Ca²⁺-homeostasis plays a critical role for insulin-secretion in pancreatic b-cells. There are two main sources of Ca²⁺, the extracellular space and intracellular stores. In pancreatic b-cells the L-type voltage gated Ca²⁺-channel is the main influx pathway upon stimulation by glucose and the main mechanism for insulin secretion. Ca²⁺-signaling is further modified by Ca²⁺ release from internal stores and the accompanied Ca²⁺ entry via store-operated Ca²⁺-channels (SOCs). In the last few years it was shown in a number of different cell types that the stromal interacting molecule 1 (Stim1) and the Orai proteins represent the key-proteins responsible for SOCE. It was already demonstrated that in mouse MIN6 cells (1) Stim1 is expressed and translocated from the ER to the peri-PM space upon ER depletion. The aim of this study was to investigate the role of Stim1 and Orai1 for basal Ca²⁺-homeostasis and SOCE in rat pancreatic b-cells and to elucidate their possible contribution to insulin secretion. The expression of Stim1 and Orai1 mRNA in the rat b-cell line INS1-R9 was positively tested with RT-PCR. Subsequently, changes in the Ca²⁺ homeostasis under resting conditions and upon cell stimulation in b-cells, that transiently overexpressed Stim1 and Orai1, were assessed. In Stim1 and Orai1 overexpressing cells, an increase in resting cytosolic Ca²⁺ and an enhanced Ca²⁺-entry upon store-depletion with different agonists (carbachol, caffeine, BHQ) was found, while Ca²⁺-influx via L-type voltage gated Ca²⁺-channels upon depolarization was unmodified. Down-regulation of Stim1 supports the data mentioned above. The increase of basal cytosolic Ca²⁺ was not accompanied by a significantly increased depletion of the ER in Ca²⁺ containing medium. Furthermore we have indications that knockdown of Stim1 reduces the ER leak, which is known to be strong in b-cells. These data point to a Stim1/Orai1-dependent basal Ca²⁺ cycling in b-cells that may contribute not only to Ca²⁺ homeostasis but also for insulin secretion.

(1) Tamarina N.A. et al.; Cell Calcium, 44, 2008.