FIGURE 1.

Schematic diagram of the model of a single IP₃R channel subunit. Each subunit has an activation Ca²⁺-binding site, an inhibitory Ca²⁺-binding site, and an IP₃-binding site. We label the binding sites by the notation (ijk), where the index i represents the IP₃-binding site, j the activation Ca²⁺-binding site, and k the inhibitory Ca²⁺-binding site. The number 1 represents an occupied binding site and 0 a nonoccupied site. Additionally we introduce a conformational change between “active” (A-state) and “inactive” (110) states of the subunit. Values for the forward (a) and backward (b) rate constants associated with each transition are listed in Table 1. In the figure, C and I represent the concentrations of Ca²⁺ and IP₃, respectively. The bold arrows indicate the binding of ligands to different sites.