Two Ca-dependent K-channels classified by the application of tetraethylammonium distribute to smooth muscle membranes of the rabbit portal vein

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Abstract
Dispersed single smooth muscle cells of rabbit portal vein were prepared by treatment with collagenase and trypsin. The muscle cells were 100–300 μm in length, 5–10 μm in maximum width and cylindrical in shape. In insideout membrane patches, two different amplitudes of ionic currents were recorded, and these single channel conductances were 273 pS (Kl-channel) and 92 pS (Ks-channel), when both sides of the membrane were exposed to 142 mM K+ solution. The channel conductances depended on concentrations of K+ on both sides of the membrane. When K+ were replaced with Na+ or Tris+, these single-channel currents were abolished. When the concentration of Ca2+ inside the membrane was greater than 10−7 M, the channel activity was enhanced but there was enhancement when Ca2+ was applied to the extracellular membrane surface, in concentrations ranging between 10−9 and 10−3 M. During application of tetraethylammonium (TEA+; 1–10 mM) to the intracellular membrane surface, amplitudes of the single-channel current of both types of the K-channel were not modified. By contrast application of TEA+ (0.1–1 mM) to the extracellular membrane surface, reduced the amplitudes of the current and increased noise levels during the open-state of the Kl-channels, but did not have such an effect on the Ks-channel. We conclude that there are at least two different Ca-dependent K-channels distributed on the smooth muscle membrane of the rabbit portal vein. TEA+ applied to the extracellular membrane surface blocks activation of the Kl-channel, but not that of the Ks-channel. These two Ca-dependent K-channels do not seem to be important for maintenance of the resting membrane potential, but do play an important role in the repolarizing stage of the Ca spikes, in the rabbit portal vein.