Transendothelial electrical potential across pial vessels in anaesthetised rats: a study of ion permeability and transport at the blood-brain barrier.

Brain pial microvessels have previously been demonstrated to have blood-brain barrier properties. The potential difference (PD) across exposed brain pial microvessels, 20-60 microns in diameter and superfused with artificial CSF, has been measured in anaesthetised rats using glass microelectrodes. 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. Superfusion with high K(+)-CSF, made by replacing Na+ with K+, caused a positive deflection in PD, VK+, whereas reducing the Na+ alone, by replacing Na+ by Tris-HCl, made the lumen more negative. These two effects were additive. Studies on venous vessels showed that ouabain had no effect on V(in) and only affected VK+ under conditions of low Na pre-exposure. Neither histamine nor cimetidine had any effect on V(in) or VK+ whereas tetraethylammonium, a K(+)-channel blocker, reduced VK+ by 20%. These experiments demonstrate that changes in PD caused by changing abluminal Na+ or K+ are due predominantly to movement of ions through channels in the endothelial cell membranes, and that actions that alter 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.