Clinical Evaluation of Segmental Tubular Reabsorption of Sodium and Fluid in Man: Lithium vs Free Water Clearances

摘要

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<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). Vfwas lower than DDClf(P<0.001) and FELi(P<0.005), while DDClfand FELidid 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±12.2when evaluated as FELi−FECl; 24.8±8.3 ml/dl GFR when evaluated as CH2Of/GFR+ΔFECl (i.e. free water clearance during frusemide plus the frusemide-induced absolute increase in FECl); 19.5±6.7 ml/dl GFR (P<0.001 vs FELi−FECl and CH2Of/GFR+ΔFECl when evaluated as (CH2O+CH2OBD)/GFR (i.e. CH2Obefore frusemide plus the frusemide-induced absolute increase in urine flow rate); and 10.0±4.8 ml/dl GFR when evaluated as CH2O(P<0.001 vs all the other evaluations).FELishowed a mean 40increase after frusemide administration; the percentage increase correlated significantly with the percentage increase of chloride excretion (r=0.83;P<0.001), but not of phosphate excretion. These results would suggest that either frusemide substantially increases distal delivery (so that free water clearance studies should not be taken as a good index of proximal tubule sodium reabsorption), or that lithium itself might undergo reabsorption in post-proximal sites (so that even lithium might not be a specific proximal