Vasopressin and renin response to dehydration in aged rats.

Abnormalities in neurohypophyseal function have been postulated to contribute to the alterations in fluid and electrolyte balance observed during aging. In this study, parameters of fluid and electrolyte balance were evaluated during chronic water deprivation in old (30 months) and young (3 months) Fischer 344 rats. The increase in serum vasopressin (VP) and renin concentrations observed in the 3 month animals following chronic water deprivation were absent in the aged rats (p less than 0.05 and p less than 0.02, respectively). This occurred in spite of apparently comparable alterations in fluid volume and osmolality (assessed by changes in body weight, hematocrit and plasma osmolality). Relative to body weight, VP content of the neural lobe was significantly reduced and was more severely depleted by dehydration in aged rats than in young rats. Thus, inadequate neurohypophyseal hormone stores may contribute to the inability of the aged animals to attain elevated serum VP concentrations during chronic stimulation. Several parameters of renal function were examined in the aged rats. Although none of the old rats were in renal failure, they all showed some indication of reduced renal function. In spite of renal abnormalities including reduced concentrating capabilities, the old rats did demonstrate a significant antidiuretic response to dehydration. However, with prolonged fluid deprivation, they were unable to attain serum VP or renin concentrations comparable to that achieved by the young rats.