
Figure 2. Fluctuation analysis of hIKs currents. (A) Mean current and timedependent variance computed from 30 sweeps filtered at 100 Hz. The current was induced by +50mV depolarizing pulses from a −80 mV holding potential in a cellattached patch recording. Pulses were delivered every 32 s. (B) Mean–variance plot of 100 Hz–filtered currents. A fit of Eq. 1 yields unitary current iv = 0.28 pA and number of channels (n) = 368; the corresponding apparent open probability Pmax = 0.43. (C) Mean–variance plot from a total of 20 sweeps at 10 kHz bandwidth from the same patch. In this case, 12s depolarizations to +50 mV were delivered every 32 s. The fit yields iv = 0.51 and Pmax = 0.24. (D) A pair of successive current traces and their difference filtered at 1 kHz; same set of data as in A. (E) The top trace is the resulting power spectrum of currents from 30 sweeps after correction for background noise. The contribution Sshot from ion transport during channels opening was estimated according to Sshot = 2e0I = 1.4 · 10−29 A2/Hz and was subtracted. The solid line indicates a powerlaw fit S(f) = 10−24/f 0.9 A2/Hz. The mean current was 45 pA, and the unitary current estimate is = 0.47 pA at 20 kHz bandwidth. The lower trace shows the spectrum (values plotted one decade lower in the graph for clarity) computed from another representative data set, where the mean current was 39 pA; the unitary current is = 0.44 pA at 20 kHz. (F) Dependence of apparent unitary current is (Eq. 3) on filter cutoff frequency at +50 mV. Variance was calculated from the numeric integral of the power spectrum over the frequency range 0.1 Hz–10 kHz.


Figure 3. Frequency distribution of hIKs patch currents. Current was measured at the end of a 5s depolarization to +50 mV in each of 128 patches, and histograms were constructed. The inset shows an expanded histogram, where the bin at zero represents the 43 patches that showed no IKs current.


Figure 4. Singlechannel hIKs current. (A) One sweep showing a putative hIKs singlechannel opening, recorded from a threechannel patch at +50 mV. Data were filtered at 500 Hz. (B) Allpoints amplitude histogram, which yields an apparent unitary current 0.34 pA (indicated by the dashed line in A). (C) Single channel currents estimated from amplitude histograms at 200 Hz bandwidth (n = 4 for +50 mV, n = 1 for other voltages). A linear fit yields 3 pS for the singlechannel conductance.


Figure 5. Fluctuation analysis of ILQT. (A) Ensemble mean current was calculated from pairs of sweeps from a total of 10 sweeps, obtained in a giant cellattached patch recording from an oocyte injected with KvLQT1 RNA alone. Currents were induced by depolarizing pulses to +50 mV from a −80mV holding potential and repolarized to −60 mV. Data were filtered at 200 Hz. (B) Time course of variance. (C) Variance–mean plot. A linear fit for the current range <50 pA yields unitary current iv = 0.04 pA; iv = 0.03 pA from a linear fit to the entire current range. (D) A pair of aligned current traces and the subtracted current trace filtered at 1 kHz. Same data as in A. (E) Power spectrum calculated from subtracted currents after background noise subtraction. Solid curve is the sum of three Lorentzians with corner frequencies 5, 141, and 5,000 Hz and amplitudes 3.6 · 10−25, 3.7 · 10−26 and 1.6 · 10−27 A2/Hz, respectively. The corresponding mean current was 240 pA, and the estimated singlechannel current is = 0.09 pA at 20 kHz. The bottom trace is the spectrum from another recording, displaced downward by one decade for clarity. The mean current in this case was 370 pA and is(20 kHz) = 0.08 pA. (F) Unitary current is was calculated from the integral of the power spectrum; points beyond 10 kHz were computed from the fitted function. In the patch recording, the bath solution was 93 mM Kaspartate, 7 mM KCl, 1 mM EGTA, 10 HEPES, pH 7.4; the pipette solution contained 100 mM NaCl, 1 mM MgCl2, 0.1 mM CaCl2, 5 mM HEPES.


Figure 6. Fluctuation analysis of rhIKs currents. (A) A pair of successive current traces, filtered at 1 kHz. The current was induced by +50mV depolarizing pulses from −80mV holding potential, and repolarized to –60 mV in a cellattached patch recording with 140 mM Kaspartate, 1 mM EGTA, 10 mM HEPES, pH 7.4 in the bath; 96 mM NaCl, 2 mM KCl, 1 mM MgCl2, 0.1 mM CaCl2, 5 mM HEPES in the pipette. Pulses were delivered every 33 s; mean current during the depolarizing pulse was 34 pA. (B) The corrected power spectrum of currents, computed from 36 traces. The solid curve is a fitted powerlaw function plus four Lorentzian components, of the form S = 1.1 · 10−29 + 6.1 · 10−25/f 1.2 + 6 · 10−26/[1 + (f/11)2] + 2.9 · 10−27/[1 + (f/141)2] + 2.2 · 10−27/[1 + (f/1,195)2] + 8.8 · 10−28/[1 + (f/3,535)2], where f is in Hz and S is in A2/Hz. The unitary current is = 0.67 pA at 20 kHz bandwidth. The lower trace is the spectrum from another recording in which the mean current was 7 pA and is = 0.51 pA at 20 kHz. (C) Frequency dependence of unitary current calculated from the numeric integral of power spectrum over the frequency range 0.1 Hz–20 kHz. (D) Ensemble mean current and variance, from the same data filtered at 100 Hz. The variance trace was calculated from pairs of sweeps to minimize error due to drift. (E) Mean–variance plot. Superimposed is a parabolic fit, which yields the unitary current estimate iv = 0.28 pA.


Figure 7. A threechannel, insideout patch recording from rhIKs channels. (A) A trace with three channels opening during +50mV depolarization. (B) Eight successive sweeps from the same patch, showing long first latencies in response to depolarizations to +50 mV from the −80mV holding potential. Pulses were delivered every 8 s. Data were filtered at 200 Hz. (C) Comparison of the time course of the rhIKs macroscopic current seen in another patch (noisy trace) and the first latency function at +50 mV. (D) The ensemble mean time courses of open probability at 50 mV obtained from 60 sweeps. The open probability at the end of the 5s depolarization was ∼0.5. The superimposed stepwise curve is the firstlatency distribution F1 scaled by the factor 0.8. It was computed according to \documentclass[10pt]{article}
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\begin{equation*}F_{1}=1 \root{3} \of {1F_{3}},\end{equation*}\end{document} where F3 is the observed firstlatency distribution from 60 sweeps recorded from the threechannel patch. (E) Diary plot of the threechannel first latency F3 in the patch recording. Latency values of 5 s correspond to the case in which no channel opens. (F) Diary plot of the timeaveraged open probability from this patch. For this recording, the bath solution was 130 mM Kaspartate, 10 mM KCl, 1 mM EGTA, 10 mM HEPES, pH 7.4; the pipette solution was 100 mM NaCl, 0.2 mM KCl, 1 mM MgCl2, 1.8 mM CaCl2, 5 mM HEPES, pH 7.4.


Figure 8. Behavior of rhIKs channels at 20 mV; same threechannel patch as in Fig. 7. (A) Five successive sweeps showing activity from only one channel. (B) The ensemble mean time course obtained from 77 sweeps (noisy trace) shows an instantaneous onset followed by a slow activation phase. The maximum open probability was 0.17. Superimposed is the corrected onechannel first latency F1 scaled by 0.8. (C) Diary plot of nPo. Currents were elicited by depolarizations to +20 mV, 5s duration, delivered at 8s intervals from a holding potential of −80 mV. Of a total 77 sweeps, 37 showed some activity; 11 of those showed a second channel opening during depolarization, and one sweep showed three channels open simultaneously.


Figure 9. Effect of extracellular potassium on peak current of hIKs, rhIKs, and ILQT in the twoelectrode voltage clamp. (A) Magnitude of hIKs current at the end of 5s depolarizations to +30 mV as the bath solution was switched between 0.2 and 10 mM K+. The lower panel shows current traces corresponding to the times indicated at top. (B) rhIKs current; (C) ILQT current; (D) the amino acid sequences of human and rat minK in the vicinity of the putative transmembrane region (box).


Figure 10. Singlechannel conductance of rIKs channels with different external K+ solutions. (A) Representative current traces induced by +50mV depolarizing pulses from −80mV holding potential. Data were filtered at 100 Hz. (B) Singlechannel current as a function of voltage obtained from doubleGaussian fits to amplitude histograms of traces. Fitted lines have slopes of 1.9, 3.2, and 4.8 pS for 0, 0.2, and 10 mM external K+, respectively. (C) External potassium dependence of single channel conductance. Error bars represent SEM (n = 2 for 0 mM and n = 4 for 10 mM; the points at 0.2 and 2 mM K+ are single observations). The superimposed fit is the function γ = 1.8 + 2.8/(1 + 0.5 mM/[K]) pS.
