Adenosine exerts a variety of partly divergent effects during postischemic reperfusion. Since the intravascular retention of neutrophils (PMN) is a crucial step in the development of organ reperfusion injury, we tested whether stimulation of human endothelial adenosine A 1 -receptors enhances adhesion of PMN, as described previously for guinea pig coronary and feline coronary and pulmonary endothelium. Cultured human umbilical vein endothelial cells (HUVECs) were stimulated with different concentrations (0.01-10 mM) of the A 1 -receptor agonist N 6 -cyclopentyladenosine (CPA), thrombin (1U/ml) serving as con- trol stimulus. Adhesion of isolated human PMN on HUVECs was studied by photometrically determining the amount of the phagocyte-specific enzyme myeloperoxidase in culture plate lysates after sedimentation of PMN and removal of nonadherent PMN. The expression of CD11b, a PMN adhesion molecule and activation marker, was measured flow cytometrically on nonadherent PMN. The presence of the endothelial adhesion molecule P-selectin was also quantified by flow cytometry. Adhesion rose concentration-dependently after stimulation with CPA, reaching a maximum (55% increase vs. control) at 1 mM, then declining to 18% below control at 10 mM. Thrombin increased adhesion by 69%. CD11b on neutrophils rose after contact with the supernatant of stimulated endothelial cells, with a maximum increase of 136% at 1 mM CPA (thrombin: + 54%). P-selectin was upregulated on HUVECs upon stimulation: 25, 46, 159, and 29%, respectively, for 0.01, 0.1, 1, and 10 mM CPA (thrombin: 73%). Thus, activation of human endothelial adenosine A 1 -receptors, e.g., by CPA, causes increased adhesion and activation of PMN, and enhances expression of endothelial P-selectin. CPA partially reverses its proinflammatory actions due to a loss of specificity at high concentrations (10 mM).