The possible role of fixed membrane surface charges in acetylcholine release at the frog neuromuscular junction
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Bass and Moore [Proc. Nat. Acad. Sci.55:1214 (1966)] proposed that the vesicles containing acetylcholine undergo Brownian motion in the nerve terminals. Acetylcholine is released whenever a vesicle touches the inner face of the axolemma of the nerve terminal. The frequency at which contact is made is limited by an energy barrier that must be overcome before the vesicle can touch the axolemma. The energy barrier has two components. (1) An electrostatic repulsion between positive, fixed charges on the vesicles and a relatively positive potential at the face of the axolemma that is generated by the resting potential. (2) A layer of water molecules held to the vesicle by the surface charge. This model is inconsistent with experimental data. A modification of the model is presented. Both the vesicle and the inner face of the axolemma are assumed to have fixed, negative surface charges that are responsible for the energy barrier. By a series of simplifications, the model leads to two predictions. (1) A plot of the ln (miniature end plate potentials/sec) as a function of the concentration of ions in the axoplasm)−0.5 should give a straight line. (2) A plot of ln (end plate potential amplitudes) as a function of (extracellular Ca++ concentration)−0.5 should give a straight line. These predictions are shown to agree reasonably well with experimental data.
KeywordsWater Molecule Acetylcholine Brownian Motion Surface Charge Energy Barrier
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