Glomerular Filtration, Renal Blood Flow, and Solute Excretion in Conscious Aging Rats

Measurement of daily food and water intakes and of renal water and solute excretion in 10-, 20-, and 30-mo-old female rats revealed that they ate the same amounts of food and excreted the same amounts of glucose, sodium, and potassium irrespective of age but that in the 30-mo-old rats phosphate, calcium, and magnesium excretion rose. The glomerular filtration rate and p-aminohippuric acid clearance measured in conscious animals following moderate volume expansion did not differ with age, when expressed per 100 g body wt, but decreased markedly in the 30-mo-old rats when compared with kidney weight. This was related to the senile hypertrophy of the kidneys in the 30-mo-old group, which was reflected by increases in glomerular volume and length of the proximal tubules and by enlargement of the medulla, with the number of nephrons remaining constant with age. There was no evidence of glomerulosclerosis or kidney disease, and proteinuria remained at a low level in all three age groups. These results suggest that the diminished renal filtration rate observed in both conscious and anesthetized senescent rats is due to a reduced supply of blood per gram of kidney tissue and not to loss of nephrons.