Normoxic Values Research Articles

Naked mole-rat skeletal muscle mitochondria exhibit minimal functional plasticity in acute or chronic hypoxia.

Oxidative phosphorylation is compromised in hypoxia, but many organisms live and exercise in low oxygen environments. Hypoxia-driven adaptations at the mitochondrial level are common and may enhance energetic efficiency or minimize deleterious reactive oxygen species (ROS) generation. Mitochondria from various hypoxia-tolerant animals exhibit robust functional changes following in vivo hypoxia and we hypothesized that similar plasticity would occur in naked mole-rat skeletal muscle. To test this, we exposed adult subordinate naked mole-rats to normoxia (21% O2) or acute (4h, 7% O2) or chronic hypoxia (4-6weeks, 11% O2) and then isolated skeletal muscle mitochondria. Using high-resolution respirometry and a fluorescent indicator of ROS production, we then probed for changes in: i) lipid- (palmitoylcarnitine-malate), ii) carbohydrate- (pyruvate-malate), and iii) succinate-fueled metabolism, and also iv) complex IV electron transfer capacity, and v) H2O2 production. Compared to normoxic values, a) lipid-fueled uncoupled respiration was reduced ~15% during acute and chronic hypoxia, b) complex I-II capacity and the rate of ROS efflux were both unaffected, and c) complex II and IV uncoupled respiration were supressed ~16% following acute hypoxia. Notably, complex II-linked H2O2 efflux was 33% lower after acute hypoxia, which may reduce deleterious ROS bursts during reoxygenation. These mild changes in lipid- and carbohydrate-fueled respiratory capacity may reflect the need for this animal to exercise regularly in highly variable and intermittently hypoxic environments in which more robust plasticity may be energetically expensive.

On the effects of opposed flow conditions on non-buoyant flames spreading over polyethylene-coated wires – Part II: Soot oxidation quenching and smoke release

Smoke release in the limited volume of a spaceship poses a major threat to the life of astronauts in long range exploration missions. If the absence of buoyant flows fundamentally affects combustion mechanisms, the possibility of atmospheric design in spacecraft environment provides a new leverage, not usually available on Earth. Investigating a non-buoyant flame spreading over the polyethylene coating of an electrical wire in an opposed laminar flow, the previous paper highlighted how flow conditions, namely oxygen content, flow velocity, and ambient pressure, affected spread rate and soot formation rate. The implementation of the Broadband Modulated Absorption/Emission (B-MAE) technique provided mappings of soot temperature and volume fraction in the spreading flames during parabolic flights. In this second paper, the link between these microscopic observations and new macroscopic findings regarding the influence of flow conditions on the smoke production of a flame in the same configuration is presented. Taking into account other requirements of space exploration, atmospheric conditions below normoxic values present a clear interest from a fire safety perspective. In the process, a threshold temperature at which soot oxidation reactions are frozen is identified. The value of 1400 K brought forward conforms with past measurements performed at normal gravity, while discrepancies with previous microgravity measurements are addressed. Given the broad capability of human lungs to adapt to various conditions, the overall mapping of smoke production as a function of flow conditions is a valuable tool for atmospheric design considerations.

Left ventricular function and remodelling in rats exposed stepwise up to extreme chronic intermittent hypoxia

The main aim was to find out whether long-lasting stepwise exposure to extreme hypoxia affects left ventricular (LV) geometry and systolic function. Adult male rats were exposed to intermittent hypobaric hypoxia (8 h/day) with increasing altitude in steps of 1000 m every 3 weeks up to 8000 m. While the LV cavity diastolic diameter did not change over the whole range of hypoxia, the wall thickness increased significantly at the altitude of 8000 m. LV fractional shortening ranged between 48.1 % and 50.1 % and remained unaffected even at the most severe hypoxia. At the end of experiment, haematocrit reached 83 %, mean systemic arterial pressure 120 % and relative LV weight 154 % of normoxic values while RV systolic pressure and relative RV weight doubled. Myocyte hypertrophy and myocardial fibrosis were more pronounced in RV than in LV. In conclusion, LV systolic function was preserved after chronic stepwise exposure of rats to extreme intermittent hypoxia despite moderate concentric hypertrophy and myocardial remodelling.

EXPRESS: Gremlin1 blocks vascular endothelial growth factor signalling in the pulmonary microvascular endothelium.

The bone morphogenetic protein (BMP) antagonist gremlin 1 plays a central role in the pathogenesis of hypoxic pulmonary hypertension (HPH). Recently, non‐canonical functions of gremlin 1 have been identified, including specific binding to the vascular endothelial growth factor receptor‐2 (VEGFR2). We tested the hypothesis that gremlin 1 modulates VEGFR2 signaling in the pulmonary microvascular endothelium.We examined the effect of gremlin 1 haploinsufficiency on the expression of VEGF responsive genes and proteins in the hypoxic (10% O2) murine lung in vivo. Using human microvascular endothelial cells in vitro we examined the effect of gremlin 1 on VEGF signaling.Gremlin 1 haploinsufficiency (Grem1+/–) attenuated the hypoxia‐induced increase in gremlin 1 observed in the wild‐type mouse lung. Reduced gremlin 1 expression in hypoxic Grem1+/– mice restored VEGFR2 expression and endothelial nitric oxide synthase (eNOS) expression and activity to normoxic values. Recombinant monomeric gremlin 1 inhibited VEGFA‐induced VEGFR2 activation, downstream signaling, and VEGF‐induced increases in Bcl‐2, cell number, and the anti‐apoptotic effect of VEGFA in vitro.These results show that the monomeric form of gremlin 1 acts as an antagonist of VEGFR2 activation in the pulmonary microvascular endothelium. Given the previous demonstration that inhibition of VEGFR2 causes marked worsening of HPH, our results suggest that increased gremlin 1 in the hypoxic lung, in addition to blocking BMP receptor type‐2 (BMPR2) signaling, contributes importantly to the development of PH by a non‐canonical VEGFR2 blocking activity.

Navigating oxygen deprivation: liver transcriptomic responses of the red eared slider turtle to environmental anoxia.

The best facultative anaerobes among vertebrates are members of the genera Trachemys (pond slider turtles) and Chrysemys (painted turtles), and are able to survive without oxygen for up to 12 to 18 weeks at ∼3 °C. In this study, we utilized RNAseq to profile the transcriptomic changes that take place in response to 20 hrs of anoxia at 5 °C in the liver of the red eared slide turtle (Trachemys scripta elegans). Sequencing reads were obtained from at least 18,169 different genes and represented a minimum 49x coverage of the C. picta bellii exome. A total of 3,105 genes showed statistically significant changes in gene expression between the two animal groups, of which 971 also exhibited a fold change equal to or greater than 50% of control normoxic values. This study also highlights a number of anoxia-responsive molecular pathways that are may be important to navigating anoxia survival. These pathways were enriched in mRNA found to significantly increase in response to anoxia and included molecular processes such as DNA damage repair and metabolic reprogramming. For example, our results indicate that the anoxic turtle may utilize succinate metabolism to yield a molecule of GTP in addition to the two molecules that results from lactate production, and agrees with other established models of anoxia tolerance. Collectively, our analysis provides a snapshot of the molecular landscape of the anoxic turtle and may provide hints into the how this animal is capable of surviving this extreme environmental stress.

Effects of anoxia on ATP, water, ion and pH balance in an insect (Locusta migratoria).

When exposed to anoxia, insects rapidly go into a hypometabolic coma from which they can recover when exposed to normoxia again. However, prolonged anoxic bouts eventually lead to death in most insects, although some species are surprisingly tolerant. Anoxia challenges ATP, ion, pH and water homeostasis, but it is not clear how fast and to what degree each of these parameters is disrupted during anoxia, nor how quickly they recover. Further, it has not been investigated which disruptions are the primary source of the tissue damage that ultimately causes death. Here, we show, in the migratory locust (Locusta migratoria), that prolonged anoxic exposures are associated with increased recovery time, decreased survival, rapidly disrupted ATP and pH homeostasis and a slower disruption of ion ([K+] and [Na+]) and water balance. Locusts could not fully recover after 4 h of anoxia at 30°C, and at this point hemolymph [K+] was elevated 5-fold and [Na+] was decreased 2-fold, muscle [ATP] was decreased to ≤3% of normoxic values, hemolymph pH had dropped 0.8 units from 7.3 to 6.5, and hemolymph water content was halved. These physiological changes are associated with marked tissue damage in vivo and we show that the isolated and combined effects of hyperkalemia, acidosis and anoxia can all cause muscle tissue damage in vitro to equally large degrees. When locusts were returned to normoxia after a moderate (2 h) exposure of anoxia, ATP recovered rapidly (15 min) and this was quickly followed by recovery of ion balance (30 min), while pH recovery took 2-24 h. Recovery of [K+] and [Na+] coincided with the animals exiting the comatose state, but recovery to an upright position took ∼90 min and was not related to any of the physiological parameters examined.

Hypoxia-reoxygenation of primary astrocytes results in a redistribution of mitochondrial size and mitophagy

Astrocytes serve to maintain proper neuronal function and support neuronal viability, but remain largely understudied in research of cerebral ischemia. Astrocytic mitochondria are core participants in the metabolic activity of astrocytes. The objective of this study is to assess astrocyte mitochondrial competence during hypoxia and post-hypoxia reoxygenation and to determine cellular adaptive and pathological changes in the mitochondrial network. We hypothesize that during metabolic distress in astrocytes; mitochondrial networks undergo a shift in fission-fusion dynamics that results in a change in the morphometric state of the entire mitochondrial network. This mitochondrial network shift may be protective during metabolic distress by priming mitochondrial size and facilitating mitophagy. We demonstrated that hypoxia and post-hypoxia reoxygenation of rat primary astrocytes results in a redistribution of mitochondria to smaller sizes evoked by increased mitochondrial fission. Excessive mitochondrial fission corresponded to Drp-1 dephosphorylation at Ser 637, which preceded mitophagy of relatively small mitochondria. Reoxygenation of astrocytes marked the initiation of elevated mitophagic activity primarily reserved to the perinuclear region where a large number of the smallest mitochondria occurred. Although, during hypoxia astrocytic ATP content was severely reduced, after reoxygenation ATP content returned to near normoxic values and these changes mirrored mitochondrial superoxide production. Concomitant with these changes in astrocytic mitochondria, the number of astrocytic extensions declined only after 10-hours post-hypoxic reoxygenation. Overall, we posit a drastic mitochondrial network change that is triggered by a metabolic crisis during hypoxia; these changes are followed by mitochondrial degradation and retraction of astrocytic extensions during reoxygenation.

Oxygen is rocket fuel for the human brain; a radical perspective!

Oxygen is rocket fuel for the human brain; a radical perspective!

Hypoxia and IF₁ Expression Promote ROS Decrease in Cancer Cells.

The role of reactive oxygen species (ROS) in the metabolic reprogramming of cells adapted to hypoxia and the interplay between ROS and hypoxia in malignancy is under debate. Here, we examined how ROS levels are modulated by hypoxia in human cancer compared to untransformed cells. Short time exposure (20 min) of either fibroblasts or 143B osteosarcoma cells to low oxygen tension down to 0.5% induced a significant decrease of the cellular ROS level, as detected by the CellROX fluorescent probe (−70%). Prolonging the cells’ exposure to hypoxia for 24 h, ROS decreased further, reaching nearly 20% of the normoxic value. In this regard, due to the debated role of the endogenous inhibitor protein (IF1) of the ATP synthase complex in cancer cell bioenergetics, we investigated whether IF1 is involved in the control of ROS generation under severe hypoxic conditions. A significant ROS content decrease was observed in hypoxia in both IF1-expressing and IF1- silenced cells compared to normoxia. However, IF1-silenced cells showed higher ROS levels compared to IF1-containing cells. In addition, the MitoSOX Red-measured superoxide level of all the hypoxic cells was significantly lower compared to normoxia; however, the decrease was milder than the marked drop of ROS content. Accordingly, the difference between IF1-expressing and IF1-silenced cells was smaller but significant in both normoxia and hypoxia. In conclusion, the interplay between ROS and hypoxia and its modulation by IF1 have to be taken into account to develop therapeutic strategies against cancer.

Abstract 676: Hypoxia Upregulates NADPH Oxidase 4-Mediated Hydrogen Peroxide Release by a HIF-Independent Mechanism in Human Endothelial Cells

NADPH oxidases are important sources of reactive oxygen species in the vascular wall. Recent evidence supports a vasoprotective role of H 2 O 2 produced by the main endothelial isoform Nox4. The impact of hypoxia on NOX4 expression in human endothelial cells and the underlying mechanism remains to be elucidated. In this study, we show that NOX4 mRNA and protein expression was upregulated by hypoxia (1 % O 2 ) in human umbilical vein endothelial cells (HUVEC). Correspondingly, H 2 O 2 production was 2-fold elevated in HUVEC after hypoxia. In contrast to rotenone and oxypurinol, lentiviral downregulation via shNOX4 abolished the elevated hypoxic hydrogen peroxide levels to normoxic values. Hypoxia stabilized the hypoxia-inducible factor (HIF)-1α protein in endothelial cells. Furthermore, VEGF promoter activity and a control promoter containing 3 hypoxia-responsive elements (HRE) were induced by hypoxia. NOX4 promoter deletions up to -119/+239 had an increased basal activity compared to control vector. A full-length and a terminally deleted NOX4 promoter construct missing a putative HRE showed a comparable activity under hypoxic and normoxic conditions, suggesting that NOX4 is not induced on the transcriptional level by hypoxia in endothelial cells. In addition, stabilization of HIF-1α protein under normoxic conditions using DMOG did not change NOX4 mRNA expression in HUVEC. Furthermore, overexpression of HIF-1α did not alter NOX4 promoter activity. Blockade of active transcription by actinomycin D revealed an increased stability of the NOX4 mRNA under hypoxic conditions. In conclusion, this study demonstrates an HIF-independent upregulation of NOX4 as major source of endothelial hydrogen peroxide generation in response to hypoxia. Our data support as a novel mechanism an increased NOX4 mRNA stability under hypoxic conditions in human endothelial cells.

Dynamic and steady-state oxygen-dependent lung relaxometry using inversion recovery ultra-fast steady-state free precession imaging at 1.5 T.

To demonstrate the feasibility of oxygen-dependent relaxometry in human lung using an inversion recovery ultra-fast steady-state free precession (IR-ufSSFP) technique. Electrocardiogram-triggered pulmonary relaxometry with IR-ufSSFP was performed in 7 healthy human subjects at 1.5 T. The data were acquired under both normoxic and hyperoxic conditions. In a single breath-hold of less than 9 seconds, 30 transient state IR-ufSSFP images were acquired, yielding longitudinal (T1) and transversal (T2) relaxometry parameter maps using voxel-wise nonlinear fitting. Possible spatial misalignments between consecutive IR-ufSSFP parameter maps were corrected using elastic image registration. Furthermore, dynamic relaxometry oxygen wash-in and wash-out scans were performed in one volunteer. From this, T1 -related wash-in and wash-out time constants (τwi , τwo ) were calculated voxel-wise on registered maps using an exponential fitting model. For healthy lung, observed T1 values were 1399 ± 77 and 1290 ± 76 ms under normoxic and hyperoxic conditions, respectively. Oxygen-related reduction of T1 was statistically significant in every volunteer. No statistically significant change, however, was observed in T2, with normoxic and hyperoxic T2 values of 55 ± 16 and 56 ± 17 ms, respectively. The observed average τwi was 87.0 ± 28.7 seconds, whereas the average τwo was 73.5 ± 21.6 seconds. IR-ufSSFP allows fast, steady-state, and dynamic oxygen-dependent relaxometry of the human lung. Magn Reson Med 79:839-845, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Metabolic plasticity in development: Synergistic responses to high temperature and hypoxia in zebrafish, Danio rerio.

This study investigated interactions of temperature and hypoxia on metabolic plasticity and regulation in zebrafish, Danio rerio, in the first week of development. Larval morphometry, oxygen consumption, and metabolic responses to acute changes in temperature and oxygen were measured in larvae reared under four conditions, including control (28°C and partial pressures of oxygen [PO2] of 21kPa), high temperature (31°C), hypoxia (11kPa), and the two stressors combined. Rearing conditions did not result in consistent morphometric changes; substantial metabolic adjustments, however, were evident. While acute temperature increase resulted in elevated oxygen consumption, with a Q10 of 2.2 ± 0.08, early-staged larvae were able to compensate to chronic temperature rise as routine metabolic rates did not differ between 28°C and 31°C chronic treatments. In contrast, larval responses to chronic and acute hypoxia were similar, with ∼30% decrease in metabolic rates from normoxic values at both temperatures. Further, prior exposure to chronic hypoxia in conjunction with acute high temperature increased Q10 by a factor of 2.5 from 2.2 ± 0.08 to 5.6 ± 0.19. Metabolic suppression by acute hypoxia was independent of any prior exposure conditions. In short, results from this study showed that zebrafish larvae exhibited surprising temperature resilience and metabolic plasticity to a 3°C temperature rise even in their first week of life. Yet exposure to a second stressor (hypoxia) resulted in elevated sensitivity to temperature change that may lead to bioenergetic imbalance due to synergetic effects of temperature and hypoxia on metabolic rates.

Weak electromagnetic fields alter Ca(2+) handling and protect against hypoxia-mediated damage in primary newborn rat myotube cultures.

Weak electromagnetic fields (WEF) enhance Ca(2+) entry into cells via voltage-gated Ca(2+) channels and affect various aspects of metabolism, structure, and function. However, little information is available on the effect of WEF on skeletal muscle, which depends primarily on intracellular Ca(2+) stores for function and metabolism. Here, we examine the effects of 30min exposure of rat primary myotube cultures to WEF (1.75μT, 16Hz) on Ca(2+) handling and creatine kinase (CK) release. Free myoplasmic Ca(2+) concentration ([Ca(2+) i]) was measured with the ratiometric dye indo-1. WEF did not affect basal [Ca(2+)]i but decreased the twitch [Ca(2+)]i transient in a time-dependent manner, and the twitch amplitude was decreased to ∼30% after 30min. WEF completely abolished the increase in [Ca(2+)]i induced by potassium chloride (∼60mM) but had no effect on the increase induced by caffeine (∼6mM). Hypoxia (2h exposure to 100% argon) resulted in a marked loss of CK into the medium (400% of normoxic value), as well as a rapid (within 20min) and sustained increase in basal [Ca(2+)]i (∼20% above baseline). However, during exposure to WEF, basal [Ca(2+)]i remained constant during the initial 60min of hypoxia and, thereafter, increased to levels similar to those observed in the absence of WEF. Finally, WEF blocked about 80% of hypoxia-mediated CK release (P < 0.05). These data demonstrate that WEF inhibits increases in [Ca(2+)]i by interfering with muscle excitation and protects against muscle damage induced by hypoxia. Thus, WEF may have therapeutic/protective effects on skeletal muscle.

Effect of acute hypoxia on inspiratory muscle oxygenation during incremental inspiratory loading in healthy adults.

To non-invasively examine the effect of acute hypoxia and inspiratory threshold loading (ITL) on inspiratory muscles [sternocleidomastoid (SCM), scalene (SA) and parasternal (PS)] oxygenation in healthy adults using near-infrared spectroscopy (NIRS). Twenty healthy adults (12 M/8 F) were randomly assigned to perform two ITL tests while breathing a normoxic or hypoxic (FIO2 = 15 %) gas mixture. NIRS devices were placed over the SCM, PS, SA, and a control muscle, tibialis anterior (TA), to monitor oxygenated (O2Hb), deoxygenated (HHb), total hemoglobin (tHb) and tissue saturation index (TSI). With the nose occluded, subjects breathed normally for 4 min through a mouthpiece that was connected to a weighted threshold loading device. ITL began by adding a 100-g weight to the ITL device. Then, every 2 min 50-g was added until task failure. Vital signs, ECG and ventilatory measures were monitored throughout the protocol. Participants were 31 ± 12 year and had normal spirometry. At task failure, the maximum load and ventilatory parameters did not differ between the hypoxic and normoxic ITL. At hypoxic ITL task failure, SpO2 was significantly lower, and ∆HHb increased more so in SA, SCM and PS than normoxic values. SCM ∆TSI decreased more so during hypoxic compared to normoxic ITL. ∆tHb in the inspiratory muscles (SCM, PS and SA) increased significantly compared to the decrease in TA during both hypoxic and normoxic ITL. The SCM, an accessory inspiratory muscle was the most vulnerable to deoxygenation during incremental loading and this response was accentuated by acute hypoxia.

Rebuttal from Peter D. Wagner.

Lundby & Montero's claim that diffusion limitation does not affect (Lundby & Montero, 2015) is based essentially on two papers from the group where Lundby trained, papers they accept uncritically (Calbet et al. 2003, 2009), and the dismissal of two papers (Hogan et al. 1991; Richardson et al. 1998), which they assert are not to be believed because they contain false positive outcomes – a claim with which we disagree. They ignore Fig. 4F in Calbet et al. (2003) where, comparing normoxia (N) to chronic hypoxia (CH), leg is in fact directly proportional to mean leg capillary , precisely as predicted for diffusion limitation. Those same data show higher in normoxia than CH despite statistically identical arterial O2 concentrations (higher [Hb] in CH compensating for hypoxaemia), thereby dissociating O2 content from . Thus, this paper actually supports the importance of diffusion limitation. Lundby & Montero reasonably question metabolism/perfusion heterogeneity rather than diffusion limitation as constraining extraction, but recent work (Vogiatzis et al. 2015) shows minimal heterogeneity, which therefore is highly unlikely to be the explanation. The 2009 Calbet paper does not evaluate diffusion limitation, and accordingly cannot elucidate its role in limiting – absence of evidence is not evidence of absence. It does show higher [Hb] in CH restoring O2 delivery to normoxic values despite lower leg blood flow and arterial hypoxaemia. Piiper & Scheid (1981) showed how diffusive exchange depends on the compound ratio D/(β•) where D is diffusing capacity, β is the slope of the O2Hb dissociation curve and is blood flow. In CH, D probably increased because of higher [Hb], β also increased with [Hb], and was lower. Thus, it is entirely possible that D/(β•) increased, allowing extraction to increase to restore . What must be understood is that O2 transport, as an integrated system involving the lungs, heart, circulation and muscles, has to be analysed as a system: all components must be measured before any can be dismissed as unimportant. Assessing only selected steps sheds no light on those com-ponents not examined. There remains no argument about the importance of the lungs, heart and circulation in transporting O2 into the muscle vasculature, but the evidence is clear that the final transport step – diffusion of O2 from the vasculature to the mitochondria – is also an important contributor to limiting . Readers are invited to give their views on this and the accompanying CrossTalk articles in this issue by submitting a brief (250 word) comment. Comments may be submitted up to 6 weeks after publication of the article, at which point the discussion will close and the CrossTalk authors will be invited to submit a ‘Last Word’. Please email your comment, including a title and a declaration of interest to jphysiol@physoc.org. Comments will be moderated and accepted comments will be published online only as ‘supporting information’ to the original debate articles once discussion has closed. Disclaimer: Supplementary materials have been peer-reviewed but not copyedited. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. The author has no conflicts of interest associated with this manuscript. Funding was provided by NIH HL091830.

A lab-on-a-chip for hypoxic patch clamp measurements combined with optical tweezers and spectroscopy- first investigations of single biological cells.

The response and the reaction of the brain system to hypoxia is a vital research subject that requires special instrumentation. With this research subject in focus, a new multifunctional lab-on-a-chip (LOC) system with control over the oxygen content for studies on biological cells was developed. The chip was designed to incorporate the patch clamp technique, optical tweezers and absorption spectroscopy. The performance of the LOC was tested by a series of experiments. The oxygen content within the channels of the LOC was monitored by an oxygen sensor and verified by simultaneously studying the oxygenation state of chicken red blood cells (RBCs) with absorption spectra. The chicken RBCs were manipulated optically and steered in three dimensions towards a patch-clamp micropipette in a closed microfluidic channel. The oxygen level within the channels could be changed from a normoxic value of 18% O 2 to an anoxic value of 0.0-0.5% O 2. A time series of 3 experiments were performed, showing that the spectral transfer from the oxygenated to the deoxygenated state occurred after about 227 ± 1 s and a fully developed deoxygenated spectrum was observed after 298 ± 1 s, a mean value of 3 experiments. The tightness of the chamber to oxygen diffusion was verified by stopping the flow into the channel system while continuously recording absorption spectra showing an unchanged deoxygenated state during 5400 ± 2 s. A transfer of the oxygenated absorption spectra was achieved after 426 ± 1 s when exposing the cell to normoxic buffer. This showed the long time viability of the investigated cells. Successful patching and sealing were established on a trapped RBC and the whole-cell access (Ra) and membrane (Rm) resistances were measured to be 5.033 ± 0.412 M Ω and 889.7 ± 1.74 M Ω respectively.

Body temperature regulation during acclimation to cold and hypoxia in rats

Extreme environmental conditions present challenges for thermoregulation in homoeothermic organisms such as mammals. Such challenges are exacerbated when two stressors are experienced simultaneously and each stimulus evokes opposing physiological responses. This is the case of cold, which induces an increase in thermogenesis, and hypoxia, which suppresses metabolism conserving oxygen and preventing hypoxaemia. As an initial approach to understanding the thermoregulatory responses to cold and hypoxia in a small mammal, we explored the effects of acclimation to these two stressors on the body temperature (Tb) and the daily and ultradian Tb variations of Sprague-Dawley rats. As Tb is influenced by sleep-wake cycles, these Tb variations reflect underlying adjustments in set-point and thermosensitivity. The Tb of rats decreased precipitously during initial hypoxic exposure which was more pronounced in cold (Tb=33.4±0.13) than in room temperature (Tb=35.74±0.17) conditions. This decline was followed by an increase in Tb stabilising at a new level ~0.5°C and ~1.4°C below normoxic values at room and cold temperatures, respectively. Daily Tb variations were blunted during hypoxia with a greater effect in the cold. Ultradian Tb variations exhibited daily rhythmicity that disappeared under hypoxia, independent of ambient temperature. The adjustments in Tb during hypoxia and/or cold are in agreement with the hypothesis that an initial decrease in the Tb set-point is followed by its partial re-establishment with chronic hypoxia. This rebound of the Tb set-point might reflect cellular adjustments that would allow animals to better deal with low oxygen conditions, diminishing the drive for a lower Tb set-point. Cold and hypoxia are characteristic of high altitude environments. Understanding how mammals cope with changes in oxygen and temperature will shed light into their ability to colonize new environments along altitudinal clines and increase our understanding of how Tb is regulated under stimuli that impose contrasting physiological constraints.

Effect of vitamin C on hyperoxia-induced vasoconstriction in exercising skeletal muscle.

Hyperoxia can cause substantial reductions in peripheral and coronary blood flow at rest and during exercise, which may be caused by reactive oxygen species (ROS) generated during hyperoxia. The aim of this study was to investigate the role of ROS in hyperoxia-induced reductions in skeletal muscle blood flow during forearm exercise. We hypothesized that infusion of vitamin C would abolish the effects of hyperoxia on the forearm blood flow (FBF) responses to exercise. Twelve young healthy adults performed rhythmic forearm handgrip exercise (10% of maximum voluntary contraction for 5 min) during normoxia and hyperoxia. For each condition, two trials were conducted with intra-arterial administration of saline or vitamin C. FBF was measured using Doppler ultrasound. During hyperoxia with saline, FBF and forearm vascular conductance (FVC) were 86.3 ± 5.1 and 86.8 ± 5.2%, respectively, of the normoxic values (100%) (P < 0.05). During vitamin C, hyperoxic FBF and FVC responses were 90.9 ± 4.2 and 90.9 ± 4.1%, respectively, of the normoxic values (P = 0.57 and 0.59). Subjects were then divided into three subgroups based on their percent decrease in FBF (>20, 10-20, and <10%) during hyperoxia. In the subgroup that demonstrated the greatest hyperoxia-induced changes (>20%), FBF and FVC during hyperoxia were 67.1 ± 4.0 and 66.8 ± 3.6%, respectively, of the normoxic values. Vitamin C abolished these effects on FBF and FVC with values that were 102.0 ± 5.2 and 100.8 ± 6.1%, respectively. However, vitamin C had no effect in the other two subgroups. This analysis is consistent with the idea that ROS generation blunts the FBF responses to exercise in the subjects most affected by hyperoxia.

Prolonged Hypoxia Increases Survival Even in Zebrafish (Danio rerio) Showing Cardiac Arrhythmia

Tolerance towards hypoxia is highly pronounced in zebrafish. In this study even beneficial effects of hypoxia, specifically enhanced survival of zebrafish larvae, could be demonstrated. This effect was actually more pronounced in breakdance mutants, which phenotypically show cardiac arrhythmia. Breakdance mutants (bre) are characterized by chronically reduced cardiac output. Despite an about 50% heart rate reduction, they become adults, but survival rate significantly drops to 40%. Normoxic bre animals demonstrate increased hypoxia inducible factor 1 a (Hif-1α) expression, which indicates an activated hypoxic signaling pathway. Consequently, cardiovascular acclimation, like cardiac hypertrophy and increased erythrocyte concentration, occurs. Thus, it was hypothesized, that under hypoxic conditions survival might be even more reduced. When bre mutants were exposed to hypoxic conditions, they surprisingly showed higher survival rates than under normoxic conditions and even reached wildtype values. In hypoxic wildtype zebrafish, survival yet exceeded normoxic control values. To specify physiological acclimation, cardiovascular and metabolic parameters were measured before hypoxia started (3 dpf), when the first differences in survival rate occurred (7 dpf) and when survival rate plateaued (15 dpf). Hypoxic animals expectedly demonstrated Hif-1α accumulation and consequently enhanced convective oxygen carrying capacity. Moreover, bre animals showed a significantly enhanced heart rate under hypoxic conditions, which reached normoxic wildtype values. This improvement in convective oxygen transport ensured a sufficient oxygen and nutrient supply and was also reflected in the significantly higher mitochondrial activity. The highly optimized energy metabolism observed in hypoxic zebrafish larvae might be decisive for periods of higher energy demand due to organ development, growth and increased activity. However, hypoxia increased survival only during a short period of development and starting hypoxia before or after this phase reduced survival, particularly in bre animals. Thus, the physiological plasticity, which enables zebrafish larvae to benefit from a hypoxia, occurs only within a narrow developmental window.

Effect of end-tidal CO2 clamping on cerebrovascular function, oxygenation, and performance during 15-km time trial cycling in severe normobaric hypoxia: the role of cerebral O2 delivery

During heavy exercise, hyperventilation-induced hypocapnia leads to cerebral vasoconstriction, resulting in a reduction in cerebral blood flow (CBF). A reduction in CBF would impair cerebral O2 delivery and potentially account for reduced exercise performance in hypoxia. We tested the hypothesis that end-tidal Pco2 (PETCO2) clamping in hypoxic exercise would prevent the hypocapnia-induced reduction in CBF during heavy exercise, thus improving exercise performance. We measured PETCO2, middle cerebral artery velocity (MCAv; index of CBF), prefrontal cerebral cortex oxygenation (cerebral O2Hb; index of cerebral oxygenation), cerebral O2 delivery (DO2), and leg muscle oxygenation (muscle O2Hb) in 10 healthy men (age 27 ± 7 years; VO2max 63.3 ± 6.6 mL/kg/min; mean ± SD) during simulated 15-km time trial cycling (TT) in normoxia and hypoxia (FIO2 = 0.10) with and without CO2 clamping. During exercise, hypoxia elevated MCAv and lowered cerebral O2Hb, cerebral DO2, and muscle O2Hb (P < 0.001). CO2 clamping elevated PETCO2 and MCAv during exercise in both normoxic and hypoxic conditions (P < 0.001 and P = 0.024), but had no effect on either cerebral and muscle O2Hb (P = 0.118 and P = 0.124). Nevertheless, CO2 clamping elevated cerebral DO2 during TT in both normoxic and hypoxic conditions (P < 0.001). CO2 clamping restored cerebral DO2 to normoxic values during TT in hypoxia and tended to have a greater effect on TT performance in hypoxia compared to normoxia (P = 0.097). However, post hoc analysis revealed no effect of CO2 clamping on TT performance either in normoxia (P = 0.588) or in hypoxia (P = 0.108). Our findings confirm that the hyperventilation-induced hypocapnia and the subsequent drop in cerebral oxygenation are unlikely to be the cause of the reduced endurance exercise performance in hypoxia.