Increase in reactivity of human platelet guanylate cyclase during aggregation potentiates the disaggregating capacity of sodium nitroprusside

The Controversial Role of Glucose-6-Phosphate Dehydrogenase Deficiency on Cardiovascular Disease: A Narrative Review

Cardiovascular disorders (CVD) are highly prevalent and the leading cause of death worldwide. Atherosclerosis is responsible for most cases of CVD. The plaque formation and subsequent thrombosis in atherosclerosis constitute an ongoing process that is influenced by numerous risk factors such as hypertension, diabetes, dyslipidemia, obesity, smoking, inflammation, and sedentary lifestyle. Among the various risk and protective factors, the role of glucose-6-phosphate dehydrogenase (G6PD) deficiency, the most common inborn enzyme disorder across populations, is still debated. For decades, it has been considered a protective factor against the development of CVD. However, in the recent years, growing scientific evidence has suggested that this inherited condition may act as a CVD risk factor. The role of G6PD deficiency in the atherogenic process has been investigated using in vitro or ex vivo cellular models, animal models, and epidemiological studies in human cohorts of variable size and across different ethnic groups, with conflicting results. In this review, the impact of G6PD deficiency on CVD was critically reconsidered, taking into account the most recent acquisitions on molecular and biochemical mechanisms, namely, antioxidative mechanisms, glutathione recycling, and nitric oxide production, as well as their mutual interactions, which may be impaired by the enzyme defect in the context of the pentose phosphate pathway. Overall, current evidence supports the notion that G6PD downregulation may favor the onset and evolution of atheroma in subjects at risk of CVD. Given the relatively high frequency of this enzyme deficiency in several regions of the world, this finding might be of practical importance to tailor surveillance guidelines and facilitate risk stratification.

Centrality of G6PD in COVID-19: The Biochemical Rationale and Clinical Implications

Introduction: COVID-19 is a novel and devastating disease. Its manifestations vary from asymptomatic to lethal. Moreover, mortality rates differ based on underlying health conditions and ethnicity. We investigated the biochemical rationale behind these observations using machine reasoning by the sci.AI system (https://sci.ai/). Facts were extracted and linked from publications available in nlm.nih.gov and Europe PMC to form the dataset which was validated by medical experts. Results: Based on the analysis of experimental and clinical data, we synthesized detailed biochemical pathways of COVID-19 pathogenesis which were used to explain epidemiological and clinical observations. Clinical manifestations and biomarkers are highlighted to monitor the course of COVID-19 and navigate treatment. As depicted in the Graphical Abstract, SARS-CoV-2 triggers a pro-oxidant (PO) response leading to the production of reactive oxygen species (ROS) as a normal innate defense. However, SARS-CoV-2's unique interference with the antioxidant (AO) system, through suppression of nitric oxide (NO) production in the renin- angiotensin-aldosterone system (RAAS), leads to an excessive inflammatory PO response. The excessive PO response becomes critical in cohorts with a compromised AO system such as patients with glucose-6-phosphate dehydrogenase deficiency (G6PDd) where NO and glutathione (GSH) mechanisms are impaired. G6PDd develops in patients with metabolic syndrome. It is mediated by aldosterone (Ald) which also increases specifically in COVID-19. Conclusion: G6PD is essential for an adequate immune response. Both G6PDd and SARS-CoV-2 compromise the AO system through the same pathways rendering G6PDd the Achilles' heel for COVID-19. Thus, the evolutionary antimalarial advantage of the G6PDd cohort can be a disadvantage against SARS-CoV-2.

Amlodipine, Nitric Oxide, and Platelet Aggregation

This study examined the effect of a single dose of 0.4 mg·kg−1 amlodipine on platelet aggregation with and without the nitric oxide synthase inhibitor, Nw-nitro-L-arginine. Blood samples were collected from rhesus monkeys at 0, 1, 2, and 6 hours after administration of amlodipine. Aggregation in platelet-rich plasma was stimulated by 10 μM adenosine diphosphate, 20 μM epinephrine, or 2 μg·mL−1 collagen. Administration of Nw-nitro-L-arginine alone significantly increased platelet aggregation for up to 2 hours. This effect was antagonized by amlodipine administered 30 minutes after Nw-nitro-L-arginine. The findings suggest that in platelet-rich plasma, the inhibition of platelet aggregation by amlodipine might be mediated by nitric oxide, a potent endogenous inhibitor of aggregation.

Evaluation of refrigerated platelet concentrates supplemented with low doses of second messenger effectors

With the goal of producing haemostatically effective platelet concentrates (PCs) with a longer shelf-life, we aimed to identify a simple combination of platelet inhibitors, with a low pharmacological load, which could avoid the unacceptable loss of platelets stored under refrigerated conditions. PCs stored with different combinations of second messenger effectors were analysed at days 5, 10 and 15 of storage and compared with those supplemented with ThromboSol--a combination of six platelet inhibitors that protects cells from cold damage. The following parameters were analysed: platelet counts, biochemical parameters (glucose, pH, bicarbonate, lactate), cell lysis (lactic dehydrogenase, LDH), membrane glycoproteins (GPs), platelet aggregation, fibrinogen binding and hypotonic shock response. We characterized the combination of amiloride and sodium nitroprusside (at 1/2 the dose included in ThromboSol). This was found to be similar to ThromboSol and superior to nontreated units in the prevention of cold-induced platelet aggregation at day 15 of storage (maintenance of 78% and 80% of initial platelet counts, respectively), preservation of GPIbalpha (11% and 12% better maintenance of mean fluorescence intensity compared with control units, respectively), and reduced cell lysis (13% and 11% decrease in supernatant LDH, respectively). The reduced pharmacological load with the identified solution compared with ThromboSol is an argument in favour of the potential use of these agents when designing strategies to improve PC storage.

L-arginine modulates aggregation and intracellular cyclic 3,5-guanosine monophosphate levels in human platelets: direct effect and interplay with antioxidative thiol agent

Platelet nitric oxide is involved in the control of aggregability via cyclic 3',5'-guanosine monophosphate synthesis. Since L-arginine provides a guanidino nitrogen group for nitric oxide synthesis through nitric oxide synthase activity, we tried to clarify whether an increased availability of this amino acid can directly modulate the response of human platelets. In our conditions, L-arginine (at 100-6000 micromol/L) was able to influence the response of human platelets stimulated with adenosine 5-diphosphate and collagen both in PRP and in whole blood. The anti-aggregating effect was not present when D-arginine was used. Permeabilized platelets exhibited an increased sensitivity to L-arginine. Also, an increased availability of Ca2+ enhanced L-arginine effect. L-arginine (at 120-500 micromol/L) increased cyclic 3',5'-guanosine monophosphate levels in resting platelets; the amino acid also determined an increase of cyclic 3',5'-guanosine monophosphate in platelets at the end of adenosine 5-diphosphate-induced aggregation. Nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine prevented L-arginine effects on aggregation and cyclic 3',5'-guanosine monophosphate synthesis. Phosphodiesterase III inhibitor milrinone and antioxidative thiol N-acetyl-L-cysteine enhanced the effect of L-arginine on cyclic 3',5'-guanosine monophosphate. In conclusion, L-arginine exerts inhibitory effects on human platelet response through a nitric oxide-dependent synthesis of cyclic 3',5'-guanosine monophosphate. A positive interplay on platelet response between L-arginine and milrinone or antioxidative thiol N-acetyl-L-cysteine was evidenced.

Modulation of human platelet function by L-canavanine: differential effects of low and high concentrations

L-Canavanine is a naturally occurring L-amino acid that interferes with L-arginine-utilizing enzymes owing to its structural analogy with this L-amino acid. In macrophages and polymorphonuclear leukocytes, which express inducible nitric oxide synthase (iNOS), L-canavanine is able to prevent the L-arginine-derived synthesis of nitric oxide (NO). Its effects on constitutive NOS (cNOS) are far less clear. Because human platelets synthesize NO from L-arginine through a cNOS and because intracellular NO levels modulate platelet function, we have investigated the effects of L-canavanine on parameters potentially influenced by NO, such as platelet levels of 3',5'-cyclic guanosine monophosphate (cGMP) and responses to different aggregating agents. In our experimental conditions, L-canavanine was able to influence the response of human platelets to different aggregating agents such as catecholamines, 5-hydroxytryptamine, and ADP. Low L-canavanine concentrations (10-100 micromol/l) decreased platelet responses, whereas a high concentration (1 mmol/l) was unable to exert antiaggregating effects. In resting platelets, L-canavanine reduced the levels of cGMP, starting from a concentration of 1 mmol/l; furthermore, at the same concentrations, it was able to reduce cGMP levels at the end of the aggregation induced by collagen. In conclusion, L-canavanine exerts differential effects on human platelets in relation to the concentrations: at low levels, it exerts antiaggregating effects by actions independent of NOS inhibition, whereas, at high levels, it inhibits NO synthesis and does not exert antiaggregating effects.

Nitric oxide inhalation inhibits platelet aggregation and platelet-mediated pulmonary thrombosis in rats

Endothelium-derived nitric oxide (NO) inhibits in vitro platelet aggregation via a cGMP-dependent mechanism. The effect of inhaled NO on platelet-mediated pulmonary thrombosis following intravenous thrombotic challenge with collagen was examined in rats and compared with the effect of G4120, a cyclic Arg-Gly-Asp-containing synthetic pentapeptide that binds to the platelet glycoprotein IIb/IIIa receptor. Intraplatelet cGMP dose-dependently increased from 39 +/- 6 fmol/10(8) platelets in control to 46 +/- 6, 68 +/- 13, and 81 +/- 13 fmol/10(8) platelets after inhalation with 20, 40, and 80 ppm NO, respectively (P < .05 for 40 and 80 ppm). Ex vivo platelet aggregation of platelet-rich plasma induced by 1 microgram/mL collagen was reduced from 75 +/- 4% in control rats to 22 +/- 10% and 20 +/- 7% in rats ventilated with 40 and 80 ppm NO, respectively, and to 30 +/- 9% in G4120-treated rats (each P < .05 versus control). Circulating platelet counts 3 minutes after collagen injection were significantly higher in the inhaled NO and G4120 groups compared with control rats (250,000 +/- 18,000 and 223,000 +/- 10,000/microL versus 160,000 +/- 18,000/microL, each P < .05). The rise in pulmonary arterial pressure after collagen injection was significantly reduced in NO- and G4120-treated rats (26 +/- 1 and 27 +/- 1 versus 32 +/- 1 mm Hg in control rats, each P < .05). The number of pulmonary resistance vessels containing platelet thrombi was significantly smaller after inhaled NO and G4120 treatment compared with control (56 +/- 3% and 50 +/- 3% versus 68 +/- 3%, respectively; P < .05). Thus, NO inhalation reduces in vivo activation of circulating platelets and platelet-rich thrombosis in thromboembolic pulmonary hypertension. Inhalation of NO may be useful in cardiovascular diseases associated with platelet activation.

Nitroglycerin tolerance at the platelet level in patients with angina pectoris

Suppression of platelet aggregation may be an important component of the therapeutic effect of nitroglycerin (NTG). Because of the phenomenon of hemodynamic tolerance to NTG, we tested the hypothesis that the anti-platelet effects of NTG in humans are also subject to tolerance induction. In patients with stable angina who had not received nitrates for at least 24 hours before study, sublingual administration of NTG (300 microg; n = 17) attenuated the reversal of adenosine diphosphate-induced platelet aggregation by NTG applied in vitro. Three minutes after in vivo NTG administration, concentration of NTG producing 50% reversal of aggregation (C50) increased from 7.9 +/- 1.9 x 10(-5) to 5.5 +/- 0.3 x 10(-4) M (p <0.01); this change persisted for at least 60 minutes. There was no concomitant change in C50 values for sodium nitroprusside applied in vitro. Basal activity of platelet guanylate cyclase and its response to sodium nitroprusside were not affected after administration of NTG. Brief intravenous infusion of NTG (10 microg/min for 10 minutes) produced no significant changes in platelet responses to NTG in vitro. However, prolonged infusion of NTG (5 microg/min for 24 hours, patients with unstable angina pectoris, n = 11) caused suppression of in vitro platelet response to NTG. Platelets from patients receiving prophylactic nitrates (n = 19) were less responsive to the antiaggregatory effects of NTG in vitro than those from patients who had not received nitrates in the previous 24 hours (n = 21). Thus, clinical exposure to NTG, even in very low doses, induces tolerance to antiaggregatory effects of NTG. This phenomenon is not associated either with cross tolerance to sodium nitroprusside or with down-regulation of platelet guanylate cyclase.

Suppressed anti-aggregating and cGMP-elevating effects of sodium nitroprusside in platelets from patients with stable angina pectoris

Platelet hyperactivity plays an important role in the pathogenesis of cardio-vascular diseases. In patients with stable angina pectoris, we have recently demonstrated that nitroglycerin suppressed the increased platelet aggregability. The anti-aggregating effect of NTG and other nitrovasodilators is mediated by platelet guanylate cyclase, which generates cyclic GMP (cGMP) in response to nitric oxide (NO) liberated from the nitrovasodilator molecule. In the current study we utilised a more "direct" NO donor, sodium nitroprusside (SNP), to examine reversal of ADP-induced platelet aggregation in comparison with intraplatelet cGMP elevation in platelets from normal subjects (n = 22) and patients with stable angina pectoris (n = 23). Concentrations of SNP associated with 50% reversal of aggregation were 2.7 +/- 0.4 x 10(-7) mol/L with normal subjects and 4.5 +/- 0.5 x 10(-6) mol/L with patients (P < 0.01). SNP produced a concentration-dependent elevation of intraplatelet cGMP content: with 10(-4) mol/L SNP this was 17-fold for normals and 5-fold for patients (P < 0.01). An increase in cAMP content was seen only with 10(-4) mol/L SNP, and was 157 +/- 11% of baseline in platelets from normal subjects and 138 +/- 14% in patients. There was a strong interrelationship between cGMP-stimulating and anti-aggregating effects of SNP. The decrease in cGMP responsiveness to SNP was not related to a dysfunction of platelet guanylate cyclase; neither basal nor SNP-stimulated activity of the enzyme varied significantly between normal subjects and patients. Lipophilic derivatives of cGMP (db-cGMP) and cAMP (db-cAMP) caused reversal of aggregation; there was a nonsignificant trend towards decreased responsiveness of platelets from patients to both db-cGMP and db-cAMP. The observed decrease in responsiveness of platelets from angina patients to anti-aggregating effects of the exogenous NO donor, SNP, can therefore be attributed to suppressed cGMP accumulation. These results imply reduced platelet sensitivity to endogenous NO (endothelium-derived relaxing factor): this might contribute to platelet hyperaggregability observed in angina pectoris.

Involvement of guanylate cyclase and phosphodiesterases in the functional heterogeneity of human blood platelet subpopulations

Cyclic nucleotides are involved in the regulation of platelet activation, shape change, and aggregation. In this study we have investigated the role of guanylate cyclase and phosphodiesterase in three functional heterogeneous human blood platelet subpopulations separated according to density. In low-density platelets aggregation was enhanced and inhibited less when cyclic GMP was increased by sodium nitroprusside, compared to high-density platelets. Low-density platelets possessed a lower basal level of cyclic GMP and exhibited a small increase in cyclic GMP after stimulation with sodium nitroprusside. Cyclic GMP-dependent phosphodiesterase activity was similar in high, low, and intermediate-density platelets. In contrast, the activity of the cyclic AMP-dependent phosphodiesterase was higher in low-density compared to high and intermediate-density platelets. These results suggest that regulation of cyclic GMP and cyclic AMP levels plays an important role in the functional heterogeneity of human blood platelets.

Mechanisms of action of nitrates

Glyceryl trinitrate, isosorbide dinitrate, and isosorbide-5-mononitrate are organic nitrate esters commonly used in the treatment of angina pectoris, myocardial infarction, and congestive heart failure. Organic nitrate esters have a direct relaxant effect on vascular smooth muscles, and the dilation of coronary vessels improves oxygen supply to the myocardium. The dilation of peripheral veins, and in higher doses peripheral arteries, reduces preload and afterload, and thereby lowers myocardial oxygen consumption. Inhibition of platelet aggregation is another effect that is probably of therapeutic value. Effects on the central nervous system and the myocardium have been shown but not scrutinized for therapeutic importance. Both the relaxing effect on vascular smooth muscle and the effect on platelets are considered to be due to a stimulation of soluble guanylate cyclase by nitric oxide derived from the organic nitrate ester molecule through metabolization catalyzed by enzymes such as glutathione S-transferase, cytochrome P-450, and possibly esterases. The cyclic GMP produced by the guanylate cyclase acts via cGMP-dependent protein kinase. Ultimately, through various processes, the protein kinase lowers intracellular calcium; an increased uptake to and a decreased release from intracellular stores seem to be particularly important.

Reversal of human platelet aggregation by low concentrations of nitroglycerin in vitro in normal subjects

The potential reversal of platelet aggregation in vitro by nitroglycerin in low concentrations was explored using both optical aggregometry and electron microscopy. Venous blood was collected from a cohort of normal volunteers (20 men and 10 women) aged 21 to 65 years. Aggregation in platelet-rich plasma was induced by adenosine diphosphate in concentrations just sufficient to maintain a steady state of aggregation, without a spontaneous disaggregation phase (3.5 to 5 microM). Administration of nitroglycerin after the induction of aggregation caused both inhibition of the primary wave of developing aggregation and marked disaggregation. This combined effect was maximal when nitroglycerin was added at 0.5 minute after the beginning of aggregation. The observed reversal of platelet aggregation by nitroglycerin was concentration-dependent. Significant effects occurred with nitroglycerin concentrations greater than or equal to 10(-8) M. Concentration associated with 50% reversal of aggregation was 1.52 +/- 0.24 (SEM) x 10(-6) M. Electron microscopy revealed that 10(-6) M nitroglycerin induced a significant reduction in both platelet clumping and morphologic changes associated with aggregation. The results of the current study suggest a beneficial antiplatelet effect of nitroglycerin in restoring homeostasis in the face of incipient platelet aggregation. The clinical use of nitroglycerin in patients with acute ischemic syndromes may rest on this action.