Summary:The energy-dependent transport of ions across the blood-brain barrier and the blood-cerebro spinal fluid barrier by N a + ,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 spe cifically 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 Abbreviation used: QNB, quinuclidinyl benzylate.
156the 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 oua bain 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.
(Na+-K+)-ATPase is an enzyme responsible for ionic homeostasis in excitable tissues. Thus, the activity of this enzyme is essential for the maintenance of optimal function of the cerebral cortex. Although abnormalities of cerebral oxidative metabolism have been described in aging, these abnormalities do not appear to be associated with changes in the in vitro enzyme characteristics of (Na+-K+)-ATPase. Specifically, there were no significant differences between preparations from the cerebral cortex of 3- and 28-month-old Fischer-344 rats in their: (i) high-affinity binding to [3H]-ouabain, (ii) number of ouabain-binding sites, or (iii) K+p-nitrophenylphosphatase activity.
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