To the Editor:—

Lipid-based theories of general anesthetic action have long endured because numerous studies have shown that the in vivo  pharmacology of an anesthetic correlates remarkably well with its ability to perturb the structural properties of simple lipid bilayers. The Meyer-Overton correlation between anesthetic potency and hydrophobicity, the inactivity of nonanesthetic long chain alcohols and highly halogenated volatile compounds (nonimmobilizers), and pressure reversal have all been demonstrated in studies using protein-free lipid bilayers. 1–6Nevertheless, a most persuasive and often mentioned argument against lipid-based theories is that at clinically relevant concentrations, anesthetics induce only small perturbations in lipid bilayer structure. 7,8For example, halothane reduces the order parameter (increase the “fluidity”) of lipid bilayers by only 1% at clinically relevant concentrations. 9An equivalent reduction in order parameter may be obtained by raising the temperature of the bilayer by less than 1°C. Similarly, halothane reduces the transition temperature between a lipid bilayer’s liquid and gel phases by only 0.5°C at anesthetic concentrations and by only 5°C even at 10 times the minimum aveolar concentration (MAC). 10I was, therefore, very interested to read the study by Johansson et al.  quantifying the effects of isoflurane and halothane on structural properties of bovine serum albumin, a lipid-free protein model used in mechanistic studies of anesthetic action. 11What did their studies show? At approximately 1 MAC, isoflurane and halothane increased the fluorescence anisotropy of bovine serum albumin by 1%. An equivalent reduction was obtained by raising the temperature of bovine serum albumin by approximately 1° C. Similarly, isoflurane and halothane raised the transition temperature between the folded and unfolded states of bovine serum albumin by less than 1°C at anesthetic concentrations and by only 3–4°C even at 10 times MAC. Studies of anesthetic binding to other protein models have been similarly unable to demonstrate significant anesthetic-induced changes in protein structure. 12,13Thus, anesthetics induce similarly small changes in the structural properties of lipids and proteins. For consistency, shouldn’t we now conclude that such insensitivity argues strongly against a protein site of anesthetic action?

The inability to detect significant anesthetic-induced structural changes in either lipid or protein model systems highlights the practical (and obvious) limitations of such studies: we can only measure what we can measure. Fluorescence anisotropy, denaturation temperature, phase transition temperature, and order parameter have been used by biophysicists for many years as indicators of lipid bilayer and protein structure in large part because they are easily quantitated. There is no compelling theoretical reason to believe that changes in these properties directly  accounts for the functional effects of anesthetics on relevant targets in the central nervous system. In fact, it seems quite likely that the anesthetic state results from changes in other lipid and/or protein physical properties that are not so easily measured.

Janoff AS, Pringle MJ, Miller KW: Correlation of general anesthetic potency with solubility in membranes. Biochim Biophys Acta 1981; 649:125–8
Chiou JS, Ma SM, Kamaya H, Ueda I: Anesthesia cutoff phenomenon: Interfacial hydrogen bonding. Science 1990; 248:583–5
Raines DE, Korten SE, Hill WAG, Miller KW: Anesthetic cutoff in cycloalkanemethanols: A test of current theories. A nesthesiology 1993; 78:918–27
North C, Cafiso DS: Contrasting membrane localization and behavior of halogenated cyclobutanes that follow or violate the Meyer-Overton hypothesis of general anesthetic potency. Biophys J 1997; 72:1754–61
Tang P, Yan B, Xu Y: Different distribution of fluorinated anesthetics and nonanesthetics in model membrane: A 19F NMR study. Biophys J 1997; 72:1676–82
Galla HJ, Trudell JR: Asymmetric antagonistic effects of an inhalation anesthetic and high pressure on the phase transition temperature of dipalmitoyl phosphatidic acid bilayers. Biochim Biophys Acta 1980; 599:336–40
Franks NP, Lieb WR: Molecular and cellular mechanisms of general anaesthesia. Nature 1994; 367:607–14
Harrison NL: Optical isomers open a new window on anesthetic mechanism. A nesthesiology 1998; 88:566–8
Mastrangelo CJ, Trudell JR, Edmunds HN, Cohen EN: Effect of clinical concentrations of halothane on phospholipid-cholesterol membrane fluidity. Mol Pharmacol 1978; 14:463–7
Craig NC, Bryant GJ, Levin IW: Effects of halothane on dipalmitoylphosphatidylcholine liposomes: A Raman spectroscopic study. Biochemistry 1987; 26:2449–58
Johansson JS, Zou H, Tanner JW: Bound volatile general anesthetics alter both local protein dynamics and global protein stability. A nesthesiology 1999; 90:235–45
Johansson JS, Rabanal F, Dutton PL: Binding of the volatile anesthetic halothane to the hydrophobic core of a tetra-alpha-helix-bundle protein. J Pharmacol Exp Ther 1996; 279:56–61
Franks NP, Jenkins A, Conti E, Lieb WR, Brick P: Structural basis for the inhibition of firefly luciferase by a general anesthetic. Biophys J 1998; 75:2205–11