Ruthenium Red and Magnesium Ion Partially Inhibit Silver Ion-Induced Release of Calcium from Sarcoplasmic Reticulum of Frog Skeletal Muscles

Abstract

Effects of Ca²⁺ -induced Ca²⁺ release blockers, ruthenium red (RR) and Mg²⁺, on Ag⁺-induced Ca²⁺ release were studied using skinned muscle fibers or fragmented heavy SR (HSR) prepared from frog muscle, and compared with those on caffeine-induced one. Exposure of the skinned fibers to 5μM Ag⁺ produced a rapid and large contraction in the presence of 0.043mM free Mg²⁺. When Mg²⁺ concentration was increased to 0.86mM, Ag⁺ led to a large transient contraction, combined with a small tonic one. The transient component was completely blocked by high Mg²⁺ (3.64mM), but the tonic one was not. Ca²⁺-ATPase activity was not stimulated by increase of Mg²⁺ from 0.86 to 3.64mM. Ag⁺ and caffeine induced a rapid Ca²⁺ efflux from HSR in a dose-dependent manner. RR over a range from 1 to 10μM dose-dependently inhibited the Ca²⁺ efflux induced by 10μM Ag⁺. Despite increase of RR to 30μM, however, further inhibition of the Ca²⁺ efflux was not produced any more (77.8±12.2% inhibition). A 10mM caffeine-induced efflux of Ca²⁺ was blocked slightly by only 0.5μM RR and almost completely by 3μM. A slight inhibition (about 28%) of the Ca²⁺-ATPase activity was observed in the presence of 10μM Ag⁺ in 0.5mg SR protein/ml of medium. RR and caffeine did not affect the enzyme activity. These results indicate that frog SR could induce a rapid release of Ca²⁺ upon Ag⁺ and caffeine, suggesting that Ag⁺ may have two different binding sites to release Ca²⁺; one is on Ca²⁺-induced Ca²⁺ release channel and the other on RR-insensitive site.