Occupancy of adenosine receptors raises cyclic AMP alone and in synergy with occupancy of chemoattractant receptors and inhibits membrane depolarization

Abstract

We have recently demonstrated that adenosine, acting via adenosine A2 receptors, inhibits generation of superoxide anions (O2-) by stimulated neutrophils. To determine the mechanism(s) by which adenosine inhibits O2- generation stimulated by the chemoattractant N-formylmethionylleucylphenylalanine (FMLP), we examined cyclic AMP (cAMP) concentrations, stimulated membrane depolarization and Ca2+ movements. Neither adenosine nor 5'-N-ethylcarboxamidoadenosine (NECA), the most potent agonist at adenosine A2 receptors, increases neutrophil cAMP content. However in the presence of the non-methylxanthine phosphodiesterase inhibitor, Ro-20-1724, both adenosine and NECA elicit a reversible increase in intracellular cAMP concentration. The chemoattractant FMLP also elicits an increment in the neutrophil cAMP content. NECA, in the presence of Ro-20-1724, synergistically enhances the increment in cAMP following stimulation by FMLP. However Ro-20-1724 does not potentiate the inhibition of O2- generation by NECA. Unlike other agents which increase neutrophil cAMP concentrations, NECA, even in the presence of a phosphodiesterase inhibitor, only trivially inhibits degranulation. We also found that adenosine markedly inhibits stimulated membrane depolarization but does not affect the stimulated increment in free ionized intracellular calcium. Moreover, inhibition by adenosine of O2- generation does not vary with the concentration of extracellular calcium. These results fulfil the last criterion for the demonstration of an A2 receptor on human neutrophils, and indicate that adenosine occupies an A2 receptor on neutrophils to raise intracellular cAMP in synergy with occupancy of the FMLP receptor. The results reported here also indicate that cAMP is not the second messenger for inhibition of O2- generation by adenosine and its analogues.

Functional characterization of three adenosine receptor types

“…tissues (see Kennedy et al, 1992a,b for releva: particular the potency of N6-substituted comi GR79236, R-PIA and CPA, was similar to that of NECA. In addition, in this study, we CV1808 and CGS21680 were substantiall' NECA in these tissues, consistent with da binding studies (Bruns et al, 1986; Jarvis et X ever, the results obtained in this study and previous work (Kennedy et al, 1992a) (Collis & Brown, 1983;Cronstein et al, 1988;Haslam & Cusack, 1981;Kusachi et al, 1983). Thus, in dog coronary artery, human neutrophils and human platelets, CV1808 and CGS21680 were similar in potency to NECA, whilst in guinea-pig aorta, both compounds were substantially less active than NECA.…”

The adenosine/neutrophil paradox resolved: human neutrophils possess both A1 and A2 receptors that promote chemotaxis and inhibit O2 generation, respectively.

“…Previous studies have shown that adenosine receptor occupancy does not inhibit°2 generation by altering stimulated Ca2l metabolism, Na'-H' pumps or increasing intracellular levels ofcAMP (31)(32)(33). Our results suggest one mechanism by which adenosine receptor occupancy inhibits O°generation by activated (53).…”

Adenosine A2 Receptor Occupancy Regulates Stimulated Neutrophil Function via Activation of a Serine/Threonine Protein Phosphatase

“…generation, cAMP is not the second messenger. Although originally described as a single receptor that signals via cAMP (26,27), it has been demonstrated previously that adenosine A 2 receptors signal some events via cAMP-independent pathways in the neutrophil (8,9). Similar observations have been made recently for mast cells as well (28).…”