title: Time course of the attenuation of sympathetic nervous activity during active heat acclimation.
abstract: The purpose of this study was to determine the time course of the attenuation in sympathetic nervous activity during active heat acclimation (HA) in healthy humans. Eight volunteers completed a maximal graded exercise test followed by 8 days of active HA. Heat acclimation consisted of 90 min of walking at 40% of maximal oxygen uptake in a heated environmental chamber at 35 °C. The mean (±SD) ending core temperature during exercise was significantly reduced during the 8 days of HA. Specifically, it decreased from 38.3 ± 0.4 °C on day 1 to 37.9 ± 0.3 °C on day 8. In addition, ending HR during exercise was also significantly reduced from 152 ± 18 bpm on day 1 to 135 ± 15 bpm on day 8 of HA. The most important new finding was that plasma norepinephrine concentration following the first day of exercise in the heat was 1.58 ± 0.22 ng/ml. It significantly decreased to 1.01 ± 0.20, 0.98 ± 0.15, and 0.89 ± 0.11 on days 3, 5, and 8, respectively. The results of the current study show that active HA causes a rapid and significant reduction in NE during exercise in the heat. Such a result suggests that SNA was likewise reduced during HA and may be partially responsible for the reductions in HR that occur with HA since end-exercise HR and NE were found to be highly correlated (r=0.79).
authors.affiliation: Department of Biology and the School of Exercise and Nutritional Sciences, San Diego State University, San Diego, CA 92182-7251, United States.
authors.forename: Daniel
authors.initials: D
authors.lastname: Hodge
authors.forename: Douglas
authors.initials: D
authors.lastname: Jones
authors.forename: Ricardo
authors.initials: R
authors.lastname: Martinez
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Buono
mesh_terms: D000064:Acclimatization / Q000502:physiology*; D000328:Adult; D015415:Biomarkers; D001831:Body Temperature / Q000502:physiology*; D001833:Body Temperature Regulation / Q000502:physiology*; D005080:Exercise Test / Q000379:methods*; D005260:Female; D006358:Hot Temperature / Q000009:adverse effects; D006801:Humans; D008297:Male; D009638:Norepinephrine / Q000097:blood*; D013997:Time Factors; D016138:Walking / Q000502:physiology; D055815:Young Adult
chemical_list: D015415:Biomarkers; D009638:Norepinephrine
keywords: Exercise; Heat acclimation; Sympathetic nervous system