Density of functional Ca2+ release-activated Ca2+ (CRAC) channels declines after T-cell activation.

CRAC channel-mediated Ca(2+) entry plays a crucial role in T lymphocyte activation. Activated T cells display enhanced Ca(2+) signaling compared with resting T cells; this is partially attributed to activation-induced upregulation of CRAC channel expression. Orai and Stim family genes encode CRAC channel structural elements and regulatory proteins, respectively, but studies of their expression in T cells have led to controversial results. We re-examined Orai and Stim gene expression in resting, activated, and Jurkat T cells. Levels of Orai1 transcripts, encoding the human T cell CRAC channel subunit, were not significantly different between resting T and activated T cells. The total amount of all Orai transcripts was 2-fold higher in activated T cells than in resting T cells. Orai1 and total Orai transcript levels were significantly higher in Jurkat T cells than those in resting T cells. Stim expression did not vary significantly among cell types. Maximal whole-cell CRAC current amplitudes were 1.4-fold and 2.3-fold higher in activated and Jurkat T cells, respectively, than in resting T cells. Due to the small size of resting T cells, the surface CRAC channel density was 2.5-fold and 1.6-fold higher in resting T cells than in activated and Jurkat T cells, respectively. Predicted the rates of cytosolic Ca(2+) elevation calculated using the average values of CRAC channel currents and cell volumes showed that < 2-fold increase in the functional CRAC channel expression level cannot account for the enhanced rate of store-operated Ca(2+) entry in activated T cells compared with resting T cells.