Clinical concentrations of chemically diverse general anesthetics minimally affect lipid bilayer properties

Abstract

General anesthetics have revolutionized medicine by facilitating invasive procedures, and have thus become essential drugs. However, detailed understanding of their molecular mechanisms remains elusive. A mechanism proposed over a century ago involving unspecified interactions with the lipid bilayer known as the unitary lipid-based hypothesis of anesthetic action, has been challenged by evidence for direct anesthetic interactions with a range of proteins, including transmembrane ion channels. Anesthetic concentrations in the membrane are high (10-100 mM), however, and there is no experimental evidence ruling out a role for the lipid bilayer in their ion channel effects. A recent hypothesis proposes that anesthetic-induced changes in ion channel function result from changes in bilayer lateral pressure that arise from partitioning of anesthetics into the bilayer. We examined the effects of a broad range of chemically diverse general anesthetics and related nonanesthetics on lipid bilayer properties using an established fluorescence assay that senses drug-induced changes in lipid bilayer properties. None of the compounds tested altered bilayer properties sufficiently to produce meaningful changes in ion channel function at clinically relevant concentrations. Even supra-anesthetic concentrations caused minimal bilayer effects, although much higher (toxic) concentrations of certain anesthetic agents did alter lipid bilayer properties. We conclude that general anesthetics have minimal effects on bilayer properties at clinically relevant concentrations, indicating that anesthetic effects on ion channel function are not bilayer-mediated but rather involve direct protein interactions.

Synthetic Analogues of the Snail Toxin 6-Bromo-2-mercaptotryptamine Dimer (BrMT) Reveal That Lipid Bilayer Perturbation Does Not Underlie Its Modulation of Voltage-Gated Potassium Channels

“…These conclusions rely on interpretations that have caveats:

The bilayer partitioning and perturbation experiments were conducted with bilayers formed by synthetic phosphocholine lipids. Results in multi-component bilayers,(62-64) and whole cells,(10) have been found to be similar to those obtained in single-component bilayers. On the basis of these previous validations, we expect the impact of compounds on single-component bilayers will extend to living cells more generally.

…”

“…Results in multi-component bilayers,(62-64) and whole cells,(10) have been found to be similar to those obtained in single-component bilayers. On the basis of these previous validations, we expect the impact of compounds on single-component bilayers will extend to living cells more generally.…”

Divergent effects of anesthetics on lipid bilayer properties and sodium channel function

“…direct membrane protein effects, which often are not immediately evident. To distinguish between these contributions, we tested a variety of general anesthetics, including inhaled ether and alkane anesthetics, intravenous anesthetics and experimental fluorobenzene anesthetics, for their effects on lipid bilayer properties using the gramicidin channel functional assay (Tibbs et al 2013; Herold et al 2014; Herold et al 2017). …”

“…This finding has the important implication that any effects observed on ion channel function at clinical anesthetic concentrations are not due to changes in lipid bilayer properties (as detected by the gramicidin channel based assay). Certain anesthetics were found to alter bilayer properties at supratherapeutic (toxic) concentrations, such that bilayer perturbations might contribute to the many undesirable off-target effects of anesthetics (data not shown, see (Herold et al 2017)).…”

“…We therefore also tested anesthetic effects on gramicidin channel function in multi-component bilayers that form coexisting l d and l o domains. Even in this more complex bilayer preparation, there were no effects of representative anesthetics on gramicidin channel function (Herold et al 2017). …”