Molecular mechanism of acetylcholine receptor-controlled ion translocation across cell membranes

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Abstract
Molecular processes (2), the binding of acetylcholine to the membrane-bound acetylcholine receptor protein and the receptor-controlled flux rates of specific inorganic ions are essential in determining the electrical membrane potential of nerve and muscle cells. The measurements reported establish the relationship between the 2 processes: the acetylcholine receptor-controlled transmembrane ion flux of 86Rb+ and the concentration of carbamoylcholine, a stable analog of acetylcholine. A 200-fold concentration range of carbamoylcholine was used. The flux was measured in the millisecond-to-minute time region by using a quench flow technique with membrane vesicles prepared from the electic organ of Electrophorus electricus in eel Ringer''s solution at pH 7.0 and 1.degree. C. The technique makes possible the study of the transmembrane transport of specific ions, with variable known internal and external ion concentrations, in a system in which a determinable number of receptors is exposed to a known concentration of ligand. The response curve of ion flux to ligand was sigmoidal, with an average maximum rate of 84 s-1. Carbamoylcholine induced inactivation of the receptor with a maximum rate of 2.7 s-1 and a different ligand dependence so that it was fast relative to ion flux at low ligand concentration, but slow relative to ion flux at high ligand concentration. The simplest model that fits the data consists of receptor in the active and inactive states in ligand-controlled equilibra. Receptor inactivation occurs with 1 or 2 ligand molecules bound. For channel opening, two ligand molecules bound to the active state are required, and cooperativity results from the channel opening process itself. With carbamoylcholine, apparently, the equilibrium position for the channel opening step is only 1/4 open. The integrated rate equation, based on the model, predicts the time dependence of receptor-controlled ion flux over the concentration range of carbamoylcholine. The values of the constants in the rate equation form the basis for predicting receptor-controlled changes in the transmembrane potential of cells and the conditions leading to transmission of signals between cells.