Plasma Cortisol, Renin, and Aldosterone During an Intense Heat Acclimation Program

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Abstract
The physiologic responses to an intense heat acclimation (HA) regimen (treadmill, 41,2 .degree.C, 8 days, 56 min exercise/44 min rest) and the effects on stress and fluid balance hormone responses were examined in 13 unacclimated male volunteers. Venous blood samples were collected before (PRE) and after (POST) exercise (days 1, 4, 8) and analyzed for plasma renin activity (PRA), aldosterone (ALD), cortisol (PC), plasma volume shifts (SDPV%), sodium concentration (Na+), and potassium concentration (K+). HA responses (day 1 vs day 8) indicated reduced strain (P < 0.05): decreased heart rate, rectal temperature, skin temperature, improved defense of PV, and attenuated PC responses. While plasma Na+ demonstrated no change during daily exercise, K+ (P < 0.01), PC, PRA, and ALD increased (P < 0.05) more than .DELTA.PV% (day 1: -7%, day 8: -5.1%) accounted for. Na+ and K+ did not change as a result of HA, and there was no change in fluid balance hormones (e.g., PRA, ALD). It was concluded that this intense heat acclimation regimen reduced physiologic strain by mechanisms other than alterations in fluid balance hormones and offered few physiologic advantages which cannot be gained through conventional heat acclimation techniques (e.g., walking).