Immunocytochemical localization of Na+,K+-ATPase catalytic polypeptide in mouse choroid plexus

Na+,K+-ATPase plays a central role in the mechanism of cerebrospinal fluid secretion by the choroid plexus. We have used an antiserum to the 100 KD catalytic polypeptide of the enzyme purified from mouse brain (30) to localize the catalytic unit in mouse choroid plexus at the light and electron microscopic levels. Pre-embedding immunostaining with the peroxidase-conjugated second antibody technique showed that microvillar borders facing the ventricle were intensely reactive. In contrast, basal and lateral plasma membrane surfaces were devoid of activity. Identical localization was obtained with a post-embedding procedure in which protein A-gold was used to stain immunoreactive sites on thin sections of Lowicryl-embedded tissue. For comparison, immunogold staining was shown to be restricted to basolateral membranes of kidney medullary ascending thick limbs. The apical localization of Na+,K+-ATPase in choroid plexus is in striking contrast to the almost exclusive basolateral localization seen in other ion-transporting tissues. The immunocytochemical data are completely consistent with physiological data on choroidal epithelial transport and with light microscopic autoradiographic localization of [3H]-ouabain binding sites.