To investigate the time course and mechanism of the increase in blood volume (BV) during isotonic exercise training, blood hemoglobin, hematocrit, and plasma volume (PV), osmotic, electrolyte, renin activity (PRA), vasopressin (AVP), and protein fractions were measured periodically in eight trained men 20-22 yr (Vo2max = 57 ml . min-1 . kg-1) before, during, and after ergometer exercise training (approximately 160 W, 65% Vo2max) for 2 h/day for 8 days. During training, plasma total osmolar and albumin contents increased to maintain a constant plasma osmolality and protein concentration during PV expansion. After training, BV increased by 457 ml (+8.1% P less than 0.05), due to an increase in PV of 427 ml (+12.1%, P less than 0.05); red cell volume was essentially constant (delta = +30 ml, NS). Plasma hypervolemia during training was associated with two major factors: 1) a ninefold elevation in PRA and AVP during exercise that facilitated Na+ and H2O retention, and 2) a progressive, chronic increase in plasma albumin content that provided increased H2O-binding capacity for the blood. Thus an efficient procedure for increasing PV is the daily performance of high-intensity isotonic leg exercise (65% Vo2max) for 2 h/day.

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Plasma volume (PV), protein, and electrolyte shifts were measured in two groups of five men in the supine position during rest, exercise (40-47% VO2max), and recovery in cool (Ta = 26.5 degrees C) and hot (Ta = 39.4 degrees C) environments. They drank 16-17 ml/kg hypertonic (1.5%) NaCl, isotonic (0.9%) NaCl, or hypertonic (1.5%) calcium gluconate solutions during the preexercise rest period. Hyper-Na consumption retarded the rise in PV (hypervolemia) at rest in the cool but not in the heat. Ingestion of iso-Na and hyper-Na resulted in twofold greater hypervolemia at rest in the heat. During exercise and recovery, PV was highest with hyper-Na in the heat. Hyper-Ca prevented the normal hypervolemic response at rest in both environments. The normal hypervolemic responses during exercise were not influenced by any drink composition. The results suggest hypertonic drinks may be better for maintaining PV levels during exercise in heat; but calcium must be used with care.

To determine the function of changes in plasma volume (PV), plasma renin activity (PRA), and arginine vasopressin (AVP) in the mechanism of the reduction of resting blood pressure during exercise training, resting supine, sitting, and standing systolic (SBP) and 5th-phase diastolic (DBP) blood pressures were measured in 10 men (19-24 yr) before and after an 8-day (2 h/day) training period on a cycle ergometer. The control group (5 men) exercised at 1.4 1/min [44% peak O2 uptake (VO2 max)] at 23.8 degrees C Tdb and 50% rh, and the acclimation group at 1.5 1/min (46% VO2 max) at 39.8 degrees C Tdb and 50% rh. After acclimation, resting supine and sitting DPB decreased (P less than 0.05) by 6 and 9 mmHg, respectively. There were no significant changes in DBP in the controls or in SBP in either group. After training, PV increased by 12.2% in controls and by 17.6% after acclimation. The resting hypotension could not be attributed to changes in resting levels of PV, AVP, or PRA. However, large decreases in PV and large increases in AVP and especially PRA during acclimation exposures suggest these responses may play a role in the chronic hypotensive response.