Importance of Aerobic Fitness in Extending Thermotolerance in Extreme Environments: Connecting Molecular Biology to the Whole Body Response

摘要

Current military operations involve exposure to high ambient temperatures while wearing protective clothing and/or the use of biological and chemical ensembles. Under heat stress, aerobic fitness is a critical factor in explaining the higher core temperature (Tc) that can be tolerated by endurance trained (TR) versus untrained (UT) individuals. Tolerating higher Tc not only extends exposure time but it ensures that individuals can safely continue to be mobile while carrying their own weight and additional loads. The purpose of this research was to examine whether differences in the immuno-inflammatory activation existed between TR and UT that might help explain the different Tc tolerated at exhaustion (EXH). Twelve TR and eleven UT walked at 4.5 km/h and 2% grade to EXH in 40 C and 30% RH while wearing a protective overgarment. Blood was sampled at 0.5 C increments until Tc reached 40.0 C or the trial was terminated due to high heart rates, nausea, dizziness or volition. Plasma endotoxin and serum lipopolysaccharide binding protein (LBP) were determined as were intracellular pro and anti-inflammatory cytokines and heat stress protein (HSP) in circulating monocytes using flow cytometry. Blood volume (BV) was determined before the trial. There were no differences between groups in the rate of Tc increase; the longer tolerance time for TR vs UT reflected differences in Tc tolerated at EXH. BV was increased for TR vs UT but these differences were not related to the differences in Tc tolerated. Exercise and heat stress invoked significant increases in plasma endotoxin and LBP but the increase was greater for UT vs TR. Plasma endotoxin was more than 2-fold greater for UT than TR. Pro-inflammatory cytokines increased for both groups with increasing Tc yet values were greater for TR compared with UT. However, at higher Tc these cytokine expressions decreased for TR but continued to increase for UT. Anti-inflammatory cytokines were also increased for TR vs UT. Intracellular.