Specific ouabain binding to brain microvessels and choroid plexus.

The energy-dependent transport of ions across the blood-brain barrier and the blood-cerebrospinal fluid barrier by Na+, K+-ATPase is credited with an important role in brain homeostasis. In this study, we have assessed the relative enrichment of Na+, K+-ATPase in regional brain capillary preparations and in the choroid plexus by the quantitative determination of the cardiac glycoside binding sites in these preparations using [3H]ouabain as a ligand. We find that ouabain binds specifically to brain microvessels of the rat and the pig and to the choroid plexus of the pig in a saturable manner. The maximum density of ouabain binding sites in brain microvessels of both species is about one-fourth that of the crude membranes of the cerebrum and cerebellum. The density of ouabain binding sites in the pig choroid plexus is intermediate between that of the brain and brain microvessels. We do not find regional differences in ouabain binding to membrane fractions of the cerebrum and cerebellum, nor any significant differences in ouabain binding to cerebral and cerebellar microvessels. These findings provide quantitative estimates of Na+, K+-ATPase in brain capillaries and choroid plexus.