Sodium transport in dogs with acute remnant and glomerulonephritic kidneys

The nephron site responsible for the different patterns of sodium excretion in response to extracellular volume expansion observed in dogs with bilateral acute remnant kidneys, acute NTN, and normal kidneys was studied with clearance and micropuncture techniques. Mean kidney GFR's in the remnant and NTN kidneys were similar at 14 and 15 ml/min, respectively, compared to 28 ml/min for the normal kdineys. However, SNGFR's were normal in the remnant kidneys but markedly reduced in dogs with NTN. Mean absolute sodium excretion was similar for the remnant and normal kidneys both during the control phase and after volume expansion. Because of the reduced GFR, FENa of the remnant kidneys was significantly higher in each situation. In contrast, mean absolute sodium excretion was markedly less in dogs with NTN than in normal dogs both before and after volume expansion. Although FENa during the control phase was similar to that in normal dogs, it increased significantly less with volume expansion. Despite these differences in urinary sodium excretion, the percent sodium reabsorption at the end of the proximal convoluted tubule was similar in all three groups. In addition, volume expansion depressed proximal SFENa to the same degree in each group. Therefore the different patterns of urinary FENa were the result of differences in fractional sodium reabsorption by the nephron segments distal to the proximal convoluted tubule. Decreased distal delivery of sodium secondary to the reduced SNGFR also contributed to the decreased sodium excretion in acute NTN.