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Department of Physiology, Yale University School of Medicine, New Haven, CT 06510.

1. Single potassium channels have been studied in the apical membrane of isolated diluting segment tubules of Amphiuma using the patch clamp technique in the cell-attached mode. A kinetic analysis of the interaction of the channel with barium is presented. 2. The channel shows inward rectification, with an inward conductance of 29.2 pS and an outward conductance of 7.5 pS (cell attached, 100 mM-KCl as pipette solution). 3. The channel is mildly voltage sensitive, the open probability of 0.8 at the resting membrane potential being reduced to 0.38 mV at a hyperpolarizing potential of 120 mV. The mean open time also decreased with hyperpolarization (from 51.3 to 11.6 ms at the above potentials). 4. Inclusion of barium in the pipette solution causes a concentration- and voltage-dependent block of the channel. The open probability was reduced by 40 and 79% of control with 0.1 and 1 mM-barium respectively (0 mV pipette potential). This inhibition increases to 70 and 95% at a pipette potential of 80 mV. 5. From the voltage dependence of the blockade by barium, it is estimated that barium ions block at a site one-third of the way across the membrane. 6. Given the high open probability of the channel at resting membrane potentials and the density of the channels on the apical surface of the cells it is concluded that the channels can account for the apical potassium conductance of the diluting segment of Amphiuma.