We have studied modulation of purinergic receptors (P2Y and P2X subtypes) on changes in intracellular Ca2+ [Ca2+]i and expression and production of COX-2 in human microglia. Measurements using Ca2+-sensitive spectrofluorometry showed adenosine triphosphate (ATP) to cause rapid transient increases in [Ca2+]i. Application of ATP plus the P2X antagonist, pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS), or treatment with adenosine diphosphate-β-S (ADP-β-S), a selective P2Y agonist, led to a considerable prolongation in [Ca2+]i responses compared with ATP. The prolonged time courses were consistent with sustained activation of store-operated channels (SOC) since SKF96365, an inhibitor of SOC, blocked this component of the response. RT-PCR data showed that microglia expressed no COX-2 either constitutively or following treatment of cells with ATP (100 μM for 8 h). However, treatment using ATP plus PPADS or with ADP-β-S led to marked expression of COX-2. The enhanced COX-2 with ATP plus PPADS treatment was absent in the presence of SKF96365 or using Ca2+-free solution. Immunocytochemistry, using a specific anti-COX-2 antibody, also revealed a pattern of purinergic modulation whereby lack of P2X activation enhanced the production of COX-2 protein. These results suggest that modulation of subtypes of purinergic receptors regulates COX-2 in human microglia with a link involving SOC-mediated influx of Ca2+. © 2003 Wiley-Liss, Inc.