REGULATION OF OXYGEN RADICAL PRODUCTION OF HUMAN POLYMORPHONUCLEAR LEUKOCYTES BY ADENOSINE - THE ROLE OF CALCIUM

In polymorphonuclear leukocytes (PMNL) adenosine is a potent inhibitor of stimulus/response coupling, as demonstrated by its adverse action on phagocytosis, degranulation and oxygen radical production. Because this nucleoside can reduce several cell functions by counteracting intracellular calcium ions (Ca2+), the present study investigates the effect of adenosine on oxygen radical production in human PMNL stimulated by N-formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe), zymosan-activated serum (ZAS) and ionophore A 23 187 in a Ca2+-dependent manner, and stimulated by phorbol myristate acetate (PMA) and latex in a Ca2+-independent manner. The results demonstrate that all the Ca2+-dependent stimuli were concentration-dependently inhibited by adenosine. In contrast, leukocyte stimulation by the Ca2+-independent activator PMA was not affected by adenosine. Surprisingly, radical formation stimulated by latex beads was concentration-dependently reduced by adenosine. When intracellular Ca2+ was modified by the ionophore clamping technique or the Ca2+-buffering capacity of quin-2, latex-induced radical formation could be separated into two parts, one showing Ca2+-dependent and the other Ca2+-independent activation. In the presence of intracellular Ca2+, adenosine exerted a strong inhibition on the latex-induced cell activation but failed to inhibit in the Ca2+-depleted state. In order to elucidate a direct reduction of Ca2+ as the underlying mechanism of adenosine-mediated inhibition, intracellular Ca2+ was measured in PMNL by quin-2 fluorescence. When PMNL were activated by latex, fMet-Leu-Phe and ionophore A 23 187, adenosine significantly reduced the stimulated rise in intracellular Ca2+. Activation of PMNL by PMA was followed by a slight decrease of intracellular Ca2+, upon which adenosine had no effect. In the presence of the enzyme adenosine deaminase or the adenosine-receptor antagonist 8-phenyltheophylline, the increase in intracellular Ca2+ was enhanced, indicating that endogenously formed adenosine is capable of exerting an inhibitory effect. Moreover, 8-phenyltheophylline could also reverse the effects of exogenously applied adenosine on oxygen radical formation. In conclusion, adenosine's inhibitory action on oxygen radical production appears to result from receptor-mediated inhibition of stimulus-induced increases in intracellular Ca2+, as (a) it is restricted to activation mechanisms that are closely dependent on intracellular Ca2+, (b) it occurs with a concomitant decrease of intracellular Ca2+ and (c) the inhibition of both is reversed by 8-phenyltheophylline.