The Ultrastructural Localization of Transport ATPase in the Rat Liver at Non-bile Canalicular Plasma Membranes

Na,K-ATPase in rat livers was localized cytochemically at the ultrastructural level. The Ernst technique, a method using p-nitrophenylphosphate (pNPP) substrate, was used to demonstrate ouabain-sensitive, K-dependent phosphatase, an enzyme of the Na,K-ATPase reaction sequence. Reaction product was localized predominantly on the sinusoidal and non-bile canalicular (intercellular) surfaces. This localization contrasts with previous histo-chemical studies using ATP substrate and with models that have considered the transport enzyme to be localized at the canalicular surface. If Na,K-ATPase is of importance in bile salt independent flow, a significant presence of the enzyme at sites other than the canalicular membrane suggests that a paracellular movement of sodium and water into the canaliculus must be considered.