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It has been shown in a previous report that volatile anesthetics enhance the fluorescence of 1-anilino-8-naphthalene sulphonate (ANS) bound to biological membranes, and this enhancement is caused by the increase of the number of binding sites (n) for ANS (1). It has been also confirmed that such an increase in the n value induced by anesthetics is seen only in lipid vesicles containing cholesterol (2). Since ANS is thought to be bound to the membrane surface because of its negative charge, the above findings suggest that the electrostatic change of the membrane surface may be induced by anesthetics through the interaction with lipid components, particularly with cholesterol. On the other hand, it has been reported that the fluidizing effect of various anesthetics on membranes is modulated by the cholesterol and/or negatively charged phospholipids (3). In the present study, we have examined the possible effect of chloroform, a volatile anesthetic, on the fluidity of membrane using the fluorescence polarization of 1,6-diphenyl 1,3,5-hexatriene (DPH), which is thought to be localized almost exclusively in the hydro carbon core of the membrane (4), under the same conditions as used in the ANS studies (1, 2). Egg yolk phosphatidylcholine (PC) or dipalmitoyl phosphatidylcholine (DPPC), one of the constituents of egg yolk PC which has saturated hydrocarbon chains, was mixed with various amounts of cholesterol dissolved in chloroform. Chloroform was carefully removed using N2 gas and subsequently by a vacuum pump for more than 3 hr. Lipid vesicles were prepared in 0.1 M Tris-HCI buffer (pH 7.4) by ultrasonication (30 secx 5) using a tip-type sonicator, at room temper ature and above the transition temperature (approx. 41 'C) for PC and DPPC vesicles, respectively. The steady-state fluorescence polarization measurement was done accord ing to the method of Shinitzky and Barenholz (4). Briefly, lipid vesicles were incubated with 1 W DPH for 1 hr at 20'C, and the fluorescence was measured by a Shimadzu fluorescence spectrometer Model RF-502, equipped with polarizing filters, FORMULA 105UV (Shimadzu Seisakusho, Kyoto, Japan). The apparent microviscosity was calculated according to the following equation which was derived by Shinitzky and Barenholz (4) from the Perrin's equation (5). =2 .4 r/(0.362-r)

Chloroform counteracts effect of cholesterol on microviscosity of lipid vesicle.