UNSTIMULATED PLATELETS EVOKE CALCIUM RESPONSES IN HUMAN UMBILICAL VEIN ENDOTHELIAL-CELLS

Interactions between human platelets and human umbilical vein endothel ial cells (HUVEC) were studied by monitoring changes in cytosolic [Ca2 +](i) in both cell types. Confluent monolayers of Fura-2-loaded HUVEC, grown on gelatin-coated coverslips, responded to repeated addition of a suspension of unstimulated platelets by transient increases in cyto solic [Ca2+](i). These platelet-evoked Ca2+ responses were not caused by products secreted from the platelets and were insensitive to inhibi tors of platelet activation and/or platelet aggregation. The platelet- evoked rises in [Ca2+](i) in endothelial cells, similarly as the throm bin-evoked rises, were blocked by preincubation of HUVEC with the phos pholipase C inhibitor U73122 or the Ca2+ influx blocker Ni2+. In contr ast, treatment with the protein tyrosine kinase inhibitor genistein wa s without effect. Video imaging experiments, in which the fluorescence signal was analysed from the individual cells of an endothelial monol ayer, showed that only 2-20% of the cells, scattered over the monolaye r, responded to the addition of platelets by a transient increase in [ Ca2+](i), whereas most of the cells responded to thrombin. This leads to the conclusion that unstimulated platelets can activate HUVEC putat ively by mechanical interaction with individual endothelial cells in t he monolayer.