Abstract 17632: STIM1 Increases Calcium Stores in the Sarcoplasmic Reticulum of Adult Feline Ventricular Myocytes

Abstract

Background: STIM1 is a Sarcoplasmic Reticulum (SR) membrane resident protein implicated in sensing and maintaining SR Ca 2+ levels. The role of STIM1 in the regulation of SR Ca 2+ stores in the normal and diseased heart is not well described. Previous reports confirm that STIM1 is present at low levels in the healthy adult heart, but expression levels increase after cardiac injury. Objective: To determine if increased STIM1 expression after cardiac injury may be involved in the disturbed Ca 2+ cycling present within diseased cardiomyocytes. Results: Adenovirus was used to express either STIM1 or red fluorescent protein (RFP) in freshly isolated adult feline ventricular myocytes (AFMs). We found that in normally quiescent myocytes, STIM1 induced high resting SR Ca 2+ load and induced Ca 2+ sparks, which lead to spontaneous contractions by triggering action potentials. Concurrently, pacing of AFMs revealed STIM1 also increased fractional shortening and Ca 2+ transient amplitude. However, STIM1 also induced cell death (60% versus only 5% in RFP controls). Blockade of the L-type Ca 2+ channel (LTCC) with verapamil rescued the cell death caused by STIM1, but was unable to reduce SR load or Ca 2+ sparks. In contrast, inhibition of ORAI channels using BTP2 rescued cell death and abolished Ca 2+ sparks, while canonical transient receptor potential channel inhibition had no effect. STIM1 expression reduced baseline LTCC current amplitude by 20%, but there was no difference in current amplitude after Bay K8644 addition, suggesting channel number was not altered. Conclusions: STIM1 induced Ca 2+ influx can load the SR of large mammalian myocytes, causing Ca 2+ sparks and waves that can trigger action potentials. The induced APs lead to cell death and this effect could be blocked LTCC blockade. STIM1-mediated Ca 2+ influx was dependent on ORAI channel function. Similar to previous reports, we found that STIM1 reduced LTCC current but did not alter LTCC availability. These data show that after cardiac injury, STIM1 and ORAI may provide Ca 2+ influx that positively influences contractility but negatively impacts cell survival and arrhythmias.