Properties of unitary ATP-sensitive K^+ channels were studied using planar lipid bilayer technique. Vesicles were prepared from bullfrog (Rana cates5riana) skeletal muscle. ATP-sensitive K^+ (K(ATP)) channels were identified by their unitary conductance and sensitivity to ATP. In the symmetrical solution containing 200mM KCI, l0mM Hepes, 1mM EGTA and pH 7.2, single K(ATP) channels showed a linear current-voltage relations with slight inward rectification. Slope conductance at reversal potential was 60.1±0.43 pS(n=3)). Micromolar ATP reversibly inhibited the channel activity when applied to the cytoplasmic side. In the range of -50∼＋50㎷, the channel activity was not voltage-dependent, but the channel gating within a burst was more frequent at negative voltage range.
Varying the concentrations of external/internal KCl(mM) to 40/200, 200/200, 200/100 and 200/40 shifted reversal potentials to -30.8±2.9(n=3), -1.1±2.7(n=3), 10.5 and 30.6(㎷), respecrivety. These reversal potentials were close to the expected values by the Nernst equation, indicating nearly ideal selectivity for K^+ over Cl^- . Under bi-ionic conditions of 200mM external test ions and 200mM internal K^+, the reversal potentials for each test ion/K pair were measured. The measured reversal potentials were used for the calculation of the releative permeability of alkali cations to K^+ ions using the GoldmanHodgkin-Katz equation. The permeability sequence of 5 cations relative to K^+ was K^+ 1), Rb^+(0.49), -Cs^+(0.27), Na^+(0.027) and Li^+(0.021). This sequence was recognized as Eisenman´s selectivity sequence Ⅳ. In addition, modelling the permeation of K^+ ion through ATP-sensitive K^+ channel revealed that a 3-barrier 2-site multiple occupancy model can reasonably predict the observed current-voltage relations.
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