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+) of less than 0.2 microM. However, a decrease of the binding became significant with raising Zn(2+) to 0.5 microM. The inhibitory effect of Zn(2+) was Zn(2+)-dependent, with IC(50/ZnI) of 2.1+/-0.4 microM (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+) dependence was not clearly changed by Ca(2+), 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.