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The differences in the intracellular Ca2+ responses to hormones in platelets from systemic lupus erythematosus (SLE) patients compared to normal humans have not been explored. This study examined the Ca2+ signaling and density of platelets in normal, inactive and active SLE patients. The platelet number per µl in inactive and normal groups did not differ, whereas the number in active SLE patients was smaller than the other two groups by 60%. The intracellular free Ca2+ levels ([Ca2+]i) in response to stimulation of four endogenous Ca2+ mobilizing hormones, 100 µM arachidonic acid (AA), 10 µM ADP, 10 nM platelet activation factor (PAF) and 1 µM thrombin, were investigated using the Ca2+-sensitive fluorescent dye, fura-2. The AA-induced [Ca2+]i rises in normal and inactive groups were similar. In contrast, the AA-induced [Ca2+]i rises in the active SLE group were significantly smaller than in the normal and inactive groups. The defect in the AA-induced [Ca2+]i rises in active SLE groups appears to be caused by defective Ca2+ influx and Ca2+ releasing pathways because the AA-induced responses were not altered by removal of extracellular Ca2+, whereas the AA-induced responses in normal and inactive SLE groups were reduced by removal of extracellular Ca2+, and the AA-induced Ca2+ release was smaller in the active SLE group. PAF, ADP and thrombin all induced [Ca2+]i rises in the three groups, but no significant differences were found among the three groups. Together, the results indicate that cell density and Ca2+ signaling in platelets from active SLE patients are altered in response to particular stimulators. In these regards, platelets from inactive SLE patients appear to be similar to those from normal humans.