Origins of open-channel noise in the large potassium channel of sarcoplasmic reticulum.

Abstract

A B S T R A C TOpen-channel noise was studied in the large potassium channel of the sarcoplasmic reticulum (SR). Inside-out patches were excised directly from the SR of split skeletal muscle fibers of lobster, with lobster relaxing ringer (LRR) in bath and pipette. The power spectrum of open-channel noise is very low and approximately flat in the 100 Hz-10 kHz frequency range. At 20~ with an applied voltage of 50 mV, the mean single-channel current (i) is 9 pA (mean single-channel conductance --180 pS) and the mean power spectral density 1.1 x 10 -29 AZ/Hz. The latter increases nonlinearly with (i), showing a progressively steeper dependence as (i) increases. At 20 mV, the mean power spectral density is almost independent of (i) and ~ 1.4 times that of the Johnson noise calculated for the equivalent ideal resistor with zero net current; at 70 mV it increases approximately in proportion to (i) 2. The mean power spectral density has a weak temperature dependence, very similar to that of (i), and both are well described by a Ql0 of 1.3 throughout the range 3-40~ Discrete ion transport events are thought to account for a significant fraction of the measured open-channel noise, probably ~ 30-50% at 50 inV. Brief interruptions of the single-channel current, due either to blockage of the open channel by an extrinsic aqueous species, or to intrinsic conformational changes in the channel molecule itself, were a possible additional source of open-channel noise. Experiments in modified bathing solutions indicate, however, that open-channel noise is not affected by any of the identified aqueous species present in LRR. In particular, magnesium ions, the species thought most likely to cause brief blockages, and calcium and hydrogen ions, have no detectable effect. This channel's openings exhibit many brief closings and substates, due to intrinsic gating of the channel. Unresolved brief full closings are calculated to make a negligible contribution (< 1%) to the measured power spectral density. The only significant source of noise due to band width-limited missed events is brief, frequent 80% substates (mean duration 20 ~s, mean frequency 1,000 s -1) which account for a small part of the measured power spectral density (~ 14%, at 50 mV, 20~ We conclude that a large fraction of the measured open-channel noise results from intrinsic conductance fluctuations, with a corner frequency higher than the resolution of our recordings, in the range 104-107 Hz. We infer the presence of motions within the channel molecule in this frequency range. INTRODUCTIONOpen-channel noise means any fluctuation in a single-channel current recording that is present only when the channel is open. Gaussian, apparently continuous current fluctuations are seen in the nicotinic end-plate channel (Sigworth, 1985) and in a stretch-activated cation channel (Eisenberg, Hainsworth, and Levis, 1987). These are thought to reflect fluctuations in the channel conductance, as a result of vibration within the channel molecule. In addition, discontinuous step changes in...

Properties of Single Voltage-gated Proton Channels in Human Eosinophils Estimated by Noise Analysis and by Direct Measurement

“…The 1/ f component dominates at large depolarizing voltages because of the nonmonotonic voltage dependence of \documentclass[10pt]{article} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{pmc} \usepackage[Euler]{upgreek} \pagestyle{empty} \oddsidemargin -1.0in \begin{document} \begin{equation*}{\mathrm{{\sigma}}}_{{\mathrm{L}}}^{{\mathrm{2}}}\end{equation*}\end{document} at low pH i . For reference, the sum of the calculated shot noise of current flow and Johnson noise of the 50 GΩ feedback resistor in parallel with the patch resistance ( Hainsworth et al, 1994 ), both of which are white noise, is shown by the dashed lines without symbols in Figs. 10 A and 11.…”

“…Both of these components increased monotonically with V. The 1/f component dominates at large depolarizing voltages because of the nonmonotonic voltage dependence of at low pH i . For reference, the sum of the calculated shot noise of current flow and Johnson noise of the 50 G⍀ feedback resistor in parallel with the patch resistance (Hainsworth et al, 1994), both of which are white noise, is shown by the dashed lines without symbols in Figs. 10 A and 11.…”

Atomic Biology, Electrostatics, and Ionic Channels

“…The RR trajectories significantly complicate analysis because they (unlike the others) do not form a stochastic process that can directly analyzed by renewal theory (Cox, 1962). The cis trajectories RR and LL are likely to be important under normal conditions; indeed, they may dominate the normal open channel noise in at least some channels (Hainsworth, Levis, and Eisenberg, 1994). If a 'slow' ion is present (i.e., an ion with a small diffusion constant) the RR and LL trajectories are expected to determine most of the behavior observed experimentally.…”

Rectifying conductance substates in a large conductance Ca(2+)-activated K+ channel: evidence for a fluctuating barrier mechanism.

“…These authors proposed a flutter model involving a weighted average of two current levels as the basis for the substate. Recently, Hainsworth et al (1994) characterized excess open-state noise in SR K-channels from native lobster SR and concluded that motions of the channel molecule in the range of 104-107 Hz are a likely source of this behavior.…”