Sodium transport in the proximal tubules and the collecting ducts during variation in the sodium concentration of the surrounding interatitium / Sodium transport in the proximal tubules and the collecting ducts during variation of the sodium concentration in the surrounding interstitium

Micropuncture experiments were performed on proximal tubules and collecting ducts of rat kidneys with and without artificial perfusion of surrounding capillaries or vasa recta respectively. Net sodium flux (ΆNa iso) was estimated under conditions of varying but equal sodium concentrations on both sides of the tubular wall. The transtubular wall equilibrium concentration difference of sodium (Δc Na) was also measured in these nephron segments under conditions of zero volume and solute fluxes. In the proximal tubule ΆNa iso of 8.4 and 7.9×10−5 μeq × mm−2 × sec−1 at sodium concentrations of 155 and 300 meq/l in the perfusion fluid and Δc Na of 24 and 24 meq/l respectively did not vary significantly. Variations of sodium concentrations in blood produced by hypertonic saline infusion or peritoneal dialysis with mannitol resulted in essentially similar values of ΆNa iso and Δc Na. In the collecting ducts also ΆNa iso and Δc Na remained uninfluenced by induced variations in sodium concentrations of the perfusion fluid. ΆNa iso at sodium concentration of 145 and 300 meq/l in the perfusion fluid of vasa recta was 4.1 and 4.1×10−5 μeq × mm−2 × sec−1 respectively and Δc Na was 98 and 104 meq/l respectively. Water reabsorption in the collecting ducts depends on the increase of sodium concentration produced in the medulla by the countercurrent multiplier system. The results represented here indicate that, in mammals sodium reabsorption is independent from the sodium concentration of the interstitial fluid. Therefore the sodium reabsorption and the water reabsorption can be varied independently from each other. Both, sodium and water reabsorption in the collecting ducts are however dependent upon the concentrations of permeable nonelectrolytes such as urea in the collecting duct fluid