Effect of phosphate deprivation on renal phosphate transport in the dog

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Abstract
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 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 degree of phosphate deprivation. Acute infusion of phosphate to raise plasma phosphate slightly above normal did not completely restore responsiveness to ECVE. The adaptive response to phosphate deprivation occurred in proximal and distal nephron segments, and factors other than plasma phosphate were primarily responsible for such an adaptation.