title: Age and regulation of fluid and electrolyte balance during repeated exercise sessions.
abstract: 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.
authors.affiliation: Noll Physiological Research Center, Pennsylvania State University, University Park 16802, USA.
authors.forename: D H
authors.initials: DH
authors.lastname: Zappe
authors.forename: G W
authors.initials: GW
authors.lastname: Bell
authors.forename: H
authors.initials: H
authors.lastname: Swartzentruber
authors.forename: R F
authors.initials: RF
authors.lastname: Wideman
authors.forename: W L
authors.initials: WL
authors.lastname: Kenney
mesh_terms: D000064:Acclimatization; D000293:Adolescent; D000368:Aged; D000375:Aging / Q000502:physiology*; D001826:Body Fluids / Q000378:metabolism*; D004231:Diuresis; D004326:Drinking; D004573:Electrolytes / Q000378:metabolism*; D006358:Hot Temperature; D006801:Humans; D007668:Kidney / Q000502:physiology; D008297:Male; D008875:Middle Aged; D009318:Natriuresis; D010101:Oxygen Consumption; D005082:Physical Exertion; D013997:Time Factors
chemical_list: D004573:Electrolytes