{"id":{"repo_id":"southwales","oai_identifier":"oai:pure.atira.dk:studenttheses/a4905434-54df-4901-86e9-94204ffa74df"},"canonical_url":"https://search.dev.ndltd.org/etd/southwales/oai:pure.atira.dk:studenttheses/a4905434-54df-4901-86e9-94204ffa74df","repository":{"repo_id":"southwales","name":"University of South Wales","base_url":"https://pure.southwales.ac.uk/ws/oai"},"display":{"title":"The influence of acute, chronic and lifelong hypoxia on the functional and structural integrity of the neurovascular unit","abstract":"Background. The current thesis adopts a whole-body, integrated approach to investigate the impact of acute, chronic and life-long exposure to hypoxia on the functional and structural integrity of the neurovascular unit (NVU).
Methods. Experimental Study 1: In a randomised, single-blind cross-over design, 12 healthy, physically active male participants (age, 23 ± 2 yrs; BMI, 25 ± 4 kg/m2) were exposed to 6 hours of normoxia (FIO2 = 0.21) and 6 hours of acute normobaric hypoxia (FIO2= 0.12) on separate visits. Experimental measures were conducted at baseline (0 h) and following 6-hours of each exposure. Experimental Study 2: In a repeated-measures, cross-sectional study, nine native lowlanders (28 ± 8 yrs, 24.3 ± 1.7 kg∙m-2) were examined at sea-level (normoxia) and following 14 days of high-altitude (HA) acclimatisation to 4,300 m in Cerro De Pasco, Peru (chronic HA hypoxia). Age- and BMI-matched healthy native highlanders of the Altiplano (~4300 m) (n = 9, 27 ± 7 yrs, 26.1 ± 6.0 kg/m2) were also examined (life-long HA hypoxia). Experimental measures for Study 1 and 2: Venous blood samples were assayed for biomarkers specific to oxidative-nitrosative stress (OXNOS; ascorbate free radical [A•−] and nitric oxide [NO] metabolites via electron paramagnetic resonance spectroscopy and ozone-based chemiluminescence, respectively) and structural integrity of the NVU (Study 1: S100ß and neuron-specific enolase [NSE]; Study 2 only: S100ß, NSE, ubiquitin carboxy-terminal hydrolase L1 [UCH-L1], glial fibrillary acidic protein [GFAP], neurofilament light-chain [NF-L] and total-tau [T-Tau] determined via automated clinical grade ELISA and Single Molecule Array technology). Cardiopulmonary variables included mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), cardiac output (Q), peripheral oxygen saturation (SpO2), minute ventilation (V̇E), partial pressure of end-tidal oxygen (PETO2) and carbon dioxide (PETCO2), haematocrit (Hct), haemoglobin (Hb), arterial oxygen content (CaO2) and blood viscosity (Study 2 only). Duplex and transcranial Doppler ultrasonography were employed to measure extracranial (internal carotid artery blood flow [ICAQ] and vertebral artery blood flow [VAQ]) and intracranial (middle cerebral artery blood velocity [MCAV] and posterior cerebral artery [PCAV]) haemodynamics. Global cerebral blood flow (gCBF) was calculated and used to quantity global cerebral substrate delivery of oxygen (gCDO2) and glucose (gCDGlu). NVU function was examined via: 1, dynamic cerebral autoregulation (dCA) assessed using transfer function analysis during spontaneous oscillations in MAP and MCAV/ PCAV; and 2, neurovascular coupling (NVC) via PCAV responses to repeated dark (eyes closed) and light (eyes open + visual stimulation) activation.
Results. Experimental Study 1: Hypoxia failed to alter A•− , total plasma NO and total RBC NO. Owing to the reduction in SpO2 and unaltered Hb, caO2 was decreased in hypoxia. An increase in V̇E following 6 h of hypoxia facilitated an increase in PETO2 and a reduction in PETCO2 . In hypoxia, HR and Q̇ were elevated and MAP was reduced. ICAQ and VAQ were both elevated in hypoxia. Despite an increase in gCBF in hypoxia, gCDO2 was lower in hypoxia and was unaffected by time. While ICAQ and ICA DO2 was reduced in hypoxia, VAQ and VA DO2 remained well preserved. gCDGlu was elevated in hypoxia owing to increase ICA DGlu and VA DGlu. Hypoxia reduced dCA (lower VLF Phase in the MCA and PCA, increased VLF Gain in the PCA) but had no effect on NVC. S100β remained unchanged and NSE decreased in hypoxia. Experimental Study 2: Chronic HA hypoxia in lowlanders increased Hct, Hb and blood viscosity, restoring CaO2 to baseline normoxic values. While highlanders were more polycythaemic, they were equally hypoxaemic albeit slightly less alkalotic, hypocapneac and hypertensive. Chronic HA hypoxia in lowlanders decreased A•− and was accompanied by a reciprocal elevation in plasma and RBC NO bioavailability. Highlanders presented with similar levels of A•− to lowlanders at sea-level, however NO bioavailability (nitrite [NO2−] and S-nitrosothiols) was further elevated. Following chronic HA hypoxia in lowlanders, gCBF, gCDGlu, gCD O2 and NVC were comparable to normoxic values and equivalent to those observed in highlanders. Although MCA/PCA phase estimates were also comparable to sea-level following chronic HA hypoxia in lowlanders, gain metrics were consistently lower in the highlanders. Chronic HA hypoxia increased NF-L and decreased T-tau whereas UCHL-1, NSE, GFAP and S100ß did not change. With the exception of NSE and GFAP, all NVU biomarkers were lower in highlanders (lifelong HA hypoxia) compared to lowlanders following chronic HA hypoxia.
Discussion: By utilising an integrated approach, this thesis examined to what extent acute, chronic and life-long hypoxia alters the functional (regional CBF substrate delivery, dCA, and NVC) and structural (BBB permeability and neuronal-axonal damage) integrity of the NVU, whilst examining potential mechanisms (OXNOS) underlying these changes. Contrary the original working hypothesis, acute hypoxia was associated with marked reductions in global substrate delivery and diminished anterior and posterior cerebral autoregulatory capacity, but it was not associated with changes in OXNOS and further, did not translate to impaired NVC, increased BBB permeability or provide any evidence for on-going neuronal-axonal damage. Study 2 supported the working hypothesis whereby following chronic HA hypoxia, it would appear that global substrate delivery and NVC remained well-preserved and dCA improved. Following life-long HA hypoxia in Andean highlanders, the consistently lower levels of S100ß, NF-L and T-Tau observed indicate that the NVU is structurally ‘tighter’ when compared with lowlanders who present with evidence of on-going axonal damage. That global cerebral perfusion and substrate delivery remained well preserved and the lower (LF) gain observed in the anterior/posterior circulation highlights improved pressure-flow coupling (potentially mediated by elevated systemic concentrations of basal plasma and RBC NO), this may indicate that the highlander brain is better equipped to buffer perfusion in response to rapid increases in BP, further ‘bolstering’ the NVU against the potentially damaging effects of BBB disruption. Evidence of such neuroprotection may be presented in the form of the comparable NVC responses between lowlanders at sea-level and highlanders. Further study of the mechanisms governing NVU adaptation in the hypoxia-tolerant human may help inform the pathophysiology and treatment of neurodegenerative diseases notwithstanding hypoxia-related illnesses such as stroke, heart failure, lung disease and cancer.
Abbreviations
A•−– Ascorbate free radical
ACA – Anterior cerebral artery
AMS – Acute mountain sickness
ATP – Adenosine tri-phosphate
β93 Cys – β93 cysteine residue
BBB – Blood-brain-barrier
BMI – Body mass index
BOLD – blood oxygenation level-dependant
CA – Cerebral autoregulation
cAMP – Cyclic adenosine monophosphate
CaO2– Arterial oxygen content
CBF – Cerebral blood flow
CDO2 – Cerebral oxygen delivery
cGMP – Cyclic guanosine monophosphate
CMRO2 – Cerebral metabolic rate of oxygen
CSF – Cerebrospinal fluid
CVR – Cerebrovascular reactivity
DO2 – Delivery of oxygen
EET – epoxyeicosatrienoic acid
eNOS – Endothelial nitric oxide synthase
FICO2 – Fraction of inspired carbon dioxide
FIO2– Fraction of inspired oxygen
GABA - γ-Aminobutyric acid
gCBF – Global cerebral blood flow
gCDGlu – Global cerebral delivery of glucose
gCDO2 – Global cerebral delivery of oxygen
HA – High-altitude
Hb – Haemoglobin
HbFe(II) – Ferrous haemoglobin
HbFe(III) – Ferric haemoglobin or methaemoglobin
HCO3- – Concentration of bicarbonate
Hct – Haematocrit
I3- – Triiodide
ICA – Internal carotid artery
ICAv – Internal carotid artery blood velocity
ICP – Intracranial blood pressure
iNOS – Inducible nitric oxide synthase
L-NMMA – NG monomethyl-L-arginine
MCA – Middle cerebral artery
MCAv – Middle cerebral artery blood velocity
MHz – Megahertz
MLCK – Myosin light chain kinase
mmHg – Milimettres of mercury
MRI – Magnetic resonance imaging.
NaOH – Sodium hydroxide
NF-L – Neurofilament Light Chain
NINDS – National Institute of Neurological Disorders and Stroke
NIH – National Institutes of Health
nNOS – Neuronal nitric oxide synthase
NO – Nitric oxide
NO3− – Nitrate
NO2− – Nitrite
NVC – Neurovascular coupling
NVU – Neurovascular unit
N2O – Nitrous oxide
OH- – Hydroxide
PaCO2 – Partial pressure of arterial carbon dioxide
PaO2 – Partial pressure of arterial oxygen
PCA – Posterior cerebral artery
PCAv – Posterior cerebral artery blood velocity
PET – Positron emission tomography
PETO2 – Partial pressure of end-tidal oxygen
PETCO2 – Partial pressure of end-tidal carbon dioxide
PKA – Protein kinase A
PO2 – Partial pressure of oxygen
PjvO2 – Partial pressure of jugular venous oxygen
PvCO2 – Partial pressure of venous carbon dioxide
PvO2 – Partial pressure of venous oxygen
ROS – Reactive oxygen species
RSNO – S-nitrosothiols
SL – Sea-level
SMCs - Smooth muscle cells
SNA – Sympathetic nervous activity
SNO-Hb – S-nitrosohemoglobin
SpO2 – Peripheral oxyhaemoglobin saturation
TCD – Transcranial Doppler ultrasound
TPR – Total peripheral resistance
T-Tau – Total tau
VA – Vertebral artery
VAv – Vertebral artery blood velocity
V̇E – Minute ventilation
","abstract_html":"<b>Background. </b>The current thesis adopts a whole-body, integrated approach to investigate the impact of acute, chronic and life-long exposure to hypoxia on the functional and structural integrity of the neurovascular unit (NVU). <br/><br/><b>Methods.</b> <i>Experimental Study 1: </i>In a randomised, single-blind cross-over design, 12 healthy, physically active male participants (age, 23 ± 2 yrs; BMI, 25 ± 4 kg/m<sup>2</sup>) were exposed to 6 hours of normoxia (F<sub>I</sub>O<sub>2</sub> = 0.21) and 6 hours of acute normobaric hypoxia (F<sub>I</sub>O<sub>2</sub>= 0.12) on separate visits. Experimental measures were conducted at baseline (0 h) and following 6-hours of each exposure. <i>Experimental Study 2:</i> In a repeated-measures, cross-sectional study, nine native lowlanders (28 ± 8 yrs, 24.3 ± 1.7 kg∙m-<sup>2</sup>) were examined at sea-level (normoxia) and following 14 days of high-altitude (HA) acclimatisation to 4,300 m in Cerro De Pasco, Peru (<i>chronic HA hypoxia</i>). Age- and BMI-matched healthy native highlanders of the Altiplano (~4300 m) (n = 9, 27 ± 7 yrs, 26.1 ± 6.0 kg/m<sup>2</sup>) were also examined (<i>life-long HA hypoxia</i>). <i>Experimental measures for Study 1 and 2: </i>Venous blood samples were assayed for biomarkers specific to oxidative-nitrosative stress (OXNOS; ascorbate free radical [A<sup>•−</sup>] and nitric oxide [NO] metabolites via electron paramagnetic resonance spectroscopy and ozone-based chemiluminescence, respectively) and structural integrity of the NVU (<i>Study 1:</i> S100<i>ß</i> and neuron-specific enolase [NSE]; <i>Study 2 only: </i>S100<i>ß</i>, NSE, ubiquitin carboxy-terminal hydrolase L1 [UCH-L1], glial fibrillary acidic protein [GFAP], neurofilament light-chain [NF-L] and total-tau [T-Tau] determined via automated clinical grade ELISA and Single Molecule Array technology). Cardiopulmonary variables included mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), cardiac output (<i>Q</i>), peripheral oxygen saturation (SpO<sub>2</sub>), minute ventilation (V̇E), partial pressure of end-tidal oxygen (P<sub>ET</sub>O<sub>2</sub>) and carbon dioxide (P<sub>ET</sub>CO<sub>2</sub>), haematocrit (Hct), haemoglobin (Hb), arterial oxygen content (CaO<sub>2</sub>) and blood viscosity (<i>Study 2 </i>only). Duplex and transcranial Doppler ultrasonography were employed to measure extracranial (internal carotid artery blood flow [ICA<sub>Q</sub>] and vertebral artery blood flow [VA<sub>Q</sub>]) and intracranial (middle cerebral artery blood velocity [MCA<sub>V</sub>] and posterior cerebral artery [PCA<sub>V</sub>]) haemodynamics. Global cerebral blood flow (gCBF) was calculated and used to quantity global cerebral substrate delivery of oxygen (gCDO<sub>2</sub>) and glucose (gCD<sub>Glu</sub>). NVU function was examined via: 1, dynamic cerebral autoregulation (dCA) assessed using transfer function analysis during spontaneous oscillations in MAP and MCA<sub>V</sub>/ PCA<sub>V</sub>; and 2, neurovascular coupling (NVC) via PCA<sub>V</sub> responses to repeated dark (eyes closed) and light (eyes open + visual stimulation) activation. <br/><br/><i><b>Results.</b> Experimental Study 1: </i>Hypoxia failed to alter A<sup>•−</sup> , total plasma NO and total RBC NO. Owing to the reduction in SpO<sub>2</sub> and unaltered Hb, <i>ca</i>O<sub>2</sub> was decreased in hypoxia. An increase in V̇E following 6 h of hypoxia facilitated an increase in P<sub>ET</sub>O<sub>2</sub> and a reduction in P<sub>ET</sub>CO<sub>2</sub> . In hypoxia, HR and <i>Q̇</i> were elevated and MAP was reduced. ICA<sub>Q</sub> and VA<sub>Q</sub> were both elevated in hypoxia. Despite an increase in gCBF in hypoxia, gCDO<sub>2</sub> was lower in hypoxia and was unaffected by time. While ICA<sub>Q</sub> and ICA DO<sub>2</sub> was reduced in hypoxia, VA<sub>Q</sub> and VA DO<sub>2</sub> remained well preserved. gCD<sub>Glu</sub> was elevated in hypoxia owing to increase ICA D<sub>Glu</sub> and VA D<sub>Glu</sub>. Hypoxia reduced dCA (lower VLF Phase in the MCA and PCA, increased VLF Gain in the PCA) but had no effect on NVC. S100β remained unchanged and NSE decreased in hypoxia. <i>Experimental Study 2: </i>Chronic HA hypoxia in lowlanders increased Hct, Hb and blood viscosity, restoring CaO<sub>2</sub> to baseline normoxic values. While highlanders were more polycythaemic, they were equally hypoxaemic albeit slightly less alkalotic, hypocapneac and hypertensive. Chronic HA hypoxia in lowlanders decreased A<sup>•−</sup> and was accompanied by a reciprocal elevation in plasma and RBC NO bioavailability. Highlanders presented with similar levels of A<sup>•−</sup> to lowlanders at sea-level, however NO bioavailability (nitrite [NO<sub>2</sub><sup>−</sup>] and <i>S</i>-nitrosothiols) was further elevated. Following chronic HA hypoxia in lowlanders, gCBF, gCD<sub>Glu</sub>, gCD O<sub>2</sub> and NVC were comparable to normoxic values and equivalent to those observed in highlanders. Although MCA/PCA phase estimates were also comparable to sea-level following chronic HA hypoxia in lowlanders, gain metrics were consistently lower in the highlanders. Chronic HA hypoxia increased NF-L and decreased T-tau whereas UCHL-1, NSE, GFAP and S100<i>ß</i> did not change. With the exception of NSE and GFAP, all NVU biomarkers were lower in highlanders (lifelong HA hypoxia) compared to lowlanders following chronic HA hypoxia. <br/><br/><b><i>Discussion:</i></b> By utilising an integrated approach, this thesis examined to what extent acute, chronic and life-long hypoxia alters the functional (regional CBF substrate delivery, dCA, and NVC) and structural (BBB permeability and neuronal-axonal damage) integrity of the NVU, whilst examining potential mechanisms (OXNOS) underlying these changes. Contrary the original working hypothesis, acute hypoxia was associated with marked reductions in global substrate delivery and diminished anterior and posterior cerebral autoregulatory capacity, but it was not associated with changes in OXNOS and further, did not translate to impaired NVC, increased BBB permeability or provide any evidence for on-going neuronal-axonal damage. Study 2 supported the working hypothesis whereby following chronic HA hypoxia, it would appear that global substrate delivery and NVC remained well-preserved and dCA improved. Following life-long HA hypoxia in Andean highlanders, the consistently lower levels of S100<i>ß</i>, NF-L and T-Tau observed indicate that the NVU is structurally ‘tighter’ when compared with lowlanders who present with evidence of on-going axonal damage. That global cerebral perfusion and substrate delivery remained well preserved and the lower (LF) gain observed in the anterior/posterior circulation highlights improved pressure-flow coupling (potentially mediated by elevated systemic concentrations of basal plasma and RBC NO), this may indicate that the highlander brain is better equipped to buffer perfusion in response to rapid increases in BP, further ‘bolstering’ the NVU against the potentially damaging effects of BBB disruption. Evidence of such neuroprotection may be presented in the form of the comparable NVC responses between lowlanders at sea-level and highlanders. Further study of the mechanisms governing NVU adaptation in the hypoxia-tolerant human may help inform the pathophysiology and treatment of neurodegenerative diseases notwithstanding hypoxia-related illnesses such as stroke, heart failure, lung disease and cancer.<br/><br/><b>Abbreviations</b><br/>A<sup>•−</sup>– Ascorbate free radical<br/>ACA – Anterior cerebral artery<br/>AMS – Acute mountain sickness<br/>ATP – Adenosine tri-phosphate<br/>β93 Cys – β93 cysteine residue<br/>BBB – Blood-brain-barrier<br/>BMI – Body mass index<br/>BOLD – blood oxygenation level-dependant<br/>CA – Cerebral autoregulation<br/>cAMP – Cyclic adenosine monophosphate<br/>CaO<sub>2</sub>– Arterial oxygen content<br/>CBF – Cerebral blood flow<br/>CDO<sub>2</sub> – Cerebral oxygen delivery<br/>cGMP – Cyclic guanosine monophosphate<br/>CMRO<sub>2</sub> – Cerebral metabolic rate of oxygen<br/>CSF – Cerebrospinal fluid<br/>CVR – Cerebrovascular reactivity<br/>DO<sub>2</sub> – Delivery of oxygen<br/>EET – epoxyeicosatrienoic acid<br/>eNOS – Endothelial nitric oxide synthase<br/>F<sub>I</sub>CO<sub>2</sub> – Fraction of inspired carbon dioxide<br/>F<sub>I</sub>O<sub>2</sub>– Fraction of inspired oxygen<br/>GABA - γ-Aminobutyric acid<br/>gCBF – Global cerebral blood flow<br/>gCD<sub>Glu</sub> – Global cerebral delivery of glucose<br/>gCDO<sub>2</sub> – Global cerebral delivery of oxygen<br/>HA – High-altitude<br/>Hb – Haemoglobin<br/>HbFe(II) – Ferrous haemoglobin<br/>HbFe(III) – Ferric haemoglobin or methaemoglobin<br/>HCO<sub>3</sub><sup>-</sup> – Concentration of bicarbonate<br/>Hct – Haematocrit<br/>I<sub>3</sub><sup>- </sup>– Triiodide<br/>ICA – Internal carotid artery<br/>ICAv – Internal carotid artery blood velocity<br/>ICP – Intracranial blood pressure<br/>iNOS – Inducible nitric oxide synthase<br/>L-NMMA – N<sup>G</sup> monomethyl-L-arginine<br/>MCA – Middle cerebral artery<br/>MCAv – Middle cerebral artery blood velocity<br/>MHz – Megahertz<br/>MLCK – Myosin light chain kinase<br/>mmHg – Milimettres of mercury<br/>MRI – Magnetic resonance imaging.<br/>NaOH – Sodium hydroxide<br/>NF-L – Neurofilament Light Chain<br/>NINDS – National Institute of Neurological Disorders and Stroke<br/>NIH – National Institutes of Health<br/>nNOS – Neuronal nitric oxide synthase<br/>NO – Nitric oxide<br/>NO<sub>3</sub><sup>− </sup>– Nitrate<br/>NO<sub>2</sub><sup>− </sup>– Nitrite<br/>NVC – Neurovascular coupling<br/>NVU – Neurovascular unit<br/>N<sub>2</sub>O – Nitrous oxide<br/>OH<sup>-</sup> – Hydroxide<br/>PaCO<sub>2</sub> – Partial pressure of arterial carbon dioxide<br/>PaO<sub>2</sub> – Partial pressure of arterial oxygen<br/>PCA – Posterior cerebral artery<br/>PCAv – Posterior cerebral artery blood velocity<br/>PET – Positron emission tomography<br/>P<sub>ET</sub>O<sub>2</sub> – Partial pressure of end-tidal oxygen<br/>P<sub>ET</sub>CO<sub>2</sub> – Partial pressure of end-tidal carbon dioxide<br/>PKA – Protein kinase A<br/>PO<sub>2</sub> – Partial pressure of oxygen<br/>PjvO<sub>2</sub> – Partial pressure of jugular venous oxygen<br/>PvCO<sub>2</sub> – Partial pressure of venous carbon dioxide<br/>PvO<sub>2</sub> – Partial pressure of venous oxygen<br/>ROS – Reactive oxygen species<br/>RSNO – S-nitrosothiols<br/>SL – Sea-level<br/>SMCs - Smooth muscle cells<br/>SNA – Sympathetic nervous activity<br/>SNO-Hb – S-nitrosohemoglobin<br/>SpO<sub>2</sub> – Peripheral oxyhaemoglobin saturation<br/>TCD – Transcranial Doppler ultrasound<br/>TPR – Total peripheral resistance<br/>T-Tau – Total tau<br/>VA – Vertebral artery<br/>VAv – Vertebral artery blood velocity<br/>V̇<sub>E</sub> – Minute ventilation <br/>","abstract_has_math":false,"creators":["Stacey, Benjamin"],"institution":null,"degree_name":"Doctoral Thesis","degree_level":"Student thesis","degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Bailey, Damian","Marley, Christopher","Fall, Lewis"],"committee_chairs":[],"committee_members":[],"year":2024,"date_issued":"2024","date_published":"2024","updated_at":"2026-07-24T04:39:57Z","subjects":[],"languages":["eng"],"rights":[],"rights_urls":[],"identifier_entries":[{"key":"dc:identifier","label":"Identifier","values":["oai:pure.atira.dk:studenttheses/a4905434-54df-4901-86e9-94204ffa74df"],"render_values":[{"text":"oai:pure.atira.dk:studenttheses/a4905434-54df-4901-86e9-94204ffa74df","href":null,"code":true}]}]},"links":{"outbound_url":"https://pure.southwales.ac.uk/en/studentTheses/a4905434-54df-4901-86e9-94204ffa74df","outbound_label":"Repository record","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Bailey, Damian","Marley, Christopher","Fall, Lewis"]},{"key":"dc:creator","label":"Author","values":["Stacey, Benjamin"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date","label":"Dc Date","values":["2024"]},{"key":"dc:date.issued","label":"Date","values":["2024"]},{"key":"dc:relation.isreferencedby","label":"Dc Relation Isreferencedby","values":["https://pure.southwales.ac.uk/en/studentTheses/a4905434-54df-4901-86e9-94204ffa74df"]},{"key":"dc:type","label":"Dc Type","values":["Thesis"]},{"key":"dc:type.qualificationlevel","label":"Dc Type Qualificationlevel","values":["Student thesis"]},{"key":"dc:type.qualificationname","label":"Dc Type Qualificationname","values":["Doctoral Thesis"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["eng"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["oai:pure.atira.dk:studenttheses/a4905434-54df-4901-86e9-94204ffa74df","https://pure.southwales.ac.uk/en/studentTheses/a4905434-54df-4901-86e9-94204ffa74df"]},{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://pure.southwales.ac.uk/files/26808308/Benjamin_Stacey_PhD_thesis.pdf"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["Background. The current thesis adopts a whole-body, integrated approach to investigate the impact of acute, chronic and life-long exposure to hypoxia on the functional and structural integrity of the neurovascular unit (NVU).
Methods. Experimental Study 1: In a randomised, single-blind cross-over design, 12 healthy, physically active male participants (age, 23 ± 2 yrs; BMI, 25 ± 4 kg/m2) were exposed to 6 hours of normoxia (FIO2 = 0.21) and 6 hours of acute normobaric hypoxia (FIO2= 0.12) on separate visits. Experimental measures were conducted at baseline (0 h) and following 6-hours of each exposure. Experimental Study 2: In a repeated-measures, cross-sectional study, nine native lowlanders (28 ± 8 yrs, 24.3 ± 1.7 kg∙m-2) were examined at sea-level (normoxia) and following 14 days of high-altitude (HA) acclimatisation to 4,300 m in Cerro De Pasco, Peru (chronic HA hypoxia). Age- and BMI-matched healthy native highlanders of the Altiplano (~4300 m) (n = 9, 27 ± 7 yrs, 26.1 ± 6.0 kg/m2) were also examined (life-long HA hypoxia). Experimental measures for Study 1 and 2: Venous blood samples were assayed for biomarkers specific to oxidative-nitrosative stress (OXNOS; ascorbate free radical [A•−] and nitric oxide [NO] metabolites via electron paramagnetic resonance spectroscopy and ozone-based chemiluminescence, respectively) and structural integrity of the NVU (Study 1: S100ß and neuron-specific enolase [NSE]; Study 2 only: S100ß, NSE, ubiquitin carboxy-terminal hydrolase L1 [UCH-L1], glial fibrillary acidic protein [GFAP], neurofilament light-chain [NF-L] and total-tau [T-Tau] determined via automated clinical grade ELISA and Single Molecule Array technology). Cardiopulmonary variables included mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), cardiac output (Q), peripheral oxygen saturation (SpO2), minute ventilation (V̇E), partial pressure of end-tidal oxygen (PETO2) and carbon dioxide (PETCO2), haematocrit (Hct), haemoglobin (Hb), arterial oxygen content (CaO2) and blood viscosity (Study 2 only). Duplex and transcranial Doppler ultrasonography were employed to measure extracranial (internal carotid artery blood flow [ICAQ] and vertebral artery blood flow [VAQ]) and intracranial (middle cerebral artery blood velocity [MCAV] and posterior cerebral artery [PCAV]) haemodynamics. Global cerebral blood flow (gCBF) was calculated and used to quantity global cerebral substrate delivery of oxygen (gCDO2) and glucose (gCDGlu). NVU function was examined via: 1, dynamic cerebral autoregulation (dCA) assessed using transfer function analysis during spontaneous oscillations in MAP and MCAV/ PCAV; and 2, neurovascular coupling (NVC) via PCAV responses to repeated dark (eyes closed) and light (eyes open + visual stimulation) activation.
Results. Experimental Study 1: Hypoxia failed to alter A•− , total plasma NO and total RBC NO. Owing to the reduction in SpO2 and unaltered Hb, caO2 was decreased in hypoxia. An increase in V̇E following 6 h of hypoxia facilitated an increase in PETO2 and a reduction in PETCO2 . In hypoxia, HR and Q̇ were elevated and MAP was reduced. ICAQ and VAQ were both elevated in hypoxia. Despite an increase in gCBF in hypoxia, gCDO2 was lower in hypoxia and was unaffected by time. While ICAQ and ICA DO2 was reduced in hypoxia, VAQ and VA DO2 remained well preserved. gCDGlu was elevated in hypoxia owing to increase ICA DGlu and VA DGlu. Hypoxia reduced dCA (lower VLF Phase in the MCA and PCA, increased VLF Gain in the PCA) but had no effect on NVC. S100β remained unchanged and NSE decreased in hypoxia. Experimental Study 2: Chronic HA hypoxia in lowlanders increased Hct, Hb and blood viscosity, restoring CaO2 to baseline normoxic values. While highlanders were more polycythaemic, they were equally hypoxaemic albeit slightly less alkalotic, hypocapneac and hypertensive. Chronic HA hypoxia in lowlanders decreased A•− and was accompanied by a reciprocal elevation in plasma and RBC NO bioavailability. Highlanders presented with similar levels of A•− to lowlanders at sea-level, however NO bioavailability (nitrite [NO2−] and S-nitrosothiols) was further elevated. Following chronic HA hypoxia in lowlanders, gCBF, gCDGlu, gCD O2 and NVC were comparable to normoxic values and equivalent to those observed in highlanders. Although MCA/PCA phase estimates were also comparable to sea-level following chronic HA hypoxia in lowlanders, gain metrics were consistently lower in the highlanders. Chronic HA hypoxia increased NF-L and decreased T-tau whereas UCHL-1, NSE, GFAP and S100ß did not change. With the exception of NSE and GFAP, all NVU biomarkers were lower in highlanders (lifelong HA hypoxia) compared to lowlanders following chronic HA hypoxia.
Discussion: By utilising an integrated approach, this thesis examined to what extent acute, chronic and life-long hypoxia alters the functional (regional CBF substrate delivery, dCA, and NVC) and structural (BBB permeability and neuronal-axonal damage) integrity of the NVU, whilst examining potential mechanisms (OXNOS) underlying these changes. Contrary the original working hypothesis, acute hypoxia was associated with marked reductions in global substrate delivery and diminished anterior and posterior cerebral autoregulatory capacity, but it was not associated with changes in OXNOS and further, did not translate to impaired NVC, increased BBB permeability or provide any evidence for on-going neuronal-axonal damage. Study 2 supported the working hypothesis whereby following chronic HA hypoxia, it would appear that global substrate delivery and NVC remained well-preserved and dCA improved. Following life-long HA hypoxia in Andean highlanders, the consistently lower levels of S100ß, NF-L and T-Tau observed indicate that the NVU is structurally ‘tighter’ when compared with lowlanders who present with evidence of on-going axonal damage. That global cerebral perfusion and substrate delivery remained well preserved and the lower (LF) gain observed in the anterior/posterior circulation highlights improved pressure-flow coupling (potentially mediated by elevated systemic concentrations of basal plasma and RBC NO), this may indicate that the highlander brain is better equipped to buffer perfusion in response to rapid increases in BP, further ‘bolstering’ the NVU against the potentially damaging effects of BBB disruption. Evidence of such neuroprotection may be presented in the form of the comparable NVC responses between lowlanders at sea-level and highlanders. Further study of the mechanisms governing NVU adaptation in the hypoxia-tolerant human may help inform the pathophysiology and treatment of neurodegenerative diseases notwithstanding hypoxia-related illnesses such as stroke, heart failure, lung disease and cancer.
Abbreviations
A•−– Ascorbate free radical
ACA – Anterior cerebral artery
AMS – Acute mountain sickness
ATP – Adenosine tri-phosphate
β93 Cys – β93 cysteine residue
BBB – Blood-brain-barrier
BMI – Body mass index
BOLD – blood oxygenation level-dependant
CA – Cerebral autoregulation
cAMP – Cyclic adenosine monophosphate
CaO2– Arterial oxygen content
CBF – Cerebral blood flow
CDO2 – Cerebral oxygen delivery
cGMP – Cyclic guanosine monophosphate
CMRO2 – Cerebral metabolic rate of oxygen
CSF – Cerebrospinal fluid
CVR – Cerebrovascular reactivity
DO2 – Delivery of oxygen
EET – epoxyeicosatrienoic acid
eNOS – Endothelial nitric oxide synthase
FICO2 – Fraction of inspired carbon dioxide
FIO2– Fraction of inspired oxygen
GABA - γ-Aminobutyric acid
gCBF – Global cerebral blood flow
gCDGlu – Global cerebral delivery of glucose
gCDO2 – Global cerebral delivery of oxygen
HA – High-altitude
Hb – Haemoglobin
HbFe(II) – Ferrous haemoglobin
HbFe(III) – Ferric haemoglobin or methaemoglobin
HCO3- – Concentration of bicarbonate
Hct – Haematocrit
I3- – Triiodide
ICA – Internal carotid artery
ICAv – Internal carotid artery blood velocity
ICP – Intracranial blood pressure
iNOS – Inducible nitric oxide synthase
L-NMMA – NG monomethyl-L-arginine
MCA – Middle cerebral artery
MCAv – Middle cerebral artery blood velocity
MHz – Megahertz
MLCK – Myosin light chain kinase
mmHg – Milimettres of mercury
MRI – Magnetic resonance imaging.
NaOH – Sodium hydroxide
NF-L – Neurofilament Light Chain
NINDS – National Institute of Neurological Disorders and Stroke
NIH – National Institutes of Health
nNOS – Neuronal nitric oxide synthase
NO – Nitric oxide
NO3− – Nitrate
NO2− – Nitrite
NVC – Neurovascular coupling
NVU – Neurovascular unit
N2O – Nitrous oxide
OH- – Hydroxide
PaCO2 – Partial pressure of arterial carbon dioxide
PaO2 – Partial pressure of arterial oxygen
PCA – Posterior cerebral artery
PCAv – Posterior cerebral artery blood velocity
PET – Positron emission tomography
PETO2 – Partial pressure of end-tidal oxygen
PETCO2 – Partial pressure of end-tidal carbon dioxide
PKA – Protein kinase A
PO2 – Partial pressure of oxygen
PjvO2 – Partial pressure of jugular venous oxygen
PvCO2 – Partial pressure of venous carbon dioxide
PvO2 – Partial pressure of venous oxygen
ROS – Reactive oxygen species
RSNO – S-nitrosothiols
SL – Sea-level
SMCs - Smooth muscle cells
SNA – Sympathetic nervous activity
SNO-Hb – S-nitrosohemoglobin
SpO2 – Peripheral oxyhaemoglobin saturation
TCD – Transcranial Doppler ultrasound
TPR – Total peripheral resistance
T-Tau – Total tau
VA – Vertebral artery
VAv – Vertebral artery blood velocity
V̇E – Minute ventilation
"]},{"key":"dc:title","label":"Title","values":["The influence of acute, chronic and lifelong hypoxia on the functional and structural integrity of the neurovascular unit"]}]}],"canonical_facts":{"dc:contributor.advisor":["Bailey, Damian","Marley, Christopher","Fall, Lewis"],"dc:creator":["Stacey, Benjamin"],"dc:date":["2024"],"dc:date.issued":["2024"],"dc:description.abstract":["Background. The current thesis adopts a whole-body, integrated approach to investigate the impact of acute, chronic and life-long exposure to hypoxia on the functional and structural integrity of the neurovascular unit (NVU).
Methods. Experimental Study 1: In a randomised, single-blind cross-over design, 12 healthy, physically active male participants (age, 23 ± 2 yrs; BMI, 25 ± 4 kg/m2) were exposed to 6 hours of normoxia (FIO2 = 0.21) and 6 hours of acute normobaric hypoxia (FIO2= 0.12) on separate visits. Experimental measures were conducted at baseline (0 h) and following 6-hours of each exposure. Experimental Study 2: In a repeated-measures, cross-sectional study, nine native lowlanders (28 ± 8 yrs, 24.3 ± 1.7 kg∙m-2) were examined at sea-level (normoxia) and following 14 days of high-altitude (HA) acclimatisation to 4,300 m in Cerro De Pasco, Peru (chronic HA hypoxia). Age- and BMI-matched healthy native highlanders of the Altiplano (~4300 m) (n = 9, 27 ± 7 yrs, 26.1 ± 6.0 kg/m2) were also examined (life-long HA hypoxia). Experimental measures for Study 1 and 2: Venous blood samples were assayed for biomarkers specific to oxidative-nitrosative stress (OXNOS; ascorbate free radical [A•−] and nitric oxide [NO] metabolites via electron paramagnetic resonance spectroscopy and ozone-based chemiluminescence, respectively) and structural integrity of the NVU (Study 1: S100ß and neuron-specific enolase [NSE]; Study 2 only: S100ß, NSE, ubiquitin carboxy-terminal hydrolase L1 [UCH-L1], glial fibrillary acidic protein [GFAP], neurofilament light-chain [NF-L] and total-tau [T-Tau] determined via automated clinical grade ELISA and Single Molecule Array technology). Cardiopulmonary variables included mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), cardiac output (Q), peripheral oxygen saturation (SpO2), minute ventilation (V̇E), partial pressure of end-tidal oxygen (PETO2) and carbon dioxide (PETCO2), haematocrit (Hct), haemoglobin (Hb), arterial oxygen content (CaO2) and blood viscosity (Study 2 only). Duplex and transcranial Doppler ultrasonography were employed to measure extracranial (internal carotid artery blood flow [ICAQ] and vertebral artery blood flow [VAQ]) and intracranial (middle cerebral artery blood velocity [MCAV] and posterior cerebral artery [PCAV]) haemodynamics. Global cerebral blood flow (gCBF) was calculated and used to quantity global cerebral substrate delivery of oxygen (gCDO2) and glucose (gCDGlu). NVU function was examined via: 1, dynamic cerebral autoregulation (dCA) assessed using transfer function analysis during spontaneous oscillations in MAP and MCAV/ PCAV; and 2, neurovascular coupling (NVC) via PCAV responses to repeated dark (eyes closed) and light (eyes open + visual stimulation) activation.
Results. Experimental Study 1: Hypoxia failed to alter A•− , total plasma NO and total RBC NO. Owing to the reduction in SpO2 and unaltered Hb, caO2 was decreased in hypoxia. An increase in V̇E following 6 h of hypoxia facilitated an increase in PETO2 and a reduction in PETCO2 . In hypoxia, HR and Q̇ were elevated and MAP was reduced. ICAQ and VAQ were both elevated in hypoxia. Despite an increase in gCBF in hypoxia, gCDO2 was lower in hypoxia and was unaffected by time. While ICAQ and ICA DO2 was reduced in hypoxia, VAQ and VA DO2 remained well preserved. gCDGlu was elevated in hypoxia owing to increase ICA DGlu and VA DGlu. Hypoxia reduced dCA (lower VLF Phase in the MCA and PCA, increased VLF Gain in the PCA) but had no effect on NVC. S100β remained unchanged and NSE decreased in hypoxia. Experimental Study 2: Chronic HA hypoxia in lowlanders increased Hct, Hb and blood viscosity, restoring CaO2 to baseline normoxic values. While highlanders were more polycythaemic, they were equally hypoxaemic albeit slightly less alkalotic, hypocapneac and hypertensive. Chronic HA hypoxia in lowlanders decreased A•− and was accompanied by a reciprocal elevation in plasma and RBC NO bioavailability. Highlanders presented with similar levels of A•− to lowlanders at sea-level, however NO bioavailability (nitrite [NO2−] and S-nitrosothiols) was further elevated. Following chronic HA hypoxia in lowlanders, gCBF, gCDGlu, gCD O2 and NVC were comparable to normoxic values and equivalent to those observed in highlanders. Although MCA/PCA phase estimates were also comparable to sea-level following chronic HA hypoxia in lowlanders, gain metrics were consistently lower in the highlanders. Chronic HA hypoxia increased NF-L and decreased T-tau whereas UCHL-1, NSE, GFAP and S100ß did not change. With the exception of NSE and GFAP, all NVU biomarkers were lower in highlanders (lifelong HA hypoxia) compared to lowlanders following chronic HA hypoxia.
Discussion: By utilising an integrated approach, this thesis examined to what extent acute, chronic and life-long hypoxia alters the functional (regional CBF substrate delivery, dCA, and NVC) and structural (BBB permeability and neuronal-axonal damage) integrity of the NVU, whilst examining potential mechanisms (OXNOS) underlying these changes. Contrary the original working hypothesis, acute hypoxia was associated with marked reductions in global substrate delivery and diminished anterior and posterior cerebral autoregulatory capacity, but it was not associated with changes in OXNOS and further, did not translate to impaired NVC, increased BBB permeability or provide any evidence for on-going neuronal-axonal damage. Study 2 supported the working hypothesis whereby following chronic HA hypoxia, it would appear that global substrate delivery and NVC remained well-preserved and dCA improved. Following life-long HA hypoxia in Andean highlanders, the consistently lower levels of S100ß, NF-L and T-Tau observed indicate that the NVU is structurally ‘tighter’ when compared with lowlanders who present with evidence of on-going axonal damage. That global cerebral perfusion and substrate delivery remained well preserved and the lower (LF) gain observed in the anterior/posterior circulation highlights improved pressure-flow coupling (potentially mediated by elevated systemic concentrations of basal plasma and RBC NO), this may indicate that the highlander brain is better equipped to buffer perfusion in response to rapid increases in BP, further ‘bolstering’ the NVU against the potentially damaging effects of BBB disruption. Evidence of such neuroprotection may be presented in the form of the comparable NVC responses between lowlanders at sea-level and highlanders. Further study of the mechanisms governing NVU adaptation in the hypoxia-tolerant human may help inform the pathophysiology and treatment of neurodegenerative diseases notwithstanding hypoxia-related illnesses such as stroke, heart failure, lung disease and cancer.
Abbreviations
A•−– Ascorbate free radical
ACA – Anterior cerebral artery
AMS – Acute mountain sickness
ATP – Adenosine tri-phosphate
β93 Cys – β93 cysteine residue
BBB – Blood-brain-barrier
BMI – Body mass index
BOLD – blood oxygenation level-dependant
CA – Cerebral autoregulation
cAMP – Cyclic adenosine monophosphate
CaO2– Arterial oxygen content
CBF – Cerebral blood flow
CDO2 – Cerebral oxygen delivery
cGMP – Cyclic guanosine monophosphate
CMRO2 – Cerebral metabolic rate of oxygen
CSF – Cerebrospinal fluid
CVR – Cerebrovascular reactivity
DO2 – Delivery of oxygen
EET – epoxyeicosatrienoic acid
eNOS – Endothelial nitric oxide synthase
FICO2 – Fraction of inspired carbon dioxide
FIO2– Fraction of inspired oxygen
GABA - γ-Aminobutyric acid
gCBF – Global cerebral blood flow
gCDGlu – Global cerebral delivery of glucose
gCDO2 – Global cerebral delivery of oxygen
HA – High-altitude
Hb – Haemoglobin
HbFe(II) – Ferrous haemoglobin
HbFe(III) – Ferric haemoglobin or methaemoglobin
HCO3- – Concentration of bicarbonate
Hct – Haematocrit
I3- – Triiodide
ICA – Internal carotid artery
ICAv – Internal carotid artery blood velocity
ICP – Intracranial blood pressure
iNOS – Inducible nitric oxide synthase
L-NMMA – NG monomethyl-L-arginine
MCA – Middle cerebral artery
MCAv – Middle cerebral artery blood velocity
MHz – Megahertz
MLCK – Myosin light chain kinase
mmHg – Milimettres of mercury
MRI – Magnetic resonance imaging.
NaOH – Sodium hydroxide
NF-L – Neurofilament Light Chain
NINDS – National Institute of Neurological Disorders and Stroke
NIH – National Institutes of Health
nNOS – Neuronal nitric oxide synthase
NO – Nitric oxide
NO3− – Nitrate
NO2− – Nitrite
NVC – Neurovascular coupling
NVU – Neurovascular unit
N2O – Nitrous oxide
OH- – Hydroxide
PaCO2 – Partial pressure of arterial carbon dioxide
PaO2 – Partial pressure of arterial oxygen
PCA – Posterior cerebral artery
PCAv – Posterior cerebral artery blood velocity
PET – Positron emission tomography
PETO2 – Partial pressure of end-tidal oxygen
PETCO2 – Partial pressure of end-tidal carbon dioxide
PKA – Protein kinase A
PO2 – Partial pressure of oxygen
PjvO2 – Partial pressure of jugular venous oxygen
PvCO2 – Partial pressure of venous carbon dioxide
PvO2 – Partial pressure of venous oxygen
ROS – Reactive oxygen species
RSNO – S-nitrosothiols
SL – Sea-level
SMCs - Smooth muscle cells
SNA – Sympathetic nervous activity
SNO-Hb – S-nitrosohemoglobin
SpO2 – Peripheral oxyhaemoglobin saturation
TCD – Transcranial Doppler ultrasound
TPR – Total peripheral resistance
T-Tau – Total tau
VA – Vertebral artery
VAv – Vertebral artery blood velocity
V̇E – Minute ventilation
"],"dc:identifier":["oai:pure.atira.dk:studenttheses/a4905434-54df-4901-86e9-94204ffa74df","https://pure.southwales.ac.uk/en/studentTheses/a4905434-54df-4901-86e9-94204ffa74df"],"dc:identifier.uri":["https://pure.southwales.ac.uk/files/26808308/Benjamin_Stacey_PhD_thesis.pdf"],"dc:language":["eng"],"dc:relation.isreferencedby":["https://pure.southwales.ac.uk/en/studentTheses/a4905434-54df-4901-86e9-94204ffa74df"],"dc:title":["The influence of acute, chronic and lifelong hypoxia on the functional and structural integrity of the neurovascular unit"],"dc:type":["Thesis"],"dc:type.qualificationlevel":["Student thesis"],"dc:type.qualificationname":["Doctoral Thesis"]},"updated_at":"2026-07-24T04:39:57Z"}