ISOFLURANES ENHANCEMENT OF CHLORIDE FLUX THROUGH RAT-BRAIN GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTORS IS STEREOSELECTIVE

Background: Recent evidence is consistent with the view that volatile anesthetics interact directly with excitable membrane;bound channel pr oteins. If these agents interact directly with chiral centers in the n euronal cell membrane, then their effects should be stereoselective. U sing rat brain membranes enriched in gamma-aminobutyric acid type A (G ABA(A)) receptors, we investigated the hypothesis that the permeabilit y response of this well-characterized central nervous system channel p rotein to stereoisomers of isoflurane is stereoselective. Methods: Rat brain synaptic microvesicles were prepared by differential centrifuga tion. Agonist-stimulated Cl-36(-) flux through membrane-bound GABA(A) receptors was assayed in the presence of (+)- and (-)-isoflurane and c ompared with control conditions. Results: Both isomers increased the p otency and efficacy of GABA; however, (+)-isoflurane was significantly more potent and efficacious than the (-)-isomer. For example, the (+) -isomer (140 mu M) reduced the median effective concentration of GABA from 12.7 +/- 1.0 to 5.4 +/- 0.5 mu M, whereas the (-)-isomer reduced it to 9.6 +/- 1.0 mu M (P < 0.001). The (+)-isomer also was 1.6 times as potent as the (-)-isomer in augmenting 5 mu M GABA-gated flux (79 /- 11 vs. 130 +/- 17 mu M, respectively; P = 0.01). In addition, the ( +)-isomer produced significantly greater maximal enhancement of flux ( 9.4 +/- 0.4 vs. 7.0 +/- 0.3 nmol . mg(-1). 3s(-1); P < 0.001). Conclus ions: Isoflurane's effects on GABA-gated chloride nux were stereoselec tive. This result supports direct interaction with a stereoselective s ite, possibly the GABA, channel protein itself, rather than a nonspeci fic perturbation of the surrounding membrane lipid. In addition, these findings, from a functional assay using mammalian brain, agree with r ecent observations in invertebrate ion channels and mammalian neuronal cell cultures.