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

Exposing the apical membrane of toad urinary bladder to the ionophore nystatin lowers its resistance to less than 100 omega cm2. The basolateral membrane can then be studied by means of transepithelial measurements. If the mucosal solution contains more than 5 mM 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 SO2-(4) Ringer's solution it is the major conductive pathway in the basolateral membrane even though the serosal side has 60 mM K+. This pathway can be blocked by serosal amiloride (Ki = 13.1 microM) or serosal Na+ ions (Ki approximately 10 to 20 mM). It also conducts Li+ and shows a voltage-dependent relaxation with characteristic rates of 10 to 20 rad sec-1 at 0 mV.