Regulation of the Tubular Transport of Phosphate in the Rat: Role of Parathyroid Hormone and 1,25-Dihydroxyvitamin D3
In the growing rat, the kidney responds to variation in the supply (1,2) and probably also to the demand (3) of the organism for inoganic phosphate (Pi) by changing its tubular capacity to transport Pi. The adaptive response in reducing the net tubular reabsorption of Pi by animals fed a high Pi diet can be observed in both intact (1) and thyroparathyroidectomized (TPTX) rats (1,2). However, after thyroparathyroidectomy the capability of the renal tubule to adapt to a high Pi intake is reduced (4). The disappearance of the direct influence by parathyroid hormone (PTH) on the tubular Pi transport may explain this phenomenon. However, removal of the parathyroid glands leads to a reduced production of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) (5), a metabolite which influences markedly Pi homeostasis (6,7). Therefore the question arises whether or not the diminished production of 1,25-(OH)2D3 in TPTX rats contributes to the reduced tubular response due to a high Pi supply. To investigate this problem, the influence of 1,25(OH)2D3 on the renal handling of Pi was studied in TPTX rats which were fed a high (1.2 g %) or low (0.2 g %) phosphorus diet. As reported in detail elsewhere (8) 1,25-(OH)2D3 was administered chronically at the dose of 13 pmol twice daily during the 7 days preceding the clearance study. This dose was selected because it normalized without overcorrecting the low intestinal Ca (9) and P (10) absorption in TPTX rats.
KeywordsParathyroid Hormone Parathyroid Gland Fractional Excretion Renal Handling Tubular Transport
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