Influence of membrane viscosity on the lateral and transverse mobility of carboxylic ionophores.

The rate of 45Ca or 22Na exchange-diffusion in multilamellar liposomes formed of dipalmitoyl-phosphatidylcholine (DPPC) and cholesterol and containing the ionophore A23187 or Br-X537A was dramatically increased when the temperature and, hence, fluidity of the lipid bilayer 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 degrees C) than in the low range of temperature (22-28 degrees 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 45Ca transport by the same ionophore at high temperatures. A decrease in the ionophore content of the liposomes shifted to a higher temperature the transition point between the flat and steep lines characterizing the temperature dependency of 45Ca transport. 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.