Apical Membrane K Conductance in the Toad Urinary Bladder

The conductance of the apical membrane of the toad urinary bladder was studied under voltage-clamp conditions at hyperpolarizing potentials (mucosa negative to serosa). The serosal medium contained high KCl concentrations to reduce the voltage and electrical resistance across the basal-lateral membrane, and the mucosal solution was Na free, or contained amiloride, to eliminate the conductance of the apical Na channels. As the mucosal potential (Vm) was made more negative the slope conductance of the epithelium increased, reaching a maximum at Vm = -100 mV. This rectifying conductance activated with a time constant of 2 msec when Vm was changed abruptly from 0 to -100 mV, and remained elevated for at least 10 min, although some decrease of current was observed. Returning Vm to +100 mV deactivated the conductance within 1 msec. Ion substitution experiments showed that the rectified current was carried mostly by cations moving from cell to mucosa. Measurement of K flux showed that the current could be accounted for by net movement of K across the apical membrane, implying a voltage-dependent conductance to K (GK). Mucosal addition of the K channel blockers TEA and Cs had no effect on GK, while 29 mM Ba diminished it slightly. Mucosal Mg (29 mM) also reduced GK, while Ca (29 mM) stimulated it. GK was blocked by lowering the mucosal pH with an apparent pKI of 4.5. Quinidine (0.5 mM in the serosal bath) reduced GK by 80%. GK was stimulated by ADH (20 mU/ml), 8-Br-cAMP (1 mM), carbachol (100 microM), aldosterone (5 X 10(-7) M for 18 hr), intracellular Li and extracellular CO2.