BARBITURATES INHIBIT HEXOSE-TRANSPORT IN CULTURED-MAMMALIAN-CELLS ANDHUMAN ERYTHROCYTES AND INTERACT DIRECTLY WITH PURIFIED GLUT-1

Barbiturates reduce cerebral blood flow, metabolism, and Glc transfer across the blood-brain barrier. The effect of barbiturates on hexose t ransport in cultured mammalian cell Lines and human erythrocytes was s tudied. Pentobarbital inhibits [H-3]-2-dGlc uptake in 3T3-C2 murine fi broblasts by similar to 95% and similar to 50% at 10 and 0.5 mM, respe ctively. Uptake of [H-3]-2-dGlc is linear with time in the presence or absence of pentobarbital, and the percent inhibition is constant. Thi s suggests that hexose transport, not phosphorylation, is inhibited by barbiturates. Inhibition by pentobarbital of hexose transport in 3T3- C2 cells is rapid (<1 min), is not readily reversible, is not altered by the presence of albumin [1% (w/v)], and is independent of temperatu re (4-37 degrees C) and the level of cell surface GLUT-1. The IC50's f or inhibition of hexose transport in 3T3-C2 cells by pentobarbital, th iobutabarbital, and barbital are 0.8, 1.0, and 4 mM, respectively. Thi s is consistent with both the Meyer-Overton rule and the pharmacology of barbiturates. Neither halothane (less than or equal to 10 mM) nor e thanol [less than or equal to 0.4% (v/v)] significantly inhibits hexos e transport. Inhibition by pentobarbital (0.5 mM) of [H-3]-2-dGlc upta ke by 3T3-C2 cells decreases the apparent V-max (similar to 50%) but d oes not alter the apparent K-m (similar to 0.5 mM). Inhibition of hexo se transport by barbiturates, but not ethanol [less than or equal to 0 .4% (v/v)], is also observed in human erythrocytes and four other cult ured mammalian cell lines. Pentobarbital quenches (Q(max) similar to 7 5%) the intrinsic fluorescence of purified and reconstituted GLUT-1 (K -d similar to 3 mM). Quenching is independent of Glc occupancy, is unc hanged by mild proteolytic inactivation, and does not appear to direct ly involve perturbations of the lipid bilayer. We propose that barbitu rates can interact directly with GLUT-1 and inhibit the intrinsic acti vity of the carrier. Glc crosses the blood-brain barrier primarily via the GLUT-1 of the endothelial cells of cerebral capillaries. Partial inhibition of this process by barbiturates may be of significance to c erebral protection.