Background: The intravenous anesthetic etomidate is optically active and exists in two mirror-image enantiomeric forms. However, although the R(+) isomer is used as a clinical anesthetic, quantitative information on the relative potencies of the R(+) and S(-) isomers is lacking. These data could be used to test the relevance of putative molecular targets.
Methods: The anesthetic concentrations for a half-maximal effect (EC50) needed to induce a loss of righting reflex in tadpoles (Rana temporaria) were determined for both etomidate enantiomers. The effects of the isomers on gamma-aminobutyric acid (GABA)-induced currents in stably transfected mouse fibroblast cells was also investigated using the patch-clamp technique. In addition, the effects of the isomers on a lipid chain-melting phase transition were determined.
Results: The EC50 concentrations for general anesthesia for the R(+) and S(-) isomers were 3.4 +/- 0.1 microM and 57 +/- 1 microM, with slopes of n = 1.9 +/- 0.1 and n = 2.9 +/- 0.2, respectively. The R(+) isomer was also much more effective than the S(-) isomer at potentiating GABA-induced currents, although the degree of stereoselectivity varied with anesthetic concentration. R(+) etomidate potentiated the GABA-induced currents by increasing the apparent affinity of GABA for its receptor. Both isomers were equally effective at disrupting lipid bilayers.
Conclusions: These data are consistent with the idea that the GABA(A) receptor plays a central role in the actions of etomidate. Etomidate exerts its effects on the receptor by binding directly to a specific site or sites on the protein and allosterically enhancing the apparent affinity of GABA for its receptor.