Age-associated alterations in thirst and arginine vasopressin in response to a water or sodium load

Age and Ageing,  March, 1995  by Ioan Davies,  Paul A. O'Neill,  Kathleen A. McLean,  James Catania,  Derek Bennett

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Age-associated changes in thirst: There was no age-associated difference in the overall recording of thirst during the water-loading experiment. This confirms a recent study of thirst in healthy elderly men [20], showing that after a water load thirst was reduced in both young and old groups. According to Phillips et al. [20], since plasma AVP fell immediately after drinking only in the young group, the results demonstrate reduced oropharyngeal inhibition of AVP secretion after drinking in healthy elderly men but maintained inhibition of thirst. During the osmotic loading used in the study reported here, a significant difference was detected in the way that the young and old age groups recorded their thirst over the complete time course of the experiment. In the osmotic loading protocol we observed a marked fall in the thirst response of the young subjects after drinking, but not of the old. There was no age-associated difference in the slopes, or intercepts, of the relationship between increasing plasma osmolality and rising thirst scores (in both water loading and during the first 3 hours of osmotic loading). However, we detected change in the thirst response between young and old subjects only when the elderly people were asked to score their thirst response during rapidly changing conditions. There are interesting parallels with studies on temperature regulation where old subjects respond slowly to environmental fluctuations [21]. Thus, this study only partially supports previous reports of a change in thirst with age [8, 10, 11], since we detected an alteration only in the recording of thirst between the two age groups investigated, not in the relationship between rising plasma osmolality and thirst.

The following discussion is restricted to a comparison of the findings produced by the hypertonic saline infusion, which is a well-validated technique for increasing plasma osmolality and stimulating thirst [22-24]. We have found, like others [16], a straight-line relationship between thirst and plasma osmolality with an x-intercept of around 290 mOsm/kg and a slope of 0.42. In the investigation by Phillips et al. [8] using this technique the published x-intercept was 261 mOsm/kg for the young subjects and 276 mOsm/kg for the old. The slopes were 2.75 for the young and 1.38 for the old group [8]. These data are different from previously published material for young subjects. In the study reported here there was no difference in the slopes of the relationship between thirst and plasma osmolality for young and old subjects although variation was higher in the former. This may be explained by the more careful screening of the elderly group and their familiarity with scientific studies, although they had no previous experience of these methods. Furthermore, we could not show a difference in the drinking behaviour of the two age groups because almost identical volumes of fluid were drunk by both age groups after the cessation of the infusion. There is doubt in our minds about the precise relationship between VAS and plasma osmolality, particularly when drinking after an osmotic load leads to a prompt drop in VAS score, without a change in plasma osmolality.