Segmental analysis of sodium reabsorption during renal vein constriction.

Clearance and micropuncture studies were performed in rats to identify the nephron site(s) of altered sodium reabsorption during partial renal vein constriction in the presence and absence of volume expansion. Renal vein constriction increased fractional deliver of sodium to the late proximal tubule from 52 +/- 3 to 61 +/- 4% in euvolemia and from 59 +/- 3 to 65 +/- 3% in volume expansion. In euvolemia, fractional delivery of sodium to the early distal tubule was 13 +/- 3% before and 24 +/- 6% after renal vein constriction. In volume expansion, fractional delivery of sodium to the early distal tubule decreased from 15 +/- 3 to 9 +/- 2% in response to renal vein constriction. We conclude that renal vein constriction decreases sodium reabsorption by the proximal tubule both in euvolemia and in volume expansion but that it increases sodium reabsorption by the superficial loop of Henle only in the presence of volume expansion.