Micropuncture and clearance measurements of segmental reabsorption by the rat nephron

Background. We wanted to verify whether the calculations of segmental tubular reabsorption obtained during water diuresis were supported by direct micropuncture measurements. Methods. Experiments were performed on 18 rats during baseline water diuresis (B) and after the administration of frusemide (F), 10 mg/kg, by whole-kidney clearance measurements and micropuncture collections from early distal (ED) and last proximal (LP) tubular segments. Results. GFR was 957 ± 79 in B, 1053 ± 77 μl/min in F, P > 0.13. SNGFR was 38 ± 1 in 166 and 38 ± 1 nl/min in 165 tubules respectively, P > 0.77. In LP collections the percentage reabsorption was 71 ± 2 in B and 76 ± 2% during F (P > 0.07) in 99 and 95 samples respectively. The absolute proximal reabsorption was not changed by F (27.6 ± 1.5 versus 27.7 ± 1.3 nl/min, P > 0.96). The data were superimposable when the analysis was restricted to paired data. The difference between ED and LP resorption was 17 ± 3 during B and fell significantly (P < 0.008) to 5 ± 3% during F. The percentage of GFR excreted during F, measured by clearance techniques, and the percentage delivery of filtrate beyond the proximal tubule, measured independently by micropuncture, were not different (27 ± 2 versus 24 ± 2%, P > 0.10), while they were significantly correlated (P < 0.04). The calculations of segmental Na reabsorption along the different nephron segments by clearance techniques were not statistically different from and were significantly correlated with the reabsorptions measured directly by micropuncture. Conclusions. The present experiments validate the calculations of reabsorption by techniques applicable to human studies of clinical physiology.