Résumé :
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Communication n° 567 Xestospongin B, a macrocyclic bis-1-oxaquinolizidine alkaloid extracted from the marine sponge Xestospongia exigua, was highly purified and tested for its ability to block inositol 1,4,5-trisphosphate (IP3)-induced calcium release. In a concentration-dependent manner xestospongin B displaced [3H]IP3 from both rat cerebellar membranes and rat skeletal myotube homogenates with an EC50 of 44.6 Æ 1.1 µM and 27.4 Æ 1.1 µM respectively. Xestospongin B, selectively blocked the slow intracellular calcium signals induced by membrane depolarization with high external K+ (47 mM) in skeletal myotubes. These slow calcium signals are unrelated to muscle contraction, and involve IP3 receptors. In rat myotubes exposed to a calcium-free medium, xestospongin B neither depleted sarcoplasmic reticulum calcium stores, nor modified thapsigargin action, and did not affect capacitative calcium entry after thapsigargin induced-depletion of calcium stores. Since results obtained in intact cultured cells lack precision in terms of the effective concentration of the blocker at the active intracellular binding site, experiments were performed on highly purified isolated myonuclei in which xestospongin B concentration could be more effectively monitored. In isolated myonuclei xestospongin B reduced, or suppressed IP3-induced calcium oscillations with an EC50 of 15.9 Æ 1.42 µM. Calcium-ATPase activity measured in skeletal myotube homogenates was unaffected by xestospongin B. We conclude that xestospongin B is not only a new membrane-permeable inhibitor of IP3-dependent Calcium signals in skeletal muscle, but constitutes a pharmacological tool for studying IP3-dependent signal-transduction in living cells and isolated subcellular organelles. This work was made possible by the ECOS Sud-CONYCIT exchange program (C03S02) and was supported in part by the AFM (grant to J.M.) and FONDAP (grant to E.J). C.C. was supported by a short-term fellowship from AFM.
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