Two types of K⁺ channel in thick ascending limb of rat kidney

We have used the patch-clamp technique to study the apical K⁺ channels in the thick ascending limb (TAL) of the rat kidney. Two types of K⁺ channels, a low-conductance and an intermediate-conductance K⁺ channel, were identified in both cell-attached and inside-out patches. We confirmed the previously reported intermediate-conductance K⁺ channel (72 pS), which is inhibited by millimolar cell ATP, acidic pH, Ba²⁺, and quinidine (4). We now report a second K⁺ channel in apical membrane of the TAL. The slope conductance of this low-conductance K⁺ channel is 30 pS, and its open probability is 0.80 in cell-attached patches. This channel is not voltage dependent, and application of 2 mM ATP in the bath inhibits channel activity in inside-out patches. In addition, 250 μM glyburide, an ATP-sensitive K⁺ channel inhibitor, blocks channel activity, whereas the same concentration of glyburide has no inhibitory effect on the 72-pS K⁺ channel. Channel activity of the 30-pS K⁺ channel decreases rapidly upon excision of patches (channel run down). Application of 0.1 mM ATP and the catalytic subunit of adenosine 3′,5′-cyclic monophosphate (cAMP)-dependent protein kinase A (PKA) restores channel activity. Furthermore, addition of 0.1 mM 8-(4-chlorophenylthio)-cAMP or 50-100 pM vasopressin in the cell-attached patches increases channel activity. In conclusion, two types of K⁺ channels are present in the apical membrane of TAL of rat kidney, and PKA plays an important role in modulation of the low-conductance K⁺ channel activity.