Effects of Atropine and Β-blockade on temperature regulation and performance during prolonged exercise

Summary

The effects of intravenous injections of Atropine (1.8 mg) and practolol (15 mg) on the thermoregulatory responses to 1 h of exercise on a motordriven treadmill have been investigated on six healthy subjects.

The results show that Β-blockade had little effect on thermal responses to work except for a small but significant (p<0.05) decrease in mean skin temperature (¯T _sk ) and peripheral tissue heat conductance (K). Metabolic (M) and total heat (H) production, and evaporative sweat loss (E) and rectal temperature (T _re ) were similar to control values. In contrast, atropine, particularly at work loads beyond 60% maximal aerobic power output (VO_{2 max}), raised T _re (p<0.001), ¯T _sk (p<0.001) and reduced E by approximately 50%. At the highest work loads T _re increased as a linear function of time during the latter part of exercise, and at the 60th min was almost independent of relative stress (expressed as % VO_{2 max}) imposed on the subjects. At the lower work loads the majority of subjects reached thermal equilibrium before the end of exercise by maintaining their convective heat transfer from core to periphery by increasing peripheral blood flow (as indicated by K), and raising their heat losses to environment by convection and radiation. The latter pathways for heat dissipation were enhanced by the subjects ability to sustain a ¯T _sk ∼4‡ C above control values independently of M. Atropine had no effect on M or H but greatly affected work performance, no subject was able to exercise at loads >70% VO_{2 max} for 1 h. These results demonstrate the ability of the thermoregulatory system to adapt to Β-adrenergic and to parasympathetic blockade during light exercise, and underline the effects of a reduction in the capacity of the sweating mechanism on physiological performance at higher rates of work.