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

Abstract

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.

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