Divalent cations cooperatively stabilize close membrane contacts in myelin.
Intact nerve myelin compacts to a dehydrated structure of closely apposed membranes when exposed to isotonic solutions at least 10 mM in calcium or tetracaine. The repeat period of the membrane pair in the compacted structure measured by X-ray diffraction is about 126 A in both central and peripheral mammalian nerve myelins whereas the normal periods are about 158 and 178 A, respectively. The electron density profile of compacted myelin shows an asymmetric membrane unit with thickness similar to that of the symmetric bilayer of flocculated myelin lipids. The centrosymmetrically averaged myelin membrane profile is similar to that of the lipid bilayer except at the surface where residual protein is concentrated. Dispersions of extracted total myelin lipids flocculate under similar conditions to those causing myelin compaction, indicating that similar forces act in both processes. Compaction is always accompanied by lateral segregation of intramembrane particles out of the close-packed domains. Lateral displacement of intramembrane proteins form compacted domains can be driven by the attraction of the lipid surfaces for each other. Rates of compaction vary with compacting reagent, concentration, tissue, and temperature, and probably reflect the permeability of the tissue. Extensive compaction by calcium or tetracaine leads to disruption and vesiculation of the spirally wrapped myelin membranes.