Inhalation Anesthetics Change the Domain Structure of Model Ternary Lipid Raft Membranes

Abstract

The mechanism of action for volatile anesthetics remains obscure despite clinical use for over 150 years. No single ion channel or protein receptor appears necessary and sufficient to account for anesthetic action, and the physical effects of volatile anesthetics on homogeneous model membranes appear too small to produce anesthesia. We recently reported that halothane changes the domain structure of a binary lipid mixture1, increasing the ratio of disordered phase to ordered phase.We have now studied two ternary model lipid raft mixtures with X-ray diffraction: Dioleoylphosphatidylcholine (DOPC)/dipalmitylphosphatidylcholine (DPPC)/cholesterol, and Dioleoylphosphatidylcholine (DOPC)/sphingomyelin (porcine)/cholesterol. Multi-layers were prepared upon glass slides, hydrated overnight at 98% relative humidity, and maintained at 27.0 ± 0.1 C on a Peltier-controlled stage in a sealed X-ray chamber. Volatile anesthetics were introduced as solutions in hexadecane. For both raft mixtures, two series of lamellar diffraction peaks are observed, with d-spacings differing by about 10%. These correspond to the liquid ordered phase and the liquid disordered phase. The relative intensities of diffraction for these phases change with increasing temperature and anesthetic concentration, both favoring the liquid disordered phase.A variety of different volatile anesthetics--halothane, isoflurane, chloroform, and hexane_all produce significant increases in the ratios of liquid disordered to liquid ordered lipid phases in these mixtures. These shifts occur at clinically relevant concentrations and are reversible upon withdrawal of anesthetic. There were no consistent effects of the anesthetics on the d-spacings of the lipid layers.These findings suggest that some effects of volatile anesthetics may be mediated through physical changes in membrane domain structures that interact with membrane proteins.1. Weinrich, M., Nanda, H., et al., Halothane changes the domain structure of a binary lipid membrane. Langmuir, 2012. 28(10): p. 4723-8.