Plasma volume, renin, and vasopressin responses to graded exercise after training

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Abstract
To investigate the effects of exercise training on fluid shifts and endocrine responses during exercise, plasma volume (PV), renin activity (PRA), osmolality (Osm) and vasopressin (PVP) concentrations were measured in venous blood obtained during rest and sitting ergometer exercise (100, 175 and 225 W) from 4 men before and after training on a cycle ergometer (2 h/day for 8 days, Tdb 25.degree. C, at 65% of their maximal O2 uptake [.ovrhdot.VO2 max]). After training, resting PV increased by 12.3% (440 ml, P < 0.05) and .ovrhdot.VO2 max increased by 11.2% (P < 0.05). Consequently, the relative work intensities at 100, 175 and 225 W decreased (P < 0.05) from 48, 75 and 89% of .ovrhdot.VO2 max before training to 42, 69 and 83% of .ovrhdot.VO2 max, respectively, after training. With increasing work intensity, PV decreased linearly while the PRA and Osm and PVP concentrations increased in a curvilinear fashion. Compared with pretraining values, the levels of Osm, PVP, PRA and the change (.DELTA.) in PV were reduced (P < 0.05) at 175 and 225 W after training and they were elevated (P < 0.05) above resting levels only when work intensity exceeded 50% .ovrhdot.VO2 max. The .DELTA. Osm was correlated with %.DELTA.PV (r = -0.80, P < 0.05) and with .DELTA.PVP (r = +0.88, P < 0.05). Fluid-endocrine response to the absolute work level is reduced by exercise training, while a similar relative exercise intensity produces similar PV, Osm, PVP and PRA changes independent of training level. A threshold of 50% .ovrhdot.VO2 max is required to increase PRA, Osm, and PVP concentrations during exercise.