Figure 1.

The structure of the Siekmann IP₃R model. C represents closed state and O is open state; qs are transition rates connecting two adjacent states and indicating how fast an IP₃R switches between the two states. The entire structure comprises two parts: One is the high-activity part or drive mode, containing three closed states $C_1$, $C_2$, $C_3$, and one open state $O_6$. The other is the low-activity part or park mode, which includes one closed state $C_4$ and one open state $O_5$.