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Effect of Blood Volume Expansion on Tubule Sodium Transport in the Isolated Dog Kidney 1

Summary Nonrecollection end-proximal tubule micropuncture technique and the microsphere method for estimating fractional distribution of renal cortical blood flow were applied to further define the mechanism of the natriuresis in the isolated dog kidney in response to volume expansion with equilibrated blood. Following volume expansion sodium excretion increased +79 ± 24 μEq/min (P < 0.01) in the face of significant decreases in inulin clearance (CIN) and renal blood flow (RBF) and in the absence of changes in renal perfusion pressure, plasma protein concentration or packed cell volume. (TF/P)IN of end-proximal tubular fluid decreased from 1.65 ± 0.03 to 1.53 ± 0.04, P < 0.025, and proximal tubule absolute reabsorption decreased from 36 ± 3 to 29 ± 2 nl/min, P < 0.05. The decrease in absolute reabsorption, however, was balanced by a decrease in single nephron GFR (SNGFR) so that no increase in distal delivery of fluid (VTF) out of the proximal tubule was detected. SNGFR/CIN remained constant. No change in fractional distribution of RBF was detected. The data indicate that volume expansion with equilibrated blood depresses proximal tubule fractional and absolute reabsorptive rates in the isolated kidney but since VTF did not increase, they imply that the natriuresis derives from a decrease in sodium transport along more distal nephron segments.