Molecular Mechanisms of Neuroprotection after the Intermittent Exposures of Hypercapnic Hypoxia.

The review introduces the stages of formation and experimental confirmation of the hypothesis regarding the mutual potentiation of neuroprotective effects of hypoxia and hypercapnia during their combined influence (hypercapnic hypoxia). The main focus is on the mechanisms and signaling pathways involved in the formation of ischemic tolerance in the brain during intermittent hypercapnic hypoxia. Importantly, the combined effect of hypoxia and hypercapnia exerts a more pronounced neuroprotective effect compared to their separate application. Some signaling systems are associated with the predominance of the hypoxic stimulus (HIF-1α, A1 receptors), while others (NF-κB, antioxidant activity, inhibition of apoptosis, maintenance of selective blood-brain barrier permeability) are mainly modulated by hypercapnia. Most of the molecular and cellular mechanisms involved in the formation of brain tolerance to ischemia are due to the contribution of both excess carbon dioxide and oxygen deficiency (ATP-dependent potassium channels, chaperones, endoplasmic reticulum stress, mitochondrial metabolism reprogramming). Overall, experimental studies indicate the dominance of hypercapnia in the neuroprotective effect of its combined action with hypoxia. Recent clinical studies have demonstrated the effectiveness of hypercapnic-hypoxic training in the treatment of childhood cerebral palsy and diabetic polyneuropathy in children. Combining hypercapnic hypoxia with pharmacological modulators of neuro/cardio/cytoprotection signaling pathways is likely to be promising for translating experimental research into clinical medicine.

Hypercapnic hypoxia as a rehabilitation method for patients after ischemic stroke.

INTRODUCTION: Experimental studies on animals have demonstrated a higher neuroprotective efficacy of hypercapnic hypoxia compared to normocapnic hypoxia. Respiratory training with hypercapnic hypoxia has shown a positive impact on the functional state of the nervous system in children with cerebral palsy (CP). It can be presumed that the combined effect of moderate hypercapnia and hypoxia will be promising for clinical application within the context of early rehabilitation after ischemic stroke. METHODS: A randomized triple-blind placebo-controlled study was conducted on 102 patients with ischemic stroke, aged 63.07 ± 12.1 years. All patients were diagnosed with ischemic stroke based on neuroimaging criteria and/or clinical criteria within the 48-72 hour timeframe. The experimental group (n = 50) underwent daily respiratory training with hypercapnic hypoxia (FetCO2 5-6%, FetO2 15-16%) using the 'Carbonic' device for 7-11 sessions of 20 minutes each day during the treatment process. The control group (placebo, n = 52) underwent training on a similar device modified for breathing atmospheric air. Neurological examinations were conducted on all patients before the study and on the day after completing the training course. RESULTS: The standard treatment demonstrated effectiveness in terms of neurological status scales in both groups. Intermittent exposure to hypercapnic hypoxia proved more effective in improving neurological function indicators in patients compared to the placebo group: NIHSS scale scores were 40% lower than in the placebo group (p < 0.001); mRS scale scores were 35% lower (p < 0.001); B-ADL-I and RMI indices were higher by 26% (p < 0.01) and 36% (p < 0.001), respectively; MoCA scale results were 13% higher (p < 0.05); HADS and BDI-II scale scores were lower by 35% (p < 0.05) and 25% (p < 0.05), respectively. The increase in MMSE scale scores in the intervention group was 54% higher (p < 0.001), and MoCA scale scores increased by 25% (p < 0.001). CONCLUSION: Respiratory training with hypercapnic hypoxia improves the functional state of the nervous system in patients with ischemic stroke. After conducting further clarifying studies, hypercapnic hypoxia can be considered as an effective method of neurorehabilitation, which can be used as early as 48-72 hours after the onset of stroke.

Ultrastructural Changes in Hippocampal Region CA1 Neurons After Exposure to Permissive Hypercapnia and/or Normobaric Hypoxia.

Isolated exposure to intermittent hypoxia and permissive hypercapnia activates signaling mechanisms that induce ultrastructural changes in mitochondria and endoplasmic reticulum, accompanied by the development of maximal ischemic tolerance in neurons under the combined influence of these factors. However, there are a lack of data on the combined impact of these factors on the ultrastructure of neuronal organelles. The present study aims to comparatively assess the ultrastructural changes in neurons following isolated and combined exposure to hypoxia and hypercapnia, as well as to correlate these changes with the neuroprotective potential previously observed for these factors. Following a 15-session course of 30-min exposures to permissive hypercapnia (PCO2 ≈ 50 mmHg) and/or normobaric hypoxia (PO2 ≈ 150 mmHg), morphometric assessment was conducted to evaluate the extent of ultrastructural changes in hippocampal neurons (mitochondria, perinuclear space, and granular endoplasmic reticulum). It was found that in hippocampal neurons from the CA1 region, permissive hypercapnia resulted in increased mitochondrial size, expansion of membranous compartments of the granular endoplasmic reticulum, and perinuclear space. Normobaric hypoxia affected only mitochondrial size, while hypercapnic hypoxia specifically widened the perinuclear space. These ultrastructural changes objectively reflect varying degrees of the influence of hypoxia and hypercapnia on organelles responsible for energy metabolism, anti-apoptotic, and synthetic functions of neurons. This confirms the effect of potentiation of their neuroprotective effects under combined exposure and highlights the dominant role of the hypercapnic component in this mechanism.

Permissive hypercapnia and hypercapnic hypoxia inhibit signaling pathways of neuronal apoptosis in ischemic/hypoxic rats.

INTRODUCTION: In the present study, we aimed to test the hypothesis that hypercapnia, independently and/or in combination with hypoxia, can activate signaling pathways related to the inhibition of proapoptotic (caspase-dependent and caspase-independent) factors and the induction of antiapoptotic factors in facilitating adaptation to hypoxia/ischemia. MATERIALS AND METHODS: Following exposure to permissive hypercapnia and/or normobaric hypoxia, the degree of apoptosis was evaluated in experimental ischemia models in vivo and in vitro. The percentages of caspase-3, apoptosis-inducing factor (AIF), Bax, and Bcl-2 in astrocytes and neurons derived from male Wistar rats were also calculated. In vitro, cells were subjected to various types of respiratory exposure (hypoxia and/or hypercapnia for 24 or 12 h) as well as further sublethal chemical hypoxia. The percentages of these molecules in nerve cells in the ischemic penumbra of the brain after photothrombotic injury were also calculated. RESULTS: The degree of apoptosis was found to decrease in ischemic penumbra, mostly due to the hypercapnic component. It was also discovered that the levels of caspase-3, AIF, and Bax decreased in this region, whereas the Bcl-2 levels increased following exposure to hypercapnia and hypercapnic hypoxia. CONCLUSIONS: This integrative assessment of the rate of apoptosis/necrosis in astrocyte and neuron cultures shows that the combination of hypercapnia and hypoxia resulted in the maximum neuroprotective effect. The levels of apoptosis mediators in astrocyte and neuron cultures were calculated after modeling chemical hypoxia in vitro. These results show that the exposure models where permissive hypercapnia and normobaric hypoxia were combined also had the most pronounced inhibitory effects on apoptotic signaling pathways.

Hypercapnic hypoxia improves cognitive and motor functions of children with cerebral palsy.

A randomized, triple-blind, placebo-controlled trial involving 42 patients between 3 and 7 years of age with spastic cerebral palsy was performed. For the treatment test group (n=22), daily respiratory exercises with hypercapnic hypoxia were performed using a Carbonic training apparatus for 20 minutes per day; a total of 14 to 16 sessions were performed. Before the start of the study and the day after training was completed, the patients underwent neurological and neurophysiological examinations (electroencephalography, magnetic stimulation of the pyramidal tract, and cognitive potentials Р300).The evoked potentials showed a decrease in the peak latency of the P3 component of the test group after treatment (302 ms) that was more pronounced than that of the placebo group (305 ms; p<0.05). Magnetic stimulation showed that hypercapnic hypoxic training resulted in reductions in central motor conduction time by 2.2 to 2.5 ms (p<0.05) and in the excitation threshold of the motor cortex by 12% to 16% (р<0.01) depending on the lateralization; The strategy of adjusting to hypercapnic hypoxia, either unfavorable (hyperventilation and avoidance) or favorable (homeostatic with the achievement of preset values for hypercapnia and hypoxia), did not change during the process of training in the placebo group; however, it shifted considerably toward favorable (from 33% to 57%; р<0.05) in the test group.Respiratory training with hypercapnic hypoxia can have a positive impact on the functional state of the nervous system of children with cerebral palsy and can be considered a method of improving the efficiency of standard therapy.

Voice and Emphasis in Arabic Coronal Stops: Evidence for Phonological Compensation.

The current study investigates multiple acoustic cues-voice onset time (VOT), spectral center of gravity (SCG) of burst, pitch (F0), and frequencies of the first (F1) and second (F2) formants at vowel onset-associated with phonological contrasts of voicing and emphasis in production of Arabic coronal stops. The analysis of the acoustic data collected from eight native speakers of the Qatari dialect showed that the three stops form three distinct modes on the VOT scale: [d] is (pre)voiced, voiceless [t] is aspirated, and emphatic [t] is voiceless unaspirated. The contrast is also maintained in spectral cues. Each cue influences production of coronal stops while their relevance to phonological contrasts varies. VOT was most relevant for voicing, but F2 was mostly associated with emphasis. The perception experiment revealed that listeners were able to categorize ambiguous tokens correctly and compensate for phonological contrasts. The listeners' results were used to evaluate three categorization models to predict the intended category of a coronal stop: a model with unweighted and unadjusted cues, a model with weighted cues compensating for phonetic context, and a model with weighted cues compensating for the voicing and emphasis contrasts. The findings suggest that the model with phonological compensation performed most similar to human listeners both in terms of accuracy rate and error pattern.

Non-steady-state photo-EMF in ß-Ga2O3 crystals at λ = 457 nm.

The non-steady-state photoelectromotive force is excited in a monoclinic gallium oxide crystal at wavelength λ = 457 nm. The crystal grown in an oxygen atmosphere is insulating and highly transparent for a visible light, nevertheless, the formation of dynamic space-charge gratings and observation of the photo-EMF signal is achieved without application of any electric field to the sample. The dependencies of the signal amplitude on the frequency of phase modulation, light intensity, spatial frequency and light polarization are measured. The material demonstrates the anisotropy along the [100] and [010] directions, namely, there is a small difference in the transport parameters and a pronounced polarization dependence of the signal. The crystal's photoconductivity, responsivity and diffusion length of electrons are estimated for the chosen light wavelength and compared with the ones for other wide-bandgap crystals.

Hypercapnia potentiates HIF-1α activation in the brain of rats exposed to intermittent hypoxia.

The mechanisms and signalling pathways of the neuroprotective effect of hypercapnia and its combination with hypoxia are poorly understood. The study aims to test the hypothesis about the potentiating effect of hypercapnia on hypoxia adaptation systems directly related to hypoxia-induced factor 1α (HIF-1α). In this study we assessed HIF-1α content in hippocampal extracts and astrocytes obtained from Wistar male rats exposed to different respiratory conditions (7- or 15-fold of hypoxia and/or hypercapnia). In addition, HIF-1α content in astrocytes was assessed in in vitro model of chemical hypoxia as well as in the cerebral cortex after photothrombotic damage of this brain region. This study indicates increased levels of HIF1α in hippocampal extracts, astrocytes, and in cells of the near-stroke region of the cerebral cortex in rats exposed to hypoxia and hypercapnic hypoxia, but not hypercapnia alone. In in vitro study, hypercapnia facilitates the effects of acute chemical hypoxia observed in astrocytes. Thus, hypercapnia does not increase the level of transcription factor HIF-1α. However, the combined effects of hypercapnia and hypoxia in in vitro simulations of acute chemical hypoxia potentiate the accumulation of HIF-1α.

Laryngeal Contrast in Qatari Arabic: Effect of Speaking Rate on Voice Onset Time.

Beckman and colleagues claimed in 2011 that Swedish has an overspecified phonological contrast between prevoiced and voiceless aspirated stops. Yet, Swedish is the only language for which this pattern has been reported. The current study describes a similar phonological pattern in the vernacular Arabic dialect of Qatar. Acoustic measurements of main (voice onset time, VOT) and secondary (fundamental frequency, first formant) cues to voicing are based on production data of 8 native speakers of Qatari Arabic, who pronounced 1,380 voiced and voiceless word-initial stops in the slow and fast rate conditions. The results suggest that the VOT pattern found in voiced Qatari Arabic stops b, d, g is consistent with prevoicing in voice languages like Dutch, Russian, or Swedish. The pattern found in voiceless stops t, k is consistent with aspiration in aspirating languages like English, German, or Swedish. Similar to Swedish, both prevoicing and aspiration in Qatari Arabic stops change in response to speaking rate. VOT significantly increased by 19 ms in prevoiced stops and by 12 ms in voiceless stops in the slow speaking rate condition. The findings suggest that phonological overspecification in laryngeal contrasts may not be an uncommon pattern among languages.

Helical Blood Flow in Hemodynamically Significant Carotid Stenosis.

OBJECTIVES: To define helical blood flow (HBF) characteristics in hemodynamically significant atherosclerotic stenosis of the internal carotid artery (ICA) by means of duplex scanning. METHODS: Twenty-five hemodynamically significant (65.0% [range, 63.0%-69.0%]) carotid stenoses were examined in 23 patients. The severity of the stenosis was calculated by the European Carotid Surgery Trial grading method by transverse section scanning in the B-mode. Rotational components were estimated in color flow mapping by transverse-section scanning of a vessel at the most narrowed site, as well as in the prestenotic and poststenotic segments. A quantitative evaluation of HBF was performed on the basis of pulsed wave Doppler imaging of longitudinal and transverse sections of the arterial lumen. RESULTS: Helical blood flow was most often (68%) registered in the poststenotic segment of the ICA as a single vortex (52%) or double vortices (16%). At the most narrowed site, HBF was registered in 48% of the cases (44% single vortex and 4% double vortices), whereas in the prestenotic segment of the blood vessel, it was registered in only 16% of the cases (8% single vortex and 8% double vortices). The time-averaged maximum blood flow velocities at the most narrowed site were 88.5 cm/s (25th-75th percentiles, 73.8-127.8 cm/s) for the axial component and 33.1 cm/s (22.7-40.9 cm/s) for the rotational component. The calculated summary velocity of motion of the blood particles in helical paths was 92.2 cm/s (75.7-144.2 cm/s). CONCLUSIONS: It has been shown that HBF can be registered by Doppler ultrasound in atherosclerotic stenosis, and its registration rate increases while passing through the narrowed segment of the ICA.

Hypercapnic hypoxia as a potential means to extend life expectancy and improve physiological activity in mice.

The application of combined hypoxia and hypercapnia (hypercapnic hypoxia) during respiratory exercises results in a maximum increase in resistance to acute hypoxia and ischemic tolerance of the brain. The results of those researches allow the assumption that hypercapnic hypoxia is a promising method for prophylaxis, treatment, and rehabilitation, as well as a means to increase life expectancy. The study was conducted to verify the hypothesis that it is possible to extend the life span through regular courses of respiratory exercises with hypercapnic hypoxia. In the present experimental research carried out on mice, the geroprotective effect of regular hypercapnic-hypoxic exercises (PO2-90 mm Hg and PCO2-50 mm Hg) was assessed in the context of the average life expectancy and the main criteria of its quality (reproductive function, muscle strength, and behavior). Results suggest that with regular training, life span is extended significantly by 16%. This result was accompanied by improved reproductive and cognitive functions, increased motor and search activities, and physical stamina in old age mices. This important phenomenon is accompanied by improved reproductive and cognitive functions, high motor function and search activity, as well as better physical stamina in old-aged mices. Recurring respiratory training under combined hypoxia and hypercapnia (hypercapnic hypoxia) during the lifetime significantly extended the life span of mice in the experiments.

Combined exposure to hypercapnia and hypoxia provides its maximum neuroprotective effect during focal ischemic injury in the brain.

BACKGROUND: In the present research, we compared the neuroprotective efficiency of combined and isolated exposure to hypoxia and hypercapnia preceding focal cerebral ischemic injury in rats. The study was conducted to verify the hypothesis of a possible increase in normobaric hypoxia (NbH; 90 mm Hg) efficiency when combined with permissive hypercapnia (PH; 50 mm Hg). METHODS: The rats from the test groups were subjected to a 15-fold exposure to NbH (90 mm Hg) and/or PH (50 mm Hg). After the 15th exposure, cerebral ischemic injury was induced by photochemical thrombosis. Seventy-two hours later, neurologic deficit was determined on the Neurological Severity Score scale and by the rotarod test, and the volume of cerebral infarction was measured after focal photochemical thrombosis. RESULTS: The neurologic deficit decreased most efficiently in rats that underwent PH and hypercapnic hypoxia (HH) exposure, whereas NbH had no impact on the neurologic status of the animals. On the contrary, motor coordination disturbances were minimal during exposure to hypoxia and HH. All respiratory interventions reduced the cerebral ischemic infarction volume in rats. The smallest infarction volumes were registered in the area of photochemical thrombosis in rats from the hypercapnic-hypoxic impact group, whereas exposure to NbH or PH did not show any cross difference. CONCLUSIONS: The impact of PH has greater neuroprotective potential compared with NbH. Thus, we can assume that hypercapnia is a predominant factor in providing neuroprotection in combination with hypoxia.

Cerebrovascular and systemic hemodynamic response to carbon dioxide in humans.

BACKGROUND: Arterial partial pressure alteration of CO2 ((Equation is included in full-text article.)) affects not only the cerebral blood flow velocity but also the systemic arterial blood pressure (BP). At the same time, BP can affect the cerebral blood flow. The objective of the present research is to study the impact of the (Equation is included in full-text article.)level on cerebrovascular CO2 reactivity ((Equation is included in full-text article.)) and BP as well as the impact of BP upon (Equation is included in full-text article.)alteration by hypercapnia and hypocapnia. MATERIALS AND METHODS: Cerebral blood flow velocity was recorded by means of transcranial Doppler in both middle cerebral arteries (MCAv left and right). The mean arterial pressure (MAP) was studied using the finger photoplethysmography method, arterial blood oxygen saturation was estimated by the pulse oximetry method, and end-tidal (Equation is included in full-text article.)((Equation is included in full-text article.)) was measured with an infrared capnograph. After a recording of the reference values of all the parameters, all the volunteers underwent a rebreathing as well as a hyperventilation. RESULTS: At rest, (Equation is included in full-text article.)was 33.6 (SD 3.1) mmHg. At rebreathing, MCAv increased at 38 mmHg (Equation is included in full-text article.), MAP - at 43 mmHg (Equation is included in full-text article.). By hyperventilation, MCAv decreased at 28 mmHg (Equation is included in full-text article.), MAP - at 26 mmHg (Equation is included in full-text article.). When (Equation is included in full-text article.)reached 43 mmHg, (Equation is included in full-text article.)increased from 2.3 (SD 1.4) to 3.3 (SD 1.2)%/mmHg (P<0.01). When (Equation is included in full-text article.)decreased to 26 mmHg, (Equation is included in full-text article.)increased from -3.6 (SD 2.5) to -5.9 (SD 3.9)%/mmHg (P<0.01). CONCLUSION: Within the alteration of (Equation is included in full-text article.)above 43 and under 26 mmHg, BP increased and decreased, respectively, leading to a change in (Equation is included in full-text article.).

Tolerance to acute hypoxia maximally increases in case of joint effect of normobaric hypoxia and permissive hypercapnia in rats.

INTRODUCTION: We studied the comparative efficacy of independent and combined effects of normobaric hypoxia (90mmHg) and permissive hypercapnia (50mmHg) in increasing the tolerance of rats to acute hypobaric hypoxia. METHODS: We determined the time to loss of pose and life duration as a measure to assess the degree of tolerance of animals to hypobaric hypoxia by exposing them to an altitude of 11,500m (barometric=180mmHg). RESULTS: Exposure to hypercapnic hypoxia increased the tolerance to acute hypobaric hypoxia compared to exposure to normobaric hypoxia or permissive hypercapnia alone. DISCUSSION: The positive effects of hypercapnia and hypercapnic hypoxia occurred after one exposure, and increasing the number of exposures proportionally increased the tolerance to acute hypobaric hypoxia. The effect of permissive hypercapnia on increasing the tolerance to acute hypobaric hypoxia was found to be significantly greater than that of exposure to normobaric hypoxia. Therefore, we propose that hypercapnia is the dominant factor in increasing tolerance to acute hypobaric hypoxia. CONCLUSION: Tolerance to acute hypoxia maximally increases in case of joint effect of normobaric hypoxia and permissive hypercapnia.