Age-related changes in plasma aldosterone and corticosterone concentrations as well as in plasma renin activity in response to 10 days of sodium deprivation were studied in old as compared to adult male Long-Evans rats. Chronic sodium deprivation greatly increased plasma concentrations of aldosterone both in old and in adult animals. However, this effect was significantly higher (+85.2%) in old (+3,574 pmol/l) as compared to adult (+1,820 pmol/l) rats. Concomitantly, adrenal weights were statistically increased in sodium-deprived old rats (+25%) whereas they were unchanged in adult animals; plasma corticosterone concentration was unchanged by sodium restriction in the two age groups. Because a putative modest decline with age of the metabolic clearance rate of aldosterone could not account totally for such an important increase in plasma concentration, it is assumed that it is, in its most part, due to an increased production. Furthermore, although plasma renin activity of senescent rats, fed either a normal or a sodium-deprived diet, was lower as compared to adult rats, the absolute and percent increases of this activity in response to sodium deprivation were, respectively, similar and higher in old as compared to adult rats and so could partially contribute to the higher aldosterone response.

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