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Transient Loss of Voltage Control of Ca²⁺ Release in the Presence of Maurocalcine in Skeletal Muscle

Sandrine Pouvreau ^*, Laszlo Csernoch , Bruno Allard ^*, Jean Marc Sabatier , Michel De Waard , Michel Ronjat  and Vincent Jacquemond ^*

^* Physiologie Intégrative Cellulaire et Moléculaire, Université Claude Bernard Lyon 1, UMR CNRS 5123, 69622 Villeurbanne, France; Department of Physiology, Medical and Health Science Center, University of Debrecen, Debrecen H-4012, Hungary; ERT 62, Université de la Méditerranée–Ambrilia Biopharma S.A., Faculté de Médecine Nord, 13916 Marseille, France; and INSERM U607/DRDC, CEA, 38054 Grenoble, France

Correspondence: Address reprint requests to Vincent Jacquemond, Physiologie Intégrative Cellulaire et Moléculaire, Université Claude Bernard Lyon 1, UMR CNRS 5123, Bât. Raphael Dubois, 43 boulevard du 11 novembre 1918, F 69622 Villeurbanne Cedex, France. Tel.: 33-4-72-44-81-64; Fax: 33-4-72-44-79-37; E-mail: vincent.jacquemond{at}univ-lyon1.fr.

In skeletal muscle, sarcoplasmic reticulum (SR) calcium release is controlled by the plasma membrane voltage through interactions between the voltage-sensing dihydropyridine receptor (DHPr) and the ryanodine receptor (RYr) calcium release channel. Maurocalcine (MCa), a scorpion toxin peptide presenting some homology with a segment of a cytoplasmic loop of the DHPr, has been previously shown to strongly affect the activity of the isolated RYr. We injected MCa into mouse skeletal muscle fibers and measured intracellular calcium under voltage-clamp conditions. Voltage-activated calcium transients exhibited similar properties in control and in MCa-injected fibers during the depolarizing pulses, and the voltage dependence of calcium release was similar under the two conditions. However, MCa was responsible for a pronounced sustained phase of Ca²⁺ elevation that proceeded for seconds following membrane repolarization, with no concurrent alteration of the membrane current. The magnitude of the underlying uncontrolled extra phase of Ca²⁺ release correlated well with the peak calcium release during the pulse. Results suggest that MCa binds to RYr that open on membrane depolarization and that this interaction specifically alters the process of repolarization-induced closure of the channels.

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