Effect of haptoglobin (Hp) on calcium release in U937 cells. Upper panel: fura-2 acetoxymethyl ester (fura-2 AM) preloaded U937 cells untreated or pretreated with 500 ng/ml monocyte chemoattractant protein 1 (MCP1) or 5 μM RS102895 were stimulated with 0.5 mg/ml Hp. Untreated cells were stimulated with 250 ng/ml MCP1 as a positive control. Bottom panel: quantification of [Ca2+]i rise in U937 cells. The rate of [Ca2+]i rise (percentage fura-2 AM saturation/s) induced by Hp was set to 100% and the rate after pretreatments was calculated. The result is representative of three independent experiments.