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

Figure 1

Diagram of the components and fluxes in a minimal subspace model. Calcium influx 𝛼 (in units of μM/s) leads to increased calcium concentration 𝑐 (units of μM) in a diadic subspace of volume 𝑣 (liters). Subspace calcium moves to the bulk passively via diffusion at rate 𝛽 (given by 0.01 ms⁻¹). Bulk calcium at the concentration 𝑐_∞=0.1 μM returns to the subspace at the same rate. The equilibration time of subspace calcium is 𝜏=1/𝛽=100ms [27].