Nonanesthetic Volatile Drugs Obey the Meyer-Overton Correlation in Two Molecular Protein Site Models

Abstract

Nonanesthetic volatile compounds fail to inhibit movement in response to noxious stimulation at concentrations predicted to induce anesthesia from their oil-water partitioning. Thus they represent tools to determine whether molecular models behave like the targets that mediate in vivo anesthetic actions. The effects of volatile anesthetics and non-anesthetics were examined in two experimental models in which anesthetics interact directly with proteins: the pore of the nicotinic acetylcholine receptor and human serum albumin.

Wild-type mouse muscle nicotinic receptors and receptors containing pore mutations ([Greek small leter alpha] S252I + [Greek small letter beta] T263I) were studied electrophysiologically in membrane patches from Xenopus oocytes. Patch currents evoked by brief pulses of acetylcholine were measured in the presence of enflurane and two nonanesthetics, 1,2-dichlorohexafluorocyclobutane and 2,3-dichlorooctafluorobutane. Nonanesthetic interactions with human serum album were assessed by quenching of intrinsic protein fluorescence.

Both anesthetic and nonanesthetic volatile compounds inhibited wild-type and [Greek small letter alpha] S252I + [Greek small letter beta] T263I mutant nicotinic channels but displayed different selectivity for open versus resting receptor states. Median inhibitory concentrations (IC50 s) in wild-type nicotinic receptors were 870 +/- 20 [micro sign]M for enflurane, 37 +/- 3 [micro sign]M for 1,2-dichlorohexafluorocylcobutane, and 11.3 +/- 5.6 [micro sign]M for 2,3-dichlorooctafluorobutane. For all three drugs, ratios of wild-type IC50 s to mutant IC50mut ranged from 7–10, and ratios of wild-type IC50 s to predicted anesthetic median effective concentrations (EC50 s) ranged from 1.8–2.3. 1,2-Dichlorohexafluorocyclobutane quenched human serum albumin with an apparent dissociation constant (Kd) of 160 +/- 11 [micro sign]M. The ratios of dissociation constants to predicted EC50 s for the nonanesthetics were within a factor of two of the dissociation constant:EC (50) ratios calculated for halothane and chloroform from previous published results.

In two models in which anesthetics bind to protein sites, both anesthetic and nonanesthetic volatile drugs cause similar steady state effects with potencies that are predicted by hydrophobicity. These protein sites do not sterically discriminate between anesthetic and nonanesthetic drugs. However, differential state-selective actions on ion channel targets may underlie the distinct in vivo effects of anesthetics and nonanesthetics.