A differential catecholamine response during prolonged exercise and passive heating

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Abstract
Plasma catecholamines (Ca) increase over time during prolonged exercise at a constant work rate. Possible reasons for this include hypoglycemia, dehydration, increased perceived exertion and rising body temperature. In an effort to ascertain the contribution of elevated core temperature on the plasma Ca response to prolonged exercise 4 trained men were studied under 2 thermal conditions during 45 min of bicycle exercise (.apprx. 60% .ovrhdot.VO2 max) and during passive heating. On one testing day each subject was heat-stressed during exercise wearing a nylon pullover shell and cotton pants, (heat stress treatment); on the 2nd day fans were directed on the subjects exercising in a thermoneutral environment (fan treatment). The passive heating experiment was conducted in a hot water bath (41.degree. C). The exercise heat stress treatment produced a higher plasma norepinephrine (NE) concentration (P < 0.05) in all subjects than the fan treatment. The plasma epinephrine (E) concentration was not significantly (P > 0.05) different during exercise in the heat stress treatment from that found during fan treatment. A rise in rectal temperature (Tre) during passive heating resulted in quantitatively small changes in plasma E and NE. The heat load combined with the exercise task produced an increase in plasma NE concentration greater than the sum of the values measured during exercise in the thermoneutral environment and passive heating. This suggested that the combination of stressors caused an augmentation of the sympathetic nervous system response. Finally, the added heat load caused changes in plasma NE and little change in plasma E, suggesting a differential Ca response with heat stress.

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