Functional association between K⁺-Cl^- cotransporter-4 and H^+,K⁺-ATPase in the apical canalicular membrane of gastric parietal cells

We studied whether K⁺-Cl^- cotransporters (KCCs) are involved in gastric HCl secretion. We found that KCC4 is expressed in the gastric parietal cells more abundantly at the luminal region of the gland than at the basal region. KCC4 was found in the stimulation-associated vesicles (SAV) derived from the apical canalicular membrane but not in the intracellular tubulovesicles, whereas H⁺,K⁺-ATPase was expressed in both of them. In contrast, KCC1, KCC2, and KCC3 were not found in either SAV or tubulovesicles. KCC4 coimmunoprecipitated with H⁺,K ⁺-ATPase in the lysate of SAV. Interestingly the MgATP-dependent uptake of ³⁶Cl^- into the SAV was suppressed by either the H⁺,K⁺-ATPase inhibitor (SCH28080) or the KCC inhibitor ((R)-(+)-(2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl) oxy]acetic acid). The KCC inhibitor suppressed the H⁺ uptake into SAV and the H⁺,K⁺-ATPase activity of SAV, but the inhibitor had no effects on these activities in the freeze-dried leaky SAV. These results indicate that the K⁺-Cl^- cotransport by KCC4 is tightly coupled with H⁺/K⁺ antiport by H+,K⁺-ATPase, resulting in HCl accumulation in SAV. In the tetracycline-regulated expression system of KCC4 in the HEK293 cells stably expressing gastric H ⁺,K⁺-ATPase, KCC4 was coimmunoprecipitated with H ⁺ ,K⁺ -ATPase. The rate of recovery of intracellular pH in the KCC4-expressing cells after acid loading through an ammonium pulse was significantly faster than that in the KCC4-nonexpressing cells. Our results suggest that KCC4 and H⁺ ,K⁺-ATPase are the main machineries for basal HCl secretion in the apical canalicular membrane of the resting parietal cell. They also may contribute in part to massive acid secretion in the stimulated state.