Different Capacity for Store-Operated Ca2+ Entry and Ca2+ Extrusion Across the Plasma Membrane of Wild-Type and Dystrophic mdx Mouse Muscle

Abstract

Store-operated Ca2+ entry (SOCE) is a ubiquitously expressed signalling system that is highly specialized in skeletal muscle. “Deregulated” SOCE had been proposed as a pathway for Ca2+ entry into dystrophic muscle that leads to fibre degradation. We recently showed that this mechanism remains tightly regulated in mdx mouse muscle but the integral SOCE proteins, STIM1 and Orai1, are upregulated 3-fold (Edwards et al., 2010). We now report that the newly identified isoform STIM1L (Darbellay et al., 2011) is upregulated 1.8-fold in mdx muscle. We found SOCE recorded in skinned fibres was 2-fold greater in mdx compared with WT for the same SR Ca2+ release amplitude. However cytoplasmic fluo-4 transients in depleted intact fibres showed SOCE in the absence of a sarcoplasmic reticulum (SR) Ca2+ pump blocker to be of reduced influx rate in mdx compared to WT fibres. A similar level of SR Ca2+ reloading was detemined in both muscle types following SOCE deactivation. Fura-2 imaging in intact fibres in the presence of 50 μM cyclopiazonic acid (CPA) and no external Ca2+ showed that more Ca2+ remained in the cytoplasm of mdx compared to WT fibres following SR depletion suggesting that Ca2+ extrusion by the plasma membrane Ca2+-ATPase (PMCA) is restricted in mdx. This helps explain reduced SOCE in intact mdx fibres, as the washout of CPA following SR depletion resulted in the greater amount of trapped cytoplasmic Ca2+ re-entering SR to cause a greater degree of SOCE deactivation before externally applied Ca2+ could enter the fibre. These results suggest that store-dependent Ca2+ influx is greater and PMCA is restricted in its capacity to extrude Ca2+ in mdx compared to WT fibres.