1. In order to examine the handling of sodium by the nephrotic kidney when separated from the immediate influences of renal nerves and humoral factors, kidneys were taken from nephrotic rats (puromycin aminonucleo-side) and studied over a range of perfusion pressures using the isolated perfused kidney technique.
2. When perfused with medium containing 6.7 g/dl albumin, the nephrotic kidneys performed differently from controls with a reduction in sodium excretion at all pressures [(mean ± sem) 1.14 ± 0.43 vs 4.20 ± 0.69 μmol/min at 105 mmHg (14 kPa); 6.32 ± 1.56 vs 44.60 ± 5.30 μmol/min at 150 mmHg (20 kPa)]. Renal vascular resistance, inulin clearance, fractional sodium excretion and fractional lithium excretion were also reduced.
3. When kidneys were perfused without oncotic agent these differences between nephrotic and control kidneys remained. Perfusion with medium containing 10 g/dl albumin, designed to prevent glomerular filtration, abolished the difference in vascular resistance between the two groups. Captopril had no effect on the sodium retention or vascular resistance of nephrotic kidneys.
4. It was concluded that (a) the isolated nephrotic kidney demonstrates increased avidity for sodium, (b) the abnormality of sodium handling is not dependent on the presence of altered oncotic forces, and (c) the alteration in vascular resistance is conditional upon glomerular filtration.