Biochemical localization of hepatic surface-membrane Na+,K+-ATPase activity depends on membrane lipid fluidity

Membrane proteins of transporting epithelia are often distributed between apical and basolateral surfaces to produce a functionally polarized cell. The distribution of Na+,K+-ATPase [ATP phosphohydrolase (Na+/K+-transporting), EC 3.6.1.37] between apical and basolateral membranes of hepatocytes has been controversial. Because Na+,K+-ATPase activity is fluidity dependent and the physiochemical properties of the apical membrane reduces its fluidity, we investigated whether altering membrane fluidity might uncover cryptic Na+,K+-ATPase in bile canalicular (apical) surface fractions free of detectable Na+,K+-ATPase and glucagon-stimulated adenylate cyclase activities. Apical fractions exhibited higher diphenylhexatriene-fluorescence polarization values when compared with sinusoidal (basolateral) membrane fractions. When 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl)octanoate (A2C) was added to each fraction, Na+,K+-ATPase, but not glucagon-stimulated adenylate cyclase activity, was activated in the apical fraction. In contrast, further activation of both enzymes was not seen in sinusoidal fractions. The A2C-induced increase in apical Na+,K+-ATPase approached 75% of the sinusoidal level. Parallel increases in apical Na+,K+-ATPase were produced by benzyl alcohol and Triton WR-1339. All three fluidizing agents decreased the order component of membrane fluidity. Na+,K+-ATPase activity in each subfraction was identically inhibited by the monoclonal antibody 9-A5, a specific inhibitor of this enzyme. These findings suggest that hepatic Na+,K+-ATPase is distributed in both surface membranes but functions more efficiently and, perhaps, specifically in the sinusoidal membranes because of their higher bulk lipid fluidity.