Douglas M. Jones

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).

Blocking the beta-adrenergic system does not affect sweat gland function during heat acclimation

The purpose of the current study was to test the hypothesis that the beta-adrenergic innervation of the human eccrine sweat gland facilitates greater sweat production following heat acclimation. Eight healthy subjects (mean ± SD age: 25.1 ± 4.1 years, weight: 79.0 ± 16.1 kg, and VO(2)max: 48.5 ± 8.0 ml/kg/min) underwent active heat acclimation by walking at 40% of their VO(2)max for 8 days (90 min a day) in an environmental chamber (35.3 ± 0.8°C and 40.2 ± 2.1% rH). To test the hypothesis, the adrenergic component of sweat gland innervation was inhibited by continuously administering a 0.5% solution of the beta-adrenergic antagonist propranolol via iontophoresis to a 5 cm(2) area of one forearm during each 90-min exercise bout. The opposing control forearm underwent iontophoresis with a saline solution. Following heat acclimation, mean sweat rate in the inhibited and control forearm was 0.47 ± 0.30 mg/cm(2)/min and 0.44 ± 0.25mg/cm(2)/min, respectively. Findings of the current study fail to support the hypothesis that adrenergic innervation facilitates human eccrine sweat gland function during heat acclimation, as no significant differences in sweating were observed. In light of the above, the physiological significance of the dual cholinergic and adrenergic innervation of the eccrine sweat gland has yet to be determined.

Cold acclimation and cognitive performance: A review

Athletes, occupational workers, and military personnel experience cold temperatures through cold air exposure or cold water immersion, both of which impair cognitive performance. Prior work has shown that neurophysiological pathways may be sensitive to the effects of temperature acclimation and, therefore, cold acclimation may be a potential strategy to attenuate cold-induced cognitive impairments for populations that are frequently exposed to cold environments. This review provides an overview of studies that examine repeated cold stress, cold acclimation, and measurements of cognitive performance to determine whether or not cold acclimation provides beneficial protection against cold-induced cognitive performance decrements. Studies included in this review assessed cognitive measures of reaction time, attention, logical reasoning, information processing, and memory. Repeated cold stress, with or without evidence of cold acclimation, appears to offer no added benefit of improving cognitive performance. However, research in this area is greatly lacking and, therefore, it is difficult to draw any definitive conclusions regarding the use of cold acclimation to improve cognitive performance during subsequent cold exposures. Given the current state of minimal knowledge on this topic, athletes, occupational workers, and military commands looking to specifically enhance cognitive performance in cold environments would likely not be advised to spend the time and effort required to become acclimated to cold. However, as more knowledge becomes available in this area, recommendations may change.