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¹ University of Ulm, Department of Applied Physiology, Albert-Einstein-Allee 11, D-89069 Ulm, Germany
² Basel University Hospital, Departments of Anaesthesia and Research, Hebelstrasse 20, 4031 Basel, Switzerland
³ University of Ferrara, Department of Experimental and Diagnostic Medicine, Via Borsari 46, 44100 Ferrara, Italy

We investigated the functional role of JP-45, a recently discovered protein of the junctional face membrane (JFM) of skeletal muscle. For this purpose, we expressed JP-45 C-terminally tagged with the fluorescent protein DsRed2 by nuclear microinjection in myotubes derived from the C2C12 skeletal muscle cell line and performed whole-cell voltage-clamp experiments. We recorded in parallel cell membrane currents and Ca²⁺ signals using fura-2 during step depolarization. It was found that properties of the voltage-activated Ca²⁺ current were not significantly changed in JP-45–DsRed2-expressing C2C12 myotubes whereas the amplitude of depolarization-induced Ca²⁺ transient was decreased compared to control myotubes expressing only DsRed2. Converting Ca²⁺ transients to Ca²⁺ input flux using a model fit approach to quantify Ca²⁺ removal, the change could be attributed to an alteration in voltage-activated Ca²⁺ permeability rather than to altered removal properties or a lower Ca²⁺ content of the sarcoplasmic reticulum (SR). Determining non-linear capacitive currents revealed a reduction of Ca²⁺ permeability per voltage-sensor charge. The results may be explained by a modulatory effect of JP-45 related to its reported in vitro interaction with the dihydropyridine receptor and the SR Ca²⁺ binding protein calsequestrin (CSQ).

(Received 23 December 2005; accepted after revision 18 January 2006; first published online 19 January 2006)
Corresponding author W. Melzer: University of Ulm, Department of Applied Physiology, Albert-Einstein-Allee 11, D-89069 Ulm, Germany. Email: werner.melzer{at}uni-ulm.de

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