The physiological relevance of hypoxia and its adaptation

Introduction : In a medical point of view, hypoxia is the deficiency of dioxygen (O₂) in blood, tissue but also at the cellular level. Hypoxia is a recurrent phenomenon in high-altitude environment. Above 3000 m of altitude, vertebrates start to express physiological changes due to hypobaric hypoxia (Gilbert-Kawai, et al., 2014). Depending on the duration of this phenomenon, those physiological changes can lead to an acclimatization of the organism to its new environment. That is why high-altitude environment is a good ground to study the physiological mechanisms of the adaptation to hypoxia at different levels.

Changes of standard datas due to hypoxia

In this kind of environment, a Ventilatory Adaptation to Hypoxia (VAH) happens with a progressive increase in the baseline ventilation. This allows an adequate O₂ supply, maintains high alveolar O₂ partial pressure, and increases the hypoxic ventilator response (HVR).

Hypoxia also stimulates endothelial and smooth muscle cells in the pulmonary vessels, which contract: Hypoxic Pulmonary Vasoconstriction (HPV) occurs. The pulmonary blood pressure is increased, which overburden of the right ventricle of the heart and induce the Chronic Mountain Sickness (CMS). Regulation of Ca²⁺ ions flux by cardiomyocytes enhances the strength and rate of cardiac contractions during hypoxia. It leads to an increase of heart rate (HR) and cardiac output (CO), which have an effect on the blood flow through blood vessels and then on the adaptation to hypoxia (Gary Sieck, 2014)

Adaptation to high-altitude environment enables the decrease of those symptoms. Highlander people have a lower HVR because they have lower ventilator recruitment threshold (Gary Sieck, 2014). Moreover, they don’t need an important VAH because they have larger static lung volumes, which provide adequate O₂ supply in case hypobaric hypoxia. They also have a decreased HPV, which minimizes the effects of CMS. Acclimatization also auto regulates the cerebral blood flow : the Internal Carotid Artery blood flow velocity increase, which enhances the cerebral O₂ delivery.

Pulmonary oxygen diffusion

The phenotypic plasticity describes the ability of an organism to change its phenotype in response to changes in the environment. In the case of hypoxia, it is characterized by an increased pulmonary surface area and a reduced thickness of the pulmonary blood/gas interface which improve O₂ diffusion at high altitude, according to the Fick’s Law. Angiogenesis also occurs : changes in the structure and the number of the blood vessels network facilitate the diffusion of gas.

Blood oxygen transport

The erythropoeitic activity is enhanced
Hypoxia increases hematocrit (Hct) and Hemoglobin (Hb) concentration in lowlander species. Due to this higher Hct, greater quantity of O₂ is carried (Storz, et al., 2010). Adaptation to hypoxia regulates the PAS domain protein 1 (EPAS1). It encodes hypoxia inducible factor 2α, which determines the set point for hypoxic induction of erythropoietin (EPO) and so the erythropoiesis (Gilbert-Kawai, et al., 2014). That is why highlander species, who don’t need any acclimatization to hypoxia, have a higher threshold and produce less erythrocytes.

Adjustement of the blood-O₂ affinity, an increased blood O₂ saturation

• Changes in intrinsic Hb-O₂ affinity and in the allosteric cofactors that modulates the affinity

• Changes in the concentration of allosteric cofactors within the erythrocyte (like 2,3-DPG). Those cofactors reduce Hb-O₂ affinity by binding to the deoxygenated Hb and stabilizing it. It facilitates the liberation of O₂ by decreasing the Hb-O₂ affinity.

• Additional changes in Hb structure (for high-altitude native species only). It increases intrinsic O₂ affinity, the Hb is less sensitive to allosteric cofactors which lead to a hypoxia tolerance .

• Bohr effect : decreased Hb-O₂ affinity at low pH. The deoxygenation of the blood promotes the release of O₂.

Tissue oxygen diffusion

The organism has to maintain an adequate delivery of O₂ to the tissues and an adequate PO₂ to allow diffusion between the blood and tissues. PvO₂=PaO₂-(specific blood conductance) : an increase of the specific conductance minimizes the decline of PO₂. So, the O₂ diffusion in tissues is maintained (Storz, et al., 2010). Mitochondria get closer to capillaries to reduce the intracellular diffusion distance and a regional tissue perfusion occurs depending on the metabolic demand of the tissue. In that way, the O₂ diffusion is facilited. This regional tissue perfusion is regulated by paracrine local signals (NO) that regulates vasodilation and central signals (sympathetic neural activity) which regulate vasoactivity in a tissue specific manner. The blood flow is increased and more O₂ is provided to the tissues. Furthermore, during hypoxia, a rapid increase of intracellular 2,3-DPG can be observed and so an enhanced tissue oxygenation. It also compensates the hypoxia induced respiratory alkalosis.

Sensing hypoxia

Carotid bodies (glomus caroticum) are the primary sensory organs able to detect hypoxia. They are located in the bifurcation of the carotid artery/trunk. Physiological responses occur through acute reflex changes which leads to a cardiorespiratory adaptation.
There are two types of chemoreceptors in the glomus caroticum :

• type I. cells : glomus cells : central role in the fast oxygen-sensory transduction

• type II. cells : glia cells

Afferentation : sensory fibers of n. sinus caroticus
Efferentation : n. glossopharygeus (n. IX) There is a sympathetic activation due to hypoxia which ellicits an increasing breathing through a « chemosensory reflex ».

The Membrane hypothesis and the role of gasotransmitters in the chemotransduction (Nanduri R. Prabhakar and Chris Peers, 2014)
In each species, there are special O₂-sensitive K⁺ channels in O₂-sensory tissues like in type I. cells which activity decreases as O₂ levels decrease. Hypoxia → O₂-sensitive K⁺ channels close → depolarization of type I. cells → voltage gated Ca²⁺ open → release of neurotransmitters → excitation of afferent sensory fibers in the nervus sinus caroticus → breathing increase. Moreover, mitochondria of type I. cells also act as sensors of O₂ levels through the regulation of the AMP/ATP ratio. If it increases, the AMP-activated protein kinase (AMPK) is activated and phosphorylates/inhibits K⁺ channels. There are three major gasotransmitters which have a main role on this membrane hypothesis :

• Inhibitory : NO and CO. Both gases inhibit afferent sensor nerve activity. During hypoxia, there is an inhibition of neuronal Nitric Oxide Synthetase (nNOS) and Heme Oxygenase (HO) which are O₂-dependent. It reduces the formation of NO and CO, and so increases sensory activity.

• Excitatory : H₂S. Hypoxia activates Cystathionine β-synthetase (CBS) and Cystathionine Ɣ-lyase (CSE), responsible of the formation of H₂S. The sensor activity is increased.

Gasotransmitters act in concert with ion channels in type I. cells and form a chemosome responsible of the chemotransduction of hypoxia.
During the adaptation to hypoxia, there is a decreased central and minimal peripheral “chemosensory reflex”.

Genetic approach to complex phenotypes and high-altitude adaptation

In early days the assumptions of genetic ancestry were made by using geographically isolated high-altitude residents. Migrant ancestry estimations were run through surname analysis or assessment of skin reflectance. New technology studying genotypes and phenotypes have rapidly developed methods that increase our ability to find genetic basis of complex phenotypes and infer the effect of natural selection on gene region. Already the early studied and research support the fact that the populations have lived a longer period at high altitude for natural selection to have occurred. Natural selection in their part acts upon individual based on their adaptive phenotypes. Gene identification and localization, Mapping , assisted to complex phenotypes typically exploit the link between phenotypic variation among individuals with genetic markers. Ancestry informative markers (AIMs) or loci that show population-specific allele frequencies can then be used to confirm or question individual-assessed genetic ancestry (Shriver, et al. 2003)

There is a differences between migrants and native high-altitude populations and their maternal physiological characteristic. Physiological characteristic are influenced by genetic origin and these have an influence on reproductive fitness.

Reduction in festal growth increases infant morbidity and mortality. Populations living in high altitude have relative “advantage” as their specific phenotype is reliant in some combinations of genetic and environmental factors as well as that maternal genotype modifies the effect is an environmental attribute on a particular phenotype.

Genetic study of phenotypic plasticity

Best way to identify adaptive mechanims of hypoxia tolerance : analysis of sequence variation and expression profiles for a set of genes responsible of structural and regulatory mechanisms of adaptation.

Genomic approach: evaluate the relative contributions of phenotypic plasticity and genotypic speciaization in high-altitude adaptation thanks to its whole RNA-sequence to characterize transcript abundance and sequence polymorphism in thousands of expressed mRNAs. It enables the identification of molecular mechanisms of phenotypic plasticity and genotypic adaptation for loci related to hypoxia adaptation and then measure the physiological effects of altered protein function or expression (Feder and Walser, 2005 ; Storz, et al., 2010)

Summary

To conclude, sensing hypoxia is primordial for the maintenance of homeostatic mechanisms in an organism. The lack of O₂ induces a chain reaction in the carotid bodies that leads to an increase of the standard physiological data like ventilation or cardiac rate. Those changes require a phenotypic plasticity based on the correlation between O₂ supply and demand through the O₂ cascade (Storz, et al., 2010). In that way, survival is possible. If the organism stays in an hypoxic environment, new phenotypes can be identified, genotypes can be changed according to the environmental-specific optimum. Some of the responses of an organism to hypoxia are qualified as maladaptive response : they would move the phenotype further away from the new optimum. They are counterproductive in case of severe hypoxia. One possible explanation is that those responses to environmental hypoxia are in fact misdirected. They originally evolved as a response to anemia, which could explain why they can be non advantageous if the hypoxia becomes too serious (Storz, et al., 2014).

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