Optimising heat acclimation to attenuate physiological stress in hypoxia

The purpose of this thesis was to determine optimal strategies for heat acclimation (HA) and to investigate whether HA attenuated the physiological responses, when acclimated, to acute hypoxia. Study one considered the optimal exogenous conditions for effective HA. Data identified that during acute exercise-heat stress a minimum degree of physiological strain was required to elevate blood extracellular heat shock protein (eHSP72, used as an indicator of cellular heat stress). The increase in eHSP72 was due to several endogenous factors achieved in 40°C/40% R.H., and not 20°C/63% R.H. or 30°C/51% R.H. Thus, the hottest external condition was used for subsequent HA methods. Study two tested the implementation of two isothermic HA models compared with a validated, traditional fixed intensity method (90 min; 50% V % O2peak). The isothermic models targeted and achieved specific core temperatures (Trec) (90 min; continuous = 38.5°C, day 1–10; progressive = 38.5°C day 1-5 then 39.0°C, day 6-10). Endogenous strain was sustained better using isothermic HA during long term timescales compared with the fixed intensity HA with a greater mean Trec and duration Trec≥38.5°C. During the early phases of acclimation, greater strain was not at the expense of greater work, or less palatable exercise intensity prescription, which were more favourable using isothermic versus fixed intensity methods, largely due to the greater initial heat production. Despite differences in the HA administration, no difference was found for resting or exercising phenotypic adaptations between methods. Additionally, using a progressive isothermic method offered no additional physiologically adaptive benefit in comparison to the fixed, or continuous isothermic methods. With Hsp72 increases being an important mechanism by which heat-hypoxic cross tolerance is augmented, study three demonstrated that the continuous isothermic HA methods from study 2 sustained the leukocyte Hsp72mRNA responses to the greatest extent. The sustained influence on physiological measures and HSP concentrations evoked by the long term continuous isothermic method, alongside the favourable administration, supported this method’s application for the final experiment, aimed at measuring the efficacy of optimised HA in reducing physiological strain during acute hypoxia. Study four data demonstrated accelerated physiological adaptations which accompany long term isothermic HA in comparison to normothermic training. Data collected within an acute hypoxic exposure during rest, then lowmoderate and moderate-high intensity exercise, identified physiological strain is reduced following HA, in comparison to normothermic training or when not acclimated, and that the Hsp72mRNA increase which occurred prior to HA was subsequently attenuated following the intervention. No increase in Hsp72 mRNA was observed in either pre or post trials in the normothermic training (control) group, suggesting insufficient stimuli to initiate the heat shock response within this group. This evidence supports the notion that HA provides a suitable strategy to decrease the physiological strain during acute hypoxic exposures.