The influence of BK channel kinetics on STOC amplitude lies in the fact that BK channel kinetics is a rate-limiting step for STOC activation. As shown in Fig. S1 B, in response to a step function of [Ca2+] between 1 and 320 µM, BK channel currents develop with time constants between 40 and 5 ms at 0 mV. Therefore, if [Ca2+] near the BK channels dissipates before their activations reach the steady state, the STOC amplitude would be smaller than its maximal (i.e., steady state) value. Longer duration sparks allow BK channels more time to reach their steady-state Po and therefore could produce a bigger STOC. This is proportionally a larger effect on BK channels further from the spark (but still <600 nm away at 0 mV) because the lower [Ca2+] there causes them to have a slower activation rate. However, in absolute terms, it may not be as important because, if far enough away, the steady-state Po may be small, and thus the BK cluster wouldn’t contribute much to the total STOC.
