Urea concentrations in tubular fluid and in renal tissue of nondiuretic rats

Abstract

Concentrations of urea and of inulin or polyfructosan were measured in proximal and distal tubular fluid of nondiuretic rats [plasma] (Purea 5.5 [plus or minus] 0.6 mM) and rats infused with small doses of urea (Purea 11.8 [plus or minus] 1.1 mM). Papillas and samples of cortical tissue were simultaneously analyzed for urea. Fractional water reabsorption was similar to that found by other investigators in nondiuretic rats. Infusion of urea induced a slight rise in urine flow and shortened the passage time of distal tubular fluid. Proximal tubular urea TF/P [tubular fluid/plasma] and fractional reabsorption were similar to those measured by other investigators. Approximately 50% of the filtered urea was reabsorbed from proximal tubules. Evidence for the entrance of urea into tubular fluid between late proximal and early distal tubules was present in 25-33% of fluid specimens from the first half of the distal tubules. Recircula-tion of urea from collecting ducts, thus, did not appear to be quantitatively important. TF/P values and fractional reabsorption of urea in the urea-infused rats were similar to those in unloaded animals. Papillary tissue water urea concentration rose with increasing plasma levels of urea, but always remained lower than in final urine. Urea concentration in cortical tissue water exceeded that in plasma by a factor of 5 in unloaded rats, and by a factor of 3 in rats infused with urea. In addition it was always 2-3 times higher than in proximal tubular fluid from any site. It also exceeded the concentration of urea in most samples of distal tubular fluid in unloaded rats, but was equal to the mean distal tubular fluid concentration in infused animals. The data appear to demonstrate a high urea concentration in tubular cells, which may be established either by active transport or by sequestration or inactivation of urea.