Phosphorus Excretion in Renal Failure1

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Abstract
The excretion of P was studied in 20 males whose renal function ranged from normal through various gradations of impairment including terminal uremia. In an attempt to elucidate the mechanism of P excretion in the urine as kidney function became progressively worse, simultaneous measurements of inulin and P clearances were performed before and after intravenous injection of 500 units of parathyroid hormone (PTH). Glomerular filtration rates (GFR) varied from 142 to 1.5 ml/minute. With but one exception, the serum P remained within the normal range of 2.5 - 4.5 mg% until the GFR was reduced to about 25 ml/ minute. As GFR was further reduced the concentration of serum P increased markedly. P filtered at the glomerulus (GFP) was calculated as the product of the concentration of inorganic P in serum and the volume of glomerular filtrate. P reabsorbed by the tubules (TRP) was assumed to be the difference between the filtered P and the urinary P (UP). Evidence for TRP was found, except in terminal uremia, when GFP closely approximated UP, suggesting a quantitative transfer of P filtered at the glomeruli to the urine. Giving PTH caused an increase in UP which was apparently due both to an increase in GFR and a decrease in TRP. The responsiveness to PTH was lost in renal disease when the GFR was reduced below 10 ml/minute. With further renal impairment it was impossible to determine whether there was a preexisting maximum effect of endogenous PTH or whether the tubules failed to respond.