THE ROLE OF CYCLIC-AMP, CALCIUM AND FILAMENTOUS ACTIN IN ADENOSINE MODULATION OF FC RECEPTOR-MEDIATED PHAGOCYTOSIS IN HUMAN NEUTROPHILS

The role of cyclic AMP, calcium and filamentous actin (F-actin) conten t during adenosine modulation of Fc receptor (FcR)-mediated phagocytos is in adherent human neutrophils was investigated. Phagocytosis of IgG -opsonized yeast particles was found to be enhanced by pico- to nanomo lar concentrations of adenosine or the A(1)-agonist N-6-cyclopentyl-ad enosine (CPA) but reduced by micromolar concentrations of adenosine or the A(2)-agonist 5'-N-ethylcarboxamidoadenosine (NECA). NECA, in the presence of the cAMP-specific phosphodiesterase inhibitor Ro 20-1724, increased the intracellular content of cAMP during phagocytosis. Ro 20 -1724 potentiated the NECA-induced reduction of the phagocytic capacit y. These observations indicate that cAMP elevations are involved in A( 2)-receptor-mediated inhibition of phagocytosis. NECA, in the presence of Ro 20-1724, markedly enhanced the actin polymerization associated with adhesion to the substrate and contact with the phagocytic prey. D uring advanced phagocytosis, however, the F-actin content reached leve ls clearly below those observed in control cells. This prolonged depol ymerization phase correlated with the A(2)-receptor-induced cAMP eleva tion. Depletion of intracellular free calcium abolished the cAMP-eleva ting effects of NECA, and also completely abrogated the A(1)- and A(2) -receptor-mediated effects on phagocytosis. However, since NECA reduce d the F-actin content even in Ca2+-depleted cells, A(2)-receptor-media ted inhibition of phagocytosis could not be directly coupled to change s in the overall content of F-actin. Our results indicate that adenosi ne modulates FcR-mediated phagocytosis in a calcium-dependent way, and does so through 'stimulatory' A(1) and 'inhibitory' A(2) receptors, a nd also that cAMP elevation is linked to the A(2)-receptor-induced inh ibition of phagocytosis.