Effect of expansion of extracellular fluid volume on the maximal reabsorptive capacity for inorganic phosphate in parathyroidectomized and intact rats.

Clearance studies were performed in order to examine the effect of expansion of extracellular fluid volume (ECFV) on the maximal reabsorptive capacity for inorganic phosphate (TmPi) in acutely parathyroidectomized (PTX) and intact rats. TmPi values were obtained in both control and volume expansion. In PTX rats, the TmPi values in control and expansion were 8.25 +/- 1.52 and 6.14 +/- 1.02 micronmol/min (mean values +/- S.D.), respectively; the TmPi/GFR values were 2.96 +/- 0.31 and 2.09 +/- 0.30 micronmol/ml, respectively. In intact rats, the TmPi values in control and expansion were 3.56 +/- 0.94 and 2.98 +/- 0.94 micronmol/min, respectively, and the TmPi/GFR values were 1.34 +/- 0.23 and 1.05 +/- 0.23 micronmol/ml, respectively. From these results it is concluded that expansion of ECFV decreases the TmPi values both in the absence and presence of parathyroid hormone.