The calcium loading of secretory granules. A possible key event in stimulus‐secretion coupling

Abstract

The review focuses on calcium accumulation by secretory organelles. The observation that secretory granules contain variable and often important quantities of calcium (1-200 mM of total calcium) can be interpreted as a maturation index. A progressive loading with calcium would be permitted by a Ca2(+)-transport mechanism on the granular membrane and calcium-binding molecules in the granular core. The saturation of this store by the stimulus-induced calcium transient would permit in mature (calcium-loaded) granules the ionic crisis leading to exocytosis. The inside of secretory organelles being acidic, calcium influx into the granule can be driven by calcium-proton exchange. The calcium-proton exchanger could be a Ca2(+)-ATPase.

Caffeine-Induced Ca 2+ Transients and Exocytosis in Paramecium Cells. A Correlated Ca 2+ Imaging and Quenched-Flow/Freeze-Fracture Analysis

“…This is due to the fact that trichocyst contents require Ca 2+ for decondensation during expulsion (Bilinski, Plattner & Matt, 1981), as confirmed independently (Kerboeuf & Cohen, 1990;Chilcoat et al, 1996). This is opposite to most other secretory organelles, like chromaffin granules (Winkler, 1993), whose contents need abundant Ca 2+ in tightly bound form at rest, i.e., to maintain the condensed state (Nicaise et al, 1992). In contrast, when the trichocyst matrix ''sees'' Ca 2+ , even in submicromolar concentrations, they undergo explosive decondensation.…”

Novel Types of Ca2+ Release Channels Participate in the Secretory Cycle of Paramecium Cells

“…In Paramecium, just as in other secretory systems, a cytosolic Ca 2ϩ signal is required for membrane fusion (24,47), while exogenous Ca 2ϩ is required for contents release (10 (15,67), but organellar Ca 2ϩ is not detectable in trichocysts by X-ray microanalysis, in contrast to that in alveolar sacs of Paramecium (33). Only access of extracellular Ca 2ϩ to trichocyst contents via the fusion pore can cause their "decondensation" (severalfold stretching upon vigorous exocytotic discharge) (10).…”

Quantitative Energy-Dispersive X-Ray Microanalysis of Calcium Dynamics in Cell Suspensions during Stimulation on a Subsecond Time Scale: Preparative and Analytical Aspects as Exemplified with Paramecium Cells

“…There are no analyses concerning ciliary beat regulation and none dealing directly with secretion regulation. As far as secretory systems have been analyzed, data reported aimed mainly at establishing concentrations of different ions particularly in secretory organelles, such as pancreatic zymogen granules (Roos and Barnard, 1985), and particularly the high [Ca] in a variety of secretory vesicles (Nicaise et al, 1992). In previous EDX analyses of Paramecium cells, during AED stimulation, use of freeze-dried sections impeded any clear-cut correlation between CaK a signals and alveolar sacs due to poor structural resolution, while it allowed, most importantly, one to establish the absence of any detectable [Ca] in trichocysts in the presence of high [NaJ (Schmitz and Zierold, 1989).…”