Brain blood flow and blood pressure during hypoxia in the epaulette shark Hemiscyllium ocellatum, a hypoxia-tolerant elasmobranch

The endemic and endangered Maugean Skate (Zearaja maugeana) exhibits short-term severe hypoxia tolerance

The endangered and range-restricted Maugean skate (Zearaja maugeana) is subjected to large environmental variability coupled with anthropogenic stressors in its endemic habitat, Macquarie Harbour, Tasmania. However, little is known about the basic biology/physiology of this skate, or how it may respond to future environmental challenges predicted from climate change and/or increases in human activities such as aquaculture. These skate live at a preferred depth of 5-15 m where the dissolved oxygen (DO) levels are moderate (~55% air saturation), but can be found in areas of the Harbour where DO can range from 100% saturation to anoxia. Given that the water at their preferred depth is already hypoxic, we sought to investigate their response to further decreases in DO that may arise from potential increases in anthropogenic stress. We measured oxygen consumption, haematological parameters, tissue-enzyme capacity and heat shock protein (HSP) levels in skate exposed to 55% dissolved O2 saturation (control) and 20% dissolved O2 saturation (hypoxic) for 48 h. We conclude that the Maugean skate appears to be an oxyconformer, with a decrease in the rate of O2 consumption with increasing hypoxia. Increases in blood glucose and lactate at 20% O2 suggest that skate are relying more on anaerobic metabolism to tolerate periods of very low oxygen. Despite these metabolic shifts, there was no difference in HSP70 levels between groups, suggesting this short-term exposure did not elicit a cellular stress response. The metabolic state of the skate suggests that low oxygen stress for longer periods of time (i.e. >48 h) may not be tolerable and could potentially result in loss of habitat or shifts in their preferred habitat. Given its endemic distribution and limited life-history information, it will be critical to understand its tolerance to environmental challenges to create robust conservation strategies.

Identification of HIF-1 signaling pathway in Pelteobagrus vachelli using RNA-Seq: effects of acute hypoxia and reoxygenation on oxygen sensors, respiratory metabolism, and hematology indices

Oxygen is a vital element in aquatic environments. The concentration of oxygen to which aquatic organisms are exposed is influenced by salinity, water temperature, weather, and surface water runoff. Hypoxia has a serious effect on fish populations, and can lead to the loss of habitat and die-offs. Therefore, in the present study we used next-generation sequencing technology to characterize the transcriptomes of Pelteobagrus vachelli and identified 70 candidate genes in the HIF-1 signaling pathway that are important for the hypoxic response in all metazoan species. For the first time, the present study reported the effects of acute hypoxia and reoxygenation on oxygen sensors, respiratory metabolism, and hematology indices in P. vachelli. The predicted physiological adjustments show that P. vachelli's blood oxygen-carrying capacity was increased through increased RBC, HB, and SI after hypoxia exposure. Glycolysis-related enzyme activities (PFK, HK, and PK) and LDH in the brain and liver also increased, indicating a rise in anaerobic metabolism. The observed reduction in oxidative enzyme level (CS) in the liver during hypoxia suggests a concomitant depression in aerobic metabolism. There were significant increases in oxygen sensor mRNA expression and HIF-1α protein expression during hypoxia and reoxygenation exposure, suggesting that the HIF-1 signaling pathway was activated in the liver and brain of P. vachelli in response to acute hypoxia and reoxygenation. Our findings suggest that oxygen sensors (e.g., HIF-1α) of P. vachelli are potentially useful biomarkers of environmental hypoxic exposure. These data contribute to a better understanding of the molecular mechanisms of the hypoxia signaling pathway in fish under hypoxia and reoxygenation.

Mass Transport: Circulatory System with Emphasis on Nonendothermic Species

Mass transport can be generally defined as movement of material matter. The circulatory system then is a biological example given its role in the movement in transporting gases, nutrients, wastes, and chemical signals. Comparative physiology has a long history of providing new insights and advancing our understanding of circulatory mass transport across a wide array of circulatory systems. Here we focus on circulatory function of nonmodel species. Invertebrates possess diverse convection systems; that at the most complex generate pressures and perform at a level comparable to vertebrates. Many invertebrates actively modulate cardiovascular function using neuronal, neurohormonal, and skeletal muscle activity. In vertebrates, our understanding of cardiac morphology, cardiomyocyte function, and contractile protein regulation by Ca2+ highlights a high degree of conservation, but differences between species exist and are coupled to variable environments and body temperatures. Key regulators of vertebrate cardiac function and systemic blood pressure include the autonomic nervous system, hormones, and ventricular filling. Further chemical factors regulating cardiovascular function include adenosine, natriuretic peptides, arginine vasotocin, endothelin 1, bradykinin, histamine, nitric oxide, and hydrogen sulfide, to name but a few. Diverse vascular morphologies and the regulation of blood flow in the coronary and cerebral circulations are also apparent in nonmammalian species. Dynamic adjustments of cardiovascular function are associated with exercise on land, flying at high altitude, prolonged dives by marine mammals, and unique morphology, such as the giraffe. Future studies should address limits of gas exchange and convective transport, the evolution of high arterial pressure across diverse taxa, and the importance of the cardiovascular system adaptations to extreme environments. © 2017 American Physiological Society. Compr Physiol 7:17-66, 2017.

The physiological tolerance of the grey carpet shark (Chiloscyllium punctatum) and the epaulette shark (Hemiscyllium ocellatum) to anoxic exposure at three seasonal temperatures

The epaulette shark (Hemiscyllium ocellatum) and the grey carpet shark (Chiloscyllium punctatum) are commonly found in periodically hypoxic environments. The ecophysiological time available for these animals to safely exploit these niches during different seasonal temperatures was examined. The time to loss of righting reflex (T (LRR)) was examined in response to an open ended anoxic challenge at three seasonal temperatures (23, 25 and 27°C). Ventilation rates were measured in an open ended anoxic challenge at 23°C and during 1.5 h of anoxia followed by 2 h of re-oxygenation at 23 and 25°C. The mean T (LRR) of epaulette and grey carpet sharks was inversely proportional to temperature. The T (LRR) was similar between species at 23°C; however, grey carpet sharks had significantly reduced T (LRR) at higher temperatures. During the standardised anoxic challenge, epaulette sharks entered into ventilatory depression significantly earlier at 25°C. During re-oxygenation, epaulette sharks exposed to anoxia at 23°C had no significant increase in ventilation rates. However, after anoxic challenge and re-oxygenation at 25°C, epaulette sharks showed a significant increase in ventilation rates during re-oxygenation. Grey carpet sharks displayed no evidence of ventilatory depression during anoxia. However, during re-oxygenation, grey carpet sharks had significantly elevated ventilation rates above pre-experimental levels and control animals. These data demonstrate that the anoxia tolerance times of both species were temperature dependent, with a significant reduction in the T (LRR) occurring at higher temperatures. Epaulette sharks had a significantly greater T (LRR) at higher temperatures than grey carpet sharks, which did not enter into a ventilatory depression.

Adenosinergic modulation of neuronal activity in the pond snail Lymnaea stagnalis

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

Ecophysiology of neuronal metabolism in transiently oxygen-depleted environments: evidence that GABA is accumulated pre-synaptically in the cerebellum

Interactions between coral reef topography, tide cycles, and photoperiod provided selection pressure for adaptive physiological changes in sheltered hypoxic niches to be exploited by specialized tropical reef fish. The epaulette shark Hemiscyllium ocellatum withstands cyclic hypoxia in its natural environment, many hours of experimental hypoxia, and anoxia for up to 5h. It shows neuronal hypometabolism in response to 5% oxygen saturation. Northern-hemisphere hypoxia- and anoxia-tolerant vertebrates that over-winter under ice alter their inhibitory to excitatory neurotransmitter balance to forestall brain ATP depletion in the absence of oxidative phosphorylation. GABA immunochemistry, HPLC analysis and receptor binding studies in H. ocellatum cerebellum revealed a heterogeneous regional accumulation of neuronal GABA despite no change in its overall concentration, and a significant increase in GABA(A) receptor density without altered binding affinity. Increased GABA(A) receptor density would protect the cerebellum during reoxygenation when transmitter release resumes. While all hypoxia- and anoxia-tolerant teleosts examined to date respond to low oxygen levels by elevating brain GABA, the phylogenetically older epaulette shark did not, suggesting that it uses an alternative neuroprotective mechanism for energy conservation. This may reflect an inherent phylogenetic difference, or represent a novel ecophysiological adaptation to cyclic variations in the availability of oxygen.

Cloning and in vivo expression of vascular endothelial growth factor receptor 2 (Flk1) in the naturally hypoxia-tolerant subterranean mole rat

Vascular endothelial growth factor receptor (VEGF) plays a critical role in blood vessel formation and affects nerve growth and survival. VEGF receptor 2 (Flk1) functions as the major signal transducer of angiogenesis, mediating VEGF induction of endothelial tubulogenesis. We have cloned and analyzed expression of Flk1 in the blind subterranean mole rat Spalax ehrenbergi. Spalax experience abrupt and sharp changes in oxygen supply in their sealed underground niche and, hence, are genetically adapted to hypoxia and serve as a unique, natural mammalian model organism for hypoxia tolerance. Spalax Flk1 is relatively conserved at the nucleic acid and amino acid level compared to human, mouse, and rat orthologs. Reverse transcription-quantitative polymerase chain reaction was used to analyze Flk1 expression in muscle and brain of animals exposed to ambient or variant hypoxic oxygen levels at multiple stages of development. Transcript levels were compared with those obtained from Rattus, a primary model for human physiology. Our findings demonstrate that under normoxic conditions Flk1 patterns of expression correlate well with our previous investigations of VEGF expression. Exposure to hypoxic conditions resulted in divergent patterns of Flk1 expression between Spalax and Rattus and between muscle and brain. It appears that the regulatory mechanisms differentiating expression between the species and between tissues are most likely unique, suggesting that Flk1 expression may be regulated by multiple processes, including both angiogenesis and neurogenesis.

Hypoxia tolerance in reptiles, amphibians, and fishes: life with variable oxygen availability

The ability of fishes, amphibians, and reptiles to survive extremes of oxygen availability derives from a core triad of adaptations: profound metabolic suppression, tolerance of ionic and pH disturbances, and mechanisms for avoiding free-radical injury during reoxygenation. For long-term anoxic survival, enhanced storage of glycogen in critical tissues is also necessary. The diversity of body morphologies and habitats and the utilization of dormancy have resulted in a broad array of adaptations to hypoxia in lower vertebrates. For example, the most anoxia-tolerant vertebrates, painted turtles and crucian carp, meet the challenge of variable oxygen in fundamentally different ways: Turtles undergo near-suspended animation, whereas carp remain active and responsive in the absence of oxygen. Although the mechanisms of survival in both of these cases include large stores of glycogen and drastically decreased metabolism, other mechanisms, such as regulation of ion channels in excitable membranes, are apparently divergent. Common themes in the regulatory adjustments to hypoxia involve control of metabolism and ion channel conductance by protein phosphorylation. Tolerance of decreased energy charge and accumulating anaerobic end products as well as enhanced antioxidant defenses and regenerative capacities are also key to hypoxia survival in lower vertebrates.

Differences in hematocrit of blood samples obtained from two venipuncture sites in sharks

OBJECTIVE

To evaluate differences in Hct between 2 venipuncture sites in captive and free-ranging sharks.

ANIMALS

32 healthy adult captive sharks (Carcharhinus melanopterus, Carcharhinus plumbeus, Stegastoma fasciatum, Orectolobus japonicus, and Triaenodon obesus) and 15 captured free-ranging adult sharks (Carcharhinus limbatus and Carcharhinus acronotus).

PROCEDURES

Blood samples were collected from the caudal tail artery followed by collection from the sinus located immediately caudal to the cranial dorsal fin. The Hct was determined for each sample and results were compared. Additionally, results for sharks that were highly active and used aerobic metabolism were compared with results for sharks that were less active and tolerant of anaerobic conditions.

RESULTS

Mean Hct for all sharks was significantly less (8% less) in blood samples obtained from the cranial dorsal fin sinus, compared with the Hct for samples obtained from the caudal tail artery. When compared on the basis of metabolic class, sharks that were more tolerant of anaerobic conditions had lower Hct values and smaller differences between the 2 venipuncture sites.

CONCLUSIONS AND CLINICAL RELEVANCE

Hct values were significantly lower in blood samples collected from the cranial dorsal fin sinus compared with values for samples collected from the caudal tail artery. It is important to recognize this difference when evaluating hematologic variables in sharks and when establishing reference ranges for Hcts for shark populations. Sharks that were more active and relied on aerobic metabolism had higher Hct values than did anaerobic-tolerant sharks, and the difference in Hct values between venipuncture sites was more pronounced.

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.

Hypoxic survival strategies in two fishes: extreme anoxia tolerance in the North European crucian carp and natural hypoxic preconditioning in a coral-reef shark

Especially in aquatic habitats, hypoxia can be an important evolutionary driving force resulting in both convergent and divergent physiological strategies for hypoxic survival. Examining adaptations to anoxic/hypoxic survival in hypoxia-tolerant animals may offer fresh ideas for the treatment of hypoxia-related diseases. Here, we summarise our present knowledge of two fishes that have evolved to survive hypoxia under very different circumstances.

The crucian carp (Carassius carassius) is of particular interest because of its extreme anoxia tolerance. During the long North European winter, it survives for months in completely oxygen-deprived freshwater habitats. The crucian carp also tolerates a few days of anoxia at room temperature and, unlike anoxia-tolerant freshwater turtles, it is still physically active in anoxia. Moreover, the crucian carp does not appear to reduce neuronal ion permeability during anoxia and may primarily rely on more subtle neuromodulatory mechanisms for anoxic metabolic depression.

The epaulette shark (Hemiscyllium ocellatum) is a tropical marine vertebrate. It lives on shallow reef platforms that repeatedly become cut off from the ocean during periods of low tides. During nocturnal low tides, the water [O2] can fall by 80% due to respiration of the coral and associated organisms. Since the tides become lower and lower over a period of a few days, the hypoxic exposure during subsequent low tides will become progressively longer and more severe. Thus, this shark is under a natural hypoxic preconditioning regimen. Interestingly, hypoxic preconditioning lowers its metabolic rate and its critical PO2. Moreover, repeated anoxia appears to stimulate metabolic depression in an adenosine-dependent way.

α-Adrenergic regulation of systemic peripheral resistance and blood flow distribution in the turtleTrachemys scriptaduring anoxic submergence at 5°C and 21°C

Anoxic exposure in the anoxia-tolerant freshwater turtle is attended by substantial decreases in heart rate and blood flows, but systemic blood pressure (Psys) only decreases marginally due to an increase in systemic peripheral resistance (Rsys). Here,we investigate the role of the α-adrenergic system in modulating Rsys during anoxia at 5°C and 21°C in the turtle Trachemys scripta, and also describe how anoxia affects relative systemic blood flow distribution(%Q̇sys) and absolute tissue blood flows. Turtles were instrumented with an arterial cannula for measurement of Psys and ultrasonic flow probes on major systemic blood vessels for determination of systemic cardiac output(Q̇sys). α-Adrenergic tone was assessed from vascular injections of α-adrenergic agonists and antagonists (phenylephrine and phentolamine, respectively) during normoxia and following either 6 h (21°C) or 12 days (5°C) of anoxic submergence. Coloured microspheres, injected through a left atrial cannula during normoxia and anoxia, as well as after α-adrenergic stimulation and blockade during anoxia at both temperatures, were used to determine relative and absolute tissue blood flows.

Anoxia was associated with an increased Rsys and functional α-adrenergic vasoactivity at both acclimation temperatures. However, while anoxia at 21°C was associated with a high systemicα-adrenergic tone, the progressive increase of Rsysat 5°C was not mediated by α-adrenergic control. A redistribution of blood flow away from ancillary vascular beds towards more vital circulations occurred with anoxia at both acclimation temperatures.%Q̇sys and absolute blood flow were reduced to the digestive and urogenital tissues (approximately 2- to 15-fold), while %Q̇sys and absolute blood flows to the heart and brain were maintained at normoxic levels. The importance of liver and muscle glycogen stores in fueling anaerobic metabolism were indicated by increases in%Q̇sys to the muscle at 21°C (1.3-fold) and liver at 5°C (1.7-fold). As well, the crucial importance of the turtle shell as a buffer reserve during anoxic submergence was indicated by 40-50% of Q̇sys being directed towards the shell during anoxia at both 5°C and 21°C. α-Adrenergic stimulation and blockade during anoxia caused few changes in%Q̇sys and absolute tissue blood flow. However, there was evidence of α-adrenergic vasoactivity contributing to blood flow regulation to the liver and shell during anoxic submergence at 5°C.

Energy metabolism of fish brain

This review focuses on recent research on the metabolic function of fish brain. Fish brain is isolated from the systemic circulation by a blood-brain barrier that allows the transport of glucose, monocarboxylates and amino acids. The limited information available in fishes suggests that oxidation of exogenous glucose and oxidative phosphorylation provide most of the ATP required for brain function in teleosts, whereas oxidation of ketones and amino acids occurs preferentially in elasmobranchs. In several agnathans and benthic teleosts brain glycogen levels rather than exogenous glucose may be the proximate glucose source for oxidation. In situations when glucose is in limited supply, teleost brains utilize other fuels such as lactate or ketones. Information on use of lipids and amino acids as fuels in fish brain is scarce. The main pathways of brain energy metabolism are changed by several effectors. Thus, several parameters of brain energy metabolism have been demonstrated to change post-prandially in teleostean fishes. The absence of food in teleosts elicits profound changes in brain energy metabolism (increased glycogenolysis and use of ketones) in a way similar to that demonstrated in mammals though delayed in time. Environmental factors induce changes in brain energy parameters in teleosts such as the enhancement of glycogenolysis elicited by pollutants, increased capacity for anaerobic glycolysis under hypoxia/anoxia or changes in substrate utilization elicited by adaptation to cold. Furthermore, several studies demonstrate effects of melatonin, insulin, glucagon, GLP-1, cortisol or catecholamines on energy parameters of teleost brain, although in most cases the results are quite preliminary being difficult to relate the effects of those hormones to physiological situations. The few studies performed with the different cell types available in the nervous system of fish allow us to hypothesize few functional relationships among those cells. Future research perspectives are also outlined.

Exposure to hypoxia primes the respiratory and metabolic responses of the epaulette shark to progressive hypoxia

The majority of vertebrates are not tolerant to hypoxia but epaulette sharks (Hemiscyllium ocellatum) living on shallow reef platforms appear to tolerate hypoxic periods during tidal fluctuations. The effects of progressive hypoxia on the metabolic and ventilatory responses of these elasmobranchs were examined in a closed respirometer. In order to determine whether repeated exposure to hypoxia primes these sharks to alter their metabolism, one group of sharks was exposed to repeated sub-lethal hypoxia, at 5% of air saturation, prior to respirometry. In response to falling oxygen concentration [O(2)], the epaulette shark increased its ventilatory rate and maintained its O(2) consumption rate (VO(2)) down to 2.2 mg O(2) l(-1) at 25 degrees C. This is the lowest critical [O(2)] ([O(2)](crit)) ever measured for any elasmobranch. After reaching the [O(2)](crit), the shark remained in the respirometer for a further 4-5 h of progressive hypoxia. Only after the [O(2)] fell to 1.0 mg l(-1) was there a decrease in the ventilatory rate followed by a rise in blood lactate levels, indicating that the epaulette shark responds to severe hypoxia by entering a phase of metabolic and ventilatory depression. Interestingly, hypoxia tolerance was dynamic because hypoxic pre-conditioning lowered the VO(2) of the epaulette shark by 29%, which resulted in a significantly reduced [O(2)](crit) (1.7 mg O(2) l(-1)), revealing that hypoxic pre-conditioning elicits an enhanced physiological response to hypoxia.

The role of adenosine in the anoxic survival of the epaulette shark, Hemiscyllium ocellatum

The epaulette shark (Hemiscyllium ocellatum) is among the few vertebrates that can tolerate extreme hypoxia for prolonged periods and, as shown here, anoxia. We examined how anoxia affected this shark's level of responsiveness, concentration of brain ATP and adenosine -- an endogenous neuronal depressant. In addition, we investigated how these variables were affected by aminophylline, an adenosine receptor antagonist. Epaulette sharks placed in an anoxic environment (<0.02 mg O2 l(-1)) lost their righting reflex after 46.3 +/- 2.8 min, but immediately regained vigilance upon return to normoxia. Then 24 h later, the same sharks were injected with either saline or aminophylline (30 mg kg(-1)) in saline and re-exposed to anoxia. In this second anoxic episode, controls sharks showed a 56% decrease in the time taken to lose their righting reflex but maintained their brain ATP levels; conversely, aminophylline-treated epaulette sharks displayed a 46% increase in the time to loss of righting reflex and had significantly lower brain ATP levels. Since anoxia also caused a 3.5-fold increase in brain adenosine levels, these results suggest that adenosine receptor activation had a pre-emptive role in maintaining brain ATP levels during anoxia. Perhaps because adenosine receptor activation initiates metabolic depression, indicated by the early loss of responsiveness (righting reflex), such a mechanism would serve to reduce ATP consumption and maintain brain ATP levels.

Effect of anoxia and adenosine on cerebral blood flow in the leopard frog (Rana pipiens)

The effect of anoxia on cerebral blood velocity (CBV) on the dorsal surface of telencephalon was examined in the leopard frog, Rana pipiens, using a stereomicroscope. During exposure to anoxia, a transient 228% increase in CBV velocity was seen after 20 min, but CBV fell back to basal values after a further 20 min of anoxia. Topical application of 50 microM adenosine during normoxia caused a 52% increase in CBV, while 250 microM adenosine caused no further increase. At both concentrations, the effect was completely inhibited by the adenosine receptor blocker aminophylline (250 microM). Superfusing the brain with aminophylline during anoxia did not affect the anoxia-induced increase in CBV. We conclude that adenosine can stimulate CBV in R. pipiens. However, unlike in other anoxia-tolerant animals, adenosine seems not to be a main mediator of the anoxia induced increase in CBV in the frog.

Neuronal oxidative hypometabolism in the brainstem of the epaulette shark (Hemiscyllium ocellatum) in response to hypoxic pre-conditioning

Reduced oxidative demand or neuronal hypometabolism is a neuroprotective strategy used by several anoxia and hypoxia-tolerant species. The epaulette shark, Hemiscyllium ocellatum inhabits shallow reef platforms that can become hypoxic. Hypoxic pre-conditioning (eight cycles of 0.34 mg O(2)/l for 120 min, 12 h apart) was used to determine whether a reduction in oxidative metabolism could be elicited in the epaulette shark brain. Hypoxic pre-conditioning resulted in a significant overall reduction in oxidative activity in coronal sections of the brainstem, but key nuclei displayed heterogeneous levels of oxidative metabolism. Motor nuclei had significantly lower levels of oxidative activity while sensory nuclei did not. The epaulette shark's ability to enter this state of hypometabolism in response to hypoxic pre-conditioning revealed a neuroprotective mechanism, which would not only reduce neuronal damage during hypoxic exposure but also minimise re-oxygenation injury.