Enzyme cytochemistry of blood-brain barrier (BBB) disturbances.

Alkaline phosphatase (AP) is one of the enzymes which is highly active in the plasmalemma of endothelial cells (ECs) of BBB-type microvessels. In the ECs of non-BBB type vessels, the reaction for AP (and other phosphatases) is negative (e.g. choroid plexus, area postrema, hypophysis). After BBB damage, the leakage of the vessels can be demonstrated by the use of horseradish peroxidase (HRP). Concomitantly, changes in polar distribution of AP in the ECs occur, paralleled by the appearance of numerous pinocytic vesicles, deep invaginations of the plasmalemma and channel-like structures. The delimiting membranes of these structures possess AP, 5'-nucleotidase, nucleoside diphosphatase and Na+, K+-ATPase activities. These observations suggest that the redistribution of plasmalemma bound enzymes from luminal to abluminal surface results from membrane flow associated with formation of pinocytic vesicles and channel-like structures in affected ECs. In the area of brain where the process of resolution of brain edema occurs, the shift of the enzymatic activity from luminal to abluminal plasmalemma of the ECs is observed probably because of the need to remove various solutes present in the edematous fluid. The appearance of positive reaction for AP in the abluminal side of the EC can be a reflection of the changed functional polarity of these cells associated with reverse transport of solutes from brain, back into the blood stream.