Fig. 5.

Effect of caffeine on activities of G4733E and R4736W RyR1 mutants. Caffeine-dependent changes in the activities of wild-type, G4733E, and R4736W RyR1s were determined at 0.4 μM Ca²⁺ by [³H]ryanodine binding assays. Changes in the amount of bound [³H]ryanodine were normalized and fit by Eq. 1 as described in materials and methods. Data are shown as means ± SE (n = 4–15). There are no significant differences among three types of RyR1 at any tested caffeine concentrations by one-way ANOVA. Maximal amounts of bound [³H]ryanodine are 121 ± 9, 109 ± 9, and 111 ± 9% (10 mM caffeine as 100%); activation Hill constants of caffeine are 2.2 ± 0.5, 1.4 ± 0.4, and 1.4 ± 0.3 mM, and Hill coefficients are 1.1 ± 0.2, 1.0 ± 0.2, and 1.0 ± 0.2 for WT-RyR1, G4733E, and R4736W, respectively (fitted value ± regression SE).