Tubular handling of Pi in young growing and adult rats

Abstract

The tubular transport of Pi is controlled by a parathyroid hormone[PTH]-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. The maximal net Pi reabsorption per volume of glomerular filtrate (max TRPi/ml GF) in young growing (2 mo.) and adult (8 to 9 mo.) rats. Max TRPi/ml GF was significantly lower in intact adult (1.44 .+-. 0.06 .mu.mol/ml) compared with intact young growing animals (2.22 .+-. 0.12 .mu.mol/ml GF). The difference was maintained after removal of the thyroparathyroid glands: adult, 2.89 .+-. 0.25; young, 4.56 .+-. 0.25 .mu.mol/ml. It was not associated with a difference in the urinary excretion of cAMP, GFR [gloneralar filtration rate], renal handling of Na, plasma Ca, 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 P 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 response observed in young growing rats. The decrease in tubular reabsorption of Pi that occurs when rats become adult is a PTH-independent phenomenon. This change may be all adaptation of the tubular Pi transport to a reduction in the utilization of Pi in relation to the diminished growth rate of the animals.