Renal response to low and high phosphate intake in weanling, adolescent and adult rats

The renal response to low and high phosphate intake was studied in weanling, young and adult rats. Weanling rats were started on experimental diets containing 0.37%, 0.7%, or 1.7% phosphate at 24 days and adult rats at 60 days of age. After 21 days, clearance studies were done in anaesthetized animals. Urine was collected during basal conditions and following a phosphate infusion. Urinary excretion of calcium, phosphate and creatinine, and plasma levels of phosphate and creatinine were determined. Plasma phosphate was slightly higher in the younger rats in all dietary groups but was not influenced by phosphate intake in either age group. Urinary phosphate excretion and fractional phosphate excretion increased significantly in both age groups with increasing phosphate intake. After high phosphate intake, both net and fractional phosphate excretions were significantly higher in younger rats (0.97 +/- 0.08 and 0.24 +/- 0.06 mumol min-1 100 g-1, P less than 0.01, and 47.5 +/- 3.84 and 18.15 +/- 5.59%, P less than 0.01, respectively). The urinary excretion of calcium related to creatinine was higher in younger rats in all dietary groups with the highest value found after low phosphate intake. During an acute phosphate infusion, fractional phosphate excretion increased significantly in both age groups after normal phosphate intake but remained unchanged after low or high phosphate intake. Plasma phosphate increased significantly only in younger rats with high phosphate intake (2.9 +/- 0.18, 3.88 +/- 0.43, P less than 0.05). It is suggested that hypercalciuria reflects early stages of phosphate depletion and that in young rats stabilized on a high phosphate intake, phosphate retention may occur during an acute phosphate load.