We observed the phase behaviors, and the changes of membrane fluidity in lipid membranes containing local anesthetics (LAs), lidocaine and tetracaine. The phase separation in DOPC/DPPC binary membranes had not been changed significantly by addition of LAs, whereas that in DOPC/DPPC/ Chol ternary membranes was suppressed by LAs. In order to evaluate the membrane fluidity, Laurdan generalized polarization values of membranes were measured. The fluidity of DOPC-rich phase in the ternary membrane became lower as LAs concentration had increased. Our results indicated the correlation between physical property change on membrane and pharmacological functions of LAs.Local anesthetics (LAs) are essential drugs for modern medicine that are often used to suppress patients' pain. LAs are known to target functional proteins, voltage-dependent sodium channels, which are located on cellular membranes. The mechanism how LAs shield pain signals was believed to be that LAs interact with the channel protein directly, and deactivate it.1 However, binding sites on the channel proteins had not been elucidated completely. In addition, according to past reports, LAs exhibit toxicity against human cells. The presence of LAs sometimes induces fatal side effects such as allergy, liver malfunction, and analgesia. 2 The mechanisms by which these side effects are induced by LAs are also not fully understood. Recently, however, some groups are expecting that these mechanisms can be explained by the change of the physical properties of lipid membranes where the proteins are buried induced by addition of LAs. 3Cell membranes, which are composed of lipid bilayer structure, are known to relate with signal transductions via membrane dynamics such as vesicular formation. In fact, some studies are providing correlation between physical property changes on biomembrane and behavior of channel proteins. 4 Moreover, a "lipid raft model" was proposed by Simons and Ikonen in 1997. 5,6 Lipid raft is a region where a large amount of saturated lipids and cholesterols are included and some functional molecules such as ion channels and membrane receptors can be localized, and lipid raft is speculated as a platform for signal transduction.In order to reveal the mechanism of raft formation, liposome, which is mainly composed of phospholipids, is used as a mimetic model of a biomembrane system. Liposome has similar structure to actual biomembranes, so we can directly observe similar phenomena on liposome with optical microscopy. Especially, raft formation can be regarded as the phase separation between saturated lipid and cholesterol-rich and unsaturated lipid-rich phases in a model membrane system, so from a biological aspect, it is important to discuss phase behavior of model membrane. 7In this study, we observed the phase separation in lipid membranes containing typical LAs, lidocaine and tetracaine, by optical microscopy. In order to understand the phase separation, the membrane fluidity is estimated by Laurdan generalized polarization measurement. Finally, w...

The functional mechanisms of local anesthetics (LAs) have not yet been fully explained, despite their importance in modern medicine. Recently, an indirect interaction between channel proteins and LAs was proposed as follows: LAs alter the physical properties of lipid membranes, thus affecting the channel proteins. To examine this hypothesis, we investigated changes in thermal stability in lipid membranes consisting of dioleoylphosphocholine, dipalmitoylphosphocholine, and cholesterol by adding the LAs, lidocaine and tetracaine. The miscibility temperature of liquid-ordered (Lo) and liquid-disordered (Ld) phase separation was lowered, whereas that of phase separation between solid-ordered (So) and Ld phases was unchanged by LAs. Furthermore, we measured the line tension at the Lo/Ld interface from domain boundary fluctuation and found that it was significantly decreased by LAs. Finally, differential scanning calorimetry (DSC) revealed a change in the lipid main transition temperature on the addition of LAs. Based on the DSC measurements, we considered that LAs are partitioned into two coexisting phases.