Renal tubular reabsorption of urea in normal and protein-depleted rats

Abstract

The concentration of inulin and urea was measured in samples of proximal and distal tubular fluid from normal and protein-depleted rats. The concentration of urea rose above that of plasma very early in the proximal tubule and showed a further rise in the distal tubule and collecting ducts of the normal rats. Proximal and distal TF/P [tubularfluid/plasma] urea ratios in protein-depleted rats were comparable to those in normal rats. However, in contrast to normal rats, the urea TF/P ratio fell from 6.9 (SD [plus or minus] 1.7) in the distal tubule to 5.2 (SD [plus or minus] 0.9) by the end of the collecting ducts. The papillary tissue water urea concentration in protein-depleted rats was consistently higher than that in final urine. The data strongly suggest that urea is actively reabsorbed from the collecting ducts of protein-depleted rats. However, the possibility that a portion of the papillary urea may exist in a bound or inactive form could not be excluded.