Orai1 proteins have been recently identified as subunits of SOCE (store-operated Ca²⁺ entry) channels. In primary isolated PACs (pancreatic acinar cells), Orai1 showed remarkable co-localization and co-immunoprecipitation with all three subtypes of IP₃Rs (InsP₃ receptors). The co-localization between Orai1 and IP₃Rs was restricted to the apical part of PACs. Neither co-localization nor co-immunoprecipitation was affected by Ca²⁺ store depletion. Importantly we also characterized Orai1 in basal and lateral membranes of PACs. The basal and lateral membranes of PACs have been shown previously to accumulate STIM1 (stromal interaction molecule 1) puncta as a result of Ca²⁺ store depletion. We therefore conclude that these polarized secretory cells contain two pools of Orai1: an apical pool that interacts with IP₃Rs and a basolateral pool that interacts with STIM1 following the Ca²⁺ store depletion. Experiments on IP₃R knockout animals demonstrated that the apical Orai1 localization does not require IP₃Rs and that IP₃Rs are not necessary for the activation of SOCE. However, the InsP₃-releasing secretagogue ACh (acetylcholine) produced a negative modulatory effect on SOCE, suggesting that activated IP₃Rs could have an inhibitory effect on this Ca²⁺ entry mechanism.

Depletion of the endoplasmic reticulum (ER) calcium store triggers translocation of stromal interacting molecule one (STIM1) to the sub-plasmalemmal region and formation of puncta-structures in which STIM1 interacts and activates calcium channels. ATP depletion induced the formation of STIM1 puncta in PANC1, RAMA37, and HeLa cells. The sequence of events triggered by inhibition of ATP production included a rapid decline of ATP, depletion of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and a slow calcium leak from the ER followed by formation of STIM1 puncta. STIM1 puncta induced by ATP depletion were co-localized with clusters of ORAI1 channels. STIM1-ORAI1 clusters that developed as a result of ATP depletion were very poor mediators of Ca(2+) influx. Re-translocation of STIM1 from puncta back to the ER was observed during total ATP depletion. We can therefore conclude that STIM1 translocation and re-translocation as well as formation of STIM1-ORAI1 clusters occur in an ATP-independent fashion and under conditions of PI(4,5)P(2) depletion.