INSULIN STIMULATES NITRIC-OXIDE SYNTHESIS IN HUMAN PLATELETS AND, THROUGH NITRIC-OXIDE, INCREASES PLATELET CONCENTRATIONS OF BOTH GUANOSINE-3',5'-CYCLIC MONOPHOSPHATE AND ADENOSINE-3',5'-CYCLIC MONOPHOSPHATE

The insulin-induced platelet anti-aggregating effect is attributed to a nitric oxide (NO)-mediated increase of cyclic guanosine monophosphat e (cGMP). The aim of this work, carried out in human platelets, is to show whether insulin increases NO synthesis in platelets and whether i t enhances not only cGMP but also cyclic adenosine monophosphate (cAMP ) in these cells. We observed that 1) insulin dose-dependently increas es NO production, evaluated as citrulline synthesis from L-arginine (n = 4, P = 0.015); 2) insulin dose-dependently increases not only cGMP but also cAMP: for instance, after 8 min of insulin incubation at 1,92 0 pmol/l, cAMP increased from 39.8 +/- 1.4 to 121.3 +/- 12.6 pmol/10(9 ) platelets (n = 16, P = 0.0001); 3) when insulin is incubated for 120 min, the increase of cGMP and cAMP shows a plateau between 2 and 20 m in, and while the effect on cGMP is significant until 120 min, the eff ect on cAMP is no more significant at 60 and 120 min; 4) insulin incre ases the effects on cAMP of the adenylate cyclase agonists Iloprost an d forskolin (n = 5, P = 0.0001) and enhances their platelet anti-aggre gating effects (n = 6 and 8, respectively; P = 0.0001); and 5) the inh ibition of NO synthase by N-G-monomethyl-L-arginine blunts both the in sulin effects on basal cGMP and cAMP (n = 4) and those on the Iloprost - and forskolin-induced cAMP increase (n = 5). Thus, insulin increases NO synthesis in human platelets, and, through NO, enhances both cGMP and cAMP. The platelet antiaggregating effect exerted by insulin is, t herefore, a NO-mediated phenomenon involving both cGMP and cAMP.