Ambient Hypoxia Research Articles

Critical Role of cAMP Response Element Binding Protein Expression in Hypoxia-elicited Induction of Epithelial Tumor Necrosis Factor-α

Tissue hypoxia is intimately associated with a number of chronic inflammatory conditions of the intestine. In this study, we investigated the impact of hypoxia on the expression of a panel of inflammatory mediators by intestinal epithelia. Initial experiments revealed that epithelial (T84 cell) exposure to ambient hypoxia evoked a time-dependent induction of the proinflammatory markers tumor necrosis factor-alpha (TNF-alpha), interleukin-8 (IL-8), and major histocompatibility complex (MHC) class II (37 +/- 6.1-, 7 +/- 0.8-, and 9 +/- 0.9-fold increase over normoxia, respectively, each p < 0.01). Since the gene regulatory elements for each of these molecules contains an NF-kappaB binding domain, we investigated the influence of hypoxia on NF-kappaB activation. Cellular hypoxia induced a time-dependent increase in nuclear p65, suggesting a dominant role for NF-kappaB in hypoxia-elicited induction of proinflammatory gene products. Further work, however, revealed that hypoxia does not influence epithelial intercellular adhesion molecule 1 (ICAM-1) or MHC class I, the promoters of which also contain NF-kappaB binding domains, suggesting differential responses to hypoxia. Importantly, the genes for TNF-alpha, IL-8, and MHC class II, but not ICAM-1 or MHC class I, contain cyclic AMP response element (CRE) consensus motifs. Thus, we examined the role of cAMP in the hypoxia-elicited phenotype. Hypoxia diminished CRE binding protein (CREB) expression. In parallel, T84 cell cAMP was diminished by hypoxia (83 +/- 13.2% decrease, p < 0.001), and pharmacologic inhibition of protein kinase A induced TNF-alpha and protein release (9 +/- 3.9-fold increase). Addback of cAMP resulted in reversal of hypoxia-elicited TNF-alpha release (86 +/- 3.2% inhibition with 3 mM 8-bromo-cAMP). Furthermore, overexpression of CREB but not mutated CREB by retroviral-mediated gene transfer reversed hypoxia-elicited induction of TNF-alpha defining a causal relationship between hypoxia-elicited CREB reduction and TNF-alpha induction. Such data indicate a prominent role for CREB in the hypoxia-elicited epithelial phenotype and implicate intracellular cAMP as an important second messenger in differential induction of proinflammatory mediators.

O2 consumption and heart rate in developing zebrafish (Danio rerio): influence of temperature and ambient O2.

Body mass, length, oxygen consumption (MO2) and heart rate (fH) were measured in "embryos" (prior to hatching), "larvae" (days 10-20), "juveniles" (days 30-70 in 10-day intervals), and "adults" (day 100) of the zebrafish Danio rerio. Fish were chronically reared at either 25, 28, or 31 degreesC and then acutely exposed to hypoxia at different developmental stages. We hypothesized that at any given rearing and measurement temperature, D. rerio would maintain MO2 at lower ambient PO2 [i.e., have a lower critical partial pressure (Pcrit)] as development progressed and that at any given developmental stage individuals reared and measured at higher temperatures would show a more pronounced hypoxic bradycardia. MO2 in normoxic fish at 28 degreesC peaked at approximately 40 micromol. g-1. h-1 at day 10, thereafter falling to 4-5 micromol. g-1. h-1 at day 100. The Q10 for MO2 was 4-5 in embryos, falling to 2-3 from day 10 to day 60 and rising again to 4-5 at day 100. Pcrit at 28 degreesC was approximately 80 mmHg in embryos but decreased sharply to 20 mmHg at 100 days, supporting the hypothesis that more mature fish would be better able to oxygen regulate to lower ambient PO2 levels. Pcrit increased sharply with measurement temperature. Heart rate (fH) at 28 degreesC increased from about 125 beats/min in embryos to a peak of approximately 175 beats/min at days 10-30 and then fell to approximately 130 beats/min by day 100. Unlike for MO2, the Q10 for fH was more constant at 1.2-2.5 throughout development. Hypoxic exposure at any temperature had no effect on fH until approximately day 30, after which time a hypoxic bradycardia was evident. As evident for MO2, the bradycardia in older larvae was more profound at higher temperatures. On the assumption that bradycardia is indicative of hypoxic stress, the increasing prevalence of a hypoxic bradycardia in older, warmer individuals supports the hypothesis that increasing hypoxic susceptibility with development would be exacerbated by increasing temperature. Collectively, these data indicate that the ability to regulate MO2 and fH in response to the compounding demands of increased temperature and/or decreased oxygen availability first develops after approximately 20 days in D. rerio and, thereafter, the ability to maintain MO2 in the face of ambient hypoxia progressively builds through to adulthood. Additionally, the temperature responses of metabolism and heart rate differ substantially at different phases of development, suggesting a loose coupling between the respiratory and cardiovascular systems, at least early in development.

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This Month in the Journal

Neuromodulators and hypoxic hypothermia in the rat

This study was designed to assess if opioids or adenosine are involved in the hypometabolism induced by hypoxia in the rat. Accordingly, antagonists such as naloxone (NLX) for opioids or theophylline (THEO) for adenosine were injected into conscious adult rats acutely exposed to either ambient hypoxia (AHx, F iO 2: 12%) at ambient temperatures of 26 or 9°C, or to CO hypoxia (COHx, F iCO=0.05%) at an ambient temperature (Ta) of 9°C. Oxygen consumption, ventilation, colonic temperature and shivering were recorded. The results show that with NLX, the degree of hypoxic hypometabolism was reduced with AHx at 26°C and slightly decreased with COHx at 9°C. With THEO, hypoxic hypometabolism was slightly reduced with AHx and COHx at 9°C. The ventilatory response to AHx and COHx was not consistently affected by either NLX or THEO. It is concluded that adenosine and opioids play a minor role, in mediating AHx or COHx hypothermia, especially during cold exposure.

O2 EXTRACTION DURING EXERCISE IN NORMOXIA AND HYPOXIA

382 Exercise performance is impaired in ambient hypoxia. To test whether this impairment is due to inadequate muscle oxygenation, we studied small muscle exercise where blood flow would not be limited centrally by cardiac output. Five subjects completed rhythmic handgrip exercise (5 sec contraction / 5 sec rest) for 3 min at 17%, 33% and 50% of maximal voluntary contraction (MVC), while breathing 21% O2 or 10% O2 in a randomized, repeated measures fashion. Near-infrared spectroscopy measured relative concentrations of oxy- and deoxy-Hb+Mb in the flexor digitorum profundus. Values were normalized to the signal range observed from rest (defined = 100) and during 10 min of resting ischemia (defined = 0). We used the magnitude of the pre- to post-contraction signal change during a contraction cycle as an index of fractional O2 extraction. Mean ± SD values for the oxy-Hb+Mb signal were: (* different from normoxia, P < 0.05) TableTableHandgrip force was not different between conditions and did not decline over any 3 min exercise bout, suggesting that total muscle VO2 at a fixed%MVC was the same in normoxia and hypoxia; yet O2 extraction was reduced by ≈ 40%. We conclude that even in severe hypoxia, total oxygen uptake must be maintained in small muscle exercise by increasing muscle blood flow to compensate for decreased fractional oxygen extraction.

Hypoxia Regulates Basal and Induced DNA Synthesis and Collagen Type I Production in Human Cardiac Fibroblasts: Effects of Transforming Growth Factor-β1, Thyroid Hormone, Angiotensin II and Basic Fibroblast Growth Factor

Analysis of post-infarct ventricular remodeling consistently shows the accumulation of collagen in failing heart. The goal of this study was to gain insights into the underlying mechanisms of this event. We determined the effect of hypoxia, caused as the result of ischemia, on biological responses including cell viability, basal and growth factor-stimulated proliferative capacity and collagen type I production in cardiac fibroblasts obtained from adult human heart. The cell viability, as examined by light microscopy and analysis of DNA, did not change by hypoxia (2% oxygen). Basal level of protein synthesis, as determined by measuring the incorporation of3H-leucine, decreased (30%,P<0.05) under hypoxia. Transforming growth factor-beta (TGF-β1)- and thyroid hormone (T3)-induced increases in protein synthesis did not change under hypoxia. In contrast, basic fibroblast growth factor (bFGF)-stimulated protein synthesis enhanced significantly under hypoxia. Angiotensin II (Ang II)-treatment, which did not induce significant changes in protein synthesis under ambient conditions, led to moderate but significant increase under hypoxia. Basal level of DNA synthesis, as determined by measuring the incorporation of3H-thymidine into DNA, decreased (32%,P<0.05) under hypoxia. The TGF-β1-induced inhibition of DNA synthesis which was observed under ambient conditions was reversed [61% (P<0.005) increase under hypoxia]. Under ambient conditions, T3, Ang II and bFGF stimulated DNA synthesis and their effects were enhanced under hypoxia. Northern analysis showed a 46% (P<0.05) increase in the level of proα1(l) collagen mRNA under hypoxia. The TGF-β1-induced increase in the level of proα1(l) collagen mRNA, under ambient conditions, was not observed under hypoxia. On the other hand, the T3-induced decrease in proα1(l) collagen mRNA was reversed under hypoxia. Ang II- and bFGF-treatment of human cardiac fibroblasts did not cause detectable changes in the level of proα1(l) collagen mRNA under ambient conditions or hypoxia. At the protein level, the amount of immunoreactive collagen type I, as determined by immunoslot blot analysis, was increased (33%,P<0.05) under hypoxia. Treatment of human cardiac fibroblasts with TGF-β1and T3under ambient conditions led to diminished level of collagen type I. Under hypoxia, however, effect of both factors was reversed. The level of immunoreactive collagen type I in Ang II- and bFGF-treated cells, which was comparable to that in untreated cells under ambient conditions, remained unchanged under hypoxia. Together, these results provide evidence that hypoxia regulates growth, proliferative capacity and collagen type I production in human cardiac fibroblasts, and that although hypoxia alone may not be a stimulus for human cardiac fibroblast proliferation, it enhances growth factor-induced DNA synthesis in those cells. Furthermore, hypoxia by increasing the basal levels of collagen type I and by reversing the TGF-β1- and T3-induced inhibition of collagen type I gene expression in human cardiac fibroblasts can enhance overall collagen type I production. Combinatorial effects of hypoxia on proliferation and collagen type I production in cardiac fibroblasts contribute to the post-infarct remodeling of the collagen matrix in failing human heart.

Hypoxia-Controlled Winter Metabolism in Honeybees ( Apis mellifera)

Experiments during three winters have revealed a metabolism controlling function of bee-induced hypoxia in the winter cluster. Permanent low oxygen levels around 15% were found in its core. This hypoxia was actively controlled, probably via indirect mechanisms. Varying ambient oxygen levels demonstrated a causal relationship between lowered oxygen and reduced metabolic rate (MR). Under deeper ambient hypoxia the bees switched to ultra low MR (ULMR), optional-occasional at 15% oxygen, obligatory at 7.5% oxygen. This dormancy status resembled deep diapause in insects. It stayed reversible after at least several days, and was terminated under normal oxygen at 15°C. Reduced MR via core-hypoxia is essential in water conserving thermoregulation of the wintering cluster. It allows bees to reconcile warm wintering in alert state—for defence of stores—with energy saving and longevity. Two further hypotheses discussed are that winter MR of bees might be related to insect diapause, and that in-body hypoxia might be functional in insect diapause.

Ventilatory and metabolic responses to ambient hypoxia or hypercapnia in rats exposed to CO hypoxia.

We have investigated at ambient temperatures (Tam) of 25 and 5 degrees C the effects of ambient hypoxia (Hxam; fractional inspired O2 = 0.14) and hypercapnia (fractional inspired CO2 = 0.04) on ventilation (V), O2 uptake (VO2), and colonic temperature (Tc) in 12 conscious rats before and after carotid body denervation (CBD). The rats were concomitantly exposed to CO hypoxia (HxCO; fractional inspired CO = 0.03-0.05%), which decreases arterial O2 saturation by approximately 25-40%. The results demonstrate the following. 1) At Tam of 5 degrees C, in both intact and CBD rats, V/VO2 is larger when Hxam or CO2 is associated with HxCO than with normoxia. At Tam of 25 degrees C, this is also the case except for CO2 in CBD rats. 2) At Tam of 5 degrees C, the changes in VO2 and Tc seem to result from additive effects of the separate changes induced by Hxam, CO2, and HxCO. It is concluded that, in conscious rats, central hypoxia does not depress respiratory activity. On the contrary, particularly when VO2 is augmented during a cold stress, both V/VO2 during HxCO and the ventilatory responses to Hxam and CO2 are increased. The mechanisms involved in this relative hyperventilation are likely to involve diencephalic integrative structures.

Developmental changes in oxygen consumption regulation in larvae of the South African clawed frog Xenopus laevis

Well-developed larval Xenopus laevis (NF stages 58&shy;66) are oxygen regulators, at least during mild hypoxia. When and how they change from oxygen conformers (the presumed condition of the fertilized egg) to oxygen regulators is unknown. Also unknown is how anaerobic metabolic capabilities change during development, especially in response to acute hypoxia, and to what extent, if any, anaerobiosis is used to supplement aerobic metabolism. Consequently, we have investigated resting rates of oxygen consumption (M.O2) and concentrations of whole-body lactate (lactic acid) during development in normoxia and in response to acute hypoxia in Xenopus laevis. M.O2 increased in an episodic, non-linear fashion during development. Resting, normoxic M.O2 increased about tenfold (to approximately 0.20 &micro;mol g-1 h-1) between NF stages 1&shy;39 and 40&shy;44, and then another tenfold between NF stages 45&shy;48 and 49&shy;51 (to approximately 2.0 &micro;mol g-1 h-1), remaining at about 2 &micro;mol g-1 h-1 for the remainder of larval development. M.O2 reached its highest level in newly metamorphosed frogs (nearly 4 &micro;mol g-1 h-1), before decreasing to about 1.0 &micro;mol g-1 h-1 in large adults. X. laevis embryos and larvae up to NF stage 54&shy;57 were oxygen conformers when exposed to variable levels of acute hypoxia. The only exception was NF stage 45&shy;48 (external gills present yet body mass still very small), which showed some capability of oxygen regulation. All larvae older than stage 54&shy;57 and adults were oxygen regulators and had the lowest values of Pcrit (the oxygen partial pressure at which M.O2 begins to decline). Whole-body lactate concentration in normoxia was about 1 &micro;mol g-1 for all larval groups, rising to about 12 &micro;mol g-1 in adults. Concentrations of lactic acid in NF stages 1&shy;51 were unaffected by even severe ambient hypoxia. However, whole-body lactate levels in NF stages 52&shy;66 increased in response to severe hypoxia, indicating that some anaerobic metabolism was being used to supplement diminishing aerobic metabolism. The largest increases in concentration of lactate occurred in late larvae and adults.

Cardiovascular changes during acute episodic repetitive hypoxic and hypercapnic breathing in rats.

It has been shown that chronic repetitive ambient hypoxia, simulating pulmonary gas disturbances observed in apnoea, leads to systemic hypertension in rats. However, the relative roles of hypoxia (HO), hypercapnia (HC), gas stress and vigilance on the cardiovascular changes have not yet been demonstrated. The aim of this study was to investigate the acute haemodynamic changes observed during the repetitive inhalation of various gas mixtures in rats for HO alone and HO+HC, and to analyse the effects of vigilance and of the stress of gas administration. We studied 6 unanaesthetized Wistar rats chronically instrumented with an aortic catheter. Nitrogen, nitrogen+CO2 mixtures and compressed air were randomly administered in a Plexiglass chamber for 10 s and then flushed by compressed air for 20 s. Two cycles were repeated every min for 10 to 12 min. The inhaled gas fractions (FI,O2,FI,CO2) were monitored by O2 and CO2 analysers. Blood pressure (BP) was measured by a P23XL transducer. The blood gases were analysed by a 1306 IL meter. In control experiments, with compressed air alone, there were no significant acute changes in heart rate (HR) and BP. During HO there were no changes in HR or BP at FI,O2 values from 0.05-0.14, whilst at FI,O2 values from 0-0.05 systolic blood pressure (SBP) rose significantly (+25.3 +/- 25.7 (SD) mmHg) and HR decreased (-93.8 +/- 124.1 bpm). During HOHC, SBP rose (+35.1 +/- 26.4 mmHg) and HR decreased (-139.3 +/- 75.7 bpm), significantly more than in HO alone. SBP was linearly correlated with Pa,O2 during HO (r = 0.53) and also during HOHC (r = 0.44) and was not directly related to Pa,CO2 which has, nevertheless, an additive effect to HO. SBP rose with each challenge significantly more when the rats were awake than when asleep (behavioural sleep). We conclude that in this acute repetitive inhalation model, the rise in SBP is not related to gas stress or to Pa,CO2 but to a decrease in Pa,O2 and is enhanced by wakefulness.

Ventilatory response to severe acute hypoxia in guinea-pigs and rats with high hemoglobin-oxygen affinity induced by cyanate

Baseline ventilation, hemoglobin concentration (Hb) and P50 were significantly lower in guinea-pigs than in rats. Chronic sodium cyanate (NaOCN) administration did not significantly increase hemoglobin concentration in either guinea-pigs or rats. It decreased the P50 significantly less in guinea-pigs than in rats. The high Hb-O 2 affinity experimentally induced did not modify the hypoxic ventilatory response (HVR) of guinea-pigs and rats. At the same level of acute hypoxia, HVR was significantly lower in NaOCN guinea-pigs than in NaOCN rats. Guinea-pigs, genotypically adapted animals to high altitude, displayed relatively minor ventilatory and Hb-O 2 affinity changes to NaOCN, and a relatively minor HVR to acute hypoxia. They probably use tissue and biochemical adaptive mechanisms, in addition to their limited extracellular responses to successfully tolerate ambient hypoxia.

Ventilatory and metabolic responses to cold and CO-induced hypoxia in awake rats

Experiments were carried out in awake rats to compare the effects of ambient and CO-induced hypoxia on thermoregulation and ventilatory control. Measurements of metabolic rate (V̇ O 2 ), ventilation (V̇), shivering (EMG) and colonic temperature (Tc) were made at fixed ambient temperature (Ta) of 25, 15 and 5°C. Animals were exposed to ambient hypoxia (F I O 2 of 21, 17, 14, 12 and 10%) or to CO hypoxia (F I CO of 0.03% in air). The results show that: (1) Both ambient and CO-induced hypoxia provoked decreases in V̇ O 2 and Tc which were more marked at low Ta values; non-shivering thermogenesis was depressed with both types of hypoxia, whereas shivering was depressed only with ambient hypoxia; (2) Ventilatory response to ambient hypoxia was blunted at low Ta values and CO-induced hypoxia did not affect ventilation. It is concluded that: (1) hypoxia affects markedly the control of Tc by altering thermogenesis: inhibition of non-shivering thermogenesis seems to result from a decrease in Ca O 2 whereas inhibition of shivering seems to result from a decrease in Pa O 2 ; (2) during hypoxia, ventilation is controlled by the opposite stimulation from chemoreceptors and inhibition from hypometabolism. However, as revealed by CO-induced hypoxia, another stimulatory factor may also interact with the control of breathing.

Oxygen tension regulates osteoblast function

Abrupt changes in oxygen availability within the periodontium have been suggested to have a regulatory role in alveolar bone remodeling during tooth movement; arguably, similar to that seen in bone growth or fracture healing. The purpose of this investigation was therefore to study the effects of ambient hypoxia and hyperoxia on osteoblast function in vitro. Osteoblast-enriched cultures from fetal rat calvariae were exposed to atmospheres of hyperoxia (90% O 2) and hypoxia (10% O 2) and assayed for media pH, PO 2, pCO 2, cellular proliferation, alkaline phosphatase (AP) activity, and collagen synthesis. Results of this study show that in low ambient oxygen tension cellular proliferation increases, whereas the AP activity, collagen synthesis, media PO 2, PCO 2 decreases. In contrast, in hyperoxic conditions cellular proliferation is suppressed with concomitant increases in: AP activity, collagen synthesis, and partial pressures for oxygen and carbon dioxide. Media pH remained unaffected. In crossover experiments, where cells were initially grown in hypoxic conditions and were switched to hyperoxic conditions, their metabolic activities were abruptly reversed. These findings in conjunction with earlier reports, suggest a triggering role for oxygen tension (an environmental factor) in bone remodeling. (A M J O RTHOD D ENTOFAC O RTHOP 1994;105:457-63.)

RAPID AND SIMULTANEOUS MEASUREMENT OF ANODIC AND CATHODIC HAEMOGLOBINS AND ATP AND GTP CONCENTRATIONS IN MINUTE QUANTITIES OF FISH BLOOD

Oxygen transport to vertebrate tissues is dependent upon (a) the intrinsic O2-binding properties of haemoglobin (Hb) and (b) modulation by allosteric effectors, such as nucleoside triphosphates (NTP), that depress Hb O2-affinity in fish red cells. While some species, such as flatfish, primarily use adenosine triphosphate (ATP) to modulate O2-binding to Hb, others, such as eel and carp, also use guanosine triphosphate (GTP) (Kono and Hashimoto, 1977; Leray, 1982; Weber and Jensen, 1988). Unlike mammals, fish exhibit high degrees of molecular and functional Hb heterogeneity, which is manifested interspecifically but is also evident from polymorphism in different individuals of the same species (Weber, 1990). While some species such as carp have multiple Hbs that migrate anodally in normal electrophoresis and exhibit similar intrinsic oxygenation properties and sensitivities to NTP (Gillen and Riggs, 1972), others such as trout and eel have both anodic and cathodic components that exhibit different intrinsic O2-binding properties and different sensitivities to cofactors. Having higher O2 affinities and lower Bohr effects, the cathodic Hbs may be better adapted for O2 transport under hypoxic, hypercapnic or acidotic conditions than the anodic ones from the same species (Hashimoto et al. 1960; Binotti et al. 1971; Weber et al. 1975; Weber, 1990). Ambient hypoxia induces decreases in erythrocytic NTP (Wood and Johansen, 1972), which safeguards blood O2-loading in the gills. The concentrations of the major NTPs and anodic or cathodic Hbs are thus important indices of respiratory and adaptational status in fish. Erythrocytic ATP and GTP levels are generally assayed by thin layer or column chromatography or enzymatically, total Hb is measured by spectrophotometry, and the anodic and cathodic composition of the Hbs present by electrophoresis or isoelectric focusing. We here report a method for rapid and simultaneous assay of ATP and GTP, and of anodic and cathodic Hbs, in minute (approximately 3 &micro;l) quantities of fish blood. We used trout [Oncorhynchus mykiss, 101&plusmn;8 g (s.e.m.); N=9], eel (Anguilla anguilla, 114&plusmn;17 g; N=9) and carp (Cyprinus carpio, 1407&plusmn;104 g; N=9) supplied by local pisciculturists. The fish were kept for at least 1 week in 1 m3 glass fibre tanks with aerated running fresh water at 15 &deg;C before experimentation. Blood samples were drawn from the caudal blood vessels into heparinised syringes.

Metabolic response to ambient temperature and hypoxia in sinoaortic-denervated rats.

We tested the hypothesis that the sinoaortic afferents may contribute to normoxic thermogenesis and to the magnitude of the hypometabolic response to hypoxia. Adult rats were either sinoaortic denervated (SAX; n = 20) or sham operated (Sham; n = 20). A few days after the operation, gaseous metabolism [O2 uptake (VO2) and CO2 production (VCO2)] was measured with an open-flow system at ambient temperatures (Tamb) of 20, 25, 30, and 35 degrees C as the animal was resting awake. At thermoneutrality (Tamb 30 degrees C) or higher Tamb there was no difference in VO2 or VCO2. Below thermoneutrality, metabolic rate was significantly lower in SAX than in Sham animals (-14 and -16% at 20 and 25 degrees C, respectively). Colonic temperature and arterial PO2 were also slightly less, whereas arterial PCO2 and pH, mean arterial pressure, and heart rate did not differ significantly between the two groups. Exposure to acute hypoxia (10% inspired O2, 20-30 min) at Tamb 20 and 25 degrees C significantly reduced VO2 in both groups to a similar value; hence, at either Tamb, the metabolic drop during hypoxia in Sham animals was larger than that in SAX animals. Hypercapnia (5% CO2 breathing) did not change VO2 in either group. We conclude that in the rat at Tamb slightly below thermoneutrality, the sinoaortic afferents 1) provide a small but significant contribution to normoxic thermogenesis and 2) are not required for the manifestation of the drop in metabolism during hypoxia.

Respiratory responses to progressive ambient hypoxia in the sturgeon, Acipenser baeri

Changes in respiratory and acid-base variables were studied in siberian sturgeon, Acipenser baeri, during progressive deep hypoxia followed by recovery under normoxic conditions. During hypoxia, both ventilatory frequency and amplitude increased and this sturgeon was able to maintain standard oxygen consumption down to a low critical level of ambient P O 2 (P W O 2 <40 mmHG). During the posthypoxic period an O 2 debt was repaid by an elevated oxygen consumption (nearly double control value at 1 h), indicating that a shift to anaerobic metabolism had occurred during exposure to severe hypoxia. Gradually increasing ambient hypoxia initially induced a respiratory alkalosis. Below the critical P W O 2 level and during normoxic recovery, a sudden flush of lactate into the blood was associated with a typical metabolic acidosis which was almost totally compensated 3.5 h after return to normoxia. Thus, as for most other fish, respiratory responses of the sturgeon to progressive hypoxia reveal a typical O 2 regulatory behavior.

Control of metabolic and ventilatory responses to cold in anesthetized cats

Interactions between the control of thermogenesis and ventilation were studied during normoxia, hyperoxia, and ambient or CO hypoxia in adult anesthetized intact or carotid-denervated cats. Shivering, metabolic and ventilatory responses to cold stress were studied. In addition, the effects of transient pharmacological stimulation (NaCN) or inhibition (Dopamine) of arterial chemoreceptor activity were studied under different levels of oxygenation. In intact animals, cold exposure provoked increases in V̇ O2 and ventilation which were directly proportional to the intensity of shivering. During ambient or CO hypoxia, V̇ O2 was less than in normoxia for all values of shivering intensity, suggesting that a non-shivering thermogenesis component may also be inhibited by hypoxia. The decrease in V̇ O2 was associated with a smaller decrease in ventilation in ambient than in CO hypoxia because of the presence of the chemoreflex drive during ambient hypoxia. Pharmacological changes in chemoreceptor activity induced transient and opposite changes in ventilation and shivering intensity, confirming their role in the control of thermogenesis. After carotid denervation, when the drug effects were incosistent or absent, changes in levels of oxygenation were still followed by changes in shivering activity and associated changes in V̇ O2 and ventilation. We conclude that control of thermogenesis and ventilation and their interaction may be mediated by chemoreceptors as well as by direct effects upon central, possibly diencephalic structures.

Adaptations in skeletal muscle capillarity following changes in oxygen supply and changes in oxygen demands

The effects of changes in oxygen supply and oxygen demands on fiber cross-sectional areas, capillary densities and capillary to fiber ratios were determined in three skeletal muscles of rat. The muscles examined were the vastus lateralis, soleus, and diaphragm. Reduced oxygen supply was produced by subjecting rats to ambient hypoxia, and increased oxygen demands were produced by subjecting rats to low ambient temperatures or treatment with thyroxin. Capillaries were visualized by injecting fluorescent dyes into the circulation. Muscles were quick frozen at resting lengths to preserve normal fiber geometry and were subsequently sectioned on a cryostat. All of the muscles sampled from animals in the experimental groups had elevated capillary densities. However, capillary to fiber ratios were not increased significantly in any muscle, for any experimental condition. Thus, all of the observed differences in capillarity were due to changes in the intrinsic rate of muscle fiber growth. Further, the relations of capillary density and capillary to fiber ratio to fiber area were the same as those obtained during normal maturation, suggesting that capillary growth is closely linked to the intrinsic rate of fiber growth.

Respiratory Pumping Behavior in the Marine Snail Aplysia californica as a Function of Ambient Hypoxia

Respiratory pumping (RP) behavior was monitored in intact, unrestrained Aplysia californica. Respiratory pumping rates in ambient normoxic seawater (4.3-4.8 mL O₂/L) were compared with those during (1) long-term (24 h) exposure to rapidly developed extreme hypoxic seawater (≤ 1.8 mL O₂/L), (2) short-term (3-4 h) exposure to three different rapidly induced hypoxic levels (i.e., "mild" hypoxia of 3.0-3.8 mL O₂/L, "medium" hypoxia of 1. 8-2. 7 mL O₂/L, and "extreme" hypoxia of 0.5-1.0 mL O₂/L), and (3) exposure to slowly developed (approximately 5 h) "extreme" hypoxia. Long-term exposure to hypoxic seawater depressed all forms of RP activity. However, during the time of developing hypoxia, RP activity increased and was inversely related to the level of hypoxia developed; the most effective hypoxic level was ≤ 1.8 mL O₂/L. Attenuation of this developing-hypoxia-induced rise in RP occurred by slowing down the rate of developing hypoxia. Aquarium reoxygenation to normoxia reestablished normoxic RP rates although the occurrence of a patterned series of RP responses, termed an RP seizure, was more frequent during reoxygenation than during exposure to normoxic or hypoxic seawater. These results suggest that A. californica at first attempts to maintain its O₂ uptake through increased RP when ambient dissolved O₂ declines, but when such hypoxia persists, RP and associated aerobic metabolism may be limited by O₂ availability.

Oxygen uptake and chorioallantoic blood flow changes during acute hypoxia and hyperoxia in the 16 day chicken embryo

Oxygen consumption rate (Ṁ O 2 of hen eggs was measured on incubation day 16 (37.8°C, 55% humidity) during acute exposure (90 min) to ambient hyperoxia (Fl O 2 = 0.42) or hypoxia (Fl O 2 = 0.105). During the last part of these exposures, an H 2 washout method was used to estimate relative changes in chorioallantoic membrane (CAM) blood flow, taking as an index the net change in the H 2 washout rate constant between any experimental condition and the circulation arrested egg. Doubling normoxic Fl O 2 increased Ṁ O 2 to an asymptotic value which was 4% above the normal ( P < 0.05; Ṁ O 2 in normoxia = 890 μmol/h) even after correcting for the normoxic increase in Ṁ O 2 with time during development ( Δ M ̇ O 2 /Δ t = 21.5 μ mol/h 2; P < 0.001 ). Halving Fl O 2 ralculated in the same way to 388 μmol/h. The estimate of the CAM blood flow, relative to normaxia, was 1.12 in hyperoxia (not significant, P = 0.05) and 0.68 in hypoxia ( P < 0.001). The limited changes in CAM blood flow and Ṁ O 2 during hyperoxia indicate that they are both already close to their maximal values in normoxia. During acute hypoxia the 16 day embryo behaves as an oxygen-conformer; however, the small relative decrease in Ṁ O 2 per unit of the flow index observed during hyperoxia suggests that the embryo can regulate its CAM blood flow to a small extent. The survival of the embryo and its recovery from hypoxia without a detectable O 2 repayment suggest small if any anaerobic regulatory pathways and indicate a true metabolic depression.