Specific reaction rate of acetylcholine receptor-controlled ion translocation: a comparison of measurements with membrane vesicles and with muscle cells.

Abstract

The specific reaction rate (.hivin.J) of the acetylcholine receptor-controlled ion translocation was determined. In eel Ringer''s solution (pH 7.0) at 1.degree. C, .hivin.J = 3 .times. 107 M -1 s-1. .hivin.J is an intrinsic constant that is characteristic of the receptor and independent of other properties of a receptor-containing cell that also determine the rates of ion translocation. Membrane vesicles (prepared from the electric organ of Electrophorus electricus) and a flow-quench technique that has a millisecond time resolution were used to measure the receptor-controlled ion translocation. The value of .hivin.J and the molar concentrations of receptor sites and inorganic ions were used to calculate that 6 .times. 103 ions are translocated per ms per receptor. Analysis of electrical noise in frog muscle cells at temperatures above 8.degree. C (Neher and Stevens) gave a value of about 1 .times. 104 ions ms-1 per channel. Each technique gives essentially the same result. It is now possible to correlate the results obtained when receptor function is measured in 2 different ways in membrane vesicles and in muscle cells: chemical kinetic measurements, using membrane vesicles, which relate the ligand binding and ion translocation processes and analysis of acetylcholine noise in muscle cells (Katz and Miledi), which allows measurement of elementary steps in the formation of ion channels through the cell membrane.