Scd. Vanijzendoorn et al., UNSTIMULATED PLATELETS EVOKE CALCIUM RESPONSES IN HUMAN UMBILICAL VEIN ENDOTHELIAL-CELLS, Biochimica et biophysica acta. Molecular cell research, 1311(1), 1996, pp. 64-70
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.