Heat stress and hydration may both alter plasma volume (PV) responses during acute exercise; potential interactions have not been fully studied. The purpose of this study was to determine the effect of graded elevations in skin temperature (Tsk) on PV changes during steady-state exercise under conditions of euhydration (EU) and hypohydration (HYPO, -4% of body mass). Thirty-two men (22 ± 4 yr) were divided into four cohorts (n = 8 each) and completed EU and HYPO trials in one environment [ambient temperature (Ta) 10, 20, 30, and 40°C]. Thirty minutes of cycle ergometry (50% V̇o2peak) was performed. Core (Tre) and mean skin (Tsk) temperatures were measured; changes in PV, total circulating protein (TCP), and mean arterial pressure (MAP) were calculated; and skin blood flow (SkBF) was estimated. Hypohydration decreased (P < 0.05) PV by 200 ml (-5.7%) but did not alter TCP. Plasma loss was not different between EU and HYPO during exercise at any Ta. Plasma losses were greater (P < 0.05) with elevated Ta with an average -130, -174, -294, and -445 ml losses during the 10, 20, 30, and 40°C trials, respectively. Significant (P < 0.05) correlations (r = 0.50 to 0.84) were found between ΔTCP and ΔPV during exercise when Tsk was cool/warm (<33°C; Ta 10, 20, and 30°C), but not at 40°C (high Tsk). We conclude that 1) graded skin warming proportionally accentuated plasma loss; 2) plasma loss was associated with plasma protein efflux at lower Tsk and SkBF; 3) at high Tsk, additional plasma loss likely results from increased net filtration at the capillaries; and 4) HYPO did not alter vascular fluid loss during exercise in any environment.
Keywords: hydration state; plasma volume loss; vasculature fluid shifts.