The influence of uremia on fractional sodium reabsorption by the proximal tubule of rats

The influence of uremia on fractional sodium reabsorption by the proximal tubule. Micropuncture and clearance studies were performed on four groups of rats. All had one remnant kidney in which the nephron population was reduced by approximately 80%. In the first two groups, the contralateral kidney was intact; in one of these, micropuncture studies were performed on the intact kidneys and in the other on the remnant kidneys. In the remaining two groups the intact kidneys were removed and the remnant kidneys were studied at one of two time intervals: 1) within 50 hours of removing the intact kidneys; 2) two to three weeks post-nephrectomy. Free flow micropuncture techniques were used to measure single nephron GFR and proximal tubular sodium reabsorption by superficial nephrons. All animals were maintained on a constant daily salt intake by tube feeding. During the studies sodium was infused at a rate adjusted to produce a modest natriuresis. In both acutely and chronically uremic animals fractional sodium excretion was markedly greater than in control animals with the same estimated degree of ECF volume expansion. Single nephron GFR was increased in the uremic animals but absolute sodium reabsorption was not significantly different from the values in the control group. A substantial reduction in fractional sodium reabsorption by proximal tubules was observed. The data suggest that both hyperfiltration and inhibition of proximal tubular sodium reabsorption contributed to the increased rate of sodium excretion per nephron in the uremic rats