INSULIN INCREASES GUANOSINE-3',5'-CYCLIC MONOPHOSPHATE IN HUMAN PLATELETS - A MECHANISM INVOLVED IN THE INSULIN ANTI-AGGREGATING EFFECT

To investigate whether insulin reduces platelet aggregability through a modulation of the guanosine-3',5'-cyclic monophosphate (cGMP) concen trations, we determined by a radioimmunoassay the cGMP values in the p latelet-rich plasma (PRP) obtained hom 17 healthy volunteers and incub ated for 3 min with different concentrations of human recombinant insu lin (0, 240, 480, 720, 960, and 1,920 pM). Insulin induced a dose-depe ndent cGMP increase, from 18.5 +/- 3.3 to 42.0 +/- 6.4 pmol/10(9) plat elets (P = 0.0001). This increase was completely blunted when PRP was preincubated for 20 min with the tyrosine kinase inhibitor genistein ( 10 mu M) or with the guanylate cyclase inhibitor methylene blue (10 mu M), but the increase remained highly significant (P = 0.003 and 0.009 ) when PRP was preincubated for 20 min with the phosphodiesterase inhi bitor 3-isobutyl-1-methyl-xanthine (IBMX, 500 mu M) or with the nitric oxide synthase inhibitor N-G-monomethyl-L-arginine (L-NMMA, 30 mu M). Finally, the insulin-induced decrease of platelet aggregability to co llagen and ADP was completely blunted when PRP was preincubated with 1 0 mu M of the guanylate cyclase inhibitor methylene blue. This study d emonstrates that the platelet antiaggregatory effect exerted by insuli n is attributable to the insulin-induced increase of cGMP that is due to a direct receptor-mediated platelet guanylate cyclase activation.