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

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 Ca2+-dependent K+ channels. Cells were bathed in high-K+ (150 mM) solution with [Ca2+] 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, similar to 40 pS) and large conductance(BK, > 200 pS) K+ channels. The open probability (P-0) of IK channels increased with depolarization, the voltage for half-maximal activation averaging + 12 mV in 80 nM Ca-bath(2+). Raising the [Ca2+] in the high-K+ solution from 80 nM to 150 nM increased the P-0 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 Ca2+-dependent K+ conductance in visceral smooth muscle. (C) 1998 Elsevier Science B.V. All rights reserved.