Cyclic adenosine monophosphate-stimulated anion transport in rabbit cortical collecting duct. Kinetics, stoichiometry, and conductive pathways

Cyclic AMP stimulates HCO₃ secretion and Cl self-exchange in rabbit cortical collecting tubule. We found that varying peritubular [Cl] changed the Cl self-exchange rate with saturation kinetics (K(m), 3-4 mM). HCO₃ secretion also showed saturation kinetics as a function of mean luminal [Cl] (K(m), 4-11 mM). Both Cl self-exchange and Cl-HCO₃ exchange thus appear to be carrier-mediated. Addition/removal of basolateral HCO₃ qualitatively changed Cl and HCO₃ transport as expected for Cl-HCO₃ exchange, but quantitatively changed Cl absorption more than HCO₃ secretion. The diffusive Cl permeability and the transepithelial conductance in the presence of HCO₃/CO₂ and cAMP were higher than in their absence suggesting that HCO₃/CO₂ and cAMP together increase a conductive Cl pathway parallel to a 1:1 Cl-HCO₃ exchanger. Thus, cAMP not only stimulates the overall process of anion exchange (probably by increasing an electroneutral exchanger and/or a series Cl conductance), but also stimulates a Cl conductance parallel to the exchange process.