Human isometric force production and electromyogram activity of knee extensor muscles in water and on dry land

This study was designed to determine trial-to-trial and day-to-day reproducibility of isometric force and electromyogram activity (EMG) of the knee extensor muscles in water and on dry land as well as to make comparisons between the two training conditions in muscle activity and force production. A group of 20 healthy subjects (12 women and 8 men) were tested three times over 2 weeks. A measurement session consisted of recordings of maximal and submaximal isometric knee extension force with simultaneous recording of surface EMG from the vastus medialis, vastus lateralis and biceps femoris muscles. To ensure identical measurement conditions the same patient elevator chair was used in both the dry and the wet environment. Intraclass correlation coefficients (ICC) and coefficients of variation (CV) showed high trial-to-trial (ICC = 0.95-0.99, CV = 3.5%-11%) and day-to-day reproducibility (ICC=0.85-0.98, CV=11%-19%) for underwater and dry land measurements of force and EMG in each muscle during maximal contractions. The day-to-day reproducibility for submaximal contractions was similar. The interesting finding was that underwater EMG amplitude decreased significantly in each muscle during maximal (P < 0.01-P < 0.001) and submaximal contractions (P < 0.05-P < 0.001). However, the isometric force measurements showed similar values in both wet and dry conditions. The water had no disturbing effect on the electrodes as shown by slightly lowered interelectrode resistance values, the absence of artefacts and low noise levels of the EMG signals. It was concluded that underwater force and EMG measurements are highly reproducible. The significant decrease of underwater EMG could have electromechanical and/or neurophysiological explanations.     

Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71 +/- 31.3 to 505 +/- 188 microV, p less than 0.01) and mean power frequency (89 +/- 13.3 to 123 +/- 23.5 Hz, p less than 0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike amplitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412 +/- 79 to 972 +/- 117 microV, p less than 0.01) and mean firing frequency (17.8 +/- 5.4 to 24.7 +/- 4.1 Hz, p less than 0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.     

Effect of inspired air temperature on genioglossus activity during nose breathing in awake humans

Experimental data suggest the presence of sensory receptors specific to the nasopharynx that may reflexly influence respiratory activity. To investigate the effects of inspired air temperature on upper airway dilator muscle activity during nose breathing, we compared phasic genioglossus electromyograms (EMGgg) in eight normal awake adults breathing cold dry or warm humidified air through the nose. EMGgg was measured with peroral bipolar electrodes during successive trials of cold air (less than or equal to 15 degrees C) and warm air (greater than or equal to 34 degrees C) nasal breathing and quantified for each condition as percent activity at baseline (room temperature). In four of the subjects, the protocol was repeated after topical nasal anesthesia. For all eight subjects, mean EMGgg was greater during cold air breathing than during baseline (P less than 0.005) or warm air breathing (P less than 0.01); mean EMGgg during warm air breathing was not significantly changed from baseline. Nasal anesthesia significantly decreased the mean EMGgg response to cold air breathing. Nasal airway inspiratory resistance, measured by posterior rhinomanometry in six subjects under similar conditions, was no different for cold or warm air nose breathing [cold 1.4 +/- 0.7 vs. warm 1.4 +/- 1.1 (SD) cmH2O.l-1.s at 0.4 l/s flow]. These data suggest the presence of superficially located nasal cold receptors that may reflexly influence upper airway dilating muscle activity independently of pressure changes in awake normal humans.     

Respiratory metabolism of mosquitofish,Gambusia affinis: effects of temperature,dissolved oxygen,and sex difference

Synopsis Routine respiratory metabolic rates of mosquitofish (0.2 g live weight) were determined at 10, 15, 20, 25, 30, and 35°C at normoxic, hypoxic (= 40 torr P_O2), and extreme hypoxic (= 25 torr P_O2) conditions. Rates generally increased with increases in temperature (overall Q₁₀ = 2.11 at normoxia). Significant depressions (P<0.05) in rates were measured at 30 and 35°C at extreme hypoxic conditions. Males exhibited higher mortality at extreme hypoxia than females, especially at 35°C. Metabolic rates were significantly elevated at hyperoxic conditions (= 300 torr P_O2) at 25 and 30°C, but not at 35°C. Resting routine rates of less-confined fish were determined at 20, 25, and 30°C at normoxia and were significantly lower than the routine rates at the same temperatures. Behavioral experiments showed that aquatic surface respiration is initiated by mosquitofish at 20–65 torr P_O2 and is obligatory below 20 torr at 20° C. Overall, respiratory metabolic rates provide a quantitative metabolic basis from which predation rates of mosquitofish in various environments can be estimated.     

Effects of feeding and hypoxia on cardiac performance and gastrointestinal blood flow during critical speed swimming in the sea bass Dicentrarchus labrax

Previous studies have shown that if European sea bass are exercised after feeding, they can achieve a significantly higher maximum metabolic rate (MMR) than when fasted. They can meet combined metabolic demands of digestion (specific dynamic action, SDA) and maximal aerobic exercise, with no decline in swimming performance. If, however, exposed to mild hypoxia (50% saturation), bass no longer achieve higher MMR after feeding but they swim as well fed as fasted, due to an apparent ability to defer the SDA response. This study explored patterns of cardiac output (Q_A) and blood flow to the gastrointestinal tract (Q_GI) associated with the higher MMR after feeding, and with the ability to prioritise swimming in hypoxia. Sea bass (mean mass ~ 325 g, forklength ~ 27 cm) were instrumented with flow probes to measure Q_A and Q_GI during an incremental critical swimming speed (U_crit) protocol in a tunnel respirometer, to compare each animal either fasted or 6 h after a meal of fish fillet equal to 3% body mass. Feeding raised oxygen uptake (M_O2) prior to exercise, an SDA response associated with increased Q_A (+ 30%) and Q_GI (+ 100%) compared to fasted values. As expected, when exercised the fed bass maintained the SDA load throughout the protocol and achieved 14% higher MMR than when fasted, and the same U_crit (~ 100 cm s^-1). Both fed and fasted bass showed pronounced increases in Q_A and decreases in Q_GI during exercise and the higher MMR of fed bass was not associated with higher maximum Q_A relative to when fasted, or to any differences in Q_GI at maximum Q_A. In hypoxia prior to exercise, metabolic and cardiac responses to feeding were similar compared to normoxia. Hypoxia caused an almost 60% reduction to MMR and 30% reduction to U_crit, but neither of these traits differed between fed or fasted bass. Despite hypoxic limitations to MMR and U_crit, maximum Q_A and patterns of Q_GI during exercise in fasted and fed bass were similar to normoxia. Estimating GI oxygen supply from Q_GI indicated that the ability of bass to prioritise aerobic exercise over SDA when metabolically limited by hypoxia was linked to an ability to defer elements of the SDA response occurring outside the GI tract.     

Effects of hyperoxia periodic training on free radicals production,biological antioxidants potential and lactate dehydrogenase activity in the lungs of rats,Rattus norvigicus

Oxygen therapy has been widely used in lung injury (Li), adult respiraotory syndrome (ARDS) and inflammatory lung diseases as well as in mechanical ventilation in intensive care units. Exposure to hyperoxia is known to induct the production of reactive oxygen species (ROS) by mitochondria. Despite decades of research, the role of hyperoxia training in oxidative stress and ROS formation in the lungs is not known. The purpose of this study was to examine the effects of periodic-hyperoxia training on biological antioxidants (BAP) and lactate dehydrogenase (LDH) activities and free radicals (FR) production. Thirty adult male rats, matched with age and body weigh, were randomly assigned to three groups. The first group served as control (C) and the second (HP) was exposed to hyperoxia for 48. Animals in the third group (HP-T) were trained on hyperoxia for 1.5 h daily for three weeks. Following the exposure period for each group animals were sacrificed and lungs tissues were homogenized for BAP, LDH and FR determinations. LDH activity was determined by Randox protocol (Randox – UK). BAP and FR were determined using dROM method (H&D – Italy). Results showed that mean (±SD) BAP activity increased significantly (p < 0.05) from the baseline control of 7105.88 ± 2021.49 to 8611.20 ± 1245.26 (U/L) after hyperoxia training; then dropped to 6784.00 ± 1879.50 during hyperoxia exposure for 48 h. Whereas mean (±SD) FR production increased significantly (p < 0.05) from the baseline control of 262.50 ± 67.52 to 339.90 ± 64.84 during HP exposure for 48 h, then dropped to 211.13 ± 52.05 (Carr), during HP training. Similarly, LDH activity increased significantly (p < 0.05) from the baseline control of 210.31 ± 70.93 to 339.90 ± 64.84 during HP exposure for 48 h, then dropped to 159.30 ± 20.61(U/L), following HP-periodic training. Furthermore, the correlation (r = 0.67×) of LDH on FR was significant (p < 0.05), implying that reduction in ROS generation induced by HP-periodic training is related to reduced rate of cell apoptosis caused oxidative stress. Based on the results of the present study HP-periodic training is recommended in order to resist oxidative damage in the lungs.     

Effects of acute hypobaric hypoxia on the appearance of ingested deuterium from a deuterium oxide-labelled carbohydrate beverage in body fluids of humans during prolonged cycling exercise

To determine whether or not acute hypobaric hypoxia alters the rate of water absorption from a carbohydrate beverage ingested during exercise, six men cycled for 80 min on three randomly assigned different occasions. In one trial, exercise was performed in hypoxia (barometric pressure, P(B) = 594 hPa, altitude 4,400 m) at an exercise intensity selected to elicit 75% of the individual's maximal oxygen uptake (VO2max) previously determined in such conditions. In the two other experiments, the subjects cycled in normoxia (P(B) = 992 hPa) at the same absolute and the same relative intensities as in hypoxia, which corresponded to 55% and 75%, respectively, of their VO2max determined in normoxia. The subjects consumed 400 ml of a 12.5% glucose beverage just prior to exercise, and 250 ml of the same drink at 20, 40 and 60 min from the beginning of exercise. The first drink contained 20 ml of deuterium oxide to serve as a tracer for the entry of water into body fluids. The heart rate (HR) during exercise was higher in hypoxia than in normoxia at the same absolute exercise intensity, whereas it was similar to HR measured in normoxia at the same relative exercise intensity. Both in normoxia and hypoxia, plasma noradrenaline concentrations were related to the relative exercise intensity up to 40 min of exercise. Beyond that duration, when exercise was performed at the highest absolute power in normoxia, the noradrenaline response was higher than in hypoxia at the same relative exercise intensity. No significant differences were observed among experimental conditions, either in temporal profiles of plasma D accumulation or in elimination of water ingested in sweat. Conversely, elimination in urine of the water ingested appeared to be related to the severity of exercise, either high absolute power or the same relative power combined with hypoxia. We concluded that water absorption into blood after drinking a 12.5% glucose beverage is not altered during cycling exercise in acute hypobaric hypoxia. It is suggested that the elimination of water ingested in sweat and urine may be dependent on local circulatory adjustments during exercise.     

Comparison of Postasphyxial Resuscitation with 100% and 21% Oxygen on Cortical Oxygen Pressure and Striatal Dopamine Metabolism in Newborn Piglets

Abstract: The present study tests the hypothesis that ventilation with 100% O₂ during recovery from asphyxia leads to greater disturbance in brain function, as measured by dopamine metabolism, than does ventilation with 21% oxygen. This hypothesis was tested using mechanically ventilated, anesthetized newborn piglets as an animal model. Cortical oxygen pressure was measured by the oxygen-dependent quenching of phosphorescence, striatal blood flow by laser Doppler, and the extracellular levels of dopamine and its metabolites by in vivo microdialysis. After establishment of a baseline, both the fraction of inspired oxygen (FiO₂) and the ventilator rate were reduced in a stepwise fashion every 20 min over a 1-h period. For the subsequent 2-h recovery, the animals were randomized to breathing 21 or 100% oxygen. It was observed that during asphyxia cortical oxygen pressure decreased from 36 to 7 torr, extracellular dopamine increased 8,300%, and dihydroxyphenylacetic acid and homovanillic acid decreased by 65 and 60%, respectively, compared with controls. During reoxygenation after asphyxia, cortical oxygen pressure was significantly higher in the piglets ventilated with 100% oxygen than in those ventilated with 21% oxygen (19 vs. 11 torr). During the first hour of reoxygenation, extracellular dopamine levels decreased to ～200% of control in the 21% oxygen group, whereas these levels were still much higher in the 100% oxygen group (～500% of control). After ～2 h of reoxygenation, there was a secondary increase in extracellular dopamine to ～750 and ～3,000% of baseline for the animals ventilated with 21 and 100%, respectively. It is concluded that although 100% FiO₂ after asphyxia increases cortical oxygenation compared with 21% FiO₂, it also results in poorer recovery in dopamine metabolism and higher secondary release of striatal dopamine. The resulting increased extracellular levels of dopamine may exacerbate posthypoxic cerebral injury.     

Effect of oxygen radicals and hyperoxia on rat heart ornithine decarboxylase activity

Rat heart ornithine decarboxylase activity from isoproterenol-treated rats was inactivated in vitro by reactive species of oxygen generated by the reaction xanthine/xanthine oxidase. Reduced glutathione, dithiothreitol and superoxide dismutase had a protective effect in homogenates and in partially purified ornithine decarboxylase exposed to the xanthine/xanthine oxidase reaction, while diethyldithiocarbamate, which is an inhibitor of superoxide dismutase, potentiated the damage induced by O₂^? on enzyme activity. Dithiothreitol at concentrations above 1.25 mM had an inhibitory effect oupon supernatant ornithine decarboxylase activity, while at 2.5 mM it was most effective in the recovery of ornithine decarboxylase activity, after the purification of the enzyme by the ammonium sulphate precipitation procedure. The ornithine decarboxylase inactivated by the xanthine/xanthine oxidase reaction showed a higher value of K_m and a reduction of V_max with respect to control activity. The exposure of rates to 100% oxygen for 3 h reduced significantly the isoproterenol-induced heart ornithine decarboxylase activity. The injection with diethyldithiocarbamate 1 h before hyperoxic exposure further reduced heart ornithine decarboxylase activity.     

Comparison of muscle force,muscle endurance,and electromyogram activity during an expedition at high altitude

Handgrip force (HF), maximal pinch force (MF), muscle endurance (ME), and the median power frequency (MdPF) of the activity shown in the electromyogram (EMG) were studied at various altitudes in eight normal healthy subjects. MF and ME were measured between the index finger and thumb, and all measurements were obtained at altitudes ranging from 610 to 4860 m during an expedition in the Qinghai Plateau in China. With the change in altitude HF, ME, and MF showed no significant change. Compared to the MdPF at 2260 m on ascent, the MdPF at other altitudes showed a significant decrease (P<0.01). Thus, we conclude that muscle performance (HF, MF, and ME) was not affected by the environment at high altitude. However, MdPF was affected and the mean MdPF at 610 m after the expedition did not recover to initial values of MdPF. We suggest these results may have been affected by fatigue and chronic exposure to the hypobaric hypoxic environment, since the members of the expedition party expressed feelings of sluggishness and fatigue after the expedition.     

Maximal power and torque-velocity relationship on a cycle ergometer during the acceleration phase of a single all-out exercise

Seven subjects pedalled on a Monark cycle ergometer as fast as possible for approximately 7 s against four different resistances which corresponded to braking torques (T _B) equal to 19, 38, 57 and 76 N · m at the crank level. Exercise periods were separated by 5-min recovery periods. Pedal velocity was recorded every 50 ms by means of a disc with 360 slots fixed on the flywheel, passing in front of a photo-electric cell linked to a microcomputer which processed the data. Every 50 ms, the time necessary to perform half a pedal revolution (t _1/2) was computed by adding the 50-ms periods necessary to reach 669 slots (the number of slots corresponding to half a pedal revolution). To measuret _1/2 to an accuracy better than 50 ms, this time was computed by a linear interpolation of the time-slot number relationship. Power (P) was averaged duringt ₁₂ by adding the power dissipated against braking torque and the power necessary to accelerate the flywheel. The torque-velocity (T-) relationship was studied during the acceleration phase of a sprint against a single TB by computing every 50 ms the relationship between and T (N · m), equal to the sum ofT _B and the torque necessary to accelerate the flywheel at the same time. The T- relationships calculated from the acceleration phase of a single all-out exercise were linear and similar to the previously described relationships between peak velocity and braking force. These relationships can be expressed as follows: = _0,acc (1 –T/T _0,acc) where is pedal velocity,T the torque exerted on the crank andT _0,acc and _0,acc have the dimensions of maximal torque and maximal velocity respectively. Based on this model, maximal power (P _max,acc) is calculated as 0.257_{0, acc} T _{0, acc}. Maximal powerP _max,acc measured with the acceleration method was independent of braking torqueT _B and slightly higher thanP _max calculated from the relationship between peak velocity andT _B.     

Hypoxia reduced upper thermal limits causing cellular and nuclear abnormalities of erythrocytes in Nile tilapia,Oreochromis niloticus

Global warming is a threat across the world that leads to estimates of the upper thermal limits of ectothermic species. Increased water temperature up-regulates oxygen consumption and metabolic rates, and alters the physiological processes. In this study, we identified the critical thermal maxima (CTmax) and physiological responses under normoxia and hypoxia in Nile tilapia, Oreochromis niloticus. CTmax was 41.25 °C under hypoxia and 44.50 °C under normoxia. Compared to normoxia, lower values of hemoglobin (Hb) and red blood cells (RBCs) were observed at the CTmax under hypoxia. In contrast, higher values of white blood cells (WBCs) and blood glucose (Glu) levels were observed at the CTmax under hypoxia. Consequently, higher frequencies of micronucleus, cellular and nuclear abnormalities of erythrocytes were observed at the CTmax under hypoxia. These results suggest that high temperature tolerance and subsequent physiology are significantly affected by the oxygen supply in Nile tilapia. As climate vulnerability is intensifying day by day, this data will be helpful in successful management practice for the aquatic environment having low oxygen content.     

Effects of prolonged cycle ergometer exercise on maximal muscle power and oxygen uptake in humans

The mechanical power (W_tot, W·kg^–1) developed during ten revolutions of all-out periods of cycle ergometer exercise (4–9 s) was measured every 5–6 min in six subjects from rest or from a baseline of constant aerobic exercise [50%–80% of maximal oxygen uptake (VO_2max)] of 20–40 min duration. The oxygen uptake [VO₂ (W·kg^–1, 1 ml O₂ = 20.9 J)] and venous blood lactate concentration ([la]_b, mM) were also measured every 15 s and 2 min, respectively. During the first all-out period, W_tot decreased linearly with the intensity of the priming exercise (W_tot = 11.9–0.25·VO₂). After the first all-out period (i greater than 5–6 min), and if the exercise intensity was less than 60% VO_2max, W_tot, VO₂ and [la]_b remained constant until the end of the exercise. For exercise intensities greater than 60% VO_2max, VO₂ and [la]_b showed continuous upward drifts and W_tot continued decreasing. Under these conditions, the rate of decrease of W_tot was linearly related to the rate of increase of V [(d W_tot/dt) (W·kg^–1·s^–1) = 5.0·10^–5 –0.20·(d VO₂/dt) (W·kg^–1·s^–1)] and this was linearly related to the rate of increase of [la]_b [(d VO₂/dt) (W·kg^–1·s^–1) = 2.310^–4 + 5.910^–5·(d [la]_b/dt) (mM·s^–1)]. These findings would suggest that the decrease of W_tot during the first all-out period was due to the decay of phosphocreatine concentration in the exercising muscles occurring at the onset of exercise and the slow drifts of VO₂ (upwards) and of W_tot (downwards) during intense exercise at constant W_tot could be attributed to the continuous accumulation of lactate in the blood (and in the working muscles).     

Effect of temperature on hemolymph lactate and glucose concentrations in spiny lobster Panulirus interruptus during progressive hypoxia

Some metabolic fuels in hemolymph samples from the sinuses at the base of the third walking leg (pre-branchial blood) and from the cardiac sinus (post-branchial blood) in the red lobster, Panulirus interruptus, were evaluated during normoxia, hypoxia, and at the critical oxygen point (P_cr) at two temperatures of acclimatization. Three-way ANOVA indicated a significant effect of oxygen saturation R(6140)=19.84 in metabolic fuel concentrations. Lactate varied from 0.01 to 0.29 mg/ml at 20 °C, from 0.02 to 0.29 mg/ml at 27 °C, and increased significantly during hypoxia (P<0.05). Glucose varied from 0.10 to 0.27 mg/ml at 20 °C, from 0.13 to 0.33 mg/ml at 27 °C, and increased with temperature. Proteins varied from 80.78 to 119.99 mg/ml at 20 °C and from 82.99 to 130.30 mg/ml at 27 °C.     

P-31 Nuclear Magnetic Resonance Analysis of Brain: II. Effects of Oxygen Deprivation on Isolated Perfused and Nonperfused Rat Brain

Phosphatic metabolite (perchloric acid extractable) concentrations of cerebral tissues were analyzed by phosphorus-31 nuclear magnetic resonance (P-31 NMR) spectroscopy following external perfusion of the isolated rat brain (30 min or 60 min) under the following conditions: (a) constant perfusion pressure with either fluorocarbon- or erythrocyte-based medium, and (b) constant perfusate flow rate (3 ml/min) with the erythrocyte-based medium. Metabolite concentrations of control perfused brains were compared with those in nonperfused controls to provide a basis for detecting any qualitative or quantitative changes in cerebral metabolite composition. Metabolic responses of perfused brains to ischemia (incomplete ischemia, 83% reduction in flow for 10 min; transient complete ischemia for 1.5 or 2 min) were evaluated immediately after the ischemic episode and at selected time points during reperfusion (3 and 15 min). Alterations in cerebral metabolite levels induced by hypoxia were analyzed using a nonperfused rat brain model. Irrespective of the perfusion method employed, the phosphatic metabolites of control perfused rat brains were identical quantitatively to those of the nonperfused controls. Cerebral ischemia resulted in significantly increased levels of ADP, AMP + IMP, Pi, fructose 1,6-diphosphate, and glycerol 3-phosphate (global ischemia only), whereas ATP and phosphocreatine (PCr) levels declined significantly. The magnitude of these changes varied with the severity of the ischemia; however, following 15 min of control reperfusion metabolite levels had reverted to preischemic values. Significant perturbations in tissue phosphoethanolamine (3.84 delta resonance) content were evident at various time points during ischemia and postischemic recovery, which varied according to the perfusion conditions. In contrast to the changes observed in response to ischemia, hypoxia affected only cerebral high-energy phosphate levels. ATP and PCr levels were reduced, while a concomitant, essentially equimolar, increase in Pi and ADP was observed. The present studies indicate that in terms of phosphatic metabolites, the control equilibrated isolated perfused rat brain is quantitatively and qualitatively indistinguishable from the nonperfused rat brain in vivo regardless of the perfusion conditions (constant flow versus constant pressure). The metabolic responses to ischemia and hypoxia, as measured by P-31 NMR, were consistent with the pattern of changes reported elsewhere. Overall, P-31 NMR spectroscopic evaluation of the intact rat brain provides a potential experimental context for dynamic measures of cerebral metabolism under exogenously controlled conditions. Th     

Effects of ambient PO2 and temperature on oxygen uptake in Nautilus pompilius

This study employs closed-circuit respirometry to evaluate the effect of declining ambient oxygen partial pressure (PO₂) and temperature on mass specific rates of oxygen uptake (V˙O₂) in Nautilus pompilius. At all temperatures investigated (11, 16, and 21 °C), V˙O₂ is relatively constant at high PO₂ (oxyregulation) but declines sharply at low PO₂ (oxyconformation). The critical PO₂ below which oxyconformation begins (P _c) is temperature dependent, higher at 21 °C (49 mmHg) than at 11 °C or 16 °C (21.7 mmHg and 30.8 mmHg respectively). In resting, post-absorptive animals, steady-state resting V˙O₂ increases significantly with temperature resulting in a Q₁₀ value of approximately 2.5. The metabolic strategy of N. pompilius appears well suited to its lifestyle, providing sufficient metabolic scope for its extensive daily vertical migrations, but allowing for metabolic suppression when PO₂ falls too low. The combination of low temperatures and low PO₂ may suppress metabolic rate 16-fold (assuming negligible contributions from anaerobic metabolism and internal O₂ stores), enhancing hypoxia tolerance. Accepted: 20 January 2000