EMODIN-INDUCED MUSCLE-CONTRACTION OF MOUSE DIAPHRAGM AND THE INVOLVEMENT OF CA2-RETICULUM( INFLUX AND CA2+ RELEASE FROM SARCOPLASMIC)

1 The effects on skeletal muscle of emodin, an anthracluinone, were st udied 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 my ogenically. 3 The contraction induced by emodin persisted in a Ca2+ fr ee medium with a slight reduction in the maximal force of contraction. The contraction induced by emodin in the Ca2+ free medium was complet ely blocked when the internal Ca2+ pool of the muscle as depleted by r yanodine. 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 mu scle in Krebs medium. The contraction was blocked in the presence of d ithiothreitol and was partially blocked by nifedipine, suggesting that oxidation of a sulphhydryl group on the external site of dihydropyrid ine receptor is involved. 5 Emodin dose-dependently increased Ca2+ rel ease from actively loaded SR vesicles and this effect was blocked by r uthenium red, a specific Ca2+ release channel blocker, and the thiol r educing agent, DTT, suggesting that emodin induced Ca2+ release throug h oxidation of the critical SH of the ryanodine receptor. 6 [H-3]-ryan odine binding was dose-dependently potentiated by emodin in a biphasic manner. The degree of potentiation of ryanodine binding by emodin inc reased dose-dependently at concentrations up to 10 mu M but decreased at higher concentrations of 10-100 mu M. 7 These data suggest that mus cle contraction induced by emodin is due to Ca2+ release from the SR o f skeletal muscle, as a result of oxidation of the ryanodine receptor and influx of extracellular Ca2+ through voltage-dependent Ca2+ channe ls of the plasma membrane.