Segmental chloride reabsorption during volume expansion and recovery

Clearance and micropuncture studies were performed in Sprague-Dawley and Munich-Wistar rats to examine the nephron sites of chloride reabsorption in animals undergoing sustained volume expansion (SVE) (10% of body wt) with isotonic saline and in animals in which the extracellular fluid volume was expanded to 10% of body wt over 60 min, after which the sustaining saline volume was abruptly discontinued (recovery) (R). Net sodium and chloride balances were not significantly different in R compared with SVE. The fractional excretion of chloride, however, was significantly lower (2.62 +/- 0.25 vs. 4.18 +/- 0.62%, P less than 0.05). In the Sprague-Dawley rats, chloride delivery to the early distal tubule average 18% in both groups. Chloride delivery to the late distal tubule was significantly lower in R (4.39 +/- 0.79%) than in SVE (8.55 +/- 0.76%, P less than 0.005). In the Munich-Wistar rats, samples were obtained from the late distal tubule and base and tip of the papilla. Chloride delivery to the base did not differ from that to the late distal tubule in either group, but was significantly lower in R compared with SVE. These results indicate that the so-called "post-volume expansion" antinatriuresis is the result, at least in part, of enhanced reabsorption in the distal tubule.