Store-operated calcium entry is present in HL-1 cardiomyocytes and contributes to resting calcium.

Store-operated Ca(2+) entry (SOCE) has recently been shown to be of physiological and pathological importance in the heart, particularly during cardiac hypertrophy. However, measuring changes in intracellular Ca(2+) during SOCE is very difficult to study in adult primary cardiomyocytes. As a result there is a need for a stable and reliable in vitro model of SOCE which can be used to test cardiac drugs and investigate the role of SOCE in cardiac pathology. HL-1 cells are the only immortal cardiomyocyte cell line available that continuously divides and spontaneously contracts while maintaining phenotypic characteristics of the adult cardiomyocyte. To date the role of SOCE has not yet been investigated in the HL-1 cardiac cell line. We report for the first time that these cells expressed stromal interaction molecule 1 (STIM1) and the Ca(2+) release-activated Ca(2+) (CRAC) channel Orai1, which are essential components of the SOCE machinery. In addition, SOCE was tightly coupled to sarcoplasmic reticulum (SR)-Ca(2+) release in HL-1 cells, and such response was not impaired in the presence of voltage dependent Ca(2+) channels (L-type and T-type channels) or reverse mode Na(+)/Ca(2+) exchanger (NCX) inhibitors. We were able to abolish the SOCE response with known SOCE inhibitors (BTP-2 and SKF-96365) and by targeted knockdown of Orai1 with RNAi. In addition, knockdown of Orai1 resulted in lower baseline Ca(2+) and an attenuated response to thapsigargin (TG) and caffeine, indicating that SOCE may play a role in Ca(2+) homeostasis during unstressed conditions in cardiomyocytes. Currently, there is little knowledge about SOCE in cardiomyocytes, and the present results suggest that HL-1 cells will be of great utility in investigating the role of SOCE in the heart.