Immunocytochemical localization of Na+, K+-ATPase in the canine choroid plexus.

To determine the localization of Na+, K+-ATPase in the choroid plexus of the canine fourth ventricle, antisera against canine renal Na+, K+-ATPase holoenzyme and the alpha and beta subunits of the enzyme were employed and the distribution of reaction product examined at the light and electron microscopic level by means of the peroxidase-antiperoxidase immunohistochemical method. In the choroidal epithelial cells, antigenic sites were restricted to the outer surface of the apical plasmalemma, including the membranes of the microvilli. No reaction product was detected in the cytoplasm or along the lateral and basal plasmalemma. These findings strengthen the hypothesis that the apical border of the choroidal epithelial cell plays an important role in cerebrospinal fluid secretion as a result of active transport mediated by Na+, K+-ATPase.