INWARD-RECTIFIER POTASSIUM CHANNELS IN BASOLATERAL MEMBRANES OF FROG-SKIN EPITHELIUM

Inward-rectifier K channel: using macroscopic voltage clamp and single -channel patch clamp techniques we have identified the K+ channel resp onsible for potassium recycling across basolateral membranes (BLM) of principal cells in intact epithelia isolated from frog skin. The spont aneously active K+ channel is an inward rectifier (K-ir) and is the ma jor component of macroscopic conductance of intact cells. The current- voltage relationship of BLM in intact cells of isolated epithelia, mou nted in miniature Ussing chambers (bathed on apical and basolateral si des in normal amphibian Ringer solution), showed pronounced inward rec tification which was K+-dependent and inhibited by Ba2+, H+, and quini dine. A 15-pS K-ir channel was the only type of K+-selective channel f ound in BLM in cell-attached membrane patches bathed in physiological solutions. Although the channel behaves as an inward rectifier, it con ducts outward current (K+ exit from the cell) with a very high open pr obability (P-o = 0.74-1.0) at membrane potentials less negative than t he Nernst potential for K+. The K-ir channel was transformed to a pure inward rectifier (no outward current) in cell-attached membranes when the patch pipette contained 120 mM KCl Ringer solution (normal NaCl R inger in bath). Inward rectification is caused by Mg2+ block of outwar d current and the single-channel current-voltage relation was linear w hen Mg2+ was removed from the cytosolic side. Whole-cell current-volta ge relations of isolated principal cells were also inwardly rectified. Power density spectra of ensemble current noise could be fit by a sin gle Lorentzian function, which displayed a K dependence indicative of spontaneously fluctuating Ki, channels. Conclusions: under physiologic al ionic gradients, a 15-pS inward-rectifier K+ channel generates the resting BLM conductance in principal cells and recycles potassium in p arallel with the Na+/K+ ATPase pump.