Three-phase re-collection
micropuncture experiments were undertaken to study the effect of 2.5% and 5%
urea infusion on tubular handling of
sodium,
potassium,
calcium, magnesium, phosphate, and
chloride in 10 acutely parathyroidectomized dogs. The fractional excretions of water and
electrolytes were increased in response to graded
urea infusion. The late proximal TF/P
inulin fell from 1.59 to 1.25, but there was no change in TF/UF Osm, TF/UF
calcium, or TF/P
sodium, so that the proximal fractional reabsorption of water decreased by 12% and that of
sodium and
calcium by 11% and 13%, respectively. Proximal TF/UF
magnesium fell from 1.33 to 1.16, and fractional
magnesium reabsorption decreased by 9%. Distal TF/UF Osm increased from 0.31 in control to 0.67 with 2.5%
urea to 0.80 with 5%
urea infusion. Distal TF/P
inulin ratios fell strikingly (3.89 to 2.05 to 1.52), accompanied by similar increases in TF/P
sodium (0.24 to 0.46 to 0.57) and TF/UF
calcium (0.31 to 0.51 to 0.62), whereas TF/UF
magnesium did not change (0.90 to 0.79 to 0.94). The fraction of
potassium remaining at the distal tubule exceeded that measured at the late proximal tubule, indicating
potassium secretion between the proximal and distal
puncture sites during
urea administration.
Urea-induced
phosphaturia was mainly a result of decreased proximal
phosphate reabsorption, with additional inhibition occurring beyond the proximal sampling site. Thus
urea infusion (1) inhibits reabsorption of
sodium,
potassium, and
calcium proportionately more than of
magnesium in the proximal tubule, (2) inhibits
sodium,
calcium, and
magnesium reabsorption in the loop, (3) promotes
potassium secretion into the pars recta of descending limb in the loop of Henle, (4) has little effect on ion transport beyond the distal sampling site, and (5) causes a slight
phosphaturia at both levels of
urea infusion by inhibiting proximal
phosphate reabsorption.