Template:Normoxia - state or rate

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Aerobic and anaerobic from normoxia to anoxia: oxygen availability and metabolic state or rate

Oxygen availability

  • Normoxia - "Mitochondrial p50 values are 5 to 20 times less than the half-saturation point of myoglobin, which then suggests that mitochondrial respiration operates at the edge of oxygen limitation under normoxic intracellular oxygen pressures" [4]. "Compared with ambient oxygen pessure of 20 kPa (150 mmHg), oxygen levels are low within active tissues and are under tight control by microcirculatory adjustments to match oxygen supply and demand. Alveolar normoxia of 13 kPa (100 mmHg) contrasts with a corresponding 1 to 5 kPa (10 to 40 mmHg) extracellular pO2 in solid organs such as heart, brain, kidney and liver" [5]. "Even in comparative physiology, the traditional perspective on transitions to anoxia is dominated by an anthropomorphic or "anthropophysiologic" recognition which centers around the normoxic condition. We are aerobically poised and view the world from the preferred normoxic environment" [3]. "It remains to be defined, how low the pO2 needs to be set in the incubation medium to provide a “normoxic” environment for embryonic cardiomyocytes" [5].
  • Hyperoxia - Hyperoxic conditions may impose oxidative stress and may increase maximum aerobic performance. "In the intracellular microenvironment, mitochondria are well separated from air-level oxygen pressure, and high rates of oxidative phosphorylation must be maintained near or at limiting oxygen levels in some tissues. On the other hand, mitochondria are routinely isolated and studied at unphysiologically high oxygen concentrations with limited additions of antioxidants, despite the fact that mitochondria in tissues are protected from oxidative stress by both low oxygen levels and complex defence systems against reactive oxygen species" [4].
  • Hypoxia - "Metabolic hypoxia is indicated as a reduced oxygen flux below the critical oxygen pressure and is either fully or partially anaerobic" [2]. Metabolism under hypoxia may be fully aerobic even below the critical oxygen pressure pc and becomes partially or fully anaerobic below the limiting oxygen pressure pl [2]. This functional or physiological definition of hypoxia is compared to environmental hypoxia defined as environmental oxygen pressures below the normoxic reference level. "The high efficiency of oxidative phosphorylation at low oxygen emphasizes that even trace amounts of oxygen can make a vital energetic contribution when ATP limitation threatens cellular survival under severe hypoxia encountered at high altitude, in aquatic habitats, and during pathological states of ischemia" [6]. — "O2 limitation (due to environmental hypoxia, tissue-work-related hypoxia, or tissue ischemia)" [7]
  • Microxia - "Microxic regulation .. effectively increases the slope of the flux-pressure relation in the microxic region" [2]. "The pattern of microxic regulation is characterized by a steep oxygen flux/pressure slope at very low oxygen, despite some degree of conformation at mild hypoxia [3].
  • Anoxia - "When strictly anoxic conditions are not achieved, anaerobic metabolism proceeds simultaneously with oxygen consumption" [8]. "The difficulties involved in defining an absolute limit between microxic and anoxic conditions are best illustrated by a logarithmic pO2 scale [2]. "The terms anoxic and microxic should be rigorously applied to conditions characterized by actual oxygen measurements, with reference to the sensitivity limit of the method for oxygen detection or to the tested limits of the respective oxygen removal technique" [3].

Metabolism

  • Aerobic - Whereas anaerobic metabolism may proceed in the absence or presence of oxygen (anoxic or oxic conditions), aerobic metabolism is restricted to oxic conditions.
  • Anaerobic - "In zoophysiology, 'anaerobic' (without air) is rarely defined in terms of controlled measurements of the actual extent of anaerobic conditions [2]. "In contrast to the aerobically balanced metabolism of animals, tissues and harvested cells under normoxic and a wide range of hypoxic states, many cultured cells are frequently below the limiting pO2 under standard aerobic culture conditions, incurring simultaneous aerobic and anaerobic metabolism" [3]. "Two disparate features characterize animal anaerobiosis: high power output in the case of physiologically induced hypoxia, and high efficiency of energy conversion under environmental anoxia [1]. Terms shown in italics have been updated for consistency of nomenclature.

Critical and limiting pO2

  • Critical oxygen pressure - "Metabolic hypoxia is indicated as a reduced oxygen flux below the critical oxygen pressure [2].
  • Limiting oxygen pressure - "Below the critical oxygen pressure, the aerobic ATP production decreases, and below the limiting oxygen pressure anaerobic processes compensate increasingly for the diminished aerobic flux." Then "there is an extended phase of fully aerobic hypoxia" [2].

References

  1. Gnaiger E (1983) Heat dissipation and energetic efficiency in animal anoxibiosis. Economy contra power. J Exp Zool 228:471-90. - »Bioblast link«
  2. Gnaiger E (1991) Animal energetics at very low oxygen: Information from calorimetry and respirometry. In: Strategies for gas exchange and metabolism. Woakes R, Grieshaber M, Bridges CR (eds), Soc Exp Biol Seminar Series 44, Cambridge Univ Press, London:149-71. - »Bioblast link«
  3. Gnaiger E (1993) Homeostatic and microxic regulation of respiration in transitions to anaerobic metabolism. In: The vertebrate gas transport cascade: Adaptations to environment and mode of life. Bicudo JEPW (ed), CRC Press, Boca Raton, Ann Arbor, London, Tokyo:358-70. - »Bioblast link«
  4. Gnaiger E (2001) Bioenergetics at low oxygen: dependence of respiration and phosphorylation on oxygen and adenosine diphosphate supply. Respir Physiol 128:277-97. - »Bioblast link«
  5. Gnaiger E (2003) Oxygen conformance of cellular respiration. A perspective of mitochondrial physiology. Adv Exp Med Biol 543:39-55. - »Bioblast link«
  6. Gnaiger E, Méndez G, Hand SC (2000) High phosphorylation efficiency and depression of uncoupled respiration in mitochondria under hypoxia. Proc Natl Acad Sci U S A 97:11080-5. - »Bioblast link«
  7. Hochachka PW, Somero GN (2002) Biochemical adaptation: mechanism and process in physiological evolution. Oxford Univ Press, New York: 466 pp. - »Bioblast link«
  8. Gnaiger E, Staudigl I (1987) Aerobic metabolism and physiological responses of aquatic oligochaetes to environmental anoxia. Heat dissipation, oxygen consumption, feeding and defecation. Physiol Zool 60:659-77. - »Bioblast link«