Ca(2+) Homeostasis in the Endoplasmic Reticulum: Coexistence of High and Low [Ca(2+)] Subcompartments in Intact HeLa Cells

Two recombinant aequorin isoforms with different Ca(2+) affinities, specifically targeted to the endoplasmic reticulum (ER), were used in parallel to investigate free Ca(2+) homeostasis in the lumen of this organelle. Here we show that, although identically and homogeneously distributed in the ER system, as revealed by both immunocytochemical and functional evidence, the two aequorins measured apparently very different concentrations of divalent cations ([Ca(2+)](er) or [Sr(2+)](er)). Our data demonstrate that this contradiction is due to the heterogeneity of the [Ca(2+)] of the aequorin-enclosing endomembrane system. Because of the characteristics of the calibration procedure used to convert aequorin luminescence into Ca(2+) concentration, the [Ca(2+)](er) values obtained at steady state tend, in fact, to reflect not the average ER values, but those of one or more subcompartments with lower [Ca(2+)]. These subcompartments are not generated artefactually during the experiments, as revealed by the dynamic analysis of the ER structure in living cells carried out by means of an ER-targeted green fluorescent protein. When the problem of ER heterogeneity was taken into account (and when Sr(2+) was used as a Ca(2+) surrogate), the bulk of the organelle was shown to accumulate free [cation(2+)](er) up to a steady state in the millimolar range. A theoretical model, based on the existence of multiple ER subcompartments of high and low [Ca(2+)], that closely mimics the experimental data obtained in HeLa cells during accumulation of either Ca(2+) or Sr(2+), is presented. Moreover, a few other key problems concerning the ER Ca(2+) homeostasis have been addressed with the following conclusions: (a) the changes induced in the ER subcompartments by receptor generation of InsP(3) vary depending on their initial [Ca(2+)]. In the bulk of the system there is a rapid release whereas in the small subcompartments with low [Ca(2+)] the cation is simultaneously accumulated; (b) stimulation of Ca(2+) release by receptor-generated InsP(3) is inhibited when the lumenal level is below a threshold, suggesting a regulation by [cation(2+)](er) of the InsP(3) receptor activity (such a phenomenon had already been reported, however, but only in subcellular fractions analyzed in vitro); and (c) the maintenance of a relatively constant level of cytosolic [Ca(2+)], observed when the cells are incubated in Ca(2+)-free medium, depends on the continuous release of the cation from the ER, with ensuing activation in the plasma membrane of the channels thereby regulated (capacitative influx).