TRANSENDOTHELIAL ELECTRICAL POTENTIAL ACROSS PIAL VESSELS IN ANESTHETIZED RATS - A STUDY OF ION PERMEABILITY AND TRANSPORT AT THE BLOOD-BRAIN-BARRIER

Brain pial microvessels have previously been demonstrated to have bloo d-brain barrier properties. The potential difference (PD) across expos ed brain pial microvessels, 20-60 mu m in diameter and superfused with artificial CSF, has been measured in anaesthetised rats using glass m icroelectrodes. The PD on insertion into venous vessels, V-in, was 3.2 mV lumen negative, and in arterial vessels it was higher at 4.5 mV. S uperfusion with high K+-CSF, made by replacing Na+ with K+, caused a p ositive deflection in PD, V-K+, whereas reducing the Na+ alone, by rep lacing Na+ by Tris-HCl, made the lumen more negative. These two effect s were additive. Studies on venous vessels showed that ouabain had no effect on V-in and only affected V-K+ under conditions of low Na pre-e xposure. Neither histamine nor cimetidine had any effect on V-in, or V -K+ whereas tetraethylammonium, a K+-channel blocker, reduced V-K+ by 20%. These experiments demonstrate that changes in PD caused by changi ng abluminal Na+ or K+ are due predominantly to movement of ions throu gh channels in the endothelial cell membranes, and that actions that a lter the activity of the Na+,K+-ATPase or reduce the resistance of the paracellular pathway in parallel with increased membrane permeability have less effect on the PD.