Summary
K currents and K-current fluctuations were recorded in inwardly rectifying K channels of frog skeletal muscle under voltage-clamp conditions. External application of 0.2 to 10mm Cs reduces the inward mean K current but produces a distinct increase of the spectral density of K-current fluctuations. The additional fluctuations arise from the random blocking by Cs ions. From the variance of current fluctuations, the steady-state current and the probability of the open unblocked channel an effective single-channel conductanceγ * was calculated.γ * strongly depends on the external Cs concentration (7.8 pS at 0.2mm Cs, 2.1 pS at 10mm Cs). This dependence is interpreted in terms of a two-step blocking process: (1) a fast exchange of Cs ions between the external solution and a first binding site inside the channel which leads to the Cs-modulated effective single-channel conductance, and (2) a slow Cs binding to a second site deeper in the channel which produces the observed current fluctuations. With this hypothesis we obtained a real single-channel conductance of γ≈10 pS and a real density ofn≈4 inwardly rectifying channels per μm2 of muscle surface area.
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Schwarz, W., Neumcke, B. & Palade, P.T. K-current fluctuations in inward-rectifying channels of frog skeletal muscle. J. Membrain Biol. 63, 85–92 (1981). https://doi.org/10.1007/BF01969449
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DOI: https://doi.org/10.1007/BF01969449