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

Mechanism of action and neuroprotective role of nicorandil in ischemic stroke

Nicorandil is a dual mechanism anti-anginal agent that acts as a nitric oxide (NO) donor and a potassium (K+) channel opener. Recent studies have evaluated the effect of nicorandil on ischemic stroke. Neurons have a low tolerance to hypoxia and therefore the brain tissue is significantly vulnerable to ischemia. Current approved treatments for ischemic stroke are tissue plasminogen activators and clot retrieval methods. The narrow therapeutic time window and lack of efficacy in restoring the dying neurons urge researchers to develop an alternative approach. In the terminal stages of anoxia, K+ channels induce hyperpolarization in various types of neuronal cells, leading to decreased neuronal activity and the preservation of the brain's energy. Nicorandil can open these K+ channels and sustain the hyperpolarization phase, which may have a neuroprotective effect during hypoxia. Additionally, we review how nicorandil can improve overall stroke outcomes through its anti-inflammatory, anti-oxidative, and edema-reducing effects. One of the major components evaluated in stroke patients is blood pressure. Studies have demonstrated that the effect of nicorandil on blood pressure is related to both its K+ channel opening and NO donating mechanisms. Since both hypertension and hypotension need correction before stroke intervention, it's crucial to consider the role of nicorandil and its impact on blood pressure. Previously published studies indicate that the right dosage of nicorandil can improve cerebral blood flow without significant changes in hemodynamic profiles. In this review, we discuss how nicorandil may contribute to better stroke outcomes based on previously published literature and laboratory findings.

Invited review - the effects of anthropogenic abiotic stressors on the sensory systems of fishes

Climate change is a growing global issue with many countries and institutions declaring a climate state of emergency. Excess CO₂ from anthropogenic sources and changes in land use practices are contributing to many detrimental changes, including increased global temperatures, ocean acidification and hypoxic zones along coastal habitats. All senses are important for aquatic animals, as it is how they can perceive and respond to their environment. Some of these environmental challenges have been shown to impair their sensory systems, including the olfactory, visual, and auditory systems. While most of the research is focused on how ocean acidification affects olfaction, there is also evidence that it negatively affects vision and hearing. The effects that temperature and hypoxia have on the senses have also been investigated, but to a much lesser extent in comparison to ocean acidification. This review assembles the known information on how these anthropogenic challenges affect the sensory systems of fishes, but also highlights what gaps in knowledge remain with suggestions for immediate action. Olfaction, vision, otolith, pH, freshwater, seawater, marine, central nervous system, electrophysiology, mechanism.

Goldfish and crucian carp are natural models of anoxia tolerance in the retina

Vertebrates need oxygen to survive. The central nervous system has an especially high energy demand, so brain and retinal neurons quickly die in anoxia. But fish of the genus Carassius are exceptionally anoxia-tolerant: the crucian carp (C. carassius) can survive months without oxygen in ice-covered ponds, and the common goldfish (C. auratus) can withstand hours of anoxia at room temperature. These fish previously offered insights into anoxia tolerance in the brain, heart, and liver. Here, we advance Carassius spp. as models to study anoxia tolerance in the retina. Electroretinogram and evoked potential recordings show that crucian carp reversibly downregulate their visual systems in anoxia, probably to save ATP. Notably, Carassius suppress their visual systems nearly twice as much as anoxia-tolerant turtles, Trachemys and Chrysemys spp., which are often promoted as the champions of anoxia tolerance. We summarize what is known about anoxia tolerance in the goldfish and crucian carp retinas, including cellular pathways which may protect retinal neurons from excitotoxic cell death. We compare the Carassius retina with two relevant models: natural anoxia tolerance in the turtle brain, and ischemic preconditioning in the rat retina. All three models include mitochondria as oxygen sensors: mitochondria depolarize due to mitochondrial ATP-dependent K⁺ channels, possibly to trigger neuroprotective second messenger cascades. The Carassius retina is an accessible and inexpensive model, with over 70 fruitful years of history in vision research. As a model for anoxia tolerance, it may provide new insights into diseases of the eye (like diabetes, macular degeneration, and eye stroke).

Molecular and Cellular Response of the Myocardium (H9C2 Cells) Towards Hypoxia and HIF-1α Inhibition

Introduction

Oxidative phosphorylation is an essential feature of Animalian life. Multiple adaptations have developed to protect against hypoxia, including hypoxia-inducible-factors (HIFs). The major role of HIFs may be in protecting against oxidative stress, not the preservation of high-energy phosphates. The precise mechanism(s) of HIF protection is not completely understood.

Materials and Methods

To better understand the role of hypoxia-inducible-factor-1, we exposed heart/myocardium cells (H9c2) to both normoxia and hypoxia, as well as cobalt chloride (prolyl hydroxylase inhibitor), echniomycin (HIF inhibitor), A2P (anti-oxidant), and small interfering RNA to beclin-1. We measured cell viability, intracellular calcium and adenosine triphosphate, NADP/NADPH ratios, total intracellular reactive oxidative species levels, and markers of oxidative and antioxidant levels measured.

Results

Hypoxia (1%) leads to increased intracellular Ca2+ levels, and this response was inhibited by A2P and echinomycin (ECM). Exposure of H9c2 cells to hypoxia also led to an increase in both mRNA and protein expression for Cav 1.2 and Cav 1.3. Exposure of H9c2 cells to hypoxia led to a decrease in intracellular ATP levels and a sharp reduction in total ROS, SOD, and CAT levels. The impact of hypoxia on ROS was reversed with HIF-1 inhibition through ECM. Exposure of H9c2 cells to hypoxia led to an increase in Hif1a, VEGF and EPO protein expression, as well as a decrease in mitochondrial DNA. Both A2P and ECM attenuated this response to varying degrees.

Conclusion

Hypoxia leads to increased intracellular Ca2+, and inhibition of HIF-1 attenuates the increase in intracellular Ca2+ that occurs with hypoxia. HIF-1 expression leads to decreased adenosine triphosphate levels, but the role of HIF-1 on the production of reactive oxidative species remains uncertain. Anti-oxidants decrease HIF-1 expression in the setting of hypoxia and attenuate the increase in Ca2+ that occurs during hypoxia (with no effect during normoxia). Beclin-1 appears to drive autophagy in the setting of hypoxia (through ATG5) but not in normoxia. Additionally, Beclin-1 is a powerful driver of reactive oxidative species production and plays a role in ATP production. HIF-1 inhibition does not affect autophagy in the setting of hypoxia, suggesting that there are other drivers of autophagy that impact beclin-1.

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.

Mitochondrial KATP channels stabilize intracellular Ca2+ during hypoxia in retinal horizontal cells of goldfish (Carassius auratus)

Neurons of the retina require oxygen to survive. In hypoxia, neuronal ATP production is impaired, ATP-dependent ion pumping is reduced, transmembrane ion gradients are dysregulated, and intracellular Ca2+ concentration ([Ca2+]i) increases enough to trigger excitotoxic cell death. Central neurons of the common goldfish (Carassius auratus) are hypoxia tolerant, but little is known about how goldfish retinas withstand hypoxia. To study the cellular mechanisms of hypoxia tolerance, we isolated retinal interneurons (horizontal cells; HCs), and measured [Ca2+]i with Fura-2. Goldfish HCs maintained [Ca2+]i throughout 1 h of hypoxia, whereas [Ca2+]i increased irreversibly in HCs of the hypoxia-sensitive rainbow trout (Oncorhynchus mykiss) with just 20 min of hypoxia. Our results suggest mitochondrial ATP-dependent K+ channels (mKATP) are necessary to stabilize [Ca2+]i throughout hypoxia. In goldfish HCs, [Ca2+]i increased when mKATP channels were blocked with glibenclamide or 5-hydroxydecanoic acid, whereas the mKATP channel agonist diazoxide prevented [Ca2+]i from increasing in hypoxia in trout HCs. We found that hypoxia protects against increases in [Ca2+]i in goldfish HCs via mKATP channels. Glycolytic inhibition with 2-deoxyglucose increased [Ca2+]i, which was rescued by hypoxia in a mKATP channel-dependent manner. We found no evidence of plasmalemmal KATP channels in patch-clamp experiments. Instead, we confirmed the involvement of KATP in mitochondria with TMRE imaging, as hypoxia rapidly (<5 min) depolarized mitochondria in a mKATP channel-sensitive manner. We conclude that mKATP channels initiate a neuroprotective pathway in goldfish HCs to maintain [Ca2+]i and avoid excitotoxicity in hypoxia. This model provides novel insight into the cellular mechanisms of hypoxia tolerance in the retina.

Cytoskeletal Arrest: An Anoxia Tolerance Mechanism

Polymerization of actin filaments and microtubules constitutes a ubiquitous demand for cellular adenosine-5′-triphosphate (ATP) and guanosine-5′-triphosphate (GTP). In anoxia-tolerant animals, ATP consumption is minimized during overwintering conditions, but little is known about the role of cell structure in anoxia tolerance. Studies of overwintering mammals have revealed that microtubule stability in neurites is reduced at low temperature, resulting in withdrawal of neurites and reduced abundance of excitatory synapses. Literature for turtles is consistent with a similar downregulation of peripheral cytoskeletal activity in brain and liver during anoxic overwintering. Downregulation of actin dynamics, as well as modification to microtubule organization, may play vital roles in facilitating anoxia tolerance. Mitochondrial calcium release occurs during anoxia in turtle neurons, and subsequent activation of calcium-binding proteins likely regulates cytoskeletal stability. Production of reactive oxygen species (ROS) formation can lead to catastrophic cytoskeletal damage during overwintering and ROS production can be regulated by the dynamics of mitochondrial interconnectivity. Therefore, suppression of ROS formation is likely an important aspect of cytoskeletal arrest. Furthermore, gasotransmitters can regulate ROS levels, as well as cytoskeletal contractility and rearrangement. In this review we will explore the energetic costs of cytoskeletal activity, the cellular mechanisms regulating it, and the potential for cytoskeletal arrest being an important mechanism permitting long-term anoxia survival in anoxia-tolerant species, such as the western painted turtle and goldfish.

Goldfish Response to Chronic Hypoxia: Mitochondrial Respiration, Fuel Preference and Energy Metabolism

Hypometabolism is a hallmark strategy of hypoxia tolerance. To identify potential mechanisms of metabolic suppression, we have used the goldfish to quantify the effects of chronically low oxygen (4 weeks; 10% air saturation) on mitochondrial respiration capacity and fuel preference. The responses of key enzymes from glycolysis, β-oxidation and the tricarboxylic acid (TCA) cycle, and Na+/K+-ATPase were also monitored in various tissues of this champion of hypoxia tolerance. Results show that mitochondrial respiration of individual tissues depends on oxygen availability as well as metabolic fuel oxidized. All the respiration parameters measured in this study (LEAK, OXPHOS, Respiratory Control Ratio, CCCP-uncoupled, and COX) are affected by hypoxia, at least for one of the metabolic fuels. However, no common pattern of changes in respiration states is observed across tissues, except for the general downregulation of COX that may help metabolic suppression. Hypoxia causes the brain to switch from carbohydrates to lipids, with no clear fuel preference in other tissues. It also downregulates brain Na+/K+-ATPase (40%) and causes widespread tissue-specific effects on glycolysis and beta-oxidation. This study shows that hypoxia-acclimated goldfish mainly promote metabolic suppression by adjusting the glycolytic supply of pyruvate, reducing brain Na+/K+-ATPase, and downregulating COX, most likely decreasing mitochondrial density.

Na+/K+-ATPase activity in the anoxic turtle (Trachemys scripta) brain at different acclimation temperature

Survival of prolonged anoxia requires a balance between cellular ATP demand and anaerobic ATP supply from glycolysis, especially in critical tissues such as the brain. To add insight into the ATP demand of the brain of the anoxia-tolerant red-eared slider turtle (Trachemys scripta) during prolonged periods of anoxic submergence, we quantified and compared the number of Na⁺-K⁺-ATPase units and their molecular activity in brain tissue from turtles acclimated to either 21°C or 5°C and exposed to either normoxia or anoxia (6h 21°C; 14days at 5°C). Na⁺-K⁺-ATPase activity and density per g tissue were similar at 21°C and 5°C in normoxic turtles. Likewise, anoxia exposure at 21°C did not induce any change in Na⁺-K⁺-ATPase activity or density. In contrast, prolonged anoxia at 5°C significantly reduced Na⁺-K⁺-ATPase activity by 55%, which was largely driven by a 50% reduction of the number of Na⁺-K⁺-ATPase units without a change in the activity of existing Na⁺-K⁺-ATPase pumps or α-subunit composition. These findings are consistent with the "channel arrest" hypothesis to reduce turtle brain Na⁺-K⁺-ATPase activity during prolonged, but not short-term anoxia, a change that likely helps them overwinter under low temperature, anoxic conditions.

Adenosine receptors mediate the hypoxic ventilatory response but not the hypoxic metabolic response in the naked mole rat during acute hypoxia

Naked mole rats are the most hypoxia-tolerant mammals identified; however, the mechanisms underlying this tolerance are poorly understood. Using whole-animal plethysmography and open-flow respirometry, we examined the hypoxic metabolic response (HMR), hypoxic ventilatory response (HVR) and hypoxic thermal response in awake, freely behaving naked mole rats exposed to 7% O₂ for 1 h. Metabolic rate and ventilation each reversibly decreased 70% in hypoxia (from 39.6 ± 2.9 to 12.1 ± 0.3 ml O₂ min(-1) kg(-1), and 1412 ± 244 to 417 ± 62 ml min(-1) kg(-1), respectively; p < 0.05), whereas body temperature was unchanged and animals remained awake and active. Subcutaneous injection of the general adenosine receptor antagonist aminophylline (AMP; 100 mg kg(-1), in saline), but not control saline injections, prevented the HVR but had no effect on the HMR. As a result, AMP-treated naked mole rats exhibited extreme hyperventilation in hypoxia. These animals were also less tolerant to hypoxia, and in some cases hypoxia was lethal following AMP injection. We conclude that in naked mole rats (i) hypoxia tolerance is partially dependent on profound hypoxic metabolic and ventilatory responses, which are equal in magnitude but occur independently of thermal changes in hypoxia, and (ii) adenosine receptors mediate the HVR but not the HMR.

Transcriptomic Responses of the Heart and Brain to Anoxia in the Western Painted Turtle

Painted turtles are the most anoxia-tolerant tetrapods known, capable of surviving without oxygen for more than four months at 3°C and 30 hours at 20°C. To investigate the transcriptomic basis of this ability, we used RNA-seq to quantify mRNA expression in the painted turtle ventricle and telencephalon after 24 hours of anoxia at 19°C. Reads were obtained from 22,174 different genes, 13,236 of which were compared statistically between treatments for each tissue. Total tissue RNA contents decreased by 16% in telencephalon and 53% in ventricle. The telencephalon and ventricle showed ≥ 2x expression (increased expression) in 19 and 23 genes, respectively, while only four genes in ventricle showed ≤ 0.5x changes (decreased expression). When treatment effects were compared between anoxic and normoxic conditions in the two tissue types, 31 genes were increased (≥ 2x change) and 2 were decreased (≤ 0.5x change). Most of the effected genes were immediate early genes and transcription factors that regulate cellular growth and development; changes that would seem to promote transcriptional, translational, and metabolic arrest. No genes related to ion channels, synaptic transmission, cardiac contractility or excitation-contraction coupling changed. The generalized expression pattern in telencephalon and across tissues, but not in ventricle, correlated with the predicted metabolic cost of transcription, with the shortest genes and those with the fewest exons showing the largest increases in expression.

Cerebral Ischemic Preconditioning: the Road So Far…

Cerebral preconditioning constitutes the brain's adaptation to lethal ischemia when first exposed to mild doses of a subtoxic stressor. The phenomenon of preconditioning has been largely studied in the heart, and data from in vivo and in vitro models from past 2-3 decades have provided sufficient evidence that similar machinery exists in the brain as well. Since preconditioning results in a transient protective phenotype labeled as ischemic tolerance, it can open many doors in the medical warfare against stroke, a debilitating cerebrovascular disorder that kills or cripples thousands of people worldwide every year. Preconditioning can be induced by a variety of stimuli from hypoxia to pharmacological anesthetics, and each, in turn, induces tolerance by activating a multitude of proteins, enzymes, receptors, transcription factors, and other biomolecules eventually leading to genomic reprogramming. The intracellular signaling pathways and molecular cascades behind preconditioning are extensively being investigated, and several first-rate papers have come out in the last few years centered on the topic of cerebral ischemic tolerance. However, translating the experimental knowledge into the clinical scaffold still evades practicality and faces several challenges. Of the various preconditioning strategies, remote ischemic preconditioning and pharmacological preconditioning appears to be more clinically relevant for the management of ischemic stroke. In this review, we discuss current developments in the field of cerebral preconditioning and then examine the potential of various preconditioning agents to confer neuroprotection in the brain.

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).

The role of ASICS in cerebral ischemia

Cerebral ischemia is a leading cause of death and long-term disabilities worldwide. Excessive intracellular Ca(2+) accumulation in neurons has been considered essential for neuronal injury associated with cerebral ischemia. Although the involvement of glutamate receptors in neuronal Ca(2+) accumulation and toxicity has been the subject of intensive investigation, inhibitors for these receptors showed little effect in clinical trials. Thus, additional Ca(2+) toxicity pathway(s) must be involved. Acidosis is a common feature in cerebral ischemia and was known to cause brain injury. The mechanisms were, however, unclear. The finding that ASIC1a channels are highly enriched in brain neurons, their activation by ischemic acidosis, and their demonstrated Ca(2+) permeability suggested a role for these channels in Ca(2+) accumulation and neuronal injury associated with cerebral ischemia. Indeed, a number of studies have now provided solid evidence supporting the involvement of ASIC1a channel activation in ischemic brain injury.

Blunted neuronal calcium response to hypoxia in naked mole-rat hippocampus

Naked mole-rats are highly social and strictly subterranean rodents that live in large communal colonies in sealed and chronically oxygen-depleted burrows. Brain slices from naked mole-rats show extreme tolerance to hypoxia compared to slices from other mammals, as indicated by maintenance of synaptic transmission under more hypoxic conditions and three fold longer latency to anoxic depolarization. A key factor in determining whether or not the cellular response to hypoxia is reversible or leads to cell death may be the elevation of intracellular calcium concentration. In the present study, we used fluorescent imaging techniques to measure relative intracellular calcium changes in CA1 pyramidal cells of hippocampal slices during hypoxia. We found that calcium accumulation during hypoxia was significantly and substantially attenuated in slices from naked mole-rats compared to slices from laboratory mice. This was the case for both neonatal (postnatal day 6) and older (postnatal day 20) age groups. Furthermore, while both species demonstrated more calcium accumulation at older ages, the older naked mole-rats showed a smaller calcium accumulation response than even the younger mice. A blunted intracellular calcium response to hypoxia may contribute to the extreme hypoxia tolerance of naked mole-rat neurons. The results are discussed in terms of a general hypothesis that a very prolonged or arrested developmental process may allow adult naked mole-rat brain to retain the hypoxia tolerance normally only seen in neonatal mammals.

Reversible metabolic depression in lamprey hepatocytes during prespawning migration: dynamics of mitochondrial membrane potential

The lamprey (Lampetra fluviatilis L.) is an extant representative of the ancient vertebrate group of Agnathans. During the prespawning migration (the river period of life from autumn until spring) lamprey hepatocytes exhibit widely different energy states: a high-energy state in autumn and spring, corresponding to a normal physiological standard, and a low-energy state in winter, which is provoked by prolonged starvation and profound metabolic arrest. In spring the restoration of energy status (return to an active state) is associated with hormonally induced lipolysis of the lipid droplets stored in the cells. Lamprey hepatocytes demonstrate an aerobic metabolism based on oxidation of free fatty acids. The dynamics of mitochondrial membrane potential (MMP) were measured throughout the prespawning migration. Pharmacological inhibition of the electron transport chain decreased the MMP and caused extensive depletion of cellular ATP without loss of cell viability. The potential molecular mechanisms responsible for winter metabolic depression in lamprey hepatocytes are discussed.

Cardiac survival in anoxia-tolerant vertebrates: An electrophysiological perspective

Certain vertebrates, such as freshwater turtles of the genus Chrysemys and Trachemys and crucian carp (Carassius carassius), have anoxia-tolerant hearts that continue to function throughout prolonged periods of anoxia (up to many months) due to successful balancing of cellular ATP supply and demand. In the present review, we summarize the current and limited understanding of the cellular mechanisms underlying this cardiac anoxia tolerance. What emerges is that cold temperature substantially modifies cardiac electrophysiology to precondition the heart for winter anoxia. Intrinsic heart rate is slowed and density of sarcolemmal ion currents substantially modified to alter cardiac action potential (AP) characteristics. These changes depress cardiac activity and reduce the energetic costs associated with ion pumping. In contrast, anoxia per se results in limited changes to cardiac AP shape or ion current densities in turtle and crucian carp, suggesting that anoxic modifications of cardiac electrophysiology to reduce ATP demand are not extensive. Additionally, as knowledge of cellular physiology in non-mammalian vertebrates is still in its infancy, we briefly discuss the cellular defense mechanisms towards the acidosis that accompanies anoxia as well as mammalian cardiac models of hypoxia/ischemia tolerance. By examining if fundamental cellular mechanisms have been conserved during the evolution of anoxia tolerance we hope to have provided a framework for the design of future experiments investigating cardiac cellular mechanisms of anoxia survival.

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.

Anoxia tolerance in turtles: Metabolic regulation and gene expression

Freshwater turtles of the Trachemys and Chrysemys genera are champion facultative anaerobes able to survive for several months without oxygen during winter hibernation in cold water. They have been widely used as models to identify and understand the molecular mechanisms of natural anoxia tolerance and the molecular basis of the hypoxic/ischemic injuries that occur in oxygen-sensitive systems and underlie medical problems such as heart attack and stroke. Peter L. Lutz spent much of his career investigating turtle anaerobiosis with a particular focus on the mechanisms of brain ion homeostasis and neurotransmitter responses to anoxia exposure and the mechanisms that suppress brain ion channel function and neuronal excitability during anaerobiosis. Our interests intersected over the mechanisms of metabolic rate depression which is key to long term anoxia survival. Studies in my lab have shown that a key mechanism of metabolic arrest is reversible protein phosphorylation which provides coordinated suppression of the rates of multiple ATP-producing, ATP-utilizing and related cellular processes to allow organisms to enter a stable hypometabolic state. Anoxia tolerance is also supported by selective gene expression as revealed by recent studies using cDNA library and DNA array screening. New studies with both adult T. scripta elegans and hatchling C. picta marginata have identified prominent groups of genes that are up-regulated under anoxia in turtle organs, in several cases suggesting aspects of cell function and metabolic regulation that have not previously been associated with anaerobiosis. These groups of anoxia-responsive genes include mitochondrially-encoded subunits of electron transport chain proteins, iron storage proteins, antioxidant enzymes, serine protease inhibitors, transmembrane solute carriers, neurotransmitter receptors and transporters, and shock proteins.

Chronic hypobaric hypoxia induces tolerance to acute hypoxia and up-regulation in alpha-2 adrenoceptor in rat locus coeruleus

Hypoxia preconditioning has been shown to produce tolerance against brain injuries. The hypothesis of this study is that chronic hypobaric hypoxia may also induce acute hypoxia tolerance. We used intracellular recording in slices from rats exposed to chronic hypobaric hypoxia (exposed) and control to investigate the effects of chronic hypobaric hypoxia on the physiology of locus coeruleus (LC) including neuronal excitability. The results showed 35.7% reduced spontaneous firing rate and no change for membrane potential and input resistance in exposed neurons. In response to the alpha-2 adrenoceptor (A2R) agonist clonidine, both the hyperpolarizing potency and efficacy were increased indicated by a decreased EC(50) (control: 30.9 nM and exposed: 19.7 nM) and a 50.5% increase in maximum hyperpolarized potential, respectively. A2R binding sites were also increased 21% in exposed neurons measured by radioligand [(3)H]rauwolscine binding assay. When treated with acute N(2)-hypoxia, the cell survival time (ST) was longer in exposed neurons, suggesting that a tolerance was induced. In addition, the ST for both groups of LC neurons was decreased by the A2R antagonist yohimbine and increased by the glutamate receptor antagonist kynurenic acid but not by MK-801; the decreased percentage of ST by yohimbine was larger and the increased percentage by kynurenic acid was smaller in exposed neurons. The results suggested that up-regulation of A2R and altered non-NMDA glutamate receptor function induced by chronic hypobaric hypoxia may underlie, in part, the decreased LC neuronal excitability and acute hypoxia tolerance.

Fetal brain hypometabolism during prolonged hypoxaemia in the llama

In this study we looked for additional evidence to support the hypothesis that fetal llama reacts to hypoxaemia with adaptive brain hypometabolism. We determined fetal llama brain temperature, Na(+) and K(+) channel density and Na(+)-K(+)-ATPase activity. Additionally, we looked to see whether there were signs of cell death in the brain cortex of llama fetuses submitted to prolonged hypoxaemia. Ten fetal llamas were instrumented under general anaesthesia to measure pH, arterial blood gases, mean arterial pressure, heart rate, and brain and core temperatures. Measurements were made 1 h before and every hour during 24 h of hypoxaemia (n = 5), which was imposed by reducing maternal inspired oxygen fraction to reach a fetal arterial partial pressure of oxygen (P(a,O(2))) of about 12 mmHg. A normoxaemic group was the control (n = 5). After 24 h of hypoxaemia, we determined brain cortex Na(+)-K(+)-ATPase activity, ouabain binding, and the expression of NaV1.1, NaV1.2, NaV1.3, NaV1.6, TREK1, TRAAK and K(ATP) channels. The lack of brain cortex damage was assessed as poly ADP-ribose polymerase (PARP) proteolysis. We found a mean decrease of 0.56 degrees C in brain cortex temperature during prolonged hypoxaemia, which was accompanied by a 51% decrease in brain cortex Na(+)-K(+)-ATPase activity, and by a 44% decrease in protein content of NaV1.1, a voltage-gated Na(+) channel. These changes occurred in absence of changes in PARP protein degradation, suggesting that the cell death of the brain was not enhanced in the fetal llama during hypoxaemia. Taken together, these results provide further evidence to support the hypothesis that the fetal llama responds to prolonged hypoxaemia with adaptive brain hypometabolism, partly mediated by decreases in Na(+)-K(+)-ATPase activity and expression of NaV channels.

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.

Hypoxia and the regulation of mitogen-activated protein kinases: gene transcription and the assessment of potential pharmacologic therapeutic interventions

Oxygen is an environmental/developmental signal that regulates cellular energetics, growth, and differentiation processes. Despite its central role in nearly all higher life processes, the molecular mechanisms for sensing oxygen levels and the pathways involved in transducing this information are still being elucidated. Altering gene expression is the most fundamental and effective way for a cell to respond to extracellular signals and/or changes in its microenvironment. During development, the expression of specific sets of genes is regulated spatially (by position/morphogenetic gradients) and temporally, presumably via the sensing of molecular oxygen available within the microenvironment. Regulation of signaling responses is governed by transcription factors that bind to control regions (consensus sequences) of target genes and alter their expression in response to specific signals. Complex signal transduction during hypoxia (deficiency of oxygen in inspired gases or in arterial blood and/or in tissues) involves the coupling of ligand-receptor interactions to many intracellular events. These events basically include phosphorylations by tyrosine kinases and/or serine/threonine kinases, such as those of mitogen-activated protein kinases (MAPKs), a superfamily of kinases responsive to stress nonhomeostatic conditions. Protein phosphorylations imposed during hypoxia change enzyme activities and protein conformations, and the eventual outcome is rather complex, comprising of an alteration in cellular activity and changes in the programming of genes expressed within the responding cells. These molecular changes serve as signals that are crucial for cell survival under contingent conditions imposed during hypoxia. This review correlates current concepts of hypoxic sensing pathways with hypoxia-related phosphorylation mechanisms mediated by MAPKs via the genetic and pharmacologic regulation/manipulation of specific transcription factors and related cofactors.

Intracellular calcium and survival of tadpole forebrain cells in anoxia

The frog brain survives hypoxia with a slow loss of energy charge and ion homeostasis. Because hypoxic death in most neurons is associated with increases in intracellular calcium ([Ca2+]i), we examined the relationship between [Ca2+]i and survival of a mixed population of isolated cells from the forebrain of North American bullfrog Rana catesbeiana tadpoles. Forebrain cells from stage V-XV tadpoles were isolated by enzymatic digestion and loaded with one of three different calcium indicators (Fura-2, Fura 2-FF and BTC) to provide estimates of [Ca2+]i accurate at low and high[Ca2+]i. Propidium iodide (PI) fluorescence was used as an indicator of cell viability. Cells were exposed to anoxia (100%N2) and measurements of [Ca2+]i and cell survival made from 1 h to 18 h. Intracellular [Ca2+] increased significantly after 3-6 h anoxia (P&lt;0.05), regardless of the type of Ca2+ indicator used; however, there were substantial differences in the measurements of [Ca2+]i with the different indicators, reflecting their varying affinities for Ca2+. Resting[Ca2+]i was approximately 50 nmol l-1 and increased to about 9-30 μmol l-1 after 4-6 h anoxia. The significant increase in [Ca2+]i during anoxia was not associated with significant increases in cell death, with 85-95% survival over this time period. Cells exposed to anoxia for 18 h, or those made anoxic for 4-6 and reoxygenated for 12 h to 16 h, had survival rates greater than 70%,but survival was significantly less than normoxic controls. These results indicate that large increases in [Ca2+]i are not necessarily associated with hypoxic cell death in vertebrate brain cells.

Neuroprotective effects of atorvastatin against glutamate-induced excitotoxicity in primary cortical neurones

Statins [3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors] exert cholesterol-independent pleiotropic effects that include anti-thrombotic, anti-inflammatory, and anti-oxidative properties. Here, we examined direct protective effects of atorvastatin on neurones in different cell damage models in vitro. Primary cortical neurones were pre-treated with atorvastatin and then exposed to (i) glutamate, (ii) oxygen-glucose deprivation or (iii) several apoptosis-inducing compounds. Atorvastatin significantly protected from glutamate-induced excitotoxicity as evidenced by propidium iodide staining, nuclear morphology, release of lactate dehydrogenase, and mitochondrial tetrazolium metabolism, but not from oxygen-glucose deprivation or apoptotic cell death. This anti-excitototoxic effect was evident with 2-4 days pre-treatment but not with daily administration or shorter-term pre-treatment. The protective properties occurred independently of 3-hydroxy-3-methylglutaryl-CoA reductase inhibition because co-treatment with mevalonate or other isoprenoids did not reverse or attenuate neuroprotection. Atorvastatin attenuated the glutamate-induced increase of intracellular calcium, which was associated with a modulation of NMDA receptor function. Taken together, atorvastatin exerts specific anti-excitotoxic effects independent of 3-hydroxy-3-methylglutaryl-CoA reductase inhibition, which has potential therapeutic implications.

Protective role of neuronal KATP channels in brain hypoxia

During severe arterial hypoxia leading to brain anoxia, most mammalian neurons undergo a massive depolarisation terminating in cell death. However, some neurons of the adult brain and most immature nervous structures tolerate extended periods of hypoxia-anoxia. An understanding of the mechanisms underlying this tolerance to oxygen depletion is pivotal for developing strategies to protect the brain from consequences of hypoxic-ischemic insults. ATP-sensitive K(+) (K(ATP)) channels are good subjects for this study as they are activated by processes associated with energy deprivation and can counteract the terminal anoxic-ischemic neuronal depolarisation. This review summarises in vitro analyses on the role of K(ATP) channels in hypoxia-anoxia in three distinct neuronal systems of rodents. In dorsal vagal neurons, blockade of K(ATP) channels with sulfonylureas abolishes the hypoxic-anoxic hyperpolarisation. However, this does not affect the extreme tolerance of these neurons to oxygen depletion as evidenced by a moderate and sustained increase of intracellular Ca(2+) (Ca(i)). By contrast, a sulfonylurea-induced block of K(ATP) channels shortens the delay of occurrence of a major Ca(i) rise in cerebellar Purkinje neurons. In neurons of the neonatal medullary respiratory network, K(ATP) channel blockers reverse the anoxic hyperpolarisation associated with slowing of respiratory frequency. This may constitute an adaptive mechanism for energy preservation. These studies demonstrate that K(ATP) channels are an ubiquituous feature of mammalian neurons and may, indeed, play a protective role in brain hypoxia.

Cardiorespiratory and tissue adenosine responses to hypoxia and reoxygenation in the short-horned sculpinMyoxocephalus scorpius

Adenosine is a product of adenylate phosphate breakdown that can exert protective effects on tissues during energy limitation. Accumulation of cardiac adenosine under hypoxia is well documented in mammals but has not been shown in fish. Adenosine content was measured in heart and brain tissue from short-horned sculpin Myoxocephalus scorpius L. exposed to acute hypoxia and to graded hypoxia and reoxygenation at 8°C. Cardiorespiratory parameters were recorded along with plasma lactate, K+,Ca2+ and Na+ levels and their relationship to adenosine levels investigated. Sculpin exhibited a large bradycardia during hypoxia,with a concomitant drop in cardiac output that recovers fully with reoxygenation. Ventilation rate also declined with hypoxia, suggesting a depression of activity. Plasma lactate concentration was significantly elevated after 4 h at 2.0 mg l-1 dissolved oxygen while K+ levels increased during acute hypoxia. Adenosine levels were maintained in heart under acute and graded hypoxia. Brain levels fluctuated under hypoxia and showed no change with reoxygenation. It is concluded that a depression of cardiac activity in conjunction with an adequate anaerobic metabolism allow sculpin to avoid excessive adenosine accumulation under conditions of moderate hypoxia. Cardiac adenosine levels decreased and plasma K+ levels and heart rate increased significantly at reoxygenation.

Hypometabolism in reptiles: behavioural and physiological mechanisms that reduce aerobic demands

During exposure to hypoxia all vertebrates utilize a suite of cardiovascular and ventilatory responses that, in combination, strive to maintain adequate delivery of oxygen to the metabolizing tissues. In addition to maintaining oxygen delivery through cardio-respiratory responses, oxygen demands in the tissues can also be reduced. Reptiles use this alternative strategy during periods of moderate to severe hypoxia by behavioural reductions in preferred body temperature and by active down-regulation of aerobic metabolism. Below we review these two different strategies and discuss their possible mechanisms and physiological significance.

Genes induced by a high-oxygen environment in Entamoeba histolytica

Entamoeba histolytica, although a microaerophilic protozoan parasite, encounters a high-oxygen environment, during invasive amoebiasis. The parasite requires specific regulation of certain proteins to maintain its physiological functions to survive in the more oxygenated condition. Our endeavor was to know how does amoeba adapt itself in a high-oxygen environment. Reactive oxygen species (ROS) was found to accumulate in an increasing concentration within the stressed trophozoites in a time-dependent manner. Increased cytopathic activity was detected at 2h in high-oxygen-exposed E. histolytica lysate compared to lysate of normal E. histolytica trophozoites by Ussing chamber assay. The differential display and semi-quantitative polymerase chain reaction showed overexpression in the mRNA levels of thiol-dependent peroxidase (Eh29), superoxide dismutase (SOD), EhCP5, G protein, HSP70, and peptidylprolyl isomerase at different time periods of oxidative stressed trophozoites compared to normally cultured E. histolytica. Analyses of the up-regulated genes that are associated with stress response, viz., signal transduction, tissue destruction, and oxidative stress management, including enhanced expression of a 29-kDa Eh29, suggest that this organism has several protective mechanisms to deal with oxidative stress during invasion.

Functional genomics and the comparative physiology of hypoxia

Comparative physiology has proven a powerful approach to our understanding of how animals function under hypoxic conditions and to identifying potential adaptations to environmental oxygen levels. This review considers the potential for using a similar comparative approach with functional genomics to understand the genetic basis of such physiological processes and evolutionary adaptations. Comparative functional genomics is currently limited by genome data, which are available for only a few model organisms. However, comparative studies between model organisms of the same species having slightly different genomes (e.g., in-bred strains of laboratory rodents, transgenic mice, and consomic rats) demonstrate the types of results, as well as the analytical challenges, that are possible if comparative functional genomics is applied to more species. Results from wild and domestic animal studies suggest new models to investigate physiological and evolutionary responses to oxygen levels with functional genomics.

Physiologic pH changes modulate calcium ion dependence of brain nitric oxide synthase in Carassius auratus

Species of the fish genus Carassius survive prolonged anoxia. Nitric oxide (NO) regulates cerebral blood flow in these fish during normoxic conditions whereas adenosine is the main vasoregulating molecule during anoxia. We investigated the calcium ion dependence of Carassius auratus brain NO synthase (NOS) as a function of pH. The physiological pH decrease from 7.2 to 6.8, which takes place during anoxia, greatly decreases NOS activity. This strong pH dependence is mainly due to variation of the calcium sensitivity of the enzyme. The EC(50) is 0.15 microM at pH 7.2 and 2.1 microM at pH 6.8 for the soluble enzyme. The particulate enzyme is also dependent on pH variations. The reduced sensitivity to calcium ions at acidic pH decreases both NO and H(2)O(2) production, saving the cells by suppression of the formation of potentially toxic nitrogen and oxygen species. Modulation of NOS activity by variation of its calcium affinity within the range of physiological pH constitutes an important and rapid mechanism to control the formation of NO and H(2)O(2) during normoxia-anoxia and anoxia-normoxia transitions.

Oxygen-sensing mechanisms and the regulation of redox-responsive transcription factors in development and pathophysiology

How do organisms sense the amount of oxygen in the environment and respond appropriately when the level of oxygen decreases? Oxygen sensing and the molecular stratagems underlying the process have been the focus of an endless number of investigations trying to find an answer to the question: "What is the identity of the oxygen sensor?" Dynamic changes in pO2 constitute a potential signaling mechanism for the regulation of the expression and activation of reduction-oxidation (redox)-sensitive and oxygen-responsive transcription factors, apoptosis-signaling molecules and inflammatory cytokines. The transition from placental to lung-based respiration causes a relatively hyperoxic shift or oxidative stress, which the perinatal, developing lung experiences during birth. This variation in DeltapO2, in particular, differentially regulates the compartmentalization and functioning of the transcription factors hypoxia-inducible factor-1alpha (HIF-1alpha) and nuclear factor-kappaB (NF-kappaB). In addition, oxygen-evoked regulation of HIF-1alpha and NF-kappaB is closely coupled with the intracellular redox state, such that modulating redox equilibrium affects their responsiveness at the molecular level (expression/transactivation). The differential regulation of HIF-1alpha and NF-kappaB in vitro is paralleled by oxygen-sensitive and redox-dependent pathways governing the regulation of these factors during the transition from placental to lung-based respiration ex utero. The birth transition period in vivo and ex utero also regulates apoptosis signaling pathways in a redox-dependent manner, consistent with NF-kappaB being transcriptionally regulated in order to play an anti-apoptotic function. An association is established between oxidative stress conditions and the augmentation of an inflammatory state in pathophysiology, regulated by the oxygen- and redox-sensitive pleiotropic cytokines.

Adaptive mechanisms of intracellular calcium homeostasis in mammalian hibernators

Intracellular Ca2+ homeostasis is a prerequisite for a healthy cell life. While cells from some mammals may suffer dysregulation of intracellular Ca2+ levels under certain deleterious and stressful conditions, including hypothermia and ischemia, cells from mammalian hibernators exhibit a remarkable ability to maintain a homeostatic intracellular Ca2+ environment. Compared with cells from non-hibernators, hibernator cells are characterized by downregulation of the activity of Ca2+ channels in the cell membrane, which helps to prevent excessive Ca2+ entry. Concomitantly, sequestration of Ca2+ by intracellular Ca2+ stores, especially the sarcoplasmic/endoplasmic reticulum, is enhanced to keep the resting levels of intracellular Ca2+ stable. An increase in stored Ca2+ in heart cells during hibernation ensures that the levels of Ca2+messenger are sufficient for forceful cell contraction under conditions of hypothermia. Maintenance of Na+ gradients, viaNa+—Ca2+ exchangers, is also important in the Ca2+ homeostasis of hibernator cells. Understanding the adaptive mechanisms of Ca2+ regulation in hibernating mammals may suggest new strategies to protect nonhibernator cells, including those of humans, from Ca2+-induced dysfunction.

Hibernating without oxygen: physiological adaptations of the painted turtle

Many freshwater turtles in temperate climates may experience winter periods trapped under ice unable to breathe, in anoxic mud, or in water depleted of O(2). To survive, these animals must not only retain function while anoxic, but they must do so for extended periods of time. Two general physiological adaptive responses appear to underlie this capacity for long-term survival. The first is a coordinated depression of metabolic processes within the cells, both the glycolytic pathway that produces ATP and the cellular processes, such as ion pumping, that consume ATP. As a result, both the rate of substrate depletion and the rate of lactic acid production are slowed greatly. The second is an exploitation of the extensive buffering capacity of the turtle's shell and skeleton to neutralize the large amount of lactic acid that eventually accumulates. Two separate shell mechanisms are involved: release of carbonate buffers from the shell and uptake of lactic acid into the shell where it is buffered and sequestered. Together, the metabolic and buffering mechanisms permit animals to survive for 3-4 months at 3 degrees C with no O(2) and with circulating lactate levels of 150 mmol l(-1) or more.

Anoxia induces thermotolerance in the locust flight system

Heat shock and anoxia are environmental stresses that are known to trigger similar cellular responses. In this study, we used the locust to examine stress cross-tolerance by investigating the consequences of a prior anoxic stress on the effects of a subsequent high-temperature stress. Anoxic stress and heat shock induced thermotolerance by increasing the ability of intact locusts to survive normally lethal temperatures. To determine whether induced thermotolerance observed in the intact animal was correlated with electrophysiological changes, we measured whole-cell K+ currents and action potentials from locust neurons. K+ currents recorded from thoracic neuron somata were reduced after anoxic stress and decreased with increases in temperature. Prior anoxic stress and heat shock increased the upper temperature limit for generation of an action potential during a subsequent heat stress. Although anoxia induced thermotolerance in the locust flight system, a prior heat shock did not protect locusts from a subsequent anoxic stress. To determine whether changes in bioenergetic status were implicated in whole-animal cross-tolerance, phosphagen levels and rates of mitochondrial respiration were assayed. Heat shock alone had no effect on bioenergetic status. Prior heat shock allowed rapid recovery after normally lethal heat stress but afforded no protection after a subsequent anoxic stress. Heat shock also afforded no protection against disruption of bioenergetic status after a subsequent exercise stress. These metabolite studies are consistent with the electrophysiological data that demonstrate that a prior exposure to anoxia can have protective effects against high-temperature stress but that heat shock does not induce tolerance to anoxia.

Dual action of cytosolic calcium on calcium channel activity during hypoxia in hippocampal neurones

The role of cytoplasmic calcium level (Ca(i)) in regulation of Ca channel activity during hypoxia was studied in hippocampal neurones from rats. Whole-cell patch clamp recordings in combination with measurements of O(2) partial pressure (pO(2)) were used. Lowering of pO(2) induced a potentiation of HVA Ca channel activity by 25.7% at Ca(i) = 75 nM in comparison with Ca(2+)-free solution. Increase of Ca(i) up to 410 nM slightly increased the effect and significantly slowed the Ca(2+) current run-down. On the other hand, hypoxia increased a steady-state channel inactivation and speeded up the kinetics of Ca(2+) current decay by about 30%. We conclude that moderate hypoxia induces dual action on Ca channels: intracellularly mediated augmentation of Ca influx via Ca channels and their Ca(2+)-dependent inactivation.

Mechanism, origin, and evolution of anoxia tolerance in animals

Organisms vary widely in their tolerance to conditions of limiting oxygen supply to their cells and tissues. A unifying framework of hypoxia tolerance is now available that is based on information from cell-level models from highly anoxia-tolerant species, such as the aquatic turtle, and from other more hypoxia-sensitive systems. The response of hypoxia-tolerant systems to oxygen lack occurs in two (defense and rescue) phases. The first lines of defense against hypoxia include a drastic, if balanced, suppression of ATP demand and supply pathways; this regulation allows ATP levels to remain constant, even while ATP turnover rates greatly decline. The ATP requirements of ion pumping are down-regulated by generalized 'channel' arrest in hepatocytes and by the arrest of specific ion channels in neurons. In hepatocytes, the ATP demands of protein synthesis are down-regulated on exposure to hypoxia by an immediate global blockade of the process (probably through translational arrest caused by complexing between polysomes and elongation factors). 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 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 functional roles of the oxygen-sensing and signal-transduction system include significant gene-based metabolic reprogramming - the rescue process - with maintained down-regulation of energy demand and supply pathways in metabolism throughout the hypoxic period. 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, which is prerequisite for surviving prolonged hypoxia or anoxia. Because the phylogenies of the turtles and of fishes are well known, we are now in an excellent position to assess conservative vs. adaptable features in the evolution of the above hypoxia-response physiology in these two specific animal lineages.

Resistance to NMDA toxicity correlates with appearance of nuclear inclusions, behavioural deficits and changes in calcium homeostasis in mice transgenic for exon 1 of the huntington gene

Transgenic Huntington's disease (HD) mice, expressing exon 1 of the human HD gene (lines R6/1 and R6/2), are totally resistant to striatal lesions caused by the NMDA receptor agonist quinolinic acid (QA). Here we show that this resistance develops gradually over time in both R6/1 and R6/2 mice, and that it occurred earlier in R6/2 (CAG-155) than in R6/1 (CAG-115) mice. The development of the resistance coincided with the appearance of nuclear inclusions and with the onset of motor deficits. In the HD mice, hippocampal neurons were also resistant to QA, especially in the CA1 region. Importantly, there was no change in susceptibility to QA in transgenic mice with a normal CAG repeat (CAG-18). R6/1 mice were also resistant to NMDA-, but not to AMPA-induced striatal damage. Interestingly, QA-induced current and calcium influx in striatal R6/2 neurons were not decreased. However, R6/2 neurons had a better capacity to handle cytoplasmic calcium ([Ca2+]c) overload following QA and could avoid [Ca2+]c deregulation and cell lysis. In addition, basal [Ca2+]c levels were increased five-fold in striatal R6/2 neurons. This might cause an adaptation of R6 neurons to excitotoxic stress resulting in an up-regulation of defense mechanisms, including an increased capacity to handle [Ca2+]c overload. However, the increased level of basal [Ca2+]c in the HD mice might also disturb intracellular signalling in striatal neurons and thereby cause neuronal dysfunction and behavioural deficits.

Mechanisms of cell survival in hypoxia and hypothermia

Most animals experience some degree of hypoxia and hypothermia during the course of their natural life history either as a consequence of ambient ‘exposure’ per se or through metabolic, respiratory and/or circulatory insufficiency. A prevailing experimental approach has been to probe tissues from natural models of hypoxia-tolerant and cold-tolerant vertebrates to look for common mechanisms of defence against O2 lack and hypothermia. The ability to sustain vital cellular functions in severe cases of either condition varies widely amongst the vertebrates. Like humans, the vast majority of mammals are unable to survive prolonged periods of hypothermia or O2 deprivation owing to irreversible membrane damage and loss of cellular ion homeostasis in vital organs such as the brain and heart. However, numerous hibernating endotherms, neonatal and diving mammals as well as many ectotherms can tolerate prolonged periods that would, in clinical terms, be called asphyxia or deep hypothermia. The key to their survival under such conditions lies in an inherent ability to downregulate their cellular metabolic rate to new hypometabolic steady states in a way that balances the ATP demand and ATP supply pathways.

Hypoxia-dependent protein expression: erythropoietin

Normal cell homeostasis relies on the ordered flow of nutrients and substrates through metabolic pathways. Any perturbation of this flow eventually leads to dysfunction, impairment of defense mechanisms, loss of viability and death. High altitude and pathological hypoxia represent a serious and frequent cause for the loss of cell viability. Although organisms customarily respond by triggering adaptive or maladaptive mechanisms, all forms of life eventually succumb to hypoxia if it is severe enough, irrespectively of the primary cause. This paper reviews one of the mechanisms by which organisms respond to hypoxia: erythropoiesis. Although such response is not always beneficial, the discovery of the biochemical mechanisms underlying erythropoiesis has triggered an active field of research that is actually applying lessons learned in the mountains to a more clinical environment.

Neuroprotective adaptations in hibernation: therapeutic implications for ischemia-reperfusion, traumatic brain injury and neurodegenerative diseases

Brains of hibernating mammals are protected against a variety of insults that are detrimental to humans and other nonhibernating species. Such protection is associated with a number of physiological adaptations including hypothermia, increased antioxidant defense, metabolic arrest, leukocytopenia, immunosuppression, and hypocoagulation. It is intriguing that similar manipulations provide considerable protection as experimental treatments for central nervous system injury. This review focuses on neuroprotective mechanisms employed during hibernation that may offer novel approaches in the treatment of stroke, traumatic brain injury, and neurodegenerative diseases in humans.

The respiratory central pattern generator of Lymnaea: a model, measured and malleable

Great progress has been made, and continues to be made in our understanding of the neuronal mechanisms underlying respiration in a wide variety of model systems. The central pattern generator (CPG) controlling aerial respiration in the pond-snail Lymnaea is a particularly well-studied model. Using in vitro and semi-intact preparations, the neural circuitry that controls aerial respiration has been characterized as consisting of three identified interneurons. Furthermore, insight has been gained into the behavioural, cellular and synaptic mechanisms by which this circuit controls respiratory rhythmogenesis. It has also been demonstrated that aerial respiratory behaviour can be modified both by experience and by environmental factors. Studies have shown that, in a behavioural hierarchy, respiration is subservient to the whole-body withdrawal response that respiratory behaviour can be modified through operant conditioning, and that respiratory behaviour is altered by hypoxia. Through research on the Lymnaea respiratory CPG we are coming to a better understanding of the construction and malleability of a CPG network. The malleability of this CPG is of particular interest. No longer can neuronal networks underlying respiratory behaviour be considered hard-wired; they have inherent plasticity.

Surviving hypoxia without really dying

In cases of severe O(2) limitation, most excitable cells of mammals cannot continue to meet the energy demands of active ion transporting systems, leading to catastrophic membrane failure and cell death. However, in certain lower vertebrates, hypoxia-induced membrane destabilisation of the kind seen in mammals is either slow to develop or does not occur at all owing to adaptive decreases in membrane permeability (i.e. ion 'channel arrest'), that dramatically reduce the energetic costs of ion-balancing ATPases. Mammalian cells do, however, exhibit a whole host of adaptive responses to less severe shortages of oxygen, which include energy-balanced metabolic suppression, ionic-induced activation of O(2) receptors and the upregulation of certain genes, all of which enhance the systemic delivery of oxygen and promote energy conservation. Accumulating evidence suggests that the mechanisms underlying these protective effects are orchestrated into action by putative members of an O(2)-sensing pathway that most if not all cells share in common. In this review we address three major questions: (i) how do cells detect shortages of oxygen and subsequently set in motion adaptive mechanisms of either energy production or energy conservation; (ii) how do these mechanisms restructure cellular pathways of ATP supply and demand to ensure that ion-motive ATPases are given priority over other cell functions to preserve membrane integrity in energy-limited states; and (iii) what mechanisms of molecular and metabolic defence against acute and long-term shortages of oxygen set hypoxia-tolerant systems apart from their hypoxia-sensitive counterparts?

Respiratory responses to single and episodic hypoxia during development: mechanisms of adaptation

The respiratory responses of the developmental subject to single and repeated episodes of hypoxia are distinct. During a single exposure, the fetus responds with an arrest of breathing activity, and the neonate, with excitation followed by depression (the biphasic response). Mechanisms under active consideration include chemosensory resetting, hypometabolism, prevalence of inhibitory neurotransmitter/modulator influence, and supramedullary regulation of control functions. When exposed to recurrent episodic hypoxia, neonates respond with relative hypoventilation, i.e. tolerance to a subsequent hypoxic stimulus. Whereas the investigation of processes responsible for this tolerance is at its infancy, studies using chronic hypoxia appear to be a useful guide. So far, altered interstitial neuromodulator levels and central markers of programmed neuronal death are harbingers of future research in this field. The clarification of the mechanisms involved in response to recurrent episodic hypoxia during development will be of fundamental value and may be useful for the eventual treatment and/or prevention of harmful central respiratory-related processes.