Effects of changes in the composition of the serosal solution on the electrical properties of the toad urinary bladder epithelium.

1. The potential profile and the cellular and paracellular transepithelial resistances of the toad urinary bladder were measured, by means of micro-electrode techniques, as functions of the osmolality of the serosal solution. 2. Reductions in serosal osmolality (that increase the rate of active sodium transport) produced proportional decreases in the electrical resistances of the apical and basal-lateral cell membranes, while the changes in resistance of the paracellular pathway were more complex. The apical membrane potential increased. 3. Increases in serosal osmolality (that decrease sodium transport) produced increases in the electrical resistances of both cell membranes, and moderate reduction in the paracellular resistance. The polarity of the apical membrane potential reversed. 4. These results indicate that reductions in serosal solution osmolality stimulate sodium transport by increasing both the sodium permeability of the luminal cell membrane (thus increasing sodium entry), and the electromotive force generated at the serosal border of the cell, thus enhancing the rate of sodium pumping. Conversely, increases in osmolality reduced sodium transport by reducing both the sodium permeability of the luminal membrane and the serosal membrane electromotive force.