Activation of small conductance Ca2+-dependent K+ channels by purinergic agonists in smooth muscle cells of the mouse ileum

1. Whole-cell and single-channel K+ currents were recorded at room temperature (22-24 degrees C), from smooth muscle cells enzymatically dispersed from the mouse ileum, using variations of the patch-clamp technique. 2. Net outward K+ currents recorded through amphotericin-B-perforated patches in response to step depolarizations positive to -50 mV from a holding potential of -80 mV were decreased by up to 70% by external apamin (0.5 mu m). Apamin-sensitive whole-cell currents were also recorded from cells perfused internally with 150 nM Ca2+ but not from cells perfused internally with 85 nM Ca2+. 3. Three types of non-inactivating Ca2+-sensitive K+ channels were identified in cell-attached and excised patches under an asymmetrical K+ gradient: (i) large conductance (BKCa; similar to 200 pX) channels blocked by 2 mM external TEA; (ii) intermediate conductance (IKCa; similar to 39 pS) channels blocked by 2 mM external TEE and inhibited by external apamin (0.5 mu M); and (iii) small conductance (SKCa; similar to 10 pS) channels that were not blocked by 5 mM external TEA but were sensitive to extracellular apamin (0.5 mu M). 4. The TEA-resistant SKCa channels were activated by an increase in [Ca2+](i) with an EC50 of 1.5 mu M and a Hill coefficient of 1.3. 5. P-2 purinoceptor agonists 2-methylthioATP (2-MeSATP), 2-chloroATP and ATP (10-50 mu M) increased an apamin-sensitive whole-cell outward K+ current. Extra;patch application of 2-MeSATP (20-100 mu M) stimulated the apamin-sensitive IKCa and SKCa channels and activated an apamin-sensitive steady outward current at 0 mV. 6. Smooth muscle cells from the mouse ileum possess two apamin-sensitive K+ channels (IKCa and SKCa); of these, the IKCa channels are TEA sensitive while the SKCa channels are TEA resistant. These channels, along with an apamin-sensitive but TEA-resistant steady outward current, may mediate membrane hyperpolarization elicited by purinergic agonists.