Acetylcholine receptor-controlled ion fluxes in membrane vesicles investigated by fast reaction techniques

Abstract

The ability of a nerve cell to integrate its response to chemical signals (neurotransmitters) arriving from many other cells is central to the function of the nervous system. This integration process depends on the ability of the neurotransmitters to bind to specific receptors which modulate the flow of specific inorganic ions through the cell membrane, and on the ability of the cell to transmit a (chemical) signal to an adjacent cell^1–3 only if the resulting change in transmembrane potential is of appropriate sign and magnitude. The controlling sign and magnitude of the cell transmembrane potential can be predicted from measurement of the rates of movement of specific inorganic ions across the membrane^4–6 and their dependence on neurotransmitter concentration. However, there have been two main obstacles to the development of a suitable preparation for such measurements. First, Kasai and Changeux⁷ obtained a membrane vesicle preparation from the electric organ of Electrophorus electricus (electroplax vesicles) in which acetylcholine receptor-controlled fluxes of inorganic ions could be observed, but the flux rates seemed too slow to be of physiological significance⁸. We have shown that these slow flux rates were associated with a large fraction of the vesicle preparation which did not respond to acetylcholine and yet dominated the measurements⁹. We then accomplished the separation of the receptor-controlled fluxes from other processes^9,10. Second, the receptor-controlled flux was found to be biphasic, with an initial phase too fast to be measured by available techniques¹¹. We have now succeeded in applying a quench flow technique¹² to the determination of the flux rate of specific inorganic ions across vesicle membranes in the millisecond time region, a time resolution sufficient for measuring the initial phase of receptor-controlled fluxes with these vesicles.