Effects of urea on electrolyte transport in the dog kidney

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.