Extracellular ATP activates store-operated Ca(2+) entry in white adipocytes: functional evidence for STIM1 and ORAI1

In the present study, we have applied ratiometric measurements of intracellular Ca(2+) concentrations ([Ca(2+)](i)) to show that extracellularly applied ATP (adenosine triphosphate) (100 µM) stimulates store-operated Ca(2+) entry (SOCE) in 3T3-L1 adipocytes. ATP produced a rapid increase in [Ca(2+)](i) consisting of an initial transient elevation followed by a sustained elevated phase that could be observed only in the presence of extracellular Ca(2+). Gene expression data and [Ca(2+)](i) recordings with uridine-5′-triphosphate or with the phospholipase C (PLC) inhibitor U73122 demonstrated the involvement of purinergic P2Y2 receptors and the PLC/inositol trisphosphate pathway. The [Ca(2+)](i) elevation produced by reintroduction of a Ca(2+)-containing intracellular solution to adipocytes exposed to ATP in the absence of Ca(2+) was diminished by known SOCE antagonists. The chief molecular components of SOCE, the stromal interaction molecule 1 (STIM1) and the calcium release-activated calcium channel protein 1 (ORAI1), were detected at the mRNA and protein level. Moreover, SOCE was largely diminished in cells where STIM1 and/or ORAI1 had been silenced by small interfering (si)RNA. We conclude that extracellular ATP activates SOCE in white adipocytes, an effect predominantly mediated by STIM1 and ORAI1.