A phosphatidic acid-sensitive intracellular pool of calcium is released by anti-CD3 in Jurkat T cells.

Jurkat T cells, loaded with the fluorescent calcium probe Indo 1, responded to exogenous phosphatidic acid (PA) by transiently increasing their cytosolic Ca2+ concentration. This effect was dose-dependent, remained unmodified when external Ca2+ was chelated with EGTA, and was totally inhibited when cells were first exposed to CD3 monoclonal antibodies, indicating that it was solely due to the release of an intracellular pool, which is also mobilized during a stimulation via the CD3 T-cell receptor (TcR) molecular complex. CD3- and phytohaemagglutinin (PHA)-stimulated Jurkat cells also produced PA, the dose-responses and kinetics of which were consistent with those of calcium release. Moreover, diacylglycerol (DAG) kinase inhibitors abrogated PA production and lowered calcium release by CD3-stimulated cells. PA did not induce any apparent increase in inositol triphosphates (IP3), nor did it modify the increase entailed by activation of the CD3 pathway, pointing out that IP3 can be supplemented in mobilizing calcium from intracellular stores. Conversely, a first exposure to PA only partially inhibited the CD3- or ionomycin-induced internal release of calcium, suggesting either a rapid restoration of the PA-sensitive stores, or a contribution of other mediators, such as IP3, in the CD3 activation pathway.