TABLE II.

Concentrations and Diffusion Constants Assumed in the Model

1	2	3
Constituent	Resting concentration	Diffusion constant
	μM	10−6 cm2 s−1
Resting free [Ca2+]	0.050	3.19
Resting free [Mg2+]	1,000	—
ATP	8,000	1.49
Troponin (Ca2+ regulatory sites)	360	0
Parvalbumin (Ca2+/Mg2+ sites)	1,500	0.159
Protein (sites for fluo-3 binding)	3,000	0
SR Ca2+ pump (Ca2+ binding sites)	240	0
Fluo-3	100
Protein-free fluo-3	11.88	1.51
Protein-bound fluo-3	88.12	0

Concentrations are referred to the myoplasmic water volume (Baylor et al., 1983) and, except for Ca²⁺ and Mg²⁺, are total concentrations. At a sarcomere length of 3.0 μm, the estimated troponin concentration, if referred to the water volume surrounding the thin filament, is 180 μM; with two Ca²⁺ regulatory sites per troponin molecule, the concentration of sites is 360 μM. The myoplasmic diffusion constants are for 18°C; they were adjusted from 16°C based on a Q₁₀ of 1.35. The diffusion constants for Ca²⁺, ATP, and parvalbumin are taken from Baylor and Hollingworth (1998), whereas the value for protein-free fluo-3 is based on the formula given in Zhao et al. (1996), with fluo-3's molecular weight = 765. Because free [Mg²⁺] is assumed to be constant, the value of its diffusion constant is not used. The 11.88 μM protein-free fluo-3 consists of 10.82 μM Ca²⁺-free fluo-3 and 1.06 μM Ca²⁺-bound fluo-3; the 88.12 μM protein-bound fluo-3 consists of 85.88 μM Ca²⁺-free fluo-3 and 2.24 μM Ca²⁺-bound fluo-3.