Renal reabsorption of phosphate during development: tubular events

Studies performed in our laboratory on the isolated perfused kidney of the guinea pig have demonstrated that the rate of Pi reabsorption is substantially greater in the newborn than in the adult, when appropriate corrections are being made either for differences in glomerular filtration rate (GFR) or in renal tubular mass. In order to determine the location of this enhanced reabsorption along the nephron, micropuncture experiments were performed on euvolemic, non-fasted guinea pigs 5-14 and 42-49 days of age, maintained on standard guinea-pig chow diet (0.76% Pi). Concomitant measurements of overall kidney function were also obtained. The results confirmed that fractional reabsorption of Pi (TRPi%) across the entire kidney was significantly higher (P<0.01) in the newborn (89.93±2.55%) than in the adult (78.25±2.89%) animals. The difference was also significant (P<0.05) when TRPi was expressed in mol/ml GFR (1.87±0.14 vs 1.53±0.12, respectively). At comparable locations along the proximal tubule (TF/P_in of 1.90±0.16 in the newborn, and 1.79±0.15 in the adult, P>0.70), the fraction of the filtered load of Pi reabsorbed was significantly higher (P<0.001) in the immature (76.66±2.74%) than in the mature (67.21±2.74%) guinea pigs. Estimates based on the differences between proximal Pi reabsorption and the urinary excretion of Pi indicate that the reabsorption of Pi in tubular segments located beyond the proximal tubule is also enhanced in the newborn when compared with the adult (15.62±2.11% vs 10.51±1.83%, respectively, P<0.05). Alternatively, this finding may be interpreted to represent a higher fractional reabsorption of Pi by deep nephrons.