Tubular adaptation to Pi restriction in hypophysectomized rats

The renal tubule adapts its tubular transport capacity for inorganic phosphate (Pi) in response to a reduction in the Pi supply. In order to assess whether growth hormone plays a critical role in this adaptive response we have studied the change in the tubular handling of Pi which follows Pi restriction in hypophysectomized (HPX) rats and compared it to that occurring in intact counterparts. HPX and intact rats were maintained either on a 1.2 g/100 g phosphorus diet or fed a 0.2 g/100 g phosphorus diet for 3, 6 or 12 days. HPX rats received ACTH and thyroxine in doses which normalize their low glomerular filtration rate (GFR). Then the maximal net Pi reabsorption per volume of glomerular filtrate (max. TRPi/ml GFR) were determined during acute Pi infusion by clearance technique. The results indicate that HPX rats responded to Pi restriction by raising their tubular capacity to reabsorb Pi. However, the rapidity and the magnitude of the adaptive response was significantly less in HPX than in intact rats. The adaptation to Pi restriction was also observed in HPX rats after thyroparathyroidectomy. It is concluded that growth hormone and other pituitary hormones do not play a key role in the adaptive response to Pi restriction. The reduced adaptive response observed in HPX rats with intact thyroparathyroid glands could be due to the decreased Pi demand consecutive to impaired growth.