Effect of external cation concentration and metabolic inhibitors on membrane potential of human glial cells

Mitochondrial uncoupler SHC517 reverses obesity in mice without affecting food intake

AIMS

Mitochondrial uncouplers decrease caloric efficiency and have potential therapeutic benefits for the treatment of obesity and related metabolic disorders. Herein we investigate the metabolic and physiologic effects of a recently identified small molecule mitochondrial uncoupler named SHC517 in a mouse model of diet-induced obesity.

METHODS

SHC517 was administered as an admixture in food. The effect of SHC517 on in vivo energy expenditure and respiratory quotient was determined by indirect calorimetry. A dose-finding obesity prevention study was performed by starting SHC517 treatment concomitant with high fat diet for a period of 12 days. An obesity reversal study was performed by feeding mice western diet for 4 weeks prior to SHC517 treatment for 7 weeks. Biochemical assays were used to determine changes in glucose, insulin, triglycerides, and cholesterol. SHC517 concentrations were determined by mass spectrometry.

RESULTS

SHC517 increased lipid oxidation without affecting body temperature. SHC517 prevented diet-induced obesity when administered at 0.05% and 0.1% w/w in high fat diet and reversed established obesity when tested at the 0.05% dose. In the obesity reversal model, SHC517 restored adiposity to levels similar to chow-fed control mice without affecting food intake or lean body mass. SHC517 improved glucose tolerance and fasting glucose levels when administered in both the obesity prevention and obesity reversal modes.

CONCLUSIONS

SHC517 is a mitochondrial uncoupler with potent anti-obesity and insulin sensitizing effects in mice. SHC517 reversed obesity without altering food intake or compromising lean mass, effects that are highly sought-after in anti-obesity therapeutics.

Therapeutic potential of mitochondrial uncouplers for the treatment of metabolic associated fatty liver disease and NASH

Background

Mitochondrial uncouplers shuttle protons across the inner mitochondrial membrane via a pathway that is independent of adenosine triphosphate (ATP) synthase, thereby uncoupling nutrient oxidation from ATP production and dissipating the proton gradient as heat. While initial toxicity concerns hindered their therapeutic development in the early 1930s, there has been increased interest in exploring the therapeutic potential of mitochondrial uncouplers for the treatment of metabolic diseases.

Scope of review

In this review, we cover recent advances in the mechanisms by which mitochondrial uncouplers regulate biological processes and disease, with a particular focus on metabolic associated fatty liver disease (MAFLD), nonalcoholic hepatosteatosis (NASH), insulin resistance, and type 2 diabetes (T2D). We also discuss the challenges that remain to be addressed before synthetic and natural mitochondrial uncouplers can successfully enter the clinic.

Major conclusions

Rodent and non-human primate studies suggest that a myriad of small molecule mitochondrial uncouplers can safely reverse MAFLD/NASH with a wide therapeutic index. Despite this, further characterization of the tissue- and cell-specific effects of mitochondrial uncouplers is needed. We propose targeting the dosing of mitochondrial uncouplers to specific tissues such as the liver and/or developing molecules with self-limiting properties to induce a subtle and sustained increase in mitochondrial inefficiency, thereby avoiding systemic toxicity concerns.

6-Amino[1,2,5]oxadiazolo[3,4-b]pyrazin-5-ol Derivatives as Efficacious Mitochondrial Uncouplers in STAM Mouse Model of Nonalcoholic Steatohepatitis

Small molecule mitochondrial uncouplers have recently garnered great interest for their potential in treating nonalcoholic steatohepatitis (NASH). In this study, we report the structure-activity relationship profiling of a 6-amino[1,2,5]oxadiazolo[3,4-b]pyrazin-5-ol core, which utilizes the hydroxy moiety as the proton transporter across the mitochondrial inner membrane. We demonstrate that a wide array of substituents is tolerated with this novel scaffold that increased cellular metabolic rates in vitro using changes in oxygen consumption rate as a readout. In particular, compound SHS4121705 (12i) displayed an EC50 of 4.3 μM in L6 myoblast cells and excellent oral bioavailability and liver exposure in mice. In the STAM mouse model of NASH, administration of 12i at 25 mg kg-1 day-1 lowered liver triglyceride levels and improved liver markers such as alanine aminotransferase, NAFLD activity score, and fibrosis. Importantly, no changes in body temperature or food intake were observed. As potential treatment of NASH, mitochondrial uncouplers show promise for future development.

[1,2,5]Oxadiazolo[3,4-b]pyrazine-5,6-diamine Derivatives as Mitochondrial Uncouplers for the Potential Treatment of Nonalcoholic Steatohepatitis

Small molecule mitochondrial uncouplers are emerging as a new class of molecules for the treatment of nonalcoholic steatohepatitis. We utilized BAM15, a potent protonophore that uncouples the mitochondria without depolarizing the plasma membrane, as a lead compound for structure-activity profiling. Using oxygen consumption rate as an assay for determining uncoupling activity, changes on the 5- and 6-position of the oxadiazolopyrazine core were introduced. Our studies suggest that unsymmetrical aniline derivatives bearing electron withdrawing groups are preferred compared to the symmetrical counterparts. In addition, alkyl substituents are not tolerated, and the N-H proton of the aniline ring is responsible for the protonophore activity. In particular, compound 10b had an EC₅₀ value of 190 nM in L6 myoblast cells. In an in vivo model of NASH, 10b decreased liver triglyceride levels and showed improvement in fibrosis, inflammation, and plasma ALT. Taken together, our studies indicate that mitochondrial uncouplers have potential for the treatment of NASH.

Mitochondrial Uncoupling: A Key Controller of Biological Processes in Physiology and Diseases

Mitochondrial uncoupling can be defined as a dissociation between mitochondrial membrane potential generation and its use for mitochondria-dependent ATP synthesis. Although this process was originally considered a mitochondrial dysfunction, the identification of UCP-1 as an endogenous physiological uncoupling protein suggests that the process could be involved in many other biological processes. In this review, we first compare the mitochondrial uncoupling agents available in term of mechanistic and non-specific effects. Proteins regulating mitochondrial uncoupling, as well as chemical compounds with uncoupling properties are discussed. Second, we summarize the most recent findings linking mitochondrial uncoupling and other cellular or biological processes, such as bulk and specific autophagy, reactive oxygen species production, protein secretion, cell death, physical exercise, metabolic adaptations in adipose tissue, and cell signaling. Finally, we show how mitochondrial uncoupling could be used to treat several human diseases, such as obesity, cardiovascular diseases, or neurological disorders.

Identification of a novel mitochondrial uncoupler that does not depolarize the plasma membrane

Dysregulation of oxidative phosphorylation is associated with increased mitochondrial reactive oxygen species production and some of the most prevalent human diseases including obesity, cancer, diabetes, neurodegeneration, and heart disease. Chemical 'mitochondrial uncouplers' are lipophilic weak acids that transport protons into the mitochondrial matrix via a pathway that is independent of ATP synthase, thereby uncoupling nutrient oxidation from ATP production. Mitochondrial uncouplers also lessen the proton motive force across the mitochondrial inner membrane and thereby increase the rate of mitochondrial respiration while decreasing production of reactive oxygen species. Thus, mitochondrial uncouplers are valuable chemical tools that enable the measurement of maximal mitochondrial respiration and they have been used therapeutically to decrease mitochondrial reactive oxygen species production. However, the most widely used protonophore uncouplers such as carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and 2,4-dinitrophenol have off-target activity at other membranes that lead to a range of undesired effects including plasma membrane depolarization, mitochondrial inhibition, and cytotoxicity. These unwanted properties interfere with the measurement of mitochondrial function and result in a narrow therapeutic index that limits their usefulness in the clinic. To identify new mitochondrial uncouplers that lack off-target activity at the plasma membrane we screened a small molecule chemical library. Herein we report the identification and validation of a novel mitochondrial protonophore uncoupler (2-fluorophenyl){6-[(2-fluorophenyl)amino](1,2,5-oxadiazolo[3,4-e]pyrazin-5-yl)}amine, named BAM15, that does not depolarize the plasma membrane. Compared to FCCP, an uncoupler of equal potency, BAM15 treatment of cultured cells stimulates a higher maximum rate of mitochondrial respiration and is less cytotoxic. Furthermore, BAM15 is bioactive in vivo and dose-dependently protects mice from acute renal ischemic-reperfusion injury. From a technical standpoint, BAM15 represents an effective new tool that allows the study of mitochondrial function in the absence of off-target effects that can confound data interpretation. From a therapeutic perspective, BAM15-mediated protection from ischemia-reperfusion injury and its reduced toxicity will hopefully reignite interest in pharmacological uncoupling for the treatment of the myriad of diseases that are associated with altered mitochondrial function.

First order phase transition and hysteresis in a cell's maintenance of the membrane potential--An essential role for the inward potassium rectifiers

Hysteretic behavior is found experimentally in the transmembrane potential at low extracellular potassium in mouse lumbrical muscle cells. Adding isoprenaline to the external medium eliminates the bistable, hysteretic region. The system can be modeled mathematically and understood analytically with and without isoprenaline. Inward rectifying potassium channels appear to be essential for the bistability. Relations are derived to express the dimensions of the bistable area in terms of system parameters. The selective advantage and evolutionary origin of inward rectifying channels and hysteretic behavior is discussed.

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

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

Regulation by sulfanylurea receptor type 1 of a non-selective cation channel involved in cytotoxic edema of reactive astrocytes

Hypoxia-ischemia and ATP depletion are associated with cytotoxic edema of glial cells, but mechanisms involved remain incompletely characterized. We examined morphologic and electrophysiological responses of freshly isolated native reactive astrocytes (NRAs) following exposure to NaN3, which depletes cellular ATP. NaN3 caused profound and sustained depolarization due to activation of a novel 35 pS Ca2+-activated, [ATP]i-sensitive non-selective cation (NCCa-ATP) channel found in >90% of excised membrane patches. This channel exhibited significantly different properties compared with previously reported NCCa-ATP channels, including different sensitivity to block by various adenine nucleotides (EC50=0.79 microM for [ATP]i, with no block by AMP or ADP), and activation by submicromolar [Ca]i. In addition, the channel was found to be regulated in a manner identical to that of SUR1-regulated KATP channels, including high affinity block by glybenclamide and tolbutamide, and opening by diazoxide. mRNA transcription and protein expression of SUR1 but not SUR2 were confirmed in reactive astrocytes both in situ and after isolation, whereas Kir6.x, which forms the pore-forming subunit of the KATP channel, was not expressed. Channel opening by [ATP]i depletion or exposure to diazoxide caused blebbing of the cell membrane, whereas [ATP]i depletion in the presence of glybenclamide did not. These findings are consistent with participation of this channel in cation flux involved in cell swelling. This novel channel may play an important role in the pathogenesis of brain swelling.

A bistable membrane potential at low extracellular potassium concentration

In order to understand the electrochemical behavior of a living cell at a low extracellular potassium concentration, a model is constructed. The model involves only the ATP driven sodium-potassium pump, and the sodium and potassium channels. Predictions of the model fit the N-shape of the current-voltage characteristic at low extracellular potassium. The model can, furthermore, quantitatively account for the experimentally observed bistability of the membrane potential at low extracellular potassium concentration. A crucial role in the control of the transmembrane potential appears to be played by how the permeability of the inward rectifying potassium channels depends on the transmembrane potential and on the extracellular potassium concentration.

A novel signalling pathway originating in mitochondria modulates rat skeletal muscle membrane excitability

Single skeletal muscle fibres from rat and cane toad were mechanically skinned and stimulated either electrically by initiating action potentials in the sealed transverse (t-) tubular system or by ion substitution causing depolarisation of the t-system to pre-determined levels. Depression of mitochondrial ATP-producing function with three diverse mitochondrial function antagonists (azide: 1-10 mM; oligomycin 1 microg ml-1 and carbonyl cyanide 4-trifluoromethoxyphenylhydrazone (FCCP) 1 microM), under conditions in which the cytosolic ATP was maintained high and constant, invariably reduced the excitability of rat fibres but had no obvious effect on the excitability of toad fibres, where mitochondria are less abundant and differently located. The reduction in excitability linked to mitochondria in rat fibres appears to be caused by depolarisation of the sealed t-system membrane. These observations suggest that mitochondria can regulate the functional state of mammalian muscle cells and have important implications for understanding how the balance between ATP utilisation and ATP production is regulated at the cellular level in general and in mammalian skeletal muscle fibres in particular.

An aldose reductase inhibitor reverses early diabetes-induced changes in peripheral nerve function, metabolism, and antioxidative defense

Aldose reductase inhibitors (ARIs) prevent peripheral nerve dysfunction and morphological abnormalities in diabetic animal models. However, some experimental intervention studies and clinical trials of ARIs on diabetic neuropathy appeared disappointing because of either 1) their inadequate design and, in particular, insufficient correction of the sorbitol pathway activity or 2) the inability to reverse established functional and metabolic deficits of diabetic neuropathy by AR inhibition in general. We evaluated whether diabetes-induced changes in nerve function, metabolism, and antioxidative defense are corrected by the dose of ARI (sorbinil, 65 mg/kg/d in the diet), resulting in complete inhibition of increased sorbitol pathway activity. The groups included control rats and streptozotocin-diabetic rats treated with/without ARI for 2 weeks after 4 weeks of untreated diabetes. ARI treatment corrected diabetes-induced nerve functional changes; that is, decrease in endoneurial nutritive blood flow, motor and sensory nerve conduction velocities, and metabolic abnormalities (i.e., mitochondrial and cytosolic NAD+/NADH redox imbalances and energy deficiency). ARI restored nerve concentrations of two major non-enzymatic antioxidants, reduced glutathione (GSH) and ascorbate, and completely arrested diabetes-induced lipid peroxidation. In conclusion, treatment with adequate doses of ARIs (that is, doses that completely inhibit increased sorbitol pathway activity) is an effective approach for reversal of, at least, early diabetic neuropathy.

Cell swelling and a nonselective cation channel regulated by internal Ca2+ and ATP in native reactive astrocytes from adult rat brain

Hypoxia-ischemia and ATP depletion are associated with glial swelling and blebbing, but mechanisms involved in these effects remain incompletely characterized. We examined morphological and electrophysiological responses of freshly isolated native reactive astrocytes (NRAs) after exposure to NaN(3), which depletes cellular ATP. Here we report that NaN(3) caused profound and sustained depolarization attributable to activation of a novel 35 pS Ca(2+)-activated, [ATP](i)-sensitive nonselective cation (NC(Ca-ATP)) channel, found in >90% of excised membrane patches. The channel was impermeable to Cl(-), was nearly equally permeable to monovalent cations, with permeabilities relative to K(+) being P(Cs)+/P(K)+(1.06) approximately P(Na)+/P(K)+(1.04) approximately P(Rb)+/P(K)+(1.02) approximately P(Li)+/P(K)+(0.96), and was essentially impermeable to Ca(2+) and Mg(2+) (P(Ca)2+/P(K)+ approximately P(Mg)2+/P(K)+ < 0.001), with intracellular Mg(2+) (100 microm to 1 mm) causing inward rectification. Pore radius, estimated by fitting relative permeabilities of organic cations to the Renkin equation, was 0.41 nm. This channel exhibited significantly different properties compared with previously reported NC(Ca-ATP) channels, including different sensitivity to block by various adenine nucleotides (EC(50) of 0.79 microm for [ATP](i), with no block by AMP or ADP), and activation by submicromolar [Ca](i). The apparent dissociation constant for Ca(2+) was voltage dependent (0.12, 0.31, and 1.5 microm at -40, -80, and -120 mV, respectively), with a Hill coefficient of 1.5. Channel opening by [ATP](i) depletion was accompanied by and appeared to precede blebbing of the cell membrane, suggesting participation of this channel in cation flux involved in cell swelling. We conclude that NRAs from adult rat brain express a 35 pS NC(Ca-ATP) channel that may play an important role in the pathogenesis of brain swelling.

Cell swelling and a nonselective cation channel regulated by internal Ca2+ and ATP in native reactive astrocytes from adult rat brain

Hypoxia-ischemia and ATP depletion are associated with glial swelling and blebbing, but mechanisms involved in these effects remain incompletely characterized. We examined morphological and electrophysiological responses of freshly isolated native reactive astrocytes (NRAs) after exposure to NaN(3), which depletes cellular ATP. Here we report that NaN(3) caused profound and sustained depolarization attributable to activation of a novel 35 pS Ca(2+)-activated, [ATP](i)-sensitive nonselective cation (NC(Ca-ATP)) channel, found in >90% of excised membrane patches. The channel was impermeable to Cl(-), was nearly equally permeable to monovalent cations, with permeabilities relative to K(+) being P(Cs)+/P(K)+(1.06) approximately P(Na)+/P(K)+(1.04) approximately P(Rb)+/P(K)+(1.02) approximately P(Li)+/P(K)+(0.96), and was essentially impermeable to Ca(2+) and Mg(2+) (P(Ca)2+/P(K)+ approximately P(Mg)2+/P(K)+ < 0.001), with intracellular Mg(2+) (100 microm to 1 mm) causing inward rectification. Pore radius, estimated by fitting relative permeabilities of organic cations to the Renkin equation, was 0.41 nm. This channel exhibited significantly different properties compared with previously reported NC(Ca-ATP) channels, including different sensitivity to block by various adenine nucleotides (EC(50) of 0.79 microm for [ATP](i), with no block by AMP or ADP), and activation by submicromolar [Ca](i). The apparent dissociation constant for Ca(2+) was voltage dependent (0.12, 0.31, and 1.5 microm at -40, -80, and -120 mV, respectively), with a Hill coefficient of 1.5. Channel opening by [ATP](i) depletion was accompanied by and appeared to precede blebbing of the cell membrane, suggesting participation of this channel in cation flux involved in cell swelling. We conclude that NRAs from adult rat brain express a 35 pS NC(Ca-ATP) channel that may play an important role in the pathogenesis of brain swelling.

A novel barium-sensitive calcium influx into rat astrocytes at low external potassium

Cultured rat cerebellar astrocytes, loaded with the Ca2+-sensitive fluorescent dyes Fura-2 or Fluo-3, responded with cytoplasmic Ca2+ transients, when the external K+ concentration was reduced from 5 mM to below 1 mM. Ca2+ transients were generated after changing to a saline containing 0.2 mM K+ in 82% of the cells (n =303) with a delay of up to 4 min. Cultured rat cortical neurones, which responded in high-K+ saline (50 mM) with Ca2+ transients, showed no Ca2+ responses in low K+ (n =22). In acute rat hippocampal brain slices, presumed glial cells responded with Ca2+ transients in low K+ similar to astrocytes in culture (88%, n =17). The Ca2+ transients were observed both in somatic and dendritic regions of cultured astrocytes, as examined with confocal laser scanning microscopy. Patch-clamped astrocytes hyperpolarized in 0.2 mM K+ from an average resting potential of -65 +/- 4 mV to -98 +/- 20 mV (n =15). The Ca2+ transients in low K+ were suppressed in Ca2+-free saline, buffered with 0.5 mM EGTA, but not after depletion of intracellular Ca2+ stores by thapsigargin, cyclopiazonic acid or by Ruthenium Red, indicating that they were due to Ca2+ influx into the cells, and not caused by intracellular Ca2+ release. The addition of different divalent cations revealed that Ba2+, but not Ni2+, Cd2+, Sr2+ or Mg2+, reversibly blocked the Ca2+ transients in low K+. There was a significant reduction of the Ca2+ responses at micromolar Ba2+ concentrations (Ki = 3.8 microM). The application of different K+ channel blockers, tetraethylammonium, dequalinium, tolbutamide, clotrimazole, or quinidine had no effect on the Ca2+ responses. Removal of external Na+, or intracellular acidification by the addition of 40 mM propionate to the saline, had also no influence on the generation of the Ca2+ transients. The results suggest that reducing the external K+ concentration elicits a Ca2+ influx into rat astrocytes which is highly sensitive to Ba2+. It is discussed that this Ca2+ influx might occur through K+ inward rectifier channels, which become Ca2+-permeable when the extracellular K+ concentration decreases to 1 mM or below.

Effects of barium on stimulus-induced rises of [K+]o in human epileptic non-sclerotic and sclerotic hippocampal area CA1

In the hippocampus of patients with therapy-refractory temporal lobe epilepsy, glial cells of area CA1 might be less able to take up potassium ions via barium-sensitive inwardly rectifying and voltage-independent potassium channels. Using ion-selective microelectrodes we investigated the effects of barium on rises in [K+]o induced by repetitive alvear stimulation in slices from surgically removed hippocampi with and without Ammon's horn sclerosis (AHS and non-AHS). In non-AHS tissue, barium augmented rises in [K+]o by 147% and prolonged the half time of recovery by 90%. The barium effect was reversible, concentration dependent, and persisted in the presence of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid [GABA(A)] receptor antagonists. In AHS tissue, barium caused a decrease in the baseline level of [K+]o. In contrast to non-AHS slices, in AHS slices with intact synaptic transmission, barium had no effect on the stimulus-induced rises of [K+]o, and the half time of recovery from the rise was less prolonged (by 57%). Under conditions of blocked synaptic transmission, barium augmented stimulus-induced rises in [K+]o, but only by 40%. In both tissues, barium significantly reduced negative slow-field potentials following repetitive stimulation but did not alter the mean population spike amplitude. The findings suggest a significant contribution of glial barium-sensitive K+-channels to K+-buffering in non-AHS tissue and an impairment of glial barium-sensitive K+-uptake in AHS tissue.

Adenosine release mediates cyanide-induced suppression of CA1 neuronal activity

The rapid suppression of CNS function produced by cyanide (CN) was studied by field, intracellular, and whole-cell recording in hippocampal slices (at 33-34 degrees C). Population spikes and field EPSPs were depressed by 4-5 min bath applications of 50-100 microM CN (IC50 was 18 miroM for spikes and 72 microM for EPSPs). The actions of CN were reversibly suppressed by the adenosine antagonists 8-sulfophenyltheophylline (8-SPT; 10 microM) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.2 microM), potentiated by the adenosine transport inhibitor dipyridamole (0.5 microM), but unaffected by the KATP channel blocker glyburide (10 microM). Therefore the CN-induced reductions of synaptic efficacy and postsynaptic excitability-demonstrated by synaptic input:output plots-are mediated mainly by adenosine. In whole-cell or intracellular recordings, CN depressed EPSCs and elicited an increase in input conductance and an outward current, the reversal potential of which was approximately -90 mV (indicating that K+ was the major carrier). These effects also were attenuated by 8-SPT. In the presence of 1 mM Ba, CN had no significant postsynaptic action; Cs (2 mM) also prevented CN-induced outward currents but only partly blocked the increase in conductance. Another 8-SPT-sensitive action of CN was to depress hyperpolarization-activated slow inward relaxations (Q current). At room temperature (22-24 degrees C), although it did not change holding current and slow inward relaxations, CN raised the input conductance; this effect also was prevented by 8-SPT (10 microM), but not by glyburide (10 microM). Adenosine release thus appears to be the major link between acute CN poisoning and early depression of CNS synaptic function.

Effects of barium on stimulus-induced changes in [K+]o and field potentials in dentate gyrus and area CA1 of human epileptic hippocampus

The effects of barium on stimulus-induced rises in [K+]o were studied in the dentate gyrus (DG) and area CA1 of human hippocampal slices. Rises in [K+]o elicited by repetitive stimulation of the hilus, stratum moleculare, alveus, or stratum radiatum were dependent on stimulus intensity and frequency. Barium augmented rises in [K+]o in the DG by about 120% but failed to do so in area CA1. In both DG and area CA1 barium had no effects on population spikes whereas stimulus-induced slow field potentials were reduced. Since barium interferes with K+ uptake and redistribution by blocking leak conductances and inwardly-rectifying currents in astrocytes, our findings suggest that glial cells in the sclerotic hippocampal area CA1 may contribute less to K+ regulation.

Plasma membrane depolarization and disturbed Na+ homeostasis induced by the protonophore carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazon in isolated nerve terminals

The effect of the protonophore carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazon (FCCP) was studied on the intracellular [Na+], pH, and plasma membrane potential in isolated nerve terminals. FCCP induced a rise of [Na+]i at, and even below, the concentrations (0.025-1 microM) in which it is usually used in intact cells to eliminate Ca2+ uptake by mitochondria. The FCCP-induced increase of [Na+]i correlates with a fall in both the ATP level and the ATP/ADP ratio. In addition, a sudden rise of the intracellular proton concentration ([H+]i) from 83 +/- 0.4 to 124 +/- 0.7 nM was observed on the addition of FCCP (1 microM). Parallel with the rise in [H+]i, an abrupt depolarization was detected, followed by a slower decrease in the plasma membrane potential. Both the extent of the pHi change and the fast depolarization of the plasma membrane were proportional to the proton electrochemical gradient across the plasma membrane; when this gradient was increased, greater depolarization was detected. The slower decrease of the membrane potential after the fast initial depolarization was abolished when the medium contained no Na+. It is concluded that FCCP (1) gives rise to a depolarization by setting the plasma membrane potential close to the proton equilibrium potential and (2) enhances the intracellular [Na+] as a consequence of an insufficient ATP level and ATP/ADP ratio to fuel the Na+,K+/ATPase. Because both disturbed Na+ homeostasis and plasma membrane depolarization could profoundly interfere with Ca2+ homeostasis in the presence of protonophores, consideration given to these alterations may help to clarify the cellular Ca2+ sequestration processes.

Effect of metabolic inhibitors on membrane potential and ion conductance of rat astrocytes

1. The aim of this study was to elucidate the effect of metabolic inhibition on the membrane potential and ion conductance of rat astrocytes. The metabolic inhibitors investigated were dinitrophenol (DNP), carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP), cyanide, and oligomycin. 2. Primary cultures of astroglial cells from newborn rat cerebral cortex were cultivated for 13-20 days on chamber slides. The effect of metabolic inhibitors on the cellular ATP concentration was estimated from the decrease in peak chemiluminescence from the luciferin/luciferase reaction. The membrane potential and ion conductances were measured from whole-cell recordings with the patch-clamp technique. 3. After 2.0 min of incubation ATP decreased from the control level to 43% with cyanide (2 mM), 58% with DNP (1 mM), 47% with FCCP (1 microM), and 69% with oligomycin (10 microM). 4. Under normal conditions V was -74.4 +/- 1.0 mV. DNP and FCCP both caused a rapid and reversible depolarization equivalent to a shift in the I/V curve of 8.2 +/- 1.3 and 19.7 +/- 3.8 mV, respectively. DNP decreased the slope conductance (g) by 22.1% but FCCP had no significant effect on g. In contrast, neither oligomycin nor cyanide had any significant effect on the I/V curve. 5. Tetraethylammonium (TEA; 10 mM) depolarized the cells by 7.1 +/- 2.0 mV but had no significant effect on g. In the presence of TEA, DNP caused a depolarization of 52.8 +/- 3.5 mV and increased g by 45.5 +/- 9.6%. The action of FCCP was not affected by the presence of TEA. 6. Perfusion of the astrocytes with a Cl- free solution inhibited the action of DNP and FCCP. Thus the depolarization was only 4.2 +/- 1.5 mV in DNP and 3.7 +/- 0.3 mV in FCCP, which were significantly smaller effects than in the presence of a high intracellular [Cl-]. 7. Block of tentative KATP channels with tolbutamide (1 mM) or Cl- channels with Zn2+ (1 mM) did not inhibit the depolarization caused by DNP or FCCP. 8. In conclusion, DNP and FCCP have specific effects on the plasmalemma in rat astrocytes which may be due to opening of Cl- channels. This effect was not seen with cyanide or oligomycin and should be considered as a possible complication when DNP and FCCP are used for metabolic inhibition.

Adenosine release mediates cyanide-induced suppression of CA1 neuronal activity

The rapid suppression of CNS function produced by cyanide (CN) was studied by field, intracellular, and whole-cell recording in hippocampal slices (at 33-34 degrees C). Population spikes and field EPSPs were depressed by 4-5 min bath applications of 50-100 microM CN (IC50 was 18 miroM for spikes and 72 microM for EPSPs). The actions of CN were reversibly suppressed by the adenosine antagonists 8-sulfophenyltheophylline (8-SPT; 10 microM) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.2 microM), potentiated by the adenosine transport inhibitor dipyridamole (0.5 microM), but unaffected by the KATP channel blocker glyburide (10 microM). Therefore the CN-induced reductions of synaptic efficacy and postsynaptic excitability-demonstrated by synaptic input:output plots-are mediated mainly by adenosine. In whole-cell or intracellular recordings, CN depressed EPSCs and elicited an increase in input conductance and an outward current, the reversal potential of which was approximately -90 mV (indicating that K+ was the major carrier). These effects also were attenuated by 8-SPT. In the presence of 1 mM Ba, CN had no significant postsynaptic action; Cs (2 mM) also prevented CN-induced outward currents but only partly blocked the increase in conductance. Another 8-SPT-sensitive action of CN was to depress hyperpolarization-activated slow inward relaxations (Q current). At room temperature (22-24 degrees C), although it did not change holding current and slow inward relaxations, CN raised the input conductance; this effect also was prevented by 8-SPT (10 microM), but not by glyburide (10 microM). Adenosine release thus appears to be the major link between acute CN poisoning and early depression of CNS synaptic function.

MK-801 reduces uptake and stimulates efflux of excitatory amino acids via membrane depolarization

MK-801 and related compounds reduce excitotoxic neuronal injury by blocking N-methyl-D-aspartate (NMDA) receptorgated ion channels. These agents also cause neuronal vacuolization and block glutamate-induced astrocyte swelling, effects that may be unrelated to actions at the NMDA receptor. In the present study, high concentrations of MK-801 (100-1,000 microM) caused uncompetitive inhibition of glutamate uptake in astrocyte and neuronal cultures and stimulated D-aspartate efflux from astrocytes. MK-801 (500 microM) reduced the maximal velocity for glutamate uptake in astrocytes from 31 to 17 nmol.mg protein-1.min-1, whereas competitive NMDA receptor antagonists did not affect glutamate uptake. MK-801 also inhibited uptake of gamma-aminobutyric acid (GABA). Because both GABA uptake and glutamate uptake are electrogenic, one mechanism by which MK-801 could inhibit uptake is by membrane depolarization. Whole cell patch-clamp recording confirmed that MK-801 in the range of 100-1,000 microM caused dose-dependent and reversible depolarization. These concentrations are far higher than necessary to block NMDA receptors, and the findings suggest that actions at sites other than NMDA receptors could contribute to the effects of high doses of MK-801 in some experimental and clinical settings.

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.

Effect of external K+, Ca2+, and Ba2+ on membrane potential and ionic conductance in rat astrocytes

1. The purpose of this study was (a) to identify if astrocytes show a similar non-Nernstian depolarization in low K+ or low Ca2+ solutions as previously found in human glial and glioma cells, and (b) to analyze the influence of the K+ conductance on the membrane potential of astrocytes. 2. The membrane potential (Em) and the ionic conductance were studied with whole-cell patch-clamp technique in neonatal rat astrocytes (5-9 days in culture) and in human glioma cells (U-251MG). 3. In 3.0 mM K+ Em was -75 +/- 1.0 mV (mean +/- SEM, n = 39) in rat astrocytes and -79 +/- 0.7 mV (n = 5) in U-251MG cells. In both cell types Em changed linearly to the logarithm of [K+]0 between 3.0 and 160 mM K+ free medium caused astrocytes to hyperpolarize to -93 +/- 2.7 mV (n = 21) and U-251MG cells to depolarize to -27 +/- 2.1 mV (n = 3). 4. The I-E curve did not show inward rectification in astrocytes at this developmental stage. The slope conductance (g) exhibited only a small decrease (-19%) in K+ free solution and no significant change in 160 mM K+. 5. Ba2+ (1.0 mM) depolarized astrocytes to -45 +/- 2.9 mV (n = 11), decreasing the slope conductance (g) by 42.4 +/- 8.3% (n = 11). Ca2+ free solution depolarized astrocytes to -53 +/- 3.4 mV (n = 12) and resulted in a positive shift of the I-E curve, increasing g by 15.3 +/- 8.2% (n = 8). 6. Calculations indicated that a block of K+ channels explains the depolarizing effect of Ba2+. The effects of K+ free or Ca2+ free solutions on Em can be explained by a transformation of K+ channels to non-specific leakage channels. That astrocytes show a different reaction to low K+ than glioma cells can be related to the lack of inwardly rectifying K+ channels in astrocytes at this developmental stage.

Mechanism of high K+ and Tl+ uptake in cultured human glioma cells

1. The aim of this study was to elucidate if the K+ uptake was higher in cultured human glioma cells than in cells from other malignant tumors and to analyze the importance of membrane potential and K+ channels for the uptake. 2. K+ transport properties were studied with the isotopes 42K and the K-analogue 201Tl. 3. Comparison with cultured cells from other malignant tumors showed that the specific steady-state accumulation of Tl+ was significantly higher in glioma cells (U-251MG and Tp-378MG). 4. In Ringer's solution at 37 degrees C the rates of K+ and Tl+ uptake were both inhibited by about 55% in ouabain and 60% in furosemide, bumetanide, or Na(+)- or Cl(-)-free medium. This indicated that the routes for K+ and Tl+ uptake were similar and due to Na,K-ATPase-dependent transport and to Na-K-Cl cotransport. 5. About 10% of the uptake was neither ouabain nor bumetanide sensitive. Ba2+, which is known to block inward-rectifying K+ channels and to depolarize glial cells, and other K+ channel blockers (Cs+ and bupivacaine), had no effect on Tl+ uptake. 6. Metabolic inhibition with dinitrophenol reduced the uptake rate to 17%. 7. The washout of Tl+ was unaffected by bumetanide and K+ channel blockers, but dinitrophenol caused a transient increase of 75%, an effect which persisted in the presence of K+ channel blockers. 8. It was concluded that the high specific K+ and Tl+ accumulation in cultured human glioma cells was due not to the presence of inwardly rectifying K+ channels or other identified K+ channels, but to Na,K-ATPase dependent transport and Na-K-Cl cotransport.