The magnitude of nonelectrolyte selectivity in the gallbladder epithelium

The permeability of the rabbit gallbladder epithelium to nonelectrolytes was determinted by radioactive tracer techniques and by a rapid osmotic procedure. As expected from empirical and theoretical considerations, there was a good agreement between the selectivity sequences obtained by the two methods for the sixteen compounds used in this study. Although the permeability coefficients are directly related to their bulk-phase partition coefficients, the gallbladder behaves as if the membranes controlling selectivatity are more hydrophilic than isobutanol. The relation between permeability coefficients and molecular weight also show that these membranes are less viscous than other single cell membranes. Small polar solutes exhibit lower apparent activiation energies for permeation than larger solutes, and this is taken as support for the view that small polar molecules permeate across this tissue via a polar pathway. Inutin and sucrose permeability coefficients are in the ratio of their free-solution diffusion coefficients, and the apparent surcose activation energy is indistinguishable from that reported for diffusion in aqueous solution. These latter observations may be explained by the presence of a few large "pores" in the epithelium.