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Lipopolysaccharide suppresses thrombin- and histamine-stimulated rises in intracellular calcium concentration in human endothelial cells

We examined the effect of LPS on thrombin- and histamine-stimulated rises in intracellular calcium concentration ([Ca2+]i) in cultured human umbilical vein endothelial cells (HUVEC). HUVEC were incubated with LPS for up to 24 h. After harvesting and labeling with Quin-2/AM, HUVEC were stimulated with thrombin or histamine, and rises in [Ca2+] i were monitored. LPS (0.01-10 μg/ml) significantly suppressed the rise in [Ca2+]i induced by 2 U/ml thrombin or 10 μM histamine. The degree of suppression was dependent on the concentration of LPS, although LPS did not change unstimulated, basal intracellular calcium levels. After HUVEC were incubated with 10 μg/ml of LPS for 24 h, the rise in [Ca2+]i was measured in response to various concentrations of thrombin (0.5, 1, 2 and 4 U/ml). The rises in [Ca 2+]i were proportional to the concentration of thrombin, and the rises in [Ca2+]i in LPS-treated HUVEC were significantly lower than those in control HUVEC at any concentration of thrombin tested. The suppressive effect of LPS on rises in [Ca2+] i appeared after 1 h incubation of HUVEC with 10 μg/ml LPS, and reached a maximal level after 16 h. Our results raise the possibility that LPS might alter various calcium-mediated biological responses in endothelial cells in vivo.