Ionic fluxes through myelin membrane vesicles

Technical difficulties and a lack of reproducibility in procedures aimed at the production of myelin vesicles have delayed functional studies on membrane transport through myelin. Myelin vesicles could provide an excellent model for the study of the transport of ions and water, etc., across this type of membrane. They could also help improve our understanding of the molecular functions of the myelin sheath. In this investigation, a novel, nonaggressive method of producing central nervous system myelin vesicles is presented. Purified bovine myelin was incubated with iminodiacetic acid (an insoluble chelating agent that is easy to remove and does not interfere with further functional assays), and rendered insoluble on 1% crosslinked polystyrene beads (Chelex-100). Myelin vesicles obtained were impermeable to sugars (sucrose, glucose, and galactose), but showed a degree of permeability towards potassium salts as determined by light-scattering. Further experiments with fluorescent probes revealed an electrogenic K+ influx, as measured by oxonol V fluorescence quenching, and a significant H+ permeability measured using the pH-sensitive probe acridine orange. H+ permeability was not detected in control liposomes made from the same endogenous myelin lipids without protein. The results are discussed with reference to previous studies performed using purified myelin proteins in reconstituted systems. The relevance of these results with respect to ionic transport across myelin membrane is discussed.