Emodin-induced muscle contraction of mouse diaphragm and the involvement of Ca²⁺ influx and Ca²⁺ release from sarcoplasmic reticulum

1. The effects on skeletal muscle of emodin, an anthraquinone, were studied in the mouse isolated diaphragm and sarcoplasmic reticulum (SR) membrane vesicles. 2. Emodin dose-dependently caused muscle contracture, simultaneously depressing twitch amplitude. Neither tubocurarine nor tetrodotoxin blocked the contraction suggesting that it was caused myogenically. 3. The contraction induced by emodin persisted in a Ca²⁺ free medium with a slight reduction in the maximal force of contraction. The contraction induced by emodin in the Ca²⁺ free medium was completely blocked when the internal Ca²⁺ pool of the muscle was depleted by ryanodine. These data suggest that the contraction caused by emodin is due to the release of Ca²⁺ from the intracellular ryanodine-sensitive pool. 4. In contrast to the effect seen in the Ca²⁺ free medium, emodin induced a small but consisted contraction in the ryanodine-treated muscle in Krebs medium. The contraction was blocked in the presence of dithiothreitol and was partially blocked by nifedipine, suggesting that oxidation of a sulphhydryl group on the external site of dihydropyridine receptor is involved. 5. Emodin dose-dependently increased Ca²⁺ release from actively loaded SR vesicles and this effect was blocked by ruthenium red, a specific Ca²⁺ release channel blocker, and the thiol reducing agent, DTT, suggesting that emodin induced Ca²⁺ release through oxidation of the critical SH of the ryanodine receptor. 6. [³H]-ryanodine binding was dose-dependently potentiated by emodin in a biphasic manner. The degree of potentiation of ryanodine binding by emodin increased dose-dependently at concentrations up to 10 μM but decreased at higher concentrations of 10-100 μM. 7. These data suggest that muscle contraction induced by emodin is due to Ca²⁺ release from the SR of skeletal muscle, as a result of oxidation of the ryanodine receptor and influx of extracellular Ca²⁺ through voltage-dependent Ca²⁺ channels of the plasma membrane.