Tubular localization of adaptation to dietary phosphate in rats

The renal tubule of rats adapts its transport capacity for inorganic phosphate (Pi) in response to changes in dietary Pi intake. Tubular localization of this adaptation was studied in rats pair-fed for 10 days a low (LPD), i.e., 0.2 g/100 g, or a high (HPD), i.e., 1.8 g/100 g, phosphorus diet. Free-flow micropunctures under acute Pi infusion were made at elevated but similar filtered loads of Pi: LPD, 6.07 +/- 0.29 (n, 32); HPD 5.34 +/- 0.24 (n, 33) micromol/kidney per min. Fractional excretion of Pi (FEPi) for the whole kidney was: LPD, 0.26 +/- 0.02 (n, 32); HPD, 0.67 +/- 0.02 (n, 33) (P less than 0.001). The fraction of filtered Pi found at various puncture sites suggests that part of the adaptation to dietary Pi occurs in the early proximal tubule. An effect in the later portion of the proximal convoluted tubule cannot be excluded in the present experiments. Furthermore, dietary Pi affects markedly the difference in Pi delivery between distal tubule and final urine. This could be due to a greater adaptive response of deep nephrons. Alternatively, the results could also indicate that an adaptation with a secretory component takes place along the terminal part of the nephrons.