Effect of intraluminal bicarbonate and chloride on fluid absorption by the rat renal proximal tubule.

In order to study mechanisms of fluid transport in the rat renal proximal convoluted tubule, the effects of large variations in intraluminal HCO3- and Cl- concentrations were measured by microperfusion techniques. No differences in rates of fluid transport were found when intraluminal HCO3- was varied from 4 to 30 mEq/liter and Cl- from 146 to 120 mEq/liter. Inhibition of H+ secretion with benzolamide had no effect on fluid absorption when little or no HCO3- was present in the lumen, but did reduce fluid transport when 25 mEq of HCO3- was present. If several different mechanisms are responsible for proximal fluid transport, such as nonelectrogenic active NaHCO3 transport, passive chloride diffusion and active sodium transport linked to H+ secretion, the above observations imply that they all operate at approximately the same rate, since the dominant driving force would have been different with each perfusion solution. The data seem more compatible with the view that active sodium transport is the major driving force for fluid absorption in the proximal tubule, that this is not linked to H+ secretion and that anions modify the rate of absorption only to the degree that they are able to accompany sodium across the epithelium. An additional observation was that absorption of isotonic NaCl was very slow in short segments of tubule, as compared to HCO3--containing perfusion solutions. Although the mechanism is uncertain, these data suggest that a finite amount of intraluminal HCO3- is necessary for optimal proximal fluid transport.