EFFECTS OF DRINKING ON THIRST AND VASOPRESSIN IN DEHYDRATED ELDERLY MEN

Inhibition of dehydration-induced arginine vasopressin (AVP) secretion and thirst depends on removal of osmotic and hemodynamic stimuli as w ell as on preabsorptive oropharyngeal factors that reduce thirst and A VP secretion on drinking before correction of the water deficits. Plas ma atrial natriuretic peptide (ANP) levels may also change with drinki ng. Therefore, the thirst and plasma responses to oral water loads (10 ml/kg) in 10 healthy old (64-76 yr) and young (20-32 yr) 24-h water-d eprived men were investigated. After 24-h water deprivation plasma sod ium, osmolality, and AVP were increased similarly in both groups (P < 0.001). Plasma ANP levels fell after dehydration similarly in both gro ups (P < 0.05) but were always higher in the older group (P < 0.05). H owever, although thirst increased in both groups (P < 0.05), this was significantly less in the elderly (P < 0.05). After the water load, th irst was reduced in both groups throughout the study (P < 0.05). Howev er, plasma AVP fell immediately after drinking only in the young group and rose to postdeprivation levels after 15 min. Plasma AVP was not d ifferent from postdeprivation throughout in the old group and after 15 min in the young group presumably because the water load was insuffic ient to replace their water deficits. In the young group only, plasma ANP rose to 182 +/- 43% of postdeprivation levels at 3 min after drink ing (P < 0.05). These results demonstrate reduced oropharyngeal inhibi tion of AVP secretion after drinking in healthy elderly men but mainta ined inhibition of thirst.