Abstract

Xestoquinone (XQN) (3 × 10⁻⁷ to 3 × 10⁻³ M), isolated from the sea sponge Xestospongia sapra, induced a concentration-dependent Ca²⁺release from the heavy fraction of fragmented sarcoplasmic reticulum (HSR) of rabbit skeletal muscle with an EC₅₀ value of ∼30 μM. On the basis of the EC₅₀, XQN is 10 times more potent than caffeine. Dithiothreitol completely blocked XQN-induced Ca²⁺ release from HSR without affecting that induced by caffeine. Caffeine-induced Ca²⁺ release was reduced markedly by Mg²⁺, procaine, and ruthenium red, agents that are known to block release of Ca²⁺ from sarcoplasmic reticulum, whereas that induced by XQN was not inhibited. The bell-shaped profile of Ca²⁺ dependence for XQN was significantly shifted upward in a wider range of pCa (between 7 and 3), whereas that for caffeine was shifted to the left in a narrower range of pCa (between 8 and 7). The maximum response to caffeine in⁴⁵Ca²⁺ release was not affected by 9-methyl-7-bromoeudistomin D, whereas the response was further increased by XQN. XQN caused a concentration-dependent decrease in [³H]ryanodine binding to HSR. This effect of XQN also was abolished in the presence of dithiothreitol. Scatchard analysis revealed that the mode of inhibition by XQN was noncompetitive in [³H]ryanodine binding to HSR. These results indicate that sulfhydryl groups are involved in both the XQN effect on ryanodine binding and on Ca²⁺ release.