Background Increases in both leukocyte and endothelial cytosolic free (Ca sup 2+) may be involved in intercellular adhesion by regulating the affinity of surface adhesion molecules or by facilitating transendothelial leukocyte migration. The purpose of this study was to examine the effect of initial contact and subsequent adhesion of human neutrophils or monocytes on human aortic endothelial (Ca2+).Methods and Results Endothelial monolayers were loaded with the fluorescent Ca sup 2+ indicator indo 1 and exposed to isolated human peripheral blood neutrophils or to a cultured human monocyte cell line. A rapid, fourfold to fivefold increase in endothelial cytosolic (Ca2+) occurred within seconds of leukocyte contact. No increase in endothelial (Ca sup 2+) occurred on contact of 18.25- microns inert microspheres, isolated red blood cells, or suspensions of cultured human aortic endothelial cells. In experiments performed on monolayers grown in 1-mm2capillary flow tubes, the increase in endothelial cytosolic (Ca2+) on initial leukocyte contact was found to be related to the subsequent resistance to leukocyte detachment during exposure to arterial levels of shear stress (13.4 dyne x cm sup -2). The increase in endothelial cytosolic (Ca2+) during leukocyte contact was not inhibited in Ca2+ -free buffer but was abolished by prior depletion of an endoplasmic reticulum Ca2+ store by thapsigargin. Pretreatment of neutrophils with R15.7, a specific monoclonal antibody to the adhesion protein CD-18, inhibited the increase in endothelial cytosolic (Ca2+) on neutrophil contact.Conclusions Initial contact leading to subsequent adhesion of human leukocytes to human aortic endothelial cells releases an endothelial intracellular Ca sup 2+ store. This may, in part, be mediated by specific adhesion proteins and may in turn regulate the affinity of surface adhesion molecules or facilitate transendothelial migration of leukocytes. (Circulation. 1994;90:1899-1907.)