Age-related changes in the pressure diuresis and natriuresis response.

The renal-excretory responses to changes in renal perfusion pressure (RPP) were studied in anesthetized young (3 mo old), adult (12 mo old), and senescent (24 mo old) rats to evaluate whether the pressure diuresis and natriuresis mechanism is altered as a function of age. Experiments were performed in anesthetized animals in which nervous and systemic hormonal influences to the kidney were fixed. Mean arterial pressure was similar in all three groups: 97.6 +/- 2.6, 102.1 +/- 3.7, and 95.2 +/- 5.2 mmHg in young, adult, and senescent rats, respectively. The relationships between RPP and diuresis/natriuresis or fractional excretions of water and sodium were similar in young and adult rats. However, in senescent rats the pressure-diuretic and pressure-natriuretic responses were slightly shifted to the right, so that diuresis and natriuresis were significantly lower at higher levels of RPP. Glomerular filtration rate was well autoregulated, and there were no differences between young and adult rats at each level of RPP. However, a significantly lower glomerular filtration rate was observed in senescent rats. These results indicate an age-related decline in the pressure-dependent sodium and water excretion that appears to be due to a decrease in glomerular filtration and an increase in tubular sodium reabsorption.