AGE-ASSOCIATED ALTERATIONS IN THIRST AND ARGININE-VASOPRESSIN IN RESPONSE TO A WATER OR SODIUM LOAD

We have examined simultaneous changes in thirst, plasma osmolality and arginine vasopressin, after oral water loading or hypertonic saline i nfusion. The studies were carried out in the same subjects, comprising young controls aged 26.8 years (SD 4.8, n = 10) and health status-def ined elderly people aged 72.1 years (SD 3.1, n = 10). Water loading ca used significant falls in plasma osmolality (p < 0.001) and thirst (p < 0.001), but there was no variation with age. Infusion with 462 mmol/ l of sodium chloride increased plasma osmolality significantly (p < 0. 001), but there was no variation with age (p = 0.12). The perception o f thirst during the osmotic loading experiment was recorded differentl y by the two age groups (p < 0.0001). However, linear regression analy sis showed no age difference in the relationship between thirst and pl asma osmolality during osmotic loading. During osmotic loading the rel ationship between the plasma concentration of arginine vasopressin in response to increasing plasma osmolality varied significantly (slope: p = 0.02; intercept; p = 0.02). Plasma arginine vasopressin rose more rapidly with increasing plasma osmolality in old subjects.