Gating Allosterism at a Single Class of Etomidate Sites on α1β2γ2LGABAAReceptors Accounts for Both Direct Activation and Agonist Modulation
Journal of Biological Chemistry
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
... oselectivity 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.