Caffeine releases a glucose‐primed endoplasmic reticulum Ca2+ pool in the insulin secreting cell line INS‐1

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Abstract
Caffeine mobilized an intracellular Ca2+ pool in intact fura‐2‐loaded INS‐1 cells in suspension exposed to high (16 mM) [glucose], while a minor effect was observed with low (2 mM) [glucose]. Cells were kept in a medium containing diaxozide or no Ca2+ to prevent the influx of extracellular Ca2+. The caffeine‐sensitive intracellular Ca2+ pool was within the endoplasmic reticulum since it was depleted by the inhibitor of the reticular Ca2+ pumps thapsigargin and the InsP3‐dependent agonist carbachol. No effect of caffeine was observed in the parent glucose‐insensitive RINmF5 cells. In microsomes from INS‐1 but not RINmF5 cells, the type 2 ryanodine receptor was present as revealed by Western blotting. It was concluded that the endoplasmic reticulum of INS‐1 cells possesses caffeine‐sensitive type 2 ryanodine receptors Ca2+ channels.