Chloride transport by the rabbit cortical collecting duct: Dependence on H,K-ATPase

The rabbit cortical collecting duct (CCD) exhibits the capacity for active chloride absorption when basolateral NaK-ATPase is inhibited by ouabain. The present studies examine the contribution of H,K-ATPase to this ouabain-insensitive Cl absorption and to related ion fluxes. Rabbits were fed a KCl-rich diet with no measurable Na for 4 to 13 d before isolation of the CCD for microperfusion. Application of peritubular ouabain (0.1 mM) significantly increased (P < 0.001) net luminal absorptive chloride flux (J(N)(Cl)) without an effect on lumen-to-bath isotopic ³⁶Cl flux (J(lb)(Cl)). The H,K-ATPase inhibitor Sch 28080 (1 to 10 μM) abolished ouabain-insensitive J(N)(Cl), but transepithelial voltage (V(T)) was not significantly affected. The contribution of H,K-ATPase activity on active Cl flux (J(A)(Cl)) and passive Cl flux (J(P)(Cl)) was also assessed. Ouabain significantly increased J(A)(Cl) and Sch 28080 inhibited J(A)(Cl), but J(P)(Cl) was not affected by Sch 28080. To assess the contribution of changes in net bicarbonate flux (J(tCO2)) to changes in J(N)(Cl), J(tCO2) was measured under identical conditions as for J(N)(Cl). Ouabain significantly increased J(tCO2), and this ouabain-insensitive bicarbonate flux was inhibited by Sch 28080 without significantly affecting V(T). To assess the possibility that the CCD may possess mechanisms for neutral salt absorption, lumen-to-bath ⁸⁶Rb efflux (K(Rb)), and ²²Na efflux (K(Na)) were also measured. Ouabain significantly increased K(Rb), and Sch 28080 inhibited this ouabain-insensitive K(Rb). Furthermore, Sch 28080 and A80915a (a structurally distinct H,K-ATPase inhibitor) significantly inhibited K(Na) in the presence of 1 mM luminal amiloride. These observations suggest that, in addition to potassium, sodium can be transported via the H,K-ATPase. Although the CCD contains more than one cell population, the data could be fitted very well to the function of the B-type intercalated cell. A cell model is proposed for the hypothesis that ouabain-insensitive chloride absorption is mediated by the parallel operation of an apical H,K-ATPase with an apical Cl-HCO₃ exchanger and that the H,K-ATPase can function, under certain conditions, as a mechanism of Na absorption.