Comparison of [³H]ryanodine receptors and Ca⁺⁺ release from rat cardiac and rabbit skeletal muscle sarcoplasmic reticulum

Sarcoplasmic reticulum (SR) vesicles prepared from rat ventricle muscle are isolated, and their [³H]ryanodine-binding and calcium transport properties are studied in detail under active loading conditions in the presence of pyrophosphate. Experiments are performed in tandem with rabbit skeletal SR under identical conditions to allow direct comparisons of the mechanisms by which activators and inhibitors influence the calcium release channel. Ca⁺⁺-induced Ca⁺⁺ release is demonstrated with both preparations and the cardiac channel is about 1.5-fold more sensitive to activation by Ca⁺⁺, which is in excellent quantitative agreement with the ability of Ca⁺⁺ to activate [³Hryanodine-binding sites. The cardiac and skeletal receptors show major quantitative differences with respect to sensitivity to pharmacologic modulators, cations and pH. The inhibitors ruthenium red, Mg⁺⁺ and neomycin are significantly more potent in inhibiting the skeletal receptor, whereas the activators daunorubicin and caffeine are significantly more potent towards the cardiac receptor. The ATP analog, β,γ-methyleneadenosine 5'-triphosphate, enhances the binding of [³H]ryanodine to the high-affinity site in skeletal SR by a factor of 4 but has a negligible effect on the cardiac receptor, although at suboptimal Ca⁺⁺ for the binding of ryanodine, β,γ-methyleneadenosine 5'-triphosphate activates the cardiac receptor to a greater extent. High levels of salt (1 M NaCl) enhance the rate of [³H]ryanodine association with its binding sites in both preparations, although they selectively reduce the binding-site capacity in skeletal SR due to a failure to maintain a stable equilibrium. Although high- and low-affinity binding of [³H]ryanodine have a similar response to changing pH, the skeletal receptors are significantly more sensitive to pH. These results contribute to the characterization of the SR Ca⁺⁺-release channel from rat heart and demonstrate significant differences in the modulation of the ryanodine receptor-Ca⁺⁺ channel complexes of cardiac and skeletal SR.