Heat acclimation attenuates physiological strain and the HSP72, but not HSP90α, mRNA response to acute normobaric hypoxia

Abstract

Heat acclimation (HA) 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 heat shock protein (HSP) 72 and HSP90α mRNA responses in acute normobaric hypoxia. Sixteen male participants completed ten 90-min sessions of isothermic HA (40°C/40% relative humidity) or exercise training [control (CON); 20°C/40% relative humidity]. HA or CON were preceded (HYP1) and proceeded (HYP2) by a 30-min normobaric hypoxic exposure [inspired O2 fraction = 0.12; 10-min rest, 10-min cycling at 40% peak O2 uptake (V̇O2 peak), 10-min cycling at 65% V̇O2 peak]. HA induced greater rectal temperatures, sweat rate, and heart rates (HR) than CON during the training sessions. HA, but not CON, reduced resting rectal temperatures and resting HR and increased sweat rate and plasma volume. Hemoglobin 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% V̇O2 peak following HA, but not CON. O2 uptake/HR was greater at rest and 65% V̇O2 peak in HYP2 following HA, but was unchanged after CON. At rest, the respiratory exchange ratio was 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.

The Hsp72 and Hsp90α mRNA Responses to Hot Downhill Running Are Reduced Following a Prior Bout of Hot Downhill Running, and Occur Concurrently within Leukocytes and the Vastus Lateralis

“…Leukocytes are a desirable tissue site for determining thermotolerance given the relative ease by which they can be collected, and because leukocytes, as circulating cells, are exposed to both systemic signals and to signals of the perfused tissues (Sonna et al, 2007). As such Hsp72 mRNA from leukocytes has been utilized as a surrogate to skeletal muscle samples with inferences made from changes in circulating intracellular sites across many exercise, heat, and nutritional experiments whereby the cellular stress response and thermotolerance are augmented (Fehrenbach et al, 2000a,b, 2001; Niess et al, 2002; Connolly et al, 2004; Marshall et al, 2007; Selkirk et al, 2009; Gibson et al, 2015a,c; Tuttle et al, 2015; Mee et al, 2016). Consequently, determining whether the HSR occurs concurrently within both tissues (leukocytes and the vastus lateralis; VL) following an acute stressor (initial experimental trial), and whether this response is attenuated in both tissues following a second trial (i.e., following preconditioning), requires elucidation to assess the viability of the leukocyte HSR to represent the skeletal muscle HSR.…”

“…In addition to Hsp72 mRNA, Hsp90α mRNA is of interest due to its important role within restoration of proteostasis (Kourtis and Tavernarakis, 2011; van Oosten-Hawle et al, 2013), regulation of the transmission of signaling cascades (Taipale et al, 2010), recovery of global protein synthesis (Duncan, 2005) and regulation of cellular repair (Erlejman et al, 2014). Additionally it is unknown if the physiological signals e.g., increases in systemic temperature (Gibson et al, 2016), which elicit increases in leukocyte Hsp72 and Hsp90α mRNA transcription to damaging (Tuttle et al, 2015), and non-damaging exercise-heat stress (Gibson et al, 2015c), are as relevant in skeletal muscle. The current study also sought to investigate the gene transcript response of another HSP, glucose regulated protein 78 mRNA (Grp78 mRNA) given its ability to indicate when the unfolded protein response ends (Ron and Walter, 2007).…”

Leukocyte Hsp72 mRNA transcription does not differ between males and females during heat acclimation

“…Based on a priori power analyses using previous experimental data with identical methods, 7,8 4 participants in each group would result in 95% probability of detecting a difference in Hsp72 mRNA across the course of controlled hyperthermia HA. In line with power analysis, and previous work in the area, 16 5 males and 5 females (Table 1) provided written informed consent to participate in the current study.…”

“…In response to 10 d HA, baseline intracellular HSP72 has been shown to increase by 18% 6 while heat shock protein 72 messenger ribonucleic acid (Hsp72 mRNA) demonstrates a pattern whereby transcription occurs within each HA session (C195%) before returning to baseline 24 hr later. 7,8 These transient HA mediated cellular adaptations to iHSP72 can confer cytoprotection to subsequent thermal 6 and non-thermal 8 stressors in vitro 6 and in vivo.…”

“…8 Due to the consistent endogenous thermal stimulus there is an equality of Hsp72 mRNA transcription during 10 d controlled hyperthermia HA. 7 Thus, controlled hyperthermia is a preferred HA method compared with traditional exogenously prescribed HA since it induces robust Hsp72 mRNA responses, ensuring sufficient and consistent increases in endogenous stimuli throughout an in vivo chronic intervention, particularly when comparing independent groups.…”