The reduction in O2peak at altitude is well documented. Maximal exercise in hypoxia is accelerated through a reduction in O2 supply with contributions from central and peripheral origins of fatigue. Changes in cerebral and muscle oxygenation have not been well characterised during incremental exercise in hypoxia. It is possible attainment of O2peak is driven by the oxygenation profile of these tissues whilst changes in molecular biomarkers of endothelial function could provide some insight into the mechanisms driving systemic and regional O2 delivery and vascular hypoxic sensing capabilities. The first study of this thesis examined the impact of acute hypoxia (FIO2 = 0.12) on the cerebral and muscle oxygenation response to incremental cycling exercise using NIRS (n = 14; age: 23 ± 5yr; height: 1.80 ± 0.07m; weight: 84 ± 8kg). The profiles were characterised at equivalent relative and absolute exercise intensities and molecular blood-borne markers of O2 sensing and function were measured before and immediately after maximal exercise for changes in oxidative stress (A• and 3-NT), NO metabolites (NOx, NO3•, NO2• and RSNO) and cell adhesion molecules (sICAM-1 and sVCAM-1). The key observations from this study were: 1) O2peak decreased by 22% and the magnitude of cerebral and muscle deoxygenation (↓O2Hb and ↑HHb) was greater in hypoxia, 2) the slope for the relative HHb response was similar between conditions whereas there was an accelerated slope across the absolute workloads in hypoxia implying cycling performance was driven by a premature attainment of maximal O2 extraction capacity of the muscle, 3) there was no evidence suggesting cerebral O2 metabolism was impaired in hypoxia however since SaO2 was 78 ± 4% at PPO it is possible the reduction in systemic O2 delivery could have influenced central fatigue, 4) there was a tendency for a rightward shift in the cerebral THb profile in hypoxia and although muscle THb peaked at 80% PPO in both trials, the response also tended to be lower in hypoxia, 5) there was no change in oxidative stress markers and NOx after exercise, 6) RSNO increased and NO2• decreased after maximal exercise. The decline in NO2• was attenuated in hypoxia possibly due to a blunted NO2•-HHb-NO pathway and may explain the systemic hypoperfusion response, 7) The increase in sICAM-1 and sVCAM-1 after exercise was augmented in normoxia, 8) Only when normoxia and hypoxia data was pooled was there a correlation between sVCAM-1 pre-post exercise and O2peak.Intermittent hypoxia (IH) may be used to improve the efficiency of exercise training and as a pre-acclimatisation strategy prior to high altitude ascent. The purpose of the second study was to evaluate the efficacy of a 10 day IH regime consisting of 9x 5 min daily exposures of 9.5% O2 breathing followed by equal periods of normoxia on submaximal and maximal cardiorespiratory responses to exercise in hypoxia. Additionally, cerebral and muscle oxygenation was monitored throughout incremental cycling to exhaustion and changes in NO metabolites (NO3•, NO2• and RSNO) and CAMs (sICAM-1 and sVCAM-1) were measured before and immediately after maximal exercise. The key observations from this study were: 1) a tendency for IH to reduce submaximal O2 and increase O2peak in hypoxia, 2) IH increased the muscle THb response to exercise due an increased intercept for both the muscle O2Hb and HHb in the absence of any change in slope, 4) cerebral oxygenation increased (↑O2Hb) at rest and during exercise, 4) the reduction in nitrite was attenuated in the IH group whilst resting sICAM-1 decreased and the pre-post maximal exercise increase in sICAM-1 was augmented after IH. It is concluded that exercise performance in acute hypoxia is driven by the magnitude of hypoxaemia and an accelerated rate of cerebral and muscle deoxygenation. Molecular biomarkers of endothelial function in particular, NO2• and CAMs, are also influenced by hypoxia and may contribute to the reduction in O2peak. IH may be used to improve exercise economy and O2peak in hypoxia by improving cerebral and muscle oxygenation in the absence of any change in central O2 delivery. It is possible a recalibration of mechanisms that affect NO bioactivation could have enhanced vascular hypoxic sensitivity, O2 delivery and adaptation within brain and muscle tissue which ultimately translated to an improved hypoxic exercise performance. These results give motivation for athletes and mountaineers to incorporate an IH strategy prior to athletic performance at altitude.
|Date of Award
|Damian Bailey (Supervisor)