Isoflurane's 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 proteins. If these agents interact directly with chiral centers in the neuronal cell membrane, then their effects should be stereoselective. Using rat brain membranes enriched in gamma-aminobutyric acid type A (GABAA) receptors, we investigated the hypothesis that the permeability response of this well-characterized central nervous system channel protein to stereoisomers of isoflurane is stereoselective.


METHODS
Rat brain synaptic microvesicles were prepared by differential centrifugation. Agonist-stimulated 36Cl- flux through membrane-bound GABAA receptors was assayed in the presence of (+)- and (-)-isoflurane and compared with control conditions.


RESULTS
Both isomers increased the potency and efficacy of GABA; however, (+)-isoflurane was significantly more potent and efficacious than the (-)-isomer. For example, the (+)-isomer (140 microM) reduced the median effective concentration of GABA from 12.7 +/- 1.0 to 5.4 +/- 0.5 microM, whereas the (-)-isomer reduced it to 9.6 +/- 1.0 microM (P < 0.001). The (+)-isomer also was 1.6 times as potent as the (-)-isomer in augmenting 5 microM GABA-gated flux (79 +/- 11 vs. 130 +/- 17 microM, 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-1.3 s-1; P < 0.001).


CONCLUSIONS
Isoflurane's effects on GABA-gated chloride flux were stereoselective. This result supports direct interaction with a stereoselective site, possibly the GABAA channel protein itself, rather than a nonspecific perturbation of the surrounding membrane lipid. In addition, these findings, from a functional assay using mammalian brain, agree with recent observations in invertebrate ion channels and mammalian neuronal cell cultures.
Quinlan
J J
JJ
Department of Anesthesiology, University of Pittsburgh Medical Center, Pennsylvania 15213-2582, USA.
Firestone
S
S
Firestone
L L
LL
D000818:Animals; D001921:Brain / Q000187:drug effects*; D002712:Chlorides / Q000378:metabolism*; D007530:Isoflurane / Q000494:pharmacology*; D008297:Male; D010539:Permeability; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000187:drug effects*; D013237:Stereoisomerism
D002712:Chlorides; D011963:Receptors, GABA-A; D007530:Isoflurane