Review: A history and perspective of mitochondria in the context of anoxia tolerance

Symbiosis is found throughout nature, but perhaps nowhere is it more fundamental than mitochondria in all eukaryotes. Since mitochondria were discovered and mechanisms of oxygen reduction characterized, an understanding gradually emerged that these organelles were involved not just in the combustion of oxygen, but also in the sensing of oxygen. While multiple hypotheses exist to explain the mitochondrial involvement in oxygen sensing, key elements are developing that include potassium channels and reactive oxygen species. To understand how mitochondria contribute to oxygen sensing, it is informative to study a model system which is naturally adapted to survive extended periods without oxygen. Amongst air-breathing vertebrates, the most highly adapted are western painted turtles (Chrysemys picta bellii), which overwinter in ice-covered and anoxic water bodies. Through research of this animal, it was postulated that metabolic rate depression is key to anoxic survival and that mitochondrial regulation is a key aspect. When faced with anoxia, excitatory neurotransmitter receptors in turtle brain are inhibited through mitochondrial calcium release, termed "channel arrest". Simultaneously, inhibitory GABAergic signalling contributes to the "synaptic arrest" of excitatory action potential firing through a pathway dependent on mitochondrial depression of ROS generation. While many pathways are implicated in mitochondrial oxygen sensing in turtles, such as those of adenosine, ATP turnover, and gaseous transmitters, an apparent point of intersection is the mitochondria. In this review we will explore how an organelle that was critical for organismal complexity in an oxygenated world has also become a potentially important oxygen sensor.

GABA receptor inhibition and severe hypoxia induce a paroxysmal depolarization shift in goldfish neurons

Mammalian neurons undergo rapid excitotoxic cell death when deprived of oxygen; however, the common goldfish (Carassius auratus) has the unique ability of surviving in oxygen-free waters, under anoxia. This organism utilizes γ-amino butyric acid (GABA) signaling to suppress excitatory glutamatergic activity during anoxic periods. Although GABA_A receptor antagonists are not deleterious to the cellular survival, coinhibition of GABA_A and GABA_B receptors is detrimental by abolishing anoxia-induced neuroprotective mechanisms. Here we show that blocking the anoxic GABAergic neurotransmission induces seizure-like activity (SLA) analogous to a paroxysmal depolarization shift (PDS), with hyperpolarization of action potential (AP) threshold and elevation of threshold currents. The observed PDS was attributed to an increase in excitatory postsynaptic currents (EPSCs) that are normally attenuated with decreasing oxygen levels. Furthermore, for the first time, we show that in addition to PDS, some neurons undergo depolarization block and do not generate AP despite a suprathreshold membrane potential. In conclusion, our results indicate that with severe hypoxia and absence of GABA receptor activity, telencephalic neurons of C. auratus manifest a paroxysmal depolarization shift, a key feature of epileptic discharge.NEW & NOTEWORTHY This work shows that the combination of anoxia and inhibition of GABA receptors induces seizure-like activities in goldfish telencephalic pyramidal and stellate neurons. Importantly, to prevent seizure-like activity, an intact GABA-mediated inhibitory pathway is required.

Evidence of different climatic adaptation strategies in humans and non-human primates

To understand human evolution it is critical to clarify which adaptations enabled our colonisation of novel ecological niches. For any species climate is a fundamental source of environmental stress during range expansion. Mammalian climatic adaptations include changes in size and shape reflected in skeletal dimensions and humans fit general primate ecogeographic patterns. It remains unclear however, whether there are also comparable amounts of adaptation in humans, which has implications for understanding the relative importance of biological/behavioural mechanisms in human evolution. We compare cranial variation between prehistoric human populations from throughout Japan and ecologically comparable groups of macaques. We compare amounts of intraspecific variation and covariation between cranial shape and ecological variables. Given equal rates and sufficient time for adaptation for both groups, human conservation of non-human primate adaptation should result in comparable variation and patterns of covariation in both species. In fact, we find similar amounts of intraspecific variation in both species, but no covariation between shape and climate in humans, contrasting with strong covariation in macaques. The lack of covariation in humans may suggest a disconnect in climatic adaptation strategies from other primates. We suggest this is due to the importance of human behavioural adaptations, which act as a buffer from climatic stress and were likely key to our evolutionary success.

Oxygen-sensitive interneurons exhibit increased activity and GABA release during ROS scavenging in the cerebral cortex of the western painted turtle

The western painted turtle (Chrysemys picta bellii) has the unique ability of surviving several months in the absence of oxygen, which is termed anoxia. One major protective strategy that the turtle employs during anoxia is a reduction in neuronal electrical activity, which may result from a natural reduction in reactive oxygen species (ROS). We previously linked a reduction in ROS levels to an increase in γ-amino butyric acid (GABA) receptor currents. The purpose of this study is to understand how fast-spiking, GABA-releasing neurons respond to reductions in ROS and how this affects GABA release. Using a fluorescence-coupled enzymatic microplate assay for GABA, we found that anoxia, the ROS scavenger N-(2-mercaptopriopionyl)glycine (MPG), or the mitochondria-specific ROS scavenger MitoTEMPO resulted in a 2.5-, 2.0-, and 2.5-fold increase in extracellular GABA concentration, respectively. This phenomenon could be blocked with TTX, indicating that it is activity dependent. Using whole cell patch-clamping techniques, we found that fast-spiking, burst-firing GABAergic turtle neurons increase the duration and number of action potentials per burst by 26% and 42%, respectively, in response to ROS scavenging via MPG. These results suggest that the reduction in mitochondrially produced ROS that occurs during anoxia leads to increased GABA release, which promotes postsynaptic inhibitory activity through activation of GABA receptors.NEW & NOTEWORTHY This is a novel study examining the response of cerebral cortical stellate interneurons to anoxia and mitochondrial reactive oxygen species (ROS) scavenging with MitoTEMPO. Under both conditions burst firing increases in these cells, and we show that extracellular GABA release increases in the presence of the ROS scavenger. We conclude that in the anoxia-tolerant painted turtle brain, a decrease in ROS levels is an important low oxygen signal.

Mitochondrial matrix pH acidifies during anoxia and is maintained by the F1Fo-ATPase in anoxia-tolerant painted turtle cortical neurons

The western painted turtle (Chrysemys picta bellii) can survive extended periods of anoxia via a series of mechanisms that serve to reduce its energetic needs. Central to these mechanisms is the response of mitochondria, which depolarize in response to anoxia in turtle pyramidal neurons due to an influx of K⁺. It is currently unknown how mitochondrial matrix pH is affected by this response and we hypothesized that matrix pH acidifies during anoxia due to increased K⁺/H⁺ exchanger activity. Inhibition of K⁺/H⁺ exchange via quinine led to a collapse of mitochondrial membrane potential (Ψ_m) during oxygenated conditions in turtle cortical neurons, as indicated by rhodamine‐123 fluorescence, and this occurred twice as quickly during anoxia which indicates an elevation in K⁺ conductance. Mitochondrial matrix pH acidified during anoxia, as indicated by SNARF‐1 fluorescence imaged via confocal microscopy, and further acidification occurred during anoxia when the F₁F_o‐ATPase was inhibited with oligomycin‐A, indicating that ΔpH collapse is prevented during anoxic conditions. Collectively, these results indicate that the mitochondrial proton electrochemical gradient is actively preserved during anoxia to prevent a collapse of Ψ_m and ΔpH.

Anoxia-mediated calcium release through the mitochondrial permeability transition pore silences NMDA receptor currents in turtle neurons

Mammalian neurons are anoxia sensitive and rapidly undergo excitotoxic cell death when deprived of oxygen, mediated largely by Ca(2+) entry through over-activation of N-methyl-d-aspartate receptors (NMDARs). This does not occur in neurons of the anoxia-tolerant western painted turtle, where a decrease in NMDAR currents is observed with anoxia. This decrease is dependent on a modest rise in cytosolic [Ca(2+)] ([Ca(2+)]c) that is mediated by release from the mitochondria. The aim of this study was to determine whether the mitochondrial permeability transition pore (mPTP) is involved in NMDAR silencing through release of mitochondrial Ca(2+). Opening the mPTP during normoxia with atractyloside decreased NMDAR currents by releasing mitochondrial Ca(2+), indicated by an increase in Oregon Green fluorescence. Conversely, the mPTP blocker cyclosporin A prevented the anoxia-mediated increase in [Ca(2+)]c and reduction in NMDAR currents. Mitochondrial membrane potential (Ψm) was determined using rhodamine-123 fluorescence and decreased with the onset of anoxia in a time frame that coincided with the increase in [Ca(2+)]c. Activation of mitochondrial ATP-sensitive potassium (mK(+)ATP) channels also releases mitochondrial Ca(2+) and we show that activation of mK(+)ATP channels during normoxia with diazoxide leads to Ψm depolarization and inhibition with 5-hydroxydecanoic acid blocked anoxia-mediated Ψm depolarization. Ψm does not collapse during anoxia but rather reaches a new steady-state level that is maintained via ATP hydrolysis by the F1-F0 ATPase, as inhibition with oligomycin depolarizes Ψm further than the anoxic level. We conclude that anoxia activates mK(+)ATP channels, which leads to matrix depolarization, Ca(2+) release via the mPTP, and ultimately silencing of NMDARs.

Scavenging ROS dramatically increase NMDA receptor whole-cell currents in painted turtle cortical neurons

Oxygen deprivation triggers excitotoxic cell death in mammal neurons through excessive calcium loading via over-activation of N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. This does not occur in the western painted turtle, which overwinters for months without oxygen. Neurological damage is avoided through anoxia-mediated decreases in NMDA and AMPA receptor currents that are dependent upon a modest rise in intracellular Ca(2+) concentrations ([Ca(2+)]i) originating from mitochondria. Anoxia also blocks mitochondrial reactive oxygen species (ROS) generation, which is another potential signaling mechanism to regulate glutamate receptors. To assess the effects of decreased intracellular [ROS] on NMDA and AMPA receptor currents, we scavenged ROS with N-2-mercaptopropionylglycine (MPG) or N-acetylcysteine (NAC). Unlike anoxia, ROS scavengers increased NMDA receptor whole-cell currents by 100%, while hydrogen peroxide decreased currents. AMPA receptor currents and [Ca(2+)]i concentrations were unaffected by ROS manipulation. Because decreases in [ROS] increased NMDA receptor currents, we next asked whether mitochondrial Ca(2+) release prevents receptor potentiation during anoxia. Normoxic activation of mitochondrial ATP-sensitive potassium (mKATP) channels with diazoxide decreased NMDA receptor currents and was unaffected by subsequent ROS scavenging. Diazoxide application following ROS scavenging did not rescue scavenger-mediated increases in NMDA receptor currents. Fluorescent measurement of [Ca(2+)]i and ROS levels demonstrated that [Ca(2+)]i increases before ROS decreases. We conclude that decreases in ROS concentration are not linked to anoxia-mediated decreases in NMDA/AMPA receptor currents but are rather associated with an increase in NMDA receptor currents that is prevented during anoxia by mitochondrial Ca(2+) release.

Oxygen-sensitive reduction in Ca²⁺-activated K⁺ channel open probability in turtle cerebrocortex

In response to low ambient oxygen levels the western painted turtle brain undergoes a large depression in metabolic rate which includes a decrease in neuronal action potential frequency. This involves the arrest of N-methyl-D-aspartate receptor (NMDAR) and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) currents and paradoxically an increase in γ-aminobutyric acid receptor (GABAR) currents in turtle cortical neurons. In a search for other oxygen-sensitive channels we discovered a Ca(2+)-activated K(+) channel (K(Ca)) that exhibited a decrease in open time in response to anoxia. Single-channel recordings of K(Ca) activity were obtained in cell-attached and excised inside-out patch configurations from neurons in cortical brain sheets bathed in either normoxic or anoxic artificial cerebrospinal fluid (aCSF). The channel has a slope conductance of 223pS, is activated in response to membrane depolarization, and is controlled in a reversible manner by free [Ca(2+)] at the intracellular membrane surface. In the excised patch configuration anoxia had no effect on K(Ca) channel open probability (P(open)); however, in cell-attached mode, there was a reversible fivefold reduction in P(open) (from 0.5 ± 0.05 to 0.1 ± 0.03) in response to 30-min anoxia. The inclusion of the potent protein kinase C (PKC) inhibitor chelerythrine prevented the anoxia-mediated decrease in P(open) while drip application of a phorbol ester PKC activator decreased P(open) during normoxia (from normoxic 0.4 ± 0.05 to phorbol-12-myristate-13-acetate (PMA) 0.1 ± 0.02). Anoxia results in a slight depolarization of turtle pyramidal neurons (∼8 mV) and an increase in cytosolic [Ca(2+)]; therefore, K(Ca) arrest is likely important to prevent Ca(2+) activation during anoxia and to reduce the energetic cost of maintaining ion gradients. We conclude that turtle pyramidal cell Ca(2+)-activated K(+) channels are oxygen-sensitive channels regulated by cytosolic factors and are likely the reptilian analog of the mammalian large conductance Ca(2+)-activated K(+) channels (BK channels).

Painted turtle cortex is resistant to an in vitro mimic of the ischemic mammalian penumbra

Anoxia or ischemia causes hyperexcitability and cell death in mammalian neurons. Conversely, in painted turtle brain anoxia increases γ-amino butyric acid (GABA)ergic suppression of spontaneous electrical activity, and cell death is prevented. To examine ischemia tolerance in turtle neurons, we treated cortical sheets with an in vitro mimic of the penumbral region of stroke-afflicted mammalian brain (ischemic solution, IS). We found that during IS perfusion, neuronal membrane potential (V(m)) and the GABA(A) receptor reversal potential depolarized to a similar steady state (-92 ± 2 to -28 ± 3 mV, and -75 ± 1 to -35 ± 3 mV, respectively), and whole-cell conductance (G(w)) increased >3-fold (from 4 ± 0.2 to 15 ± 1 nS). These neurons were electrically quiet and changes reversed after reperfusion. GABA receptor antagonism prevented the IS-mediated increase in G(w) and neurons exhibited enhanced electrical excitability and rapid and irreversible rundown of V(m) during reperfusion. These results suggest that inhibitory GABAergic mechanisms also suppress electrical activity in ischemic cortex. Indeed, after 4 hours of IS treatment neurons did not exhibit any apparent damage; while at 24 hours, only early indicators of apoptosis were present. We conclude that anoxia-tolerant turtle neurons are tolerant of exposure to a mammalian ischemic penumbral mimic solution.

First aid kit for hypoxic survival: sensors and strategies

Survival success under conditions of acute oxygen deprivation depends on efficiency of the central and peripheral chemoreception, optimization of oxygen extraction from the hypoxic environment and its delivery to the periphery, and adjustments of energy production and consumption. This article uses a comparative approach to assess the efficiency of adaptive strategies used by anoxia-tolerant and hypoxia-sensitive species to support survival during the first minutes to 1 h of oxygen deprivation. An aquatic environment is much more demanding in terms of diurnal and seasonal variations of the ambient oxygen availability from anoxia to hyperoxia than is an air environment. Therefore, fishes and aquatic turtles have developed a number of adaptive responses, which are lacking in most of the terrestrial mammals, to cope with these extreme conditions. These include efficient central and peripheral chemoreception, acute changes in respiratory rate and amplitude, and acute increase of the gas-exchange interface. A special set of adaptive mechanisms are engaged in reduction of the energy expenditure of the major oxygen-consuming organs: the brain and the heart. Both reduction of ATP consumption and a switch to alterative energy sources contribute to the maintenance of ATP and ion balance in hypoxia-tolerant animals. Hypoxia and hyperoxia are conditions favoring development of oxidative stress. Efficient protection from oxidation in anoxia-tolerant species includes reduction in the glutamate levels in the brain, stabilization of the mitochondrial function, and maintenance of nitric oxide production under conditions of oxygen deprivation. We give an overview of the current state of knowledge on some selected molecular and cellular acute adaptive mechanisms. These include the mechanisms of chemoreception in adult and neonatal mammals and in fishes, acute metabolic adaptive responses in the brain, and the role of nitrite in the preservation of heart function under hypoxic conditions.

Regulation of AMPA receptor currents by mitochondrial ATP-sensitive K+ channels in anoxic turtle neurons

Mammalian neurons rapidly undergo excitotoxic cell death during anoxia, whereas neurons from the anoxia-tolerant painted turtle survive without oxygen for hours and offer a unique model to study mechanisms to reduce the severity of cerebral stroke. An anoxia-mediated decrease in whole cell N-methyl-D-aspartate receptor and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) currents are an important part of the turtle's natural defense. Here we investigate the role of mitochondrial ATP-sensitive K(+) (mK(ATP)) channels in the regulation of AMPAR. Whole cell AMPAR currents were stable over 90 min of normoxic recording; however, anoxia resulted in a 52% decrease in AMPAR currents. Pharmacological activation of mK(ATP) channels with diazoxide or levcromakalim resulted in a 46% decrease in normoxic AMPAR currents and the decrease was abolished with application of the antagonists 5-hydroxydecanoic acid and glibenclamide, whereas mK(ATP) antagonists blocked the anoxia-mediated decrease. Mitochondrial K(Ca) channel modulators responded similarly. The Ca(2+)-uniporter antagonist ruthenium red reduced AMPAR currents by 38% and was blocked with the agonist spermine. The calcium chelator BAPTA in the recording electrode during anoxia or diazoxide perfusion also abolished the reduction in AMPAR currents. We conclude that the mK(ATP) channel is involved in the anoxia-mediated down-regulation of AMPAR activity during anoxia and that it is a common mechanism to reduce glutamatergic excitability.

Adenosinergic modulation of neuronal activity in the pond snail Lymnaea stagnalis

Adenosine has been termed a retaliatory metabolite and its neuroprotective effects have been implicated in the hypoxia tolerance of several species; however, its role in the invertebrate CNS remains unclear. To determine if adenosine modulates neuronal activity in invertebrate neurons, we conducted whole-cell recordings from neurons in the central ring ganglia of the anoxia-tolerant pond snail Lymnaea stagnalis during exposure to adenosine and pharmacological compounds known to modulate the type I subclass of adenosine receptors (A1R). Action potential (AP) frequency and membrane potential (Vm) were unchanged under control conditions, and addition of adenosine decreased AP frequency by 47% (from 1.08±0.22 to 0.57±0.14 Hz) and caused significant hyperpolarization of Vm. The A1R agonist cyclopentyladenosine (CPA) mimicked the results obtained with adenosine whereas antagonism of the A1R with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) had no effect on AP frequency or Vm but prevented the adenosine and CPA-mediated decreases in neuronal activity. Furthermore, Ca2+ measurements with fluo-4 revealed that A1R activation led to a 12% increase in intracellular Ca2+ concentration and this elevation was also antagonized by DPCPX. Our results suggest that adenosine acting via the adenosine receptor (type I subclass) depresses neuronal activity in the adult L. stagnalis CNS and this depression is correlated with an increase in cytosolic Ca2+ levels.

Transcriptome analysis of the central nervous system of the mollusc Lymnaea stagnalis

Background

The freshwater snail Lymnaea stagnalis (L. stagnalis) has served as a successful model for studies in the field of Neuroscience. However, a serious drawback in the molecular analysis of the nervous system of L. stagnalis has been the lack of large-scale genomic or neuronal transcriptome information, thereby limiting the use of this unique model.

Results

In this study, we report 7,712 distinct EST sequences (median length: 847 nucleotides) of a normalized L. stagnalis central nervous system (CNS) cDNA library, resulting in the largest collection of L. stagnalis neuronal transcriptome data currently available. Approximately 42% of the cDNAs can be translated into more than 100 consecutive amino acids, indicating the high quality of the library. The annotated sequences contribute 12% of the predicted transcriptome size of 20,000. Surprisingly, approximately 37% of the L. stagnalis sequences only have a tBLASTx hit in the EST library of another snail species Aplysia californica (A. californica) even using a low stringency e-value cutoff at 0.01. Using the same cutoff, approximately 67% of the cDNAs have a BLAST hit in the NCBI non-redundant protein and nucleotide sequence databases (nr and nt), suggesting that one third of the sequences may be unique to L. stagnalis. Finally, using the same cutoff (0.01), more than half of the cDNA sequences (54%) do not have a hit in nematode, fruitfly or human genome data, suggesting that the L. stagnalis transcriptome is significantly different from these species as well. The cDNA sequences are enriched in the following gene ontology functional categories: protein binding, hydrolase, transferase, and catalytic enzymes.

Conclusion

This study provides novel molecular insights into the transcriptome of an important molluscan model organism. Our findings will contribute to functional analyses in neurobiology, and comparative evolutionary biology. The L. stagnalis CNS EST database is available at .

Neuronal membrane potential is mildly depolarized in the anoxic turtle cortex

Neuronal membrane potential (E(m)) regulates the activity of excitatory voltage-sensitive channels. Anoxic insults lead to a severe loss of E(m) and excitotoxic cell death (ECD) in mammalian neurons. Conversely, anoxia-tolerant freshwater turtle neurons depress energy usage during anoxia by altering ionic conductance to reduce neuronal excitability and ECD is avoided. This wholesale alteration of ion channel and pump activity likely has a significant effect on E(m). Using the whole-cell patch clamp technique we recorded changes in E(m) from turtle cortical neurons during a normoxic to anoxic transition in the presence of various ion channel/pump modulators. E(m) did not change with normoxic perfusion but underwent a reversible, mild depolarization of 8.1+/-0.2 mV following anoxic perfusion. This mild anoxic depolarization (MAD) was not prevented by the manipulation of any single ionic conductance, but was partially reduced by pre-treatment with antagonists of GABA(A) receptors (5.7+/-0.5 mV), cellular bicarbonate production (5.3+/-0.2 mV) or K(+) channels (6.0+/-0.2 mV), or by perfusion of reactive oxygen species scavengers (5.2+/-0.3 mV). Furthermore, all of these treatments induced depolarization in normoxic neurons. Together these data suggest that the MAD may be due to the summation of numerous altered ion conductance states during anoxia.

Mitochondrial ATP-sensitive K+ channels regulate NMDAR activity in the cortex of the anoxic western painted turtle

Hypoxic mammalian neurons undergo excitotoxic cell death, whereas painted turtle neurons survive prolonged anoxia without apparent injury. Anoxic survival is possibly mediated by a decrease in N-methyl-d-aspartate receptor (NMDAR) activity and maintenance of cellular calcium concentrations ([Ca(2+)](c)) within a narrow range during anoxia. In mammalian ischaemic models, activation of mitochondrial ATP-sensitive K(+) (mK(ATP)) channels partially uncouples mitochondria resulting in a moderate increase in [Ca(2+)](c) and neuroprotection. The aim of this study was to determine the role of mK(ATP) channels in anoxic turtle NMDAR regulation and if mitochondrial uncoupling and [Ca(2+)](c) changes underlie this regulation. In isolated mitochondria, the K(ATP) channel activators diazoxide and levcromakalim increased mitochondrial respiration and decreased ATP production rates, indicating mitochondria were 'mildly' uncoupled by 10-20%. These changes were blocked by the mK(ATP) antagonist 5-hydroxydecanoic acid (5HD). During anoxia, [Ca(2+)](c) increased 9.3 +/- 0.3% and NMDAR currents decreased 48.9 +/- 4.1%. These changes were abolished by K(ATP) channel blockade with 5HD or glibenclamide, Ca(2+)(c) chelation with 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) or by activation of the mitochondrial Ca(2+) uniporter with spermine. Similar to anoxia, diazoxide or levcromakalim increased [Ca(2+)](c) 8.9 +/- 0.7% and 3.8 +/- 0.3%, while decreasing normoxic whole-cell NMDAR currents by 41.1 +/- 6.7% and 55.4 +/- 10.2%, respectively. These changes were also blocked by 5HD or glibenclamide, BAPTA, or spermine. Blockade of mitochondrial Ca(2+)-uptake decreased normoxic NMDAR currents 47.0 +/- 3.1% and this change was blocked by BAPTA but not by 5HD. Taken together, these data suggest mK(ATP) channel activation in the anoxic turtle cortex uncouples mitochondria and reduces mitochondrial Ca(2+) uptake via the uniporter, subsequently increasing [Ca(2+)](c) and decreasing NMDAR activity.

AMPA receptors undergo channel arrest in the anoxic turtle cortex

Without oxygen, all mammals suffer neuronal injury and excitotoxic cell death mediated by overactivation of the glutamatergic N-methyl-D-aspartate receptor (NMDAR). The western painted turtle can survive anoxia for months, and downregulation of NMDAR activity is thought to be neuroprotective during anoxia. NMDAR activity is related to the activity of another glutamate receptor, the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR). AMPAR blockade is neuroprotective against anoxic insult in mammals, but the role of AMPARs in the turtle's anoxia tolerance has not been investigated. To determine whether AMPAR activity changes during hypoxia or anoxia in the turtle cortex, whole cell AMPAR currents, AMPAR-mediated excitatory postsynaptic potentials (EPSPs), and excitatory postsynaptic currents (EPSCs) were measured. The effect of AMPAR blockade on normoxic and anoxic NMDAR currents was also examined. During 60 min of normoxia, evoked peak AMPAR currents and the frequencies and amplitudes of EPSPs and EPSCs did not change. During anoxic perfusion, evoked AMPAR peak currents decreased 59.2 +/- 5.5 and 60.2 +/- 3.5% at 20 and 40 min, respectively. EPSP frequency (EPSP(f)) and amplitude decreased 28.7 +/- 6.4% and 13.2 +/- 1.7%, respectively, and EPSC(f) and amplitude decreased 50.7 +/- 5.1% and 51.3 +/- 4.7%, respectively. In contrast, hypoxic (Po(2) = 5%) AMPAR peak currents were potentiated 56.6 +/- 20.5 and 54.6 +/- 15.8% at 20 and 40 min, respectively. All changes were reversed by reoxygenation. AMPAR currents and EPSPs were abolished by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In neurons pretreated with CNQX, anoxic NMDAR currents were reversibly depressed by 49.8 +/- 7.9%. These data suggest that AMPARs may undergo channel arrest in the anoxic turtle cortex.

Anoxia-induced changes in reactive oxygen species and cyclic nucleotides in the painted turtle

The Western painted turtle survives months without oxygen. A key adaptation is a coordinated reduction of cellular ATP production and utilization that may be signaled by changes in the concentrations of reactive oxygen species (ROS) and cyclic nucleotides (cAMP and cGMP). Little is known about the involvement of cyclic nucleotides in the turtle's metabolic arrest and ROS have not been previously measured in any facultative anaerobes. The present study was designed to measure changes in these second messengers in the anoxic turtle. ROS were measured in isolated turtle brain sheets during a 40-min normoxic to anoxic transition. Changes in cAMP and cGMP were determined in turtle brain, pectoralis muscle, heart and liver throughout 4 h of forced submergence at 20-22 degrees C. Turtle brain ROS production decreased 25% within 10 min of cyanide or N(2)-induced anoxia and returned to control levels upon reoxygenation. Inhibition of electron transfer from ubiquinol to complex III caused a smaller decrease in [ROS]. Conversely, inhibition of complex I increased [ROS] 15% above controls. In brain [cAMP] decreased 63%. In liver [cAMP] doubled after 2 h of anoxia before returning to control levels with prolonged anoxia. Conversely, skeletal muscle and heart [cAMP] remained unchanged; however, skeletal muscle [cGMP] became elevated sixfold after 4 h of submergence. In liver and heart [cGMP] rose 41 and 127%, respectively, after 2 h of anoxia. Brain [cGMP] did not change significantly during 4 h of submergence. We conclude that turtle brain ROS production occurs primarily between mitochondrial complexes I and III and decreases during anoxia. Also, cyclic nucleotide concentrations change in a manner suggestive of a role in metabolic suppression in the brain and a role in increasing liver glycogenolysis.

Adaptive responses of vertebrate neurons to anoxia--matching supply to demand

Oxygen depleted environments are relatively common on earth and represent both a challenge and an opportunity to organisms that survive there. A commonly observed survival strategy to this kind of stress is a lowering of metabolic rate or metabolic depression. Whether metabolic rate is at a normal or a depressed level the supply of ATP (glycolysis and oxidative phosphorylation) must match the cellular demand for ATP (protein synthesis and ion pumping), a condition that must of course be met for long-term survival in hypoxic and anoxic environments. Underlying a decrease in metabolic rate is a corresponding decrease in both ATP supply and ATP demand pathways setting a new lower level for ATP turnover. Both sides of this equation can be actively regulated by second messenger pathways but it is less clear if they are regulated differentially or even sequentially with the onset of anoxia. The vertebrate brain is extremely sensitive to low oxygen levels yet some species can survive in oxygen depleted environments for extended periods and offer a working model of brain survival without oxygen. Hypoxia tolerant vertebrate brain will be the primary focus of this review; however, we will draw upon research involving hypoxia/ischemia tolerance mechanisms in liver and heart to offer clues to how brain can tolerate anoxia. The issue of regulating ATP supply or demand pathways will also be addressed with a focus on ion channel arrest being a significant mechanism to reduce ATP demand and therefore metabolic rate. Furthermore, mitochondria are ideally situated to serve as cellular oxygen sensors and mediator of protective mechanisms such as ion channel arrest. Therefore, we will also describe a mitochondria based mechanism of ion channel arrest involving ATP-sensitive mitochondrial K(+) channels, cytosolic calcium and reaction oxygen species concentrations.

Calcium and protein phosphatase 1/2A attenuate N-methyl-D-aspartate receptor activity in the anoxic turtle cortex

Excitotoxic cell death (ECD) is characteristic of mammalian brain following min of anoxia, but is not observed in the western painted turtle following days to months without oxygen. A key event in ECD is a massive increase in intracellular Ca(2+) by over-stimulation of N-methyl-d-aspartate receptors (NMDARs). The turtle's anoxia tolerance may involve the prevention of ECD by attenuating NMDAR-induced Ca(2+) influx. The goal of this study was to determine if protein phosphatases (PPs) and intracellular calcium mediate reductions in turtle cortical neuron whole-cell NMDAR currents during anoxia, thereby preventing ECD. Whole-cell NMDAR currents did not change during 80 min of normoxia, but decreased 56% during 40 min of anoxia. Okadaic acid and calyculin A, inhibitors of serine/threonine PP1 and PP2A, potentiated NMDAR currents during normoxia and prevented anoxia-mediated attenuation of NMDAR currents. Decreases in NMDAR activity during anoxia were also abolished by inclusion of the Ca(2+) chelator -- BAPTA and the calmodulin inhibitor -- calmidazolium. However, cypermethrin, an inhibitor of the Ca(2+)/calmodulin-dependent PP2B (calcineurin), abolished the anoxic decrease in NMDAR activity at 20, but not 40 min suggesting that this phosphatase might play an early role in attenuating NMDAR activity during anoxia. Our results show that PPs, Ca(2+) and calmodulin play an important role in decreasing NMDAR activity during anoxia in the turtle cortex. We offer a novel mechanism describing this attenuation in which PP1 and 2A dephosphorylate the NMDAR (NR1 subunit) followed by calmodulin binding, a subsequent dissociation of alpha-actinin-2 from the NR1 subunit, and a decrease in NMDAR activity.

Adenosine as a signal for ion channel arrest in anoxia-tolerant organisms

Certain freshwater turtles and fish are extremely anoxia-tolerant, capable of surviving hours of anoxia at high temperatures and weeks to months at low temperatures. There is great interest in understanding the cellular mechanisms underlying anoxia-tolerance in these groups because they are anoxia-tolerant vertebrates and because of the far-reaching medical benefits that would be gained. It has become clear that a pre-condition of prolonged anoxic survival must involve the matching of ATP production with ATP utilization to maintain stable ATP levels during anoxia. In most vertebrates, anoxia leads to a severe decrease in ATP production without a concomitant reduction in utilization, which inevitably leads to the catastrophic events associated with cell death or necrosis. Anoxia-tolerant organisms do not increase ATP production when faced with anoxia, but rather decrease utilization to a level that can be met by anaerobic glycolysis alone. Protein synthesis and ion movement across the plasma membrane are the two main targets of regulatory processes that reduce ATP utilization and promote anoxic survival. However, the oxygen sensing and biochemical signaling mechanisms that achieve a coordinated reduction in ATP production and utilization remain unclear. One candidate-signaling compound whose extracellular concentration increases in concert with decreasing oxygen availability is adenosine. Adenosine is known to have profound effects on various aspects of tissue metabolism, including protein synthesis, ion pumping and permeability of ion channels. In this review, I will investigate the role of adenosine in the naturally anoxia-tolerant freshwater turtle and goldfish and give an overview of pathways by which adenosine concentrations are regulated.

Effect of anoxia and pharmacological anoxia on whole-cell NMDA receptor currents in cortical neurons from the western painted turtle

The mammalian brain undergoes rapid cell death during anoxia that is characterized by uncontrolled Ca(2+) entry via N-methyl-D-aspartate receptors (NMDARs). In contrast, the western painted turtle is extremely anoxia tolerant and maintains close-to-normal [Ca(2+)](i) during periods of anoxia lasting from days to months. A plausible mechanism of anoxic survival in turtle neurons is the regulation of NMDARs to prevent excitotoxic Ca(2+) injury. However, studies using metabolic inhibitors such as cyanide (NaCN) as a convenient method to induce anoxia may not represent a true anoxic stress. This study was undertaken to determine whether turtle cortical neuron whole-cell NMDAR currents respond similarly to true anoxia with N(2) and to NaCN-induced anoxia. Whole-cell NMDAR currents were measured during a control N(2)-induced anoxic transition and a control NaCN-induced transition. During anoxia with N(2) normalized, NMDAR currents decreased to 35.3%+/-10.8% of control values. Two different NMDAR current responses were observed during NaCN-induced anoxia: one resulted in a 172%+/-51% increase in NMDAR currents, and the other was a decrease to 48%+/-14% of control. When responses were correlated to the two major neuronal subtypes under study, we found that stellate neurons responded to NaCN treatment with a decrease in NMDAR current, while pyramidal neurons exhibited both increases and decreases. Our results show that whole-cell NMDAR currents respond differently to NaCN-induced anoxia than to the more physiologically relevant anoxia with N(2).

Gap junctions do not underlie changes in whole-cell conductance in anoxic turtle brain

An acute reduction in cell membrane permeability could provide an effective strategy to prolong anoxic survival. A previous study has shown that in the western painted turtle whole-cell neuronal conductance (G(w)) decreases during anoxia, which may be mediated by the activation of adenosine A(1) receptors and calcium. Reduction in G(w) is thought to be the result of ion channel closure, but closure of gap junctions could also be responsible for this phenomenon. In our study, antibody staining of connexin 32 and 43 (Cx32 and Cx43) suggested the presence of gap junctional components in the turtle cortex. To examine if gap junctions were involved in the previously measured anoxic decrease in G(w), neuronal connectivity was assessed through the measurement of whole-cell capacitance (C(w)). Turtle cortical sheets were perfused with normoxic (95%O(2)/5%CO(2)), anoxic (95%N(2)/5%CO(2)), high calcium (4 mM) and adenosine (200 microm) artificial cerebral spinal fluid (aCSF). No significant change in C(w) was observed under any of the above conditions. However, during hypo-osmotic aCSF perfusion C(w) decreased significantly, with the lowest value of 50+/-10.4 pF (P<0.05) occurring at 30 min. To visualize changes in gap junction permeability lucifer yellow was loaded into turtle neurons during normoxic, anoxic, 0 calcium, hypo-osmotic, cold shock, (+)-isoproterenol, nitric oxide donor S-nitoso-acetyl penicillamine, and 8-bromo-guanosine 3',5'-cyclic monophosphate aCSF perfusion. Dye propagation was only observed in 3 of 20 cold shock experiments (4 degrees C). We conclude that gap junctions are not involved in the acute reduction in G(w) previously observed during anoxia and that our results support the hypothesis that ion channel arrest is involved.

Tissue-specific expression of inducible and constitutive Hsp70 isoforms in the western painted turtle

Expression of Hsp73 and Hsp72 in four tissues of the naturally anoxia-tolerant western painted turtle (Chrysemys picta) was investigated in response to a 24 h forced dive and following 1 h recovery. Of the tissues examined, brain and liver displayed approximately threefold and sevenfold higher basal Hsp73 expression than heart and skeletal muscle. Basal Hsp72 expression was relatively low in all tissues examined. After the 24 h forced dive and 1 h recovery, Hsp73 expression did not differ significantly from basal expression with the exception of liver, where expression decreased significantly after 1 h recovery. Hsp72 expression was unchanged in liver following a 24 h dive; however, it increased twofold in brain and threefold in heart and skeletal muscle. Dive-induced Hsp72 expression was found to correlate inversely with basal Hsp73 expression. Following 1 h recovery, Hsp72 expression was significantly elevated in all tissues above levels in dived animals. These data indicate a tissue-specific pattern of Hsp73 and Hsp72 expression in the western painted turtle during both unstressed and stressed conditions.

Succinate and alanine as anaerobic end-products in the diving turtle (Chrysemys picta bellii)

The western painted turtle is an extremely anoxia-tolerant vertebrate capable of tolerating blood lactate levels of 150-200 mM. Since lactate increases to such high levels, other fermentation end-products such as succinate and alanine, which have not been previously measured in this species, might also be expected to increase. Therefore, I measured turtle heart, liver, and blood concentrations of lactate, succinate, and alanine following a 28-day anoxic dive at 5 degrees C. Succinate and lactate concentrations increased significantly in all three compartments while alanine increased significantly in the liver only. Lactate was found to accumulate by a similar amount in all three compartments (66.4-80.5 micromol g or ml(-1) in the blood compartment) and was used as a reference to which alanine and succinate concentrations could be compared. Succinate and alanine levels increased by 2 and 0.9% of lactate in liver, approximately 0.3 and 0.04% of lactate in blood, and 0.6 and 0.07% of lactate in heart, respectively. The contribution of each to the total anoxic heat production was calculated and accounted for an additional 1.5% of the previously measured exothermic gap. I conclude that succinate and alanine concentrations do increase in the anoxic turtle but are minor anaerobic end-products.

Hypoxia-induced silencing of NMDA receptors in turtle neurons

Hypoxia-induced suppression of NMDA receptors (NMDARs) in western painted turtle (Chrysemys picta) cortical neurons may be critical for surviving months of anoxic dormancy. We report that NMDARs are silenced by at least three different mechanisms operating at different times during anoxia. In pyramidal neurons from cerebrocortex, 1-8 min anoxia suppressed NMDAR activity (Ca(2+) influx and open probability) by 50-60%. This rapid decrease in receptor activity was controlled by activation of phosphatase 1 or 2A but was not associated with an increase in [Ca(2+)](i). However, during 2 hr of anoxia, [Ca(2+)](i) in cerebrocortical neurons increased by 35%, and suppression of NMDARs was predicted by the increase of [Ca(2+)](i) and controlled by calmodulin. An additional mechanism of NMDAR silencing, reversible removal of receptors from the cell membrane, was found in cerebrocortex of turtles remaining anoxic at 3 degrees C for 3-21 d. When suppression of NMDARs was prevented with phosphatase inhibitors, tolerance of anoxia was lost. Silencing of NMDARs is thus critical to the remarkable ability of C. picta to tolerate life without oxygen.

Acute reduction in whole cell conductance in anoxic turtle brain

We tested the effect of anoxia, a "mimic" turtle artificial cerebrospinal fluid (aCSF) consisting of high Ca2+ and Mg2+ concentrations and low pH and adenosine perfusions, on whole cell conductance (G(w)) in turtle brain slices using a whole cell voltage-clamp technique. With EGTA in the recording electrode, anoxic or adenosine perfusions did not change Gw significantly (values range between 2.15 +/- 0.24 and 3.24 +/- 0.56 nS). However, perfusion with normoxic or anoxic mimic aCSF significantly decreased Gw. High [Ca2+] (4.0 or 7.8 mM) perfusions alone could reproduce the changes in Gw found with the mimic perfusions. With the removal of EGTA from the recording electrode, Gw decreased significantly during both anoxic and adenosine perfusions. The A1-receptor agonist N6-cyclopentyladenosine reduced Gw in a dose-dependent manner, whereas the A1-receptor specific antagonist 8-cyclopentyl-1,3-dipropylxanthine blocked both the adenosine- and anoxic-mediated changes in Gw. These data suggest a mechanism involving A1-receptor-mediated changes in intracellular [Ca2+] that result in acute changes in Gw with the onset of anoxia.

Adaptations of vertebrate neurons to hypoxia and anoxia: maintaining critical Ca2+ concentrations

Down-regulation of ion channel activity ('channel arrest'), which aids in preserving critical ion gradients in concert with greatly diminished energy production, is one important strategy by which anoxia-tolerant neurons adapt to O2 shortage. Channel arrest results in the elimination of action potentials and neurotransmission and also decreases the need for ion transport, which normally requires a large energy expenditure. Important targets of this down-regulation may be channels in which activity would otherwise result in the toxic increases in intracellular [Ca2+] characteristic of anoxia-sensitive mammalian neurons. In turtles, Na+ channels and the Ca2+-permeable ion channel of the N-methyl-d-aspartate (NMDA)-type glutamate receptor undergo down-regulation during anoxia. Inactivation of NMDA receptors during hypoxia occurs by a variety of mechanisms, including alterations in the phosphorylation state of ion channel subunits, Ca2+-dependent second messenger activation, changes in Ca2+-dependent polymerization/depolymerization of actin to postsynaptic receptors and activation of other G-protein-coupled receptors. Release of inhibitory neurotransmitters (e.g. gamma-aminobutyrate) and neuromodulators (e.g. adenosine) into the brain extracellular fluids may play an important role in the down-regulation of these and other types of ion channels.

Adenosine and anoxia reduce N-methyl-D-aspartate receptor open probability in turtle cerebrocortex

During normoxia, glutamate and the glutamate family of ion channels play a key role in mediating rapid excitatory synaptic transmission in the central nervous system. However, during hypoxia, intracellular [Ca2+] increases to neurotoxic levels, mediated largely by the N-methyl-D-aspartate (NMDA) subfamily of glutamate receptors. Adenosine has been shown to decrease the magnitude of the hypoxia-induced increase in [Ca2+]i in mammalian brain slices, delaying tissue injury. Turtle brain is remarkably tolerant of anoxia, maintaining a pre-anoxic [Ca2+]i while cerebral adenosine levels increase 12-fold. Employing cell-attached single-channel patch-clamp techniques, we studied the effect of adenosine (200 micromol l-1) and anoxia on NMDA receptor open probability (Popen) and current amplitude. After 60 min of anoxic perfusion, channel Popen decreased by 65 % (from 6.8+/-1.6 to 2.4+/-0.8 %) an effect that could also be achieved with a normoxic perfusion of 200 micromol l-1 adenosine (Popen decreased from 5.8+/-1.1 to 2.3+/-1.2 %). The inclusion of 10 micromol l-1 8-phenyltheophylline, an A1 receptor blocker, prevented the adenosine- and anoxia-induced decrease in Popen. Mean single-channel current amplitude remained at approximately 2.7+/-0.23 pA under all experimental conditions. To determine whether a change in the membrane potential could be part of the mechanism by which Popen decreases, membrane and threshold potential were measured following each experiment. Membrane potential did not change significantly under any condition, ranging from -76.8 to -80.6 mV. Therefore, during anoxia, NMDA receptors cannot be regulated by Mg2+ in a manner dependent on membrane potential. Threshold potentials did decrease significantly following 60 min of anoxic or adenosine perfusion (control -33.3+/-1.9 mV, anoxia -28.4+/-1.5 mV, adenosine -23.4+/-2.8 mV). We conclude that anoxia modulates NMDA receptor activity and that adenosine plays a key role in mediating this change. This is the first direct measurement of ion channel activity in anoxic turtle brain and demonstrates that ion channel regulation is part of the naturally evolved anoxic defence mechanism of this species.

Oxygen sensing and signal transduction in metabolic defense against hypoxia: lessons from vertebrate facultative anaerobes

Earlier studies identified two main defense strategies against hypoxia in hypoxia tolerant animals: (1) reduction in energy turnover, and (2) improved energetic efficiency of those metabolic processes that remain. We used two model systems from the highly anoxia-tolerant aquatic turtle: (1) tissue slices of brain cortex (to probe cell level electrophysiological responses to oxygen limitation), and (2) isolated liver hepatocytes (to probe signalling and defense). In the latter, a cascade of processes underpinning hypoxia defense begins with an oxygen sensor that is probably a heme protein and a signal transduction pathway that leads to the specific activation of some genes (increased expression of several proteins) and to specific down-regulation of other genes (decreased expression of several other proteins). The pathway seems to have characteristics in common with oxygen-regulated control elements in other cells. The probable roles of the oxygen sensing and signal transduction system include coordinate down-regulation of energy demand and energy supply pathways in metabolism. Because of this coordination, hypoxia tolerant cells stay in energy balance even as they down-regulate to extremely low levels of ATP turnover. The main ATP-demanding processes in normoxia (protein synthesis, protein degradation, glucose synthesis, urea synthesis and maintenance of electrochemical gradients) are all turned down to variable degrees during anoxia or extreme hypoxia. Most striking is the observation that ion pumping is the main energy sink in anoxia-despite reductions in cell membrane permeability ("channel arrest"). Neurons also show a much lower permeability than do homologous mammalian cells but, in this case under acute anoxia, there is no further change in cell membrane conductivity. We consider that, through this recent work, it is becoming evident how normoxic maintenance ATP turnover rates can be down-regulated by an order of magnitude or more-to a new hypometabolic steady state that is prerequisite for surviving prolonged hypoxia or anoxia. The implications of these developments extend to many facets of biology and medicine.

Interactive effects of pH and temperature on N-methyl-D-aspartate receptor activity in rat cortical brain slices

Low extracellular pH decreases the activity of the N-methyl-D-aspartate (NMDA) glutamate receptor, and may thus limit neuronal calcium overload during cerebral ischemia. During induced hypothermia, alkaline pH ("alphastat regulation") is often used to preserve cardiac and enzymatic function. The purpose of this study is to measure the functional activity of cerebral cortex NMDA receptors over the range of temperatures used in profound hypothermic cardiopulmonary bypass (20-37 degrees C). Extracellular pH was varied over a broad range relevant to both alphastat and pH stat acid-base management (7.0-7.8). Change in cytosolic free calcium evoked by 50 microM NMDA in brain slices was used as an index of NMDA receptor activity. Cortical slices (300 microns thick) were loaded with fura-2 Aspartate Methyl for study in a fluorometer. At 37 degrees C, a change in extracellular pH from 7.1 to 7.8 increased the NMDA-evoked change in cytosolic calcium in brain slices by a factor of 4 (p < 0.05). In contrast, at 20 degrees C there was minimal effect of changing extracellular pH from 7.1 to 7.8 (27% increase). We conclude that hypothermia results in decreased pH sensitivity of the NMDA receptor. The results predict that different strategies of pH management during induced hypothermia may have limited impact on NMDA receptor-mediated processes, such as neuronal calcium overload.

Effects of fructose-1,6-bisphosphate on glutamate release and ATP loss from rat brain slices during hypoxia

Fructose-1,6-bisphosphate (FBP), an intermediate of glucose metabolism, is neuroprotective in brain hypoxia or ischemia. Because the mechanisms for this protection are not clear, we examined the effects of FBP on two important events in brain ischemia, i.e., loss of ATP and release of the excitatory neurotransmitter glutamate. Glutamate release from cortical brain slices was measured fluorometrically (glutamate dehydrogenase-catalyzed conversion of glutamate to alpha-ketoglutarate) during hypoxia (PO2 15 mm Hg) or hypoxia plus 100 microM cyanide. FBP (3.5 mM, with glucose 20 mM) reduced glutamate release during hypoxia by 55% and during hypoxia/cyanide by 46% (p < 0.005), and prevented a significant fall in [ATP]. [ATP] was maintained in oxygenated glucose-free conditions with 20 but not 3.5 mM FBP, and fell to < 20% of normal with hypoxia. Despite the drop in [ATP], 3.5 or 20 mM FBP without glucose decreased hypoxia-evoked glutamate release. We conclude (1) FBP present without glucose preserves normal [ATP] only when oxygen is available, suggesting limited uptake and metabolism; and (2) FBP decreases hypoxia-evoked glutamate release by processes independent of [ATP]. These results suggest protective actions of FBP that are separate from augmentation of anaerobic energy production, as previously proposed.

Unifying theory of hypoxia tolerance: molecular/metabolic defense and rescue mechanisms for surviving oxygen lack

We develop a unifying theory of hypoxia tolerance based on information from two cell level models (brain cortical cells and isolated hepatocytes) from the highly anoxia tolerant aquatic turtle and from other more hypoxia sensitive systems. We propose that the response of hypoxia tolerant systems to oxygen lack occurs in two phases (defense and rescue). The first lines of defense against hypoxia include a balanced suppression of ATP-demand and ATP-supply pathways; this regulation stabilizes (adenylates) at new steady-state levels even while ATP turnover rates greatly decline. The ATP demands of ion pumping are down-regulated by generalized "channel" arrest in hepatocytes and by "spike" arrest in neurons. Hypoxic ATP demands of protein synthesis are down-regulated probably by translational arrest. In hypoxia sensitive cells this translational arrest seems irreversible, but hypoxia-tolerant systems activate "rescue" mechanisms if the period of oxygen lack is extended by preferentially regulating the expression of several proteins. In these cells, a cascade of processes underpinning hypoxia rescue and defense begins with an oxygen sensor (a heme protein) and a signal-transduction pathway, which leads to significant gene-based metabolic reprogramming-the rescue process-with maintained down-regulation of energy-demand and energy-supply pathways in metabolism throughout the hypoxic period. This recent work begins to clarify how normoxic maintenance ATP turnover rates can be drastically (10-fold) down-regulated to a new hypometabolic steady state, which is prerequisite for surviving prolonged hypoxia or anoxia. The implications of these developments are extensive in biology and medicine.

Volatile and intravenous anesthetics decrease glutamate release from cortical brain slices during anoxia

BACKGROUND

Extracellular accumulation of the excitatory neurotransmitter L-glutamate during cerebral hypoxia or ischemia contributes to neuronal death. Anesthetics inhibit release of synaptic neurotransmitters but it is unknown if they alter net extrasynaptic glutamate release, which accounts for most of the glutamate released during hypoxia or ischemia. The purpose of this study was to determine if different types of anesthetics decrease hypoxia-induced glutamate release from rat brain slices.

METHODS

Glutamate released from cortical brain slices was measured fluorometrically with the glutamate dehydrogenase catalyzed formation of the reduced form of nicotinamide adenine dinucleotide phosphate. Glutamate release was measured in oxygenated (PO2 = 400 mmHg), hypoxic ((PO2 = 20 mmHg), and anoxic ((PO2 = 20 mmHg plus 100 microM NaCN) solutions and with clinical concentrations of anesthetics (halothane 325 microM, enflurane 680 microM, propofol 200 microM, sodium thiopental 50 microM). The source of glutamate released during these stresses was defined with toxins inhibiting N and P type voltage-gated calcium channels, and with calcium-free medium.

RESULTS

Glutamate released during hypoxia or anoxia was 1.5 and 5.3 times greater, respectively, than that evoked by depolarization with 30 mM KCl. Hypoxia/anoxia-induced glutamate release was not mediated by synaptic voltage-gated calcium channels, but probably by the reversal of normal uptake mechanisms. Halothane, enflurane, and sodium thiopental, but not propofol, decreased hypoxia-evoked glutamate release by 50-70% (P < 0.05). None of the anesthetics alter basal glutamate release.

CONCLUSIONS

The authors conclude that halothane, enflurane, and sodium thiopental but not propofol, at clinical concentrations, decrease extrasynaptic release of L-glutamate during hypoxic stress.

Role of adenosine in NMDA receptor modulation in the cerebral cortex of an anoxia-tolerant turtle (Chrysemys picta belli)

Accumulation of the neuromodulator adenosine in the anoxia-tolerant turtle brain may play a key role in a protective decrease in excitatory neurotransmission during anoxia. Since excitatory neurotransmission is mediated largely by Ca2+ entry through N-methyl-D-aspartate (NMDA) receptors, we measured the effect of adenosine on NMDA-mediated Ca2+ transients in normoxic and anoxic turtle cerebrocortical sheets. Intracellular [Ca2+] was measured fluorometrically with the Ca2+-sensitive dye Fura-2. Baseline intracellular [Ca2+] and [ATP] were also measured to assess cortical sheet viability and potential toxic effects of NMDA. Baseline [Ca2+] did not change significantly under any condition, ranging from 109 +/- 22 to 187 +/- 26 nmoll-1. Throughout normoxic and 2h anoxic protocols, and after single and multiple NMDA exposures, [ATP] did not change significantly, ranging from 16.0 +/- 1.9 to 25.3 +/- 4.9 nmol ATP mg-1 protein. Adenosine caused a reduction in the normoxic NMDA-mediated increase in [Ca2+] from a control level of 287 +/- 35 to 103 +/- 22 nmoll-1 (64%). This effect is mediated by the A1 receptor since 8-phenyltheophylline (a specific A1 antagonist) effectively blocked the adenosine effect and N6-cyclopentyladenosine (a specific A1 agonist) elicited a similar decrease in the NMDA-mediated response. Cortical sheets exposed to anoxia alone exhibited a 52% decrease in the NMDA-mediated [Ca2+] rise, from 232 +/- 30 to 111 +/- 9 nmoll-1. The addition of adenosine had no further effect and 8-phenyltheophylline did not antagonize the observed decrease. Therefore, the observed down-regulation of NMDA receptor activity during anoxia must involve additional, as yet unknown, mechanisms.

Effects of isoflurane and hypothermia on glutamate receptor-mediated calcium influx in brain slices

BACKGROUND

To understand how volatile anesthetics protect neurons during cerebral ischemia, we studied the effects of isoflurane on cerebral glutamate receptor-mediated calcium influx. Calcium influx via these key excitatory receptors may mediate pain transmission, memory, and the pathophysiologic sequelae of cerebral anoxia or ischemia. Because cerebral protection by hypothermia may involve a decrease in glutamate receptor activity, we also examined the interaction of temperature and isoflurane on glutamate receptor inhibition.

METHODS

We measured glutamate receptor-mediated changes in cytosolic calcium in 300-microns-thick rat cortical brain slices. Temperature was varied to 28, 34, 37, or 39 degrees C and isoflurane partial pressure to 0.016-0.019 atm (equivalent to 1.16 minimum alveolar concentration [MAC], adjusted for temperature and age). Brain slices were loaded with fura-2 to permit measurement of cytosolic free calcium. Calcium changes due to the glutamate receptor agonist N-methyl-D-aspartate (NMDA) (50 microM), to ischemia levels of L-glutamate (1.0 mM) or to simulated ischemia (1.0 mM glutamate, 100 microM NaCN, and 3.5 mM iodoacetate) was then measured. Slice lactate dehydrogenase leakage and adenosine triphosphate were measured as indices of cellular integrity.

RESULTS

Isoflurane reduced both L-glutamate and NMDA-mediated calcium fluxes by approximately 60%. Neither the activity of the NMDA receptor nor its inhibition by isoflurane was altered by temperature. The rate of calcium influx during ischemia was significantly reduced both by temperature and by isoflurane (P < 0.05). Adenosine triphosphate loss and lactate dehydrogenase leakage were reduced by isoflurane during simulated ischemia by 37% and 73% (P < 0.05), respectively.

CONCLUSIONS

(1) At 1.16 MAC, isoflurane potently inhibits glutamate receptors and delays cellular injury induced by simulated ischemia, and (2) hypothermia does not reduce the intrinsic activity of cortical glutamate receptors but delays calcium accumulation during simulated ischemia. Isoflurane reduces the severity of key pathophysiologic events in an in vitro model of simulated cerebral ischemia.

Substrate and acute temperature effects on turtle heart and liver mitochondria

The oxidative properties of heart and liver mitochondria from the Western painted turtle (Chrysemys picta bellii) were characterized on the basis of substrate preferences and temperature sensitivity. Turtle heart and liver mitochondria oxidize most substrates at 2- and 10-fold less, respectively, than rates obtained from the corresponding rat mitochondria. Krebs cycle intermediates, ketone bodies, and glutamate were oxidized at similarly high rates by turtle heart mitochondria (70.0-121.2 nmol O.min-1.mg protein-1). Fatty acylcarnitines were oxidized at approximately one-half of the above rates, and rates of amino acid oxidation were either not detectable or very low. Heart mitochondria oxidize ketone bodies at rates as high as pyruvate plus malate. Liver mitochondria oxidized Krebs cycle intermediates, amino acids, and fatty acids at similarly low rates (3.0-8.0 nmol O.min-1.mg protein-1). Typically, succinate was oxidized at the highest rates, 20.6 +/- 4.0 and 121.2 +/- 1.2 nmol O.min-1.mg protein-1, for liver and heart, respectively. Values for the rate of change of oxidation with a 10 degrees C increase (Q10) from heart and liver mitochondria oxidizing glutamate were calculated over the range 25-5 degrees C in 5 degrees C intervals; over the range 10-5 degrees C, Q10 values were 3.2 and 19.8, respectively. Q10 values calculated over the higher temperature intervals were lower. It is suggested that the large difference in temperature sensitivity between mitochondria from these two tissues and the ability of the heart mitochondria to oxidize ketone bodies at high rates are adaptations to recover from the long anoxic overwintering bouts experienced by this species.

Anoxic suppression of Na(+)-K(+)-ATPase and constant membrane potential in hepatocytes: support for channel arrest

The maintenance of ion gradients across the plasma membrane by the Na(+)-K(+)-ATPase has been shown to utilize a large fraction of the total cellular energy demand. In view of the importance of ion gradients to cellular function, and the remarkable anoxia tolerance of Chrysemys picta bellii (western painted turtle) and hepatocytes isolated from this species, it was of interest to determine if in response to anoxia 1) ion gradients were maintained and 2) if the activity of the plasma membrane Na(+)-K(+)-ATPase changed to aid in ion gradient maintenance. From normoxic hepatocyte suspensions the ouabain-inhibitable 86Rb+ uptake (a measure of Na(+)-K(+)-ATPase activity) was determined, and the rate of ATP utilization was 19.1 mumol ATP.g cells-1.h-1 or 28% of the total normoxic cellular ATP turnover. In response to anoxic incubation the activity of the pump decreased by 75% to 4.8 mumol ATP.g cells-1.h-1 and this comprised 74% of the total anoxic ATP turnover. Presently, it is not known whether the observed reduction in Na(+)-K(+)-ATPase activity is regulated by 1) allosteric modification, 2) endocytosis from the membrane, or 3) reduced Na+ influx. Plasma membrane potential was measured during anoxia, using the distribution of 36Cl-, and was not significantly different from the normoxic measurement, -30.6 +/- 3.9 and -31.3 +/- 5.8 mV, respectively. Therefore, the plasma membrane ion gradient is maintained during anoxia, and since the activity of the Na(+)-K(+)-ATPase decreases, the influx of ions must also decrease.(ABSTRACT TRUNCATED AT 250 WORDS)

Microcalorimetric measurement of reversible metabolic suppression induced by anoxia in isolated hepatocytes

The metabolic suppression due to anoxia in hepatocytes from the anoxia-tolerant turtle Chrysemys picta bellii was measured directly using microcalorimetric techniques. The normoxic heat flux from hepatocytes in suspension (25 degrees C) was 1.08 +/- 0.08 mW/g cells and decreased by 76% to 0.26 +/- 0.03 mW/g cells in response to anoxic incubation. After an acute decrease in temperature (to 10 degrees C) anoxic heat flux dropped by 96% relative to the normoxic control at 25 degrees C. The relative decrease in heat flux at both temperatures was similar, 76% at 25 degrees C and 68% at 10 degrees C. From the caloric equivalent of glycogen fermentation to lactate the heat flux from lactate production was calculated to be -93 microW/g cells (25 degrees C), and this accounted for 36% of the anoxic heat flux. When the enthalpy change associated with the release of free glucose (from glycogen breakdown) is considered, an additional 6% of the anoxic heat flux can be accounted for. Therefore, a portion of the anoxic heat flux is unaccounted for (58%), resulting in an "exothermic gap." This differs from the normoxically incubated hepatocytes where the indirect calorimetric measurement of heat flux (hepatocyte O2 consumption) could fully account for the calorimetrically measured heat flux. When normoxic hepatocytes were inhibited with cyanide, a rapid suppression in heat flux was observed. Because rapid reequilibration to a lower, cyanide-induced steady state occurred in < 15 min, it is also assumed that there is no short-term Pasteur effect in this tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of protein synthesis to anoxia and recovery in anoxia-tolerant hepatocytes

Hepatocytes from the western painted turtle (Chrysemys picta bellii) display a profound metabolic suppression under anoxia. Fractional rates of protein synthesis fell by 92% during 12 h anoxia at 25 degrees C and were indistinguishable from the rate obtained with cycloheximide. Normoxic recovery saw protein synthesis increase to 160% of control values and return to normal after 2 h. The GTP-to-GDP ratio, implicated in the control of translation, fell threefold during anoxia. Purine nucleotide phosphate profiles suggest that this change occurs through increasing concentrations of ADP and GDP, with concentrations of ATP and GTP and total purines remaining constant. The normoxic cost for protein synthesis was calculated at 47.6 +/- 6.8 mmol ATP/g protein. Normoxic protein synthesis accounted for 36% of overall ATP turnover rates, close to the extent of O2 consumption inhibitable by cycloheximide (28%). Under anoxia, the proportion of ATP turnover utilized by protein synthesis did not change significantly. ATP turnover rates for urea synthesis reflected a similar pattern, falling 72% under anoxia. These results reflect the cell's ability to suppress protein synthesis under anoxia in a manner that is coordinated with the reduction in total metabolic rate.

Anoxia-tolerant hepatocytes: model system for study of reversible metabolic suppression

Chrysemys picta bellii is well known for its ability to survive extended anoxic periods and has been widely used as a model system to study anoxic metabolism. Described here is a method for the isolation of anoxia-tolerant hepatocytes from this species. Freshly isolated hepatocytes were determined to be viable based on trypan blue exclusion, gluconeogenic capacity from [14C]lactate, responsiveness to epinephrine and glucagon, and maintenance of cellular adenylate concentrations. Under anoxic conditions for 10 h there was no significant increase in cell staining and no decrease in cellular ATP concentration. Furthermore, the addition of cyanide at the 5-h mark did not result in any significant differences in these parameters; however, iodoacetate added at this time caused trypan blue staining to increase and ATP concentrations to fall. The rate of glucose production from the cells was threefold greater under anoxic than normoxic conditions, underscoring the important role of the liver in supplying substrate during anoxia. From the rate of O2 consumption and rate of lactate production under anaerobic conditions, ATP turnover rates were calculated to be 68.4 +/- 7.2 and 6.5 +/- 0.43 mumol ATP.g-1.h-1, respectively; this corresponds to a 90% decrease in metabolic rate during anoxia. Within a cellular system such as this the more complex regulatory mechanisms involved in a large coordinated reduction in metabolism can be probed.

Mitochondrial and peroxisomal fatty acid oxidation in elasmobranchs

In heart and red muscle of dogfish (Squalus acanthias), the maximal activities of the fatty acid catabolizing enzyme carnitine palmitoyltransferase (CPT) are less than 5% the rate in the same tissues of teleosts (carp, Cyprinus carpio; trout, Salmo gairdneri). CPT activities in these tissues of hagfish (Eptatretus stouti) are approximately 10% the rate in teleosts. However, the maximal activities of the beta-oxidation enzyme beta-hydroxyacyl-CoA dehydrogenase (HOAD) in dogfish red muscle and heart are similar to these tissues in the other species. This paradox prompted a more detailed study on the capacity of mitochondria from dogfish cardiac and red skeletal muscles to utilize fatty acids, possibly by a CPT-independent pathway. Free fatty acids were not oxidized by mitochondria from red muscle (hexanoate, octanoate, decanoate, and palmitate) or from heart (octanoate, palmitate). Neither hyposmotic incubation nor addition of 5 mM ATP could stimulate oxidation of octanoate or palmitate in either preparation, suggesting that these tissues have little capacity to oxidize fatty acids by a carnitine-independent pathway. Palmitoyl carnitine oxidation was detectable at very low rates in these mitochondria only with hyposmotic incubation. Octanoyl carnitine was oxidized at greater rates than palmitoyl carnitine, 10% the rate of pyruvate in both tissues, suggesting that medium-chain fatty acids could be physiologically relevant fuels in elasmobranchs if available to heart and red muscle. One potential source of medium-chain fatty acids is hepatic peroxisomal beta-oxidation, which occurs in dogfish liver at maximal activities similar to carp and trout liver. However, based on relative rates of oxidation, it is likely that dogfish heart and red muscle metabolism are fueled primarily by carbohydrate and ketone bodies.

Oxidative properties of carp red and white muscle

Substrate preferences of isolated mitochondria and maximal enzyme activities were used to assess the oxidative capacities of red muscle (RM) and white muscle (WM) of carp (Cyprinus carpio). A 14-fold higher activity of citrate synthase (CS) in RM reflects the higher mitochondrial density in this tissue. RM mitochondria oxidize pyruvate and fatty acyl carnitines (8:O, 12:O, 16:O) at similarly high rates. WM mitochondria oxidize these fatty acyl carnitines at 35–70% the rate of pyruvate, depending on chain length. WM has only half the carnitine palmitoyl transferase/CS ratio of RM, but similar ratios of beta-hydroxyacyl CoA dehydrogenase/CS. Ketone bodies are poor substrates for mitochondria from both tissues. In both tissues mitochondrial alpha-glycerophosphate oxidation was minimal, and alpha-glycerophosphate dehydrogenase was present at low activities, suggesting the alpha-glycerophosphate shuttle is of minor significance in maintaining cytosolic redox balance in either tissue. The mitochondrial oxidation rates of other substrates relative to pyruvate are as follows: alpha-ketoglutarate 90% (RM and WM); glutamate 45% (WM) and 70% (RM); proline 20% (WM) and 45% (RM). Oxidation of neutral amino acids (serine, glycine, alanine, beta-alanine) was not consistently detectable. These data suggest that RM and WM differ in mitochondrial properties as well as mitochondrial abundance. Whereas RM mitochondria appear to be able to utilize a wide range of metabolic fuels (fatty acids, pyruvate, amino acids but not ketone bodies), WM mitochondria appear to be specialized to use pyruvate.