Inhibition of ryanodine binding to sarcoplasmic reticulum vesicles of cardiac muscle by Zn ²⁺ ions

Using the assay of [ ³ H]ryanodine binding to the sarcoplasmic reticulum, the effect of Zn ²⁺ on ryanodine receptors (RyRs) of cardiac muscle was investigated. There was no obvious change in the binding at [Zn ²⁺ ] _f of less than 0.2 μM. However, a decrease of the binding became significant with raising [Zn ²⁺ ] _f to 0.5 μM. The inhibitory effect of Zn ²⁺ was [Zn ²⁺ ] _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 Zn ²⁺ -induced decrease of the binding. The Hill coefficient for this inhibitory effect of Zn ²⁺ was between 0.8 and 1.2. The interactions of the effects of Zn ²⁺ and various modulators of RyR indicate that the inhibitory effect of Zn ²⁺ was mostly mediated through inhibiting Ca ²⁺ activation sites (CaA) on RyR. Since the [Zn ²⁺ ] _f dependence was not clearly changed by [Ca ²⁺ ] _f, the inhibitory effect of Zn ²⁺ may not be due to competition of Zn ²⁺ with Ca ²⁺ for CaA and probably is indirect. The inhibitory effect of Zn ²⁺ could not be antagonized by 2 mM dithiothreitol, a thiol-reducing agent, suggesting that the binding of Zn ²⁺ 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 Zn ²⁺ on ryanodine binding to the sarcoplasmic reticulum of cardiac muscle show several distinct differences. It is indicated that the effect of Zn ²⁺ on RyRs may be isoform-dependent. The physiological significance of the effects of Zn ²⁺ is discussed.