Revision as of 07:45, 28 June 2022

The ABC of hypoxia - special BEC issue

Questions raised by Dal on the strategy for the ABC of hypoxia triggered the concept of a special issue of BEC on the ABC of hypoxia. The special issue is introduced by the ‘definitions paper’ on ABC of hypoxia - what is the norm? Contributors of articles to the special issue will present their (peer-reviewed) manuscripts and may contribute to the introductory definitions paper.

Living Communication

For the Living Communication (future editions), we will invite additional contributors to ensure a broad perspective of hypoxia and hyperoxia, from comparative physiology, high altitude medicine, to clinical interventions and studies of isolated mitochondria, cultured cells, to living organisms in health and disease. We should clarify step-by-step who intends to join as a contributing 'author' or as a ‘signatory’. A good example is the following reference:

Amrhein V, Greenland S, McShane B (2019) Scientists rise up against statistical significance. Nature 567:305-7. - https://www.nature.com/articles/d41586-019-00857-9 - with more than 800 signatories

An example to learn from:

BEC_2020.1_doi10.26124bec2020-0001.v1 with 666 coauthors. Many coauthors (1) ignore the message in their current publications, or (2) do not cite the paper if they use the message. The history of this paper has contributed to initiating the MitoFit Preprints server, but only exceptional coauthors have submitted a manuscript to MitoFit Preprints since 2019 (Gnaiger 2019 MitoFit_Preprints).

Points of discussion

To do

Bengt: My proposal would be that we write a short concise definition piece.
Erich: Priorities and aims:

Contribute to an improved presentation of concepts on ‘oxia’ across disciplines, particularly considering hypoxia and hyperoxia in clinical settings, high-altitude medicine and sport physiology, comparative physiology, studies of cell models and mitochondrial preparations, and evolutionary biology. If we consider obligatory and facultative anaerobes among eucaryotic parasites and microbes, definitions must be much less rigorously ‘definite’.
Critically consider previous efforts to communicate core concepts on hypoxia and hyperoxia with an attempt to harmonize the nomenclature within the framework of a theoretically founded and practical terminology.
Avoid ad-hoc recommendations which – although justified in a specific context – may fail to receive recognition in a more general context. (This is why I favor an expanded list of coauthors.) - Bengt: I have just completed a so-called Delphi study, it was quite fun. We were able to recruit and have participate 50 % of the worlds most recognized experts in altitude medicine and physiology (defined as having more than 9 papers on AMS) to help define a list of statements of desirable minimal knowledge for lay persons traveling to and at altitude. Over a series of several rounds we could reach consensus. A student of mine is writing it up. Perhaps something to keep in mind. Once we have a solid document we could summarize it in a series of statements that we could then submit to a panel of experts in a Delphi study.
If in doubt, give preference to terminology that is used already more widely – widely either in an interdisciplinary sense or in a practical sense of common use. This argument is not strictly scientific since science must progress beyond established paradigms. Scientific terminology and communication play key roles in the advancement of knowledge across disciplines and unfortunately also in resistance against new breakthroughs. To emphasize an openness for progress and innovation, any preference for established terminology should be taken into account only in cases of argumentative doubt.

Intracellular normoxia

The definitions summarized in the main text should guide the discussion towards controversies that have yet to be resolved.

Bengt: My idea is to label ‘anything’ that a healthy human can do while breathing sea level equivalent air, resting, sleeping, walking, running, one-legged kicking, maximal sprinting, can be labelled physiological, i.e. comprised in the range of what normal physiology can attain. The advantage of such is that it simplifies things so that any deviation from what is observed in all these activities when breathing low or high oxygen gas, or other ways of changing the amount of oxygen transported to the site at stake, can then be labelled as such.

One-legged kicking

Erich: One-legged kicking is the invention by Bengt Saltin as an experimental model to proof limitation by cardiovascular oxygen supply under physiological conditions of maximum aerobic work during running and cycling not restricted to one leg. One-legged kicking is not under evolutionary selection pressure (I do not include soccer players since their kicking is driven by the anaerobic mechanism of PCr splitting). Thus, one-legged kicking cannot provide an argument to define a norm for intracellular conditions nor can intracellular normoxia be a useful reference point, if it depends on training status, regulation by the brain in less motivated compared to professionally motivated maximum exercisers (several excellent papers by Bengt Kayser). With these arguments, it may well be that I must revise previous reference values of intracellular p_O₂ in view of “Thus mitochondrial respiration proceeds at 90 % of its hyperbolic maximum at the p₅₀ of myoglobin, suggesting the possibility of a small but significant oxygen limitation even under normoxia in active muscle.“ Then intracellular tissue normoxia will not extend to the very low p_O₂ observed in non-steady state of maximum aerobic activity when intracellular p_O₂ drops towards the p₅₀ of myoglobin – this is (C) functional hypoxia (defined by Critical functions and critical p_c).
Bengt: The question then becomes, when making observations in our reference object (say, the usual suspect a 70 kg 170 cm healthy and active male ….. ;-) ), what do we consider in our observations in B and C as normoxic? Resting only would be too restricted I find, since physiology needs oscillation for homeostasis (in the originally intended definition of the latter term, see e.g. https://www.frontiersin.org/articles/10.3389/fphys.2020.00200/full). So we should include some increased levels of metabolism from physical activity, certainly an aspect of human behavior that should be considered physiological. But up what level? And by what type of that archetypical male? You are right that the leg-kicking of genius Bengt Saltin (on my PhD committee !) is perhaps too much, even though experimentally so well thought through to study physiology. The idea to set the limit at what is sustainable is good, but runs into the problem of what is sustainable … the so-called critical power concept (which I have my critical thoughts about …. but that is another story) is a decay towards ‘infinity’ at some quite sub-maximal power, lower than 80-90 VO2max, and dependent on training status. Hence my reasoning, instilled by a desire to look for an internally logical framework with no need for some arbitrary cut-off that cannot be defined precisely. This yearning for simple clarity led me to wish taking the observations in environmental normoxia as the baseline of systems normoxia, i.e. any oxygen concentration anywhere in the organism, at rest up to that observed in maximal large and small muscle mass exercise paradigms.
Erich: I share your concerns against having to set an arbitrary activity level to define (B) compartmental hypoxia. Therefore, I proposed neither V_O₂max nor basal nor standard metabolism as a reference for normoxia, but routine metabolism. This is not an arbitrarily defined state but can be considered as the activity level averaged over the awake period of an organism. An attempt to define routine metabolism is lacking to my knowledge in the mammalian literature, hence my reference to the literature on fishes, and a reference to steady-state (averaged, allowing for oscillations). An alternative could be to combine from our ABC the arguments on (B) and (C): If normoxic critical functions (e.g. respiration, protein synthesis) are defined as those not limited by oxygen availability in a healthy organism, then (B) normoxia in the compartmental microenvironments (e.g. intracellular) might be defined as those oxygen levels supporting (C) normoxic function. Perhaps we would find that the two alternatives of normoxia (B) ― compartmental oxygen pressure at routine metabolism versus compartmental oxygen pressure above a critical oxygen pressure inducing oxygen limitation ― lead to closely corresponding results.

Tissue work-induced physiological hypoxia

Erich: Would we then drop the concept of tissue work-induced physiological hypoxia? We would run against a strong literature.
Bengt: That is indeed the central question that I am grappling with. I tend to wish to see the combination of ‘physiological’ and ‘hypoxia’ to be interpreted as an oxymoron (😉). As an archetypical example, lactate release from muscle fibers during exercise up to max, accompanied by conditions of lowering oxygen concentrations in working muscle which are indeed labeled hypoxic by others, but which are not the reason for the increased lactate release which has more to do with mass effects upstream from the Krebs cycle, and also being perfectly physiological with the lactate released serving important physiological mechanisms. This is something that starts to occur already at quite submaximal power outputs, that we can consider as being within the scope of usual metabolic fluctuations over the day in people engaging into the recommended physical activity levels by the WHO. As a second example we can look at the low oxygen concentrations in some specific tissus such is the renal papillae, pretty low but again perfectly physiological. I would consider both examples to be physiological, but not necessarily hypoxic, even though low in oxygen tensions as compared to arterial blood. If we would decide to go with the status quo we must define some frontier between physiological and pathological hypoxia. One way is indeed to look at what the effect functionally is of some drop in oxygen beyond what is observed in physiological conditions, but where to draw the line, and how to define some way that is applicable in various tissus? The idea of routine metabolism would not take away the problem of labeling for the two examples? So, my take remains, but I am of course OK to let go (!), is that we do need some additional terminology to distinguish between physiological and not physiological, hence my initial proposal to also use euoxia and dysoxia. In the renal papillae there are low oxygen concentrations but they are euoxic; a patient with angina pectoris has periods of low oxygen concentrations in myocardial tissu and they are dysoxic.
Erich: On your “arechetypical example, lactate release from muscle fibers during exercise up to max, accompanied by conditions of lowering oxygen concentrations in working muscle which are indeed labeled hypoxic by others, but which are not the reason for the increased lactate release which has more to do with mass effects upstream from the Krebs cycle, and also being perfectly physiological with the lactate released serving important physiological mechanisms". - Correlative and causative relations must be distinguished. The best method to do this is measurement of oxygen kinetics which reveals responses related to (1) hypoxia and hyperoxia directly and (2) mechanisms other than control by O₂.

Erich: In hyperbolic oxygen kinetics a critical function (e.g. mitochondrial respiration) is measured at kinetic oxygen saturation and p_O₂ lowered towards anoxia. Below a critical p_O₂ (p_c) the function is oxygen limited and conditions below the p_c are hypoxic. The problem of defining the p_c has been addressed in Gnaiger (2003). The function is reduced to 50 % of maximum at the oxygen pressure p₅₀; to 75 % at p₇₅; 90 % at p₉₀; 95 % at p₉₅; 99 % at p₉₉. Where is the p_c?

Terms

Happy hypoxia - https://physoc.onlinelibrary.wiley.com/doi/10.14814/phy2.14619
Silent hypoxia - https://pubmed.ncbi.nlm.nih.gov/33891272/

Current conferences

» https://www.rsc.org/events/detail/3610/keystone-symposia-hypoxia-molecular-mechanisms-of-oxygen-sensing-and-response-pathways

» https://www.hypoxia.net/

» https://www.dwscientific.com/2nd-whitley-hypoxia-symposium

» https://waset.org/hypoxia-exercise-and-hypoxic-exposure-conference-in-january-2022-in-bali

» https://conferenceindex.org/event/international-conference-on-hypoxia-inducible-factors-and-oxygen-biology-ichifob-2022-may-dubai-ae

@@ Line 55: / Line 55: @@
 ::::# Avoid ad-hoc recommendations which – although justified in a specific context – may fail to receive recognition in a more general context. (This is why I favor an expanded list of coauthors.) - Bengt: I have just completed a so-called Delphi study, it was quite fun. We were able to recruit and have participate 50 % of the worlds most recognized experts in altitude medicine and physiology (defined as having more than 9 papers on AMS) to help define a list of statements of desirable minimal knowledge for lay persons traveling to and at altitude. Over a series of several rounds we could reach consensus. A student of mine is writing it up. Perhaps something to keep in mind. Once we have a solid document we could summarize it in a series of statements that we could then submit to a panel of experts in a Delphi study.
 ::::# If in doubt, give preference to terminology that is used already more widely – widely either in an interdisciplinary sense or in a practical sense of common use. This argument is not strictly scientific since science must progress beyond established paradigms. Scientific terminology and communication play key roles in the advancement of knowledge across disciplines and unfortunately also in resistance against new breakthroughs. To emphasize an openness for progress and innovation, any preference for established terminology should be taken into account only in cases of argumentative doubt.
-=== Chemical anoxia, chemical hypoxia ===
-:::[[User:Cecatto Cristiane|Cecatto Cristiane]] ([[User talk:Cecatto Cristiane|talk]]) 16:24, 12 January 2022 (CET) Here I leave my thoughts and my little research about it. I have some PMID numbers saved if someone is particularly interested.
-:::The search in PubMed by the term “chemical anoxia” retrieved 73 results, which included several electric transfer system inhibitors, alone or in combination, used to inhibit the mitochondrial respiration and therefore making the mitochondria unable to use the O2 that is available. This does not characterize anoxia.
-:::Some papers bring the concept of true anoxia (or acute anoxic anoxia) vs chemical anoxia, being the true anoxia induced by N2 (PMID: 9249565, 16621957). Another paper brings the “true-chemical anoxia” with the combination of azide and N2 or cyanide + N2 (PMID: 18491317, 19779726).
-:::When the term searched in PubMed is “chemical hypoxia”, 444 results are retrieved and in principle, the majority shows the use of cobalt as an inductor of hypoxia. Cobalt stabilizes hypoxia-inducible factors 1α and 2α under normoxic conditions (PMID: 30484873). But still, some papers present the use of the same inhibitors mentioned before. The concept of physical hypoxia (decrease of O2 concentration – in cell culture) vs chemical hypoxia appears (PMID: 32922298, PMID: 31136722).
-:::There was also interchangeability between chemical hypoxia and chemical ischemia (PMID: 29987461).
-:::In all of them, besides decreasing the O2 concentration, using N2, or Dithionite, the one that makes more sense for me is FCCP (which I saw in only one paper) if there is time enough to the sample consumes all the O2 available. But this will only work in a closed measurement.
-:::I assume that more inhibitors could fall into this misleading classification if we could have the time to read all 517 papers.
-:::The term “anoxic anoxia” retrieved 48 results, one from 2000 and all others from the 1990’s and older.
-:::Surprisingly, no papers were mentioning CO, maybe due to handling difficulties???
-:::{| class="wikitable"
-|- style="font-weight:bold;"
-! Chemicals or combinations for “chemical anoxia”
-|-
-| Cyanide
-|-
-| Azide
-|-
-| Cyanide + deoxyglucose
-|-
-| Rotenone
-|-
-| Rotenone + deoxyglucose
-|-
-| Antimycin A + deoxyglucose
-|-
-| Cyanide + fusicoccin + SHAM
-|-
-| Cyanide + iodoacetate
-|-
-| Azide + N2
-|-
-| 3-Nitropropionic acid
-|-
-| Dithionite
-|-
-| FCCP
-|-
-| Cobalt
-|-
-| Antimycin A
-|-
-| Rotenone + Antimycin A + Azide   + Oligomycin
-|-
-| Cobalt + deferoxamine + azide
-|}
 === Intracellular normoxia ===
@@ Line 131: / Line 82: @@
-= To be replaced by manuscript pdf =
+= Current conferences =
-== Introduction ==
-:::: For explaining normoxia and deviations from normoxia, we distinguish ('''A''') ambient oxygen conditions in the environment and ('''B''') biological compartments, from ('''C''') critical functions and signaling affected by oxygen availability below a critical oxygen pressure ('''Figure 1'''). This leads to three different but connected definitions of normoxia, which provide a reference for deriving three corresponding causes for deviations from normoxic conditions and normoxic function. The '''ABC''' of hypoxia links the three letters to the meaning of three complementary perspectives on normoxia across scientific disciplines. Several articles under the umbrella of ''ABC of oxygen'' (Bateman 1998; Leach 1998; Peacock 1998; Williams 1998; Wilmshurst 1998) use the ABC symbolically and provide overviews on specific areas related to normoxia, hypoxia, and hyperoxia.
-[[File:TheHypeOfOxia 001.jpg|left|500px|thumb|'''Figure 1'''. '''The ABC of hypoxia''': Ambient oxygen pressure ''p''<sub>O<sub>2</sub></sub> and intracellular ''p''<sub>O<sub>2</sub></sub> in tissues in vivo exert different limitations on critical physiological functions. At any ambient ''p''<sub>O<sub>2</sub></sub>, intracellular partial oxygen pressure ''p''<sub>iO<sub>2</sub></sub> varies widely between tissues and as a function of the level of physiological activities (not shown). Graphics by Paolo Cocco.]]
-:::: Conditions called 'hypoxic' from one perspective ('''A''' ambient) are classified from another perspective as 'hyperoxic' ('''B''' biological compartments) or normoxic ('''C''' critical functions). For instance, slightly hypoxic culture conditions ('''A''') of stem cells may actually be hyperoxic compared to tissue conditions in vivo ('''B'''), but are normoxic ('''C''') if the ''p''<sub>O<sub>2</sub></sub> is above the ''p''<sub>c</sub> of mitochondrial respiration. Respiration drops sharply below the ''p''<sub>c</sub>. If we consider as normoxic any intracellular ''p''<sub>O<sub>2</sub></sub> ('''B''') that is obtained at any physiological activity level of a healthy organism at ambient normoxia ('''A'''), then this definition of normoxia contrasts with the notion of physiologically induced tissue hypoxia, if the low intracellular ''p''<sub>O<sub>2</sub></sub> drops below the ''p''<sub>c</sub> of mitochondrial respiration ('''C''').
-:::: The concept of harmonization instead of standardization of terminology pursues a strategy that may be commonly acceptable across apparently incompatible points of view: Instead of proposing a guideline on terminology, the '''ABC''' of hypoxia and corresponding norms is intended to provide recommendations to clarify (''1'') the point of view ('''A''' versus '''B''' versus '''C''') and (''2'') the causes and processes of altered oxygen availability and supply ('''a''' versus '''b''' versus '''c'''). Then every author may consider if the important field of oxygen-regulated biological function will gain (''3'') from a consensus on general definitions potentially provided by the '''ABC''' of hypoxia.
-== Systematic definitions of normoxia as a reference for hypoxia ==
-=== Categories of normoxia ===
-:::: <big>'''A. Ambient normoxia'''</big>
-:::: Comparable to referring to sea level for expressions of altitude, ambient normoxia may be defined arbitrarily as ''p''<sub>O<sub>2</sub></sub> of c. 20 kPa (150 mmHg) at sea level in present air or in the air-saturated aqueous environment of an organism. We take the SI definition of standard pressure of 100 kPa as the reference barometric pressure and water vapour saturated air as the reference for the normoxic ''p''<sub>O<sub>2</sub></sub>.
-:::: <big>'''B. Biological compartments of the respiratory cascade – compartmental normoxia'''</big>
-:::: We propose to define biological compartmental normoxia as the ''p''<sub>O<sub>2</sub></sub> in any compartment of a living organism (alveolar, arterial, venous, mixed-venous, intracellular) obtained under ambient normoxia (i.e. sea level equivalent). At environmental normoxia, compartmental ''p''<sub>O<sub>2</sub></sub> is a function of aerobic metabolic activity and O<sub>2</sub> transport from the environment to the various compartments of an organism (Weibel 2000; Keeley, Mann 2019; Ortiz-Prado et al 2019). Compartmental ''p''<sub>O<sub>2</sub></sub> may be far lower compared to ambient oxygen levels ('''A''') but may be normoxic in terms of specific critical functions (defined in '''C''').
-:::: In isolated living cells and mitochondrial preparations – including isolated mitochondria, tissue homogenates, and permeabilized cells and tissues – ambient normoxia at air saturation of the incubation media ('''A''') must be distinguished from biologically relevant normoxia as defined by the oxygen pressure prevailing in the cellular and mitochondrial microenvironment in the tissue of the intact organism ('''B''').
-:::: Biological compartmental normoxia ranges from ('''B<sub>r</sub>''') aerobic resting or routine steady-state activity up to ('''B<sub>max</sub>''') at maximum aerobic activity ''V''<sub>O<sub>2</sub>max</sub> sustained for only a few minutes. Routine respiration (Chabot et al 2016; Nelson 2016) is higher than standard or basal respiration due to the oxygen consumption required to sustain various routine activities, not restricted to locomotory activity but including the effects of food intake and perception. In isolated living cells studied in culture and isolated mitochondria, the concept of ‘environmental normoxia’ becomes ambiguous without a clear distinction between ''ambient normoxia'' from the perspective of the living organism and ''experimental normoxia'' that mimics the extracellular or intracellular microenvironment in vivo (Figure 1). ROUTINE respiration of living cells is under physiological control of energy turnover in the range from LEAK respiration to OXPHOS capacity (Gnaiger et al 2020). Correspondingly, experimental normoxia needs to be adjusted to the activity-dependent compartmental ''p''<sub>O<sub>2</sub></sub> in the tissues of origin.
-:::: <big>'''C. Critical function – normoxia evaluated by function, functional normoxia'''</big>
-:::: For any critical function, normoxic performance is defined as the biological response that does not deviate from the physiological function measured under environmental or compartmental normoxia. Whereas normoxic respiration of isolated mitochondria can be measured as a constant rate in a wide range of O<sub>2</sub> concentrations (oxyregulators; Gnaiger 2003), H<sub>2</sub>O<sub>2</sub> production is a continuous function of O<sub>2</sub> concentration (oxyconformance; Komlódi et al 2021). H<sub>2</sub>O<sub>2</sub> production, therefore, cannot be used as a critical function for defining functional normoxia.
-:::: ‘The high affinity of cytochrome ''c'' oxidase for oxygen implies independence of mitochondrial respiration of oxygen over a wide range of oxygen levels, which gives rise to the paradigm of “oxygen regulation”, although “kinetic oxygen saturation” describes more accurately the underlying mechanism’ (Gnaiger 2003). Normoxic respiration can thus be defined as respiration at kinetic oxygen saturation, and hypoxic respiration as respiration below a critical oxygen pressure ''p''<sub>c</sub>, when the ''p''<sub>O<sub>2</sub></sub> becomes limiting and respiration shows oxyconformance. “Intracellular hypoxia is defined as ('''B''') local oxygen pressure below normoxic reference states, or ('''C''') limitation of mitochondrial respiration by oxygen levels below kinetic saturation, resulting in oxyconformance” (Gnaiger 2003).
-=== Causes of deviations from normoxia ===
-:::: Based on definitions of the categories ('''A''') environmental normoxia, ('''B''') compartmental normoxia, and ('''C''') functional normoxia, the causes for deviations from normoxia are separated into three categories:
-:::: <big>'''a. Environmental hypoxia and hyperoxia'''</big>
-::::* Hypobaric conditions: high altitude or low-pressure chamber
-::::* Hyperbaric conditions: high-pressure chamber, diving
-::::* Normobaric conditions: O<sub>2</sub> deprivation in the environment (environmental normobaric hypoxia), O<sub>2</sub> supplementation (environmental normobaric hyperoxia).
-:::: <big>'''b. Compartmental hypoxia and hyperoxia'''</big>
-::::* Environmentally induced hypoxia or hyperoxia on the compartmental level (living organism)
-::::* Pathologically and toxicologically induced hypoxia or hyperoxia on the compartmental level (living organism)
-::::* Experimental conditions for isolated organs, tissues, cells, and organelles: deviations of incubation ''p''<sub>O<sub>2</sub></sub> of experimental preparations from ('''B''') compartmental normoxia in the intact organism
-:::: <big>'''c. Functionally induced hypoxia and hyperoxia'''</big>
-::::* Environmental: respiratory O<sub>2</sub> depletion or photosynthetic O<sub>2</sub> accumulation in eutrophic semi-closed aqueous environments (Gnaiger 1983).
-::::* Physiologically induced on the compartmental level. Hypoxia: tissue-work related; living organism at high workload of a tissue; (mal)adaptive responses of the respiratory cascade to (de)training and lifestyle. Hyperoxia: endosymbiotic algae at high light intensities (e.g. corals).
-::::* Pathological-pharmacological-toxicological O<sub>2</sub>-transport related hypoxia (ischemia and stroke, anaemia, chronic heart disease, chronic obstructive pulmonary disease, disordered regional distribution of blood flow, severe COVID-19, obstructive sleep apnea, CO poisoning), inhibition or acceleration of O<sub>2</sub>-linked pathways (cyanide, rotenone, NO, ..; doping, ..).
-::::* Genetic: inhibition or acceleration of O<sub>2</sub>-linked pathways (mutations, inherited diseases, knockout, knock-in).
-:::: Maximum activity may induce compartmental hypoxia, gauged from a comparison of intracellular ''p''<sub>O<sub>2</sub></sub> ― which declines in skeletal muscle at ''V''<sub>O<sub>2</sub>max</sub> relative to routine activity (Richardson et al 2006) ― and oxygen kinetics of isolated mitochondria (Gnaiger 2001; Harris et al 2015). Since OXPHOS capacity of isolated mitochondria (Gnaiger et al 2020) is already slightly limited at intracellular tissue ''p''<sub>O<sub>2</sub></sub> observed at ''V''<sub>O<sub>2</sub>max</sub>, a high workload can entail physiological hypoxia. ''V''<sub>O<sub>2</sub>max</sub> cannot be maintained over prolonged periods of time, such that upon functionally induced hypoxia the organism returns to a normoxic steady state.
-:::: Categories '''A''' and '''a''' appear similar, distinguished only as ('''A''') a description of a state in terms of a given ''p''<sub>O<sub>2</sub></sub>, in contrast to ('''a''') including the causes for deviations of the ''p''<sub>O<sub>2</sub></sub> from normoxia. Therefore, '''A''' and '''a''' are tightly linked.
-== Far from normoxia ==
-[[File:O2 log scale.png|left|500px|link=Gnaiger 2003 Adv Exp Med Biol|thumb|'''Figure 2'''. '''Deviations from ambient normoxia''': logarithmic scale of partial pressure of oxygen ''p''<sub>O<sub>2</sub></sub> (1 kPa = 0.133322 mmHg). The corresponding oxygen concentrations ''c''<sub>O<sub>2</sub></sub> [µM] is given for pure water at 25 °C. The volume fractions of air ''Φ''<sub>air</sub> [% air] and volume fractions of O<sub>2</sub> ''Φ''<sub>O<sub>2</sub></sub> [% O<sub>2</sub>] are for the standard barometric pressure of 100 kPa and corrected for a 3.2 kPa water vapor saturation pressure at 25 °C. Left: some critical and limiting ''p''<sub>O<sub>2</sub></sub>, ''p''<sub>c</sub> and ''p''<sub>l</sub>, for mammalian tissues and mitochondria. Right: limit of detection of selected methods (O2k: Oroboros Oxygraph-2k). Modified after Gnaiger (1991).]]
-=== Extents of hypoxia: approaching anoxia ===
-:::: There is a continuous transition of hypoxia to anoxia, which is best represented on a logarithmic scale of ''p''<sub>O<sub>2</sub></sub> ('''Figure 2'''). Respiration declines below the critical ''p''<sub>O<sub>2</sub></sub>, ''p''<sub>c</sub>, and anaerobic metabolism is stimulated below the limiting ''p''<sub>O<sub>2</sub></sub>, ''p''<sub>l</sub> (Gnaiger 1991). Only if the transition to anoxia is of interest, then further differentiation of microxia and anoxia is of technical and physiological interest taking into account the limit of detection of methods applied for determining ''p''<sub>O<sub>2</sub></sub> and different methods to detect functional responses to the presence (microxia) or absence (anoxia) of trace amounts of oxygen (Gnaiger 1993; Harrison et al 2015). Oxic versus anoxic conditions (in the presence or absence of molecular oxygen) must be distinguished from aerobic and anaerobic metabolism. Aerobic metabolism requires oxic conditions, whereas anaerobic metabolism may proceed under oxic conditions (aerobic glycolysis) or under anoxia.
-=== Extents of hyperoxia: Experimental conditions for studies of cultured cells and isolated mitochondrial ===
-::::* Difference between environmental and intracellular tissue normoxia (compartmental)
-::::* Extend the focus on the respiratory response to effective ‘tissue hyperoxia and hypoxia’ in studies of isolated mitochondria and cells, to oxygen sensing, redox states, molecular signaling, oxidative stress, P»/O<sub>2</sub> ratios (Gnaiger et al 2000), ...
-=== Partial pressure and concentration of oxygen ===
-:::: Discontinuous differences of oxygen pressure, Δ''p''<sub>O<sub>2</sub></sub>, are caused by diffusion limitation across compartmental barriers, and discontinuous oxygen concentration differences, Δ''c''<sub>O<sub>2</sub></sub>, are additionally a function of oxygen solubilities in different compartments, such as the gaseous-aqueous compartments in the lung and the aqueous-membraneous compartments in the cell.
-:::: In O<sub>2</sub> transport by convection, the total O<sub>2</sub> concentration matters in the medium that is moved from the source to the sink. O<sub>2</sub> is transported with the medium. O<sub>2</sub> carriers such as hemoglobin enhance the convectional efficacy by effectively increasing the total amount of O<sub>2</sub> transported by a volume of blood. Just having a carrier is not sufficient, but the loading of the carrier with O<sub>2</sub> at the source and unloading at the sink are essential. A larger amount of molecular O<sub>2</sub> is transported per volume of gas compared to a volume of aqueous solution. In O<sub>2</sub> diffusion by molecular dispersion of O<sub>2</sub> in a gradient of O<sub>2</sub> pressure, d''p''<sub>O<sub>2</sub></sub>/d''z'' is the driver, where Fick's law of diffusion represents a special case of the linear flux-pressure relationships which can be extended to discontinuous descriptions based on pressure differences Δ''p''<sub>O<sub>2</sub></sub> (Gnaiger 2020). In diffusion, O<sub>2</sub> is transferred across the medium, which may be facilitated by O<sub>2</sub> carriers such as myoglobin, again dependent on the loading/unloading kinetics. The O<sub>2</sub> solubility is a decisive component of O<sub>2</sub> transfer by diffusion (Hitchman, Gnaiger 1983), implicit in the diffusion coefficient or mobility (Gnaiger 2020).
-[[File:O2 solubility-gaslaw.jpg|left|500px|thumb|'''Table 1'''. '''Temperature dependence of oxygen levels in the gas and aqueous phases at ambient normoxia''': ''S''<sub>G</sub>(g) and ''S''<sub>O<sub>2</sub></sub>(aq) are the gas solubilities in the gas and aqueous phase. ''γ''<sub>O<sub>2</sub></sub>(aq) is the activity coeffient of O<sub>2</sub>(aq). ''p''<sub>O<sub>2</sub></sub>* and ''c''<sub>O<sub>2</sub></sub>(aq)* are the partial pressure and concentration of O<sub>2</sub>(aq) at ambient normoxia of 100 kPa barometric pressure, at water vapour saturation and equilibrium* with the gas phase.]]
-:::: At ambient normoxia, the concentration of oxygen in dry air at 25 °C equals ''Φ''<sub>O<sub>2</sub></sub>·(''RT'')<sup>-1</sup> = 0.20946·(100-3.17) kPa·(2.479 kJ·mol<sup>-1</sup>)<sup>-1</sup> = 8.18 mM. In contrast, the oxygen concentration in air-saturated pure water is 0.255 mM or 254.7 µM. Under these conditions, the oxygen partial pressure is identical at 20.28 kPa in air and air-saturated water, but the oxygen concentration in air is 32-fold higher than in water (https://wiki.oroboros.at/index.php/Oxygen_solubility). The concept of compartmental normoxia ― and consideration of terrestrial and aqueous organisms ― raises the issue of the preference of expressing O2 ‘levels’ in terms of amount concentration ''c''<sub>O<sub>2</sub></sub> in units [mol∙L<sup>-1</sup> = M] or gas pressure ''p''<sub>O<sub>2</sub></sub> in SI units [J∙m<sup>-3</sup> = Pa]. At room temperature or 37 °C, the concentration of oxygen is 30- to 40-fold higher in the gas phase compared to the aqueous phase in equilibrium with the gas phase. Compared to the oxygen solubility ''S''<sub>O<sub>2</sub></sub> [µM∙kPa<sup>-1</sup>] in water, ''S''<sub>O<sub>2</sub></sub> in serum is 0.89 at 37 °C (Baumgärtl, Lübbers 1983) and in mitochondrial respiration medium MiR05 this solubility factor of the medium (''F''<sub>M</sub>, solubility in salt solution divided by solubility in pure water) is 0.92. Taken together, these are the physicochemical reasons why tracheal oxygen supply through the gas phase is very effective in supporting high oxygen demand of flying insects, and why we need red blood cells with hemoglobin to boost the total amount of oxygen carried per volume of blood.
-:::: Physicochemical concepts on the oxygen solubility in gas (g) versus aqueous solution (aq) are summarized in '''Table 1'''. In aqueous extracellular and intracellular environments, climate change is intrinsically linked to concepts of normoxic O<sub>2</sub> concentration ''c''<sub>O<sub>2</sub></sub> [µM] versus O<sub>2</sub> partial pressure ''p''<sub>O<sub>2</sub></sub> [kPa] due to the decline of O<sub>2</sub> solubility [µM/kPa] with increasing temperature.
-== Notes ==
-=== Quotes on hypoxia and normoxia ===
-:::: Several definitions of normoxia or hypoxia are restricted to a single category or specific combination of categories and lack, therefore, generality.
-::::* '''ABC''': ''Compared with ambient oxygen pressure 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 ''p''<sub>O<sub>2</sub></sub> in solid organs such as heart, brain, kidney, and liver (19). Considering the respiratory cascade and oxygen in the microenvironment of tissue (23, 26), it appears surprising that protein synthesis becomes inhibited in hepatocytes incubated at a “hypoxic” ''p''<sub>O<sub>2</sub></sub> of 11 kPa (80 mmHg) compared with 95 % oxygen (41), hepatocyte respiration is reduced at 9 kPa (70 mmHg (54)), and cytochrome ''c'' oxidase is reversibly inhibited at 50 μM (4 kPa or 30 mmHg (16)). Does this suggest substantial oxygen limitation of aerobic ATP production and protein synthesis to prevail under normoxia in vivo, or are responses to oxygen altered in vitro?'' ([[Gnaiger 2003 Adv Exp Med Biol]]).
-::::* '''B''' and '''C''': ''Intracellular hypoxia is defined as local oxygen pressure below normoxic reference states, or limitation of mitochondrial respiration by oxygen levels below kinetic saturation, resulting in oxyconformance'' ([[Gnaiger 2003 Adv Exp Med Biol]]). - These are definitions for ('''B''') or ('''C''').
-::::* '''B''' and '''C''': ''The high affinity of cytochrome ''c'' oxidase for oxygen implies independence of mitochondrial respiration of oxygen over a wide range of oxygen levels, which gives rise to the paradigm of “oxygen regulation”, although “kinetic oxygen saturation” describes more accurately the underlying mechanism''  ([[Gnaiger 2003 Adv Exp Med Biol]]). - Normoxic respiration can thus be defined as respiration at kinetic oxygen saturation, and hypoxic respiration as respiration below a critical oxygen pressure ''p''<sub>c</sub>, when the ''p''<sub>O<sub>2</sub></sub> becomes limiting and respiration shows oxyconformance.
-::::* '''C''': Richalet (2021) refers to Opitz (1941): ‘In 1941, Opitz says, in the introduction of his review paper “Über akute Hypoxie” (About acute hypoxia): “Die Bezeichnung ‘Hypoxie’ soll immer dann verwendet werden, wenn die Sauerstoffversorgung der Gewebe gegenüber der Norm erschwert ist.” (The term “hypoxia” should always be used when the oxygen supply to the tissues is more difficult than the norm.) (40).’
-::::* '''A''' and '''B''' versus '''C''': ''Thus mitochondrial respiration proceeds at 90 % of its hyperbolic maximum at the ''p''<sub>50</sub> of myoglobin, suggesting the possibility of a small but significant oxygen limitation even under normoxia in active muscle'' ([[Gnaiger 2003 Adv Exp Med Biol]]). - According to Opitz (1941), even oxygen-limited ''V''<sub>O<sub>2</sub>max</sub> under normoxia ('''A''') would be considered a physiological norm for the healthy trained person, and hence functionally normoxic - in terms of normal oxygen delivery to the active muscle tissue ('''B'''). From the perspective of mitochondrial respiration at such a physiologically induced intracellular low ''p''<sub>O<sub>2</sub></sub>, this is a hypoxic condition relative to the critical ''p''<sub>O<sub>2</sub></sub> (''p''<sub>c</sub>) and kinetic oxygen saturation ('''C'''). In this context it is important to consider the following four points:
-::::::* (''1'')	At OXPHOS capacity the ''p''<sub>50</sub> and hence ''p''<sub>c</sub> is higher than under conditions of ADP-limited lower respiratory activity with a minimum at LEAK respiration. Mitochondrial respiration becomes more ‘sensitive’ to ''p''<sub>O<sub>2</sub></sub> at high levels of activation (Gnaiger et al 1998; Gnaiger 2001). Therefore, low intracellular tissue ''p''<sub>O<sub>2</sub></sub> at ''V''<sub>O<sub>2</sub>max</sub> would be considered functionally normoxic for LEAK respiration but functionally hypoxic for OXPHOS capacity. At ''V''<sub>O<sub>2</sub>max</sub>, muscle mitochondria operate slightly below OXPHOS capacity if respiration is oxygen limited, and at OXPHOS capacity in untrained persons who are not oxygen limited but mitochondrial-capacity limited (Richardson et al 1999; Gifford et al 2016). This relates to apparent mitochondrial excess capacities in muscle (Gnaiger et al 1998).
-::::::* (''2'')	''even under normoxia'' – this refers to ambient normoxia ('''A'''), perhaps not made sufficiently clear – therefore, we need the '''ABC'''.
-::::::* (''3'')	''in active muscle'' (from the point of view of aerobic activity the maximum is at ''V''<sub>O<sub>2</sub>max</sub>): intracellular ''p''<sub>O<sub>2</sub></sub> is lower than at rest or routine activity. Therefore, muscle ‘runs’ into the paradox that ''p''<sub>O<sub>2</sub></sub> is lower while mitochondria become more limited at low ''p''<sub>O<sub>2</sub></sub> in the OXPHOS state compared to the LEAK state. This explains the adaption to aerobic training under (sea-level) normoxia with increased mt-biogenesis, in contrast to high-altitude adaption with lower mt-density.
-::::::* (''4'')	‘Intracellular tissue normoxia’ must be used with further specification in terms of either ('''B''') if short-term maximum activity is included without restriction to steady-state routine states, or ('''C''') defining tissue-work induced hypoxia in terms of critical functions limited by ''p''<sub>O<sub>2</sub></sub>.
-::::* '''A''' versus '''B''': ''.. stem cells are conventionally incubated under non-physiological air O<sub>2</sub> tension (21 %). Therefore, the study of mechanisms and signaling activated at lower O<sub>2</sub> tension, such as those existing under native microenvironments (referred to as hypoxia), represent an effective strategy to define if O<sub>2</sub> is essential in preserving naïve stemness potential as well as in modulating their differentiation'' ([[Di 2021 Cells]]).
-::::* Unclear: ('''B''' and '''C'''). Tissue hypoxia is defined as a condition in which the cells of a tissue have abnormal oxygen utilization such that the tissue is experiencing anaerobic metabolism (4) [[Third European Consensus Conference in Intensive Care Medicine 1996 Am J Respir Crit Care Med]].
-::::* '''A''' or '''B.c''': Hypoxia - ‘a condition in which there is not enough oxygen available to the blood and body tissues’ (https://dictionary.cambridge.org/fr/dictionnaire/anglais/hypoxia, retrieved 2021-12-18).
-::::* '''A''' or '''B.c''': Hypoxia – ‘deficiency in the amount of oxygen delivered to the body tissues’ (https://www.collinsdictionary.com/dictionary/english/hypoxia, retrieved 2021-12-19)
-::::* '''A''' or '''B.c''': Hypoxia – ‘a deficiency of oxygen reaching the tissues of the body’ (https://www.merriam-webster.com/dictionary/hypoxia, retrieved 2021-12-19)
-::::* '''C.c''': Hypoxia is defined as functional hypoxia by the European Environmental Agency as “a state of low oxygen concentration in water and sediments, relative to the needs of most aerobic species” (https://www.eea.europa.eu/help/glossary/chm-biodiversity/hypoxia; retrieved 2021-12-14).
-=== Various 'oxia' terms ===
-:::: For harmonization, the following ‘oxia’ terms are linked to the '''ABC''' categories.
-::::* '''B''': “Physoxia: physiological oxygen level in peripheral tissues with an average of approximately 6 % (ranging from approximately 7.5 % to 4 % depending on the tissue; lower limit approximately 1 %). For experimental studies, 5 % is the proposed compromise since this is often used” (McKeown 2014). ― The term ‘physoxia’ or ‘physioxia’ (Carreau et al 2011) suggests physiological control in contrast to abiotic environmental control. Without further specification, physoxia may be interpreted as ('''B.a''') compartmental oxygen level under any environmental conditions, ('''B.c''') for any level of physiological activity, and ('''Ba.c''') their combination (e.g. muscle ''p''<sub>O<sub>2</sub></sub> at ''V''<sub>O<sub>2</sub>max</sub> at high altitude). In addition, physoxia does not separate the categories '''B''' and '''C''' of normoxia, and it may include any pathological cause of deviation from normoxia.
-::::* '''C.b''': “Pathological hypoxia: shows persistence of poor oxygenation suggesting disruption to normal homeostasis. Below this level pathological hypoxia applies” (McKeown 2014). Besides regulation of hypoxia response genes, the critical physiological function should be specified. ― High altitude exposure may result in prolonged poor oxygenation of tissues. But is this pathological hypoxia?
+::::» https://www.rsc.org/events/detail/3610/keystone-symposia-hypoxia-molecular-mechanisms-of-oxygen-sensing-and-response-pathways
+::::» https://www.hypoxia.net/
+::::» https://www.dwscientific.com/2nd-whitley-hypoxia-symposium
+::::» https://waset.org/hypoxia-exercise-and-hypoxic-exposure-conference-in-january-2022-in-bali
+::::» https://conferenceindex.org/event/international-conference-on-hypoxia-inducible-factors-and-oxygen-biology-ichifob-2022-may-dubai-ae

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