Tubular handling of Pi in young growing and adult rats.

The tubular transport of inorganic phosphate (Pi) is controlled by a parathyroid hormone-independent mechanism that responds to variations in the Pi intake. This adaptation mechanism could also respond to growth-mediated variation in the utilization of Pi by the organism. In the present work we have determined the maximal net Pi reabsorption per volume of glomerular filtrate (max TRPi/ml GF) in the young growing (2-mo) and adult 8- to 9-mo) rats. Max TRP[i/ml GF was significantly lower in intact adult (1.44 +/- 0.06 mumol/ml) compared with intact young growing animals (2.22 +/- 0.12 mumol/ml GF). This difference was maintained after removal of the thyroparathyroid glands; adult, 2.89 +/- 0.25, young, 4.56 +/- 0.25 mumol/ml. It was not associated with a difference in the urinary excretion of cAMP, GFR, renal handling of sodium, plasma calcium, or acid-base status. Administration of growth hormone preparations to adult rats did not raise max TRPi/ml GF to the level observed in young intact animals. With regard to the tubular Pi adaptation to Pi restriction, lowering the phosphorus content in the diet from 0.8 to 0.2 g/100 g resulted in an attenuated and delayed enhancement in max TRPi/ml in adult as compared with the response observed in young growing rats. These results show that the decrease in tubular reabsorption of Pi that occurs when rats become adult in a parathyroid hormone-independent phenomenon. It is suggested that this change is an adaptation of the tubular Pi transport to a reduction in the utilization of Pi in relation to the diminished growth rate of the animals.