Modulation of alamethicin-induced conductance by membrane composition.

The effect of cholesterol on the opening and closing of the alamethicin-channel was studied varying the cholesterol content of glycerolmonooleate membranes. Increasing the cholesterol mole fraction in the membrane shifted the log of the steady-state conductance-voltage curve to the right along the voltage axis. The shift was found to be 80-100 mV when the cholesterol mole fraction was increased from 0 to 0.5. The kinetic for the approach to steady-state conductance during voltage clamp was also studied in membranes under these conditions. At all the different cholesterol contents the on kinetic is well described by a single exponential. The off kinetic, on the other hand, is described by a double exponential time course. At the same voltage the time constant describing the on current relaxation is the same as the one found for the slow relaxation present in the off relaxation. Increasing the membrane cholesterol content increases the magnitude of the time constant describing the fast and slow process by several orders of magnitude. Cholesterol also increases the voltage dependence of the slow time constant. The effect of cholesterol on the steady-state conductance can be explained, but not in a unique way, by an increase in the membrane dipole potential. Phenomenological comparison of the fast and slow kinetic processes seen in multi-channel membranes with single channel characteristics indicates that: (i) cholesterol increases the mean life of the single channel and (ii) cholesterol increases the mean life of the different conductance levels.