Ion channel-mediated fluxes in membrane vesicles: selective amplification of isotope uptake by electrical diffusion potentials.

The procedure we have described provides a simple, convenient, and sensitive method to assay conductive ion fluxes in membrane vesicles. It is particularly useful for detecting channels in heterogeneous populations of vesicles. The principal advantages are similar to those of sensitive enzyme assays, namely, screening for existence of channels in different membrane fractions, assaying purified channel proteins, large-scale testing of pharmacological agents, antibodies, etc. and in studies of macroscopic regulatory features, including channel activity or density in different states and interaction with regulatory ligands. In the future one can expect further applications in detecting synthesis of channel proteins, gene expression, etc. The tracer assay does not provide much information on molecular characteristics such as single-channel conductance, voltage sensitivity, and ion specificity. It therefore serves other purposes to those of the modern biophysical methods such as patch-clamp, noise analysis, and study of channels incorporated into bilayers.