Figure 3.

Single channel conductance of Ca_V2.1 and Ca_V2.2 channels. (A) Average tail currents for 79 Ca_V2.1 traces (green trace) from a prepulse to +40 mV and 45 Ca_V2.2 traces from a prepulse to +80 mV (red trace). (B) Variance associated with the decay phase between successive tail currents shown in (A). (C) Mean versus variance plots for Ca_V2.1 (solid green symbols) and Ca_V2.2 (open red symbols) from the traces in (B). The data corresponding to the first 300 μs after the voltage jump were discarded. The relationship in panel C was obtained from the subsequent 10 ms after the voltage jump. Data were fitted with the expression: ν = i^∗I−I²/n + ν_b, where ν is the variance, i is the single channel amplitude, I is the average current amplitude during the tail, n is the number of channels, and ν_b is the variance associated with the background noise. For these particular measurements, the fitting parameters were i = −0.42 ± 0.02 pA, n = 7747 ± 641, ν_b = 0.135 ± 0.08 nA² for Ca_V2.1; and i = −0.34 ± 0.01 pA, n = 4238 ± 138, and ν_b = 0.015 ± 0.002 nA² for Ca_V2.2. (D) The current voltage relationship from the estimated single channel currents for Ca_V2.1 (green solid symbols, 4 cells) and Ca_V2.2 currents (red open symbols, 7 cells). For Ca_V2.1, the data taken at −60 and −50 mV, and at −40 and −30 mV were pooled, thus each data point represents the average of at least four experiments. The continuous lines were generated using a GHK curve for a divalent cation assuming a reversal potential of +80 mV for both channels.