Fine structure of ultrathin artificial membranes : I. Changes by acetylcholine addition in lipid proteolipid membranes.

A special technique for the electron-microscope study of the fine structure of ultrathin artificial membranes is described. Membranes made of total phospholipids of the cerebral cortex and cholesterol showed globular elements of 40 Å embedded in a denser and diffuse matrix. These same elements were also seen organized in a periodic banded pattern. Identical patterns were observed with and without supporting films. Lipidic membranes containing small amounts of proteolipid fromElectrophorus showed a lower electron density, a finer and smoother texture and a decrease in electrical resistance.Lipidic membranes containing the cholinergic receptor proteolipid fromElectrophorus, upon addition of acetylcholine, showed a rapid and transient rise in conductance which was accompanied by changes in fine structure, consisting in a more uneven corrugated appearance of the membrane and the presence of dense spots of 20 Å. These results are discussed in relation to the channel hypothesis of ion permeability. It is postulated that the binding of acetylcholine by the receptor proteolipid results in conformational changes in this protein that facilitate the translocation of ions through the membrane.