AGONISTS FOR ENDOTHELIAL P-2 PURINOCEPTORS TRIGGER A SIGNALING PATHWAY PRODUCING CA2+ RESPONSES IN LYMPHOCYTES ADHERENT TO ENDOTHELIAL-CELLS

Recirculation of lymphocytes through the body involves their frequent adhesion to endothelial cells but little is known of the signalling pathways between these two cell types. Lymphocytes from patients with chronic lymphocytic leukaemia were loaded with the Ca2+-sensitive indicator, fura 2, and allowed to adhere to either glass or monolayers of human umbilical-vein endothelial cells. Addition of ATP or UTP (1-10 mu M) to the superfusate produced a transient rise in cytosolic Ca2+ concentration in the lymphocytes adherent to endothelium (24 of 35 cells). In contrast, ATP or UTP (1-10 mu M) had no effect on the cytosolic Ca2+ of lymphocytes attached to glass. As the only lymphocyte receptor for ATP (P-2Z class) requires higher ATP concentrations (> 50 mu M) for Ca2+ influx and is unresponsive to UTP, the involvement of a lymphocyte P-2Z purinoceptor is unlikely. Various agonists including ATP, UTP, 2-methylthioATP, ADP and histamine all stimulated increases in endothelial cytosolic Ca2+ but only ATP and UTP (both agonists for endothelial P-2U purinoceptors) triggered Ca2+ transients in adherent lymphocytes. Removal of extracellular Ca2+ did not abolish the ATP-induced rise in cytosolic Ca2+ concentration in lymphocytes adherent to endothelial cells. These findings show that stimulation of endothelial P-2U purinoceptors triggers an endothelial-lymphocyte signalling pathway which releases internal Ca2+ in adherent lymphocytes.