Because ASM cells have a resting potential of ∼40 mV, we further estimated the number of BK channel puncta activated by RYRs at this physiological potential by the same technique, but using a Po appropriate for −40 mV. Fig. 6 (B and C, green lines) shows that ∼40–50% of RYR1 and RYR2 did not have an associated BK channel punctum at this potential. On average, an RYR2 punctum activated 0.78 ± 0.03 BK channel puncta, and an RYR1 punctum activated 1.25 ± 0.05 BK channel puncta (n = 5 cells for RYR1 and 6 cells for RYR2). These estimates suggest that although the physical relationship between RYRs and BK channels in a spark microdomain does not change at different potentials, the functional size of a spark microdomain becomes smaller as the potentials become more negative.
