Medullary collecting duct function in the remnant kidney before and after volume expansion

The role of the medullary collecting ducts (CD) in the regulation of water and electrolyte excretion by the remnant kidney has not been determined. Medullary CD function was therefore studied by the microcatheterization technique during hydropenia and after volume expansion with an isotonic saline load in rats with sham-operated normal kidney (stage I), remnant kidney (stage II), remnant kidney after contralateral nephrectomy (stage III), and sham-operated normal kidney after contralateral nephrectomy (stage-III control). Sodium, potassium, water, and solute reabsorption along the medullary CD were not altered in stage II during hydropenia when compared to normal control (stage I), but water reabsorption proximal to the medullary CD was decreased. In stage III, where the uremia was mild (BUN, 41 mg/dl), the fractions of the filtered load of sodium (72%) and water (62%) which were reabsorbed along the medullary CD were reduced in comparison to stage II (94% and 83%, respectively), stage I, or stage-III control. The fraction of filtered potassium entering the medullary CD was increased to 53% in stage III, compared to 16% in stage II, and 10 or 11% in stage I or stage-III control. No change in the fraction of filtered potassium remaining along the collecting ducts was observed. After volume expansion, there was no significant change in the fraction of filtered sodium and water remaining along the medullary CD in stage I, II, or III. The greater fractional excretion in stage III resulted from decreased reabsorption in more proximal nephron segments. The results indicate that (a) during hydropenia, fractional reabsorption of sodium and water is decreased along the medullary CD of the stage-III remnant kidney in the presence of mild uremia, (b) the increased fractional excretion of potassium in the remnant kidney with uremia is not determined by altered potassium handling in the medullary CD but occurs proximal to this nephron segment, and (c) extracellular fluid volume expansion with isotonic saline results in similar inhibition of fractional sodium and water reabsorption in the collecting ducts of both remnant and normal kidneys.