Response to isotonic saline infusion after selective renal cortical ablation in the rat

The response to progressive volume expansion with isotonic saline was studied in sham-operated (CS) rats and in rats with reduced nephron mass after unilateral removal of cortical tissue (CX) or amputation of the kidney poles (NX). Glomerular filtration rate (GFR) in CS kidneys was not statistically different from that in control kidneys. CX and NX kidneys had GFR values markedly lower than control and CS kidneys. CX kidneys had FENa similar to that of control and CS kidneys until very high rates of urine flow were reached (volume expansion greater than 15% of body weight), when it was greater in CX than in CS or control kidneys (25 +/- 14% vs. 11 +/- 2%; p less than 0.05). The same relationship was found between NX kidneys and their controls: FENa only exceeded the values in control kidneys at high rates of urine flow. A defect in free water reabsorption was present in CX when compared to CS and control kidneys, while NX kidneys had TcH2O values which were not different from control kidneys. This different response of CX and NX kidneys suggests that the behavior of CX kidneys cannot be attributed simply to a nonspecific reduction of renal mass. Instead, these results suggest a heterogeneity of nephron function between superficial short-loop nephrons and deep long-loop nephrons primarily with respect to water handling.