Systemic hypoxemia catalyzes cerebral oxidative-nitrosative stress during extreme apnea in humans: implications for cerebral bioenergetic function

BACKGROUND Voluntary asphyxia induced by apnea in competitive breath hold (BH) divers affords a unique opportunity to examine integrated mechanisms underlying the preservation of cerebral bioenergetic function. This study examined to what extent physiological extremes of oxygen (O2) demand and carbon dioxide (CO2) production impact redox homeostasis and corresponding red blood cell (RBC)-mediated cerebral vasodilation. METHODS Ten ultra-elite apneists (6 men, 4 women) aged 33 ± 9 (mean ± SD) years old performed two maximal dry apneas preceded by, [1] normoxic normoventilation resulting in severe hypoxemic hypercapnia apnea (HHA) and [2] hyperoxic hyperventilation designed to prevent hypoxemia resulting in isolated hypercapnic apnea (IHA). Transcerebral exchange kinetics of ascorbate radicals (A·-, electron paramagnetic resonance spectroscopy), lipid hydroperoxides (LOOH, spectrophotometry) and nitric oxide metabolites (NO, tri-iodide reductive chemiluminescence) were calculated as the product of global cerebral blood flow (gCBF, duplex ultrasound) and radial arterial (a) to internal jugular venous (v) concentration gradients determined at eupnea and after apnea. RESULTS Apnea duration increased from 306 ± 62 s during HHA to 959 ± 201 s during IHA (P = <0.001), resulting in individual nadirs of 29 mmHg and 40 % for PaO2 and SaO2 respectively in HHA and PaCO2 peak of 68 mmHg in IHA. Apnea resulted in a more pronounced elevation in the net cerebral output (v>a) of A·- and LOOH in HHA (P = <0.05 vs. IHA). This coincided with a lower apnea-induced increase in gCBF (P = <0.001 vs. IHA) and related suppression in plasma nitrite (![Graphic][1]) uptake (a>v) (P = < 0.05 vs. IHA), implying reduced consumption and delivery of NO consistent with elevated cerebral oxidative-nitrosative stress (OXNOS). While apnea-induced gradients consistently reflected plasma ![Graphic][2] consumption (a>v) and RBC iron nitrosylhemoglobin formation (v>a), we failed to observe equidirectional gradients consistent with S -nitrosohemoglobin consumption and plasma S -nitrosothiol delivery. CONCLUSIONS These findings highlight a key catalytic role for hypoxemia in cerebral OXNOS with ![Graphic][3] reduction the more likely mechanism underlying endocrine NO vasoregulation with the capacity to transduce physiological O2-CO2 gradients into graded vasodilation. ### Competing Interest Statement Dr Bailey is Editor-in-Chief of Experimental Physiology, Chair of the Life Sciences Working Group, member of the Human Spaceflight and Exploration Science Advisory Committee to the European Space Agency, member of the Space Exploration Advisory Committee to the UK Space Agency, member of the National Cardiovascular Network for Wales and South East Wales Vascular Network and is affiliated to the companies FloTBI, Inc. and Bexorg, Inc. focused on the technological development of novel biomarkers of brain injury in humans. ### Clinical Trial The current study does not constitute a clinical trial. ### Funding Statement This study was funded by a Royal Society Wolfson Research Fellowship (#WM170007) and Higher Education Funding Council for Wales (Dr Bailey), Canada Research Chair (CRC) and Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery grant (Dr Ainslie), NSERC (Drs Bain and Hoiland), Autonomic Province of Vojvodina, Serbia (#142-451- 2541, Dr Barak) and Croatian Science Foundation Grant (#IP-2014-09-1937, Drs Barak, Drvis and Ainslie). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Experimental procedures were approved by the University of Split, Ethics Committee (#H14-00922). All procedures were carried out in accordance with the most (7th) recent amendment of the Declaration of Helsinki of the World Medical Association7 (with the exception that it was not registered in a publicly accessible database prior to recruitment) with verbal and written informed consent obtained from all participants. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes The data that support the findings of this study are not openly available due to reasons of sensitivity and are available from the corresponding author upon reasonable request. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif