Abstract 34: 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 Ca2+ levels. The role of STIM1 in the regulation of SR Ca2+ 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 Ca2+ cycling present within diseased cardiomyocytes. Results: We used adenovirus to express either STIM1 or red fluorescent protein (RFP) in freshly isolated adult feline ventricular myocytes (AFMs). After 48 hours in culture, STIM1 induced cell death in 60% of myocytes versus only 5% in RFP controls. Addition of nifedipine rescued the cell death caused by STIM1, but block of transient receptor potential canonical (TRPC) channels was unable to improve viability. AFMs expressing STIM1 exhibited increased fractional shortening and Ca2+ transient amplitude, which was associated with increased SR load. Interestingly, high SR load levels caused by STIM1 were not reduced by nifedipine addition. We also found that baseline L-type channel current amplitude was significantly reduced by 20%, but no difference in current amplitude was found after Bay K8644 addition. Conclusions: In this study, we found that STIM1 caused Ca2+ overload leading to cell death. This process was L-type channel dependent, but TRPC channels were not involved. STIM1 also caused increased SR load at rest, which was not altered by nifedipine revealing that STIM1 mediated Ca2+ influx does not require L-type channel activity but Ca2+ influx through L-type channels is essential for STIM1 induced cellular death. Furthermore, STIM1 reduced L-type current activity but did not alter channel availability. These data show that after cardiac injury, STIM1 is likely to play a role in regulating cell survival as well as excitation-contraction coupling.