The target calcium store of nicotinic acid adenine dinucleotide phosphate (NAADP), the most potent endogenous calcium-mobilizing compound known to date, has been proposed to reside in the lysosomal compartment or in the endo/sarcoplasmic reticulum. This study was performed to test the hypothesis of a lysosomal versus an endoplasmic reticular calcium store sensitive to NAADP in T-lymphocytes. Pretreatment of intact Jurkat T cells with glycyl-phenylalanine 2-naphthylamide largely reduced staining of lysosomes by LysoTracker Red and abolished NAADP-induced Ca(2+) signaling. However, the inhibitory effect was not specific since Ca(2+) mobilization by d-myo-inositol 1,4,5-trisphosphate and cyclic ADP-ribose was abolished, too. Bafilomycin A1, an inhibitor of the lysosomal H(+)-ATPase, did not block or reduce NAADP-induced Ca(2+) signaling, although it effectively prevented labeling of lysosomes by LysoTracker Red. Further, previous T cell receptor/CD3 stimulation in the presence of bafilomycin A1, assumed to block refilling of lysosomal Ca(2+) stores, did not antagonize subsequent NAADP-induced Ca(2+) signaling. In contrast to bafilomycin A1, emptying of the endoplasmic reticulum by thapsigargin almost completely prevented Ca(2+) signaling induced by NAADP. In conclusion, in T-lymphocytes, no evidence for involvement of lysosomes in NAADP-mediated Ca(2+) signaling was obtained. The sensitivity of NAADP-induced Ca(2+) signaling toward thapsigargin, combined with our recent results identifying ryanodine receptors as the target calcium channel of NAADP (Dammermann, W., and Guse, A. H. (2005) J. Biol. Chem. 280, 21394-21399), rather suggest that the target calcium store of NAADP in T cells is the endoplasmic reticulum.
