Stimulation of T cell receptor in lymphocytes enhances Ca2+ signaling and accelerates membrane trafficking. The relationships between these processes are not well understood. We employed membrane-impermeable lipid marker FM1-43 to explore membrane trafficking upon mobilization of intracellular Ca2+ in Jurkat T cells. We established that liberation of intracellular Ca2+ with T cell receptor agonist phytohemagglutinin P or with Ca2+-mobilizing agents ionomycin or thapsigargin induced accumulation of FM1-43 within the lumen of the endoplasmic reticulum (ER), nuclear envelope (NE), and Golgi. FM1-43 loading into ER-NE and Golgi was not mediated via the cytosol because other organelles such as mitochondria and multivesicular bodies located in close proximity to the FM1-43-containing ER were free of dye. Intralumenal FM1-43 accumulation was observed even when Ca2+ signaling in the cytosol was abolished by the removal of extracellular Ca2+. Our findings strongly suggest that release of intracellular Ca2+ may create continuity between the extracellular leaflet of the plasma membrane and the lumenal membrane leaflet of the ER by a mechanism that does not require global cytosolic Ca2+ elevation.
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