Drop In PO2 Research Articles

Capillary oxygen regulates demand–supply coupling by triggering connexin40-mediated conduction: Rethinking the metabolic hypothesis

Coupling red blood cell (RBC) supply to O2 demand is an intricate process requiring O2 sensing, generation of a stimulus, and signal transduction that alters upstream arteriolar tone. Although actively debated, this process has been theorized to be induced by hypoxia and to involve activation of endothelial inwardly rectifying K+ channels (KIR) 2.1 by elevated extracellular K+ to trigger conducted hyperpolarization via connexin40 (Cx40) gap junctions to upstream resistors. This concept was tested in resting healthy skeletal muscle of Cx40-/- and endothelial KIR2.1-/- mice using state-of-the-art live animal imaging where the local tissue O2 environment was manipulated using a custom gas chamber. Second-by-second capillary RBC flow responses were recorded as O2 was altered. A stepwise drop in PO2 at the muscle surface increased RBC supply in capillaries of control animals while elevated O2 elicited the opposite response; capillaries were confirmed to express Cx40. The RBC flow responses were rapid and tightly coupled to O2; computer simulations did not support hypoxia as a driving factor. In contrast, RBC flow responses were significantly diminished in Cx40-/- mice. Endothelial KIR2.1-/- mice, on the other hand, reacted normally to O2 changes, even when the O2 challenge was targeted to a smaller area of tissue with fewer capillaries. Conclusively, microvascular O2 responses depend on coordinated electrical signaling via Cx40 gap junctions, and endothelial KIR2.1 channels do not initiate the event. These findings reconceptualize the paradigm of blood flow regulation in skeletal muscle and how O2 triggers this process in capillaries independent of extracellular K+.

Conducted Capillary Signaling Enables Oxygen Responses in Skeletal Muscle Independent of Metabolite Production

Red blood cell (RBC) flow through the microvasculature is closely matched to tissue O2 requirements. At a fundamental level, O2 demand‐supply coupling entails the sensing of PO2, and the generation of a stimulus that alters upstream arteriolar tone. However, where and how O2 needs are sensed in the microcirculation is presently contested. One idea centers on hypoxia triggering the release of K+, activating endothelial KIR2.1 channels and initiating a hyperpolarization that conducts upstream via gap junctions, comprised of connexins (Cx). We tested this idea in skeletal muscle where we controlled the local tissue O2 environment using live animal imaging in Cx40‐/‐and endothelial KIR2.1‐/‐ mice. The extensor digitorum longus muscle positioned overtop a gas control chamber allowed second‐by‐second monitoring of capillary RBC flow responses as O2 was altered around its physiological set point of 53 mmHg. A stepwise drop in PO2 at the muscle surface (53 to 15 or 0 mmHg) increased RBC supply rate in control capillaries while elevated chamber O2 elicited the opposite response; these capillaries robustly expressed Cx40. The RBC flow responses were rapid and tightly coupled to O2 levels as readily observed when chamber O2 was oscillated in a sinusoidal manner. In contrast, this blood flow response was significantly diminished in Cx40‐/‐ mice and translated into lower capillary RBC O2 saturation. KIR2.1‐/‐mice, on the other hand, had normal resting RBC O2 saturation and reacted normally to O2 changes, albeit an oscillation or a sustained stepwise decrease. Furthermore, we confirmed that RBC flow responses are conserved in endothelial KIR2.1‐/‐mice even when the low O2 challenge is applied to a restricted number of surface capillaries in the muscle; interestingly, these responses were dominated by capillary hematocrit changes in both control and endothelial KIR2.1‐/‐mice. Modelling this phenomenon supported the idea that the number of capillaries stimulated is indeed tied to conduction distance and tissue RBC distribution. In conclusion, we demonstrate that microvascular O2 responses depend on coordinated electrical signaling via gap junctions comprised of Cx40 and that endothelial KIR2.1channels do not drive the initiating electrical event. These findings reconceptualize our understanding of blood flow regulation and how O2 initiates this process at the capillary level independent of metabolite production.

Intracranial pressure spikes trigger spreading depolarizations.

Spreading depolarizations are highly prevalent and spatiotemporally punctuated events worsening the outcome of brain injury. Trigger factors are poorly understood but may be linked to sudden worsening in supply-demand mismatch in compromised tissue. Sustained or transient elevations in intracranial pressure are also prevalent in the injured brain. Here, using a mouse model of large hemispheric ischaemic stroke, we show that mild and brief intracranial pressure elevations (20 or 30 mmHg for just 3 min) potently trigger spreading depolarizations in ischaemic penumbra (4-fold increase in spreading depolarization occurrence). We also show that 30 mmHg intracranial pressure spikes as brief as 30 s are equally effective. In contrast, sustained intracranial pressure elevations to the same level for 30 min do not significantly increase the spreading depolarization rate, suggesting that an abrupt disturbance in the steady state equilibrium is required to trigger a spreading depolarization. Laser speckle flowmetry consistently showed a reduction in tissue perfusion, and two-photon pO2 microscopy revealed a drop in venous pO2 during the intracranial pressure spikes suggesting increased oxygen extraction fraction, and therefore, worsening supply-demand mismatch. These haemodynamic changes during intracranial pressure spikes were associated with highly reproducible increases in extracellular potassium levels in penumbra. Consistent with the experimental data, a higher rate of intracranial pressure spikes was associated with spreading depolarization clusters in a retrospective series of patients with aneurysmal subarachnoid haemorrhage with strong temporal correspondence. Altogether, our data show that intracranial pressure spikes, even when mild and brief, are capable of triggering spreading depolarizations. Aggressive prevention of intracranial pressure spikes may help reduce spreading depolarization occurrence and improve outcomes after brain injury.

Caffeine Exacerbates Postictal Hypoxia

A stroke-like event follows seizures which may be responsible for the postictal state and a contributing factor to the development of seizure-induced brain abnormalities and behavioral dysfunction associated with epilepsy. Caffeine is the world’s most popular drug with ∼85% of people in the USA consuming it daily. Thus, persons with epilepsy are likely to have caffeine in their body and brain during seizures. This preclinical study investigated the effects of acute caffeine on local hippocampal tissue oxygenation pre and post seizure. We continuously measured local oxygen levels in the CA1 region of the hippocampus and utilized the electrical kindling model in rats. Rats were acutely administered either caffeine, or one of its metabolites, or agonists and antagonists at adenosine sub-receptor types or ryanodine receptors prior to the elicitation of seizures. Acute caffeine administration caused a significant drop in pre-seizure hippocampal pO2. Following a seizure, caffeine, as well as two of its metabolites paraxanthine, and theophylline, increased the time below the severe hypoxic threshold (10 mmHg). Likewise, the specific A2A receptor antagonist, SCH-58261, mimicked caffeine by causing a significant drop in pre-seizure pO2 and the area and time below the severe hypoxic threshold. Moreover, the A2A receptor agonist, CGS-21680 was able to prevent the effect of both caffeine and SCH-58261 adding further evidence that caffeine is likely acting through the A2A receptor. Clinical tracking and investigations are needed to determine the effect of caffeine on postictal symptomology and blood flow in persons with epilepsy.

Hypoxia and Inflammation as a Consequence of β-Fibril Accumulation: A Perspective View for New Potential Therapeutic Targets.

Amyloidoses are heterogeneous diseases that result from the deposition of toxic insoluble β-sheet fibrillar protein aggregates in different tissues. The cascade of molecular events leading to amyloidoses and to the related clinical manifestations is not completely understood. Nevertheless, it is known that tissue damage associated to this disease involves alteration of tissue architecture, interaction with cell surface receptors, inflammation elicited by the amyloid protein deposition, oxidative stress, and apoptosis. However, another important aspect to consider is that systemic protein massive deposition not only subverts tissue architecture but also determines a progressive cellular hypertrophy and dilation of the extracellular space enlarging the volume of the organ. Such an alteration increases the distance between cells and vessels with a drop in pO2 that, in turn, causes both necrotic cell death and activation of the hypoxia transcription factor HIF-1α. Herewith, we propose the hypothesis that both cell death and hypoxia represent two important events for the pathogenesis of damage and progression of amyloidoses. In fact, molecules released by necrotic cells activate inflammatory cells from one side while binding to HIF-1α-dependent membrane receptors expressed on hypoxic parenchymal cells on the other side. This latter event generates a signaling cascade triggering NFκB activation and chronic inflammation. Finally, we also suggest that this scenario, once proved and detailed, might suggest important targets for new therapeutic interventions.

Blunt Chest Trauma in Mice after Cigarette Smoke-Exposure: Effects of Mechanical Ventilation with 100% O2.

Cigarette smoking (CS) aggravates post-traumatic acute lung injury and increases ventilator-induced lung injury due to more severe tissue inflammation and apoptosis. Hyper-inflammation after chest trauma is due to the physical damage, the drop in alveolar PO2, and the consecutive hypoxemia and tissue hypoxia. Therefore, we tested the hypotheses that 1) CS exposure prior to blunt chest trauma causes more severe post-traumatic inflammation and thereby aggravates lung injury, and that 2) hyperoxia may attenuate this effect. Immediately after blast wave-induced blunt chest trauma, mice (n=32) with or without 3-4 weeks of CS exposure underwent 4 hours of pressure-controlled, thoraco-pulmonary compliance-titrated, lung-protective mechanical ventilation with air or 100 % O2. Hemodynamics, lung mechanics, gas exchange, and acid-base status were measured together with blood and tissue cytokine and chemokine concentrations, heme oxygenase-1 (HO-1), activated caspase-3, and hypoxia-inducible factor 1-α (HIF-1α) expression, nuclear factor-κB (NF-κB) activation, nitrotyrosine formation, purinergic receptor 2X4 (P2XR4) and 2X7 (P2XR7) expression, and histological scoring. CS exposure prior to chest trauma lead to higher pulmonary compliance and lower PaO2 and Horovitz-index, associated with increased tissue IL-18 and blood MCP-1 concentrations, a 2-4-fold higher inflammatory cell infiltration, and more pronounced alveolar membrane thickening. This effect coincided with increased activated caspase-3, nitrotyrosine, P2XR4, and P2XR7 expression, NF-κB activation, and reduced HIF-1α expression. Hyperoxia did not further affect lung mechanics, gas exchange, pulmonary and systemic cytokine and chemokine concentrations, or histological scoring, except for some patchy alveolar edema in CS exposed mice. However, hyperoxia attenuated tissue HIF-1α, nitrotyrosine, P2XR7, and P2XR4 expression, while it increased HO-1 formation in CS exposed mice. Overall, CS exposure aggravated post-traumatic inflammation, nitrosative stress and thereby organ dysfunction and injury; short-term, lung-protective, hyperoxic mechanical ventilation have no major beneficial effect despite attenuation of nitrosative stress, possibly due to compensation of by regional alveolar hypoxia and/or consecutive hypoxemia, resulting in down-regulation of HIF-1α expression.

Oxygen diffusion: an enzyme-controlled variable parameter.

Previous oxygen microelectrode studies have shown that the oxygen diffusion coefficient (DO₂) increases during extracellular PO₂ decreases, while intracellular PO₂ remained unchanged and thus cell function (spike activity of neurons). Oxygen dependency of complex multicellular organisms requires a stable and adequate oxygen supply to the cells, while toxic concentrations have to be avoided. Oxygen brought to the tissue by convection diffuses through the intercellular and cell membranes, which are potential barriers to diffusion. In gerbil brain cortex, PO₂ and DO₂ were measured by membrane-covered and by bare gold microelectrodes, as were also spike potentials. Moderate respiratory hypoxia was followed by a primary sharp drop of tissue PO₂ that recovered to higher values concomitant with an increase of DO₂. A drop in intracellular PO₂ recovered immediately. Studies on the abdominal ganglion of aplysia californica showed similar results.Heterogeneity is a feature of both normal oxygen supply to tissue and supply due to a wide range of disturbances in oxygen supply. Oxygen diffusion through membranes is variable thereby ensuring adequate intracellular PO₂. Cell-derived glucosamine oxidase seems to regulate the polymerization/depolymerisation ratio of membrane mucopolysaccharides and thus oxygen diffusion.Variability of oxygen diffusion is a decisive parameter for regulating the supply/demand ratio of oxygen supply to the cell; this occurs in highly developed animals as well as in species of a less sophisticated nature. Autoregulation of oxygen diffusion is as important as the distribution/perfusion ratio of the capillary meshwork and as the oxygen extraction ratio in relation to oxygen consumption of the cell. Oxygen diffusion resistance is the cellular protection against luxury oxygen supply (which can result in toxic oxidative species leading to mutagenesis).

In vivophotoacoustic lifetime imaging of tumor hypoxia in small animals

Tumor hypoxia is an important factor in assessment of both cancer progression and cancer treatment efficacy. This has driven a substantial effort toward development of imaging modalities that can directly measure oxygen distribution and therefore hypoxia in tissue. Although several approaches to measure hypoxia exist, direct measurement of tissue oxygen through an imaging approach is still an unmet need. To address this, we present a new approach based on in vivo application of photoacoustic lifetime imaging (PALI) to map the distribution of oxygen partial pressure (pO2) in tissue. This method utilizes methylene blue, a dye widely used in clinical applications, as an oxygen-sensitive imaging agent. PALI measurement of oxygen relies upon pO2-dependent excitation lifetime of the dye. A multimodal imaging system was designed and built to achieve ultrasound (US), photoacoustic, and PALI imaging within the same system. Nude mice bearing LNCaP xenograft hindlimb tumors were used as the target tissue. Hypoxic regions were identified within the tumor in a combined US/PALI image. Finally, the statistical distributions of pO2 in tumor, normal, and control tissues were compared with measurements by a needle-mounted oxygen probe. A statistically significant drop in mean pO2 was consistently detected by both methods in tumors.

Effects of Sepiapterin Infusion on Renal Oxygenation and Early Acute Renal Injury After Suprarenal Aortic Clamping in Rats

Acute kidney injury (AKI) can occur after aortic clamping due to microvascular dysfunction leading to renal hypoxia. In this rat study, we have tested the hypothesis that the administration of the precursor of the nitric oxide synthase essential cofactor tetrahydrobiopterin (BH4) could restore renal oxygenation after ischemia reperfusion (I/R) and prevent AKI. We randomly distributed rats into 4 groups: sham group; ischemia-reperfusion group; I/R + sepiapterin, the precursor of BH4; and I/R + sepiapterin + methotrexate, an inhibitor of the pathway generating BH4 from sepiapterin. Cortical and outer medullary microvascular oxygen pressure, renal oxygen delivery, renal oxygen consumption were measured using dual-wavelength oxygen-dependent quenching phosphorescence techniques during ischemia and throughout 3 hours of reperfusion. Kidney injury was assessed using myeloperoxidase staining for leukocyte infiltration and urine neutrophil gelatinase-associated lipocalin levels. Ischemia reperfusion induced a drop in microvascular PO2 (P < 0.01 vs. Sham, both), which was prevented by the infusion of sepiapterin. Sepiapterin partially prevented the rise in renal oxygen extraction (P < 0.001 vs. I/R). Finally, treatment with sepiapterin prevented renal infiltration by inflammatory cells and decreased urine neutrophil gelatinase-associated lipocalin levels indicating a decrease of renal injury. These effects were blunted when adding methotrexate, except for myeloperoxidase. In conclusion, the administration of sepiapterin can prevent renal hypoxia and AKI after suprarenal aortic clamping in rats.

Intermittent hypoxic training: risks versus benefits. A biomedical engineering point of view

A recent Letter to the Editor ‘‘Intermittent hypoxic training: risks versus benefits’’ (Hinghofer-Szalkay 2010) recommended inquiring into the long-term risks of intermittent hypoxic training (IHT), pointing to the potential risk of reactive oxygen species (ROS) formation. In this context, it is important to separate the hazards associated with the use of a device/system and the physiological risks and benefits of the therapy itself. Devices and systems for hypoxia intervention are called ‘‘hypoxicators’’. Hypoxicators fall under the common definition of a medical device because of their intended use, the ‘‘modification of physiological process’’ (Council Directive 93/42/EEC 1993). Any device for human or animal treatment must naturally meet essential requirements for medical devices, e.g. electrical medical safety (IEC 60601-1 2007); and the Australian Therapeutic Goods Administration recently recommended regulating all hypoxicators (Therapeutic Goods Administration 2008). The working mechanism underpinning IHT is intimately linked to oxygen sensing mechanisms (Renshaw and Nikinmaa 2007). If there is a significant drop in cellular pO2, the signalling protein responsible for oxygen level monitoring hypoxia-inducible factor 1 alpha (HIF-1a) triggers the transcription of a large number of genes involved in diverse processes such as angiogenesis, erythropoiesis, vascular tone, metal transport, glycolysis, mitochondrial function, cell growth and survival (e.g. Webb et al. 2009). The main advantage of IHT is that it creates a hypoxic challenge that is sufficiently strong to produce the desired adaptive responses in a drug-free fashion (Hellemans 1999; Hamlin et al. 2007, 2009), while avoiding any damaging effects. Indeed, more-prolonged stays at extreme altitudes can cause oxidative stress (Huang et al. 2008). However, this adverse effect should not be automatically extrapolated onto other hypoxic protocols, such as IHT, since it is important to recognise that severe chronic hypoxia and shorter-term hypoxia often have directly opposite physiological effects (Bailey et al. 2000; Serebrovskaya et al. 2008). In a recent review of various hypoxia training regimes (Millet et al. 2010), ‘‘medically supervised treatments’’ using ‘‘altitude houses’’ and ‘‘altitude domes’’ were given a preference over home-use ‘‘altitude tents’’. Such risk analysis is of some concern, since, in either system, there is typically no continuous physiological monitoring of subjects undergoing hypoxic training. Altitude houses and domes, driven by nitrogen inflow, create the hazard of a potentially life-threatening low-oxygen environment. In our view, the greatest single hazard of IHT is the potential for prolonged levels of low arterial O2 saturation (SpO2) to or below the safe threshold of 70% (Westerman 2004). However, nowadays, noninvasive pulse-oximetry readily allows measurement of SpO2. Properly designed hypoxicators thus address the residual risks by providing safety cutoffs, automatically preventing potentially dangerous reduction of inspired pO2 and SpO2 and limiting exposure time (Bassovitch and Serebrovskaya 2009). To the best of our knowledge, there have been no reported adverse physiological effects when users have followed the recommendations of ‘‘a few minutes of targeted SpO2 75–88% at rest, alternated with reoxygenation’’ Communicated by Susan Ward.

Using carbon fibre microelectrodes to monitor the oxidative metabolism of blowfly eyes

The oxidative metabolism in animal tissues can be conveniently monitored by measuring tissue PO2 with a carbon fibre microelectrode. We have established a recording configuration in a living animal by insertion of a carbon fibre electrode (CFE) into the retina of a blowfly (Calliphora vicina – chalky). The current flowing over an exposed carbon disc at the tip of an insulated carbon fibre with 5 µm diameter is linearly proportional to PO2 when the PO2 was varied between 0 kPa (100% N2) and 100 kPa (100% O2) in the recording chamber. The slight changes in sensitivity of CFE during the recording time were corrected by calibrationsperformed at the start and at the end of the experiments. Exposure of the eye to bright light caused a drop in tissue PO2. Hypoxia increased with the stimulation time, reaching a maximum after about 20 s (∆PO2=11.6 kPa). These results are in good agreement with direct measurements of O2 consumption in isolated eyes.

Reduction of hypoxia by perfluorocarbon emulsion in a traumatic spinal cord injury model

Few therapies have consistently demonstrated effectiveness in preserving O2 delivery after spinal cord injury (SCI). Perfluorocarbons (PFCs) offer great promise to carry and deliver O2 more efficiently than conventional measures. The authors investigated the use of Clark-type microelectrodes to monitor spinal cord oxygenation directly (intraparenchymal [IP] recording) and indirectly (cerebrospinal fluid [CSF] recording) in the context of SCI, O2 therapy, and PFC treatment. After placement of a subdural/CSF Licox probe in rats, incremental increases in the fraction of inspired O2 (FiO2) up to 100% were administered to establish a dose-response curve. The probe was then placed in the parenchyma of the same animals for a second dose-response curve. In a second study, rats with CSF or IP probes underwent SCI with the NYU Impactor and treatment with O2, followed by administration of PFC, or saline in the control group. All animals in the first experiment responded to the FiO2 dose increase, with changes in PO2 evident in both CSF and IP levels. The SCI in the second experiment caused a marked drop in PO2 from a mean of 21.4 to 10.4 mm Hg, with most animals dropping to less than half their preinjury value. All animals responded to 100% O2 treatment. Every animal that received PFCs showed significant improvement, with a mean increase in PO2 of 23.3 mm Hg. Only 1 saline-treated animal showed any benefit. Oxygen values in the PFC treatment group reached up to 6 times the normal level. Oxygen levels in SCI show a profound drop almost immediately postinjury. Administration of PFCs combined with 100% O2 therapy can reverse tissue hypoxia and holds promise for reducing ischemic injury.

Apparent diffusion time of oxygen from blood to tissue in rat cerebral cortex: implication for tissue oxygen dynamics during brain functions

To investigate the dynamics of tissue oxygen demand and supply during brain functions, we simultaneously recorded Po(2) and local cerebral blood flow (LCBF) with an oxygen microelectrode and laser Doppler flowmetry, respectively, in rat somatosensory cortex. Electrical hindlimb stimuli were applied for 1, 2, and 5 s to vary the duration of evoked cerebral metabolic rate of oxygen (CMR(O(2))). The electrical stimulation induced a robust increase in Po(2) (4-9 Torr at peak) after an increase in LCBF (14-26% at peak). A consistent lag of approximately 1.2 s (0.6-2.3 s for individual animals) in the Po(2) relative to LCBF was found, irrespective of stimulus length. It is argued that the lag in Po(2) was predominantly caused by the time required for oxygen to diffuse through tissue. During brain functions, the supply of fresh oxygen further lagged because of the latency of LCBF onset ( approximately 0.4 s). The results indicate that the tissue oxygen supports excess demand until the arrival of fresh oxygen. However, a large drop in Po(2) was not observed, indicating that the evoked neural activity demands little extra oxygen or that the time course of excess demand is as slow as the increase in supply. Thus the dynamics of Po(2) during brain functions predominantly depend on the time course of LCBF. Possible factors influencing the lag between demand and supply are discussed, including vascular spacing, reactivity of the vessels, and diffusivity of oxygen.

O2 sensing at the mammalian carotid body: why multiple O2 sensors and multiple transmitters?

Carotid bodies are the sensory organs for detecting systemic hypoxia and the ensuing reflexes prevent the development of tissue/cellular hypoxia. Although every mammalian cell responds to hypoxia, O2 sensing by the carotid body is unique in that it responds instantaneously (within seconds) to even a modest drop in arterial PO2. Sensing hypoxia in the carotid body requires an initial transduction step involving O2 sensor(s) and transmitter(s) for subsequent activation of the afferent nerve ending. This brief review focuses on: (a) whether the transduction involves 'single' or 'multiple' O2 sensors; (b) the identity of the excitatory transmitter(s) responsible for afferent nerve activation by hypoxia; and (c) whether inhibitory transmitters have any functional role. The currently proposed O2 sensors include various haem-containing proteins, and a variety of O2-sensitive K+ channels. It is proposed that the transduction involves an ensemble of, and interactions between, haem-containing proteins and O2-sensitive K+-channel proteins functioning as a 'chemosome'; the former for conferring sensitivity to wide range of PO2 values and the latter for the rapidity of the response. Hypoxia releases both excitatory and inhibitory transmitters from the carotid body. ATP is emerging as an important excitatory transmitter for afferent nerve activation by hypoxia. Whereas the inhibitory messengers act in concert with excitatory transmitters like a 'push-pull' mechanism to prevent over excitation, conferring the 'slowly adapting' nature of the afferent nerve activation during prolonged hypoxia. Further studies are needed to test the interactions between putative O2 sensors and excitatory and inhibitory transmitters in the carotid body.

HIGHLIGHTED TOPIC: Pulmonary Circulation and Hypoxia

In 1946, Ulf von Euler and Goran Liljestrand published revolutionary findings on the impact of hypoxia as a stimulant for vasoconstriction in the pulmonary circulation. More than 50 years later, exploring the mechanistic basis for this relationship is still a vital area of research. Among the many

Temperature‐dependent changes in energy metabolism, intracellular pH and blood oxygen tension in the Atlantic cod

The effect of acute increase in temperature on oxygen partial pressure (Po2) was measured in the gill arches of Atlantic cod Gadus morhua between 10 and 19° C by use of oxygen microoptodes. Oxygen saturation of the gill blood under control conditions varied between 90 and 15% reflecting a variable percentage of arterial or venous blood in accordance with the position of each optode in the gill arch. The data obtained suggested that arterial Po2 remained more or less constant and arterial oxygen uptake did not become limiting during warming. A progressive drop in venous Po2, however, was observed at &gt;10° C indicating that excessive oxygen uptake from the blood is not fully compensated for by circulatory performance, until finally, Po2 levels fully collapse. In a second set of experiments energy and acid–base status of white muscle of Atlantic cod in vivo was measured by magnetic resonance (31P‐NMR) spectroscopy in unanaesthetized and unimmobilized fish in the temperature range between 13 and 21° C. A decrease in white muscle intracellular pH (pHi) with temperature occurred between 10 and 16° C (ΔpH per ° C = −0·025 per ° C). In white muscle temperature changes had no influence on high‐energy phosphates such as phosphocreatine (PCr) or ATP except during exposure to high critical temperatures (&gt;16° C), indicating that white muscle energy status appears to be relatively insensitive to thermal stress if compared to the thermal sensitivity of the whole animal. The data were consistent with the hypothesis of an oxygen limitation of thermal tolerance in animals, which is set by limited capacity of oxygen supply mechanisms. In the case of Atlantic cod circulatory rather than ventilatory performance may be the first process to cause oxygen deficiency during heat stress.

Peripheral oxygen transport in skeletal muscle of Antarctic and sub-Antarctic notothenioid fish.

Transcellular oxygen flux was modelled mathematically in the aerobic skeletal muscles of perciform fish species living at widely different temperatures (Antarctica, sub-Antarctica and the Mediterranean Sea). Using structural data derived from stereological analysis of electron micrographs, mean fibre P(O(2)) was calculated on the basis of temperature-corrected rates of mitochondrial respiration and oxygen diffusion. The mean muscle fibre diameter (MFD) among Antarctic notothenioids was in the range 17-61 microm and mitochondrial volume density, Vv(mit,f), was 0.27-0.53, but capillary-to-fibre ratio varied only between 1.2 and 1.5. For a mean capillary P(O(2)) of 6 kPa, the model predicted a mean tissue P(O(2)) in the range 0.7-5.8 kPa at the estimated maximum aerobic capacity (M(O(2)max)). The lowest levels of tissue oxygenation were found in the pectoral muscle fibres of the icefish Chaenocephalus aceratus, which lacks the respiratory pigments haemoglobin and myoglobin. Red-blooded notothenioids found in the sub-Antarctic had a similar muscle fine structure to those caught south of the Antarctic Convergence, with an MFD of 20-41 microm and Vv(mit,f) of 0.27-0.33, resulting in an estimated mean P(O(2)) of 4-5 kPa at M(O(2)max). Mean tissue P(O(2)) in the sub-Antarctic icefish Champsocephalus esox, with greater MFD and Vv(mit,f), 56 microm and 0.51, respectively, was calculated to exceed 1 kPa at winter temperatures (4 degrees C), although oxidative metabolism was predicted to be impaired at the summer maximum of 10 degrees C. At the high end of the thermal range, related perciform species from the Mediterranean had a negligible drop in intracellular P(O(2)) across their small-diameter fibres, to a minimum of 5.4 kPa, comparable with that predicted for Trematomus newnesi from the Antarctic (5.6 kPa) with a similar MFD. These data suggest that, within a single phylogenetic group, integrative structural adaptations potentially enable a similar degree of tissue oxygenation over a 20 degrees C range of environmental temperature in the red-blooded notothenioids, and that this is compromised by the lack of respiratory pigments in the icefishes. The mean capillary radius was 1.5 times greater in the two icefish than in the other notothenioids, and the model simulations indicate that the evolution of wide-bore capillaries is essential to maintain tissue oxygenation in the absence of respiratory pigments.

Multiparameter brain tissue monitoring--correlation between parameters and identification of CPP thresholds.

Continuous monitoring of brain interstitial gas concentrations allows direct regional evaluation of the pathophysiology of cerebral tissues. We have incorporated the Paratrend 7 (P7) multiparameter sensor into our established multimodal monitoring of head injured patients, to investigate the relationship between brain and arterial pO2, pCO2, and pH, as well as defining thresholds for cerebral perfusion pressure (CPP). A P7 sensor was inserted into the brain tissue of 40 adult head injured patients via a modified Camino bolt or triple lumen bolt. A second sensor was placed in the femoral artery for continuous monitoring of blood gases. Data signals from 19 monitored parameters were collected onto computer at the bedside for up to 14 days. No complications were seen. For individual patients the changes in brain tissue parameters showed large variations over 24 hours and the relationship between parameters varied considerably both between patients and during the period of monitoring any one individual. Changes related to periods of arterial desaturation, cerebral hypoperfusion and therapeutic manoeuvres could be seen. Good correlation was seen between brain pCO2 and arterial pCO2 (r = 0.58). Poor correlation was seen between CPP and brain pO2, and between brain pO2 and ICP. However, by grouping values for intracranial pressure (ICP) and CPP, thresholds for brain tissue pO2 were identified in 16 patients where CPP fell below 60 mmHg. No patients where CPP was always > 60 mmHg showed a significant threshold for a drop in brain pO2 (n = 16). In conclusion, the P7 shows potential as a monitor of regional brain oxygenation and for detection of potentially damaging secondary insults. The results must be interpreted whilst considering catheter position, autoregulation and systemic arterial changes for each individual.

Fetal cerebral, circulatory, and metabolic responses during heart rate decelerations with umbilical cord compression

OBJECTIVE: The purpose of this study was to determine the cerebral, circulatory, and metabolic responses of the ovine fetus near term to umbilical cord compression with variable-type fetal heart rate decelerations. STUDY DESIGN: Nine fetal sheep, at 0.9 of gestation, were studied before, during, and after umbilical cord occlusion for 1-minute and again after repetitive 1-minute cord occlusions every 5 minutes for 1 hour, with resultant fetal heart rate decelerations of ~90 beats/min. Brachiocephalic arterial and sagittal venous blood was analyzed for oxygen content, blood gases and pH, glucose, and lactate. Cerebral and upper body blood flow was measured with the microsphere technique. RESULTS: Umbilical cord occlusion with moderate to severe variable-type fetal heart rate deceleration resulted in an immediate drop in arterial PO2 by ~7 torr, an increase in PCO2 by ~9 torr, and a small but significant increase in lactate levels. Cerebral oxidative metabolism was well maintained but required an increase in fractional oxygen extraction because the variable change in cerebral blood flow was insufficient to maintain oxygen delivery. A redistribution of upper body blood flow was evident, with that to the brain and heart variably maintained or increased whereas that to muscle tissue was markedly decreased. Repetitive umbilical cord occlusion over 1 hour resulted in a significant drop in fetal arterial pH, with the acidemia mixed as PCO2 increased ~6 torr, whereas lactate levels increased almost fourfold. CONCLUSION: Although cerebral oxidative metabolism appears to be well maintained during moderate to severe variable-type fetal heart rate decelerations with umbilical cord occlusion, the need to increase fractional oxygen extraction and the redistribution of blood flow from carcass tissues may contribute to an accumulation of lactic acid both within the brain and systemically when such an insult occurs repeatedly. (Am J Obstet Gynecol 1996;175:929-36.)

BMS-181168 for Protection of the Human Brain against Hypoxia: Double-Blind, Placebo-Controlled EEG Mapping Studies

In a double-blind, placebo-controlled, cross-over trial, the antihypoxidotic properties of BMS-181168 (previously BMY 21502)--a 1-[[1-[2-(trifluoromethyl)-4-pyrimidinyl]-4-piperidinyl]methyl]-2- pyrrolidinone alleviating impairment of learning and memory in the animal--were studied utilizing EEG mapping under an experimental hypoxic hypoxidosis. The latter was induced by a fixed gas combination of 9.8% oxygen (O2) and 90.2% nitrogen (N2) (found at 6000 m altitude), which was inhaled for 23 minutes under normobaric conditions by 16 healthy male volunteers (aged 23-35 years, mean 27.2 years). After an adaptation session, they received in randomized order at weekly intervals oral single doses of placebo, or of 100 mg, 200 mg, and 400 mg BMS-181168. Evaluation of blood gases (PO2, PCO2, SO2), adverse events, and EEG mapping was carried out prior to drug administration and 2, 4, 6 and 8 hours post-drug, on each occasion under normoxic and transient hypoxic conditions. Hypoxemia was controlled by drawing arterialized capillary blood samples from the earlobes after hyperemization of the latter (after 0, 14, and 23 minutes of hypoxic gas inhalation) and by oximetry. After 23 minutes of inhalation, analysis showed a drop in PO2 from 98 to 48 mm Hg, in PCO2 from 41 to 31 mm Hg, and in SO2 from 97 to 80%. Descriptive statistical analyses of EEG mapping data demonstrated under hypoxia/placebo conditions an increase in delta/theta activity and a decrease in alpha activity as well as a slowing of the delta/theta centroid and an increase in the alpha and beta centroid, which suggests a marked deterioration in physiological vigilance.(ABSTRACT TRUNCATED AT 250 WORDS)