Urine concentration during solute diuresis in potassium-depleted rabbits. Evidence for a defect in tubular sodium transport.

1. The relationship between osmolal clearance (C(osm)) and the reabsorption of solute-free water by the kidney (T(H2O) (c)) was examined during 10% mannitol and 2.3% saline diuresis in normal and potassium-depleted rabbits.2. In normal rabbits at osmolal clearances close to 3.0 ml./min, T(H2O) (c) during mannitol diuresis was 0.87 +/- 0.06 ml./min and during saline diuresis 1.19 +/- 0.07 ml./min. The mean difference in T(H2O) (c) of 0.32 +/- 0.05 ml./min was highly significant (P < 0.001).3. In one group of potassium-depleted rabbits with a reduction in maximal urinary concentration, T(H2O) (c) during both mannitol and saline diureses was reduced significantly below normal and the increment in T(H2O) (c) normally seen during saline diuresis was abolished.4. In a second group of potassium-depleted rabbits maximal urinary concentration (1253 +/- 88 m-osmole/kg H(2)O) was not significantly different from that in normal rabbits (1272 +/- 116 m-osmole/kg H(2)O). In these animals, T(H2O) (c) at osmolal clearances close to 3.0 ml./min was not significantly different during mannitol diuresis (0.83 +/- 0.07 ml./min) from that in normal animals, whereas it was reduced significantly during saline diuresis (0.89 +/- 0.07 ml./min, P < 0.001) and the difference in T(H2O) (c) normally seen between mannitol and saline diuresis was abolished.5. The inability to increase T(H2O) (c) during saline diuresis above that achieved during mannitol appears to be the earliest manifestation of the concentrating defect associated with potassium depletion. It probably results from an impairment of sodium transport by the ascending limb of the loop of Henle. This is supported by the fact that potassium-depleted rabbits excreted a greater percentage of the filtered load of sodium than did normal controls.