Changes in plasma catecholamines in response to variable rectal temperature during prolonged exercise and passive heating

All subjects exercised on the bicycle ergometer at approximately 60 percent of their maximal oxygen uptake on two occasions, two days apart. On one testing day the subjects wore a nylon shell and cotton pants to retard evaporation and thus impose a heat load. In the second exercise test the subjects wore only shorts while fans were directed on the subjects during the ride. The order of the experimental treatments was counterbalanced in hopes of preventing an ordering effect. Measurements of oxygen uptake, rated perceived exertion, and withdrawal of blood samples were taken at rest, after a five-minute warmup, and every 15 minutes during the 45-minute exercise task.

Approximately two weeks following completion of the exercise tests, four of the five subjects took part in a passive heating experiment in a warm water bath (41°c). Physiological measures and blood samples were again taken at rest and at two subsequent times when the individual subject's T_re had reached the level obtained within the final measurement periods during the exercise test with the nylon shell.

Both epinephrine and norepinephrine increased across time during exercise. The shell treatment produced higher plasma concentrations of epinephrine in four out of five subjects (P> 0.11) when compared to the fan treatment. Plasma norepinephrine levels were greater during the shell test in all subjects (P<0.003) when compared to exercise during the fan condition.

Although increases in rectal temperature in both exercise and passive heating were found to be associated with changes in plasma catecholamine levels, the correlations and differences in the slopes between treatments suggest that other factors must be involved in the increase in plasma catecholamine concentrations during exercise. The measured variable with closest association to plasma catecholamine levels was heart rate. Plasma catecholamines changed as a curvilinear function of heart rate, independent of the experimental treatment. This suggests that the rise in plasma catecholamines and heart rate during exercise and thermal stress may reflect the degree of vasoconstriction in the "nonactive" tissues. This catecholamine and heart rate association ensures the efficacy of the body to maintain the integrity of the systemic blood pressure in the face of increased demands of the skin and exercising muscle for blood.