical ion permeation–independent functions. Given their cell surface localization and well-known pharmacology, pharmacological strategies to target potassium channel could prove to be promising cancer therapeutics.

Potassium channels in cell cycle and cell proliferation

Normal cell-cycle progression is a crucial task for every multicellular organism, as it determines body size and shape, tissue renewal and senescence, and is also crucial for reproduction. On the other hand, dysregulation of the cell-cycle progression leading to uncontrolled cell proliferation is the hallmark of cancer. Therefore, it is not surprising that it is a tightly regulated process, with multifaceted and very complex control mechanisms. It is now well established that one of those mechanisms relies on ion channels, and in many cases specifically on potassium channels. Here, we summarize the possible mechanisms underlying the importance of potassium channels in cell-cycle control and briefly review some of the identified channels that illustrate the multiple ways in which this group of proteins can influence cell proliferation and modulate cell-cycle progression.

Cell volume regulation in epithelial physiology and cancer

The physiological function of epithelia is transport of ions, nutrients, and fluid either in secretory or absorptive direction. All of these processes are closely related to cell volume changes, which are thus an integrated part of epithelial function. Transepithelial transport and cell volume regulation both rely on the spatially and temporally coordinated function of ion channels and transporters. In healthy epithelia, specific ion channels/transporters localize to the luminal and basolateral membranes, contributing to functional epithelial polarity. In pathophysiological processes such as cancer, transepithelial and cell volume regulatory ion transport are dys-regulated. Furthermore, epithelial architecture and coordinated ion transport function are lost, cell survival/death balance is altered, and new interactions with the stroma arise, all contributing to drug resistance. Since altered expression of ion transporters and channels is now recognized as one of the hallmarks of cancer, it is timely to consider this especially for epithelia. Epithelial cells are highly proliferative and epithelial cancers, carcinomas, account for about 90% of all cancers. In this review we will focus on ion transporters and channels with key physiological functions in epithelia and known roles in the development of cancer in these tissues. Their roles in cell survival, cell cycle progression, and development of drug resistance in epithelial cancers will be discussed.

K(+) channels and cell cycle progression in tumor cells

K⁺ ions play a major role in many cellular processes. The deregulation of K⁺ signaling is associated with a variety of diseases such as hypertension, atherosclerosis, or diabetes. K⁺ ions are important for setting the membrane potential, the driving force for Ca²⁺ influx, and regulate volume of growing cells. Moreover, it is increasingly recognized that K⁺ channels control cell proliferation through a novel signaling mechanisms triggered and modulated independently of ion fluxes. In cancer, aberrant expression, regulation and/or sublocalization of K⁺ channels can alter the downstream signals that converge on the cell cycle machinery. Various K⁺ channels are involved in cell cycle progression and are needed only at particular stages of the cell cycle. Consistent with this idea, the expression of Eag1 and HERG channels fluctuate along the cell cycle. Despite of acquired knowledge, our understanding of K⁺ channels functioning in cancer cells requires further studies. These include identifying the molecular mechanisms controlling the cell cycle machinery. By understanding how K⁺ channels regulate cell cycle progression in cancer cells, we will gain insights into how cancer cells subvert the need for K⁺ signal and its downstream targets to proliferate.

Noninvasive determination of cell nucleoplasmic viscosity by fluorescence correlation spectroscopy

Noninvasive and reliable quantification of rheological characteristics in the nucleus is extremely useful for fundamental research and practical applications in medicine and biology. This study examines the use of fluorescence correlation spectroscopy (FCS) to noninvasively determine nucleoplasmic viscosity (eta(nu)), an important parameter of nucleoplasmic rheology. Our FCS analyses show that eta(nu) of lung adenocarcinoma (ASTC-a-1) and HeLa cells are 1.77+/-0.42 cP and 1.40+/-0.27 cP, respectively, about three to four times larger than the water viscosity at 37 degrees C. eta(nu) was reduced by 31 to 36% upon hypotonic exposure and increased by 28 to 52% from 37 to 24 degrees C. In addition, we found that eta(nu) of HeLa cells reached the lowest value in the S phase and that there was no significant difference of eta(nu) between in the G1 and G2 phases. Last, nucleoplasmic viscosity was found to be larger than cytoplasmic viscosity in both HeLa and ASTC-a-1 cells. These results indicate that FCS can be used as a noninvasive tool to investigate the microenvironment of living cells. This is the first report on the measurement of eta(nu) in living cells synchronized in the G1, S, and G2 phases.

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.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

KCa3.1 Ca2+ activated K+ channels regulate human airway smooth muscle proliferation

Airway smooth muscle cell hyperplasia contributes to airway remodeling and hyperreactivity characteristic of asthma. Changes to potassium channel activity in proliferating human airway smooth muscle (HASM) cells have been described, but no regulatory role in proliferation has been attributed to them. We sought to investigate the expression of the intermediate conductance calcium-activated potassium channel K(Ca)3.1 in HASM cells and investigate its role in proliferation. Smooth muscle cells derived from human airways were grown in vitro and K(Ca)3.1 channel expression was measured using Western blot, RT-PCR, and patch clamp electrophysiology. Pharmacologic inhibitors of the channel were used in assays of cellular proliferation, and flow cytometry was used to identify cell cycle regulation. HASM cells expressed K(Ca)3.1 channel mRNA, protein, and activity with up-regulation evident after transforming growth factor-beta stimulation. Pharmacologic inhibition of K(Ca)3.1 led to growth arrest in cells stimulated to proliferate with mitogens. These inhibitors did not cause cellular toxicity or induce apoptosis. We have demonstrated, for the first time, the expression of K(Ca)3.1 channels in HASM cells. In addition, we have shown that K(Ca)3.1 channels are important in HASM cell proliferation, making these channels a potential therapeutic target in airway remodeling.

Cell size-proliferation relationship in rat glioma cells

The homeostasis of the central nervous system is highly controlled by glial cells and is dramatically altered in the case of glioma. In this respect, the complex connection between cell size and division is of particular importance and needs clarifying. In order to investigate this connection, cell number and volume were measured in C6 rat glioma cells under different experimental conditions, including continuous cell culture, Cl- channel blockade, and anisotonicity, and in the presence of an inhibitory conditioned medium collected from cell cultures or in a medium containing a low level of fetal calf serum. The rate of cell proliferation changed with cell volume in a bell-shaped manner, so that it is optimal within a cell volume window and appears to be controlled by low and high cell size checkpoints. The cell size-proliferation relationship can be defined by Boltzmann-like equations, which may reflect the effects of macromolecular crowding on proteins controlling the cell cycle progression. Altogether, these observations indicate that glioma cell proliferation is controlled predominantly but not exclusively by cell size-dependent mechanisms.

Reduced store-operated Ca(2+) currents in rat basophilic leukaemia cells cultured under serum-free conditions

Influx of Ca(2+) represents an important regulatory signal in the process of cell proliferation. However, little is known about how Ca(2+) entry changes during the cell-cycle. Patch-clamp experiments and microfluorimetry show that store-operated Ca(2+) entry was substantially reduced in rat basophilic leukaemia cells cultured for 24h under serum-free conditions. Likewise, retinoic acid treatment blocked Ca(2+) influx activated by store depletion via inositol 1,4,5-trisphosphate. Both procedures are known to arrest cells at the G0/G1 boundary of the cell-cycle and induced a reduction in 5-bromo 2'-deoxyuridine incorporation into DNA. Ca(2+) release from the stores remained unaltered and two types of K(+) currents were not affected in cells after serum starvation. The specific reduction in Ca(2+) entry was not detected when using aphidicolin, 5-fluorouracil or thymidine to synchronise the cell-cycle. These data suggest that store-operated Ca(2+) influx changed during cell-cycle progression which might have important implications for cell growth.

Cell cycle-related changes in regulatory volume decrease and volume-sensitive chloride conductance in mouse fibroblasts

Cell cycle-related changes in the ability to regulate cell volume following hyposmotic swelling were studied in mouse fibroblasts using videomicroscopy and the whole-cell patch clamp technique. Regulatory volume decrease (RVD) and volume-sensitive Cl- conductance (G(Cl,vol)) were measured: (1) in proliferating cells of different sizes; (2) in cells arrested in defined phases of the cell cycle (G1, G1/S, S, and M phases) using mevastatin, mimosine, hydroxyurea, aphidicolin, cytosine beta-D-arabinofuranoside, and taxol; and (3) in serum-starved cells (G(0) state). Cells in all groups were able to undergo RVD, although the cells approaching mitosis (i.e., the largest cells in proliferating cultures and the taxol-treated cells) had the lowest rates of shrinkage during RVD. In agreement with this finding, the density of G(Cl,vol) was stable in proliferating and cell cycle-arrested cells for most of the cell cycle, with the exception of the cells approaching mitosis and the new daughter cells where the density was decreased to half. The impairment of RVD was greatest in serum-starved cells which also had the lowest density of G(Cl,vol). We conclude that proliferating cells maintain an ability to recover from osmotic swelling as they progress through the cell cycle, although this ability may be compromised during mitosis.

Effects on Rb(+)(K+) uptake of HeLa cells in a high K(+) medium of exposure to a switched 1.7 Tesla magnetic field

Effects of a switched, time-varying 1.7 T magnetic field on Rb(+)(K+) uptake by HeLa S3 cells incubated in an isosmotic high K(+) medium were examined. The magnetic flux density was varied intermittently from 0.07-1.7 T at an interval of 3 s. K(+) uptake was activated by replacement of normal medium by high K(+) medium. A membrane-permeable Ca(2+) chelating agent (BAPTA-AM) and Ca(2+)-dependent K(+) channel inhibitors (quinine, charibdotoxin, and iberiotoxin) were found to reduce the Rb(+)(K+) uptake by about 30-40%. Uptake of K(+) that is sensitive to these drugs is possibly mediated by Ca(2+)-dependent K(+) channels. The intermittent magnetic field partly suppress ed the drug-sensitive K(+) uptake by about 30-40% (P < 0.05). To test the mechanism of inhibition by the magnetic fields, intracellular Ca(2+) concentration ([Ca(2+)]c) was measured using Fura 2-AM. When cells were placed in the high K(&plus;) medium, [Ca(2+)]c increased to about 1.4 times the original level, but exposure to the magnetic fields completely suppressed the increase (P < 0.01). Addition of a Ca(2+) ionophore (ionomycin) to the high K(+) medium increased [Ca(2+)]c to the level of control cells, regardless of exposure to the magnetic field. But the inhibition of K(+) uptake by the magnetic fields was not restored by addition of ionomycin. Based on our previous results on magnetic field-induced changes in properties of the cell membrane, these results indicate that exposure to the magnetic fields partly suppresses K(+) influx, which may be mediated by Ca(2+)-dependent K(+) channels. The suppress ion of K(+) fluxes could relate to a change in electric properties of cell surface and an inhibition of Ca(2+) influx mediated by Ca(2+) channels of either the cell plasma membrane or the inner vesicular membrane of intracellular Ca(2+) stores.

A cytoplasmic cell cycle controls the activity of a K+ channel in pre-implantation mouse embryos

We previously have reported that the activity of a 240 pS K+ channel varies during the cell cycle in pre-implantation mouse embryos. In the present study, we show that: (i) the cycling of channel activity is not prevented by inhibiting protein synthesis and hence does not involve cyclin-dependent kinase 1 (cdk1)-cyclin B; and (ii) the cycling of channel activity continues in anucleate zygote fragments with a time course similar to that observed in nucleate fragments. We further demonstrate that: (i) persistent activation of the K+ channel in one-cell embryos arrested in metaphase requires the maintenance of an active cdk1-cyclin B complex; and (ii) both DNA synthesis inhibition with aphidicolin and DNA damage produced by mitomycin C prevent the down-regulation of the channel at the start of S phase by a mechanism that requires tyrosine kinase activation. Thus, the 240 pS K+ channel in these cells is controlled by a previously unsuspected cytoplasmic clock that functions independently of the well-known clock controlling the chromosomal cell cycle, but can interact with it.

The development of voltage-gated ion channels and its relation to activity-dependent development events

Spontaneous activity is an essential feature in the development of the nervous system. The patterns of activity and the waveform and ionic dependence of the action potentials that occur during such activity are fine-tuned to carry out certain developmental functions, and are therefore generally not compatible with the mature physiological function of the cell. For this reason, the patterns of ion channel development that create spontaneous activity early in the development of a given cell type are complex and not easily predicted from the mature properties of that same cell. Ion channels are often found that are specific to early stages of development, and that either are not retained in the mature cell or whose properties are greatly changed during later differentiation. The exact significance of such patterns of channel development is just now becoming clear, as we understand more about the mechanisms linking spontaneous activity to later developmental events.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Potassium conductance in the androgen-sensitive prostate cancer cell line, LNCaP: involvement in cell proliferation

BACKGROUND

Very little is known about the expression of ion channels in prostate cells (both normal and malignant), and their possible role in physiological and pathological functions. We therefore studied ion conductances and their role in the proliferation of LNCaP cells, an androgen-sensitive human prostate cancer cell line.

METHODS

We applied patch-clamp recording techniques for electrophysiological studies, and 3H-thymidine incorporation and protein content assays for cell growth studies.

RESULTS

Only one type of voltage-dependent ion conductance, a potassium K+ conductance, was identified. This current, which was depressed by a rise in intracellular Ca2+, had a high sensitivity to tetraethylammonium (TEA) (with half-block at 2 mM) and was also inhibited by 2 nM alpha-dendrotoxin (DTX) and 20 nM mast-cell degranulating peptide (MCDP). K+ channel inhibitors inhibited [3H]thymidine incorporation and protein content, in a dose-dependent fashion, indicating that K+ channels are involved in cell growth.

CONCLUSIONS

We conclude from our findings that the human cancer prostate cell line LNCaP has a new type of K+ channel, likely to play an essential role in the physiology of these cells and, more specifically, in cell proliferation.

The major surface protein complex of Treponema denticola depolarizes and induces ion channels in HeLa cell membranes

The oral spirochete Treponema denticola is closely associated with periodontal diseases in humans. The 53-kDa major surface protein (Msp) located in the outer membrane of T. denticola serovar a (ATCC 35405) has both pore-forming activity and adhesin activity. We have used standard patch clamp recording methods to study the effects of a partially purified outer membrane complex containing Msp on HeLa cells. The Msp complex was free of the chymotrypsin-like proteinase also found in the outer membrane of T. denticola. Msp bound to several HeLa cell proteins, including a 65-kDa surface protein and a 96-kDa cytoplasmic protein. The Msp complex depolarized and increased the conductance of the HeLa cell membrane in a manner which was not strongly selective for Na+, K+, Ca2+, and Cl- ions. Cell-attached patches of HeLa cell membrane exposed to Msp complex exhibited short-lived channels with a slope conductance of 0.4 nS in physiologically normal saline. These studies show that Msp binds both a putative epithelial cell surface receptor and cytoplasmic proteins and that the Msp complex can form large conductance ion channels in the cytoplasmic membrane of epithelial cells. These properties may contribute to the cytopathic effects of T. denticola on host epithelial cells.

Cell cycle--dependent expression of L- and T-type Ca2+ currents in rat aortic smooth muscle cells in primary culture

The expression of L- and T-type Ca2+ channels has been reported to change during various biological events, including cellular differentiation and proliferation. The present study aimed to examine whether or not the expression of L- and T-type Ca2+ channels depends on the cell cycle in rat aortic smooth muscle cells in primary culture. Both the phase of the cell cycle and the functional expression of Ca2+ channels were determined in the same single cell, using an immunocytochemical analysis of cell cycle-specific nuclear antigens and a whole-cell voltage-clamp method, respectively. In the G0 (n = 130) and M (n = 75) phases, all cells showed only L-type Ca2+ currents. The cells showing a T-type Ca2+ current appeared in the G1 phase (37%, n = 85) and increased in the S phase (90%, n = 21). For L-type Ca2+ channels, the current density was significantly greater in the G1 phase than in the G0 and M phases. However, either the voltage-dependent properties or the dose-response relationships of Bay K 8644- and second messenger-induced modulations of L-type Ca2+ current did not differ in the four phases of the cell cycle. These findings thus indicate that the expression of L- and T-type Ca2+ channels depends on the cell cycle, whereas the characteristics of L-type Ca2+ channels do not differ between the phases of the cell cycle.

Properties of Ca(2+)-dependent K+ channels of human gingival fibroblasts

Cells in the oral cavity are normally exposed to different temperatures. Ion transport systems are influenced by temperature in other tissues: In particular, changes in intracellular K+ ion can affect cell growth and synthesis of macromolecules. The purpose of this investigation was to identify K+ channels in human gingival fibroblast cells and analyze the effect of temperature on their K+ conduction properties. Ca(2+)-dependent K+ channels with a large conductance (125 pS in symmetrical K(+)-rich solutions) were identified in human gingival fibroblasts and studied by the patch-clamp technique. The open probability of the channels varied with membrane potential between +40 and -100 mV. When the bath temperature was decreased from 40 to 4 degrees C, channel conductance was reduced, but the mean open time of the channels was increased. The activation energies for the conductance and the reciprocal of the mean open time were estimated to be 9.1 and 22.9 kJ/mol, respectively. These values are lower than those reported for these and other types of channels in cells from other tissues. The open probability of the channels was nearly constant in the temperature range studied. These results suggest that the properties of Ca(2+)-dependent K+ channels of gingival fibroblasts remain relatively unchanged when the cells are exposed to a wide range of temperatures.

Characterisation of Ca(2+)-dependent inwardly rectifying K+ currents in HeLa cells

The whole-cell configuration of the patch-clamp technique was used to examine K+ currents in HeLa cells. Under quasi-physiological ionic gradients, using an intracellular solution containing 10(-7) mol/l free Ca2+, mainly outward currents were observed. Large inwardly rectifying currents were elicited in symmetrical 145 mmol/l KCl. Replacement of all extracellular K+ by isomolar Na+, greatly decreased inward currents and shifted the reversal potential as expected for K+ selectivity. The inwardly rectifying K+ currents exhibited little or no apparent voltage dependence within the range of from -120 mV to 120 mV. A square-root relationship between chord conductance and [K+] at negative potentials could be established. The inwardly rectifying nature of the currents was unaltered after removal of intracellular Mg2+ and chelation with ATP and ethylenediaminetetraacetic acid (EDTA). Permeability ratios for other monovalent cations relative to K+ were: K+ (1.0) > Rb+ (0.86) > Cs+ (0.12) > Li (0.08) > Na+ (0.03). Slope conductance ratios measured at -100 mV were: Rb+ (1.66) > K+ (1.0) > Na+ (0.09) > Li (0.08) > Cs+ (0.06). K+ conductance was highly sensitive to intracellular free Ca2+ concentration. The relationship between conductance at 0 mV and Ca2+ concentration was well described by a Hill expression with a dissociation constant, KD, of 70 nmol/l and a Hill coefficient, n, of 1.81. Extracellular Ba2+ blocked the currents in a concentration- and voltage-dependent manner. The dependence of the KD for the blockade was analysed using a Woodhull-type treatment, locating the ion interaction site at 19% of the distance across the electrical field of the membrane and a KD (0 mV) of 7 mmol/l. Tetraethylammonium and 4-aminopyridine were without effect whilst quinine and quinidine blocked the currents with concentrations for half-maximum effects equal to 7 mumol/l and 3.5 mumol/l, respectively. The unfractionated venom of the scorpion Leiurus quinquestriatus (LQV) blocked the K+ currents of HeLa cells. The toxins apamin and scyllatoxin had no detectable effect whilst charybdotoxin, a component of LQV, blocked in a voltage-dependent manner with half-maximal concentrations of 40 nmol/l at -120 mV and 189 nmol/l at 60 mV; blockade by charybdotoxin accounts for the effect of LQV. Application of ionomycin (5-10 mumol/l), histamine (1 mmol/l) or bradykinin (1-10 mumol/l) to cells dialysed with low-buffered intracellular solutions induced K+ currents showing inward rectification and a lack of voltage dependence.

Calcium signalling and cell proliferation

The orderly sequence of events that constitutes the cell cycle is carefully regulated. A part of this regulation depends upon the ubiquitous calcium signalling system. Many growth factors utilize the messenger inositol trisphosphate (InsP3) to set up prolonged calcium signals, often organized in an oscillatory pattern. These repetitive calcium spikes require both the entry of external calcium and its release from internal stores. One function of this calcium signal is to activate the immediate early genes responsible for inducing resting cells (G0) to re-enter the cell cycle. It may also promote the initiation of DNA synthesis at the G1/S transition. Finally, calcium contributes to the completion of the cell cycle by stimulating events at mitosis. The role of calcium in cell proliferation is highlighted by the increasing number of anticancer therapies and immunosuppressant drugs directed towards this calcium signalling pathway.

Effects of acute ozone exposure on the electrophysiological properties of guinea pig trachea

Acute ozone (O3) exposures produce an increase in the apparent permeability of the tracheal epithelium, but the mechanism of this response is poorly understood. Comparison of previous studies suggests that qualitative differences may exist between measurements made in vivo or in vitro. To test this possibility we used both in vitro and in vivo electrophysiological techniques to investigate the effects of O3 exposure on guinea pig tracheal epithelium. Male Hartley guinea pigs were exposed to either 1 or 2 ppm O3 or to filtered air for 3 h and were studied 0, 6, or 24 h after exposure. Air-exposed animals had in vitro mean tracheal potential (VT) -32.0 +/- 1.5 mV, conductance (GTL) 2.18 +/- 0.22 mS/cm, short-circuit current (ISCL) 62.6 +/- 3.7 microA/cm, and diameter (D) 2.44 +/- 0.10 mm. In vitro properties after 1 ppm O3 exposure did not differ at any time point from control. Two parts per million O3 increased ISCL, but only at 6 h postexposure. The effect of O3 on ISCL was abolished by amiloride. There were no significant changes in VT, GTL, or D. In vivo tracheal potential under pentobarbital anesthesia was -19.7 +/- 1.7 mV. At 6 h postexposure to 2 ppm O3, but not at 0 or 24 h, in vivo VT was increased. Thus, acute exposure of guinea pigs to a high concentration of O3 caused a delayed increase in Na+ absorption by the trachea with no change in conductance. This indicates that paracellular permeability of guinea pig tracheal epithelium was not substantially increased by acute O3 and suggests that enhanced macromolecular uptake in this species probably occurs transcellularly. Furthermore, the increase of in vivo VT following O3 exposure is consistent with the in vitro response, indicating that in vivo/in vitro differences are not responsible for the discrepancies between previous electrophysiological and "permeability" studies.