Perturbation of cellular energy state in complete ischemia: relationship to dissipative ion fluxes

Role of NAD+-Modulated Mitochondrial Free Radical Generation in Mechanisms of Acute Brain Injury

It is commonly accepted that mitochondria represent a major source of free radicals following acute brain injury or during the progression of neurodegenerative diseases. The levels of reactive oxygen species (ROS) in cells are determined by two opposing mechanisms—the one that produces free radicals and the cellular antioxidant system that eliminates ROS. Thus, the balance between the rate of ROS production and the efficiency of the cellular detoxification process determines the levels of harmful reactive oxygen species. Consequently, increase in free radical levels can be a result of higher rates of ROS production or due to the inhibition of the enzymes that participate in the antioxidant mechanisms. The enzymes’ activity can be modulated by post-translational modifications that are commonly altered under pathologic conditions. In this review we will discuss the mechanisms of mitochondrial free radical production following ischemic insult, mechanisms that protect mitochondria against free radical damage, and the impact of post-ischemic nicotinamide adenine mononucleotide (NAD⁺) catabolism on mitochondrial protein acetylation that affects ROS generation and mitochondrial dynamics. We propose a mechanism of mitochondrial free radical generation due to a compromised mitochondrial antioxidant system caused by intra-mitochondrial NAD⁺ depletion. Finally, the interplay between different mechanisms of mitochondrial ROS generation and potential therapeutic approaches are reviewed.

Contribution of a mitochondrial pathway to excitotoxic neuronal necrosis

It is traditionally thought that excitotoxic necrosis is a passive mechanism that does not require the activation of a cell death program. In this study, we examined the contribution of the cytochrome c-dependent mitochondrial death pathway to excitotoxic neuronal necrosis, induced by exposing cultured cortical neurons to 1 mM glutamate for 6 hr and blocked by the NMDA antagonist, dizocilpine. Glutamate treatment induced early cytochrome c release, followed by activation of caspase-9 and caspase-3. Preincubation with the caspase-9 inhibitor z-LEHD-fmk, the caspase-3 inhibitor z-DEVD-fmk, or the specific pan-caspase inhibitor Q-VD-oph decreased the percentage of propidium iodide-positive neurons (52.5% +/- 3.1%, 39.4% +/- 3.5%, 44.6% +/- 3%, respectively, vs. 65% +/- 3% in glutamate + vehicle). EM studies showed mitochondrial release of cytochrome c in neurons in the early stages of necrosis and cleaved caspase-3 immunoreactivity in morphologically necrotic neurons. These results suggest that an active mechanism contributes to the demise of a subpopulation of excitotoxic necrotic neurons.

Spinal cord mitochondria display lower calcium retention capacity compared with brain mitochondria without inherent differences in sensitivity to cyclophilin D inhibition

The mitochondrial permeability transition (mPT) is a potential pathogenic mechanism in neurodegeneration. Varying sensitivity to calcium-induced mPT has been demonstrated for regions within the CNS possibly correlating with vulnerability following insults. The spinal cord is selectively vulnerable in e.g. amyotrophic lateral sclerosis and increased mPT sensitivity of mitochondria derived from the spinal cord has previously been demonstrated. In this study, we introduce whole-body hypothermia prior to removal of CNS tissue to minimize the effects of differential tissue extraction prior to isolation of spinal cord and cortical brain mitochondria. Spinal cord mitochondria were able to retain considerably less calcium when administered as continuous infusion, which was not related to a general increased sensitivity of the mPT to calcium, its desensitization to calcium by the cyclophilin D inhibitor cyclosporin-A, or to differences in respiratory parameters. Spinal cord mitochondria maintained a higher concentration of extramitochondrial calcium during infusion than brain mitochondria possibly related to an increased set-point concentration for calcium uptake. A hampered transport and retention capacity of calcium may translate into an increased susceptibility of the spinal cord to neurodegenerative processes involving calcium-mediated damage.

Green tea protects cytoskeleton from oxidative injury in cardiomyocytes

Cardiac ischemia/reperfusion injury results in oxidative stress and poor physiological recovery. Episodes of hypoxia/reoxygenation (H/R) cause some subtle functional and structural alterations in sarcolemma, mithocondria, sarcoplasmic reticulum, nucleus, as well as cytoskeleton. In this report, by using cultured rat cardiomyocytes and laser confocal microscopy we have verified the possibility to counteract cytoskeleton alterations induced by H/R with the supplementation of an antioxidant agent, a green tea extract (GTE), and compared its effects to those of alpha-tocopherol. Moreover the effects of GTE on cell viability and cytosolic antioxidant activity have been evaluated. H/R induced myocardial damage occurs as histological alterations such as degeneration and disorganization of the cytoskeleton and loss of structural integrity of the nucleus. GTE supplementation increases cytosolic antioxidant activity and shows protective effects on cardiomyocyte cytoarchitecture and viability.

Oxidative metabolism, apoptosis and perinatal brain injury

Perinatal hypoxic-ischaemic injury (HII) is a significant cause of neurodevelopmental impairment and disability. Studies employing 31P magnetic resonance spectroscopy to measure phosphorus metabolites in situ in the brains of newborn infants and animals have demonstrated that transient hypoxia-ischaemia leads to a delayed disruption in cerebral energy metabolism, the magnitude of which correlates with the subsequent neurodevelopmental impairment. Prominent among the biochemical features of HII is the loss of cellular ATP, resulting in increased intracellular Na+ and Ca2+, and decreased intracellular K+. These ionic imbalances, together with a breakdown in cellular defence systems following HII, can contribute to oxidative stress with a net increase in reactive oxygen species. Subsequent damage to lipids, proteins, and DNA and inactivation of key cellular enzymes leads ultimately to cell death. Although the precise mechanisms of neuronal loss are unclear, it is now clear both apoptosis and necrosis are the significant components of cell death following HII. A number of different factors influence whether a cell will undergo apoptosis or necrosis, including the stage of development, cell type, severity of mitochondrial injury and the availability of ATP for apoptotic execution. This review will focus on some pathological mechanisms of cell death in which there is a disruption to oxidative metabolism. The first sections will discuss the process of damage to oxidative metabolism, covering the data collected both from human infants and from animal models. Following sections will deal with the molecular mechanisms that may underlie cerebral energy failure and cell death in this form of brain injury, with a particular emphasis on the role of apoptosis and mitochondria.

Metabolic stages, mitochondria and calcium in hypoxic/ischemic brain damage

Cerebral hypoxia/ischemia leads to mitochondrial dysfunction due to lack of oxygen leaving the glycolytic metabolism as a main pathway for ATP production. Inhibition of mitochondrial respiration thus triggers generation of lactate and hydrogen ions (H+), and furthermore dramatically reduces ATP generation leading to disregulation of cellular ion metabolism with subsequent intracellular calcium accumulation. Upon reperfusion, when mitochondrial dysfunction is (at least partially) reversed by restoring cerebral oxygen supply, bioenergetic metabolism recovers and brain cells are able to re-institute their normal ionic homeostatic mechanisms. However, the initial restoration of normal mitochondrial function may be only transient and followed by a secondary, delayed perturbation of mitochondrial respiratory performance seen as a decrease in cellular ATP levels and known as "secondary energy failure". There have been several mechanisms considered responsible for delayed post-ischemic mitochondrial failure, the mitochondrial permeability transition (MPT) being one that is considered important. Although the amount of calcium available during early reperfusion in vivo is limited, relative to the amount needed to trigger the MPT in vitro; the additional intracellular conditions (of acidosis, high phosphate, and low adenine nucleotideae levels) prevailing during reperfusion, favor MPT pore opening in vivo. Furthermore, the cellular redistribution and/or changes in the intracellular levels of pro-apoptotic proteins can alter mitochondrial function and initiate apoptotic cell death. Thus, mitochondria seem play an important role in orchestrating cell death mechanisms following hypoxia/ischemia. However, it is still not clear which are the key mechanisms that cause mitochondrial dysfunction and lead ultimately to cell death, and which have more secondary nature to brain damage acting as aggravating factors.

Pharmacological interventions for stroke: failures and future

Given the few options currently available for patients following ischaemic stroke, the recent disappointing failures of several large-scale Phase III clinical trials has made the search for novel therapeutic approaches even more critical. Experimental evidence has suggested that the majority of stroke patients have a slow evolution of brain injury which can occur over several hours. Progressive microcirculatory failure following the initial onset of ischaemia may contribute to the expansion of brain injury. Included among the pathophysiological changes that are speculated to occur as a secondary response to the initial ischaemia are free radical production, excitotoxicity (for example, glutamate) disruption of ionic homeostasis (for example, sodium and calcium influx), enzymatic changes, stimulation of the inflammatory process, endothelin release, activation of platelets and leukocytes, delayed coagulation and endothelial dysfunction. All of these pathophysiological reactions could contribute to an increase in local vascular resistance and therefore cause progressive hypoperfusion of the brain following the onset of stroke. The scope of this review will focus on recent clinical failures in addition to agents currently in clinical development, comparing vascular targets to the common neuroprotective strategies.

Acidosis promotes the permeability transition in energized mitochondria: implications for reperfusion injury

We have studied the influence of pH on opening of the mitochondrial permeability transition pore (PTP) in both deenergized and energized mitochondria in the presence of Pi. In deenergized mitochondria from rat brain and heart, we observed the expected inhibition of Ca2+-induced PTP opening at increasingly acidic pH values. Unexpectedly, mitochondria energized with either electron transport complex I or complex II substrates displayed the opposite behavior, acidic pH promoting rather than inhibiting PTP opening. We show that the potentiating effect of acidic pH is due to an increased rate of Pi uptake. The data also revealed that brain mitochondria are more heterogeneous than heart or liver mitochondria in relation to onset of a permeability transition, and that this heterogeneity depends on their Pi transport capacity. Taken together, these results indicate that the inhibitory effects of acidic pH on the PTP may be overcome in situ by an increased rate of Pi uptake, and that ischemic and postischemic acidosis may worsen rather than relieve PTP-dependent tissue damage.

Neuronal cell death and reactive oxygen species

1. We have investigated the role of reactive oxygen species (ROS) in cell death induced by ischemia or application of the excitatory amino acid agonist, N-methyl-D-aspartate (NMDA) or kainate (KA), in acutely isolated rat cerebellar granule cell neurons, studied by flow cytometry. Various fluorescent dyes were used to monitor intracellular calcium concentration, ROS concentration, membrane potential, and viability in acutely dissociated neurons subjected to ischemia and reoxygenation alone, NMDA or kainate alone, and ischemia and reoxygenation plus NMDA or kainate. 2. With ischemia followed by reoxygenation, ROS concentrations rose slightly and there was only a modest increase in cell death after 60 min. 3. When NMDA or kainate alone was applied to the cells there was a large increase in ROS and in intracellular calcium concentration but only a small loss of cellular viability. However, when NMDA or kainate was applied during the reoxygenation period there was a large loss of viability, accompanied by membrane depolarization, but the elevations of ROS and intracellular calcium concentration were not greater than seen with the excitatory amino acids alone. 4. These observations indicate that other factors beyond ROS and intracellular calcium concentration contribute to cell death in cerebellar granule cell neurons.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

Early metabolic inhibition-induced intracellular sodium and calcium increase in rat cerebellar granule cells

1. Possible mechanisms responsible for the increases in intracellular calcium ([Ca2+]i) and sodium ([Na+]i) levels seen during metabolic inhibition were investigated by continuous [Ca2+]i and [Na+]i measurement in cultured rat cerebellar granule cells. An initial small mitochondrial Ca2+ release was seen, followed by a large influx of extracellular Ca2+. A large influx of extracellular Na+ was also seen. 2. The large [Ca2+]i increase was not due to opening of voltage-dependent or voltage-independent calcium channels, activation of NMDA/non-NMDA channels, activation of the Na+i-Ca2+o exchanger, or inability of plasmalemmal Ca2+-ATPase to extrude, or mitochondria to take up, calcium. 3. The large [Na+]i increase was not due to activation of the TTX-sensitive Na+ channel, the Na+i-Ca2+o exchanger, the Na+-H+ exchanger, or the Na+-K+-2Cl- cotransporter, or an inability of Na+-K+-ATPase to extrude the intracellular sodium. 4. Phospholipase A2 (PLA2) activation may be involved in the large influx, since both were completely inhibited by PLA2 inhibitors. Moreover, melittin (a PLA2 activator) or lysophosphatidylcholine or arachidonic acid (both PLA2 activation products) caused similar responses. Inhibition of PLA2 activity may help prevent the influx of these ions that may result in serious brain injury and oedema during hypoxia/ischaemia.

Attenuation of hypoxic current by intracellular applications of ATP regenerating agents in hippocampal CA1 neurons of rat brain slices

Hypoxia-induced outward currents (hyperpolarization) were examined in hippocampal CA1 neurons of rat brain slices, using the whole-cell recording technique. Hypoxic episodes were induced by perfusing slices with an artificial cerebrospinal fluid aerated with 5% CO2/95% N2 rather than 5% CO2/95% O2, for about 3 min. The hypoxic current was consistently and reproducibly induced in CA1 neurons dialysed with an ATP-free patch pipette solution. This current manifested as an outward shift in the holding current in association with increased conductance, and it reversed at -78 +/- 2.5 mV, with a linear I-V relation in the range of -100 to -40 mV. To provide extra energy resources to individual neurons recorded, agents were added to the patch pipette solution, including MgATP alone, MgATP + phosphocreatine + creatine kinase, or MgATP + creatine. In CA1 neurons dialysed with patch solutions including these agents, hypoxia produced small outward currents in comparison with those observed in CA1 neurons dialysed with the ATP-free solution. Among the above agents examined, whole-cell dialysis with MgATP + creatine was the most effective at decreasing the hypoxic outward currents. We suggest that the hypoxic hyperpolarization is closely related to energy metabolism in individual CA1 neurons, and that the energy supply provided by phosphocreatine metabolism may play a critical role during transient metabolic stress.

State of actin filaments is changed by anoxia in cultured rat neocortical neurons

Cultured neocortical neurons respond rapidly to oxygen deprivation. Within minutes they demonstrate an increase in intracellular calcium and pronounced changes in their morphology. These changes include swelling, bleb formations, process retraction and a change in shape of the soma from pyramidal or ellipsoidal to round. Since the cytoskeleton is responsible for the maintenance of cell shape, we investigated the changes in state of a major component of the neuronal cytoskeleton, the actin filaments. Actin exists in a dynamic equilibrium between the monomeric and filamentous states. This equilibrium is dependent, in part, upon intracellular ATP, which is reduced during anoxia. We differentially labelled monomeric actin with Texas Red-tagged DNase-1 and the polymeric form with Bodipy-phallicidin. Using confocal microscopy and image reconstruction, we have found that the ratio of filamentous to monomeric actin increases on average three-fold following 10 min of anoxia. In addition, filamentous actin redistributes within the soma and appears to have a more homogeneous distribution than in normoxic neurons. Our results show that, in cultured neocortical neurons, actin filaments are modulated by anoxia. We believe that, although the filamentous/monomeric ratio is modulated, at least in part, by the levels of ATP and ADP, the relative distribution and amount of the filamentous to monomeric form within neurons are likely regulated by other factors such as calcium-sensitive actin-binding and severing proteins.

O2-sensing mechanisms in excitable cells: role of plasma membrane K+ channels

Although carotid chemosensitive glomus cells have been the most extensively studied from the vantage point of how cells sense the lack of O2, it is clear that all tissues sense O2 deprivation. In addition, all mammalian cells can trigger a cascade of events that, depending on the severity and duration of hypoxia-induced stress, can lead to permanent injury and death or to adaptation and survival. Crucial in this cascade, we believe, how the cascade is initiated, how O2 lack is detected by cells, and how these initial steps can activate further processes. In this chapter, we focus on the initial steps of O2 sensing in tissues most commonly studied, i.e. carotid glomus cells, central neurons, smooth muscle cells, and neuro-epithelial bodies of the airways. Recently it has become clear that plasma membranes of various tissues can sense the lack of O2, not only indirectly via alterations in the intracellular milieu (such as pH, Ca, ATP, etc), but also directly through an unknown mechanism that involves plasma-membrane K channels and possibly other membrane proteins. This latter mechanism is suspected to be totally independent of cytosolic changes because excised patches from plasma membranes were used in these experiments from carotid cells and neurons. There are a number of questions in this exciting area of research that pertain to the role of this plasma-membrane O2-sensing mechanism in the overall cell response, identification of all the important steps in O2 sensing, differences between O2-tolerant and O2-susceptible cells, and differences between acute and chronic cell responses to lack of O2.

Changes in extracellular acid-base homeostasis in cerebral ischemia

The purpose of this study was to examine the changes in extracellular CO32- and lactate concentration produced by ischemia, especially in relation to the occurrence of anoxic depolarization, and how some of these changes are altered by the inhibition of organic acid transport systems with probenecid. These data demonstrate that (i) the transmembrane mechanisms contributing to intracellular acid-base regulation (Na+/H+ and HCO3-/Cl- exchanges, and lactate/H+ cotransport) are markedly activated during ischemia; (ii) the efficacy of these mechanisms is abolished as the cellular membrane permeability to ions, including H+ and pH-changing anions, suddenly increases with anoxic depolarization; and (iii) efflux of intracellular lactate during ischemia, and its reuptake with reperfusion, mainly occur via a transporter. These findings imply that residual cellular acid-base homeostasis persists as long as cell depolarization does not occur, and strengthen the concept that anoxic depolarization is a critical event for cell survival during ischemia.

Rat brain glucose and energy metabolites: effect of +Gz (head-to-foot inertial load) exposure in a small animal centrifuge

A unique small animal centrifuge with on-line physiological monitoring and brain tissue collection (in < 1 s) capability was used to investigate the effect of increasing +Gz levels, exposure duration, number of exposures, and time course of metabolic changes in the rat brain. To determine the +Gz tolerance, rats were exposed to +7.5 to 25 Gz (30 s each) and EEG was monitored. G-induced loss of consciousness (G-LOC) defined as isoelectric EEG (I-EEG) occurred only at +22.5 and 25 Gz within 14.5 +/- 3 s. To study the effect of increasing +Gz levels on metabolism, rats were exposed to either 0.5 (control) or +7.5 to 25 Gz (30 s each), and brains were collected 1 min postcentrifugation by freeze fixation. A significant increase in lactate (> or = +7.5 Gz) and a decrease in glucose, creatine phosphate (Cr-P), and ATP levels were observed at +15 Gz and higher. The effect of exposure duration was investigated by exposing the rats to +22.5 Gz for 15-60 s. Brain lactate levels increased six-fold while glucose decreased (75%) following the 60-s exposure. The level of Cr-P and ATP decreased significantly after the 15- and 30-s exposures with no further changes at longer +Gz exposures. For time course studies, brains were collected both during (5-35 s) and after (1-15 min) a +25 Gz exposure. A significant decrease in Cr-P occurred within 5 s, but changes in glucose, ATP, and lactate required 15 s. All metabolites returned to control levels within 3 min, except lactate and adenosine, which required 15 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Mediators of injury in neurotrauma: intracellular signal transduction and gene expression

Membrane lipid-derived second messengers are generated by phospholipase A2 (PLA2) during synaptic activity. Overstimulation of this enzyme during neurotrauma results in the accumulation of bioactive metabolites such as arachidonic acid, oxygenated derivatives of arachidonic acid, and platelet-activating factor (PAF). Several of these bioactive lipids participate in cell damage, cell death, or repair-regenerative neural plasticity. Neurotransmitters may activate PLA2 directly when linked to receptors coupled to G proteins and/or indirectly as calcium influx or mobilization from intracellular stores is stimulated. The release of arachidonic acid and its subsequent metabolism to prostaglandins are early responses linked to neuronal signal transduction. Free arachidonic acid may interact with membrane proteins, i.e., receptors, ion channels, and enzymes, modifying their activity. It can also be acted upon by prostaglandin synthase isoenzymes (the constitutive prostaglandin synthase PGS-1 or the inducible PGS-2) and by lipoxygenases, with the resulting formation of different prostaglandins and leukotrienes. Glutamatergic synaptic activity and activation of postsynaptic NMDA receptors are examples of neuronal activity, linked to memory and learning processes, which activate PLA2 with the consequent release of arachidonic acid and platelet-activating factor (PAF), another lipid mediator. Both mediators may exert presynaptic and postsynaptic effects contributing to long-lasting changes in glutamate synaptic efficacy or long-term potentiation (LTP), PAF, a potential retrograde messenger in LTP, stimulates glutamate release. The PAF antagonist BN 52021 competes for receptors in presynaptic membranes and blocks this effect. PAF may also be involved in plasticity responses because PAF leads to the expression of early response genes and subsequent gene cascades. The PAF antagonist BN 50730, selective for PAF intracellular binding, blocks PAF-mediated induction of gene expression. A consequence of neural injury induced by ischemia, trauma, or seizures is an increased release of neurotransmitters, that in turn generates an overproduction of second messengers. Glutamate, a key player in excitotoxic neuronal damage, triggers increased permeation of calcium mediated by NMDA receptors and activation of PLA2 in postsynaptic neurons. NMDA receptor antagonists reduce the accumulation of free fatty acids and elicit neuroprotection in ischemic damage. Increased production of free arachidonic acid and PAF converges to exacerbate glutamate-mediated neurotransmission. These neurotoxic actions may be brought about by arachidonic acid-induced potentiation of NMDA receptor activity and decreased glutamate reuptake. On the other hand, PAF stimulates the further release of glutamate at presynaptic endings. The neuroprotective effects of the PAF antagonist BN 52021 in ischemia-reperfusion are due, at least in part, to an inhibition of presynaptic glutamate release. PAF also induces expression of the inducible prostaglandin synthase gene, and PAF antagonists selective for the intracellular sites inhibit this effect. The PAF antagonist also inhibits the enhanced abundance, due to vasogenic cerebral edema and ischemia-reperfusion damage, of inducible prostaglandin synthase mRNA in vivo. Therefore, PAF, an injury-generated mediator, may favor the formation of other cell injury and inflammation mediators by turning on the expression of the gene that encodes prostaglandin synthase.

Action of adenosine receptor antagonists on hypoxia-induced effects in the rat hippocampus in vitro

1. We have studied three hypoxia-induced phenomena in the CA1 stratum pyramidale of the rat hippocampal slice: (a) the increase in extracellular potassium ion concentration ([K+]e) measured with ion-sensitive microelectrodes, (b) the intracellularly-recorded pyramidal cell hyperpolarization and (c) the extracellularly-recorded depression of the synaptically-evoked field potential recorded in stratum pyramidale. 2. The extracellular potassium ion concentration ([K+]e) rose from 3 mM to 4.1-4.4 mM at a time when the pyramidal cells hyperpolarized by about 6 mV and neurotransmission was virtually abolished. 3. Presumed glial cells depolarized in response to hypoxia. The shape and time course of this response was remarkably similar to the rise in [K+]e so induced. This is consistent with findings that glial cell membrane potential is dependent on transmembrane K+ gradient. 4. We investigated the effects of theophylline (100 microM) and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.1 microM) on these effects. We have found that these compounds attenuated by about half the hypoxia-induced increase in [K+]e; however, they did not reduce the hypoxia-induced hyperpolarization. We have confirmed that they dramatically reduced the suppression of excitatory transmission caused by the hypoxia. We conclude that adenosine A1 receptors may be involved in the alteration of K+ homeostasis in the hippocampal slice during hypoxia.

Major differences in response to graded hypoxia between hypoglossal and neocortical neurons

Intracellular electrophysiologic recordings were performed in brain slices from adult rats to compare the response of brain stem hypoglossal neurons (XII) and layer II/III neocortical neurons (NCX) to two levels of oxygen deprivation (hypoxia, pO2 = 15-20 Torr; anoxia, pO2 = 0 Torr). These recordings were also used during re-oxygenation after hypoxia or anoxia to study neuronal recovery. Both groups of neurons showed a greater response to anoxia than hypoxia in terms of membrane potential (Vm) and input resistance (Rm). When the two groups were compared at each level of O2, XII depolarized more and in a shorter period of time than NCX. During anoxia, XII neurons responded with anoxic depolarization (AD) of > 20 mV/min by 3 min, along with a large decrease in Rm. NCX neurons, on the other hand, exhibited AD after a mean latency of approximately 9 min and 18% of NCX neurons did not even show AD. Although all neurons (both XII and NCX) recovered when re-oxygenated before or at AD, XII neurons failed to recover from periods of anoxia that were well tolerated by NCX neurons. We conclude that: (1) there are marked differences in the magnitude and trajectory of membrane depolarization between XII and NCX neurons in response to O2 deprivation, with NCX neurons showing a much longer latency to AD during anoxia than XII; and (2), when exposed to periods of anoxia of similar duration and severity, XII neurons are less likely to recover than NCX neurons and XII neurons may, therefore, be inherently more vulnerable to anoxia-induced injury than NCX neurons.

Oxygen deprivation inhibits a K+ channel independently of cytosolic factors in rat central neurons

1. K+ channel modulation has been shown to be an integral and important cellular response to O2 deprivation. Although part of this modulation occurs as a result of changes in concentrations of several cytosolic factors such as ATP and Ca2+, it is unknown whether there are mechanisms other than those originating from the cytosol. To test the hypothesis that membrane-delimited mechanisms participate in the O2-sensing process and are involved in the modulation of K+ channel activity in central neurons, we performed experiments using patch-clamp techniques and dissociated cells from the rat neocortex and substantia nigra. 2. Whole-cell outward currents were studied in voltage-clamp mode using Na(+)-free or low-Na+ (5 mM, with 1 microM tetrodotoxin) extracellular medium plus 0.5 mM Co2+. O2 deprivation produced a biphasic response in current amplitude, i.e. an initial transient increase followed by a pronounced decrease in outward currents. The reduction in outward currents was a reversible process since perfusion with a medium of PO2 > 100 mmHg (1 mmHg = 133 Pa) led to a complete recovery. 3. In cell-free excised membrane patches, we found that a specific K+ current (large conductance, inhibited by micromolar concentrations of ATP and activated by Ca2+) was reversibly inhibited by lack of O2. This was characterized by a marked decrease in channel open-state probability and a slight reduction in unitary conductance. The magnitude of channel inhibition by O2 deprivation was closely dependent on O2 tension. The PO2 level for 50% channel inhibition was about 10 mmHg with little or no inhibition at PO2 > or = 20 mmHg. 4. Single-channel kinetic analysis showed that channel open times consisted of two components and closed times were composed of three. The hypoxia-induced inhibition of K+ channel activity was mediated by selective suppression of the longer time constant channel openings without significantly affecting closed time constants. This led to an increase in frequency of opening and closing and rapid channel flickerings. 5. Our data showed that O2 deprivation had no effect on another K+ current characterized by a much smaller conductance and Ca2+ independence. This provides evidence for the selective nature of the hypoxia-induced inhibition of some species of K+ channels. 6. These results therefore provide the first evidence for regulation of K+ channel activity by O2 deprivation in cell-free excised patches from central neurons.

Coupling among energy failure, loss of ion homeostasis, and phospholipase A2 and C activation during ischemia

The objective of the present experiments was to correlate changes in cellular energy metabolism, dissipative ion fluxes, and lipolysis during the first 90 s of ischemia and, hence, to establish whether phospholipase A2 or phospholipase C is responsible for the early accumulation of phospholipid hydrolysis products. Ischemia was induced for 15-90 s in rats, extracellular K+ (K+e) was recorded, and neocortex was frozen in situ for measurements of labile tissue metabolites, free fatty acids, and diacylglycerides. Ischemia of 15- and 30-s duration gave rise to a decrease in phosphocreatine concentration and a decline in the ATP/free ADP ratio. Although these changes were accompanied by an activation of K+ conductances, there were no changes in free fatty acids until after 60 s, when free arachidonic acid accumulated. An increase in other free fatty acids and in total diacylglceride content did not occur until after anoxic depolarization. The results demonstrate that the early functional changes, such as activation of K+ conductances, are unrelated to changes in lipids or lipid mediators. They furthermore suggest that the initial lipolysis occurs via both phospholipase A2 and phospholipase C, which are activated when membrane depolarization leads to influx of calcium into cells.

Recovery of mitochondrial and plasma membrane function following hypoglycemic coma: coupling of ATP synthesis, K+ transport, and changes in extra- and intracellular pH

The primary objective of the present study was to evaluate the recovery of plasma and mitochondrial membrane functions after 30 min of hypoglycemic coma and to establish whether a lingering accumulation of free fatty acids (FFAs) delays the recovery. A secondary objective was to study whether production of metabolic acids following glucose infusion leads to a fall in intracellular pH (pHi). Phosphocreatine, creatine, ATP, ADP, and AMP, as well as glycogen, glucose, lactate, pyruvate, and FFAs of rat brain cortex and caudoputamen were measured, and "free" ADP was calculated from the creatine kinase equilibrium. Extracellular pH (pHe) and K+ concentration (K+e) were measured with ion-sensitive microelectrodes, and pHi was derived by the CO2 method. Glucose injection was followed by resumption of oxidative phosphorylation within approximately 2 min and by an equally rapid restoration of normal K+e levels. These functions recovered although tissue FFAs remained elevated for at least 7-8 min. Tissue lactate content increased only moderately and production of metabolic acids did not lead to intracellular acidosis. After 15 min of recovery, pHi was moderately increased, although pHe fell toward 7.0. It is speculated that the dissociation between intra- and extra-cellular pH is compatible with an up-regulation of an Na+/H+ antiporter, e.g., by phosphorylation.

Coupling of energy failure and dissipative K+ flux during ischemia: role of preischemic plasma glucose concentration

The present experiments were undertaken to assess the influence of preischemic hypo- or hyperglycemia on the coupling among changes in extracellular K+ concentration (K+e) and in cellular energy state, as the latter is reflected in the tissue concentrations of phosphocreatine (PCr), Cr, ATP, ADP, and AMP, and in the calculated free ADP (ADPf) concentrations. The questions posed were whether the final release of K+ was delayed because the extra glucose accumulated by hyperglycemic animals produced enough ATP to continue supporting Na(+)-K(+)-driven ATPase activity, and whether the additional acidosis altered the ionic transients. As expected, preischemic hypoglycemia shortened and hyperglycemia prolonged the phase before K+e rapidly increased. This was reflected in corresponding changes in tissue ATP content. Thus, hypoglycemia shortened and hyperglycemia prolonged the time before the fall in ATP concentration accelerated. When tissue was frozen at the moment of depolarization, the tissue contents of ATP were similar in hypo-, normo-, and hyperglycemic groups, approximately 30% of control. This suggests that hyperglycemia retards loss of ion homeostasis by leading to production of additional ATP. However, hyperglycemia did not reduce the rate at which the PCr concentration fell, and the ATP/ADPf ratio decreased. There were marked differences in the amount of lactate accumulated between the groups. Thus, massive depolarization in hypoglycemic groups occurred at a tissue lactate content of approximately 4 mM kg-1. This corresponds to a decrease in intracellular pH (pHi) from approximately 7.0 to approximately 6.9. In the hyperglycemic groups, depolarization occurred at a lactate content of about 12 mm kg-1, corresponding to a pHi of approximately 6.4.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes of labile metabolites during anoxia in moderately hypo- and hyperthermic rats: correlation to membrane fluxes of K+

The objective of this study was to assess the influence of temperature on the coupling among energy failure, depolarization, and ionic fluxes during anoxia. To that end, we induced anoxia by cardiac arrest in anesthetized rats maintained at a body temperature of either 34 degrees C or 40 degrees C, measured extracellular K+ concentration (K+e), and froze the neocortex through the exposed dura for measurements of phosphocreatine (PCr), creatine (Cr), ATP, ADP, and AMP, glucose, glycogen, pyruvate and lactate content after ischemic intervals of maximally 130 s. Free ADP (ADPf) concentrations were derived from the creatine kinase equilibrium. Hypothermia reduced the initial rate of rise in K+e, and delayed the terminal depolarization; however, both hypo- and hyperthermic animals showed massive loss of ion homeostasis at a K+e of 10-15 mM. The initial rate of rise in K+e did not correlate to changes in ATP, or ATP/ADPf ratio, suggesting that temperature changes per se may control the degree of activation of K+ conductances. The results clearly showed that, in both hyper- and hypothermic subjects, energy failure preceded the sudden activation of membrane conductances for ions. The results indicate that temperature primarily influences membrane permeability to ions like K+e (and Na+), and that cerebral energy state is secondarily affected. It is proposed that the higher rate of rise of K+e at high temperatures accelerates ATP hydrolysis primarily by enhancing metabolic rate in glial cells.