Age and regulation of fluid and electrolyte balance during repeated
exercise sessions.
Zappe, D. H., G. W. Bell, H. Swartzentruber, R. F. Wideman, and W. L.
Kenney.
Noll Physiological Research Center, The Pennsylvania State
University, University Park, PA 16802
APStracts 2:0208R, 1995.
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, 6
older (O) (67+/-1 yrs) and 6 younger (Y) men (24+/-2 yrs) cycled for
4 successive days at 50% O2max for 90 min in a warm environment (30
degrees C Tdb, 24 degrees C 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 x d-1 x kg
BW-1) compared to O (32+/-2 ml x d-1 x kg BW-1) and an increase
(p<0.05) in the total circulating protein (TCP) content in Y
(0.23+/-0.1 g x kg BW-1) but not in O (0.10+/-0.1 g x kg BW-1).
Throughout the 4 day exercise period there were similar reductions in
24-h urine flow rate (U) and urinary sodium excretion (UNa x ) in Y
and O. Additionally, acute renal clearance measures, made during
exercise on day 1 and 4, showed similar (p>0.05) reductions in
UNa x between Y (-55+/-10%) and O (-44+/-6%). However, during
exercise in O there were no changes (p>0.05) in U (2+/-12%) and
urine osmolality (UOsm) (-12+/-6%) from resting values compared to Y
where U 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) which appear necessary for
the hypervolemic response.
Received 1 May 1995; accepted in final form 18 July 1995.
APS Manuscript Number R258-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 July 1995.