Renal tubular effects of chronic phosphate depletion

The effects of chronic phosphate depletion on renal tubular function were evaluated by micropuncture and free water clearance studies in the dog. Proximal tubular punctures demonstrated that chronic hypophosphatemia led to a reduction in ratio of tubular fluid to plasma inulin in late superficial tubular from 1.59+/-0.08 in control animals to 1.29+/-0.06 in phosphate-depleted dogs, with proportional inhibition of calcium and sodium reabsorption. The chronic decrease in proximal tubular fluid reabsorption was confirmed by the analysis of sustained water diuresis in conscious, phosphate-depleted dogs, before and after repletion of body PO4 stores, and in control animals. Urine flow rate/100 ml glomerular filtration rate (V/GFR) was significantly higher in PO4 DEPLETION THAN CONTROL (15.8+/-1.1 VS. 10.7+/-0.82). In addition, acetazolamide infusion did not increase V/GFR in phosphate-depleted dogs (15.8+/-1.1 vs. 17.16+/-0.9), supporting the conclusion that inhibition of proximal tubular fluid reabsorption was responsible for the elevated urine flow rate. PO4 repletion over 5 days reduced V/GFR to 9.2+/-0.7 despite no change in urine osmolality and no change in GFR, further suggesting a specific reversible alteration in proximal tubular reabsorption in phosphate depletion. Although hypercalciuria was a constant finding in phosphate depletion (fractional excretion of calcium of 2.04+/-0.4% vs. 0.47+/-0.13% in controls), the enhanced distal delivery of calcium was not a crucial factor; acute phosphate infusion reduced urinary calcium excretion to control values without affecting the reduced proximal tubular reabsorption in either intact or thyroparathyroidectomized phosphate-depleted dogs the change in distal nephron calcium reabsorption was independent of parathyroid hormone (PTH) levels since infusion of PTH failed to alter urinary calcium excretion. We conclude that chronic phosphate depletion leads to a reversible, sustained inhibition in proximal tubular reabsorptive fuction as well as a specific decrease in distal nephron calcium reabsorption. This latter reabsorptive defect is sensitive to phosplate infusion but not corrected by PTH.