Phosphate transport by rat renal brush border membrane vesicles: influence of dietary phosphate, thyroparathyroidectomy, and 1,25-dihydroxyvitamin D3.

In the present work we have investigated whether the changes in the renal handling of inorganic phosphate (Pi) induced by 1) dietary Pi, 2) removal of parathyroid glands and 3, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], are associated with alterations in the Na-dependent Pi uptake by brush border membrane vesicles (BBMV) isolated from renal cortex. Sham-operated (SHAM) or thyroparathyroidectomized (TPTX) rats treated or not with 26 pmol/day of 1,25(OH)2D3 i.p. were fed low (0.2%) or high (1.2%) P diet for 7 days. The results showed that in SHAM, TPTX and TPTX+1,25(OH)2D3 the Pi uptake by BBMV was greater after low than high Pi diet. It was greater in TPTX than in SHAM counterparts fed either diets. In TPTX fed low or high Pi diet 1,25(OH)2D3 decreased the Pi uptake to the level observed in SHAM. A striking parallelism was found between variations in Pi uptake by BBMV and in the tubular Pi reabsorption of the whole kidney. The Na-dependent glucose, the mannitol uptake by BBMV, and the alkaline phosphatase activity in cortical homogenates and BBMV were not affected by the various treatments. Thus, dietary Pi, chronic TPTX and 1,25(OH)2D3 appear to specifically affect the Na-dependent Pi transport system bound to the brush border membranes of renal cortical tubules. The alterations observed at this membrane level could account, at least in part, for the changes induced by these factors on the overall tubular reabsorption of Pi.