Stimulation of chloride transport by HCO₃-CO₂ in rabbit cortical collecting tubule

We examined both the role of HCO₃-CO₂ in Cl transport as well as the effect of in vivo acid-base status on Cl transport by the rabbit cortical collecting tubule. The lumen-to-bath ³⁶Cl tracer flux, expressed as the rate coefficient K(Cl), was measured in either HEPES-buffered (CO₂-free) or HCO₃-CO₂-containing solutions. Amiloride was added to the perfusate to minimize the transepithelial voltage and thus the electrical driving force for Cl diffusion. Because K(Cl) fell spontaneously with time in HCO₃-CO₂ solutions in the absence but not the presence of cAMP, we used cAMP throughout to avoid time-dependent changes. Acute in vitro removal of bath HCO₃-CO₂ reduced K(Cl). Acetazolamide addition in HEPES-buffered solutions also lowered K(Cl); K(Cl) could be restored to control values by adding exogenous HCO₃-CO₂ in the presence of acetazolamide. In vivo acid-base effects on Cl transport were determined by dissecting tubules from either NaHCO₃-loaded or NH₄Cl-loaded rabbits. Tubules from HCO₃-loaded rabbits had higher rates of Cl self exchange. Acute in vitro addition of bath HCO₃-CO₂ increased K(Cl) and did so to a greater degree in tubules from HCO₃-loaded rabbits. Most of this effect of HCO₃-CO₂ addition on K(Cl) could not be accounted for by Cl-HCO₃ exchange; rather, it appeared due to stimulation of Cl self exchange. The data are consistent with ³⁶Cl transport occurring via Cl-HCO₃ exchange as well as Cl self exchange. Both processes are acutely stimulated by HCO₃ and/or CO₂ and both are chronically regulated by in vivo acid-base status.