This review focuses on the response of „stress” hormones to heat, exercise (single
or repeated bouts), and combinations of these stimuli, with particular reference to
their impact upon immune function. Very hot conditions induce a typical stress response,
with secretion of catecholamines and Cortisol. The catecholamines induce a demargination
of leukocytes, and Cortisol subsequently causes cells to migrate to lymphoid tissue.
Sustained exercise, even in a thermally comfortable environment, induces a larger
hormonal response than moderate thermal stress. With moderate exercise, increases
in leukocyte numbers are related mainly to plasma norepinephrine concentrations, but
with more intense exercise epinephrine concentrations assume a major importance. As
exercise continues, plasma Cortisol levels also rise, inducing an influx of neutrophils
from bone marrow and an efflux of other leukocyte subsets. A combination of exercise
and heat stress augments both hormonal and leukocyte responses. But these changes
seem to be reversed if temperatures are clamped by exercising in cold water. If a
second bout of excercise is performed with an inter-test interval of 30 - 45 min,
neither hormone concentrations nor immune responses show any great cumulative effect
under temperate conditions. However, in a hot environment the second exercise bout
induces a larger and more persistent neutrophilia. Training influences these various
responses mainly by decreasing the stress imposed when exercising at a given absolute
work-rate.
Key words
Cortisol - epinephrine - exercise stress - growth hormone - leukocytes - norepinephrine
- repeated exercise - thermal stress
