In antidiuretic rats plasma urea concentration (Pu)was elevated up to 35 mM by intravenous urea loading. In renal cortical tissue (T) water content and unlabeled (u) and 14-C-urea (u*) were determined.
Water content of total renal cortex and of superficial and juxtamedullary layers were not significantly different and were not changed by the elevation ofPu.
In the control group (Pu<10 mM), bothT/Pu(3.06±0.71;n=29) and plasma specific activity were significantly higher than values forT/Pu* (2.20±0.57;n=12) and tissue specific activity, respectively.T/PuandT/Pu* were decreased significantly to the same level (1.68±0.35;n=20 and 1.51±0.21;n=7, respectively) atPu>20 mM and plasma and tissue specific activity became equal.
Deproteination of tissue homogenates did not depress 14-C-activity.
JuxtamedullaryT/Puwas significantly higher than superficialT/Pu.The difference was independent ofPu.
The data suggest, that renal cortical tissue urea concentration is determined (i) by distal tubular fluid urea concentration, which is inversely correlated withPuand (ii) by a constant amount of urea in an extraluminal compartment not readily exchanging.
The mechanism by which urea concentration in this extraluminal compartment is maintained independent ofPumight be urea neogenesis or urea binding to cellular structures or proteins.