A store-operated mechanism determines the activity of the electrically excitable glucagon-secreting pancreatic α-cell

cAMP Induces Stromal Interaction Molecule 1 (STIM1) Puncta but neither Orai1 Protein Clustering nor Store-operated Ca2+ Entry (SOCE) in Islet Cells

“…The cholinergic agonist carbachol (50 M), which raises IP 3 and mobilizes ER Ca 2ϩ both in ␣-cells (22) and ␤-cells (19), induced sustained subplasmalemmal accumulation of STIM1-YFP preceded (3 of 8) or not by an initial peak in most islet cells exposed to 3 mM glucose (Fig. 1D).…”

“…These criteria should eliminate the small somatostatin-releasing ␦-cells with ␤-cell-like domination of ␣ 2 -adrenoceptors (31) as small cells and cells with small footprints were never taken as ␤-cells. Because epinephrine mobilizes intracellular Ca 2ϩ in ␣-but not ␤-cells (22), this response together with cell size was used for cell identification in most measurements of [Ca 2ϩ ] i . Data and Statistical Analysis-Image analysis was made using the MetaFluor or ImageJ (W. S. Rasband, National Institutes of Health, rsb.info.nih.gov) software.…”

“…Whereas Ca 2ϩ depletion of the ER has little effect on the membrane potential (20) from ␤-cells exposed to a sub-stimulatory glucose concentration (3 mM), such depletion further depolarizes glucose-stimulated ␤-cells (23) and amplifies insulin secretion (23,24). Activation of SOCE has much more important effects on the ␣-cell by triggering Ca 2ϩ entry through voltage-operated Ca 2ϩ channels (VOCCs) (22) and glucagon release (24), and SOCE has been attributed to be a central function in epinephrine stimulation and glucose inhibition of glucagon secretion (22,24).…”

“…Although these cells have more potent voltage-operated routes for Ca 2ϩ entry, SOCE has been found to be important for different cellular processes. However, even maximally activated SOCE has modest effects on the cytoplasmic Ca 2ϩ concentration ([Ca 2ϩ ] i ) in ␤- (19 -21) and ␣-cells (22) and has mostly been attributed a functional role based on the depolarizing effect (22)(23)(24). Whereas Ca 2ϩ depletion of the ER has little effect on the membrane potential (20) from ␤-cells exposed to a sub-stimulatory glucose concentration (3 mM), such depletion further depolarizes glucose-stimulated ␤-cells (23) and amplifies insulin secretion (23,24).…”

“…Studies of the store-operated mechanism in primary ␤-cells (21,25) and ␣-cells (22) (26). We have now utilized adenoviruses encoding STIM1-YFP and Orai1-mCherry (16) to infect pancreatic islets and study STIM1 translocation and association with Orai1 in primary pancreatic islet cells during conditions known to modulate hormone secretion and SOCE.…”