Table 2.
Parameter values used in the modelling of SR Ca2+ release (16°C)
| 1 | 2 | 3 |
|---|---|---|
| Slow-twitch | Fast-twitch | |
| A. Resting concentrations (free) | ||
| [Ca2+]R (μM) | 0.1 | 0.1 |
| [Mg2+]R (μM) | 1000 | 1000 |
| B. Site concentrations (total) | ||
| Troponin Ca2+ sites (μM) | 120 | 240 |
| ATP Ca2+ sites (μM) | 8000 | 8000 |
| Parvalbumin Ca2+ and Mg2+ sites (μM) | 0 | 1500 |
| C. Reaction rate constants | ||
| Ca2+−troponin reaction | ||
| On-rate (108M−1 s−1) | 0.885 | 0.885 |
| Off-rate (s−1) | 57.5 | 115 |
| Dissociation constant (μM) | 0.65 | 1.3 |
| Ca2+−parvalbumin reaction | ||
| On-rate (108M−1 s−1) | — | 0.417 |
| Off-rate (s−1) | — | 0.5 |
| Dissociation constant (μM) | — | 0.012 |
| Mg2+−parvalbumin reaction | ||
| On-rate (108M−1 s−1) | — | 0.00033 |
| Off-rate (s−1) | — | 3 |
| Dissociation constant (μM) | — | 90.9 |
Concentrations units are relative to the myoplasmic water volume, as with Δ [Ca2+]. Free [Mg2+] is assumed to be constant at 1mM. The reaction of Ca2+ with ATP (not shown) is assumed to be instantaneous, with Δ[CaATP] = 3.6 ×Δ[Ca2+]; this assumes that [ATP] = 8 mM and that the effective dissociation constant of ATP for Ca2+ is 2.2 mM (Baylor & Holling worth, 1998). Rate constants for the parvalbumin reaction are not listed for slow-twitch fibres because it has been reported taht the parvalbumin concentration in slow-twitch fibres is zero. For model calculations at 28°C, the reaction rate constants were twice those shown here. See Methods for justification of parameter values at 16°C.