Julien D. Périard & Patricia Ruell & Corinne Caillaud & Martin W. Thompson
Received: 4 October 2011 /Revised: 23 November 2011 /Accepted: 28 November 2011 / Published online: 6 January 2012, Cell Stress Society International 2012.
Extracellular heat-shock protein 72 (eHsp72) expression during exercise-heat stress is suggested to increase with the level of hyperthermia attained, independent of the rate of heat storage. This study examined the influence of exercise at various intensities to elucidate this relationship, and investigated the association between eHsp72 and eHsp27. Sixteen male subjects cycled to exhaustion at 60% and 75% of maximal oxygen uptake in hot conditions (40°C, 50% RH). Core temperature, heart rate, oxidative stress, and blood lactate and glucose levels were measured to determine the predictor variables associated with eHsp expression. At exhaustion, heart rate exceeded 96% of maximum in both conditions. Core temperature reached 39.7°C in the 60% trial (58.9 min) and 39.0°C in the 75% trial (27.2 min) (P<0.001). The rate of rise in core temperature was 2.1°C h−1 greater in the 75% trial than in the 60% trial (P<0.001). A significant increase and correlation was observed between eHsp72 and eHsp27 concentrations at exhaustion (P<0.005). eHsp72 was highly correlated with the core temperature attained (60% trial) and the rate of increase in core temperature (75% trial; P<0.05). However, no common predictor variable was associated with the expression of both eHsps. The similarity in expression of eHsp72 and eHsp27 during moderate and high-intensity exercise may relate to the duration (i.e., core temperature attained) and intensity (i.e., rate of increase in core temperature) of exercise. Thus, the immuno-inflammatory release of eHsp72 and eHsp27 in response to exercise in the heat may be duration and intensity dependent.
Thermotolerance during a prolonged acute bout of exercise in the heat is characterised by the heat-shock response and adaptations associated with heat acclimation. The heat-shock response confers transient thermal tolerance, in part due to the expression of heat-shock proteins (Hsps). These highly conserved ubiquitous stress proteins are found in all eukaryotes and prokaryotes, and are synthesised by the cells of an organism in response to a variety of stimuli, including heat, oxidative, metabolic and chemical stress (Welch 1992; Morimoto et al. 1994). The classification of Hsps is based on function and molecular mass, ranging from 8 to 110 kDa (Schlesinger 1990; Welch 1992). This includes the constitutively expressed and stress-inducible Hsp70 family. Hsp72 in particular is responsive to heat stress and exercise (Locke 1997). Its extracellular expression (eHsp72), measured in plasma and serum, has been suggested as a potential signal, triggering innate immunity and stimulating the release of proinflammatory cytokines (Pockley et al. 1998; Asea et al. 2000; Njemini et al. 2003; Pockley et al. 2003; Njemini et al. 2004; Asea 2005; Noble et al. 2008). In humans, the contracting muscles do not appear to be a source of eHsp72 (Febbraio et al. 2002a), whereas the brain and hepatosplanchnic tissues are capable of releasing Hsp72 in the systemic circulation (Febbraio et al. 2002b; Lancaster et al. 2004).