The Role of Store-Operated Calcium Entry in Store Repletion During Repetitive High Frequency Tetanic Stimulation of Single Skeletal Muscle Fibers

Abstract

Store-operated Ca2+ Entry (SOCE) involves a trans-sarcolemmal Ca2+ influx mechanism triggered by Ca2+ store depletion. Recently, we demonstrated that SOCE activation in skeletal myotubes involves a functional coupling between STIM1 Ca2+ sensor proteins in the sarcoplasmic reticulum (SR) and Ca2+-permeable Orai1 channels in the sarcolemma. However, the physiological role of SOCE in muscle remains unknown. Here, we monitored myoplasmic Ca2+ transients in mag-fluo-4 loaded mouse flexor digitorum brevis fibres during repetitive high frequency tetanic stimulation (60 consecutive 500ms, 50Hz stimulation trains every 2.5s). In normal Ringer's solution, tetanic Ca2+ transient amplitude decays in three phases: an initial rapid phase (trains 1-10), a second phase of maintained amplitude (trains 10-40), and a final phase of decay (trains 40-60). The maintained phase corresponds to a slightly elevated tail transient integral during each interpulse interval, consistent with activation of Ca2+ influx between tetani. Addition of 0.5mM CdCl2 plus 0.2mM LaCl3 did not alter the initial or final phases of Ca2+ transient decay, but significantly (p<0.01) compromised both the maintained Ca2+ transient (4±3% reduction from trains 10 to 40 in normal Ringer versus 30±3% reduction with Cd/La) and the increase in tail transient integral (which decreased 21±7% with Cd/La) observed during the second phase. Similar results were obtained following addition of either BTP-2 or SKF96365, two known SOCE inhibitors, consistent with SOCE mediating store repletion during the secondary phase of maintained release. Together, these results suggest that repetitive high frequency tetanic stimulation activate a SOCE flux used to replenish SR Ca2+ stores required to maintain subsequent Ca2+ release. Current experiments are testing the validity of this assertion using molecular interventions (transient STIM1 knockdown and dnOrai1 expression) to more selectively inhibit SOCE.