Endogenous and exogenous adenosine inhibit granulocyte aggregation without altering the associated rise in intracellular calcium concentration

The effects of adenosine, adenosine deaminase (ADA), and an irreversible ADA inhibitor 2'-deoxycoformycin (DCF) on granulocyte aggregation in response to four different stimuli: the synthetic chemotaxin N-formyl-met-leu-phe (FMLP), zymosan-activated plasma (ZAP), the calcium ionophore A23187, and phorbol myristate acetate (PMA) were studied. Adenosine inhibited granulocyte aggregation in response to 10^-7 mol/L FMLP in a dose-dependent fashion; inhibition in the presence of 1 μmol/L adenosine was 25% ± 3% (SD) and was 50% (the maximal inhibition observed) with 1 mmol/L adenosine. Quantatively similar results were obtained when ZAP or A23187 was used as the aggregant but the response to PMA was not affected. ADA not only reversed the inhibition due to adenosine but actually augmented the aggregation to FMLP by 118% ± 9%. Similar results were obtained with ZAP and A23187 but not with PMA. These effects of ADA depended on its enzymatic activity as they could be blocked by preincubation with DCF. Fluorescent measurements of intracellular calcium in fura-2 loaded granulocyte suspensions established that neither adenosine nor ADA affected subsequent FMLP-stimulated calcium responses. Adenosine, therefore, may inhibit granulocyte responsiveness by blocking signal transduction at a point after calcium entry/mobilization but before activation of protein kinase C. Furthermore, the augmentation of responses seen with ADA suggests that endogenous adenosine may be a physiologic autocrine regulator of granulocyte function.