The patterns of phosphate excretion in man with advancing chronic renal disease are well established. Although phosphate clearance decreases with time, it falls proportionately less than glomerular filtration rate, and the ratio of phosphate clearance to GFR increases as the disease advances (1). Thus the average rate of phosphate excretion per residual nephron increases as the nephron population diminishes, and the onset of hyperphosphatemia thereby is delayed. The explanation for this sequence is of considerable theoretical interest. The major possibilities are two: 1) The change could reflect the operation of a control system geared either to maintain external phosphate balance or to maintain plasma phosphate concentrations constant; 2) the relative phosphaturia per nephron could be a consequence of uremia or of the abnormalities in residual nephrons or both. These studies represent an attempt to distinguish between these two interpretations using the dog with intrinsic renal disease as the experimental model. The experiments also were designed to delineate the general characteristics of a control system, should one exist. phate intake of at least 0.5 g. A preliminary bladdersplitting operation was performed to permit the quantitative collection of urine from the individual kidneys without recourse to ureteral catheterization (2). Thereafter, unilateral pyelonephritis was induced by a technique described previously (3). In five dogs, the diseased kidney was studied in the presence of the control kidney, and the functions of the two organs were compared. These experiments were similar in design to those described in a previous publication (4) and were performed to verify the previous results in the present animals. In 27 dogs, the control kidneys were removed to permit investigation of the diseased kidneys under environmental conditions varying from moderate azotemia to severe uremia. Methods

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