Activation of a single WT hRyR2 channel by [Ca²⁺]_cyt. Current traces demonstrate the progressive activation of a single representative channel by [Ca²⁺]_cyt from nominally 0 to 200 µM. Luminal Ca²⁺ was fixed at 50 nM, and the bilayer was voltage clamped at +30 mV for all experiments. The permeant ion was K⁺, with both chambers having symmetrical solutions (210 mM KCl). The short bars to the right of each trace represent the closed level of the channel, and opening events are downward deflections from the baseline. The current amplitude progressively decreases with increasing [Ca²⁺]_cyt, as the relative permeability of Ca²⁺ is approximately six times that of K⁺. Although the pore is increasingly occupied by Ca²⁺, the current decreases as a result of the lower conductance of Ca²⁺ compared with K⁺ (Williams et al., 2001).