pAgonist-sensitive intracellular Casup2+/sup stores may be heterogeneous and exhibit distinct functional features. We have studied the properties of intracellular Casup2+/sup stores using targeted aequorins for selective measurements in different subcellular compartments. Both, HEK-293T [HEK (human embryonic kidney)-293 cells expressing the large T-antigen of SV40 (simian virus 40)] and HeLa cells accumulated Casup2+/sup into the ER (endoplasmic reticulum) to near millimolar concentrations and the IPsub3/sub-generating agonists, carbachol and ATP, mobilized this Casup2+/sup pool. We find in HEK-293T, but not in HeLa cells, a distinct agonist-releasable Casup2+/sup pool insensitive to the SERCA (sarco/endoplasmic reticulum Casup2+/sup ATPase) inhibitor TBH [2,5-di-(it/i-butyl)-benzohydroquinone]. TG (thapsigargin) and CPA (cyclopiazonic acid) completely emptied this pool, whereas lysosomal disruption or manoeuvres collapsing endomembrane pH gradients did not. Our results indicate that SERCA3d is important for filling the TBH-resistant store as: (i) SERCA3d is more abundant in HEK-293T than in HeLa cells; (ii) the SERCA 3 ATPase activity of HEK-293T cells is not fully blocked by TBH; and (iii) the expression of SERCA3d in HeLa cells generated a TBH-resistant agonist-mobilizable compartment in the ER. Therefore the distribution of SERCA isoforms may originate the heterogeneity of the ER Casup2+/sup stores and this may be the basis for store specialization in diverse functions. This adds to recent evidence indicating that SERCA3 isoforms may subserve important physiological and pathophysiological mechanisms./p
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