A Cytochemical Evaluation of Blood-brain Barrier Sodium, Potassium- and Calcium-Adenosine Triphosphatase Polarity

The blood-brain barrier (BBB) is formed by cerebral vascular endothelial cells. Brain ion and fluid homeotasis essential for proper neural functioning is due to the BBB. Sodium-potassium and calcium-activated adenosine triphosphatase (Na+, K+-ATPase and Ca2+-ATPase) serve as one of the main mechanisms controlling brain Na+, K+, and Ca2+ concentrations. The present accepted concept is that both of these enzymes are localized to the abluminal plasma membrane of endothelial cells although there have been contrary results from some studies. Because of these discrepancies, further work was needed. Various cytochemical procedures for Na+, K+-ATPase and Ca2+-ATPase were conducted in rat cerebral cortex to ascertain the localization of these enzymes in the microvessel endothelium. Under certain fixation, tissue processing, and incubation procedures, no abluminal polarity was demonstrable for either enzyme. Both enzymes showed equal positive reactions on both the luminal and abluminal endothelial membranes. Both enzymes were sensitive to effects of commonly used fixation protocols. Ca2+-ATPase reaction is remarkably dependent on fixation. Strong abluminal “Ca2+-ATPase” localization reported in the literature was due primarily to ecto-ATPase activity or a combination of ecto-ATPase and Ca2+ specific ATPase. Determination of Ca2+ specific ATPase localization required the use of appropriate controls to distinguish the Ca2+ specific ATPase from the ecto-ATPase. Though “Ca2+-ATPase” activity was present on both surfaces of the capillaries, the luminal membrane primarily contained the Ca2+ specific ATPase. Na+, K+-ATPase appeared to possess a gradient from the arterioles to the venules, and within the capillaries themselves. This enzymatic gradient did not exist for “Ca2+-ATPase”. The results obtained suggest that the current theory of BBB Na+, K+- ATPase and Ca2+-ATPase abluminal polarity cannot be demonstrated consistently under various cytochemical techniques. Cytochemical data discrepancies for these two enzymes as reported in the literature appear to be caused primarily by differences in fixation and incubation protocols rather than the reasons suggested by the investigators. For Ca2+- ATPase studies, interference by ecto-ATPase must be considered since current techniques do not localize the Ca2+ specific ATPase