Effect of oxytocin on electrical potential and ionic permeability of the apical membrane of frog gall bladder epithelial cells

The effect of oxytocin on the intracellular Na+ and K+ concentrations, the level of the transmembrane potential difference, and the relative ionic permeability (PNa/PK) of the apical parts of the surface membrane of the epithelial cells was studied in experiments on the isolated frog gall bladder. In a dose of 20 milliunits/ml, oxytocin, added to the external incubation solution, reduced the transmembrane potential difference, increased the PNa/PK ratio, and led to a very small shift in the Na+ and K+ concentrations in the intracellular fluid. After exposure of the organ to the hormone for 30 min the membrane potential (MP) of the cells fell from 52.7 to 38.7 mV at P<0.001 (negative charge inside the cell), whereas the value of PNa/P was increased from 0.083 (control) to 0.175 (experiment) at P<0.001. Meanwhile the intracellular Na+ concentration was increased by 18.3 meq/kg and the K+ concentration reduced by 5.2 meq/kg intracellular water. This shift in the intracellular concentrations of Na+ and K+ could result in an evident decrease of only 0.7 mV in MP whereas in fact MP fell by 14.0 mV. Consequently, the decrease in the transmembrane potential difference takes place as a result of an increase in PNa/PK under the influence of oxytocin. Electrogenic ion transport through the apical membranes of the frog gall bladder epithelial cells could not be detected.