Endogenous adenosine regulates neutrophil pro-inflammatory activities by cyclic AMP-dependent accelerated clearance of cytosolic calcium

Abstract.

Objective and design: To identify the involvement of adenosine in restoration of Ca²⁺ homeostasis to activated human neutrophils.¶Materials: Neutrophils were isolated from venous blood taken from healthy, adult, human volunteers.¶Treatment: The cells were exposed to adenosine deaminase (ADA, 0.1–2 units/ml) for 10 min at 37°C prior to activation with N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP, 1 μM).¶Methods: Cytosolic Ca²⁺ concentrations and transmembrane fluxes of the cation in FMLP-activated neutrophils ± ADA were measured using spectrofluorimetric and radiometric procedures respectively, while intracellular cAMP and inositol triphosphate were measured by radioassay, and superoxide production and elastase release by, chemiluminescence and colourimetric methods respectively. Levels of statistical significance were calculated using the Mann-Whitney U-test and ANOVA.¶Results: Although FMLP-activated generation of inositol triphosphate and mobilisation of Ca²⁺ from neutrophil internal stores, as well as the magnitude of the subsequent efflux and store-operated influx of the cation were unaffected by ADA, there was a prolonged elevation in cytosolic Ca²⁺ in the presence of the enzyme, which was associated with failure to activate adenylate cyclase and with increased production of superoxide and release of elastase. These effects of ADA were attenuated by dibutyryl cAMP (4 mM), CGS 21680 (1 μM) and rolipram (0.5 μM), as well as by EGTA (10 mM).¶Conclusions: These results are compatible with a physiological role for adenosine in promoting deactivation of neutrophils, possibly by promoting cAMP-dependent clearance of Ca²⁺ from the cytosol of the cells by the endo-membrane Ca²⁺-ATPase.