Activation of cGMP-stimulated phosphodiesterase by nitroprusside limits cAMP accumulation in human platelets: effects on platelet aggregation

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Abstract
CGMP enhances cAMP accumulation in platelets via cGMP-inhibited phosphodiesterase (PDE3) [Maurice and Haslam (1990) Mol. Pharmacol. 37, 671–681]. However, cGMP might also limit cAMP accumulation by activating cGMP-stimulated phosphodiesterase (PDE2). We therefore evaluated the role of PDE2 in human platelets by using erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) to inhibit this enzyme selectively. IC50 values for the inhibition of platelet PDE2 by EHNA, with 10 μM cAMP as substrate in the absence and in the presence of 1 μM cGMP, were 15 and 3 μM respectively. Changes in platelet cyclic [3H]nucleotides were measured after prelabelling with [3H]adenine and [3H]guanine. Nitroprusside (NP) caused concentration-dependent increases in [3H]cGMP and a biphasic increase in [3H]cAMP, which was maximal at 10 μM (49±6%) and smaller at 100 μM (32±6%) (means±S.E.). In the presence of EHNA (20 μM), which had no effects alone, NP caused much larger increases in platelet [3H]cAMP (125±14% at 100 μM). EHNA also enhanced [3H]cGMP accumulation at high NP concentrations. In accord with these results, EHNA markedly potentiated the inhibition of thrombin-induced platelet aggregation by NP. The roles of cAMP and cGMP in this effect were investigated by using 2´,5´-dideoxyadenosine to inhibit adenylate cyclase. This compound decreased the accumulation of [3H]cAMP but not that of [3H]cGMP, and diminished the inhibition of platelet aggregation by NP with EHNA. We conclude that much of the effect of NP with EHNA is mediated by cAMP. Lixazinone (1 μM), a selective inhibitor of PDE3, increased platelet [3H]cAMP by 177±15%. This increase in [3H]cAMP was markedly inhibited by NP; EHNA blocked this effect of NP. Parallel studies showed that NP suppressed the inhibition of platelet aggregation by lixazinone. EHNA enhanced the large increases in [3H]cAMP seen with 20 nM prostacyclin (PGI2), but had no effect with 1 nM PGI2. NP and 1 nM PGI2 acted synergistically to increase [3H]cAMP, an effect attributable to the inhibition of PDE3 by cGMP; EHNA greatly potentiated this synergism. In contrast, NP decreased the [3H]cAMP accumulation seen with 20 nM PGI2, an effect that was blocked by EHNA. The results show that, provided that cGMP is present, PDE2 plays a major role in the hydrolysis of low cAMP concentrations and restricts any increases in cAMP concentration and decreases in platelet aggregation caused by the inhibition of PDE3. At high cAMP, PDE2 plays the major role in cAMP breakdown, whether cGMP is present or not.

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