Figure 4.

Voltage-dependent properties of single channel currents in resting T cells. (A) Single channel activity during a voltage ramp from −120 to 50 mV. Note the very brief transitions that occur between –120 and –80 mV and longer transitions at more depolarized potentials. (B) Single channel currents recorded at −120, −80, and −40 mV. Again, note that depolarization promotes longer open and closed events. (C) Distributions of open (τ_o) and closed (τ_c) times at −120 and −40 mV. For single channel analysis, traces with stable opening of only one channel were selected. The kinetics of transitions between the open and closed levels were idealized by fitting manually controlled cursors to the two levels and setting a discriminator at 50% of the current between levels. Time histograms computed from a minimum of 60 intervals. Histograms of open and closed event durations were fitted by single Gaussian distributions; two or more components did not provide a significantly better fit. Same cell as in B. (D) Voltage dependence of mean open (τ_o, ○) and mean closed (τ_c, ▪) times (n = 4). Values of τ_o and τ_c for individual cells were calculated by fitting duration histograms with a single Gaussian function, as described in C. Data are fitted with a single exponential function with a steepness factor k = 35 mV for open times and 50 mV for closed times. (E) Single channel current amplitudes (▪) and open probabilities (P _o, ○) at varying potentials. Averages from four cells are shown; if not shown, standard error bars are smaller than symbol size. The smooth curve through P _o data was obtained from exponential fits of τ_c and τ_o (smooth lines on D) according to equation P _o = τ_o/(τ_o+ τ_c).