Fig. 7

The peak rate of Ca²⁺ release evoked by various depolarizing test pulses is linearly related to the amount of “activation” intramembrane charge moved by the same pulse. Each test pulse was immediately preceded by a depolarizing prepulse (see pulse schematic in lower left inset), which by itself did not activate Ca²⁺ release. Many of the test pulses were too short to establish the ionic current baseline for calculating charge moved during the test pulse, so Qon could not be measured for short test pulse durations due to uncertainty regarding the level of ionic current remaining during the test pulse. Charge moved by the test pulse was consequently determined as Qoff–Q pre. The test pulse charge represents an upper estimate of the activating charge moved during the test pulse; the x intercept on the graph is interpreted as representing the amount of pre-activating (i.e., precursor) charge still present in the test pulse (i.e., pre-activating charge not moved by the prepulse). The lower right inset gives the charge movement current records obtained using various duration test pulses to 0 mV, each immediately following the prepulse to − 50 mV. The upper left inset gives the Ca²⁺ release calculated from the Ca²⁺ transient recorded simultaneously with the charge movement records shown in the lower right inset for each of the same test pulses. Reproduced, with modification from ref. [50]