Discrete-State Stochastic Models of Calcium-Regulated Calcium Influx and Subspace Dynamics Are Not Well-Approximated by ODEs That Neglect Concentration Fluctuations

Figure 3

Diagram of the components and fluxes in a subspace model that includes calcium-regulated calcium influx. A single calcium channel (with two calcium binding sites) is associated with a subspace of volume 𝑣. The calcium influx rate is 𝛼₀ and 𝛼₁ when calcium is unbound and bound, respectively, and the transition rates between these states are 𝑘⁺𝑐² and 𝑘⁻, where 𝑐 is the subspace calcium concentration. Subspace calcium is passively coupled at rate 𝛽 to the bulk cytosol with constant concentration 𝑐_∞.