Application of calcein-loaded liposomes for the determination of membrane channel size.

We found that calcein-loaded liposomes can be used to evaluate the sizes of channels in membranes by measuring changes in calcein release when the molecular size of the solute added to the outer suspension medium is changed. If the solute added to the outer medium can enter the inner aqueous phase through the channel, the osmotic pressure of the inner phase increases, causing bursting of the liposomes and the release of calcein. Thus, the size of the channel formed in the liposomal membrane can be determined by examining whether the solute added to the outer medium induces calcein release. We used a series of sugars as solutes and estimated the channel size formed by polyene antibiotics. The results agreed well with those conducted in a similar manner using erythrocytes, demonstrating that this method should be useful for examining channel sizes in membranes.