Role of sodium and urea in the renal concentrating mechanism in Psammomys obesus.

Micropunctures were performed at the tip of Henle's loops and vasa recta accessible at the extrarenal surface of the papilla in a desert rodent (Psammomys obesus) studied under mild NaCl ((NaCl 4%, 0.0375 ml/min) and mild urea (urea 4%, 0.0375 ml/min) loading conditions. In Nacl loaded animals, it was confirmed that solute addition (mainly sodium) contributes in a large proportion to the concentrating process along the thin descending limb. Comparison of sodium and urea concentrations in the loops with those in vasa recta at the same level of the papilla demonstrated that 1. the transepithelial sodium gradient was compatible with a diffusion transport of this ion from the interstitium to the thin descending limb; 2. the sodium concentration higher in interstitium than in the loop fluid was not compatible with the existence of a purely passive concentrating process in inner medulla as was recently proposed [8], 3. the transepithelial urea gradient was very limited which indicates that this solute does not play an important part in the concentrating process. In urea loaded psammomys, solute addition (mainly urea) to the thin descending limb fluid was still present but water abstraction was enhanced as compared to salt loaded animals, probably on account to the higher interstitial urea concentration. It is, thus, brought to evidence that the relative contribution of water abstraction and solute addition to the concentrating process along the thin descending limb can vary in a given species as a function of the physiological state.