MECHANISM OF ATTENUATED THIRST IN AGING - ROLE OF CENTRAL VOLUME RECEPTORS

To test the hypothesis that the inhibitory action of central blood vol ume expansion on thirst and renal fluid regulation is attenuated with aging, we monitored the drinking and renal responses of dehydrated old er (70 +/- 2 yr, n = 6) and younger (24 +/- 1 yr, n = 6) subjects duri ng 195 min of head-out water immersion (HOI), which shifts blood centr ally and increases plasma volume (PV). Subjects dehydrated by exercisi ng for 2 h at 36 degrees C in the evening and refraining from fluids o vernight before HOI in 34 degrees C water or a seated control in water perfusion suit [time control (TC)] the next morning. Ad libitum water intake was allowed after 15 min of HOI. Dehydration decreased PV by 1 0.6 +/- 1 and 7.3 +/- 1.8% (P < 0.05) and increased plasma osmolality by 6 +/- 2 and 7 +/- 1 mosmol/kg H2O (P < 0.05) in older and younger s ubjects, respectively. Thirst ratings increased in both groups, but pr e-HOI thirst perception on a line rating scale was lower in older (69 +/- 8 mm) than younger (94 +/- 6 mm, P < 0.05) subjects. Fifteen minut es of HOI restored PV by 718 +/- 1.0 and 5.7 +/- 1.0% in older and you nger subjects, respectively, but suppressed thirst rating in younger s ubjects only (P < 0.05). Fluid intake was reduced in HOI compared with TC in younger (6.3 +/- 0.5 vs. 14.3 +/- 2.2 ml/kg, P < 0.05) but not in older (6.7 +/- 2.1 vs. 8.4 +/- 3.3 ml/kg) subjects. During HOI, old er subjects had smaller suppression of plasma renin activity and aldos terone concentration but a greater increase in the plasma atrial natri uretic peptide concentration (P-[ANP], P < 0.05). HOI increased fracti onal sodium excretion in both groups, but mean arterial pressure incre ased only in the older subjects (P < 0.05). We conclude that the inhib itory influence of central volume expansion on thirst and drinking beh avior is diminished with aging. Furthermore, in contrast to younger pe ople, HOI natriuresis is associated with exaggerated increases in P-([ ANP]) and arterial blood pressure in older people, suggesting arterial baroreceptors may be involved in the fluid regulatory response to cen tral blood volume expansion in older people.