Two types of ATP-sensitive potassium channels in rat portal vein smooth muscle cells

1 Single-channel recordings were made from single, enzymatically isolated smooth muscle cells of rat portal vein by the patch-clamp technique. 2 Unitary potassium currents were identified through two types of K-channels with conductances in 60:130 mM K-gradient of 50 and 22 pS; these are referred to as LR and MK channels respectively. 3 The LK channels became extremely active if isolated patches were created into nucleotide-free solution; activity was inhibited by ATP applied to the inner surface of the patch with a half maximal inhibition (K-i) of 11-23 mu M. Channel activity declined and disappeared with time and could be regenerated by a brief application of Mg-ATP or a nucleoside diphosphate such as UDP (in the presence of Mg). LK channel activity was rarely stimulated by levcromakalim and not by pinacidil (K-channel openers, KCOs) but was blocked by glibenclamide. 4 Activity of MK channels declined if isolated patches were created into nucleotide free solution; activity reappeared if UDP or ATP alone (in the presence of Mg) was applied; pinacidil or levcromakalim in the presence of ATP or UDP further increased channel activity which was blocked by glibenclamide. 5 The LK channel inhibited by ATP(i) is very similar in its conductance and other properties to the K-ATP channel described in tissues other than smooth muscle, in its conductance and properties the MK channel resembles the K-NDP channel we have previous described as present in other smooth muscles and opening in responses to KCOs.