Effect of age on renal conservation of phosphate in the rat

Renal handling of phosphate (Pi) was examined in male Wistar-derived rats, 2-3, 6, 12, 18, and 24 mo of age. We observed a significant age-related phosphaturia [i.e., elevated urinary excretion (UPi V) and fractional excretion (FEPi)] in rats fed a normal phosphorus diet (NPD; 0.5% Pi). Concomitantly, plasma Pi decreased significantly and progressively with age. The mechanism of this age-related decrement in Pi conservation was examined by determining the initial (5 s) rate of Na+ gradient-dependent uptake of Pi in renal brush-border membrane vesicles (BBMV). Pi uptake significantly declined with increasing age. No consistent age-related decrease was seen in the Na+ gradient-dependent uptakes of glucose and proline by the same BBMV preparations, demonstrating the specificity of the Pi transport decrement. Pi transport kinetics revealed a significant age-related decrease in Vmax. No difference in Km of Pi was seen between age groups. These kinetic findings suggest either a decreased number of Pi carriers or decreased turnover of Pi carriers. Elevated parathyroid hormone did not explain the alteration in Pi conservation since urinary cAMP was not elevated in the intact senescent rat, and Pi uptake was not normalized in 24-mo-old rats 3 days after parathyroidectomy. The senescent 24-mo-old rat as well as the young adult 6-mo-old animal adapted to a low-phosphorus diet (LPD; 0.1% Pi) with a striking (greater than 100%) increase in Pi uptake by BBMV compared with NPD. thus the senescent kidney retained the capacity to respond appropriately to a LPD.(ABSTRACT TRUNCATED AT 250 WORDS)