An intermediate conductance K + channel in the cell membrane of mouse intestinal smooth muscle

Abstract

Single channel currents were recorded from cell-attached and inside-out patches in smooth muscle cells of the mouse ileum in order to identify TEA-sensitive Ca 2+-dependent K + channels. Cells were bathed in high-K + (150 mM) solution with [Ca 2+] buffered to 80–150 nM with EGTA and patch pipettes were filled with low-K + (2.5 mM) physiological solution. Two distinct TEA-sensitive unitary outward current levels were identified at a holding potential ( V h) of 0 mV, corresponding to intermediate conductance (IK, ∼40 pS) and large conductance (BK, >200 pS) K + channels. The open probability ( P o) of IK channels increased with depolarization, the voltage for half-maximal activation averaging +12 mV in 80 nM Ca bath 2+. Raising the [Ca 2+] in the high-K + solution from 80 nM to 150 nM increased the P o of IK channels at V h=0 mV from 0.078 to 0.21. Likewise, the open probability of BK channels at 0 mV was increased from 0.003 to 0.026. Unlike BK channels, IK channels inactivated with maintained depolarization with a voltage for half-maximal inactivation of −66 mV. IK channels were blocked by 2–5 mM external TEA and were sensitive to both charybdotoxin (100 nM) and apamin (500 nM). Our results suggest that IK channels contribute significantly to the Ca 2+-dependent K + conductance in visceral smooth muscle.