An Amiloride-sensitive Na⁺ conductance in the basolateral membrane of toad urinary bladder

Summary

Exposing the apical membrane of toad urinary bladder to the ionophore nystatin lowers its resistance to less than 100 Ω cm². The basolateral membrane can then be studied by means of transepithelial measurements. If the mucosal solution contains more than 5mm Na⁺, and serosal Na⁺ is substituted by K⁺, Cs⁺, or N-methyl-d-glucamine, the basolateral membrane expresses what appears to be a large Na⁺ conductance, passing strong currents out of the cell. This pathway is insensitive to ouabain or vanadate and does not require serosal or mucosal Ca²⁺. In Cl-free SO ²⁻₄ Ringer's solution it is the major conductive pathway in the basolateral membrane even though the serosal side has 60mm K⁺. This pathway can be blocked by serosal amiloride (K _i=13.1 μm) or serosal Na⁺ ions (K _i∼ 10 to 20mm). It also conducts Li⁺ and shows a voltage-dependent relaxation with characteristic rates of 10 to 20 rad sec⁻¹ at 0 mV.