Tubular handling of Pi: localization of effects of 1,25(OH)2D3 and dietary Pi in TPTX rats.

Previous clearance studies have shown that chronic administration (26 pmol/day i.p. for 7 days) of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) decreases the tubular reabsorptive capacity for inorganic phosphate (Pi) in thyroparathyroidectomized (TPTX) rats. In the present study the tubular localization of this effect was examined by free-flow micropuncture in TPTX rats. At the mentioned dosage, 1,25(OH)2D3 inhibited net Pi reabsorption in the early portion of the proximal tubule. In addition, 1,25(OH)2D3 treatment altered the difference in Pi delivery between the distal tubule and the final urine, suggesting an inhibition of net Pi reabsorption along the terminal portion of the nephron, or, alternatively, admixture of tubular fluid with higher Pi concentration from deep nephrons. Finally, in TPTX rats the tubular localization of the effect of varying the dietary Pi content was found to be quite similar to that of 1,25(OH)2D3.