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.

Effect of Zinc Ions on Caffeine-Induced Contracture in Vascular Smooth Muscle and Skeletal Mu-scle of Rat

“…It is indicated that caffeine contracture of smooth muscle and skeletal muscle was differentially affected by Zn 2+ . The present result seems to be consistent with the differential effect of Zn 2+ on ryanodine binding to the SR vesicles of skeletal muscle and cardiac muscle [6,7]. However, there are many sites of action of Zn 2+ , which may be involved in modulating the contraction of muscle cells [2][3][4][5].…”

“…The decrease of the binding became obvious with [Zn 2+ ] of more than 1 µM [6]. In contrast, the effect of Zn 2+ in cardiac muscle was monophasic [7]. Ryanodine binding to the SR vesicles of cardiac muscle could not be changed by low [Zn 2+ ], while a significant decrease of the binding occurred with raising [Zn 2+ ] to 0.5 µM.…”

“…More recently, we found that ryanodine binding to the SR vesicles of skeletal muscle and cardiac muscle was differentially affected by Zn 2+ [6,7]. The binding to the SR of skeletal muscle was biphasically modulated by Zn 2+ .…”

Intracellular distributions of essential elements in cardiomyocytes

“…Under the circumstances present at »1420 s, when external Zn is non-existent and internal Zn is relatively high (»10 nM), inward calcium current may have been inhibited by intracellular rather than extracellular Zn 2+ (Atar et al, 1995;Turan, 2003). It is also possible that high [Zn 2+ ] i blocked the release of SR Ca 2+ , perhaps by inhibiting Ca 2+ -induced opening of SR Ca 2+ release channels (Wang et al, 2001). Based on our X-ray Xuorescence images and our data from Xuorescence microscopy, we speculate that there exist signiWcant internal stores of Zn 2+ in organelles, probably in the SR, and that these stores of Zn 2+ modulate Ca 2+ regulation.…”

“…These data demonstrate that external Zn 2+ inhibits systolic characteristics of cardiomyocyte [Ca 2+ ] i dynamics. The inhibition of calcium induced calcium release from the SR by external Zn 2+ may occur via the reduction in Ca 2+ inXux through sarcolemmal calcium channels (Atar et al, 1995;Turan, 2003) or via reducing the probability of opening SR Ca 2+ release channels (Wang et al, 2001). Table 1 Mean and standard errors (SEM) of measured concentrations ( g/cm 2 for P-K and ng/cm 2 for Ca-Zn) of nine elements in large (not less than 100 m 2 ) regions of interests selected in six diVerent intact rat cardiomyocytes Approximated concentrations (ppm and mM) were calculated after assuming sample width of 10 m and atomic weights shown.…”