K⁺-Cl^- cotransporter-3a up-regulates Na ⁺,K⁺-ATPase in lipid rafts of gastric luminal parietal cells

Gastric parietal cells migrate from the luminal to the basal region of the gland, and they gradually lose acid secretory activity. So far, distribution and function of K⁺-Cl^- cotransporters (KCCs) in gastric parietal cells have not been reported. We found that KCC3a but not KCC3b mRNA was highly expressed, and KCC3a protein was predominantly expressed in the basolateral membrane of rat gastric parietal cells located in the luminal region of the glands. KCC3a and the Na⁺,K⁺-ATPase α1-subunit (α1NaK) were coimmunoprecipitated, and both of them were highly localized in a lipid raft fraction. The ouabain-sensitive K ⁺-dependent ATP-hydrolyzing activity (Na⁺,K ⁺-ATPase activity) was significantly inhibited by a KCC inhibitor (R-(+)-[(2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl) oxy]acetic acid (DIOA)). The stable exogenous expression of KCC3a in LLC-PK1 cells resulted in association of KCC3a with endogenous α1NaK, and it recruited α1NaK in lipid rafts, accompanying increases of Na ⁺,K⁺-ATPase activity and ouabain-sensitive Na⁺ transport activity that were suppressed by DIOA, whereas the total expression level of α1NaK in the cells was not significantly altered. On the other hand, the expression of KCC4 induced no association with α1NaK. In conclusion, KCC3a forms a functional complex with α1NaK in the basolateral membrane of luminal parietal cells, and it up-regulates α1NaK in lipid rafts, whereas KCC3a is absent in basal parietal cells.