Figure 4.

Activation of [³H]ryanodine binding by HdCa (hadrucalcin) and synergistic effect with caffeine. (A) Heavy sarcoplasmic reticulum vesicles from rabbit skeletal muscle were incubated with 7 nmol·L⁻¹[³H]ryanodine in the absence (control) and the presence of indicated concentrations of HdCa. Binding conditions were as specified in Methods. The ED₅₀ was 37 ± 8 nmol·L⁻¹ (mean ± SE, n= 3) and was determined with the formula B= (B_max)/[1 + (ED₅₀/[HdCa])^nH], where B is specific binding of [³H]ryanodine, B_max is the maximum binding stimulated by HdCa, and nH is the Hill coefficient (1.3).