No abstract

This review focuses on the unique clinical and molecular pharmacology of etomidate. Among general anesthesia induction drugs, etomidate is the only imidazole, and it has the most favorable therapeutic index for single bolus administration. It also produces a unique toxicity among anesthetic drugs-- inhibition of adrenal steroid synthesis that far outlasts its hypnotic action and that may reduce survival of critically ill patients. The major molecular targets mediating anesthetic effects of etomidate in the central nervous system are specific γ-aminobutyric acid type A receptor subtypes. Amino acids forming etomidate binding sites have been identified in transmembrane domains of these proteins. Etomidate binding site structure models for the main enzyme mediating etomidate adrenotoxicity have also been developed. Based on this deepening understanding of molecular targets and actions, new etomidate derivatives are being investigated as potentially improved sedative-hypnotics or for use as highly selective inhibitors of adrenal steroid synthesis.

At clinical concentrations, the potent intravenous general anesthetic etomidate enhances ␥-aminobutyric acid, type A (GABA A ) receptor activity elicited with low ␥-aminobutyric acid (GABA) concentrations, whereas much higher etomidate concentrations activate receptors in the absence of GABA. Therefore, GABA A receptors may possess two types of etomidate sites: high affinity GABA-modulating sites and low affinity channelactivating sites. However, GABA modulation and direct activation share stereoselectivity for the (R)(؉)-etomidate isomer and display parallel dependence on GABA A ␤ subunit isoforms, suggesting that these two actions may be mediated by a single class of etomidate site(s) that exert one or more molecular effects. In this study, we assessed GABA modulation by etomidate using leftward shifts of electrophysiological GABA concentration responses in cells expressing human ␣ 1 ␤ 2 ␥ 2L receptors. Etomidate at up to 100 M reduced GABA EC 50 values by over 100-fold but without apparent saturation, indicating the absence of high affinity etomidate sites. In experiments using a partial agonist, P4S, etomidate both reduced EC 50 and increased maximal efficacy, demonstrating that etomidate shifts the GABA A receptor gating equilibrium toward open states. Results were quantitatively analyzed using equilibrium receptor gating models, wherein a postulated class of equivalent etomidate sites both directly activates receptors and enhances agonist gating. A Monod-Wyman-Changeux coagonist mechanism with two equivalent etomidate sites that allosterically enhance GABA A receptor gating independently of agonist binding most simply accounts for direct activation and agonist modulation. This model also correctly predicts the actions of etomidate on GABA A receptors containing a point mutation that increases constitutive gating activity.

The potent general anesthetic etomidate produces its effects by enhancing GABA A receptor activation. Its photolabel analog [ 3 H]azi-etomidate labels residues within transmembrane domains on ␣ and ␤ subunits: ␣Met236 and ␤Met286. We hypothesized that these methionines contribute to etomidate sites formed at ␣-␤ subunit interfaces and that increasing sidechain bulk and hydrophobicity at either locus would mimic etomidate binding and block etomidate effects. Channel activity was electrophysiologically quantified in ␣ 1 ␤ 2 ␥ 2L receptors with ␣ 1 M236W or ␤ 2 M286W mutations, in both the absence and the presence of etomidate. Measurements included spontaneous activation, GABA EC 50 , etomidate agonist potentiation, etomidate direct activation, and rapid macrocurrent kinetics. Both ␣ 1 M236W and ␤ 2 M286W mutations induced spontaneous channel opening, lowered GABA EC 50 , increased maximal GABA efficacy, and slowed current deactivation, mimicking effects of etomidate on ␣ 1 ␤ 2 ␥ 2L channels. These changes were larger with ␣ 1 M236W than with ␤ 2 M286W. Etomidate (3.2 M) reduced GABA EC 50 much less in ␣ 1 M236W␤ 2 ␥ 2L receptors (2-fold) than in wild type (23-fold). However, etomidate was more potent and efficacious in directly activating ␣ 1 M236W␤ 2 ␥ 2L compared with wild type. In ␣ 1 ␤ 2 M286W␥ 2L receptors, etomidate induced neither agonist-potentiation nor direct channel activation. These results support the hypothesis that ␣ 1 Met236 and ␤ 2 Met286 are within etomidate sites that allosterically link to channel gating. Although ␣ 1 M236W produced the larger impact on channel gating, ␤ 2 M286W produced more profound changes in etomidate sensitivity, suggesting a dominant role in drug binding. Furthermore, quantitative mechanistic analysis demonstrated that wild-type and mutant results are consistent with the presence of only one class of etomidate sites mediating both agonist potentiation and direct activation.