Figure 6.

Noise analysis of divalent currents. Pipette solution 9. Currents at varying times, indicated to the right of each trace, in exemplary experiments with 20 mM Ca²⁺ (A) and 20 mM Ba²⁺ (C). Data were recorded at 5 kHz sampling and post-filtered at 1 kHz. Currents are shown during the last 180 ms of a 660 ms pulse to −110 mV. Ba²⁺ currents were corrected for ∼3% residual activation by subtraction of a linear function. This procedure did not significantly affect the value of single-channel current. (B, D) Variance analysis of Ca²⁺ and Ba²⁺ currents. Background-subtracted variances (σ² − σ₀ ²) plotted as a function of the mean current during the development of Ca²⁺ current (B) or Ba²⁺ current (D). Background variance was between 0.5 and 0.6 pA² in five experiments. The labeled points correspond to traces in A and C. Data are fitted by a linear function (n = 58, correlation coefficient 0.81 for Ca²⁺; and n = 189, correlation coefficient 0.95 for Ba²⁺). The slopes indicate a single channel current i of 9.7 ± 0.4 fA for Ca²⁺ and 80.8 ± 0.8 fA for Ba²⁺.