Single-channel analysis of block by Ca²⁺_o. (A) Bursts of channel activity recorded in the cell-attached configuration, selected from long (5–10 s) depolarizations to −70 mV in 0.1, 3, and 10 μM Ca²⁺_o (n = 3–4). No leak subtraction; tick marks at the left indicate zero pipet current. (B) Mean open times (symbols defined in C). (C) Mean closed times, for the fast component (see Fig. S4, D–F). (D) Dependence of the net closing rate (the reciprocal of the mean open time) on [Ca²⁺]_o. For this analysis, open times from −40 to −90 mV were averaged at each concentration. From a fit to bimolecular kinetics (straight line), the zero-Ca²⁺_o closing rate was 1.2 ms⁻¹, and the blocking rate was 5.3 × 10⁸ M⁻¹s⁻¹. The fit minimized the sum of squared errors for all measured time constants (i.e., it is not a simple linear regression).