EXPRESSION OF CHRONIC THYROPARATHYROIDECTOMY ON PHOSPHATE-TRANSPORT IN WHOLE KIDNEY AND PROXIMAL LUMINAL MEMBRANES DURING PHOSPHATE DEPRIVATION

Both parathyroid hormone (PTH) and the dietary supply of inorganic phosphate (Pi) influence the tubular Pi reabsorptive capacity. In this study we have assessed how removal of parathyroid gland affects the response of Pi transport to Pi restriction in the whole kidney and in brush border membrane vesicles (BBMV) isolated from the renal cortex. Intact and thyroparathyroidectomized (TPTX) rats were switched from normal to low Pi diet. The maximal tubular Pi reabsorption per volume of glomerular filtrate (max. TRPi/ml GF) and the initial (3, 6 and 9 s) Na+-dependent Pi uptake by BBMV were determined before and after 1, 2, 3 and 8 days of Pi restriction. The results indicate that before Pi restriction max. TRPi/ml GF was 1.94 .+-. 0.04 and 4.03 .+-. 0.14 .mu.ol/ml GF intact and TPTX rats respectively. The corresponding Na+-dependent Pi uptake (3 s value) was 137 .+-. 23 in sham and 272 .+-. 41 pmol/mg protein in TPTX rats. During Pi restriction the initial difference in max TRPi/ml GF remained constant throughout the time course study. At 8 days max. TRPi/ml GF was in intact 3.08 .+-. 0.09 and in TPTX 5.04 .+-. 0.18 .mu.mol/ml GF. In sharp contrast, in BBMV the initial difference between intact and TPTX became completely abolished 8 days after saturating the Pi restriction (SHAM = 425 .+-. 69; TPTX = 434 .+-. pmol/mg protein). In conclusion, the overall tubular Pi transport capacity and the Na+-dependent Pi transport system located in the luminal membrane of the proximal tubule appear to be quite differently affected by changes in the PTH status during Pi restriction.