Effect Of Chronic Hypobaric Hypoxia Research Articles

Protective effect of a chronic hypobaric hypoxic environment at high altitude on cardiotoxicity induced by doxorubicin in rats: a 7 T magnetic resonance study.

Doxorubicin (DOX)-induced cardiotoxicity (DIC), a major clinical problem, has no effective preventive therapies. We hypothesized that left ventricular (LV) systolic function would be improved in a chronic hypobaric hypoxia environment at high altitude. The purpose of this study was to investigate whether cardiovascular magnetic resonance could reveal the cardioprotective effect of chronic hypobaric hypoxia on DIC. In total, 60 rats were randomly assigned to 1 of 6 groups (n=10 per group): the P group (plain), PD group (plain + DOX), HH group (high altitude), HHD4 group (high altitude + DOX for 4 weeks), HHD8 group (high altitude + DOX for 8 weeks), and HHD12 group (high altitude + DOX for 12 weeks). The rats were transported to either Yushu (altitude: 4,250 m) or Chengdu (altitude: 500 m) where they underwent intraperitoneal injection of DOX (5 mg/kg/week for 3 weeks) or saline. Preclinical 7 T cardiovascular magnetic resonance was performed at weeks 4, 8, and 12. Tissue tracking was used to measure LV cardiac function and to analyze global and segmental strains. Subsequently, histological and oxidative stress tests were performed to evaluate the protective effect of a high-altitude environment on DIC. The left ventricular ejection fraction (LVEF) and global and regional strains in the middle, apical, anterior, septal, inferior, and lateral segments (all P<0.05) were improved in the HHD4 group compared with the PD group. The global strain was significantly greater in absolute value in the HHD8 and HHD12 groups than in the HHD4 group (all P<0.05). Additionally, histological and enzyme-linked immunosorbent assay evaluations supported the in vivo results. A chronic hypobaric and hypoxic environment at high altitude partially prevented cardiac dysfunction and increased global and regional strain in DIC rat models, thereby minimizing myocardial injury and fibrosis. In addition, by increasing the total duration of chronic hypobaric hypoxia, the global strain was further increased, which was likely due to reduced oxidative stress.

Effect of chronic hypobaric hypoxia on the expression of Neuropeptide Y in an animal model of Parkinson's disease

Effect of chronic hypobaric hypoxia on the expression of Neuropeptide Y in an animal model of Parkinson's disease

Hypobaric hypoxia induced renal damage is mediated by altering redox pathway.

Systemic hypobaric hypoxia is reported to cause renal damage; nevertheless the exact pathophysiological mechanisms are not completely understood. Therefore, the present study aims to explore renal pathophysiology by using proteomics approach under hypobaric hypoxia. Six to eight week old male Sprague Dawley rats were exposed to hypobaric hypoxia equivalent to altitude of 7628 metres (pO2-282mmhg) at 28°C and 55% humidity in decompression chamber for different time intervals; 1, 3, and7 days. Various physiological, proteomic and bioinformatic studies were carried out to examine the effect of chronic hypobaric hypoxia on kidney. Our data demonstrated mild to moderate degenerative tubular changes, altered renal function, injury biomarkers and systolic blood pressure with increase in duration of hypobaric hypoxia exposure. Renal proteomic analysis showed 38 differential expressed spots, out of which 25 spots were down regulated and 13 were up regulated in 7 dayhypobarichypoxic exposure group of rats as compared to normoxia control. Identified proteins were involved in specific molecular changes pertinent to endogenous redox pathways, cellular integrity and energy metabolism. The study provides an empirical evidence of renal homeostasis under hypobaric hypoxia by investigating both physiological and proteomics changes. The identification of explicit key proteins provides a valuable clue about redox signalling mediated renal damage under hypobaric hypoxia.

Chronic Hypobaric Hypoxia Modulates Primary Cilia Differently in Adult and Fetal Ovine Kidneys.

Hypoxic environments at high altitude have significant effects on kidney injury. Following injury, renal primary cilia display length alterations. Primary cilia are mechanosensory organelles that regulate tubular architecture. The effect of hypoxia on cilia length is still controversial in cultured cells, and no corresponding in vivo study exists. Using fetal and adult sheep, we here study the effect of chronic hypobaric hypoxia on the renal injury, intracellular calcium signaling and the relationship between cilia length and cilia function. Our results show that although long-term hypoxia induces renal fibrosis in both fetal and adult kidneys, fetal kidneys are more susceptible to hypoxia-induced renal injury. Unlike hypoxic adult kidneys, hypoxic fetal kidneys are characterized by interstitial edema, tubular disparition and atrophy. We also noted that there is an increase in the cilia length as well as an increase in the cilia function in the hypoxic fetal proximal and distal collecting epithelia. Hypoxia, however, has no significant effect on primary cilia in the adult kidneys. Increased cilia length is also associated with greater flow-induced intracellular calcium signaling in renal epithelial cells from hypoxic fetuses. Our studies suggest that while hypoxia causes renal fibrosis in both adult and fetal kidneys, hypoxia-induced alteration in cilia length and function are specific to more severe renal injuries in fetal hypoxic kidneys.

Chronic hypoxia suppresses the CO2 response of solitary complex (SC) neurons from rats

We studied the effect of chronic hypobaric hypoxia (CHx; 10-11% O(2)) on the response to hypercapnia (15% CO(2)) of individual solitary complex (SC) neurons from adult rats. We simultaneously measured the intracellular pH and firing rate responses to hypercapnia of SC neurons in superfused medullary slices from control and CHx-adapted adult rats using the blind whole cell patch clamp technique and fluorescence imaging microscopy. We found that CHx caused the percentage of SC neurons inhibited by hypercapnia to significantly increase from about 10% up to about 30%, but did not significantly alter the percentage of SC neurons activated by hypercapnia (50% in control vs. 35% in CHx). Further, the magnitudes of the responses of SC neurons from control rats (chemosensitivity index for activated neurons of 166+/-11% and for inhibited neurons of 45+/-15%) were the same in SC neurons from CHx-adapted rats. This plasticity induced in chemosensitive SC neurons by CHx appears to involve intrinsic changes in neuronal properties since they were the same in synaptic blockade medium.

Long-term exposure to hypobaric hypoxia in rat affects femur cross-sectional geometry and bone tissue material properties

Hypoxia leads to an increase in erythropoiesis, which induces hypertrophy of the erythropoietic marrow and may induce bone resorption. This study investigates the effect of chronic hypobaric hypoxia (simulated high altitude, SHA) on the biomechanics of rat femurs by mechanical tests of diaphyseal stiffness and strength and calculation of some indicators of bone material properties. Adult female rats were exposed to SHA (5500 m, 23.5h/d for 60 d =HX rats; NX=normoxic). This treatment induced reticulocytosis and polycythemia. Bone mineral content did not differ between NX and HX rats at the end of the study. To determine cortical bone mechanical properties, the right femur was assessed using a 3-point bending test. Endosteum and periosteum moved away from the central axis in HX rats as deduced by an increase in the cross-sectional moment of inertia. The two indicators of bone material properties, the elastic modulus and stress at the yielding point, were significantly reduced. However, the strength of the femur as an intact organ ("load capacity" parameters) was not compromised by exposure to SHA. It is thus proposed that the negative effect of SHA on bone material quality was completely offset by an improvement in diaphyseal cross-sectional design, thus allowing a normal biomechanical response to bending of the femur as a whole.

Effect of prolonged stay at high altitude on platelet aggregation and fibrinogen levels

Increased liability for thrombosis has been suspected at high altitude. Platelet function and fibrinogen levels are known to play a major role in thrombogenic disorders. In order to investigate effect of chronic hypobaric hypoxia on platelet function and fibrinogen concentration, a study was conducted on 40 healthy men at sea level and following 3 and 13 months sojourn at high altitude (4100 m–4500 m). A consistent decline in platelet number was observed in high altitude sojourners during 13 months stay at high altitude. Platelet counts decreased by 12% after 3 months of stay at high altitude and by 31% after 13 months stay in comparison to sea level counts (266.01 ± 65.10 × 103/µl). Mean platelet volume increased from 9.18 ± 0.83 fl at sea level to 10.8 ± 0.98 fl after 3 months stay at high altitude and showed a further increase to 12.15 ± 1.18 fl following 13 months residency in the hypoxic environment. There was significant reduction in maximal platelet aggregation in response to ADP, epinephrine and collagen, the effect being most pronounced with collagen (75%) and least with ADP (24%). Plasma fibrinogen concentration was 53% higher in sojourners after 3 months stay at high altitude and 61% higher after 13 months stay at high altitude. This implies that increased platelet activity may not be responsible for precipitation of thrombotic phenomenon during prolonged stay at high altitude but increased availability of substrate for coagulation could still favor pro-coagulant tendencies.

High energy phosphate concentrations and AMPK phosphorylation in skeletal muscle from mice with inherited differences in hypoxic exercise tolerance

The effect of chronic hypobaric hypoxia (1/2 atmospheric pressure) on high energy phosphate (HEP) compounds was investigated in slow (soleus; SOL) and fast twitch (extensor digitorum longus; EDL) muscle from 3 strains of mice with large differences in hypoxic exercise tolerance (HET). Phosphocreatine concentration ([PCr]) decreased 16–29% following hypoxia in EDL and SOL in all strains, while [ADP] and [AMP] increased. In the EDL, HET was negatively correlated with the PCr/ATP ratio and positively correlated with the ATP/P i ratio. The free energy of ATP hydrolysis (Δ G obs) remained constant despite the substantial changes that occurred in HEP profiles. The alteration of HEP set points and preservation of Δ G obs are consistent with the notion that (1) maximal rates of steady-state ATP turnover are reduced under hypoxia, and (2) HEP perturbations during rest to work transitions are reduced in skeletal muscle from hypoxia acclimated animals. We therefore expected a lower phosphorylation ratio of AMP-activated protein kinase (AMPK-P/AMPK) during stimulation in hypoxic acclimated animals. However, neither the resting nor stimulated AMPK-P/AMPK was influenced by hypoxia, although there were significant differences among strains.

Time Course of Endocrine Changes in the Hypophysis-Gonad Axis Induced by Hypobaric Hypoxia in Male Rats

Chronic hypobaric hypoxia (CHH) induces a decrease in sperm output and spermatogenesis in male rats. The mechanisms that underlie these changes in testicular function are unknown and could involve changes in the hypophysis-gonad axis. We have tested the hypothesis that changes take place in the endocrine status (FSH, follicle stimulating hormone; LH, luteinizing hormone; testosterone) of rats subjected to CHH. Male Wistar rats were maintained under normobaric or hypobaric conditions (428 torr, 4,600 m). On days 0, 5, 15 and 30 post-exposure, 12 rats were anesthetized, their body weights were measured and blood samples were collected. The testicles were fixed in 4% formaldehyde and processed for histological analysis. In this time course, the FSH levels rose by day 5 post-exposure. On subsequent days, the FSH levels decreased in rats subjected to CHH with a tendency to remain higher than the normoxic group. The LH plasma levels decreased in rats exposed to CHH. Consistent with the decrease in LH levels, the plasma testosterone level decreased significantly after 30 days of CHH exposure. Integrated analysis of hormonal changes in rats subjected to CHH and the body dehydration that occurs in HH allows us to conclude that the effects of CHH on spermatogenesis may be partially related to changes in the hypophysis-gonad hormonal axis.

Chronic hypobaric hypoxia induces tolerance to acute hypoxia and up-regulation in alpha-2 adrenoceptor in rat locus coeruleus

Hypoxia preconditioning has been shown to produce tolerance against brain injuries. The hypothesis of this study is that chronic hypobaric hypoxia may also induce acute hypoxia tolerance. We used intracellular recording in slices from rats exposed to chronic hypobaric hypoxia (exposed) and control to investigate the effects of chronic hypobaric hypoxia on the physiology of locus coeruleus (LC) including neuronal excitability. The results showed 35.7% reduced spontaneous firing rate and no change for membrane potential and input resistance in exposed neurons. In response to the alpha-2 adrenoceptor (A2R) agonist clonidine, both the hyperpolarizing potency and efficacy were increased indicated by a decreased EC50 (control: 30.9 nM and exposed: 19.7 nM) and a 50.5% increase in maximum hyperpolarized potential, respectively. A2R binding sites were also increased 21% in exposed neurons measured by radioligand [3H]rauwolscine binding assay. When treated with acute N2-hypoxia, the cell survival time (ST) was longer in exposed neurons, suggesting that a tolerance was induced. In addition, the ST for both groups of LC neurons was decreased by the A2R antagonist yohimbine and increased by the glutamate receptor antagonist kynurenic acid but not by MK-801; the decreased percentage of ST by yohimbine was larger and the increased percentage by kynurenic acid was smaller in exposed neurons. The results suggested that up-regulation of A2R and altered non-NMDA glutamate receptor function induced by chronic hypobaric hypoxia may underlie, in part, the decreased LC neuronal excitability and acute hypoxia tolerance.

Effects of chronic hypobaric hypoxia on testis histology and round spermatid oxidative metabolism

In order to evaluate the effects of the exposition to continuous chronic hypobaric hypoxia (CCHH) and intermittent chronic hypobaric hypoxia (ICHH) on testis histology and on oxidative metabolism of spermatogenic cells (SC), male rats were exposed to a 4600-m simulated altitude (PO2: 89.6 mmHg). After 60 days, ICHH and CCHH groups presented a significant decrease in testicular mass, an increase in interstitial space, a decrease in height of the seminiferous epithelium, depletion of cellular elements, vacuolization in epithelial cells and folding of the basal membrane. Round spermatids from animals exposed to CCHH presented a significant decrease in energy-dependent cell shape changes. Round spermatid mitochondria of CCHH rats seem to be limited in their ability to handle reducing equivalents. These mitochondria also appear to be uncoupled under basal conditions. Round spermatids from CCHH rats evidence large oxygen consumption (QO2) insensitive to inhibition by cyanide, a process that could be partly related to lipoperoxidation. Thus, exposure of male rats to CCHH and ICHH induced evident changes in testicular morphology and loss of spermatogenic cells, in all stages of the spermatogenic cycle. This post-meiotic spermatogenic cell loss in the testis correlated well with metabolic changes in round spermatids that evidenced a strong metabolic stress in these cells.

Effects of chronic hypobaric hypoxia on contractile properties of rat sternohyoid and diaphragm muscles.

1. Chronic hypoxia occurs in a variety of circumstances, including respiratory disease and exposure to altitude, and is known to affect respiratory muscle structure. However, little is known about its effects on respiratory muscle contractile properties. 2. Rats were exposed to normoxia (n = 16) or hypobaric hypoxia (n = 16; barometric pressure 450 mmHg) for 6 weeks. Contractile properties were measured in isolated sternohyoid and diaphragm muscles in warmed, oxygenated Krebs' solution. Isometric twitch and tetanic tension, contraction time, half-relaxation time and tension-frequency relationship were determined using field stimulation with platinum electrodes. Fatigue was induced by stimulation at 40 Hz with 300 msec trains of 0.5 Hz for 5 min. 3. Chronic hypoxia had no effect on bodyweight, but did increase haematocrit. Chronic hypoxia increased specific force development in both muscles and increased sternohyoid fatigue. Chronic hypoxia had no effect on contractile kinetics in either muscle, but shifted the tension-frequency relationship to the left in the diaphragm. 4. Therefore, chronic hypoxia alters rat respiratory muscle force and fatigue, either due to the direct effects of hypoxia or to increased muscle activation.

Non-invasive measurement of the cardiovascular effects of chronic hypoxaemia on dogs living at moderately high altitude

Pulmonary hypertension is a well-recognised condition in dogs, and, among other mechanisms, is caused by hypoxia. In order to evaluate the effect of chronic hypobaric hypoxia on pulmonary arterial pressure...

Changes in MCT 1, MCT 4, and LDH expression are tissue specific in rats after long-term hypobaric hypoxia.

Little is known about the effect of chronic hypobaric hypoxia on the enzymes and transporters involved in lactate metabolism. We looked at the protein expression of monocarboxylate transporters MCT 1, MCT 2, and MCT 4, along with total lactate dehydrogenase (LDH) and LDH isozymes in skeletal muscle, cardiac muscle, and liver. Expression of these components of the lactate shuttle affects the ability to transport and oxidize lactate. We hypothesized that the expression of MCTs and LDH would increase after acclimation to high altitude (HA). The response to acclimation to HA was, however, tissue specific. In addition, the response was different in whole muscle (Mu) and mitochondria-enriched (Mi) fractions. Heart, soleus, and plantaris muscles showed the greatest response to HA. Acclimation resulted in a 34% increase in MCT 4 in heart and a decrease in MCT 1 (-47%) and MCT 4 (-47%) in plantaris Mu. In Mi fractions, the heart had an increase (+40%) and soleus a decrease (-40%) in LDH. HA also had a significant effect on the LDH isozyme composition of both the Mu and Mi fractions. Mitochondrial density was decreased in both the soleus (-17%) and plantaris (-44%) as a result of chronic hypoxia. We conclude that chronic hypoxia had a tissue-specific effect on MCTs and LDH (that form the lactate shuttle) but did not produce a consistent increase in these components in all tissues.

Prolonged exposure to hypobaric hypoxia transiently reduces GABA A receptor number in mice cerebral cortex

The central nervous system is severely affected by hypoxic conditions, which produce alterations in neural cytoarchitecture and neurotransmission, resulting in a variety of neuropathological conditions such as convulsive states, neurobehavioral impairment and motor CNS alterations. Some of the neuropathologies observed in hypobaric hypoxia, corresponding to high altitude conditions, have been correlated with a loss of balance between excitatory and inhibitory neurotransmission, produced by alterations in glutamatergic and GABAergic receptors. In the present work, we have studied the effect of chronic hypobaric hypoxia (506 hPa, 18 h/day×21 days) applied to adult male mice on GABA A receptors from cerebral cortex, to determine whether hypoxic exposure may irreversibly affect central inhibitory neurotransmission. Saturation curves for [ 3H]GABA specifically bound to GABA A receptors in isolated synaptic membranes showed a 30% decrease in maximal binding capacity after hypoxic exposure ( B max control, 4.70±0.19, hypoxic, 3.33±0.10 pmol/mg protein), with no effect on GABA binding sites affinity ( K d control: 159.3±13.3 nM, hypoxic: 164.2±15.1 nM). Decreased B max values were observed up to the 10th post-hypoxic day, returning to control values by the 15th post-hypoxic day. Pharmacological properties of GABA A receptor were also affected by hypoxic exposure, with a 45 to 51% increase in the maximal effect by positive allosteric modulators (pentobarbital and 5α-pregnan-3α-ol-20-one). We conclude that long-term hypoxia produces a significant but reversible reduction on GABA binding to GABA A receptor sites in cerebral cortex, which may reflect an adaptive response to this sustained pathophysiological state.

Color discrimination under chronic hypoxic conditions (simulated climb "Everest-Comex 97").

Hypoxia is known to alter visual functions. In the present study, the effects of chronic hypobaric hypoxia upon visual color discrimination were studied in 8 subjects participating in a simulated climb from sea level (PO2 = 210 hPa) to 8,848 m (PO2 = 70 hPa) over a 31-day period of confinement in a decompression chamber ('Everst-Comex 97'). During these investigations, the subjects were required to discriminate between colors of different hue in the red, blue, and green ranges. Alterations in color discrimination increased slightly but significantly as altitude increased. Impairments occurred mainly in the red and blue ranges. In addition, our results further indicate that color discrimination would be affected only when a minimum threshold of difference between color stimuli is not present. Methodological and physiological implications are discussed.

COLOR DISCRIMINATION UNDER CHRONIC HYPOXIC CONDITIONS (SIMULATED CLIMB 'EVEREST-COMEX 97')

Hypoxia is known to alter visual functions. In the present study, the effects of chronic hypobaric hypoxia upon visual color discrimination were studied in 8 subjects participating in a simulated climb from sea level (PO2 = 210 hPa) to 8,848 m (PO2 = 70 hPa) over a 31-day period of confinement in a decompression chamber (‘Everest-Comex 97’). During these investigations, the subjects were required to discriminate between colors of different hue in the red, blue, and green ranges. Alterations in color discrimination increased slightly but significantly as altitude increased. Impairments occurred mainly in the red and blue ranges. In addition, our results further indicate that color discrimination would be affected only when a minimum threshold of difference between color stimuli is not present. Methodological and physiological implications are discussed.

Increased brain capillaries in chronic hypoxia.

The effect of chronic hypobaric hypoxia (28 days, 455 Torr) on the organization of brain vessels was studied in Balb/c mice. In comparison to age-matched controls kept at sea level, emulsion-perfused capillaries in hypoxic mice showed marked dilation in all brain areas studied. Capillary length per unit volume of tissue (Lv) was increased in the cerebellar granular layer, the caudate nucleus, the globus pallidus, the substantia nigra, the superior colliculus, and the dentate gyrus. There was a selective increase of Lv in the hippocampus (CA1 strata pyramidale and lacunosum and CA3 strata pyramidale and oriens) and in somatosensory cortex layers V and VI, motor cortex layers II, III, V, and VI, and auditory cortex layers II and III. An increase in capillary surface area per unit volume of tissue was also determined in several brain areas, including layer IV of somatosensory cortex, where Lv was not significantly increased. The O2 diffusion conductance and PO2 in the tissues were estimated with a mathematical model. The remodeling of capillary diameter and length during chronic hypoxia accounts for the significant increase of O2 conductance to neural tissues. Also the estimated tissue PO2 in chronic brain hypoxia is markedly increased in the caudate nucleus and the substantia nigra compared with acute hypoxia. These results suggest that formation of new capillaries is an important mechanism to restore the O2 deficit in chronic brain hypoxia and that local rates of energy utilization may influence angiogenesis in different areas of the brain.

Adaptation of cardiac myosin and creatine kinase to chronic hypoxia: role of anorexia and hypertension.

The effects of chronic hypobaric hypoxia (CHH, 28 days, simulated altitude 5,500 m) on the cardiac expression of myosin heavy chain (MHC) and creatine kinase (CK) was studied in rat left (LV) and right (RV) ventricle. To separate the effects of hypoxia from its associated perturbations, anorexia and pulmonary hypertension (resulting in RV hypertrophy), CHH animals were compared with normoxic controls (C) and with rats restricted in food supply (pair fed, PF). In RV, the increased proportion of beta-MHC in CHH (20 +/- 3%) vs. C (7 +/- 2%, P < 0.01) and vs. PF (12 +/- 2%, P < 0.05) rats was mainly attributed to hypertension. In contrast, the higher beta-MHC of CHH (23 +/- 2%) vs. C (13 +/- 2%, P < 0.05) in LV was mainly ascribed to anorexia (PF = 21 +/- 3%, not significant). A major contribution of anorexia was also evidenced in the isozymic profile of CK; anorexia accounted for a 25% decrease in mito-CK specific activity in LV, whereas hypertension partly accounted for the threefold increase in BB-CK in RV. CHH specifically induced a twofold rise in LV BB-CK. This suggests that both the expression of slow myosin, improving the economy of contraction, and the changes in CK isozymic profile could provide a biochemical basis for the CHH resistance to ischemia.

Effects of chronic hypobaric hypoxia on blood oxygen binding in pigeons

Effects of chronic hypobaric hypoxia on blood O2 transport were studied using two groups of domestic pigeons (Columba livia). High-altitude (HA) birds were acclimated for 6 wk at 7 km simulated altitude (308 Torr, 25°C); sea-level (SL) pigeons were maintained for 5-6 wk at 758 Torr (25°C). Whole blood O2 equilibrium curves (O2ECs) were generated for HA and SL pi- geons at 37°C and 41°C using thin film techniques. At pH 7.50 and 41°C, half-saturation PO2s (P50) for HA and SL birds were 32.9 ± 1.4 (6); (mean ± S.E.M. (N)) and 35.6 ± 0.6 (6) Torr, respectively. At pH 7.56 and 37°C, the corresponding P50s were 26.5 ± 1.8 (4) and 29.0 ± 0.6 (5) Torr, respectively. HA and SL pigeons exhibited similar Hb-O2 temperature coefficients (ΔH = -9.8 to -10.6 kcal/mole O2) and CO2 (Bohr slopes (Δlog P50/ΔpH = -.43 to -.44 at 37°C; -.41 at 41°C). Hill's n increased with increasing saturation for both animal groups at 37°C and 41°C. Hypoxic acclimation elicited a significant increase in O2 capacity; %Hct and (Hb) increased by 38% and 43%, respectively. Chronic hypoxia had no effect on isohemoglobin composition; both groups exhibited two isoHb fractions in a 9:1 molar ratio. At PaO2 = 30 Torr, calculated in vivo CaO2 for HA (39.2°C, pH 7.53) and SL (41°C, pH 7.50) birds were 14.2 vol % and 7.6 vol %, respectively. Results suggest that altitude tolerance of pigeons is enhanced by the increased blood O2 capacity and the effect of hypoxia- induced hypothermia on Hb-O2 affinity. J. Exp. Zool. 277:293-300, 1997. © 1997 Wiley-Liss, Inc.