BODY-FLUID BALANCE IN DEHYDRATED HEALTHY OLDER MEN - THIRST AND RENALOSMOREGULATION

We examined osmotic control of thirst and free water clearance in heal thy older (65+, n = 10) and younger (Y, n = 6) subjects during a 3-h r ehydration period after an similar to 2.4% decrease in body weight. Pl asma volume (PV), plasma osmolality (P-osm), renal function, and thirs t were measured before and after dehydration and during rehydration. I n 65+, baseline PV was lower (43.1 +/- 1.6 vs. 48.1 +/- 2.5 ml/kg), P- osm was higher (287 +/- 1 vs. 281 +/- 2 mosmol/kgH(2)O), and perceived thirst was lower than in Y. During dehydration, the osmotic threshold for increased thirst was shifted to a higher P-osm in 65+. Total flui d intake was greater in Y than in 65+ (16.6 +/- 4.1 vs. 8.9 +/- 2.0 ml /kg); however, the relation between thirst and the rate of fluid intak e was identical. Thus the blunted rehydration in 65+ is related to a l ower overall sensation of thirst. The stimulus-response characteristic s of osmotic control of free water clearance was similar in 65+ and Y; however, 65+ operated around a higher P-osm and on a less-steep porti on of the stimulus-response curve. These data support the hypothesis t hat the hyperosmotic hypovolemic state of healthy older individuals is not a result of a simple water deficit but represents a shift in the operating point for control of body fluid volume and composition.