Kinetics for development of gramicidin-induced ion permeability in unilamellar phospholipid vesicles

Two kinetics for the development of gramicidin-dependent cation permeability in small unilamellar vesicles have been studied by using a vesicle-entrapped, pH-sensitive fluorescence probe to continuously report changes in intravesicular pH. The incorporation of 4-5 gramicidin dimers/vesicle was sufficient to increase the proton and counterion permeability of that vesicle by several orders of magnitude, so that ionic equilibration following a perturbation of the external medium pH occurred in less than 1 s. Once a functional gramicidin dimer has become incorporated into one vesicle, it does not readily exchange into another, so that the effects of gramicidin with regard to an individual vesicle can be considered to be essentially "all or none." The rate at which transmembrane ion permeability develops in a vesicle suspension was found to depend upon the degree of fluidity of the membrane hydrocarbon interior, being much lower at low temperatures or when cholesterol was present in the bilayer. Low temperatures and increasing bilayer cholesterol content also decreased the number of vesicles affected by a given gramicidin concentration, indicating a decreased membrane solubility for the ionophore at low bilayer fluidities.