AN IN-SITU CYTOCHEMICAL EVALUATION OF BLOOD-BRAIN-BARRIER SODIUM, POTASSIUM-ACTIVATED ADENOSINE-TRIPHOSPHATASE POLARITY

It is presently believed that sodium, potassium-activated adenosine tr iphosphatase (Na+, K+-ATPase) is localized on the abluminal plasma mem brane of brain endothelial cells. But there have been contrary reports from some cytochemical studies. We examined the localization of the e nzyme in rat cerebral microvessel endothelium using the in situ model originally employed to establish the abluminal polarity concept. Alter ations in fixation and incubation media from the original reports were conducted to determine the effect on localization pattern. With the E rnst indirect incubation method as originally used, three types of loc alization patterns were obtained: abluminal only, luminal only, and on both surfaces of endothelial cells. With the direct incubation method of Mayahara, reaction product was seen on both surfaces. Reduction in fixation time followed by the use of the indirect incubation method r esulted in a complete loss of the reaction product. The same reduction in fixation time followed by the use of the direct method did not alt er the localization pattern of the enzyme. Our results demonstrated th at Na+, K+-ATPase is localized on both surfaces of brain endothelial c ells. The localization pattern of Na+, K+-ATPase is significantly depe ndent upon fixation and the incubation medium used in the in situ mode l. Data discrepancies for the enzyme as reported in the literature app ear to be caused by differences in cytochemical protocols, rather than the biological reasons advocated by other investigators. We conclude that past cytochemical reports of blood-brain barrier (BBB) Na+, K+-AT Pase abluminal localization were incomplete. The currently held ablumi nal polarity theory of the enzyme needs to be reexamined. Past basic a nd clinical cytochemical studies of BBB Na+, K+-ATPase should be viewe d and interpreted with caution. (C) 1998 Elsevier Science B.V. All rig hts reserved.