Fig. 5.

Role of Na⁺,K⁺-ATPase and its modulation by ouabain. (A) The enzyme consists of a catalytic α-subunit and a β-subunit that has a long extracellular domain with adhesive properties (23). (B) Because of β/β adhesiveness the enzyme stabilizes at the intercellular space, which is sealed at the outermost end by the TJ and opens toward the interstitial side. Because of the firmness of the α/β union, the whole enzyme resides at the borders of the intercellular space. (C) Monolayers of MDCK in transverse optical sections show the Na⁺,K⁺-ATPase (green) at the cell–cell borders but not at the apical or basal domains. Nuclei were stained with propidium iodine (red). (D) Pumps located in the lateral cell border facing the intercellular space lower the Na⁺ concentration in the cytoplasm, and the gradient of this ion across the apical border acts as a driving force for cotransporters and countertransporters (brown and gray circles) translocating H⁺, K⁺, Ca²⁺, sugars, amino acids, etc. (E) Ouabain (pink circles) causes endocytosis of pumps as in Fig. 4B, thereby diminishing the driving force for translocating mechanisms and decreasing the flux of nutrients. (F) When one of the cells is a ouabain-resistant one (R), cell–cell attachment molecules are not retrieved from its membrane or from the membrane of its neighbor W, ouabain-binding pumps of the W cell are not endocytosed (Fig. 4C), and cotransporters remain in operation because the Na⁺ gradient across the apical membrane does not dissipate. Ionic balance of the W cells is maintained by the ouabain-resistant pumps of R cells (14) as well as the presence of gap junctions, which, because of the enhancement of connexin 32 (Fig. 3), increases cell–cell communication (Fig. 2).