Inhibition of ryanodine binding to sarcoplasmic reticulum vesicles of cardiac muscle by Zn2+ ions

Using the assay of [H-3]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 muM, However, a decrease of the binding became significant with raising [Zn2+](f) to 0.5 CIM. The inhibitory effect of Zn2+ was [Zn2+](f)-dependent, with IC50/(ZnI) of 2.1 +/-0.4 muM (mean +/-S.D.). Scatchard analysis indicates that both an increase of K, 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. Copyright (C) 2001 S. Karger AG, Basel.