What, then, determines the decay of STOCs? Our data support the notion that the kinetics of BK channels is the dominant factor in smooth muscle. First, STOCs in ASM can be fitted well with a single exponential function with a time constant similar to the mean open time of BK channels. Second, the time constants are independent of both amplitudes of STOCs and Ca2+ sparks. With computer modeling of STOCs that includes the spatial relationship between RYRs and BK channels, we observed that the [Ca2+] near the BK channels drops rapidly to the level below the EC50 (3.5 µM at 0 mV) when a Ca2+ spark terminates. The underlying reasons why BK channel kinetics determines STOC decay are that (a) RYRs and BK channels are close, though not physically associated, and (b) the Ca2+ sensitivity of BK channels is sufficiently low (Singer and Walsh, 1987; Tanaka et al., 1997; Bao and Cox, 2005).
