Calcium-activated epithelial potassium channels.

Calcium-activated K channels have now been directly demonstrated with patch clamping in many epithelia and are present in the apical cell membrane of the proximal tubule, the diluting segment and the cortical collecting tubule of the kidney. These channels with a typical conductance of 50-100 ps (3-6.10(6) ions/s), are highly selective for potassium but also allow passage of Rb, NH4 and Na. Conductance is modified by the concentration of K bathing the channel and depolarization increases the channel open probability. Increases in cell Ca, in intracellular pH or in cyclic AMP increase K channel activity and could thus stimulate secretory K flux from the cell into the lumen. In addition, increased K recycling may also affect reabsorption of other solutes, either by regulating the supply of K to the NaCl cotransporter or, alternatively, by stabilizing the apical membrane potential and thus maintaining an effective electrochemical driving force for electrogenic apical Na entry.