Figure 5.

Properties of low- and high-open-probability regimes. (A and B) Experiments with Ca_Vβ_2b subunits (alone or in mixture experiments) robustly showed high-open-probability clusters (A). Moreover, the fraction of time spent in the high-open-probability clusters increased with more of Ca_Vβ_2b being cotransfected (B). Open circles show values from each experiment. (Diamonds with error bars) Logarithmic (A) or arithmetic (B) group means ± SE. All group means were compared using the Holm-Bonferroni method. ^∗p < 0.05, †p < 0.01, and ‡p < 0.001 compared with Ca_Vβ_1a experiments; §p < 0.001 compared with Ca_Vβ_2b experiments. (C and D) High-open-probability clusters were characterized by a lower inactivation of the average current. (C) Exemplary average currents of the low-open-probability (left) and high-open-probability (right) regimes from the same single-channel experiments. For better visibility, a moving average of 15 points is shown. Graphs are scaled to the same peak current level (the lowest value of moving average of 100 points). (Scale bars) Vertical, 2fA; and horizontal, 20 ms. (D) Inactivation of average currents after 150 ms during low (half-down diamonds) and high (half-up diamonds) open-probability clusters (mean ± SE). For each experiment, all clusters with either high- or low-open-probability were pooled together. Data were used for the analysis only if the pooled duration of the respective clusters was longer than 100 episodes. The value n is the number of experiments with sufficient statistics. All group means were compared using Holm-Bonferroni method. ^∗p < 0.05 and †p < 0.01 compared with high-open-probability clusters of Ca_Vβ_2b experiments. (E and F) Lifetimes of low- and high-open-probability regimes. Low- or high-activity regimes were defined as consecutive clusters of the same type of the channel open probability (low or high, respectively). Mean lifetimes and errors were determined as described in Materials and Methods. Errors for the lifetime of the low-open-probability regime in Ca_Vβ_1a experiments could not be estimated because transitions from the low- to high-activity regime occurred only in one experiment.