Comparative effects of acute volume expansion and short-term training on thermal and cardiovascular responses to prolonged exercise.

To investigate the hypothesis that increases in plasma volume (PV) are crucial to the cardiovascular and thermal adaptations resulting from training, eight moderately active males (Vo2max = 4.10 +/- 0.32 L/min; mean +/- SE) performed prolonged cycle exercise at 60% Vo2max during a control test (CON) and following infusion (10 mL/kg) of a 6% dextran solution (DEX). These responses were also compared with short-term training (TR) involving 3 days of cycling for 90-120 min at moderate intensity (60% Vo2max). During DEX, exercise cardiac output (Q) and stroke volume (SV) were persistently higher (p < 0.05) than CON, while heart rate (HR) was unchanged. In comparison, TR resulted in a lower (p < 0.05) HR at constant Q. In contrast with TR, in which the exercise response was unchanged from CON, the DEX condition produced a lower total peripheral resistance. Rectal temperature (Tre) was lowered (p < 0.05) both by DEX and TR, but the conditions differed in the time at which the reduction occurred. For DEX, the lower Tre was manifested early and persisted throughout exercise, whereas for TR the Tre was only lower later in exercise. Forearm blood flow, mean skin temperature, and sweat rate were not affected by DEX or TR. It is concluded that acute PV expansion does not affect the time-dependent response in cardiovascular function or thermoregulation during prolonged exercise in ambient conditions. The primary effects appear to be manifested during rest or soon after the onset of exercise.