Inhibition of Ryanodine Binding to Sarcoplasmic Reticulum Vesicles of Cardiac Muscle by Zn2+ Ions

Abstract

Using the assay of [³H]ryanodine binding to the sarcoplasmic reticulum, the effect of Zn²⁺ on ryanodine receptors (RyRs) of cardiac muscle was investigated. There was no obvious change in the binding at [Zn²⁺]_f of less than 0.2 µM. However, a decrease of the binding became significant with raising [Zn²⁺]_f to 0.5 µM. The inhibitory effect of Zn²⁺ was [Zn²⁺]_f-dependent, with IC_50/ZnI of 2.1±0.4 µM (mean±S.D.). Scatchard analysis indicates that both an increase of K_d and a decrease of B_max were responsible for Zn²⁺-induced decrease of the binding. The Hill coefficient for this inhibitory effect of Zn²⁺ was between 0.8 and 1.2. The interactions of the effects of Zn²⁺ and various modulators of RyR indicate that the inhibitory effect of Zn²⁺ was mostly mediated through inhibiting Ca²⁺ activation sites (CaA) on RyR. Since the [Zn²⁺]_f dependence was not clearly changed by [Ca²⁺]_f, the inhibitory effect of Zn²⁺ may not be due to competition of Zn²⁺ with Ca²⁺ for CaA and probably is indirect. The inhibitory effect of Zn²⁺ could not be antagonized by 2 mM dithiothreitol, a thiol-reducing agent, suggesting that the binding of Zn²⁺ ions to RyRs of cardiac muscle is not accompanied by obvious change of redox state of the RyRs. In comparison with that seen in skeletal muscle [3], the effects of Zn²⁺ on ryanodine binding to the sarcoplasmic reticulum of cardiac muscle show several distinct differences. It is indicated that the effect of Zn²⁺ on RyRs may be isoform-dependent. The physiological significance of the effects of Zn²⁺ is discussed.