Inhibition of the M current in NG 108-15 neuroblastoma x glioma hybrid cells

ATP-inhibition of M current in frog sympathetic neurons involves phospholipase C but not Ins P(3), Ca(2+), PKC, or Ras

Suppression of the voltage-activated, noninactivating K(+) conductance (M conductance; g(M)) by muscarinic agonists, P(2Y) agonists or bradykinin increases neuronal excitability. All agonist effects are mediated, at least in part, via the Gq/(11) class of G protein. We found, using whole cell or perforated patch recording from bullfrog sympathetic B neurons that ATP-induced suppression of g(M) was attenuated by the phospholipase C (PLC) inhibitor, U73122 (IC(50) approximately 0.14 microM) but not by the inactive isomer, U73343. The ability of extracellularly applied U73122 to inhibit PLC was confirmed by its antagonism of ATP-induced elevation of intracellular Ca(2+) as measured by fura-2 photometry. ATP-induced g(M) suppression was not antagonized by the protein kinase C (PKC) inhibitor, chelerythrine (5 microM extracellular +10 microM intracellular), by the Ca(2+)-ATPase inhibitor, thapsigargin (5 microM), or by inositol trisphosphate (InsP(3)) receptor antagonists, heparin (approximaterly 300 microM) or xestospongin C (1.8 microM). The effect of ATP on g(M) was thus dependent on PLC yet independent of PKC and of InsP(3)-induced release of intracellular Ca(2+). We therefore tested the involvement of a PKC-independent action of diacylglycerol (DAG) that could occur via activation of Ras. This low-molecular-weight G protein is activated following DAG binding to Ras-GRP, a neuronal Ras-GTP exchange factor. However, impairment of Ras function by culturing neurons with isoprenylation inhibitors (perillic acid, 0.1 mM, or alpha-hydroxyfarnesyl-phosphonic acid, 10 microM) failed to affect ATP-induced g(M) suppression. Inhibition of MEK (mitogen-activated protein kinase), a downstream target of Ras, by using PD 98059 (10 microM) was also ineffective. The transduction mechanism used by ATP to suppress g(M) in frog sympathetic neurons therefore differs from the PLC-independent mechanism used by muscarine and from the PLC and Ca(2+)-dependent mechanism used by bradykinin and UTP in mammalian ganglia. The possibility remains that "lipid-signaling" mechanisms, perhaps involving PLC-induced depletion of phosphatidylinositol bisphosphate, are involved in PLC-mediated inhibition of g(M) by ATP in amphibian sympathetic neurons.

Modulation and genetic identification of the M channel

Potassium channels constitute a superfamily of the most diversified ion channels, acting in delicate and accurate ways to control or modify many physiological and pathological functions including membrane excitability, transmitter release, cell proliferation and cell degeneration. The M-type channel is a unique ligand-regulated and voltage-gated K(+) channel showing distinct physiological and pharmacological characteristics. This review will cover some important progress in the study of M channel modulation, particularly focusing on membrane transduction mechanisms. The K(+) channel genes corresponding to the M channel have been identified and will be reviewed in detail. It has been a long journey since the discovery of M current in 1980 to our present understanding of the mysterious mechanisms for M channel modulation; a journey which exemplifies tremendous achievements in ion channel research and exciting discoveries of elaborate modulatory systems linked to these channels. While substantial evidence has accumulated, challenging questions remain to be answered.

Two types of K(+) channel subunit, Erg1 and KCNQ2/3, contribute to the M-like current in a mammalian neuronal cell

The potassium M current was originally identified in sympathetic ganglion cells, and analogous currents have been reported in some central neurons and also in some neural cell lines. It has recently been suggested that the M channel in sympathetic neurons comprises a heteromultimer of KCNQ2 and KCNQ3 (Wang et al., 1998) but it is unclear whether all other M-like currents are generated by these channels. Here we report that the M-like current previously described in NG108-15 mouse neuroblastoma x rat glioma cells has two components, "fast" and "slow", that may be differentiated kinetically and pharmacologically. We provide evidence from PCR analysis and expression studies to indicate that these two components are mediated by two distinct molecular species of K(+) channel: the fast component resembles that in sympathetic ganglia and is probably carried by KCNQ2/3 channels, whereas the slow component appears to be carried by merg1a channels. Thus, the channels generating M-like currents in different cells may be heterogeneous in molecular composition.

Separation of M-like current and ERG current in NG108-15 cells

Differentiated NG108-15 neuroblastoma x glioma hybrid cells were whole-cell voltage-clamped. Hyperpolarizing pulses, superimposed on a depolarized holding potential (-30 or -20 mV), elicited deactivation currents which consisted of two components, distinguishable by fitting with two exponential functions. Linopirdine [DuP 996, 3,3-bis(4-pyridinylmethyl)-1-phenylindolin-2-one), a neurotransmitter-release enhancer known as potent and selective blocker of the M-current of rat sympathetic neurons, in concentrations of 5 or 10 microM selectively inhibited the fast component (IC50 = 14.7 microM). The slow component was less sensitive to linopirdine (IC50>20 microM). The class III antiarrhythmics [(4-methylsulphonyl)amido]benzenesulphonamide (WAY-123.398) and 1-[2-(6-methyl-2-pyrydinil)ethyl]-4-(4-methylsulphonylaminobenz oyl) piperidine (E-4031), selective inhibitors of the inwardly rectifying ERG (ether-à-go-go-related gene) potassium channel, inhibited predominantly the slow component (IC50 = 38 nM for E-4031). The time constant of the WAY-123.398-sensitive current resembled the time constant of the slow component in size and voltage dependence. Inwardly rectifying ERG currents, recorded in K+ -rich bath at strongly negative pulse potentials, resembled the slow component of the deactivation current in their low sensitivity to linopirdine (28% inhibition at 50 microM). The size of the slow component varied greatly between cells. Accordingly, varied the effect of WAY-123.398 on deactivation current and holding current. RNA transcripts for the following members of the ether-à-go-go gene (EAG) K+ channel family were found in differentiated NG108-15 cells: ERG1, ERG2, EAGI, EAG-like (ELK)1, ELK2; ERG3 was only present in non-differentiated cells. In addition, RNA transcripts for KCNQ2 and KCNQ3 were found in differentiated and non-differentiated cells. We conclude that the fast component of the deactivation current is M-like current and the slow component is deactivating ERG current. The molecular correlates are probably KCNQ2/KCNQ3 and ERG1/ERG2, respectively.

Low concentrations of muscarine potentiate M-current in bullfrog sympathetic B-neurones

The concentration-dependence of the effect of muscarine on M-current (IM) and the underlying M-conductance (gM) in B-cells of bullfrog sympathetic ganglion was examined using whole-cell recording techniques. High concentrations of muscarine (> or = 200 nM) produced the classical suppression and over-recovery of steady-state IM at -30 mV. By contrast, low concentrations of muscarine (< or = 30 nM) shifted the gM activation curve to more negative potentials, increased the activation time constant (tau a) and increased steady-state IM. This effect may reflect muscarine-induced changes in submembrane Ca2+ concentration.

The effects of bradykinin on K+ currents in NG108-15 cells treated with U73122, a phospholipase C inhibitor, or neomycin

1. Bradykinin has multiple effects on differentiated NG108-15 neuroblastoma x glioma cells: it increases Ins(1,4,5)P3 production and intracellular Ca2+ concentration [Ca2+]i evokes a Ca2+ activated K+ current (IK(Ca)) and inhibits M current (IM). We studied the effect of the aminosteroid U73122 and the antibiotic neomycin, both putative blockers of phospholipase C (PLC), on these four bradykinin effects. 2. Preincubation with 1 or 5 microM U73122 for 15 min partly suppressed Ins(1,4,5)P3 generation and the increase in [Ca2+]i induced by 1 microM bradykinin. U73122 10 microM caused total and irreversible inhibition. The inactive analogue U73343 was without effect. 3. Resting levels of Ins(1,4,5)P3 were not affected. However, resting [Ca2+]i was increased by 10 microM U73122, but not by U73343. Individual cells responded to 10 microM U73122 with a small increase in [Ca2+]i, followed in some cells by a large further rise. 4. Pretreatment of whole-cell clamped cells with 1 microM U73122 for 30 min reduced the bradykinin-induced IK(Ca) to a fifth of its normal size. To suppress it totally, a 7-12 min pretreatment with 5 microM U73122 was required. Again, U73343 was without effect. 5. U73122 and U73343 at concentrations of 5-10 microM irreversibly decreased the holding current (Ih) which at a holding potential of -30 or -20 mV mainly flows through open M channels. The decrease was often preceded by a transient increase. 6. M current (IM) measured with 1 s pulses, was also decreased by 5-10 microM U73122 and U73343, but short applications of U73122 could cause a small increase. The bradykinin-induced inhibition of IM was not affected by U73122. 7. Preincubation with 1 or 3 mM neomycin for 15 min did not affect Ins(1,4,5)P3 generation and the increase in [Ca2+]i induced by bradykinin. Pretreatment with 3 mM neomycin for about 20 min diminished the bradykinin-induced IK(Ca) to a fifth of its normal size. 8. The four main conclusions drawn from the results are: (a) U73122 suppresses bradykinin-induced PLC activation and IK(Ca), but not IM inhibition. (b) This indicates that the transient outward current IK(Ca), but not the decrease of IM in response to bradykinin, is mediated by PLC. (c) U73122 itself inhibits IM and mobilizes Ca2+ from intracellular stores. (d) Externally applied neomycin is not an effective inhibitor of PLC-mediated signalling pathways in NG108-15 cells.

Control of M-current

M-current is a non-inactivating potassium current found in many neuronal cell types. In each cell type, it is dominant in controlling membrane excitability by being the only sustained current in the range of action potential initiation. It can be modulated by a large array of receptor types, and the modulation can occur either by suppression or enhancement. Modulation of M-current has dramatic effects on neuronal excitability. This review discusses the numerous second messenger pathways that converge on regulation of this current: in particular, two forms of regulation of the M-current, receptor-mediated modulation and the control of macroscopic current amplitude by intracellular calcium. Both types of regulation are discussed with reference to the modulation of single-channel gating properties.

Muscarinic mechanisms in nerve cells

The receptor subtype and transduction mechanisms involved in the regulation of various neuronal ionic currents are reviewed, with some recent observations on sympathetic neurons, hippocampal cell membranes and basal forebrain cells.

Calcineurin regulates M channel modal gating in sympathetic neurons

The M current regulates neuronal excitability, with its amplitude resulting from high open probability modal M channel behavior. The M current is affected by changing intracellular calcium levels. It is proposed that internal calcium acts by regulating M channel modal gating. Intracellular application of a preactivated form of the calcium-dependent phosphatase calcineurin (CaN420) inhibited the macroscopic M current, while its application to excised inside-out patches reduced high open probability M channel activity. Addition of ATP reversed the action of CaN420 on excised patches. The change in M channel gating induced by CaN420 was different from the effect of muscarine. A kinetic model supports the proposition that shifts in channel gating induced by calcium-dependent phosphorylation and dephosphorylation control M current amplitude.

Reduction of spike frequency adaptation and blockade of M-current in rat CA1 pyramidal neurones by linopirdine (DuP 996), a neurotransmitter release enhancer

1. Linopirdine (DuP 996) has been shown to enhance depolarization-induced release of several neurotransmitters in the CNS through a mechanism which may involve K+ channel blockade. The electrophysiological effects of linopirdine were therefore investigated directly, by use of conventional voltage recording and single electrode voltage-clamp. 2. Linopirdine (10 microM) reduced spike frequency adaptation (SFA) in rat hippocampal CA1 pyramidal neurones in vitro. The reduction of SFA comprised an increase in number of spikes and a reduction in inter-spike intervals after the first, but with no effect on time to first spike. Linopirdine also caused a voltage-dependent depolarization of resting membrane potential (RMP). 3. M-current (IM), a current known to underlie SFA and to set RMP, was blocked by linopirdine in a reversible, concentration-dependent manner (IC50 = 8.5 microM). This block was not reversed by atropine (10 microM). 4. Linopirdine did not affect IQ, the slow after-hyperpolarization following a spike train, or spike duration. 5. Linopirdine may represent a novel class of K+ blocker with relative selectivity for the M-current. This block of IM is consistent with the suggestion from a previous study that linopirdine may affect a tetraethylammonium-sensitive channel, and it could be speculated that IM blockade may be involved with the enhancement of neurotransmitter release by linopirdine.

Model experiments on squid axons and NG108-15 mouse neuroblastoma x rat glioma hybrid cells

Three types of ionic current essentially determine the firing pattern of nerve cells: the persistent Na+ current, the M current and the low-voltage-activated Ca(2)+ current. The present article summarizes recent experiments concerned with the basic properties of these currents. Keynes and Meves (Proc R Soc Lond B (1993) 253, 61-68) studied the persistent or steady-state Na+ current on dialysed squid axons and measured the probability of channel opening both for the peak and the steady-state Na+ current (PF(peak) and PF(ss)) as a function of voltage. Whereas PF(peak) starts to rise at -50 mV and reaches a maximum at +40 to +50 mV, PF(ss) only begins to rise appreciably at around 0 mV and is still increasing at +100 mV. This differs from observations on vertebrate excitable tissues where the persistent Na+ current tums on in the threshold region and saturates at around 0 mV. Schmitt and Meves (Pflugers Arch (1993) 425, 134-139) recorded M current, a non-inactivating K+ current, from NGI08-15 neuroblastoma x glioma hybrid cells, voltage-clamped in the whole-cell mode, and studied the effects of phorbol 12,13-dibutyrate (PDB), an activator of protein kinase C (PKC), and arachidonic acid (AA). PDB and AA both decreased I(M), the effective concentrations being 0.1-1 mu M and 5-25 mu M, respectively; while the PDB effect was regularly observed, the M current depression by AA was highly variable from cell to cell. The PKC 19-31 peptide, an effective inhibitor of PKC, in a concentration of 1 muM almost totally prevented the effects of PDB and AA on M current, suggesting that both are mediated by PKC. Schmitt and Meves (Pflugers Arch (1994a) 426, Suppl R 59) measured low-voltage-activated (l-v-a) and high-voltage-activated (h-v-a) Ca2+ currents on NG108-15 cells and investigated the effect of AA and PDB on both types of current. At pulse potentials > -20 mV, AA (25-100 mu M) decreased 1-v-a and h-v-a I(Ca). The decrease was accompanied by a small negative shift and a slight flattening of the activation and inactivation curves of the l-v-a I(Ca). The AA effect was not prevented by 50 mu M eicosa-5,8,11,14-tetraynoic acid (ETYA), an inhibitor of AA metabolism, or PKC 19-31 peptide and not mimicked by 0.1-1 mu M PDB. Probably, AA acts directly on the channel protein or its lipid environment. The physiological relevance of these three sets of observations is briefly discussed.

Activation of nucleotide receptors inhibits M-type K current [IK(M)] in neuroblastoma x glioma hybrid cells

A phospholipase-C-linked nucleotide receptor, sensitive to both uridine and adenosine triphosphate (UTP and ATP) has been cloned from NG108-15 neuroblastoma x glioma hybrid cells. We have tested whether activation of this receptor could inhibit the voltage-dependent K+ current [IK(M) or "M-current"] in NG108-15 cells recorded using whole-cell patch-clamp methods. Both UTP and ATP inhibited IK(M) by 44% and 42%, respectively, at 100 microM. Mean IC50 values were: UTP, 0.77 +/- 0.27 microM; ATP, 1.81 +/- 0.82 microM. The order of nucleotide and nucleoside activity at 100 microM was: UTP = ATP > ATP [gamma S] = ITP > 2-MeSATP > ADP = GTP >> AMP-CPP, adenosine, where ATP[gamma S] is adenosine 5'-O-(3-thiotriphosphate), ITP is inosine 5'-triphosphate, 2-MeSATP is 2-methylthio ATP and AMP-CPP is alpha, beta methylene ATP. This rank order accords with their activities at the cloned P2U receptor. Effects were not inhibited by suramin (up to 500 microM) or by pre-incubation for 12 h in 500 ng.ml-1 Pertussis toxin. Inhibition of IK(M) was frequently preceded by a transient outward current, probably a Ca(2+)-activated K+ current, responding to Ca2+ mobilization. No effect on the delayed rectifier K+ current was observed. These observations match those expected from stimulating other phospholipase-C-linked receptors in NG108-15 cells.

M-current suppression by agonist and phorbol ester in bullfrog sympathetic neurons

Activation of protein kinase C (PKC) by phorbol esters is known to suppress M-current. 4-beta-Phorbol 12,13-dibutyrate (PDBu) irreversibly suppressed M-current in a concentration-dependent manner (Ki 38 nM). Inhibitors of PKC, the pseudo-substrate peptide PKCI (19-31), staurosporine and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) antagonized PDBu-mediated suppression of M-current. Suppression of M-current by muscarine and luteinizing hormone-releasing hormone (LHRH) was unaffected by PKCI (19-31) and H7, but was antagonized by staurosporine. The balance of data suggests that suppression of M-current by agonists is probably not mediated by activation of PKC. Addition and subsequent removal of PDBu to M-current suppressed by muscarine prevented the action of PDBu, while closing M-channels by voltage or blocking by barium did not. This suggests that M-channel closure by muscarine protects those channels from the effects of PDBu. Partial suppression of M-current by low concentrations of muscarine antagonized the response to PDBu, with the magnitude of suppression equivalent to that seen with PDBu alone. It is suggested that two interconvertable populations of M-channels exist, one that is sensitive to both agonist and PDBu and another that can only be suppressed by agonist.

Bradykinin modulates potassium and calcium currents in neuroblastoma hybrid cells via different pertussis toxin-insensitive pathways

In NG108-15 cells, bradykinin (BK) activates a potassium current (IK,BK) and inhibits the voltage-dependent calcium current (ICa,V). BK also stimulates a phosphatidylinositol-specific phospholipase C (PI-PLC). The subsequent release of inositol 1,4,5-trisphosphate and increase in intracellular calcium contribute to IK,BK, through activation of a calcium-dependent potassium current. In membranes from these cells, stimulation of PI-PLC by BK is mediated by Gq and/or G11, two homologous, pertussis toxin-insensitive G proteins. Here, we have investigated the role of Gq/11 in the electrical responses to BK. GTP gamma S mimicked and occluded both actions of BK, and both effects were insensitive to pertussis toxin. Perfusion of an anti-Gq/11 alpha antibody into the pipette suppressed IK,BK, but not the inhibition of ICa,V by BK. Thus, BK couples to IK,BK via Gq/11, but coupling to ICa,V is most likely via a different, pertussis toxin-insensitive G protein.

Contribution of a H+ pump in determining the resting potential of neuroblastoma cells

The aim of this work was to examine the effects of changes in external K+ concentration (Ko) around its physiological value, of various K+ channels blockers, including internal Cs+, of vacuolar H(+)-ATPase inhibitors and of the protonophore CCCP on the resting potential and the voltage-dependent K+ current of differentiated neuroblastoma x glioma hybrid NG108-15 cells using the whole-cell patch-clamp technique. The results are as follows: (i) under standard conditions (Ko = 5 mM) the membrane potential was -60 +/- 1 mV. It was unchanged when Ko was decreased to 1 mM and was depolarized by 4 +/- 1 mV when Ko was increased to 10 mM. (ii) Internal Cs+ depolarized the membrane by 21 +/- 3 mV. (iii) The internal application of the vacuolar H(+)-ATPase inhibitors N-ethylmaleimide (NEM), NO3- and bafilomycin A1 (BFA) depolarized the membrane by 15 +/- 2, 18 +/- 2 and 16 +/- 2 mV, respectively. (iv) When NEM or BFA were added to the internal medium containing Cs+, the membrane was depolarized by 45 +/- 1 and 42 +/- 2 mV, respectively. (v) The external application of CCCP induced a transient depolarization followed by a prolonged hyperpolarization. This hyperpolarization was absent in BFA-treated cells. The voltage-dependent K+ current was increased at negative voltages and decreased at positive voltages by NEM, BFA and CCCP. Taken together, these results suggest that under physiological conditions, the resting potential of NG108-15 neuroblastoma cells is maintained at negative values by both voltage-dependent K+ channels and an electrogenic vacuolar type H(+)-ATPase.

Tyrosine kinase-dependent suppression of a potassium channel by the G protein-coupled m1 muscarinic acetylcholine receptor

Neurotransmitter receptors alter membrane excitability and synaptic efficacy by generating intracellular signals that ultimately change the properties of ion channels. Through expression studies in Xenopus oocytes and mammalian cells, we found that the G protein-coupled m1 muscarinic acetylcholine receptor potently suppresses a cloned delayed rectifier K+ channel through a pathway involving phospholipase C activation and direct tyrosine phosphorylation of the K+ channel. Furthermore, analysis of neuroblastoma cells revealed that a similar tyrosine kinase-dependent pathway links endogenous G protein-coupled receptors to suppression of the native RAK channel. These results suggest a novel mechanism by which neurotransmitters and hormones may regulate a specific type of K+ channel that is widely expressed in the mammalian brain and heart.

Calcium-mobilizing and electrophysiological effects of bradykinin on cortical astrocyte subtypes in culture

The possible consequences of activating bradykinin (Bk) receptors on identified astrocyte subtypes derived from rat cortex have been investigated in terms of: 1) mobilization of intracellular Ca2+; and 2) electrophysiological response. Bk induced a rapid, transient rise in intracellular Ca2+ in 63% of cortical type-1-like astrocytes and 44% of type-2 astrocytes tested. Experiments involving Ca(2+)-free conditions suggested that the release occurred largely from internal stores in both astrocyte subtypes. Bk receptor activation resulted in an inward current in approximately 10% of each astrocyte subtype tested using the whole-cell patch clamp technique. The use of perforated patch recording confirmed a similar population of cells responsive to Bk. The Bk-induced current was associated with a decrease in membrane conductance. The reversal potential was close to the K+ equilibrium potential, consistent with the closure of K+ channels. The current demonstrated similar EC50 values and Hill coefficients and also a marked degree of desensitization for both astrocyte subtypes. Recordings from type-1-like astrocytes indicated that the receptor mediating the electrophysiological response was of the B2 subtype. The data were consistent with the functional translation of Bk receptor occupation to physiological responses in distinct sub-populations of cortical type-1-like and type-2 astrocytes.

Protein kinase C as mediator of arachidonic acid-induced decrease of neuronal M current

The M current, IM, of NG108-15 neuroblastoma x glioma hybrid cells, a non-inactivating K+ current, is decreased by arachidonic acid (5-25 microM), often after an initial transitory increase. To test the possibility that the decrease is caused by activation of protein kinase C (PKC) we used the PKC 19-31 peptide, which is an effective inhibitor of PKC. With 1 microM peptide in the pipette solution the normally observed strong reduction of IM by 1 microM phorbol 12,13-dibutyrate (PDB) was almost totally prevented, indicating that PKC is completely inhibited; also the voltage dependence of the M conductance, gM(V), was shifted to more negative membrane potentials. In the presence of 1 microM peptide the effect of 25 microM arachidonic acid on IM was significantly reduced, suggesting that the effect, or at least a large part of it, is mediated by PKC.

On the mechanism of M-current inhibition by muscarinic m1 receptors in DNA-transfected rodent neuroblastoma x glioma cells

1. Acetylcholine (ACh) produces two membrane current changes when applied to NG108-15 mouse neuroblastoma x rat glioma hybrid cells transformed (by DNA transfection) to express m1 muscarinic receptors: it activates a Ca(2+)-dependent K+ conductance, producing an outward current, and it inhibits a voltage-dependent K+ conductance (the M conductance), thus diminishing the M-type voltage-dependent K+ current (IK(M)) and producing an inward current. The present experiments were undertaken to find out how far inhibition of IK(M) might be secondary to stimulation of phospholipase C, by recording membrane currents and intracellular Ca2+ changes with indo-1 using whole-cell patch-clamp methods. 2. Bath application of 100 microM ACh reversibly inhibited IK(M) by 47.3 +/- 3.2% (n = 23). Following pressure-application of 1 mM ACh, the mean latency to inhibition was 420 ms at 35 degrees C and 1.79 s at 23 degrees C. Latencies to inhibition by Ba2+ ions were 148 ms at 35 degrees C and 92 ms at 23 degrees C. 3. The involvement of a G-protein was tested by adding 0.5 mM GTP-gamma-S or 10 mM potassium fluoride to the pipette solution. These slowly reduced IK(M), with half-times of about 30 and 20 min respectively, and rendered the effect of superimposed ACh irreversible. Effects of ACh were not significantly changed after pretreatment for 24 h with 500 ng ml-1 pertussis toxin or on adding up to 10 mM GDP-beta-S to the pipette solution. 4. The role of phospholipase C and its products was tested using neomycin (to inhibit phospholipase C), inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4), heparin, and phorbol dibutyrate (PDBu) and staurosporin (to activate and inhibit protein kinase C respectively). Both neomycin (1 mM external) and InsP3 (100 microM intrapipette) inhibited the ACh-induced outward current and/or intracellular Ca2+ transient but did not block ACh-induced inhibition of IK(M). Intrapipette heparin (1 mM) blocked activation of IK(Ca) and reduced Ach-induced inhibitions of IK(M), but also reduced inhibition of ICa via endogeneous m4 receptors. PDBu (with or without intrapipette ATP) and staurosporin had no significant effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of K+ channels in the quercetin-induced inhibition of neuroblastoma cell growth

The effects of the flavonoid quercetin on cell proliferation and voltage-dependent K+ current were studied on mouse neuroblastoma x glioma hybrid cells. In the presence of 1% fetal calf serum, quercetin inhibited both cell proliferation and K+ current with effective doses inducing half-maximum effects of 10 microM and 70 microM respectively. Valinomycin (1 nM) antagonized 80% of the growth-inhibitory effects of 10 microM quercetin. The results suggest that at least a part of the anti-proliferative effect of quercetin is mediated by a K+ channel blockade. They further confirm a role for K+ channels in mitogenesis and cell proliferation.

The effect of arachidonic acid on the M current of NG108-15 neuroblastoma x glioma hybrid cells

The M current, IM, a voltage-dependent non-inactivating K+ current, was recorded in NG108-15 neuroblastoma x glioma hybrid cells, using the whole-cell mode of the patch-clamp technique. We studied the effect of arachidonic acid, other fatty acids and inhibitors of the arachidonic acid metabolism. In relatively high concentrations (25-50 microM) arachidonic acid first increased and later decreased the current, Ih, which holds the membrane potential at -30 mV and mainly flows through open M channels. It shifted the midpoint potential, Vo, of the relation between M conductance, gM, and membrane potential, V, to more negative values and decreased the maximum conductance gM and the time constant tau M. In smaller concentrations (5-10 microM) arachidonic acid merely decreased Ih and gM with little effect on Vo and tau M. Eicosatetraynoic acid and docosahexaenoic acid acted similarly to arachidonic acid whereas stearic acid had no effect. Of the three enzyme inhibitors studied, nordihydroguaiaretic acid acted similarly to arachidonic acid. i.e. caused a biphasic change in Ih. Indomethacin and quinacrine caused, respectively, a pure increase and a pure decrease of Ih and gM. Possible explanations are build-up of internally produced arachidonic acid, depletion of eicosanoid products or an inhibitory effect unrelated to arachidonic acid metabolism.

Dissection of bradykinin-evoked responses by buffering intracellular Ca2+ in neuroblastoma x glioma hybrid NG108-15 cells

Signal transduction pathways from bradykinin (BK) receptors were investigated in NG108-15 neuroblastoma x glioma hybrid cells by buffering the intracellular calcium (Ca2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a Ca2+ chelator. BK increased inositol-1,4,5-trisphosphate (Ins(1, 4,5)P3) formation at the same rate in the control and in BAPTA-acetoxy methyl ester (AM)-treated NG108-15 cells. However, a transient increase of intracellular Ca2+ concentrations in response to BK was significantly suppressed in Ca(2+)-buffered hybrid cells. Accordingly the BK-induced outward current was inhibited in BAPTA-AM-treated hybrid cells, while the subsequent inward current associated with a fall in membrane conductance was apparently increased. The initial phase of acetylcholine release from NG108-15 cells in response to BK was markedly inhibited in BAPTA-AM-treated coculture dishes when detected as miniature end-plate potentials of myotubes, though the late phase of acetylcholine secretion was observed. These results indicate that BK induces two distinct responses in NG108-15 cells: Ins(1,4,5)P3-dependent intracellular Ca2+ rise-sensitive and -insensitive components.

Intracellular Mg2+ inhibits the IP3-activated IK(Ca) in NG108-15 cells. [Why intracellular citrate can be useful for recording IK(Ca)]

Receptor-mediated formation of inositol 1,4,5-trisphosphate (IP3) can induce an outward Ca(2+)-activated K+ current [IK(Ca)] in some neural cells. We have investigated IK(Ca) activated by intracellular injections of IP3 in whole-cell patch-clamped neuroblastoma x glioma hybrid cells. The current could only be recorded reliably using citrate as the anion in the pipette, but not using acetate, aspartate, chloride, fluoride, gluconate or methylsulphate. This could be attributed to buffering of intracellular Mg2+ by citrate. Theoretical calculations suggested free [Mg2+] of 1.0 and 0.07 mM respectively in the acetate- and citrate-based recording solutions. Further, IP3-activated IK(Ca) could be recorded when the free Mg2+ level in the acetate, chloride or methylsulphate solutions was lowered to the range (0.05 mM) calculated for the citrate solution. Thus, raised [Mg2+] blocks IK(Ca). This appeared to be due to inhibition of the response to released Ca2+, since high [Mg2+] also blocked the response to intracellular injections of Ca2+ ions. Mean Mg2+ levels in intact neuroblastoma x glioma hybrid cells measured by Mag-Indo-1/AM fluorescence were estimated to be less than 0.14 mM. We therefore conclude that IP3-induced IK(Ca) is expressed under normal conditions, but may be subject to regulation by intracellular Mg2+.