Salutary and deleterious effects of acidity on an indirect measure of metabolic rate and ATP concentrations in CNS cultures

Roles of Na+/H+ exchange in regulation of p38 mitogen-activated protein kinase activity and cell death after chemical anoxia in NIH3T3 fibroblasts

Activation of Na(+)/H(+) exchange (NHE) plays a major role in cell death following ischemia/hypoxia in many cell types, yet counteracts apoptotic cell death after other stimuli. To address the role of NHE activity in regulation of cell death/survival, we examined the causal relationship between NHE, p38 mitogen-activated protein kinase (MAPK), ERK1/2, p53, and Akt activity, and cell death, after chemical anoxia in NIH3T3 fibroblasts. The NHE1 inhibitor 5'-(N-ethyl-N-isopropyl) amiloride (EIPA) (5 muM), as well as removal of extracellular Na(+) [replaced by N-methyl-D: -glucamine (NMDG(+))], prevented recovery of intracellular pH (pH(i)) during chemical anoxia (10 mM NaN(3) +/- 10 mM glucose), indicating that activation of NHE was the dominating mechanism of pH(i) regulation under these conditions. NHE activation by chemical anoxia was unaffected by inhibitors of p38 MAPK (SB203580) and extracellular signal-regulated kinase (ERK) (PD98059). In contrast, chemical anoxia activated p38 MAPK in an NHE-dependent manner, while ERK1/2 activity was unaffected. Anoxia-induced cell death was caspase-3-independent, mildly attenuated by EIPA, potently exacerbated by SB203580, and unaffected by PD98059. Ser(15) phosphorylation of p53 was increased by anoxia in an NHE- and p38 MAPK-independent manner, while Akt activity was unaffected. It is suggested that after chemical anoxia in NIH3T3 fibroblasts, NHE activity is required for activation of p38 MAPK, which in turn protects the cells against anoxia-induced death. In spite of this, NHE inhibition slightly attenuates anoxia-induced cell death, likely due to the involvement of NHE in other anoxia-induced death pathways.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

Failure and function of intracellular pH regulation in acute hypoxic-ischemic injury of astrocytes

Astrocytes can die rapidly following ischemic and traumatic injury to the CNS. Brain acid-base status has featured prominently in theories of acute astrocyte injury. Failure of astrocyte pH regulation can lead to cell loss under conditions of severe acidosis. By contrast, the function of astrocyte pH regulatory mechanisms appears to be necessary for acute cell death following the simulation of transient ischemia and reperfusion. Severe lactic acidosis, and the failure of astrocytes to regulate intracellular pH (pH(i)) have been emphasized in brain ischemia under hyperglycemic conditions. Direct measurements of astrocyte pH(i) after cardiac arrest demonstrated a mean pH(i) of 5.3 in hyperglycemic rats. In addition, both in vivo and in vitro studies of astrocytes have shown similar pH levels to be cytotoxic. Whereas astrocytes exposed to hypoxia alone may require 12-24 h to die, acidosis has been found to exacerbate and speed hypoxic loss of these cells. Recently, astrocyte cultures were exposed to hypoxic, acidic media in which the large ionic perturbations characteristic of brain ischemia were simulated. Upon return to normal saline ("reperfusion"), the majority of cells died. This injury was dependent on external Ca2+ and was prevented by inhibition of reversed Na(+)-Ca2+ exchange, blockade of Na(+)-H+ exchange, or by low pH of the reperfusion saline. These data suggested that cytotoxic elevation of [Ca2+]i occurred during reperfusion due to a sequence of activated Na(+)-H+ exchange, cytosolic Na+ loading, and resultant reversal of Na(+)-Ca2+ exchange. The significance of this reperfusion model to ischemic astrocyte injury in vivo is discussed.

Treatment of metabolic acidosis

Metabolic acidosis is characterized by a decrease of the blood pH associated with a decrease in the bicarbonate concentration. This may be secondary to a decrease in the strong ion difference or to an increase in the weak acids concentration, mainly the inorganic phosphorus. From a conceptual point of view, two types of nontoxic metabolic acidosis must be differentiated: the mineral metabolic acidosis that reveals the presence of an excess of nonmetabolizable anions, and the organic metabolic acidosis that reveals an excess of metabolizable anions. Significance and consequences of these two types of acidosis are radically different. Mineral acidosis is not caused by a failure in the energy metabolic pathways, and its treatment is mainly symptomatic by correcting the blood pH (alkali therapy) or accelerating the elimination of excessive mineral anions (renal replacement therapy). On the other hand, organic acidosis gives evidence that a severe underlying metabolic distress is in process. No reliable argument exists to prove that this acidosis is harmful under these conditions in humans. Experimental data even show that hypoxic cells are able to survive only if the medium is kept acidic. The management of an acute organic metabolic acidosis is therefore primarily based on the cause of the acidosis, and no scientific argument exists to justify the correction of the acid-base imbalance in this context.

Differential and irreversible CNS ontogenic reduction in maximal MK-801 binding site number in the NMDA receptor after acute hypoxic hypoxia

CNS exposure to hypoxia impairs excitatory and inhibitory neurotransmission. Our aim was to determine variations induced by normobaric acute hypoxic hypoxia (8% O(2) for 60 min) on the NMDA receptor complex, as well as their potential reversibility after normoxic recovery. To this end, [3H]MK-801 binding assays to a synaptic membrane fraction isolated from chick optic lobes were performed. Previous studies throughout development had disclosed a characteristic age-dependent pattern. Results at embryonic day (ED) 12 and 18 indicated two distinct MK-801 binding sites. Hypoxic treatment failed to alter either the high affinity site dissociation constant (K(d)) or its maximal binding capacity (B(max)), whereas the low affinity site B(max) was significantly decreased (50% and 30% at ED12 and 18, respectively), without alteration in its K(d) values. Hypoxic embryos restored for 48 h at ED12 to normoxic conditions displayed unchanged MK-801 binding reduction, unlike those treated likewise at ED18 whose values fully recovered control levels. To conclude, hypoxic treatment reduces low affinity MK-801 B(max) in the NMDA receptor which proves irreversible up to ED12. Such early neuronal vulnerability may be due to post-transcriptional changes, to endocytosis followed by receptor degradation, or alternatively to cell death.

Continuous measurement of glucose utilization in heart myoblasts

Understanding quantitative aspects of cell energy metabolism and how it is influenced by environment is central to biology, medicine, and biotechnology. Most methods used for measuring metabolic fluxes associated with energy metabolism require considerable personnel effort or high maintenance instrumentation. The microphysiometer is a commercially available instrument that measures acid extrusion rates, which are commonly used for drug screening. With the addition of oxygen sensors, the instrument can also be used to measure cell oxygen consumption rates and thereby calculate glycolytic fluxes. In the work described here, oxygen consumption and acid extrusion rates were used to measure glucose utilization by the H9c2 rat heart myoblast cell line and these results are compared with fluxes measured with a radiometric assay. Both assays were used to investigate changes in H9c2 energy metabolism due to cell stimulation with carbachol and insulin. The results demonstrate the utility of the microphysiometer method for measuring both transient and sustained changes in partitioning of glucose utilization between glycolysis and oxidation in live cells.

The binding of zinc and copper ions to nerve growth factor is differentially affected by pH: implications for cerebral acidosis

It has recently been shown that transition metal cations Zn2+ and Cu2+ bind to histidine residues of nerve growth factor (NGF) and other neurotrophins (a family of proteins important for neuronal survival) leading to their inactivation. Experimental data and theoretical considerations indicate that transition metal cations may destabilize the ionic form of histidine residues within proteins, thereby decreasing their pK(a) values. Because the release of transition metal cations and acidification of the local environment represent important events associated with brain injury, the ability of Zn2+ and Cu2+ to bind to neurotrophins in acidic conditions may alter neuronal death following stroke or as a result of traumatic injury. To test the hypothesis that metal ion binding to neurotrophins is influenced by pH, the effects of Zn2+ and Cu2+ on NGF conformation, receptor binding and NGF tyrosine kinase (trkA) receptor signal transduction were examined under conditions mimicking cerebral acidosis (pH range 5.5-7.4). The inhibitory effect of Zn2+ on biological activities of NGF is lost under acidic conditions. Conversely, the binding of Cu2+ to NGF is relatively independent of pH changes within the studied range. These data demonstrate that Cu2+ has greater binding affinity to NGF than Zn2+ at reduced pH, consistent with the higher affinity of Cu2+ for histidine residues. These findings suggest that cerebral acidosis associated with stroke or traumatic brain injury could neutralize the Zn2+-mediated inactivation of NGF, whereas corresponding pH changes would have little or no influence on the inhibitory effects of Cu2+. The importance of His84 of NGF for transition metal cation binding is demonstrated, confirming the involvement of this residue in metal ion coordination.

Glucocorticoids exacerbate insult-induced declines in metabolism in selectively vulnerable hippocampal cell fields

Glucocorticoids (GCs), the adrenal steroids released during stress, can compromise the ability of hippocampal neurons to survive necrotic neurological insults. This GC-induced endangerment has energetic facets, in that it can be attenuated with energy supplementation. In the present report, we studied the effects of GCs on the metabolic response of specific hippocampal cell fields to necrotic insults. We used silicon microphysiometry, which allows indirect measurement of metabolism in real time in tissue explants. Aglycemia caused a significant decline in metabolism in dentate gyrus explants, but not in CA1 or CA3 explants. When coupled with our prior report of cyanide disrupting metabolism only in CA1 explants, and the glutamatergic excitotoxin kainic acid disrupting metabolism only in CA3 explants, this demonstrates that microphysiometry can detect the selective regional vulnerability that characterizes the hippocampal response to these necrotic insults. We then examined the effects of GCs on the response to these insults, monitoring explants taken from rats that were adrenalectomized, intact, or treated with corticosterone (the GC of rats) that produced circulating levels equivalent to those of major stressors. Increased exposure to GCs worsened the decline in metabolism in dentate gyrus explants induced by hypoglycemia, and in CA1 explants induced by cyanide (after eliminating the effects of glial release of lactate for the support of neuronal metabolism). Thus, GCs worsen the metabolic consequences of necrotic insults in hippocampal explants.

Effects of acidosis on the post-hypoxic recovery of synaptic transmission in gerbil hippocampal slices

We investigated the effects of acidosis on the hypoxic neuronal damage using gerbil hippocampal slices. Acidosis has delayed the onset of harmful hypoxic depolarization, resulting in a decrease in the total hypoxic period and the hypoxic depolarization. This effect has been considered to be protective. However, the synaptic recovery after reoxygenation was attenuated when acidosis (pH: 6.2-6.9) was sustained. Conversely, the synaptic recovery was potentiated when the acidosis was restored to the physiological milieu during the reoxygenation period. These results suggest that acidosis plays a protective effect against the hypoxic neuronal damage only when rapid appreciable pH recovery is achieved during reoxygenation.

Glucocorticoids exacerbate the deleterious effects of gp120 in hippocampal and cortical explants

Glucocorticoids (GCs), the adrenal steroids secreted during stress, can compromise the ability of hippocampal neurons to survive numerous necrotic insults. We have previously observed that GCs worsen the deleterious effects of gp120, the glycoprotein of the acquired immune deficiency syndrome virus, which can indirectly damage neurons and which is thought to play a role in the neuropathological features of human immunodeficiency virus infection. Specifically, GCs augment gp120-induced calcium mobilization, ATP depletion, decline in mitochondrial potential, and neurotoxicity in fetal monolayer cultures from a number of brain regions. In the present report, we demonstrate a similar gp120/GC synergy in adult hippocampal and cortical explants. We generated explants from rats that were either adrenalectomized, adrenally intact, or intact and treated with corticosterone to produce levels seen in response to major stressors. Metabolic rates in explants were then indirectly assessed with silicon microphysiometry, and cytosolic calcium concentrations were assessed with fura-2 fluorescent microscopy. We observed that basal levels of GCs tonically augment the disruptive effects of gp120 on metabolism in the CA1 cell field of the hippocampus and in the cortex. Moreover, raising GC concentrations into the stress range exacerbated the ability of gp120 to mobilize cytosolic calcium in a number of hippocampal cell fields. Finally, we observed that the synthetic GC prednisone had similarly exacerbating effects on gp120. Thus, GCs can worsen the deleterious effects of gp120 in a system that is more physiologically relevant than the fetal monolayer culture and in a region-specific manner.

Effect of extracellular pH on GABA-activated current in rat recombinant receptors and thin hypothalamic slices

We studied the effects of extracellular pH (pHo) on gamma-aminobutyric acid (GABA)-mediated Cl- current in rat hypothalamic neurons and recombinant type-A GABA (GABA(A)) receptors stably expressed in human embryonic kidney cells (HEK 293), using whole cell and outside-out patch-clamp recordings. In alpha3beta2gamma2s receptors, acidic pH decreased, whereas alkaline pH increased the response to GABA in a reversible and concentration-dependent manner. Acidification shifted the GABA concentration-response curve to the right, significantly increasing the EC50 for GABA without appreciably changing the slope or maximal current induced by GABA. We obtained similar effects of pH in alpha1beta2gamma2 receptors and in GABA-activated currents recorded from thin hypothalamic brain slices. In outside-out patches recorded from alpha3beta2gamma2 recombinant receptors, membrane patches were exposed to 5 microM GABA at control (7.3), acidic (6.4), or alkaline (8.4) pH. GABA activated main and subconductance states of 24 and 16 pS, respectively, in alpha3beta2gamma2 receptors. Alkaline pH(o) increased channel opening frequency and decreased the duration of the long closed state, resulting in an increase in open probability (from 0.0801 +/- 0.015 in pH 7.3 to 0.138 +/- 0.02 in pH 8.4). Exposure of the channels to acidic pH(o) had the opposite effects on open probability (decreased to 0.006 +/- 0.0001). Taken together, our results indicate that the function of GABA(A) receptors is modulated by extracellular pH. The proton effect is similar in recombinant and native receptors and is dependent on GABA concentration. In addition, the effect appears to be independent of the alpha-subunit isoform, and is due to the ability of H+ to alter the frequency of channel opening. Our findings indicate that GABAergic signaling in the CNS may be significantly altered during conditions that increase or decrease pH.

Neonatal tolerance to hypoxia: a comparative-physiological approach

Newborn mammals exhibit a number of physiological reactions which differ from normal adult physiology and are often regarded as signs of immaturity. However, when looked upon from a comparative point of view, it becomes obvious that some of these 'physiological peculiarities' bear striking similarity to adaptation mechanisms known from hypoxia-tolerant animals and may thus contribute to the well-established, yet poorly understood, phenomenon of neonatal hypoxia tolerance. As the mammalian fetus lives at oxygen partial pressures corresponding to 8000 m altitude, the first line of perinatal hypoxia defense consists of long-term adaptations to limited intrauterine oxygen supply: (1) improved O2 transport by fetal acclimatization to high altitude, (2) reduced metabolic rate by hibernation-like deviation from metabolic size allometry, (3) diminished cerebral vulnerability by functional analogies to diving turtle brain, and (4) enhanced metabolic flexibility by optional repartitioning of energy supply from growth to maintenance metabolism. In the case of birth asphyxia, these background mechanisms are complemented by short-term responses to acute oxygen lack: (1) reduction of body temperature as in natural torpor, (2) reduction of heart rate and redistribution of circulation as in diving mammals, (3) reduction of respiration rate typical of 'hypoxic hypometabolism', and (4) reduction of blood pH according to the concept of 'acidotic torpidity'. Although anaerobic metabolism is improved in neonatal mammals by increased glycogen stores, reduced metabolic demands, and sustained wash-out of acid metabolites, neonatal hypoxia tolerance seems to be primarily based on the ability to maintain tissue aerobiosis as long as possible. This is even reflected by isoenzyme patterns which do not consistently favour anaerobic glycolysis and, thus, are reminiscent of the 'lactate paradox' found in high altitude adaptation. Altogether, from a biological point of view, the perinatal period appears as a source of adaptive mechanisms that can be refound, in varying combinations, in many survival strategies. From a clinical point of view, the interplay of long- and short-term mechanisms offers a novel approach to estimation of the newborn's ability to withstand temporary oxygen lack. However, most of these mechanisms are not unambiguous and, above all, not unlimited in their protective effect so that they do not release obstetricians or neonatologists from their obligation to counteract fetal or neonatal hypoxia without delay.

Microphysiometric analysis of human alpha1a-adrenoceptor expressed in Chinese hamster ovary cells

1. The human recombinant alpha1a-adrenoceptor (AR) has been stably expressed in Chinese hamster ovary cells. Four stable clones, aH4, aH5, aH6 and aH7, expressing 30, 370, 940 and 2900 fmol AR mg(-1) protein, respectively, have been employed to characterize this AR subtype using radioligand binding and microphysiometry to measure extracellular acidification rates. 2. Noradrenaline (NA) gave concentration-dependent responses in microphysiometry with increasing extracellular acidification rates. The potency of NA increased as the receptor density increased; pEC50 values of NA for the clones aH4, aH5, aH6 and aH7 were 6.9, 7.5, 7.8 and 8.1, respectively. This increase of potency according to receptor density indicates the presence of spare receptor for NA. Methoxamine, phenylephrine, oxymetazoline and clonidine also gave concentration-dependent responses with various intrinsic activities. 3. Antagonists shifted concentration-response curves for NA rightward in a concentration-dependent manner. Schild analysis revealed that the affinity profile of this AR subtype to antagonists in the clone aH7 had a typical pattern for the alpha1a-AR; high affinity for prazosin and WB 4101, and low affinity for BMY7378 (pA2=9.5, 9.8 and 7.3, respectively). This profile is similar in the case of the clone aH4. These affinities were in good agreement with those obtained in binding experiments. 4. These results have demonstrated that (1) classical receptor theory can be applied in microphysiometry, and (2) microphysiometry is a useful tool to investigate the pharmacological characterization of alpha1a-AR.

Increases in [Ca2+]i and changes in intracellular pH during chemical anoxia in mouse neocortical neurons in primary culture

The effect of chemical anoxia (azide) in the presence of glucose on the free intracellular Ca2+ concentration ([Ca2+]i) and intracellular pH (pHi) in mouse neocortical neurons was investigated using Fura-2 and BCECF. Anoxia induced a reversible increase in [Ca2+]i which was significantly inhibited in nominally Ca2+-free medium. A change in pHo (8.2 or 6.6), or addition of NMDA and non-NMDA receptor antagonists (D-AP5 and CNQX) in combination, significantly reduced the increase in [Ca2+]i, pointing to a protective effect of extracellular alkalosis or acidosis, and involvement of excitatory amino acids. An initial anoxia-induced acidification was observed under all experimental conditions. In the control situation, this acidification was followed by a recovery/alkalinization of pHi in about 50% of the cells, a few cells showed no recovery, and some showed further acidification. EIPA, an inhibitor of Na+/H+ exchangers, prevented alkalinization, pointing towards anoxia-induced activation of a Na+/H+ exchanger. In a nominally Ca2+-free medium, the initial acidification was followed by a significant alkalinization. At pHo 8.2, the alkalinization was significantly increased, while at pHo 6.2, the initial acidification was followed by further acidification in about 50% of the cells, and by no further change in the remaining cells.

Hyperglycemia and focal brain ischemia

The influence of hyperglycemic ischemia on tissue damage and cerebral blood flow was studied in rats subjected to short-lasting transient middle cerebral artery (MCA) occlusion. Rats were made hyperglycemic by intravenous infusion of glucose to a blood glucose level of about 20 mmol/L, and MCA occlusion was performed with the intraluminar filament technique for 15, 30, or 60 minutes, followed by 7 days of recovery. Normoglycemic animals received saline infusion. Perfusion-fixed brains were examined microscopically, and the volumes of selective neuronal necrosis and infarctions were calculated. Cerebral blood flow was measured autoradiographically at the end of 30 minutes of MCA occlusion and after 1 hour of recirculation in normoglycemic and hyperglycemic animals. In two additional groups with 30 minutes of MCA occlusion, CO2 was added to the inhaled gases to create a similar tissue acidosis as in hyperglycemic animals. In one group CBF was measured, and the second group was examined for tissue damage after 7 days. Fifteen and 30 minutes of MCA occlusion in combination with hyperglycemia produced larger infarcts and smaller amounts of selective neuronal necrosis than in rats with normal blood glucose levels, a significant difference in the total volume of ischemic damage being found after 30 minutes of MCA occlusion. After 60 minutes of occlusion, when the volume of infarction was larger, only minor differences between normoglycemic and hyperglycemic animals were found. Hypercapnic animals showed volumes of both selective neuronal necrosis and infarction that were almost identical with those observed in normoglycemic, normocapnic animals. When local CBF was measured in the ischemic core after 30 minutes of occlusion, neither the hyperglycemic nor the hypercapnic animals were found to be significantly different from the normoglycemic group. Brief focal cerebral ischemia combined with hyperglycemia leads to larger and more severe tissue damage. Our results do not support the hypothesis that the aggravated injury is caused by any disturbances in CBF.

Benzodiazepine modulation of recombinant alpha1beta3gamma2 GABA(A) receptor function efficacy determination using the Cytosensor microphysiometer

Gamma-aminobutyric acid (GABA) dose dependently increased extracellular acidification rate in Ltk cells stably expressing human recombinant alpha1beta3gamma2 GABA(A) receptors but had no effect in non-transfected controls. Cells seeded at 1 x 10(5) cells/cup, with 4-5 days induction, had basal acidification rates of 105+/-2 microVs(-1) at 37 degrees C (mean+/-standard error of mean, n=37). GABA responses had a characteristic time-course with an initial alkalinisation followed by a peak of acidification, which was optimized by increasing agonist exposure from 15 s to 25-30 s. The maximum concentration of GABA tested (100 microM) produced a 40+/-2% increase over basal acidification rate (n=3), with an EC50 of 15.5 microM and a Hill slope of 1.5. Responses were specifically antagonized by bicuculline and could be modulated by benzodiazepine ligands with varying efficacies. Full benzodiazepine agonists flunitrazepam (1 microM) and zolpidem (10 microM) significantly potentiated the response to 10 microM GABA by 124+/-15% (n=7) and 117+/-23% (n=3), respectively. The partial agonist bretazenil (100 nM) produced a 45+/-13% (n=3) potentiation whilst the inverse agonist DMCM (10 microM) (methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) inhibited the response to 20 microM GABA by 53+/-5%. The microphysiometer offers an alternative functional measure for GABA(A) receptors with the sensitivity to measure subtle modulatory effects of benzodiazepine site ligands and to determine their relative efficacy.