Lithium and free water clearance (C-Li and C-H2O) were measured simult
aneously in 5 patients with Bartter's syndrome (BS), and the results w
ere compared with values obtained in a large group of healthy control
subjects. In line with the literature, fractional diluting segment rea
bsorption estimated from C-H2O appeared to be subnormal in BS (82.4 +/
- 4.7 versus 89.4 +/- 4.4% in controls, p < 0.01). Fractional delivery
to this segment, expressed as the maximum urine flow during water diu
resis (V-max/glomerular filtration rate, GFR), also appeared to be red
uced (11.3 +/- 1.6 versus 13.8 +/- 2.5%, p < 0.05), suggesting compens
atory increased proximal reabsorption. In disagreement with the latter
, C-Li/GFR, alleged to be a measure of fractional sodium and water out
put from the proximal tubules, was increased (36.2 +/- 1.8 versus 30.7
+/- 5.1% in controls, p < 0.01), suggesting that suppressed proximal
reabsorption could be the primary reabsorption defect in BS. During ch
ronic converting enzyme inhibition with enalapril, given to treat hypo
kalemia (3 patients), the concordance between the two methods was rest
ored because C-Li/GFR fell by approximately 50%, whereas V-Max/GFR did
not change; the alleged diluting segment rabsorption defect remained
present. These contradictory results lead to the conclusion that one o
f the two methods reflected the pattern of tubular sodium handling in
BS incorrectly. However, the present study does not unequivocally indi
cate which method yielded erroneous results in this condition.