Uncoupling of glomerular and tubular regulations of urea excretion in rat

Renal adjustments to diets varying in protein and sodium content were studied in young anesthetized Munich-Wistar rats. Diets were as follows: high (50%) protein (HP), normal (24%) protein (NP), reduced (8%) protein without extra salt (RPNS), and RP with extra salt (RPWS, 2.8% extra salt). Rats grew at the same rate on all diets. Glomerular filtration rate (GFR), urea, sodium, and osmolar clearances were measured. In series 1, groups of rats were maintained for 4 wk on the four diets and one group on changed diet (CP2d, RPWS diet for 4 wk changed to NP diet for 2 days). GFR was reduced compared with NP rats by 12% in RPNS and RPWS, and by 16% in CP2d rats, but the differences were not statistically significant. Fractional excretion of urea (FEurea) was significantly changed compared with NP. It was 40% higher in HP rats, and 50 and 56% lower in RPNS and RPWS, respectively. In the CP2d group it had increased to the NP value. In series 2, groups of rats were maintained on the RPNS diet for 1, 2, 3, and 4 wk, respectively. GFR decreased 21.6% after only 1 wk but after 4 wk it was not significantly lower than in the NP group. In contrast, no significant reduction was found in FEurea after 1 wk, whereas it decreased by 40% during the 2nd wk, with no further decrease after 3 and 4 wk. GFR was not directly related to protein or salt intake.(ABSTRACT TRUNCATED AT 250 WORDS)