Identification of RBK1 potassium channels in C6 astrocytoma cells

From the Cover: Vulnerability of C6 Astrocytoma Cells After Single-Compound and Joint Exposure to Type I and Type II Pyrethroid Insecticides

A primary mode-of-action of all pyrethroid insecticides (PYRs) is the disruption of the voltage-gated sodium channel electrophysiology in neurons of target pests and nontarget species. The neurological actions of PYRs on non-neuronal cells of the nervous system remain poorly investigated. In the present work, we used C6 astrocytoma cells to study PYR actions (0.1-50 μM) under the hypothesis that glial cells may be targeted by and vulnerable to PYRs. To this end, we characterized the effects of bifenthrin (BF), tefluthrin (TF), α-cypermethrin (α-CYP), and deltamethrin (DM) on the integrity of nuclear, mitochondrial, and lysosomal compartments. In general, 24- to 48-h exposures produced concentration-related impairment of cell viability. In single-compound, 24-h exposure experiments, effective concentration (EC)₁₅s 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT assay) were computed as follows (in μM): BF, 16.1; TF, 37.3; α-CYP, 7.8; DM, 5.0. We found concentration-related damage in several C6-cell subcellular compartments (mitochondria, nuclei, and lysosomes) at ≥ 10^-1 μM levels. Last, we examined a mixture of all PYRs (ie, Σ individual EC₁₅) using MTT assays and subcellular analyses. Our findings indicate that C6 cells are responsive to nM levels of PYRs, suggesting that astroglial susceptibility may contribute to the low-dose neurological effects caused by these insecticides. This research further suggests that C6 cells may provide relevant information as a screening platform for pesticide mixtures targeting nervous system cells by expected and unexpected toxicogenic pathways potentially contributing to clinical neurotoxicity.

The Kv1.3 potassium channel is localized to the cis-Golgi and Kv1.6 is localized to the endoplasmic reticulum in rat astrocytes

The functions of voltage-gated potassium (Kv) channels in neurons have been well defined, whereas their roles in glial cells are not fully understood. Kv1.1, Kv1.3 and Kv1.6 are endogenously expressed in C6 astrocytoma cells, but their trafficking and subcellular localization have not been well studied. In C6 cells, Kv1.1 was localized to the cell surface, Kv1.3 was predominantly localized in the cis-Golgi, and Kv1.6 was enriched in the endoplasmic reticulum. Disruption of the Golgi stacks with brefeldin A treatment redirected Kv1.3 to the endoplasmic reticulum, further confirming that Kv1.3 was localized in the Golgi. Denaturing and reducing immunoblot analysis identified an expected Kv1.3 monomer and an unexpected Kv1.3 dimer/aggregate. These two forms had different protein half-lives: that of the monomer form T1/2 was 5.1 h, whereas the dimer/aggregate form was stable over the 8-h measurement period. The Kv1.3 dimer/aggregate form on immunoblots appeared to be correlated with its Golgi retention, based on examination with several cell types that expressed Kv1.3. Glycosidase treatment showed that Kv1.3 contained complex-type N-glycans terminated with sialic acids, suggesting that Kv1.3 had traveled to the trans-Golgi network for sialylation before it was recycled to the cis-Golgi for retention. Inhibition of N-glycosylation did not affect Kv1.3 localization, indicating that N-glycans did not play a role in its Golgi retention. Thus, Kv1.3 appears to be distributed to the cis-Golgi membrane of rat astrocytes in a similar way as a Golgi resident protein, and this unusual distribution appears to be correlated with its SDS/2-mercaptoethanol-resistant dimer/aggregate forms on immunoblots.

Potassium currents in primary cultured astrocytes from the rat corpus callosum

The corpus callosum (CC) is the main white matter tract in the brain and is involved in interhemispheric communication. Using the whole-cell voltage-clamp technique, a study was made of K(+)-currents in primary cultured astrocytes from the CC of newborn rats. These cells were positive to glial fibrillary acidic protein after culturing in Dulbecco's Modified Eagle Medium (> 95% of cells) or in serum-free neurobasal medium with G5 supplement (> 99% of cells). Astrocytes cultured in either medium displayed similar voltage-activated ion currents. In 81% of astrocytes, the current had a transient component and a sustained component, which were blocked by 4-aminopyridine and tetraethylammonium, respectively; and both had a reversal potential of -66 mV, indicating that they were carried by K(+) ions. Based on the Ba(2+)-sensitivity and activation kinetics of the K(+)-current, two groups of astrocytes were discerned. One group (55% of cells) displayed a strong Ba(2+) blockade of the K(+)-current whose activation kinetics, time course of decay, and the current-voltage relationship were modified by Ba(2+). This current was greatly blocked (52%) by Ba(2+) in a voltage-dependent way. Another group (45% of cells) presented weak Ba(2+)-blockade, which was only blocked 24% by Ba(2+). The activation kinetics and time course of decay of this current component were unaffected by Ba(2+). These results may help to understand better the roles of voltage-activated K(+)-currents in astrocytes from the rat CC in particular and glial cells in general.

Twenty years of dendrotoxins

Dendrotoxins are small proteins that were isolated 20 years ago from mamba (Dendroaspis) snake venoms (Harvey, A.L., Karlsson, E., 1980. Dendrotoxin from the venom of the green mamba, Dendroaspis angusticeps: a neurotoxin that enhances acetylcholine release at neuromuscular junctions. Naunyn-Schmiedebergs Arch. Pharmacol. 312, 1-6.). Subsequently, a family of related proteins was found in mamba venoms and shown to be homologous to Kunitz-type serine protease inhibitors, such as aprotinin. The dendrotoxins contain 57-60 amino acid residues cross-linked by three disulphide bridges. The dendrotoxins have little or no anti-protease activity, but they were demonstrated to block particular subtypes of voltage-dependent potassium channels in neurons. Studies with cloned K(+) channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1, Kv1.2 and Kv1.6 channels in the nanomolar range, whereas toxin K from the black mamba Dendroaspis polylepis preferentially blocks Kv1.1 channels. Structural analogues of dendrotoxins have helped to define the molecular recognition properties of different types of K(+) channels, and radiolabelled dendrotoxins have also been useful in helping to discover toxins from other sources that bind to K(+) channels. Because dendrotoxins are useful markers of subtypes of K(+) channels in vivo, dendrotoxins have become widely used as probes for studying the function of K(+) channels in physiology and pathophysiology.

Voltage-gated sodium and potassium channels in radial glial cells of trout optic tectum studied by patch clamp analysis and single cell RT-PCR

Radial glial cells in the visual center of trout were analyzed immunocytochemically and with the whole cell mode of the patch-clamp technique in combination with RT-PCR. By immunostaining with anti-GFAP antibodies radially oriented cell processes spanning the entire width of the tectum were brightly labeled, while with anti-S-100 antiserum the cell bodies residing in a discrete layer close to the ventricular border became most clearly visible. Virtually all radial glial cells examined in brain slices exhibited voltage-gated sodium inward currents that were activated above -40 mV, blocked by micromolar concentrations of TTX and totally eliminated if sodium was substituted for Tris in the bath solution. In contrast with adjacent nerve cells of the same slices radial glial cells did not exhibit spontaneous electrical activity and could not be stimulated to generate action potentials by depolarizing current injections. Two types of voltage-gated potassium outward currents were elicited by depolarizing voltage steps: a sustained current with delayed rectifier properties and a superimposed transient "A"-type current, both being activated at a threshold potential of -40 mV. In cultured radial glial cells subtle differences were noticed regarding current density, inactivation kinetics, and TEA-sensitivity of the potassium currents. Inwardly rectifying potassium currents activating at hyperpolarized voltages were not observed. By single cell RT-PCR the transcripts of two shaker-related potassium channel genes (termed tsha1-a fish homologue to Kv1.2- and tsha3) were amplified, while transcripts for tsha 2 and tsha 4 were not detected.

Expression of Kv1.1, a Shaker-like potassium channel, is temporally regulated in embryonic neurons and glia

Kv1.1, a Shaker-like voltage-gated potassium channel, is strongly expressed in a variety of neurons in adult rodents, in which it appears to be involved in regulating neuronal excitability. Here we show that Kv1.1 is also expressed during embryonic development in the mouse, exhibiting two transient peaks of expression around embryonic day 9.5 (E9.5) and E14.5. Using both in situ hybridization and immunocytochemistry, we have identified several cell types and tissues that express Kv1.1 RNA and protein. At E9.5, Kv1.1 RNA and protein are detected transiently in non-neuronal cells in several regions of the early CNS, including rhombomeres 3 and 5 and ventricular zones in the mesencephalon and diencephalon. At E14.5, several cell types in both the CNS and peripheral nervous system express Kv1.1, including neuronal cells (sensory ganglia and outer aspect of cerebral hemispheres) and glial cells (radial glia, satellite cells, and Schwann cell precursors). These data show that Kv1.1 is expressed transiently in a variety of neuronal and non-neuronal cells during restricted periods of embryonic development. Although the functional roles of Kv1.1 in development are not understood, the cell-specific localization and timing of expression suggest this channel may play a role in several developmental processes, including proliferation, migration, or cell-cell adhesion.

Cyclic AMP regulates potassium channel expression in C6 glioma by destabilizing Kv1.1 mRNA

The tissue distributions and physiological properties of a variety of cloned voltage-gated potassium channel genes have been characterized extensively, yet relatively little is known about the mechanisms controlling expression of these genes. Here, we report studies on the regulation of Kv1.1 expressed endogenously in the C6 glioma cell line. We demonstrate that elevation of intracellular cAMP leads to the accelerated degradation of Kv1.1 RNA. The cAMP-induced decrease in Kv1.1 RNA is followed by a decrease in Kv1. 1 protein and a decrease in the whole cell sustained K+ current amplitude. Dendrotoxin-I, a relatively specific blocker of Kv1.1, blocks 96% of the sustained K+ current in glioma cells, causing a shift in the resting membrane potential from -40 mV to -7 mV. These data suggest that expression of Kv1.1 contributes to setting the resting membrane potential in undifferentiated glioma cells. We therefore suggest that receptor-mediated elevation of cAMP reduces outward K+ current density by acting at the translational level to destabilize Kv1.1 RNA, an additional mechanism for regulating potassium channel gene expression.

Molecular structure and expression of shaker type potassium channels in glial cells of trout CNS

Two shaker-related potassium channel transcripts (tsha1, tsha2) were identified in glial cells of trout central nervous system (CNS) by polymerase chain reaction (PCR)-cloning and sequencing. While tsha1 was highly similar to the mammalian Kv1.2 subtype of potassium channels, tsha2 did not show a preferential sequence homology with a particular subtype of shaker, but exhibited uniform similarity with mammalian Kv1.1, Kv1.2, and Kv1.3, respectively. Transcripts for the shaw and shal subfamilies of voltage-gated potassium channels were detected in whole brain tissue only but not in freshly dissociated glial cells. Using mRNA extracted from different glial cell types in combination with sequence specific PCR primers, tshal was found restricted to oligodendrocytes and their progenitor cells, while tsha2 was common to astrocytes, as well.

Identification of the delayed rectifier potassium channel, Kv1.6, in cultured astrocytes

Astrocytes are an abundant glial cell type of the central nervous system that appear to play a role in regulating extracellular potassium concentrations in brain, thereby contributing to the maintenance of normal neuronal activity. Voltage-gated potassium conductances, shown to be present in astrocytes, may be involved in this and other astrocytic functions. Toward defining the role of voltage-gated potassium channels in astrocytes, total RNA prepared from cultured mouse cortical astrocytes was screened, using a reverse transcriptase-polymerase chain reaction (RT-PCR) approach, for the expression of several members of the Shaker-like potassium channel subfamily (Kv1.1-Kv1.6). A relatively high level of Kv1.6 transcript was identified by RT-PCR and then confirmed and quantitated by ribonuclease protection assays using a Kv1.6-specific riboprobe. Immunocytochemical staining showed double-labeling of glial fibrillary acidic protein-positive cells with antibody specific for the Kv1.6 channel. The Kv1.6 protein expression was variable among the individual astrocytes. Outward voltage-gated currents were studied in astrocytes in primary culture using the Nystatin-perforated patch voltage clamp technique. Outward potassium currents were observed in all cells studied, and this current was partially blocked by perfusion with 100 nM dendrotoxin (DTX) in 14 of 16 cells tested. This DTX-sensitive current appeared to be a sustained outward potassium current, consistent with the suggestion that the Shaker-like potassium channel Kv1.6 underlies a portion of the delayed rectifier potassium current in cultured mouse cortical astrocytes.

Recent studies on dendrotoxins and potassium ion channels

1. Dendrotoxins are small proteins isolated from mamba (Dendroaspis) snake venoms. They block some subtypes of voltage-dependent potassium channels in neurons. 2. Dendrotoxins contain 57-60 amino acid residues crosslinked by three disulfide bridges. They are homologous to Kunitz-type serine protease inhibitors, such as aprotinin, although they have little or no antiprotease activity. 3. Dendrotoxins act mainly on neuronal K+ channels. Studies with cloned K+ channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1 and Kv1.2 channels in the nanomolar range. In native cells, dendrotoxin appears preferentially to block inactivating forms of K+ current. 4. Dendrotoxins can induce repetitive firing in neurons and facilitate transmitter release. On direct injection to the CNS, dendrotoxins can induce epileptiform activity. 5. Radiolabeled dendrotoxins are useful markers of subtypes of K+ channels in vivo, and structural analogs help to define the molecular recognition properties of different types of K+ channels.

Characterization of Ca(2+)-activated 86Rb+ fluxes in rat C6 glioma cells: a system for identifying novel IKCa-channel toxins

1. The pharmacological characteristics of a putative Ca2+ activated K+ channel (IKCa channel) in rat glioma C6 cells were studied in the presence of the Ca2+ ionophore, ionomycin and various K+ channel blockers, 86Rb+ being used as a radioisotopic tracer for K+. 2. The resting 86Rb+ influx into C6 cells was 318 +/- 20 pmol s-1. The threshold for ionomycin activation of 86Rb+ influx was approx. 100 nM. At ionomycin concentrations above the activation threshold, the initial rate of 86Rb+ influx was proportional to ionophore concentration. Ionomycin-activated 86Rb+ flux was saturable (EC50 = 0.62 +/- 0.03 microM) and was not inhibited by ouabain. 3. Intracellular Ca2+ increased within 30 s from a basal level of 42 +/- 2 nM to 233 +/- 17 nM, after addition of 2 microM ionomycin. During this period, intracellular pH fell from 7.03 +/- 0.04 to 6.87 +/- 0.03 and the cell hyperpolarized from -34 +/- 10 mV to -76 +/- 2 mV. 4. Single channel conductance measurements on inside-out patches in physiological K+ solutions identified a 14 +/- 3 pS CA(2+)-activated K+ current between -25 mV and +50 mV. In symmetrical (100 mM) K+, the single channel conductance was 26 pS. 5. Externally applied quinine (IC50 = 0.12 +/- 0.34 mM) and tetraethylammonium chloride (IC50 = 10 +/- 1.9 mM) inhibited 86Rb+ influx into C6 cells in a concentration-dependent manner. Charybdotoxin (IC50 = 0.5 +/- 0.02 nM) and iberiotoxin (IC50 = 800 +/- 150 nM), as well as the crude venoms from the scorpions Leiurus quinquestriatus and Mesobuthus tamulus, also inhibited 86Rb+ influx. In contrast, apamin and toxin I had no inhibitory effects on 86Rb+ flux. A screen of fractions from cation exchange h.p.l.c. of Mesob. tamulus venom revealed the presence of at least four charybdotoxin-like peptides. One of these was iberiotoxin; the other three are novel toxins. 6. The ionomycin-activated 86Rb+ influx into rat C6 glioma cells has proved to be a valuable pharmacological assay for the screening of toxins and crude venoms which modify intermediate conductance, Ca2+ activated K+ channel activity.

Physiology of transformed glial cells

Much of our present knowledge of glial cell function stems from studies of glioma cell lines, both rodent (C6, C6 polyploid, and TR33B) and human (1321N1, 138MG, D384, R-111, T67, Tp-276MG, Tp-301MG, Tp-483MG, Tp-387MG, U-118MG, U-251MG, U-373MG, U-787MG, U-1242MG, and UC-11MG). New methods such as patch clamp and Ca2+ imaging have lead to rapid progress the last few years in our knowledge about glial cells, where an unexpected presence and diversity of receptors and ion channels have emerged. Basic mechanisms related to membrane potential and K+ transport and the presence of voltage gated ion channels (Na+, inwardly rectifying K+, Ca(2+)-activated K+, Ca2+, and Cl- channels) have been identified. Receptor function and intracellular signaling for glutamate, acetylcholine, histamine, serotonin, cathecolamines, and a large number of neuropeptides (bradykinin, cholecystokinin, endothelin, opioids, and tachykinins) have been characterized. Such studies are facilitated in cell lines which offer a more homogenous material than primary cultures. Although the expression of ion channels and receptors vary considerably between different cell lines and comparative studies are rare, a few differences (compared to astrocytes in primary culture) have been identified which may turn out to be characteristic for glioma cells. Future identification of specific markers for receptors on glial and glioma cells related to cell type and growth properties may have great potential in clinical diagnosis and therapy.

Axons regulate the expression of Shaker-like potassium channel genes in Schwann cells in peripheral nerve

We examined potassium channel gene expression of two members of the Shaker subfamily, MK1 and MK2, in sciatic nerves from rats and mice. In Northern blot analysis, MK1 and MK2 probes detected single transcripts of approximately 8 kb and approximately 9.5 kb, respectively, in sciatic nerve and brain from both species. Polymerase chain reaction amplification of a cDNA library of cultured rat Schwann cells using MK1- and MK2- specific primers produced DNA fragments that were highly homologous to MK1 and MK2. To determine whether these channel genes were axonally regulated, we performed Northern blot analysis of developing, permanently transected, and crushed rat sciatic nerves. The mRNA levels for both MK1 and MK2 increased from P1 to P15 and then declined modestly. Permanent nerve transection in adult animals resulted in a dramatic and permanent reduction in the mRNA levels for both MK1 and MK2, whereas normal levels of MK1 and MK2 were restored when regeneration was allowed to occur following crush injury. In all cases, MK1 and MK2 mRNA levels paralleled that of the myelin gene P0. Elevating the cAMP in cultured Schwann cells by forskolin, which mimics axonal contact but not myelination, did not induce detectable levels of MK1 and MK2 mRNA by Northern blot analysis. Further, the level of MK1 mRNA in the vagus nerve, which contains relatively fewer myelinating Schwann cells and relatively more non-myelinating Schwann cells than the sciatic nerve, is reduced relative to the sciatic nerve. In conclusion, we have identified two Shaker-like potassium channel genes in sciatic nerves whose expressions are regulated by axons. We suggest that MK1 and MK2 mRNA are expressed in high levels only in myelinating Schwann cells and that these Shaker-like potassium channel genes have specialized roles in these cells.

Interactions among calcium compartments in C6 rat glioma cells: involvement of potassium channels

1. Variations in intracellular free Ca2+ concentration ([Ca2+]i) induced by alteration of the extracellular concentrations of Ca2+ ([Ca2+]o) and K+ ([K+]o) were imaged in single fluo-3-loaded C6 glioma cells. In addition, the effect of membrane potential on [Ca2+]i was investigated in fura-2-loaded, voltage-clamped cells. 2. Step alterations of [Ca2+]o from 0 to 10 nM were followed by proportional variations in [Ca2+]i, with a maximum 7-fold increase and an apparent half-maximum at [Ca2+]o of 1.5 mM. 3. The time to half-maximum change (t1/2) of [Ca2+]o-associated [Ca2+]i variations ranged between 10 and 50 s, and was inversely related to the amplitude of [Ca2+]o steps. 4. Transient, serotonin-induced [Ca2+]i elevations, used as a measure of Ca2+ availability in inositol 1,4,5-trisphosphate-sensitive stores, were diminished within 10 min in 0 mM [Ca2+]o, but were unaffected by [Ca2+]o changes in the 1-5 mM range. 5. Restoration of normal [Ca2+]i following its elevation by serotonin was delayed by removal of external Na+ or Cl- and was enhanced by warming the medium to 37 degrees C. These conditions did not affect [Ca2+]o-associated [Ca2+]i variations. 6. [Ca2+]o-associated [Ca2+]i variations were depressed by La3+ and Ba2+, while blockers of voltage-activated Ca2+ channels were ineffective. 7. Elevated [K+]o depressed the basal level of [Ca2+]i, and in high concentrations (70-140 mM) also diminished the response to serotonin. 8. Depolarizing the membrane potential of voltage-clamped cells reversibly reduced [Ca2+]i. These membrane-potential associated [Ca2+]i variations were blocked by La3+, Ba2+ and TEA, all of which also depolarized membrane resting potential. 9. Apamin (at 1-10 microM), a blocker of [Ca2+]i-activated K+ channel, totally and reversibly prevented [Ca2+]o-associated [Ca2+]i variations. 10. These studies indicate that C6 cells are responsive to variations in [Ca2+]o, and that a K+ channel is a possible path through which Ca2+ penetrates into the cell.

On the mechanism of 4-aminopyridine action on the cloned mouse brain potassium channel mKv1.1

1. This study used the whole-cell patch clamp technique to investigate the mechanism of action of the K+ channel blocker 4-aminopyridine (4-AP) on the cloned K+ channel mouse Kv1.1 (mKv1.1) expressed in Chinese hamster ovary cells. 2. Cells transfected with mKv1.1 expressed a non-inactivating, delayed rectifier-type K+ current. 4-AP induced a dose-, voltage- and use-dependent block of mKv1.1. 3. 4-AP blockade of mKv1.1 was similar whether 4-AP was administered extracellularly (IC50 = 147 microM) or intracellularly (IC50 = 117 microM). 4. Inclusion of the first twenty amino acids of the N-terminus sequence of the Shaker B K+ channel ('inactivation peptide') in the patch electrode transformed mKv1.1 into a rapidly inactivating current. The time constant of decay for the modified current was dependent on the concentration of inactivation peptide, and under these conditions extracellular 4-AP had a reduced potency (IC50 values of 471 and 537 microM for 0.5 and 2 mg ml-1 inactivation peptide, respectively). 5. A permanently charged analogue of 4-AP, 4-aminopyridine methiodide (4-APMI), was found to block mKv1.1 when applied inside the cell, but was without effect when administered externally. 6. Decreasing the intracellular pH (pHi) to 6.4 caused an increase in 4-AP potency (IC50 = 76 microM), whereas at pHi 9.0, the 4-AP potency fell (IC50 = 295 microM). Conversely, increasing extracellular pH (pHo) to 9.0 caused an increase in 4-AP potency (IC50 = 93 microM), whereas at pHo 6.4, 4-AP potency decreased (IC50 = 398 microM). 7. Taken together, these findings support the hypotheses that the uncharged form of 4-AP crosses the membrane, and that it is predominantly the cationic form which acts on mKv1.1 channels intracellularly, possibly at or near to the binding site for the inactivation peptide.

Three distinct types of voltage-dependent K+ channels are expressed by Müller (glial) cells of the rabbit retina

There is ample evidence that retinal radial glial (Müller) cells play a crucial role in retinal ion homeostasis. Nevertheless, data on the particular types of ion channels mediating this function are very rare and incomplete; this holds especially for mammalian Müller cells. Thus, the whole-cell variation of the patch-clamp technique was used to study voltage-dependent currents in Müller cells from adult rabbit retinae. The membrane of Müller cells was almost exclusively permeable to K+ ions, as no significant currents could be evoked in K(+)-free internal and external solutions, external Ba2+ (1 mM) reversibly blocked most membrane currents, and external Cs+ ions (5 mM) blocked all inward currents. All cells expressed inwardly rectifying channels that showed inactivation at strong hyperpolarizing voltages (> or = -120 mV), and the conductance of which varied with the square root of extracellular K+ concentration ([K+]e). Most cells responded to depolarizing voltages (> or = -30 mV) with slowly activating outward currents through delayed rectifier channels. These currents were reversibly blocked by external application of 4-aminopyridine (4-AP, 0.5 mM) or tetraethylammonium (TEA, > 20 mM). Additionally, almost all cells showed rapidly inactivating currents in response to depolarizing (> or = -60 mV) voltage steps. The currents were blocked by Ba2+ (1 mM), and their amplitude increased with the [K+]e. Obviously, these currents belonged to the A-type family of K+ channels. Some of the observed types of K+ channels may contribute to retinal K+ clearance but at least some of them may also be involved in regulation of proliferative activity of the cells.

Ca(2+)-dependent K+ channel activity in rat glioma cells induced by bradykinin stimulation and by inositol 1,4,5-trisphosphate injection

1. A glial cell line derived from C6 rat glioma cells has been shown previously to respond to extracellular pulses of bradykinin or intracellular injection of inositol 1,4,5-trisphosphate (Ins-P3) with a slow hyperpolarizing response due to activation of a K+ current (G. Reiser et al., Brain Res. 506, 205-214; 1990). 2. We determined the ensuing single-channel activity, which is most likely caused by Ca2+ released from internal stores after bradykinin stimulation. Bradykinin-activated channels were selectively permeable to K+, but not to Na+ or to Cl-, and exhibited conductances of mainly 40 and 50 pS. In glioma cells the same type of channel was activated by intracellular injection of Ins-P3 and by extracellular bradykinin pulses.

A Ca(2+)- and pH-dependent K+ channel of rat C6 glioma cells and its possible role in acidosis-induced cell swelling

The aim of the present study was to explore whether a change in membrane K+ conductance contributes to acidosis-induced swelling of cultured rat C6 glioma cells. Electrophysiological studies were performed using whole-cell and single-channel recordings in combination with cell volume measurements in cell suspension by flow cytometry. Whole-cell recordings revealed a voltage-dependent K+ conductance. The predominant K+ channel in single-channel recordings with symmetrical high K+ concentrations was inwardly rectifying and had conductances of 35 and 15 pS, respectively. A raised internal free Ca2+ concentration and membrane depolarization increased the open probability of this channel. Internal acidosis (pH 6.4-5.4), on the other hand, reduced open probability and single-channel conductance. Both whole-cell and single-channel K+ currents were blocked by quinidine (0.1-1 mM), which was therefore used to analyze the functional consequences of an inhibition of this conductance for cell volume. Thereby, quinidine (1 mM) produced a small (5%) and transient cell swelling of C6 glioma cells. In contrast, acidosis (pH 5.6) caused a much larger (about 20%) and maintained swelling. Since quinidine produced only a minor swelling of C6 cells, it is unlikely that inhibition of the K+ conductance caused acidosis-induced cell swelling. Other mechanisms, such as activation of ion transporters, must therefore be responsible.

Inhibition of human brain tumor cell growth by a receptor-operated Ca2+ channel blocker

SK&F 96365, a reported receptor-operated Ca2+ channel blocker, inhibited the growth of U-373 MG human astrocytoma and SK-N-MC human neuroblastoma cell lines in a dose-dependent manner. Carbachol and serum which act as growth factors for these cells induced a rapid, transient increase of intracellular Ca2+ concentration without a sustained increase. SK&F 96365 also exerted a significant inhibition of carbachol or serum-induced intracellular Ca2+ mobilization. These results suggest that SK&F 96365 is a potent inhibitor of brain tumor cell growth and that its effect may be mediated by the inhibition of agonist-induced intracellular Ca2+ mobilization.

Pharmacology of a cloned potassium channel from mouse brain (MK-1) expressed in CHO cells: effects of blockers and an 'inactivation peptide'

1. Chinese hamster ovary cells (CHO), maintained in cell culture, were stably transfected with DNA for the MK-1 voltage-activated potassium channel, previously cloned from a mouse brain library. 2. Voltage-activated currents were recorded by the whole cell patch clamp method. In CHO cells transfected with the vector only, there were no significant outward voltage activated currents. However, large outward voltage-activated potassium currents were always observed in those cells which had been transfected with the vector containing the DNA encoding for MK-1. 3. These potassium currents activated from -40 mV, and reversed at the potassium equilibrium potential. The half-maximal conductance of MK-1 was at -10 mV and had a slope factor of 11 mV when fitted with a Boltzmann function. There was only very slight (< 10%) inactivation of MK-1 even at very large positive voltages. 4. MK-1 was reversibly blocked by: 4-aminopyridine (4-AP, 0.1-4 mM), Toxin I 10-100 nM), mast cell degranulating peptide (1 microM), tetraethylammonium (TEA, 4-10 mM), tedisamil (100 microM), quinine (100 microM) and ciclazindol (100 microM); all applied to the outside of the cell from a 'U tube' rapid perfusion system. 4-AP may block closed as well as open MK-1 potassium channels. 5. A synthetic 20 amino acid peptide derived from the N-terminus sequence of the Shaker B potassium channel (the 'inactivation peptide') produced dramatic inactivation of MK-1 when applied to the inside, but not the outside of the cell. Reducing peptide concentration or 'degrading' the peptide produced less inactivation. 6. The block of MK-1 by the synthetic inactivation peptide was quite different in time dependence from block by internal TEA (0.4-4 mM), which probably blocks much more quickly but less potently than the peptide.