AJP - Regulatory, Integrative and Comparative Physiology, Vol 270, Issue 1 71-R79, Copyright © 1996 by American Physiological Society

ARTICLES

Age and regulation of fluid and electrolyte balance during repeated exercise sessions

D. H. Zappe, G. W. Bell, H. Swartzentruber, R. F. Wideman and W. L. Kenney
Noll Physiological Research Center, Pennsylvania State University, University Park 16802, USA.

A common response after only 3-4 days of repeated exercise in younger individuals is an expansion of plasma volume (PV); however, it is not known if older individuals have a similar response. In this study, six older (O) (67 +/- 1 yr) and six younger (Y) men (24 +/- 2 yr) cycled for 4 successive days at 50% maximal oxygen consumption (Vo2max) for 90 min in a warm environment [30 degrees C temperature dry bulb (Tdb), 24 degrees C temperature wet bulb (Twb)]. On day 4, PV was increased (P < 0.05) in Y (10.0 +/- 1%) but not (P > 0.05) in O (1.7 +/- 2%). The increased PV was associated with a greater (P < 0.05) daily fluid intake during the exercise period in Y (45 +/- 3 ml. day-1.kg body wt-1) compared with O (32 +/- 2 ml.day-1.kg body wt-1) and an increase (P < 0.05) in the total circulating protein (TCP) content in Y (0.23 +/- 0.1 g/kg body wt) but not in O (0.10 +/- 0.1 g/kg body wt). Throughout the 4-day exercise period there were similar reductions in 24-h urine flow rate (UV) and urinary sodium excretion (UNaV) in Y and O. Additionally, acute renal clearance measures made during exercise on days 1 and 4 showed similar (P > 0.05) reductions in UNaV between Y (-55 +/- 10%) and O (-44 +/- 6%). However, during exercise in O there were no changes (P > 0.05) in UV (2 +/- 12%) and urine osmolality (UOsm) (-12 +/- 6%) from resting values compared with Y, where UV was decreased (P < 0.05) by 41 +/- 9% and UOsm was increased (P < 0.05) by 39 +/- 8%. Therefore, the inability of the older subjects to increase PV after repeated days of exercise is not related to an impaired renal fluid and Na+ conservation ability, despite a reduced urine concentrating ability during exercise, but to other factors (e.g., fluid intake and TCP) that appear necessary for the hypervolemic response.

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