Basolateral membrane Cl/HCO3 exchange in the rat proximal convoluted tubule. Na-dependent and -independent modes.

Abstract

To examine whether Cl-coupled HCO3 transport mechanisms were present on the basolateral membrane of the mammalian proximal tubule, cell pH was measured in the microperfused rat proximal convoluted tubule using the pH-sensitive, intracellularly trapped fluorescent dye (2'',7'')-bis(carboxyethyl)-(5,6)-carboxyfluorescein. Increasing the peritubular Cl concentration from 0 to 128.6 meq/liter caused cell pH to decrease from 7.34 .+-. 0.04 to 7.21 .+-. 0.04 (p < 0.001). With more acid extracellular fluid (pH 6.62), a similar increase in the peritubular Cl concentration caused cell pH to decrease by a similar amount from 6.97 .+-. 0.04 to 6.84 .+-. 0.05 (p < 0.001). This effect was blocked by 1 mM SITS. To examine the Na dependence of Cl/HCO3 exchange, the above studies were repeated in the absence of luminal and peritubular Na. In alkaline Na-free solutions, peritubular Cl addition caused cell pH to decrease from 7.57 .+-. 0.06 to 7.53 .+-. 0.06 (p < 0.025); in acid Na-free solutions, peritubular Cl addition caused cell pH to decrease from 7.21 .+-. 0.04 to 7.19 .+-. 0.04 (p < 0.05). The effect of Cl on cell pH was smaller in the absence of luminal and peritubular Na than in its presence. To examine whether the previously described Na/(HCO3)n > 1 cotransporter was coupled to or dependent on Cl, the effect of lowering the peritubular Na concentrations from 147 to 25 meq/liter was examined in the absence of ambient Cl. Cell pH decreased from 7.28 .+-. 0.03 to 7.08 .+-. 0.03, a response similar to that observed previously in the presence of Cl. The results demonstrate that Cl/HCO3 (or Cl/OH) exchange is present on the basolateral membrane. Most of Cl/HCO3 exchange is dependent on the presence of Na and may be coupled to it. The previously described Na/(HCO3)n > 1 cotransporter is the major basolateral membrane pathway for the coupling of Na and HCO3 and is not coupled to Cl.