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Title: Monocyte heat shock protein 72 at rest and in response to environmental and exercise stress : implications for cross tolerance in vivo
Author: Taylor, Lee
Awarding Body: University of Hull
Current Institution: University of Hull
Date of Award: 2010
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The human body endures stress on a daily basis, with many occupational and recreational activities beset with such challenges to homeostasis. These challenges include that of exercise and exposure to challenging environments (hypoxia and hyperbaria). A group of specialised proteins, termed heat shock proteins (HSP) provide protection to such stressors at a cellular level. This cellular defence mechanism protects and oversees whole body protein homeostasis, which is vital to all cellular processes. One such protective HSP, is HSP72, which is present in almost all cellular compartments and has received extensive and widespread research interest – with elevations in HSP72 indicatively linked to augmented cellular and whole body resistance to various exercise and environmental stressors. Despite this extensive research interest, several fundamental areas of concern with regard to HSP72 have not been satisfactorily addressed or delineated. In general, the experimental chapters of this thesis were designed to investigate several broad research questions related to those areas that have not been sufficiently addressed, as highlighted by the Literature Review. These areas include the reliance on thermal and/or mechanical stress to induce elevations in HSP72, both in vitro and in vivo, to initiate conveyed cellular protection. No in vivo attempts have been made to use a non-thermal and/or non-mechanical based stimulus, such as a hypoxic or hyperbaric exposure, to induce elevations in basal HSP72 in an attempt to confer cellular tolerance to future episodes of stress. Additionally, any potential relationships between changes in redox balance and stress induced changes in HSP72 expression have not been investigated in vivo, this potential interplay could be important when discussing any likely mechanisms for HSP72 dependent conferred cellular tolerance. In order to investigate such hypoxic or hyperbaric mediated changes in basal HSP72 expression securely, basal expression of monocyte expressed HSP72 (mHSP72) warrants investigation (diurnal and/or circadian variation), as, at present, this has not be conducted securely or adequately. The first experimental chapter investigated basal expression of mHSP72 over a 24 h period. Seventeen recreationally active (mean ± SD: 5.9±2.2 h∙wk-1) male subjects (19.8±4.3 yr, 177±6.4 cm, 75.7±10.9 kg) had blood samples taken every 4 h from 0900 until 0900 the next day, at rest, within a temperature regulated laboratory. Core temperature, as assessed by ingestible telemetric temperature sensor pill, was obtained at 5 min intervals. Basal mHSP72 expression was found to follow a circadian rhythm, which was correlated to core temperature (rs=0.41, p<0.001). Notably, during “waking” hours (0900 – 2100), this circadian rhythm was shown to follow a quadratic trend in expression (F = 21.2, p < 0.001). The second experimental chapter investigated the repeatability of the quadratic trend in basal mHSP72 expression demonstrated within the previous experimental chapter. Twelve healthy recreationally active (mean ± SD: 5.2±1.9 h∙wk-1) male subjects (20.2±1.9 yr, 178.7±5.6 cm, 75.1±6.0 kg) had blood samples taken on three separate days (separated by three days) over a 9 h period (0800, 1100, 1400) at rest within a temperature regulated laboratory. Results supported those from the previous chapter, whereby, the quadratic trend in basal mHSP72 expression was evident on three separate days (F = 26.0; p = 0.001; partial η2 = 0.74), where mHSP72 decreased between 0800 and 1100 (mean difference = -17%; 95% CI = -24%, -10%; p < 0.001) and then increased between 1100 and 1400 (mean difference = 8%; 95% CI = 2%, 14%; p = 0.015). In conjunction with the first experimental chapter, these results demonstrate the importance in controlling the time of day interventions are administered in vivo, as differential responses may be seen due to differences in basal HSP72 expression. Furthermore, when regular blood samples are required post intervention, the timetabling of such collections needs to be stringently adhered to, due to within-day variation in basal mHSP72. Differing basal values of mHSP72 are known to determine the magnitude of post stressor mHSP72 expression and thus any variation (even minimal) in basal mHSP72 is important. The third experimental chapter investigated the potential of an environmental stressor to disrupt the quadratic trend in basal mHSP72 and explored whether any such changes in mHSP72 may have a relationship with alterations in redox balance. Six healthy recreationally active (mean ± SD: 5.9±2.3 h.wk-1) male subjects (mean ± SD: 21.3±7.2 yr, 179.2±4.8 cm, 79.3±9.9 kg) participated within the study. Control values (NA) for mHSP72 were obtained one week before the first hyperbaric air (HA) exposure with the hyperbaric oxygen (HBO) exposure following a week later (i.e. 3 study days NA, HA and HBO each separated by one week). These exposures commenced at 1500 and involved a simulated dive consisting of HA (2.8 ATA) or HBO (20 min O2, 5 min HA cycle) within a hyperbaric chamber constituting 78 min bottom time. Within each study day blood samples were taken at 0900, 1300, 1700 and 2100. The administration of HBO and HA were sufficient to disrupt the quadratic trend shown within the NA condition (F = 27.6, p < 0.001). The model demonstrated significant main effects for condition (F = 24.7, p < 0.001) and time (F = 9.6, p < 0.001), and a condition x time interaction effect was also observed (F = 7.1, p < 0.001). Decomposition of this interaction effect revealed a reduction in mHSP72 was evident post hyperbaric exposures, whereby, mHSP72 expression at 1700 was significantly higher in NA than in HA (p = 0.016) and HBO (p < 0.001), this reduction was still evident in both HA and HBO compared to NA at 2100 (p < 0.001). In addition to quantification of mHSP72, a measure of oxidative stress, thiobarbituric acid reactive substances (plasma TBARS), was also retrospectively assessed from the isolated plasma of these blood samples. There were no significant main effects observed for condition (F = 0.7; p = 0.50) or time (F = 0.06; p = 0.81), and no significant condition x time interaction effect (F = 0.5; p = 0.62) for plasma TBARS. Despite the failure of the hyperbaric environments to elicit increases in basal mHSP72, one important physiological contribution may be contrived of this reduction in mHSP72, as in vitro and in vivo low basal mHSP72 content is indicatively correlated to enhanced post stressor HSP72 expression. Such hyperbaric mediated reductions in basal content may allow enhanced HSP72 expression post stressor, an intervention which may be of benefit to hyperthermic exercise acclimation protocols which seek elevated mHSP72 as part of the in vivo heat acclimation process. The fourth experimental chapter employed an acute hypoxic exposure (75 min, 2980 m) at rest in an attempt to disrupt the previously demonstrated quadratic trend in basal mHSP72 expression and explored whether any such changes in mHSP72 may have a relationship with alterations in redox balance. Twelve healthy recreationally active (mean ± SD: 5.1±1.5 h.wk-1) male subjects (19.8±3.5 yr, 175.5±10.8 cm, 73.1±8.0 kg) participated in the study. Testing was conducted on consecutive days, with all subjects providing control samples on this first day with the hypoxic exposure administered on the second day. This exposure commenced and ceased at 0930 and 1045 respectively. Blood samples were taken at 0800, 1100, 1400, 1700 and 2000. In addition to quantification of mHSP72 a measure of oxidative stress, plasma TBARS, was also retrospectively assessed from the isolated plasma of these blood samples. There was a significant quadratic trend in mHSP72 for the control condition (F = 23.5; p = 0.002; partial η2 = 0.77) with no such trend evident for the hypoxic condition (largest F ratio was for a quadratic trend: F = 3.9; p = 0.087).
Supervisor: McNaughton, Lars; Lovell, Ric Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Sports sciences