Urinary concentration and dilution after unilateral nephrectomy in the rat.

1. The effects of unilateral nephrectomy on urinary concentration and dilution were studied in Sprague-Dawley rats. To exclude incomplete suppression of antidiuretic hormone, free water formation was also sutdied in rats with congenital diabetes insipidus (Brattleboro strain). 2. Urinary solute-free water formation was similar in Sprague-Dawley and Brattleboro rats. Uninephrectomized animals excreted a water load promptly and diluted their urine to the same degree as control rats. Maximal values for Cwater and TCwater per kidney were higher after nephrectomy, but were similar to those of control rats at comparable rates of fluid delivery to the distal nephron. Renal tissue osmolaity was similar in uninephrectomized and sham-operated animals, indicating that nonantidiuretic hormone-dependent backflux of filtrate was the same in the two groups. The only defect observed in uninephrectomized animals was a small reduction in maximal urine osmolaity. 3. These results demonstrate that free water formation and reabsorption are unaffected by unilateral nephrectomy and suggest that, in the remaining kidney, filtrate reabsorption along the entire nephron increases in proportion to the increment in glomerular filtration.