Ion permeability of rabbit intestinal brush border membrane vesicles

The ion permeability of rabbit jejunal brush border membrane vesicles was studied by measuring unidirectional fluxes with radioactive tracers and bi-ionic diffusion potentials with the potential-sensitive fluorescent dye, diS-C3-(5). Tracer measurements provide estimates of the absolute magnitudes of permeability coefficients, while fluorescence measurements provide estimates of relative and absolute ion permeabilities. The magnitudes of the permeability coefficients for Na+, K+, Rb+, and Br- were approximately 5 nanoliters/(mg protein X sec) or 10(-5) cm/sec as determined by radioactive tracer measurements. The apparent selectivity sequence, relative to Na+, as determined by bi-ionic potential measurements was: F-, isethionate, gluconate, choline (less than 0.1) less than Na+(1.0) less than Cl-(1.5) = NO-3(1.5) less than Br-(2.3) less than K+(2.4) less than Rb+(2.5) less than Cs+(2.6) less than Li+(3.9) less than NH+4(12) less than I-(40). The origin of this selectivity sequence and its relationship to the ion permeability of the brush border membrane in the intact epithelium are discussed.