Plasma Hormonal Responses at Graded Hypohydration Levels during Exercise-Heat Stress

摘要

The effects of graded levels of hypohydration (3, 5, and 7% of body weight) on hormonal responses to exercise in the heat were examined in six heat-acclimated male volunteers. On the day following dehydration, subjects performed light (approx. 25% maximal O2 consumption, 1.03 1/min) exercise in a hot (49 C, 20% relative humidity) environment for four consecutive 25-min intervals interspaced by 10-min rests; blood was obtained before exercise and at approx. 10 min before completion of each exercise period. During euhydration, plasma cortisol (PC) levels manifested significant decrements over time (e.g., time 0, 14.2 micrograms.100/ml vs. time 2, 8.9 micrograms.100/ml), probably related to its diurnal periodicity. However, during hypohydration, levels of PC were increased and correlated with hypohydration intensity (e.g., time 0, 0,3,5, and 7% hypohydration, 14.2, 16.5, 19.8, and 36.2 micrograms.100/ml, respectively). Plasma renin activity (PRA) was increased significantly by hypohydration (e.g., time 0, euhydrated vs. 3%, 3.7 vs. 6.2 units) but was unaffected by exercise in the heat. Plasma aldosterone (ALD) levels were generally increased by exercise in the heat (e.g., time 0 vs time 4, 3% hypohydration, 12.1 vs. 18.7 ng.100/ml). Regression analysis illustrated that graded intensities of hypohydration were correlated with incremented PRA and ALD through 5% hypohydration. Conversely, PC was incrementally elevated through 7% hypohydration. These results demonstrated that graded levels of hypohydration and exercise in the heat both increase hormonal levels following heat acclimation.