Heat acclimation attenuates physiological strain and the Hsp72, but not Hsp90α mRNA response to acute normobaric hypoxia

Heat acclimation attenuates physiological strain in hot conditions via phenotypic and cellular adaptation. The aim of this study was to determine whether HA reduced physiological strain, and Hsp72 and Hsp90α mRNA responses in acute normobaric hypoxia. Sixteen male participants completed ten 90 min sessions of isothermic heat acclimation (HA; 40°C/40%RH) or exercise training (CON; 20°C/40% RH). HA or CON were preceded (HYP1) and proceeded (HYP2) by a 30min normobaric hypoxic exposure (FiO2=0.12; 10min rest, 10min cycling at 40%V.O2peak, 10min cycling at 65%V.O2peak). HA induced greater rectal temperatures (Trec), sweat rate (SR) and heart rates (HR) than CON during the training sessions. HA, but not CON, reduced resting Trec, resting HR and increased SR and plasma volume. Haemoglobin mass did not change following HA nor CON. Hsp72 and Hsp90α mRNA increased in response to each HA session, but did not change with CON. HR during HYP2 was lower and O2 saturation higher at 65%V.O2peak following HA, but not CON. V.O2/HR was greater at rest and 65% V.O2peak in HYP2 following HA, but was unchanged after CON. At rest, the respiratory exchange ratio reduced during HYP2 following HA, but not CON. The increase in Hsp72 mRNA during HYP1, did not occur in HYP2 following HA. In CON, Hsp72 mRNA expression was unchanged during HYP1 and HYP2. In HA and CON, increases in Hsp90α mRNA during HYP1 were maintained in HYP2. HA reduces physiological strain, and the transcription of Hsp72, but not Hsp90α mRNA in acute normobaric hypoxia.