After effects of chronic hypoxia on VO2 kinetics and on O2 deficit and debt

Single breath O2 consumption (PB = 730, FI02 = 0.21) was measured at rest, during 10 min cycloergometric exercise at 125 W, and in the following recovery phase in seven subjects before, and 12 days after 6 weeks at 5,200 m or above. Peak blood lactate after exercise (Lab) was measured. O2 deficits and debts and half times (t1/2) of the VO2 on- and off-kinetics were calculated. Before acclimatization, the VO2 on- and off-responses were close to a single exponential with t1/2 = 30 s. After return to sea level, the VO2 on-response curves were less steep in the initial phase, becoming closer to sigmoid. The t1/2, independent of the shape of the underlying function, was approximately 10 s longer. The VO2 off-responses during the initial 4 min of recovery were the same before and after acclimatization. Average O2 deficit was approximately 320 ml larger after acclimatization: the fast component of O2 debt was similar. Since steady state VO2 and Lab were the same, the O2 deficit difference can be attributed to a greater utilization of O2 stores. Of these, about 1/3 is explained in terms of increased mixed venous blood O2 stores, due to increased [Hb] (16.6 vs 14.9 g X dl-1), while the remainder is ascribed essentially to increased Mb-bound O2. O2 stores utilization and replenishment is presumed to occur when muscle metabolism is low; as a consequence, while it is clearly detectable from the shape of the initial phase of the VO2 on-response, during recovery it is spread throughout, thus becoming more difficult to appreciate.     

Increased hemoglobin O2 affinity protects during acute hypoxia

Acclimatization to hypoxia requires time to complete the adaptation mechanisms that influence oxygen (O(2)) transport and O(2) utilization. Although decreasing hemoglobin (Hb) O(2) affinity would favor the release of O(2) to the tissues, increasing Hb O(2) affinity would augment arterial O(2) saturation during hypoxia. This study was designed to test the hypothesis that pharmacologically increasing the Hb O(2) affinity will augment O(2) transport during severe hypoxia (10 and 5% inspired O(2)) compared with normal Hb O(2) affinity. RBC Hb O(2) affinity was increased by infusion of 20 mg/kg of 5-hydroxymethyl-2-furfural (5HMF). Control animals received only the vehicle. The effects of increasing Hb O(2) affinity were studied in the hamster window chamber model, in terms of systemic and microvascular hemodynamics and partial pressures of O(2) (Po(2)). Pimonidazole binding to hypoxic areas of mice heart and brain was also studied. 5HMF decreased the Po(2) at which the Hb is 50% saturated with O(2) by 12.6 mmHg. During 10 and 5% O(2) hypoxia, 5HMF increased arterial blood O(2) saturation by 35 and 48% from the vehicle group, respectively. During 5% O(2) hypoxia, blood pressure and heart rate were 58 and 30% higher for 5HMF compared with the vehicle. In addition, 5HMF preserved microvascular blood flow, whereas blood flow decreased to 40% of baseline in the vehicle group. Consequently, perivascular Po(2) was three times higher in the 5HMF group compared with the control group at 5% O(2) hypoxia. 5HMF also reduced heart and brain hypoxic areas in mice. Therefore, increased Hb O(2) affinity resulted in hemodynamics and oxygenation benefits during severe hypoxia. This acute acclimatization process may have implications in survival during severe environmental hypoxia when logistic constraints prevent chronic acclimatization.     

Regional O2 uptake during hypoxia and recovery in hypermetabolic dogs

The regional distribution of O2 deficit in muscle and nonmuscle tissues was measured in hypermetabolic dogs ventilated with a low inspired O2 fraction and was compared with excess O2 used in these regions during normoxic recovery. O2 uptake was stimulated by 2,4-dinitrophenol (DNP). Arterial, mixed venous, and muscle venous blood samples were drawn before, during, and after severe hypoxia (9% O2-91% N2) for the calculation of hindlimb O2 uptake and cardiac output. The O2 deficit and excess O2 uptake in recovery were calculated as the cumulative differences between normoxic control and respective hypoxic and recovery O2 uptake values. The DNP data were compared with data previously obtained in our laboratory. A greater whole-body O2 deficit was incurred in the DNP group during hypoxia and was associated with a larger O2 use in recovery. The total O2 deficit was equally distributed between muscle and nonmuscle tissues, but more excess O2 use occurred in nonmuscle tissues. The greater excess O2 used by nonmuscle tissues may have been associated with the restoration of intracellular ion concentrations brought about by the increased activity of energy-using membrane pumps.     

Adrenergic and local control of O2 uptake during and after severe hypoxia

The distribution of whole-body O2 supply during severe hypoxia and recovery and its relation to the regional distribution of O2 deficit and repayment was studied. Mongrel dogs were anesthetized, paralyzed, and ventilated to maintain an end-tidal PCO2 between 35 and 40 Torr. In one group, the alpha- and beta-adrenergic receptors were blocked to eliminate neural and humoral adrenergic influences. In a second group, alpha-adrenergic receptors were stimulated to decrease O2 delivery by excessive vasoconstriction. In a third group, beta-adrenergic receptors were stimulated to increase O2 delivery. Whole-body and hindlimb muscle O2 uptake and vascular responses were measured during normoxic control, 15 or 30 min of severe hypoxia (9% O2 in N2), and 20 or 30 min of normoxic recovery, respectively. The whole-body O2 deficit and excess O2 uptake in recovery were partitioned into muscle and nonmuscle areas. The data showed that neural or humoral influences had little effect on the regional distribution of the total O2 deficit and O2 excess in recovery. The O2 deficit could be decreased somewhat by increasing delivery, but the amount of excess O2 used in recovery was unaffected. This suggested that the excess O2 use in recovery was more a function of an energy deficit during hypoxia and not an O2 deficit.     

O2 delivery to contracting muscle during hypoxic or CO hypoxia

The consequences of a decreased O2 supply to a contracting canine gastrocnemius muscle preparation were investigated during two forms of hypoxia: hypoxic hypoxia (HH) (n = 6) and CO hypoxia (COH) (n = 6). Muscle O2 uptake, blood flow, O2 extraction, and developed tension were measured at rest and at 1 twitch/s isometric contractions in normoxia and in hypoxia. No differences were observed between the two groups at rest. During contractions and hypoxia, however, O2 uptake decreased from the normoxic level in the COH group but not in the HH group. Blood flow increased in both groups during hypoxia, but more so in the COH group. O2 extraction increased further with hypoxia (P less than 0.05) during concentrations in the HH group but actually fell (P less than 0.05) in the COH group. The O2 uptake limitation during COH and contractions was associated with a lesser O2 extraction. The leftward shift in the oxyhemoglobin dissociation curve during COH may have impeded tissue O2 extraction. Other factors, however, such as decreased myoglobin function or perfusion heterogeneity must have contributed to the inability to utilize the O2 reserve more fully.     

Supply of O2 regulates O2 demand during utilization of reserves of poly-beta-hydroxybutyrate in N2-fixing soybean bacteroids.

A liquid reaction medium containing dissolved air and oxyleghaemoglobin, but no energy-yielding substrate, was supplied to bacteroids confined in a stirred flow reaction chamber. The relative oxygenation of the leghaemoglobin in the chamber was determined automatically by spectrophotometry of the effluent solution, and the concentrations of free, dissolved O2 ([O2]) and rates of O2 consumption were calculated. Dissolved CO2 and NH3 from N2 fixation were determined in fractions of the effluent solution. Bacteroids utilized endogenous reserves of poly-beta-hydroxybutyrate (PHB), which were depleted by 9.2% during a typical 5 h-long experiment. Stepwise increases in flow rate (increasing supply of O2) initially produced a drop in O2 demand and resulted in a rise in [O2] and a decline in N2 fixation. Subsequently, O2 demand rose (presumably because of increased mobilization of substrate from PHB) and [O2] declined to a low level. N2 fixation was fully restored, or even enhanced, within 15-20 min of establishment of a new, steady [O2]. This pattern of regulation by O2 supply was completely eliminated by adding low concentrations (20-50 microM) of oxidizable substrate (succinate, malate, ethanol) to the reaction medium. During endogenous activity, rates of CO2 evolution were proportional to, but less than, rates of O2 consumption up to 5.4 nmol O2 min-1 mg-1, above which CO2 evolution exceeded O2 consumption. These and other features of endogenous activity are discussed in relation to sustaining N2 fixation by nodules in vivo.     

Diurnal O2 and carbohydrate levels in wheat kernels during embryony

In vitro zygotic and somatic embryogenesis procedures for wheat have been improved by simulating in ovulo nutritional, hormonal and dissolved oxygen (dO₂) conditions. However, diurnal fluctuations in these conditions during early embryony are not well characterized. In this study, dO₂ and water-soluble carbohydrate levels in wheat kernels were determined after 8 h of light and 8 h of dark at approximately 6, 12 and 18 day post anthesis (DPA). Clark style O₂ microelectrodes, having a tip diameter of approximately 115 μm, were inserted into intact kernels immediately distil to the developing embryo, and dO₂ levels were recorded at 50 μm intervals into the center of kernels. High-performance anion exchange chromatography with pulsed amperometric detection was used to quantify carbohydrate levels in endosperm sap. dO₂ levels in the chlorophyllous layer of the pericarp reached 190 mmol m⁻³ during the day, which probably represents, because of photosynthesis, a supersaturated O₂ condition relative to the external environment (21% O₂). At the embryo surface, dO₂ levels at 6 DPA ranged from 135 to 170 mmol m⁻³. At 12 and 18 DPA, dO₂ levels at the embryo axis ranged from 100 to 150 mmol m⁻³. At all three stages, dO₂ levels in the center of the endosperm were below 13 mmol m⁻³. Extreme fluctuations in carbohydrate levels were observed diurnally during rapid seed fill (12 DPA). Levels of sucrose and short-chain fructans were much higher during the day than during the night. In contrast, fructose, glucose, and myo-inositol levels were much higher during the night than during the day. By 18 DPA (hard dough stage), carbohydrate levels tended to be similar during the day and night. These dynamic fluctuations may assist in regulating embryony in ovulo, and their simulation might improve the development of somatic and zygotic embryos in vitro.     

Effects of hypoxic hypoxia on O2 uptake and heart rate kinetics during heavy exercise

Engelen, Marielle, Janos Porszasz, Marshall Riley, KarlmanWasserman, Kazuhira Maehara, and Thomas J. Barstow. Effects ofhypoxic hypoxia on O₂ uptake andheart rate kinetics during heavy exercise. J. Appl.Physiol. 81(6): 2500-2508, 1996.It is unclearwhether hypoxia alters the kinetics ofO₂ uptake(O₂) during heavy exercise[above the lactic acidosis threshold (LAT)] and how thesealterations might be linked to the rise in blood lactate. Eight healthyvolunteers performed transitions from unloaded cycling to the sameabsolute heavy work rate for 8 min while breathing one of threeinspired O₂ concentrations: 21%(room air), 15% (mild hypoxia), and 12% (moderate hypoxia). Breathing12% O₂ slowed the time constantbut did not affect the amplitude of the primary rise inO₂ (period of first2-3 min of exercise) and had no significant effect on either thetime constant or the amplitude of the slowO₂ component (beginning2-3 min into exercise). Baseline heart rate was elevated inproportion to the severity of the hypoxia, but the amplitude andkinetics of increase during exercise and in recovery were unaffected bylevel of inspired O₂.We conclude that the predominant effect of hypoxia during heavyexercise is on the early energetics as a slowed time constant forO₂ and an additionalanaerobic contribution. However, the sum total of the processesrepresenting the slow component of O₂ is unaffected.

     

Muscle fatigue and exhaustion during dynamic leg exercise in normoxia and hypobaric hypoxia

Fulco, Charles S., Steven F. Lewis, Peter N. Frykman, RobertBoushel, Sinclair Smith, Everett A. Harman, Allen Cymerman, and Kent B. Pandolf. Muscle fatigue and exhaustion during dynamic leg exercisein normoxia and hypobaric hypoxia. J. Appl. Physiol. 81(5): 1891-1900, 1996.Using anexercise device that integrates maximal voluntary static contraction(MVC) of knee extensor muscles with dynamic knee extension, we comparedprogressive muscle fatigue, i.e., rate of decline in force-generatingcapacity, in normoxia (758 Torr) and hypobaric hypoxia (464 Torr).Eight healthy men performed exhaustive constant work rate kneeextension (21 ± 3 W, 79 ± 2 and 87 ± 2% of 1-leg kneeextension O₂ peak uptake fornormoxia and hypobaria, respectively) from knee angles of90-150° at a rate of 1 Hz. MVC (90° knee angle) wasperformed before dynamic exercise and during 5-s pauses every 2 minof dynamic exercise. MVC force was 578 ± 29 N in normoxia and 569 ± 29 N in hypobaria before exercise and fell, at exhaustion, to similar levels (265 ± 10 and 284 ± 20 N for normoxia andhypobaria, respectively; P > 0.05)that were higher (P < 0.01) thanpeak force of constant work rate knee extension (98 ± 10 N, 18 ± 3% of MVC). Time to exhaustion was 56% shorter for hypobariathan for normoxia (19 ± 5 vs. 43 ± 7 min, respectively;P < 0.01), and rate of right leg MVC fall wasnearly twofold greater for hypobaria than for normoxia (mean slope = 22.3 vs. 11.9 N/min, respectively;P < 0.05). With increasing durationof dynamic exercise for normoxia and hypobaria, integratedelectromyographic activity during MVC fell progressively with MVCforce, implying attenuated maximal muscle excitation. Exhaustion, perse, was postulated to relate more closely to impaired shorteningvelocity than to failure of force-generating capacity.

     

Measurement of rat plasma adenosine levels during normoxia and hypoxia.

A stop solution containing EDTA, EGTA, dipyridamole, erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and d,l-alpha-glycerophosphate has been used to prevent adenosine formation and loss from rat femoral arterial blood samples prepared for measurement of plasma adenosine levels. The femoral arterial plasma adenosine concentration in normoxic rats was 79.2 +/- 12.7 nM. During a 5 min period of hypoxia (8% oxygen) plasma adenosine increased to 190.2 +/- 32.2 nM. A resting plasma adenosine level of circa 80 nM, which is 10X lower than most previous estimates, approximates the threshold levels of adenosine required for arterial dilation.     

Increasing plasmalogen levels protects human endothelial cells during hypoxia

Supplementation of cultured human pulmonary arterial endothelial cells (PAEC) with sn-1-O-hexadecylglycerol (HG) resulted in an approximately twofold increase in cellular levels of plasmalogens, a subclass of phospholipids known to have antioxidant properties; this was due, primarily, to a fourfold increase in the choline plasmalogens. Exposure of unsupplemented human PAEC to hypoxia (PO(2) = 20-25 mmHg) caused an increase in cellular reactive oxygen species (ROS) over a period of 5 days with a coincident decrease in viability. In contrast, HG-supplemented cells survived for at least 2 wk under these conditions with no evidence of increased ROS. Hypoxia resulted in a selective increase in the turnover of the plasmalogen plasmenylethanolamine. Human PAEC with elevated plasmalogen levels were also more resistant to H(2)O(2), hyperoxia, and the superoxide generator plumbagin. This protection was seemingly specific to cellular stresses in which significant ROS were generated because the sensitivity to lethal heat shock or glucose deprivation was not altered in HG-treated human PAEC. HG, by itself, was not sufficient for protection; HG supplementation of bovine PAEC had no effect upon plasmalogen levels and did not rescue these cells from the cytotoxic effects of hypoxia. This is the initial demonstration that plasmalogen content can be substantially enhanced in a normal cell. These data also demonstrate that HG can protect cells during hypoxia and other ROS-mediated stress, likely due to the resulting increase in these antioxidant phospholipids.     

Maximum O2 uptake, O2 debt and deficit, and muscle metabolites in Thoroughbred horses

This study determined maximal O2 uptake (VO2max), maximal O2 deficit, and O2 debt in the Thoroughbred racehorse exercising on an inclined treadmill. In eight horses the O2 uptake (VO2) vs. speed relationship was linear until 10 m/s and VO2max values ranged from 131 to 153 ml.kg-1.min-1. Six of these horses then exercised at 120% of their VO2max until exhaustion. VO2, CO2 production (VCO2), and plasma lactate (La) were measured before and during exercise and through 60 min of recovery. Muscle biopsies were collected before and at 0.25, 0.5, 1, 1.5, 2, 5, 10, 15, 20, 40, and 60 min after exercise. Muscle concentrations of adenosine 5'-triphosphate (ATP), phosphocreatine (PC), La, glucose 6-phosphate (G-6-P), and creatine were determined, and pH was measured. The O2 deficit was 128 +/- 32 (SD) ml/kg (64 +/- 13 liters). The O2 debt was 324 +/- 62 ml/kg (159 +/- 37 liters), approximately two to three times comparative values for human beings. Muscle [ATP] was unchanged, but [PC] was lower (P less than 0.01) than preexercise values at less than or equal to 10 min of recovery. [PC] and VO2 were negatively correlated during both the fast and slow phases of VO2 during recovery. Muscle [La] and [G-6-P] were elevated for 10 min postexercise. Mean muscle pH decreased from 7.05 (preexercise) to 6.75 at 1.5 min recovery, and the mean peak plasma La value was 34.5 mmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)