MICROPUNCTURE AND CLEARANCE MEASUREMENTS OF SEGMENTAL REABSORPTION BYTHE 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 administr ation of frusemide (F), 10 mg/kg, by whole-kidney clearance measuremen ts and micropuncture collections from early distal (ED) and last proxi mal (LP) tubular segments. Results. GFR was 957+/-79 in B, 1053+/-77 m u l/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 reabso rption was 71+/-2 in B and 76+/-2% during F (P>0.07) in 99 and 95 samp les respectively. The absolute proximal reabsorption was not changed b y F (27.6+/-1.5 versus 27.7+/-1.3 nl/min, P>0.96). The data were super imposable when the analysis was restricted to paired data. The differe nce between ED and LP resorption was 17+/-3 during B and fell signific antly (P<0.008) to 5+/-3% during F. The percentage of GFR excreted dur ing F, measured by clearance techniques, and the percentage delivery o f filtrate beyond the proximal tubule, measured independently by micro puncture, were not different (27+/-2 versus 24+/-2%, P>0.10), while th ey were significantly correlated (P<0.04). The calculations of segment al Na reabsorption along the different nephron segments by clearance t echniques were not statistically different from and were significantly correlated with the reabsorptions measured directly by micropuncture. Conclusions. The present experiments validate the calculations of rea bsorption by techniques applicable to human studies of clinical physio logy.