Electrophysiology of plasma membrane vesicles

In both renal and gastrointestinal physiology, it has become popular to study epithelial transport phenomena using vesicles isolated from the apical and basolateral cell membranes. Transport in vesicle preparations is usually monitored with radioactive tracers, but more recently attention has been directed to electrophysiological methods. As it is impossible to measure the electrical properties of membranes in small vesicles (less than 500 nm diam) with classical electrophysiological techniques, indirect methods have to be employed. In this review I focus on the application of voltage-sensitive optical probes to measure membrane potentials in brush border membrane vesicles. Optical signals are calibrated with diffusion potentials generated with known ion gradients in the presence of ionophores, e.g., EKS with K gradients in the presence of valinomycin. Membrane potential measurements can be used 1) to illustrate the specificity and kinetics of sugar-, amino acid-, and carboxylic acid-Na cotransport systems in brush border membranes, and 2) to determine the ion permeability of brush border membranes. All organic solutes known to be transported by Na cotransport across brush border membranes depolarize the membrane in a Na-dependent, saturable manner. The results agree, both qualitatively and quantitatively, with electrophysiological data obtained in the intact renal tubule and with tracer uptake in vesicles. Bi-ionic potential measurements demonstrate that brush border membranes are permselective to anions and cations, but there are indications that the permeabilities are somewhat dependent on the method of vesicle preparation and the experimental conditions. However, electrical potential measurements provide insight into the mechanisms of ion transport in vesicle preparations, and the application of patch-clamp techniques should provide further gains in the future.