Stimulation of total CO₂ flux by 10% CO₂ in rabbit CCD: Role of an apical Sch-28080- and Ba-sensitive mechanism

These studies examine the effect of ambient PCO₂ on net bicarbonate (total CO₂) absorption by the in vitro perfused cortical collecting duct (CCD) from K-replete rabbits and the mechanism responsible for this effect. Exposure to 10% CO₂ increased net bicarbonate flux (total CO₂ flux, J_tCO2) by 1.8-fold (P < 0.005), and this effect was inhibited by luminal 10 μM Sch-28080, an H-K-adenosinetriphosphatase (H-K-ATPase) inhibitor. In contrast, exposure to 10% CO₂ significantly decreased Rb efflux, and this decrement in Rb efflux was blocked by luminal 2 mM Ba, a K channel blocker. Thus transepithelial tracer Rb flux did not increase upon exposure to 10% CO₂ as we have observed in this segment under K-restricted conditions. The observation that 10% CO₂ increased net bicarbonate absorption without a change in absorptive Rb flux suggested that 10% CO₂ increased apical K recycling. To test this hypothesis, we examined whether luminal Ba inhibited the stimulation of luminal acidification induced by 10% CO₂. If apical K exit were necessary for full activation of proton secretion, then inhibiting K exit should indirectly affect the stimulation of J_tCO2 by 10% CO₂. In fact, the effect of 10% CO₂ on J_tCO2 in the presence of 2 mM luminal Ba was quantitatively indistinguishable from the effect of 10% CO₂ on J_tCO2 in the presence of 10 μM luminal Sch-28080. We interpret these data to suggest that under K-replete conditions 1) acute peritubular acidosis (10% CO₂) stimulates luminal acidification by the CCD from normal rabbits via an apical Sch-28080- and Ba-sensitive mechanism that is most consistent with an activation of an H-K-ATPase, 2) the reduction of Rb efflux following exposure to 10% CO₂ and the inhibition of this response by 2 mM luminal Ba is compatible with an effect of acidosis on transcellular K conductances plausibly located in principal cells, and 3) the lack of stimulation of Rb efflux by 10% CO₂ is in distinct contrast to the effect of this same maneuver to enhance markedly Rb efflux by a Sch-28080-sensitive mechanism in the CCD of K-restricted animals. To explain these observations we propose that renal H-K-ATPase is functionally coupled to an apical Ba-sensitive pathway in the CCD of K-replete rabbits.