Electrical properties of the cellular transepithelial pathway inNecturus gallbladder: III. Ionic permeability of the basolateral cell membrane

Summary

The ionic permeability of the basolateral membrane ofNecturus gallbladder epithelium was studied with intracellular microelectrode techniques. After removal of most of the subepithelial tissue (to reduce unstirred layer thickness), impalements were performed from the serosal side, and ionic substitutions were made in the serosal solution while a microelectrode was kept in a cell. Thus, it was possible to obtain continuous (and reversible) records of transepithelial and cell membrane potentials and to measure intermittently the transepithelial resistance and the ratio of cell membrane resistances. From these data and the mean value of the equivalent resistance of the cell membranes in parallel (obtained from cable analysis in a different group of tissues), absolute cell membrane and shunt resistances and equivalent electromotive forces (emf's) were calculated. From the changes of basolateral membrane emf (E _b ) produced by the substitutions, the conductance (G) and permeability (P) of the membrane for K, Cl and Na were estimated. Potassium-for-sodium substitutions produced large reductions of both cell membrane potentials, ofE _b , and of the resistance of the basolateral membrane (R _b ), indicating highG _K andP _K . Chloride substitution with isethionate or sulfate resulted in smaller changes of cell membrane potentials andE _b and in no significant change ofR _b , indicating small but measurable values ofG _Cl andP _Cl . Sodium substitutions with N-methyl-d-glucamine (NMDG) resulted in cell potential changes entirely attributable to the biionic potential produced in the shunt pathway (P _Na >P _NMDG ), and in no significant changes ofP _b orE _b , indicating thatG _Na andP _Na are undetectable. The question of the mechanism of Cl transport across the basolateral membrane was addressed by comparing the mean rate of transepithelial Cl transport (J ^net _Cl ) and the predicted passive Cl flux across the basolateral membrane (from the membrane Cl conductance, potential, and Cl equilibrium potential). The conclusion is that only a very small fraction of the Cl flux across the basolateral membrane can be electrodiffusional. Since the paracellular Cl conductance is also too low to account forJ ^net _Cl , these results suggest the presence of a neutral mechanism of Cl extrusion from the cells. This could be a NaCl pump, a downhill KCl transport mechanism, or a Cl−HCO₃ exchange mechanism.