Slow depletion of endoplasmic reticulum Ca2+ stores and block of store-operated Ca2+ channels by 2-aminoethoxydiphenyl borate in mouse pancreatic acinar cells

The compound 2-aminoethoxydiphenyl borate (2-APB) has been used as either a specific membrane-permeable inhibitor for InsP(3) receptors or a store-operated Ca2+ channel blocker in some cells. In this study, we have investigated actions of 2-APB on Ca2+ signalling in mouse pancreatic acinar cells by measuring Ca2+ concentration in the cytosol ([Ca2+]c) and in the endoplasmic reticulum (ER). Although 2-APB (50 muM) inhibited or modulated [Ca2+]c oscillations generated by a low dose of ACh, it did not block InsP(3)-mediated Ca2+ release from the ER elicited by high doses of acetylcholine (ACh). 2-APB alone more than 70 muM tended to increase [Ca2+]c. When we directly measured Ca2+ concentration in the lumen of the ER with a low affinity Ca2+ dye, Mag-fluo-4, 2-APB itself slowly lowered ER Ca2+ concentration and ACh could further release Ca2+ from the ER in the presence of 2-APB, suggesting its lack of potency to block InsP(3) receptors. When store-operated Ca2+ entry was evoked by addition of external Ca2+ (5 mM) after depletion of Ca2+ stores, 2-APB (50 muM) substantially blocked the Ca2+ influx in a reversible manner. We conclude that (a) 2-APB is a good blocker for store-operated Ca2+ channels, (b) 2-APB could not effectively block InsP(3) receptors, and (c) low doses of 2-APB lower Ca2+ concentration in the lumen of the ER without a significant elevation of [Ca2+]c.