Lack of solvent drag of NaCl and NaHCO₃ in rabbit proximal tubules

Using in vitro microperfusion of rabbit nephron segments we measured the effects of osmotically induced water flow on net transport of HCO₃ and Cl. Measurements were made in superficial and juxtamedullary proximal convolutions and in superficial pars recta. In addition, measurements were taken in the presence and absence (hypothermia) of active transport. Using osmotic gradients of 25 mM raffinose in superficial and 50 mM in juxtamedullary segments, we observed increases in water flow equal to or greater than the normal rates of volume reabsorption observed in these tubule segments. However, there were no significant changes in HCO₃ and Cl flux. This lack of significant solvent drag was seen both when osmotic water flow was in the lumen-to-bath direction and when osmotic flow was in the bath-to-lumen direction. The results of these studies suggest that solvent drag does not contribute significantly to NaCl and NaHCO₃ reabsorption in proximal tubules. The lack of significant solvent drag of these salts can be interpreted as indicating either that osmotically induced transepithelial water flow in proximal tubules almost exclusively traverses transcellular pathways or that proximal tubule tight junction reflection coefficients for these salts are close to unity.