Time course of the attenuation of sympathetic nervous activity during active heat acclimation

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).