Evidence of Restricted Permeability to Urea in Rat Distal Tubule. ∗

Abstract

Although urea is generally considered to be a highly permeant non-electrolyte, several lines of evidence have been produced to indicate that the distal part of the nephron has a restricted permeability to urea. The urea concentration of late distal samples collected by micropuncture of rat nephrons has been found to be highly variable, and it was suggested that the variability might be a reflection of varying degrees of permeability (1). When radio-inulin and radio-urea were injected into nephron segments, recovery in the urine was higher with distal tubular injection than with proximal(2). In the former study, interpretation is clouded by the fact that urea is added to the tubular fluid at the level of the thin loop of Henle(1). In addition, differences in contact time may contribute to explaining the results found in both studies. The studies of Capek et al(3) were of a more direct nature. These workers found that the permeability of the distal tubule to urea was 20 times less than the proximal tubule. The authors did not comment, however, on the physiological meaning of this observation and, in particular, did not evaluate if the reduced distal impermeability was low enough to account for accumulation of urea in distal samples.

In dogs, the renal tiiiue concentration of urea is increased during ureteral obstruction hut only in experiments characterized by high levels of stop-flow filtration and low medullary concentration of urea (i.e., NaCl diuresis). The tissue concentration approached the flee-flow levels shortly after release of the obitruction (4).

Intraluminal trapping during stop-flow would imply that the tubule would offer a restricted permeability to urea. Therefore, experiments using the micropuncture technique in rats were designed to test the hypothesis that the increase in the cortical concentration of urea is due to its intraluminal trapping in the distal tubules.