Channel Opening Process is Responsible for the Slow Component in the Flux Measurement of Membrane Vesicles

The time course of tracer ion efflux from vesicles of uniform size, each of which contains an ion channel was theoretically analyzed. It is shown that the apparent flux rate reflects the opening rate of the channel when the flux rate is much higher than the channel gating rates. The theory was also developed for the system of vesicles whose volume is changed by an osmotic pressure jump with permeable solutes. The theory was applied to analyze the results obtained from ion flux measurements with sarcoplasmic reticulum vesicles, where ion fluxes through anion and cation channels have been observed by the tracer method, the volume change-light scattering method and the fluorescence quenching method. The results were successfully interpreted with the present model referring to the results from the electrical single channel recordings.