193 Biochimica et Biophysica Acta, 597 (1980) 193--198 © Elsevier/North-Holland Biomedical Press BBA 78706 EFFECTS OF GASEOUS ANAESTHETICS AND INERT GASES ON THE PHASE TRANSITION IN SMECTIC MESOPHASES OF DIPALMITOYL PHOSPHATIDYLCHOLINE WILLIAM MACNAUGHTAN * and ALISTER G. MACDONALD Physiology Department, Marischal College, Aberdeen AB9 1AS (U.K.) (Received May 25th, 1979) Key words: Anesthe tic; Inert gas; Smec tic mesophase; Dipalmitoyl phospha tidylch oline; Phase transition Summary The phase transition in smectic mesophases of dipalmitoyl phosphatidyl- choline was studied under high pressures of helium (340 atm), nitrogen (340 atm), nitrous oxide (43 atm), cyclopropane (4.4 atm) and n-propane (8.2 atm), using a turbidimetric technique. Helium and nitrogen increased the transi- tion temperature by 0.021 and 0.006°C/atm, respectively, compared with 0.024°C/atm for hydrostatic pressure. Nitrous oxide reduced the transition by 0.58°C/arm. The hydrocarbon gases spread the transition width and lowered the transition temperature with increasing effect at higher doses. Comparisons with other membrane probes are made and the concentration of gases in the bilayer which lower the transition temperature by I°C are estimated, in mol%: He, 10.2; N2, 13.2; N20, 9.04; n-C3H8, 6.3 and cyclopropane, 12.8. Introduction Inert gases (nitrogen, nitrous oxide) exert interesting narcotic effects, apparently by a mechanism similar to that of the more potent inhalational general anaesthetics. The narcotic potency of inert gases is related to their solubility in hydrophobic solvents and in some cases their effects may be reversed by hydrostatic pressure. A narcotic dose of inert gas, such as nitrogen, comprises a significant hydrostatic pressure in conjunction with the perturba- tion caused by the dissolved gas molecules [1]. The least soluble of the inert gases, helium, fails to narcotise animals, merely acting as a pressure-transmitting fluid. However, higher pressures of helium (>175 arm) have been shown to exert narcotic-like effects in cells by means of experiments which distinguish * Present address: MRC Cyclotron unit, Hammersmith Hospital, Du Cane Rd., London W12 OHS.