Figure 13. An R* activation model can be ruled out

A: The R* activation model readily reproduces the qualitative features of the Fain et al. [27] step-flash data regardless of whether τ_E (top panel) or τ_R (bottom panel) is rate-limiting. B: However, for neither of the rate-limiting cases (τ_E: top two panels; τ_R: bottom two panels) can the model capture the key feature of the Matthews [3] data, i.e., when a Ca⁺⁺ clamp is applied for a brief period around the time of the flash, the model does not generate any significant extension of saturation period (compare with data, Figure 2B). This failure implies that the R* activation model, and any similar model (with an early Ca⁺⁺ feedback up to and including PDE activation) that does not alter the recovery kinetics of the target reaction, can be ruled out.