Renal and systemic adaptation to sodium restriction in aging rats

The influence of age on the systemic and renal adaptation to dietary sodium restriction was assessed in 10-, 20-, and 30-mo-old female WAG/Rij rats. In control conditions, mean arterial pressure (MAP) was similar in all rats and plasma renin activity (PRA) was lower in 30- than in 10- and 20-mo-old rats (2.5 +/- 0.6, 5.1 +/- 0.4, and 3.9 +/- 1.0 ng ANG I.ml-1.h-1, respectively). Dietary sodium restriction was associated with a reduction in MAP in 30-mo-old rats, whereas no change occurred in 10- and 20-mo-old rats. Impairment in the early (days 1-6) renal adaptation to salt restriction was observed in 30- compared with 10- and 20-mo-old rats (6-day cumulative sodium excretion of 728 +/- 139, 437 +/- 53, and 478 +/- 37 mumol, respectively). During the 7- to 12-day period, MAP stabilized in the oldest rats and cumulative sodium excretion became similar to that of other age groups. The early increase in PRA and urinary aldosterone excretion observed in 10- and 20-mo-old rats was consistently blunted in 30-mo-old rats. These findings suggest that the delayed response of the renin-angiotensin-aldosterone system has a major role in the impaired renal and systemic adaptation to dietary sodium removal in senescent rats.