Dependence of serosal membrane potential on mucosal membrane potential in toad urinary bladder.

Ussing's model for sodium transport across epithelia assumes the existence of two barriers in series; the outer barrier behaves as a sodium electrode and the inner barrier as a potassium electrode (Koefoed-Johnsen and Ussing, 1958); furthermore, these membranes are supposed to be independent of one another. Although experimental determinations of the changes in transepithelial potential across frog skin and toad urinary bladder after ionic substitutions agree with this view (Koefoed-Johnsen and Ussing, 1958; Gatzy and Clarkson, 1965; Leb et al., 1965), microelectrode experiments have shown that, in the steady state, unilateral changes in the composition of the bathing solution alter the potential difference across both barriers in frog skin (Cereijido and Curran, 1965) and Amphiuma distal tubule (Wiederholt and Giebisch, 1974). However, an alternative explanation for these observations would be that significant alterations in intracellular ionic activities took place. Accordingly, it has been proposed that fast changes in the composition of the outer bathing solution in frog skin or toad urinary bladder produce changes in the transepithelial potential by altering only the potential drop across the outer barrier (Lindemann and Gebhardt, 1973). The present experiments were designed to study the alterations in potential profile across the toad urinary bladder epithelium during changes of the composition of the mucosal bathing solution. Urinary bladders from Colombian toads were studied as previously described (Reuss and Finn, 1974). The solutions employed were standard amphibian Ringer (in millimoles per liter, NaCl 109, NaHCO3 2.4, CaC12 0.9, KCI 2.5, glucose 5.5), potassium-Ringer (equimolar K-for-Na substitution), or Ringer plus 10-5 or 10-4 M amiloride. Short current pulses were passed transepithelially in order to measure the total transepithelial resistance and the ratio of the apical to basal-lateral membrane resistances. The mucosal solution was changed by gravity superfusion through a glass pipette with the microelectrode in a cell.