Clinical evaluation of segmental tubular reabsorption of sodium and fluid in man: lithium vs free water clearances.

Segmental sodium reabsorption in the proximal and distal tubule was evaluated by different methods in seven healthy subjects, seven patients with recurrent calcium nephrolithiasis, five patients with isolated renal glucosuria and three patients with Fanconi syndrome. In all the subjects, the delivery of fluid from the proximal tubule, evaluated as 'chloride' factor during maximal water diuresis (DDCl, 12.4 +/- 5.5 ml/dl GFR), was lower (P less than 0.001) than 'volume' or 'chloride' factors during maximal water diuresis plus frusemide administration (40 mg i.v.) (Vf, 22.5 +/- 7.5 and DDClf, 27.1 +/- 8.9 ml/dl GFR, respectively), and of lithium clearance (FELi, 28.1 +/- 12.6%). Vf was lower than DDClf (P less than 0.001) and FELi (P less than 0.005), while DDClf and FELi did not differ; these unequal results are likely to represent different degrees of free water back-diffusion along distal tubule segments in the free water clearance studies. Accordingly, estimation of sodium reabsorption in the distal tubule showed corresponding differences within the four methods as those observed for the distal delivery: it was 25.5 +/- 12.2% when evaluated as [FELi-FECl]; 24.8 +/- 8.3 ml/dl GFR when evaluated as [CH2Of/GFR + delta FECl] (i.e. free water clearance during frusemide plus the frusemide-induced absolute increase in FECl); 19.5 +/- 6.7 ml/dl GFR (P less than 0.001 vs [FELi-FECl] and [CH2Of/GFR + delta FECl] when evaluated as [(CH2O + CH2OBD)/GFR] (i.e. CH2O before frusemide plus the frusemide-induced absolute increase in urine flow rate); and 10.0 +/- 4.8 ml/dl GFR when evaluated as CH2O (P less than 0.001 vs all the other evaluations).(ABSTRACT TRUNCATED AT 250 WORDS)