SINGLE-CHANNEL AND WHOLE-CELL K-CURRENTS EVOKED BY LEVCROMAKALIM IN SMOOTH-MUSCLE CELLS FROM THE RABBIT PORTAL-VEIN

1 Single channel and whole-cell current recordings were made from single smooth muscle cells isolated from the rabbit portal vein. 2 Application of 10 muM levcromakalim ((-)-Ckm) to single cells held with pipettes containing 1 mM GDP induced a K-current (I(K(Ckm))) which occurred in addition to the current caused by GDP alone (I(K(GDP))) and averaged 135 pA at - 37 mV. We have investigated whether the same K channels underlie the GDP- and Ckm-induced K-currents. 3 If 1 mM GDP was in the pipette but Mg ions were omitted the effect of GDP was absent and I(K(Ckm)) averaged only 10 pA, suggesting that the action of (-)-Ckm was Mg-dependent. 4 Intracellular ATP was not observed to have much effect on I(K(-Ckm)). Loading of cells with 10 mM ATP from the recording pipette had no significant effect and flash photolysis of caged-ATP loaded into cells from the pipette, estimated to release about 1 mM free ATP, also had no effect on I(K(-Ckm)). 5 Bath-applied glibenclamide inhibited I(K(-Ckm)) with an IC50 of 200 nM, a value 8 times higher than that found for inhibition of I(K(GDP)). The delayed rectifier K-current (I(K(DR))) was also inhibited by glibenclamide but at higher Concentrations (IC50 100 muM). Bath-applied tetraethylammonium ions (TEA) inhibited I(K(-Ckm)) and I(K(GDP)) to the same extent (IC50 about 7 mM). 6 In inside-out patch recordings (-)-Ckm (10 muM) applied to the intracellular surface of the membrane potentiated the opening of K channels already stimulated by 1 mM GDP and all of the channel activity was abolished by 10 muM glibenclamide. The unitary conductance of the channels was 24 pS in a 60 mM: 130 mM K-gradient. 7 We suggest that (-)-Ckm may hyperpolarize and relax smooth muscle cells by opening K(NDP), a class of small conductance K channels that are related to the ATP-sensitive K channels seen in other tissues.