Microelectrode studies in toad urinary bladder epithelium. effects of Na concentration changes in the mucosal solution on equivalent electromotive forces.

Microelectrode techniques were employed to measure membrane potentials, the electrical resistance of the cell membranes, and the shunt pathway, and to compute the equivalent electromotive forces (EMF) at both cell borders in toad urinary bladder epithelium before and after reductions in mucosal sodium concentration. Basal electrical parameters were not significantly different from those obtained with impalements from the serosal side, indicating that mucosal impalements do not produce significant leaks in the apical membrane. A decrease in mucosal Na concentration caused the cellular resistance to increase and both apical and basolateral EMF to depolarize. When Na was reduced from 112 to 2.4 mM in bladders with spontaneously different baseline values of transepithelial potential difference (Vms), a direct relationship was found between the change in Vms brought about by the Na reduction and the base-line Vms before the change. A direct relationship was also found by plotting the change in EMF at the apical or basolateral border caused by a mucosal Na reduction with the corresponding base-line EMF before the change. These results indicate that resting apical membrane EMF (and, therefore, resting apical membrane potential) is determined by the Na selectivity of the apical membrane, whereas basolateral EMF is at least in part the result of rheogenic Na transport. These results are consistent with data of others that suggested a link between the activity of the basolateral Na pump and apical Na conductance.

Dual effects of amphotericin B on ion permeation in toad urinary bladder epithelium

The mechanisms of action of amphotericin B on the electrical properties of the toad urinary bladder epithelium were studied with microelectrode techniques. Cell membrane and transepithelial electrical potentials and resistances were measured in the absence and in the presence of the drug during exposure to bathing mediums of different ionic compositions. As observed previously by other investigators, amphotericin B produces a dramatic decrease of transepithelial electrical resistance (Rt) and an increase of the rate of sodium transport. Our results indicate that the effect of the drug on Rt depends in part on an increase in Na conductance across the luminal cell membrane (amiloride-insensitive), but is caused mainly by an increase of ionic conductances (with the sequence GK greater than GNa greater than G choline greater than GCI) at a site in parallel with the impaled cells (i.e., across a cell type not investigated by the microelectrode measurements or across the paracellular pathway.