Effect of phosphate deprivation on renal phosphate transport in the dog

In order to examine the role of dietary intake of phosphate in regulating renal phosphate transport, clearance and micropuncture studies were performed in 25 dogs with various duration of phosphate deprivation induced by low phosphate diet and aluminum hydroxide gel. In phosphate deprivation of 17-41 days duration, the phosphaturic response to extracellular volume expansion (ECVE) was blunted in the intact group and was virtually abolished in the acutely thyroparathyroidectomized (TPTX) group. With longer phosphate deprivation of 53-110 days, no phosphaturia occurred after ECVE or administration of parathyroid hormone (PTH), even with intact parathyroids. These alterations in phosphaturic responses did not correlate with the plasma phosphate but rather with the duration of phosphate deprivation. Fractional proximal tubule phosphate reabsorption was enhanced in phosphate deprivation and the proximal tubule fluid-to-ultrafilterable phosphate ratio appeared to be a good index for the degree of phosphate deprivation. Acute infusion of phosphate to raise plasma phosphate slightly above normal did not completely restore the responsiveness to ECVE. It is concluded that the adaptive response to phosphate deprivation occurs in both proximal and distal nephron segments and that factors other than plasma phosphate are primarily responsible for such an adaptation.