Differential expression of Na,K-ATPase alpha and beta subunit isoforms at the blood-brain barrier and the choroid plexus

The relative levels of expression of Na,K-ATPase alpha and beta subunit isoforms were determined in rat cerebral microvessels, choroid plexus, and capillary-depleted brain homogenates by Western blot analysis. Three alpha (alpha 1, alpha 2, alpha 3) and two beta (beta 1, beta 2) subunits were detected in brain as previously reported. All 3 alpha and 2 beta isoforms were also found in microvessels. The choroid plexus expressed alpha 1, beta 1, and beta 2, while alpha 2 and alpha 3 were not found. Autoradiographic signals were normalized to a constant amount of protein from the 3 tissues to calculate relative levels of expression. Expression of alpha 1, alpha 2, and alpha 3 was higher in microvessels than in the brain, by 1.7-, 1.2-, and 1.2-fold, respectively, but expression of beta 1 and beta 2 was 5.5-fold lower. alpha 1 was 4-fold higher in the choroid plexus than in the brain, while expression of beta 1 and beta 2 was 15- and 5.5-fold lower, respectively. Na,K-ATPase enzymatic activities in microvessels and choroid plexus homogenates were similar and 5-fold lower than in brain homogenate. We conclude that: (a) expression of functionally active Na,K-ATPase isoenzymes in microvessels and choroid plexus is restricted in comparison to the brain possibly due to a limited pool of beta 1 and beta 2 subunits; (b) six structurally distinct Na,K-ATPase isoenzymes may be expressed in cerebral microvessels and two in the choroid plexus.