Adenosine increases human platelet levels of cGMP through nitric oxide - Possible role in its antiaggregating effect

Adenosine is an endogenous antiaggregating substance that influences the platelet responses through specific A-type receptors that activate adenylate cyclase increasing the levels of 3',5'-cyclic adenosine monophosphate (cAMP). In this study, we investigated whether adenosine can also influence the levels of 3',5'-cyclic guanosine monophosphate (cGMP) and decrease the aggregating response of human platelets to adenosine-5-diphosphate (ADP) through this nucleotide. In platelet samples from healthy volunteers, we evaluated the effect of adenosine on ADP-induced aggregation and cyclic nucleotide synthesis. Some experiments were repeated in the presence of dipyridamole (inhibitor of adenosine uptake and phosphodiesterase activity), N-G-monomethyl-L-arginine (L-NMMA, nitric synthase inhibitor), ionomycin (calcium ionophore), and ambroxol (2-amine-3,5-dibromo-N-[trans-4-hydroxycyclohexyl]benzylamine, inhibitor of nitric oxide (NO)dependent activation of guanylate cyclase). Adenosine decreased the response to ADP in a concentration-dependent way (analysis of variance, ANOVA: P < .0001): cAMP levels increased from 30.0+/-2.0 (control) to 46.0+/-3.0 pmol/10(9) platelets (in the presence of 15 mumol/l adenosine) and cGMP levels increased from 5.6+/-1.0 (control) to 10.9+/-2.0 pmol/10(9) platelets (in the presence of 15 mumol/l adenosine). Also, nucleotide levels measured at the end of aggregation were higher in platelet samples exposed to adenosine than in controls. Dipyridamole at 40 mumol/l slightly increased adenosine's effects on both nucleotides. L-NMMA blunted the effect of adenosine on cGMP both in unstimulated samples and in aggregated platelets without any effect on cAMP synthesis. Platelet exposure to L-NMMA and ambroxol partially prevented adenosine's effect on ADP-induced aggregation. In conclusion, adenosine, which enhances intraplatelet cAMP levels, was determined to also cause an increase in cGMP concentrations through a mechanism that involves NO synthesis. This effect plays a direct role in the adenosine-induced antiaggregation. (C) 2002 Elsevier Science Ltd. All rights reserved.