Single channel and whole‐cell K‐currents evoked by levcromakalim in smooth muscle cells from the rabbit portal vein

Abstract

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 μm 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 μm). 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 μm) 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 μm 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.