A MICROCALORIMETRIC STUDY OF TURTLE CORTICAL SLICES: INSIGHTS INTO BRAIN METABOLIC DEPRESSION

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).

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.

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.

Proteomic changes in the brain of the western painted turtle (Chrysemys picta bellii) during exposure to anoxia

During anoxia, overall protein synthesis is almost undetectable in the brain of the western painted turtle. The aim of this investigation was to address the question of whether there are alterations to specific proteins by comparing the normoxic and anoxic brain proteomes. Reductions in creatine kinase, hexokinase, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate kinase reflected the reduced production of adenosine triphosphate (ATP) during anoxia while the reduction in transitional endoplasmic reticulum ATPase reflected the conservation of ATP or possibly a decrease in intracellular Ca(2+). In terms of neural protection programed cell death 6 interacting protein (PDCD6IP; a protein associated with apoptosis), dihydropyrimidinase-like protein, t-complex protein, and guanine nucleotide protein G(o) subunit alpha (Go alpha; proteins associated with neural degradation and impaired cognitive function) also declined. A decline in actin, gelsolin, and PDCD6IP, together with an increase in tubulin, also provided evidence for the induction of a neurological repair response. Although these proteomic alterations show some similarities with the crucian carp (another anoxia-tolerant species), there are species-specific responses, which supports the theory of no single strategy for anoxia tolerance. These findings also suggest the anoxic turtle brain could be an etiological model for investigating mammalian hypoxic damage and clinical neurological disorders.

Evolution of the oxygen sensitivity of cytochrome c oxidase subunit 4

Vertebrates possess two paralogs of cytochrome c oxidase (COX) subunit 4: a ubiquitous COX4-1 and a hypoxia-linked COX4-2. Mammalian COX4-2 is thought to have a role in relation to fine-tuning metabolism in low oxygen levels, conferred through both structural differences in the subunit protein structure and regulatory differences in the gene. We sought to elucidate the pervasiveness of this feature across vertebrates. The ratio of COX4-2/4-1 mRNA is generally low in mammals, but this ratio was higher in fish and reptiles, particularly turtles. The COX4-2 gene appeared unresponsive to low oxygen in nonmammalian models (zebrafish, goldfish, tilapia, anoles, and turtles) and fish cell lines. Reporter genes constructed from the amphibian and reptile homologues of the mammalian oxygen-responsive elements and hypoxia-responsive elements did not respond to low oxygen. Unlike the rodent ortholog, the promoter of goldfish COX4-2 did not respond to hypoxia or anoxia. The protein sequences of the COX4-2 peptide showed that the disulfide bridge seen in human and rodent orthologs would be precluded in other mammalian lineages and lower vertebrates, all of which lack the requisite pair of cysteines. The coordinating ligands of the ATP-binding site are largely conserved across mammals and reptiles, but in Xenopus and fish, sequence variations may disrupt the ability of the protein to bind ATP at this site. Collectively, these results suggest that many of the genetic and structural features of COX4-2 that impart responsiveness and benefits in hypoxia may be restricted to the Euarchontoglires lineage that includes primates, lagomorphs, and rodents.

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).

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.

Piscine insights into comparisons of anoxia tolerance, ammonia toxicity, stroke and hepatic encephalopathy

Although the number of fish species that have been studied for both hypoxia/anoxia tolerance and ammonia tolerance are few, there appears to be a correlation between the ability to survive these two insults. After establishing this correlation with examples from the literature, and after examining the role Peter Lutz played in catalyzing this convergent interest in two variables, this article explores potential mechanisms underpinning this correlation. We draw especially on the larger body of information for two human diseases with the same effected organ (brain), namely stroke and hepatic encephalopathy. While several dissimilarities exist between the responses of vertebrates to anoxia and hyperammonemia, one consistent observation in both conditions is an overactivation of NMDA receptors or glutamate neurotoxicity. We propose a glutamate excitotoxicity hypothesis to explain the correlation between ammonia and hypoxia resistance in fish. Furthermore, we suggest several experimental paths to test this hypothesis.

Adaptative response of Vitis root to anoxia

The effect of anoxia on the energy economy of root cells was studied by measuring heat production, ethanol and ATP production, K(+) fluxes and electrical activity in two Vitis species, V. riparia and V. rupestris, that differ in their tolerance to anoxia. Anoxia triggered a marked decrease of metabolic activity (measured by microcalorimetry) and of ATP levels in both species. In V. riparia after the first 2 h of anoxia, the decrease in the rate of heat production was not associated with a further significant decrease in ATP content, whereas in V. rupestris the ATP level continued to decrease until very low values were reached. The concomitant increase in the rate of ethanol production did not compensate for the decreased aerobic ATP supply. In V. rupestris, anoxia typically led to energy deficit and ATP imbalance, together with the subsequent disruption of ion homeostasis and cell death. In V. riparia, the strong decrease in K(+) membrane permeability together with the fast down-regulation of the electrical signals allowed the cells to avoid severe ion imbalances during prolonged anoxic episodes.

PETER HOCHACHKA: Adventures in Biochemical Adaptation

Peter Hochachka was one of the most creative forces in the field of comparative physiology during the past half-century. His career was truly an exploratory adventure, in both intellectual and geographic senses. His broad comparative studies of metabolism in organisms as diverse as trout, tunas, oysters, squid, turtles, locusts, hummingbirds, seals, and humans revealed the adaptable features of enzymes and metabolic pathways that provide the biochemical bases for diverse lifestyles and environments. In its combined breadth and depth, no other corpus of work better illustrates the principle of "unity in diversity" that marks comparative physiology. Through his publications, his stimulating mentorship, his broad editorial services, and his continuous-and highly infectious-enthusiasm for his field, Peter Hochachka served as one of the most influential leaders in the transformation of comparative physiology.

Effect of graded hypoxic and acidotic stress on contractile force of heart muscle from hypoxia-tolerant and hypoxia-intolerant turtles

Previous studies have shown that isometric contractile force of in vitro cardiac muscle from the anoxia-tolerant painted turtle, Chrysemys picta bellii, decreases when anoxic and when acidotic. This study sought to define the thresholds for these responses in the isolated ventricular strips of the painted turtle and in the anoxia-intolerant softshell turtles, Apalone spinifera. The ventricular strips were exposed to HCO3- Ringer's solution equilibrated at P(O2) 156, 74, 37, 19, and 0 mmHg (45 min at each grade), at both pH 7.0 and at pH 7.8. Strips were also exposed to graded lactic acidosis with intervals between pH 6.8 and pH 7.8 at P(O2) 156 mmHg (softshell) or 37 mmHg (painted). In painted turtle strips at pH 7.8, force remained at control levels until it decreased by 30% at P(O2) 19 mmHg. No further significant decrease occurred at P(O2) 0. In contrast, softshell turtle muscle force did not fall significantly until P(O2) reached 0. When graded hypoxia was imposed at pH 7.0, strips from both species were more sensitive to hypoxia, but the softshell force decreased at a higher P(O2) than the painted turtle (P(O2) 156 mmHg vs. 37 mmHg), its force fell to a lower level at P(O2) 0 (22 % of control vs. 40 % of control), and unlike painted turtle heart muscle, softshell muscle did not recover fully. In summary, these data indicate that ventricular strips of the painted turtle are no more tolerant of hypoxia alone than strips from the softshell turtle, but that when hypoxia is combined with acidosis, the painted turtle heart muscle functions significantly better during the exposure and recovers more fully after exposure.

Acid-base balance during hypoxic hypometabolism: selected vertebrate strategies

An important functional advantage of hypoxic hypometabolism is that it blunts the acid-base consequences of hypoxia. Hypoxia can lead to anaerobiosis and metabolic acidosis and, in animals that are apneic, to respiratory acidosis. A fall in blood and tissue pH is a major limiting factor in hypoxic tolerance and a variety of strategies occur in vertebrates, in concert with hypometabolism, to respond to this acid-base challenge. These include sequestering of lactic acid away from the circulating blood during the hypoxic exposure, either in underperfused tissues or in mineralized tissues, supplementing extracellular buffering by releasing bone mineral into the circulation, and utilizing alternative metabolic pathways for anaerobiosis to produce ethanol rather than lactate as the principal end-product. For submerged air-breathing ectotherms, effective cutaneous O2 and CO2 exchange can also allow an animal to avoid or minimize both anaerobiosis and respiratory acidosis. These responses serve to maintain a viable acid-base state in the body and to extend the time that the hypoxic stress can be endured.

Time-dependent changes in spontaneous respiratory activity in turtle brainstems in vitro

Our goal was to determine whether time-dependent changes in respiratory motor output in vitro could be minimized by altering bath solution composition. Adult turtle brainstems were bathed in standard solution, nutrient-rich Dulbecco's Eagle media (100 or 25% concentration), or standard solution with phenylbiguanide (PBG, 5-HT3 agonist which increases respiratory drive). Except for a 63% frequency increase in PBG solution, hypoglossal bursts were unaltered within 100 min of observation. Respiratory activity was abolished within 7 h in 100% Dulbecco's compared with a mean of 24-31 h in other test solutions. At 12 h, burst frequency decreased faster in standard solution and 25% Dulbecco's (-0.28+/-0.07 and -0.13+/-0.05 bursts/h, respectively) compared with PBG solution (-0.09+/-0.04 bursts/h); amplitude declined at approximately 2%/h in all solutions. The tendency for episodic discharge decreased gradually in standard solution, but was eliminated in 25% Dulbecco's and PBG solution. Certain bath solutions may minimize time-dependent frequency reductions but may also cause breathing pattern changes.

Cerebral metabolism during cord occlusion and hypoxia in the fetal sheep: a novel method of continuous measurement based on heat production

This study was undertaken to validate a new method of measuring cerebral metabolic rate in the fetal sheep based on heat production in a local region of the brain. Heat production was compared to oxygen use in 20 near-term fetuses during basal conditions, moderate hypoxia and cord occlusion. Thermocouples were placed to measure core and brain temperature and a composite probe placed in the parietal cortex to measure changes in cortical blood flow (CBF) using laser Doppler flowmetry and tissue PO2 using fluorescent decay. Catheters were inserted in a brachiocephalic artery and sagittal sinus for blood sampling. With moderate hypoxia, induced by administering 10-12 % oxygen to the ewes, fetal arterial PO2 declined from 23 +/- 1 to 11 +/- 1 Torr and brain tissue PO2 fell from 7.6+/- 0.7 to a nadir of 0.8 +/- 0.4 Torr, while CBF increased to 139 +/- 5 % of baseline. Cortical heat production, calculated as the product of CBF, the temperature gain from artery to brain tissue, and the specific heat of blood, decreased by 45 +/- 11 % in parallel to similar declines in oxygen uptake. With severe asphyxia induced by complete cord occlusion for 10 min, fetal arterial PO2 declined from 23 +/- 1 to 9 +/- 2 Torr and brain tissue PO2 fell from 7.0 +/- 0.7 to essentially 0 Torr while CBF decreased 40 +/- 5 %. Cortical heat production decreased by 78 +/- 6 % while oxygen use declined by 90 +/- 3 %. Glucose uptake increased significantly relative to oxygen use and lactate concentration increased in sagittal sinus blood. We conclude that local measurements of heat production in the brain provide a useful index of overall metabolic rate, closely reflecting oxygen use in moderate hypoxia and indicating a significant contribution from anaerobic metabolism during severe asphyxia.

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.

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.

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.

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?

Living without oxygen: lessons from the freshwater turtle

Freshwater turtles, and specifically, painted turtles, Chrysemys picta, are the most anoxia-tolerant air-breathing vertebrates. These animals can survive experimental anoxic submergences lasting up to 5 months at 3 degrees C. Two general integrative adaptations underlie this remarkable capacity. First is a profound reduction in energy metabolism to approximately 10% of the normoxic rate at the same temperature. This is a coordinated reduction of both ATP generating mechanisms and ATP consuming pathways of the cells. Second is a defense of acid-base state in response to the extreme lactic acidosis that results from anaerobic glycolysis. Central to this defense is an exploitation of buffer reserves within the skeleton and, in particular, the turtle's shell, its most characteristic structure. Carbonates are released from bone and shell to enhance body fluid buffering of lactic acid and lactic acid moves into shell and bone where it is buffered and stored. The combination of slow metabolic rate and a large and responsive mineral reserve are key to this animal's extraordinary anaerobic capacity.

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.

Na(+)-K(+) pump and metabolic activities of trout erythrocytes during anoxia

Metabolic activity in the red blood cells of brown trout was monitored under conditions of oxygen depletion and chemically induced anoxia. Although metabolic activity was reduced during anoxia to one-third of the normoxic value, these cells maintained their ATP contents stable and were viable for hours in the absence of oxygen. In addition, Na(+)-K(+) pump activity was not down-regulated when metabolic activity was reduced during anoxia. The compatibility of this finding with energy equilibrium and ion homeostasis was investigated.

Extracellular levels of amino acid neurotransmitters during anoxia and forced energy deficiency in crucian carp brain

The crucian carp is one of the few vertebrates that has the ability to survive long periods of anoxia. A devastating event in the anoxic mammalian brain is a massive release of excitatory neurotransmitters, particularly glutamate. Using microdialysis to measure extracellular levels of several amino acid neurotransmitters and related compounds in the telencephalon of crucian carp in vivo, we show here that this species avoids a release of glutamate during anoxia, which is probably related to its ability to maintain energy charge. Instead, 6 h of anoxia produced a doubling of the extracellular level of GABA, the major inhibitory neurotransmitter in brain. The release of GABA may be a mechanism for lowering neuronal activity and energy use, thereby facilitating the maintenance of energy charge. Perfusing the microdialysis probe with a high-K+ Ringer showed that the telencephalon had the ability to release both glutamate and GABA. Moreover, if energy deficiency was produced during anoxia, by inhibiting glycolysis with iodoacetate (IAA), the resulting release of GABA was more rapid and profound than that of glutamate, possibly reflecting a second line of anoxia defence aimed at minimising the effect of a temporary energy failure.

Reversible decreases in ATP and PCr concentrations in anoxic turtle brain

A hallmark of anoxia tolerance in western painted turtles is relative constancy of tissue adenylate concentrations during periods of oxygen limitation. During anoxia heart and brain intracellular compartments become more acidic and cellular energy demands are met by anaerobic glycolysis. Because changes in adenylates and pH during anoxic stress could represent important signals triggering metabolic and ion channel down-regulation we measured PCr, ATP and intracellular pH in turtle brain sheets throughout a 3-h anoxic-re-oxygenation transition with 31P NMR. Within 30 min of anoxia, PCr levels decrease 40% and remain at this level during anoxia. A different profile is observed for ATP, with a statistically significant decrease of 23% occurring gradually during 110 min of anoxic perfusion. Intracellular pH decreases significantly with the onset of anoxia, from 7.2 to 6.6 within 50 min. Upon re-oxygenation PCr, ATP and intracellular pH recover to pre-anoxic levels within 60 min. This is the first demonstration of a sustained reversible decrease in ATP levels with anoxia in turtle brain. The observed changes in pH and adenylates, and a probable concomitant increase in adenosine, may represent important metabolic signals during anoxia.

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.

Anoxia tolerant animals from a neurobiological perspective

This paper discusses the mechanisms for brain anoxia survival seen in crucian carp (Carassius carassius) and a few species of freshwater turtle (Chrysemys and Trachemys species). Comparisons are made with the hypoxic tolerant mammalian neonate brain. In the anoxic tolerant species the basic strategy for anoxia survival appears to be the maintenance of ion gradients, and thereby the avoidance of anoxic depolarization. Important facilitating factors involve having huge glycogen stores, increased blood supply to the brain, the suppression of electrical activity, increased release of inhibitory neuromodulators and neurotransmitters, upregulation of inhibitory neuroreceptors, the down-regulation of excitatory ion conductance and the down-regulation of Ca2+ channels. By contrast, for the mammalian neonate the most important causes of its increased hypoxia tolerance may be just simple consequences of the comparatively undifferentiated state of the brain of the newborn, with its lower energy requirements, slower decline in ATP and lower excitability levels acting to delay depolarization.