Endothelial intracellular Ca2+ release following monocyte adhesion is required for the transendothelial migration of monocytes

Although molecular changes accompanying leukocyte extravasation have been investigated intensively, the particular events following leukocyte adhesion and leading to the actual transendothelial migration process remain largely unkown. To characterize intraendothelial signals elicited by leukocyte adhesion and functionally required for their transmigration, we recorded endothelial free cytosolic intracellular Ca2+ levels ([Ca2+](i)) during the course of leukocyte adhesion. We show that monocyte and granulocyte adhesion induced Ca2+ transients in either untreated or TNF-alpha -stimulated microvascular endothelial cells (HMEC-1). The functional significance of these [Ca2+](i) rises was demonstrated by treating filter-grown endothelial monolayers with BAPTA/AM. This intraendothelial Ca2+ chelation left monocyte adhesion basically unaffected, but caused a significant and dose-dependent reduction of the transendothelial migration of monocytes. Granulocyte diapedesis, on the other hand, was hardly modified. Thapsigargin-treatment of endothelial cells almost completely inhibited the transmigration of monocytes suggesting that the necessary Ca2+ transients depended on a release from intracellular Ca2+ stores. Our results thus shaw that the transmigration of monocytes through endothelial monolayers of microvascular origin is favoured by an increase of the intraendothelial [Ca2+](i) induced by leukocyte adhesion to the endothelial cells. (C) 2001 Harcourt Publishers Ltd.