Chemistry and Physics of Lipids, 31 (1982) 227-235 227 Elsevier Scientific Publishers Ireland Ltd. INFLUENCE OF MEMBRANE VISCOSITY ON THE LATERAL AND TRANSVERSE MOBILITY OF CARBOXYLIC IONOPHORES M. DELEERS and W.J. MALAISSE Laboratory of Experimental Medt'cine, Brussels University School o/Medicine, B-1000 Brussels {Belgium) Received March 10th, 1 9 8 2 accepted May 28th, 1982 The rate of 4sCa or 2~Na exchange-diffusion in multilameUar liposomes formed of dipal- mitoyl-phosphatidycholine (DPPC) and cholesterol and containing the ionophore A23187 or Br-X537A was dramatically increased when the temperature and, hence, fluidity of the lipid bilaver were increased. In the case of 45Ca transport, i.e. when each Ca2÷ ion binds to two molecules of ionophore, the relative increment in transport velocity in response to a given increase in temperature or fluidity was much more marked in the high range of temperature (30-40°C) than in the low range of temperature (22-28°C). In the case of 22Na transport, however, i.e. when each Na ÷ ion binds to only one ionophoretic molecule, the temperature- dependency of the transport process followed a single pattern throughout the entire range of temperature. In the latter case, the slope of the temperature-dependent line was the same as that seen for 4SCa transport by the same ionopho~ at high temperatures. A decrease in the ionophore content of the liposomes shifted to a higher temperature the transition point be- tween the fiat and steep lines characterizing the temperature dependency of 4SCatransport, it is concluded that the membrane viscosity affects both the lateral mobility of the ionophoretic molecules and the transverse mobility of the cation-ionophore complex. Keywords." ionophores; viscosity; liposomes. Introduction Recent studies have indicated that the efficiency ofionophore-mediated calcium exchange-diffusion across artificial membranes is tightly dependent on the viscosity of the lipid bilayer in which the ionophores are inserted [1--3]. In these previous studies, changes in the viscosity of the lipid bilayer were achieved by using liposomes formed of different types or different combinations oflipids. In the present study, we have explored the influence of membrane viscosity upon ionophore-mediated cationic transport by using liposomes with the same lipid composition but incubated at variable temperatures. Moreover, we have extended the scope of the study by comparing the transport of 4SCa and 22Na, respectively, as mediated by the sanle ionophore, namely bromolasalocid. This allowed us to demonstrate that the viscosity of the artificial membrane exerts a dual influence upon the efficiency of the trans- port process, by affecting both the transverse and lateral motility of the iono- phoretic molecules. 0009 --3084/82/0000-0000/$02.75 © 1982 Elsevier Scientific Publishers Ireland Ltd.