K-current fluctuations in inward-rectifying channels of frog skeletal muscle.
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 10 mM 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 conductance gamma* was calculated. Gamma* strongly depends on the external Cs concentration (7.8 pS at 0.2 mM Cs, 2.1 pS at 10 mM 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 gamma approximately equal to 10 pS and a real density of n approximately equal to 4 inwardly rectifying channels per micrometer2 of muscle surface area.
D000818:Animals; D004553:Electric Conductivity; D007473:Ion Channels / Q000502:physiology*; D008433:Mathematics; D008564:Membrane Potentials; D008954:Models, Biological; D009132:Muscles / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D011893:Rana esculenta
D007473:Ion Channels; D011188:Potassium