Tubular adaptation of inorganic phosphate (Pi) transport in response to variations in dietary Pi in rats.

The renal response to variations in the dietary intake of inorganic phosphate (Pi) has been studied in intact and thyroparathyroidectomized (TPTX) rats. The results show that the renal tubule can markedly vary its capacity to transport Pi according to homeostatic requirements: the lower the prior dietary intake, the higher the capacity of the tubule to reabsorb Pi. This adaptive response also occurs in TPTX rats. It is observed as early as three days after varying the dietary intake of Pi. This diet-induced difference in the tubular transport capacity for Pi can be seen in the presence of identical plasma Ca and urinary pH and also during marked expansion of extracellular volume. Comparison between Pi intake and thyroparathyroidectomy indicates that the fractional excretion of Pi measured at similar [Pi] Pl. can vary more than 100 times according to the prior dietary intake, whereas the removal of the thyroparathyroid glands brings about only a change of 2-4 times in this parameter. Free-flow micropunctures done in sham-operated rats pair-fed diets containing either 0.2 or 1.8 g/100 g P show a difference in Pi handling along the early proximal and distal tubule. However the most striking alteration seems to take place along the terminal nephron where an apparent net secretion of Pi would occur in the rats fed the high Pi diet. In conclusion the renal tubule adapts its transport capacity for Pi according to the supply of Pi in the diet. This involves a PTH-independent mechanism of great adaptive capability.