Glucose and oxygen deprivation induces a Ca(2+)-mediated decrease in (Na(+)+K+)-ATPase activity in rat brain slices

Cerebral metabolic alterations in rats with diabetic ketoacidosis: effects of treatment with insulin and intravenous fluids and effects of bumetanide

OBJECTIVE

Cerebral edema is a life-threatening complication of diabetic ketoacidosis (DKA) in children. Recent data suggest that cerebral hypoperfusion and activation of cerebral ion transporters may be involved, but data describing cerebral metabolic alterations during DKA are lacking.

RESEARCH DESIGN AND METHODS

We evaluated 50 juvenile rats with DKA and 21 normal control rats using proton and phosphorus magnetic resonance spectroscopy (MRS). MRS measured cerebral intracellular pH and ratios of metabolites including ATP/inorganic phosphate (Pi), phosphocreatine (PCr)/Pi, N-acetyl aspartate (NAA)/creatine (Cr), and lactate/Cr before and during DKA treatment. We determined the effects of treatment with insulin and intravenous saline with or without bumetanide, an inhibitor of Na-K-2Cl cotransport, using ANCOVA with a 2 x 2 factorial study design.

RESULTS

Cerebral intracellular pH was decreased during DKA compared with control (mean +/- SE difference -0.13 +/- 0.03; P < 0.001), and lactate/Cr was elevated (0.09 +/- 0.02; P < 0.001). DKA rats had lower ATP/Pi and NAA/Cr (-0.32 +/- 0.10, P = 0.003, and -0.14 +/- 0.04, P < 0.001, respectively) compared with controls, but PCr/Pi was not significantly decreased. During 2-h treatment with insulin/saline, ATP/Pi, PCr/Pi, and NAA/Cr declined significantly despite an increase in intracellular pH. Bumetanide treatment increased ATP/Pi and PCr/Pi and ameliorated the declines in these values with insulin/saline treatment.

CONCLUSIONS

These data demonstrate that cerebral metabolism is significantly compromised during DKA and that further deterioration occurs during early DKA treatment--consistent with possible effects of cerebral hypoperfusion and reperfusion injury. Treatment with bumetanide may help diminish the adverse effects of initial treatment with insulin/saline.

Oxygen-glucose deprivation decreases glutathione levels and glutamate uptake in rat hippocampal slices

Ischemia is a transitory or permanent reduction of blood flow that may provoke an excessive release of glutamate. In that condition, increased reactive oxygen species generation and/or decreased cerebral antioxidant capacity may induce cell death. Antioxidant enzymes and thiols play an important role in the cellular defenses against oxidative stress. The purpose of this study was to evaluate cell viability, glutamate uptake and antioxidant status in rat hippocampal slices exposed to oxygen-glucose deprivation (OGD), an in vitro model of ischemia. After 15 min or 1 h of OGD, hippocampal slices showed a significant reduction of cell viability. Reperfusion during 1 or 2 h did not increase cell death. In this condition, the activities of antioxidant enzymes catalase, glutathione reductase, and peroxidase did not change. However, slices exposed to 15 min OGD and reperfused for 1 or 2 h showed higher superoxide dismutase activity. A significant reduction of glutathione levels was observed after 1 or 2 h of reperfusion in slices previously exposed to 1 h of OGD, although the protein-thiol content was unchanged. Slices exposed to 1 h of OGD and reperfused for 2 h showed reduced sodium-dependent l-[(3)H]glutamate uptake. The reduction of glutamate uptake was partially reversed by dl-dithiothreitol (DTT), a thiol-reducing agent, which may reduce thiol groups in glutamate transporters. Therefore, higher glutamate levels in the synaptic cleft could promote transporter reversal and impair glutamate uptake. Increased extracellular glutamate levels associated with decreased glutathione levels might exacerbate cell damage induced by oxygen and glucose deprivation.

Multiple functions of Na,K-ATPase in epithelial cells

The Na,K-adenosine triphosphatase (ATPase), or sodium pump, has been well studied for its role in the regulation of ion homeostasis in mammalian cells. Recent studies suggest that Na,K-ATPase might have multiple functions such as a role in the regulation of tight junction structure and function, induction of polarity, regulation of actin dynamics, control of cell movement, and cell signaling. These functions appear to be modulated by Na,K-ATPase enzyme activity as well as protein-protein interactions of the alpha and beta subunits. In this review we attempt to differentiate functions associated with enzyme activity and subunit interactions. In addition, the consequence of impaired Na,K-ATPase function or reduced subunit expression levels in kidney diseases such as cancer, tubulointerstitial fibrosis, and ischemic nephropathy are discussed.

Role of Na-K-ATPase in the assembly of tight junctions

Na-K-ATPase, also known as the sodium pump, is a crucial enzyme that regulates intracellular sodium homeostasis in mammalian cells. In epithelial cells Na-K-ATPase function is also involved in the formation of tight junctions through RhoA GTPase and stress fibers. In this review, a new two-step model for the assembly of tight junctions is proposed: step 1, an E-cadherin-dependent formation of partial tight junction strands and of the circumferential actin ring; and step 2, active actin polymerization-dependent tethering of tight junction strands to form functional tight junctions, an event requiring normal function of Na-K-ATPase in epithelial cells. A new role for stress fibers in the assembly of tight junctions is proposed. Also, implications of Na-K-ATPase function on tight junction assembly in diseases such as cancer, ischemia, hypomagnesemia, and polycystic kidney disease are discussed.

Neuroprotective effect of GMP in hippocampal slices submitted to an in vitro model of ischemia

1. Guanosine-5'-monophosphate (GMP) was evaluated as a neuroprotective agent against the damage observed in rat hippocampal slices submitted to an in vitro model of ischemia with or without the presence of the ionotropic glutamate receptor agonist, Kainic acid (KA). 2. Cellular injury was evaluated by MTT reduction, lactate dehydrogenase(LDH) release assay, and measurement of intracellular ATP levels. 3. In slices submitted to ischemic conditions, 1 mM GMP partially prevented the decrease in cell viability induced by glucose and oxygen deprivation and the addition of KA. 4. KA or N-methyl-D-aspartate (NMDA) receptor antagonists, gamma-D-glutamylamino-methylsulfonate (GAMS) or (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801, 20 microM) also prevented toxicity in hippocampal slices under ischemic conditions, respectively. 5. The association of GMP with GAMS or MK-801 did not induce additional protection than that observed with GMP or that classical glutamate receptor antagonists alone. 6. GMP, probably by interacting with ionotropic glutamate receptors, attenuated the damage caused by glucose and oxygen deprivation in hippocampal slices. This neuroprotective action of GMP in this model of excitotoxicity is of outstanding interest in the search for effective therapies against ischemic injury.

Repolarization of the plasma membrane shapes NMDA-induced cytosolic [Ca2+ ] transients

After inactivation of NMDA receptors, restoration of basal cytosolic [Ca2+] ([Ca2+]c) is delayed. This may be caused by Ca2+ influx via reverse Na/Ca exchange or voltage-gated Ca2+ channels, and/or by Ca2+ efflux from internal stores. Monitoring of [Na+]c, [Ca2+]c, and plasma membrane potential in cultured cerebellar granule cells showed that repolarization of the plasma membrane and inactivation of voltage-gated Ca channels plays the most critical role in restoration of low [Ca2+]c following NMDA receptor inactivation. During NMDA receptor activation, however, an Na-dependent mechanism enhanced NMDA-induced elevation in [Ca2+]c. This mechanism did not involve Na,K-ATPase activation by Na+, because it operated even when Na,K-ATPase was inhibited.

A late phase of exocytosis from synaptosomes induced by elevated [Ca2+]i is not blocked by Clostridial neurotoxins

Treatment of rat cerebrocortical synaptosomes with botulinum toxin types E and C1 or tetanus toxin removed the majority of intact SNAP-25, syntaxin 1A/1B, and synaptobrevin and diminished Ca(2+)-dependent K+ depolarization-induced noradrenaline secretion. With botulinum toxin type E, <10% of intact SNAP-25 remained and K(+)-evoked release of glutamate and GABA was inhibited. The large component of noradrenaline release evoked within 120 s by inclusion of the Ca2+ ionophore A23187 with the K+ stimulus was also attenuated by these toxins; additionally, botulinium neurotoxin type E blocked the first 60 s of ionophore-induced GABA and glutamate exocytosis. However, exposure to A23187 for longer periods induced a phase of secretion nonsusceptible to any of these toxins (>120 s for noradrenaline; >60 s for glutamate or GABA). Most of this late phase of release represented exocytosis because of its Ca2+ dependency, ATP requirement, and sensitivity to a phosphatidylinositol 4-kinase inhibitor. Based on these collective findings, we suggest that the ionophore-induced elevation of [Ca2+]i culminates in the disassembly of complexes containing nonproteolyzed SNAP receptors protected from the toxins that can then contribute to neuroexocytosis.

The neuroprotective agent MS-153 stimulates glutamate uptake

We investigated the effect of (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153), a novel neuroprotective agent, on L-[3H]glutamate uptake through GLT-1, a Na(+)/K(+)-dependent glial glutamate transporter, expressed in COS-7 cells. MS-153 (1-100 microM) accelerated the L-[3H]glutamate uptake through GLT-1 in a concentration-dependent and time-dependent manner. Eadie-Hofstee analysis revealed that MS-153 significantly decreased the K(m) of the glutamate uptake by COS-7 cells expressing GLT-1. In contrast, [3H]gamma-aminobutyric acid (GABA) uptake through a glial GABA transporter was not affected. In addition, MS-153 increased Na(+) currents through GLT-1 expressed in Xenopus oocytes. We also investigated the effect of MS-153 on amino acid efflux from rat hippocampal slices. The increase in glutamate efflux induced by 50 mM KCl was significantly attenuated by the treatment with MS-153 at 10 microM, while MS-153 had no significant effect on the K(+)-evoked efflux of GABA. Furthermore, the increase in glutamate efflux by ischemia (hypoxia/aglycemia) was partially, but significantly inhibited by MS-153. These results suggest that the cerebroprotective effect of MS-153 in this ischemic model in vivo is due to the specific reduction of the glutamate concentration in the extracellular space, which can probably be attributed to the acceleration of glutamate uptake by the indirect modulation of the glutamate transporter activity.

Magnesium attenuates a striatal dopamine increase induced by anoxia in the neonatal rat brain: an in vivo microdialysis study

We evaluated the effects of magnesium on extracellular dopamine (DA) and its metabolites in the striatum of 5-d-old rats submitted to 16 min of anoxia using microdialysis and HPLC. Rat pups were divided into three groups and received either 1) intrastriatal perfusion (IS) of MgSO4, 2) intraperitoneal injection (IP) of MgSO4, and 3) NaCl and Ringer's solution, respectively in place of MgSO4. After stabilization, Mg2+, saline, and Ringer's solution were administered; then, 114 animals were exposed to 100% nitrogen for 16 min. Anoxia induced a DA surge, an acutely marked increase of DA, in both the control and the IP group. In contrast, the DA surge was significantly suppressed in the IS group (p < 0.01, analysis of variance). During anoxia, the plasma Mg2+ in the IP group, but not in the IS group, maintained a significantly higher level compared with the basal level. On the other hand, Mg2+ in the perfusates in the IS group, but not in the IP group, maintained a significantly high level during anoxia. Alterations induced by anoxia in other metabolites, 3,4-dihydroxyphenylacetic acid, homovanillic acid, norepinephrine, and 5-hydroxyindole-3-acetic acid, did not significantly differ among the three groups. We propose that elevated levels of Mg2+ in the striatum had inhibitory effects on the DA surge during anoxia.

Relationships between ATP depletion, membrane potential, and the release of neurotransmitters in rat nerve terminals. An in vitro study under conditions that mimic anoxia, hypoglycemia, and ischemia

BACKGROUND AND PURPOSE

It is known that the extracellular accumulation of glutamate during anoxia/ischemia is responsible for initiating neuronal injury. However, little information is available on the release of monoamines and whether the mechanism of its release resembles that of glutamate, which may itself influence the release of monoamines by activating presynaptic receptors. This study was designed to characterize the release of both amino acids and monoamines under chemical conditions that mimic anoxia, hypoglycemia, and ischemia.

METHODS

The contents of synaptosomes in adenine nucleotides (ATP, ADP, and AMP), amino acids (aspartate, glutamate, taurine, and gamma-aminobutyric acid), and monoamines (dopamine, noradrenaline, and 5-hydroxytryptamine) were measured by high-performance liquid chromatography, after the synaptosomes were subjected to anoxia (KCN + oligomycin), hypoglycemia (2 mmol/L 2-deoxyglucose in glucose-free medium), and ischemia (anoxia plus hypoglycemia).

RESULTS

The anoxia- and ischemia-induced release or noradrenaline, dopamine, 5-hydroxytryptamine, and glutamate correlated well with ATP depletion. The correlation observed between glutamate levels and the release of dopamine and 5-hydroxytryptamine in ischemic conditions suggests a functional linkage between the two transmitter systems. However, the antagonists of presynaptic glutamate receptors failed to alter the amount of monoamines released. The inhibition of Na+,K+-ATPase by ouabain had an effect similar to that produced by ischemia.

CONCLUSIONS

The decrease in Na+ and K+ gradients resulting from the energy depletion of the synaptosomes under ischemic conditions or resulting from the inhibition of Na+, K+-ATPase by ouabain promotes the reversal of the neurotransmitter transporters. The decrease in uptake of neurotransmitters may also contribute to the rise in the extracellular concentration of different transmitters observed during brain ischemia.

Effects of serotonin1A agonists on anoxia-induced impairment of protein synthesis in rat brain slices

Earlier in vivo experiments suggest that serotonin1A (5-HT1A) agonists are new tools for the treatment of experimental cerebral ischemia. The present study examined this idea in an in vitro system. Incubation of rat brain slices under anoxic conditions for 30 min decreased protein synthesis that was assayed in a normoxic medium by measuring the incorporation of [14C]lysine into trichloroacetic acid-insoluble tissue extracts. The 5-HT1A agonists 8-hydroxy-2-(di-n-propylamino) tetralin (10-100 microM) and buspirone (50 microM) attenuated the anoxia-induced decrease in protein synthesis in the slices. Although the degree of the effect is small, it may be relevant to the neuroprotective effect in the in vivo experiments.

Ganglioside GM1 prevents N-methyl-D-aspartate neurotoxicity in rabbit hippocampus in vivo. Effects on calcium homeostasis

Microdialysis was used to apply 1 mM N-methyl-D-aspartate (NMDA) for 20 min to the hippocampus of rabbits, control and pre-treated with GM1 ganglioside (im injections of 30 mg/kg for 3 d, twice a day). Concentrations of ionized Ca2+ and 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha)-immunoreactive material in the dialyzates and 45Ca and [14C]sucrose efflux from the prelabeled hippocampus were determined. After 24 h, the morphology of the hippocampal neurons was examined. In control animals, the application of NMDA resulted in 25% decrease in Ca2+ concentration and in 1000% increase in 6-keto PGF 1 alpha concentration in the dialyzates. A 30% decrease in 45Ca efflux was accompanied by 20% increase in [14C]sucrose efflux, reflecting a corresponding reduction of the extracellular space volume. A degeneration of CA1 pyramidal neurons in the vicinity of a microdialysis probe was observed. In GM1-treated rabbits the NMDA-induced decrease in Ca2+ concentrations in the dialyzates was not reduced significantly, whereas a 70% stimulation of 45Ca efflux was noted, with a concomitant 40% reduction of 6-keto-PG F1 alpha release. NMDA-evoked increase in [14C]sucrose efflux did not differ from the control. In these animals CA1 neurons were well preserved. These results indicate that the pretreatment with GM1 results in activation of calcium extrusion from the NMDA-stimulated rabbit hippocampal neurons that alleviates destabilization of calcium homeostasis and reduces NMDA-evoked neuronal injury.

The effect of acute hypoglycemia on the cerebral NMDA receptor in newborn piglets

The effects of acute insulin-induced hypoglycemia on the cerebral NMDA receptor in the newborn were examined by determining [3H]MK-801 binding as an index of NMDA receptor function in 6 control and 7 hypoglycemic piglets. In hypoglycemic animals, the glucose clamp technique with constant insulin infusion was used to maintain a blood glucose concentration of 1.2 mmol/l for 120 min before obtaining cerebral cortex for further analysis; controls received a saline infusion. Concentrations of glucose, lactate, ATP, and PCr were measured in cortex, and Na+,K(+)-ATPase activity was determined in a brain cell membrane preparation. [3H]MK-801 binding was evaluated by: (1) saturation binding assays over the range of 0.5-50 nM [3H]MK-801 in the presence of 100 microM glutamate and glycine; and (2) binding assays at 10 nM [3H]MK-801 in the presence of glutamate and/or glycine at 0, 10, or 100 microM. Blood and brain glucose concentrations were significantly lower in hypoglycemic animals than controls. There was no change in brain ATP with hypoglycemia, but PCr was decreased 80% compared to control (P < 0.05). Na+,K(+)-ATPase activity was 13% lower in hypoglycemic animals (P < 0.05). Based on saturation binding data, hypoglycemia had no effect on the number of functional receptors (Bmax), but the apparent affinity was significantly increased, as indicated by a decrease in the Kd (dissociation constant) from the control value of 8.1 +/- 1.6 nM to 5.5 +/- 2.1 nM (P < 0.05). Augmentation of [3H]MK-801 binding by glutamate and glycine alone or in combination was also significantly greater in the hypoglycemic animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of neuronal ion exchange, not potentiated excitation, causes excitotoxicity after inhibition of oxidative phosphorylation

Neuronal cell death during impaired energy metabolism is often attributed to increased activity at glutamate receptors, but this increase has not been directly demonstrated. We recorded responses to glutamate and N-methyl-D-aspartate in hippocampal slice CA1 neurons and glia while inhibiting mitochondrial complex II with 3-nitropropionic acid. As the period of inhibition increased, neuronal depolarization following bath application of glutamate (5 mM) or N-methyl-D-aspartate (50 microM) increased dramatically. However, depolarization upon iontophoresis of glutamate and N-methyl-D-aspartate decreased with time. A transient hyperpolarization, reflecting electrogenic sodium pump activity, was present immediately after responses to iontophoretic glutamate agonists. In the presence of the inhibitor, this hyperpolarization decreased and eventually disappeared. Even the repolarization seen upon washing of the iontophoretic or bath application of glutamate or N-methyl-D-aspartate was incomplete. Glial depolarization upon bath application of glutamate increased during inhibition, while glial depolarization upon application of N-methyl-D-aspartate decreased. Application of the N-methyl-D-aspartate antagonists aminophosphonovaleric acid (100 microM) or MK-801 (20 microM) resulted in a delay of further depolarization when applied early during the terminal decay of membrane potential following metabolic inhibition. We conclude that during impaired oxidative phosphorylation the failure of repolarizing mechanisms, not potentiated neuronal depolarization by excitants, is the primary cause of the terminal depolarization. Large glial depolarization increases the demand for neuronal ion exchange which cannot be met in situations of reduced energy metabolism. Our results provide further evidence that acute and chronic blockade of energy metabolism have different effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissociation of cyclic GMP level from relaxation of the distal, but not the proximal colon of rats

The role of cyclic GMP (cGMP) in nonadrenergic, noncholinergic (NANC) relaxation of the longitudinal muscle of rat proximal and distal colon was examined. Electrical field stimulation (EFS) of preparations of longitudinal muscle from the proximal region significantly increased the cGMP content. Nitro-L-arginine inhibited this increase, and L-arginine reversed the inhibitory effect of nitro-L-arginine. Exogenously added nitric oxide (NO) and atrial natriuretic peptide (ANP) also increased the cGMP content of preparations of the proximal colon and induced muscle relaxation. From these and our previous findings suggesting an essential role of NO in NANC inhibition in the proximal colon, we conclude that the mechanism of NANC inhibition in the proximal region of rat colon involves NO and a cGMP generating system. In contrast, although exogenously added NO and ANP increased the cGMP content in the distal colon to the same extent as in the proximal colon, they did not induce any muscle relaxation. Vasoactive intestinal peptide (VIP), the most likely candidate as a NANC neurotransmitter in rat distal colon, did not increase the cGMP content in this region. Furthermore, no participation of NO in the NANC inhibitory response was observed in the distal region, but EFS increased the cGMP content significantly. Thus we conclude that relaxation of longitudinal smooth muscle in the distal portion of rat colon is not associated with a change in the cGMP content.