General Anesthetics do not Alter Lipid Bilayer Properties at Clinically Relevant Concentrations

General anesthetics are a widely used class of drugs but, despite their clinical use for >160 years, their exact molecular mechanism(s) remain to be elucidated. A mechanism proposed early on was direct interaction with the lipid bilayer, in some unspecified manner to alter cellular function, which lead to the unitary lipid-based hypothesis of anesthetic action. More recent studies show that general anesthetics interact specifically with various proteins, in particular membrane-embedded ion channels. For example, the inhibition of voltage-gated sodium channels by volatile anesthetics leads to reduced neurotransmitter release in excitable cells. But, though a number of anesthetic targets have been identified, it remains unclear whether the bilayer per se may be involved as well. We therefore examined whether various general anesthetics (isoflurane, sevoflurane, halothane, desflurane, chloroform, diethyl ether, F3, cyclopropane, ketamine and etomidate) and related nonanesthetics (F6 and flurothyl) alter lipid bilayer properties at clinically relevant concentrations. The effects on lipid bilayer properties were tested using the gramicidin-based fluorescence assay (GBFA). The results show that none of the anesthetics or nonanesthetics tested altered lipid bilayer properties at the clinical concentration of 1 MAC (minimal alveolar concentration) with a membrane mole-fraction ranging from 1x10∧-3 (for F6) to 0.1 (for diethyl ether and sevoflurane). Even at two- to four-fold higher concentrations only minimal effects on the bilayer were observed; at much higher (supratherapeutic) concentrations, however, certain anesthetic agents did alter lipid bilayer properties. These results suggest that general anesthetics do not alter ion channel function by altering lipid bilayer properties in a manner that is sensed by a bilayer-spanning channel at clinically relevant concentrations.