Effects of atrial natriuretic factor on human platelet function

Platelet Function in CKD: A Systematic Review and Meta-Analysis

BACKGROUND

Patients with CKD are at high risk for thrombotic and hemorrhagic complications. Abnormalities in platelet function are central to these complications, but reports on platelet function in relation to CKD are conflicting, and vary from decreased platelet reactivity to normal or increased platelet responsiveness. The direct effects of uremic toxins on platelet function have been described, with variable findings.

METHODS

To help clarify how CKD affects platelet function, we conducted a systematic review and meta-analysis of platelet activity in CKD, with a focus on nondialysis-induced effects. We also performed an extensive literature search for the effects of individual uremic toxins on platelet function.

RESULTS

We included 73 studies in the systematic review to assess CKD's overall effect on platelet function in patients; 11 of them described CKD's effect on ex vivo platelet aggregation and were included in the meta-analysis. Although findings on platelet abnormalities in CKD are inconsistent, bleeding time was mostly prolonged and platelet adhesion mainly reduced. Also, the meta-analysis revealed maximal platelet aggregation was significantly reduced in patients with CKD upon collagen stimulation. We also found that relatively few uremic toxins have been examined for direct effects on platelets ex vivo; ex vivo analyses had varying methods and results, revealing both platelet-stimulatory and inhibitory effects. However, eight of the 12 uremic toxins tested in animal models mostly induced prothrombotic effects.

CONCLUSIONS

Overall, most studies report impaired function of platelets from patients with CKD. Still, a substantial number of studies find platelet function to be unchanged or even enhanced. Further investigation of platelet reactivity in CKD, especially during different CKD stages, is warranted.

C2238 ANP gene variant promotes increased platelet aggregation through the activation of Nox2 and the reduction of cAMP

Subjects carrying the C2238 variant of the atrial natriuretic peptide (ANP) gene have a higher occurrence of stroke and acute coronary syndrome, suggesting an increased predisposition to acute thrombotic events in these subjects. We evaluated for the first time the direct effects of mutant ANP (C2238/αANP) on platelet activation in vitro and in human subjects. In vitro, platelets were incubated with no peptide, with T2238/αANP (WT) or with C2238/αANP at different concentrations. C2238/αANP (10⁻¹⁰ M) induced higher collagen-induced platelet aggregation with respect to both control without ANP and T2238/αANP. This effect was even stronger at a higher concentration (10⁻⁶ M). Mechanistically, C2238/αANP significantly lowered platelet cAMP levels, increased ROS production and activated Nox2, with respect to both control and T2238/αANP. Forskolin, a cAMP activator, and sNOX2-tat, a Nox2 inhibitor, significantly reduced the pro-aggregant effects of C2238/αANP. In vivo, we found that platelet aggregation resulted to be higher in patients with atrial fibrillation carrying the C2238 ANP gene variant with respect to non-carriers. In conclusions, C2238/αANP promotes platelet aggregation through the activation of Nox2 and the reduction of cAMP.

Natriuretic peptides in cardiometabolic regulation and disease

In the 30 years since the identification of the natriuretic peptides, their involvement in regulating fluid and blood pressure has become firmly established. Data indicating a role for these hormones in lifestyle-related metabolic and cardiovascular disorders have also accumulated over the past decade. Dysregulation of the natriuretic peptide system has been associated with obesity, glucose intolerance, type 2 diabetes mellitus, and essential hypertension. Moreover, the natriuretic peptides have been implicated in the protection against atherosclerosis, thrombosis, and myocardial ischaemia. All these conditions can coexist and potentially lead to heart failure, a syndrome associated with a functional natriuretic peptide deficiency despite high circulating concentrations of immunoreactive peptides. Therefore, dysregulation of the natriuretic peptide system, a 'natriuretic handicap', might be an important factor in the initiation and progression of metabolic dysfunction and its accompanying cardiovascular complications. This Review provides a summary of the natriuretic peptide system and its involvement in these cardiometabolic conditions. We propose that these peptides might have an integrating role in lifestyle-related metabolic and cardiovascular disorders.

Natriuretic peptides induce weak VASP phosphorylation at Serine 239 in platelets

Cyclic guanosine-3',5'-monophoshate (cGMP) is the common second messenger for the cardiovascular effects of nitric oxide (NO) and natriuretic peptides (NP; e.g. atrial NP [ANP]), which activate soluble and particulate guanylyl cyclases, respectively. The role of NO in regulating cGMP and platelet function is well documented, whereas there is little evidence supporting a role for NPs in regulating platelet reactivity. By studying platelet aggregation and secretion in response to a PAR-1 peptide, collagen and ADP, and phosphorylation of the cGMP-dependent protein kinase (PKG) substrate vasodilator-stimulated phosphoprotein (VASP) at serine 239, we evaluated the effects of NPs in the absence or presence of the non-selective cGMP and cAMP phosphodiesterase (PDE) inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Our results show that NPs, possibly through the clearance receptor (natriuretic peptide receptor-C) expressed on platelet membranes, increase VASP phosphorylation but only following PDE inhibition, indicating a small, localised cGMP synthesis. As platelet aggregation and secretion measured under the same conditions were not affected, we conclude that the magnitude of PKG activation achieved by NPs in platelets per se is not sufficient to exert functional inhibition of platelet involvement in haemostasis.

Atrial natriuretic peptide increases inflammation, infarct size, and mortality after experimental coronary occlusion

Acute coronary artery occlusion triggers the release of atrial natriuretic peptide (ANP) from the heart. ANP affects vasodilation, natriuresis, and inflammation, but the integrated biological effects of ANP on myocardial infarction are unknown. To elucidate these effects, the left anterior coronary artery was ligated in anesthetized, ANP-deficient (ANP(-/-)) and congenic wild-type (ANP(+/+)) mice. The survival of ANP(-/-) mice was markedly better (56%) at 30 days postinfarction than the survival of ANP(+/+) mice (20%, P < 0.01). Surviving mice were comparable initially, but ANP(-/-) mice developed more cardiac hypertrophy (P < 0.001) and had lower contractility indexes 30 days after infarction (P < 0.05). An analysis 24 h after coronary occlusion showed that ANP(-/-) mice had smaller infarcts than ANP(+/+) mice (62.6 +/- 12.1 vs. 100.8 +/- 3.8%, P < 0.001) adjusted for comparable areas at risk for ischemia. The administration of ANP to ANP(-/-) mice via osmotic minipumps significantly enlarged infarct size to levels comparable with those observed in ANP(+/+) mice (P < 0.05). There was no difference in neutrophil migration into the noninfarcted myocardium of ANP(-/-) mice undergoing actual versus sham-operated coronary occlusion. By comparison, after coronary occlusion, the neutrophil infiltration into the myocardium was enhanced in ANP(+/+) (P < 0.0005) and ANP(-/-) mice administered ANP (P < 0.0005). The expression of P-selectin, a molecule that mediates neutrophil adhesion, was significantly greater after coronary occlusion in the vasculature of ANP(+/+) or ANP(-/-) mice treated with ANP than in ANP(-/-) mice (P < 0.002). Taken together, these results indicate that ANP increases P-selectin, neutrophil infiltration, infarct size, and mortality following experimental coronary occlusion.

Cloning and characterisation of natriuretic peptides from the venom glands of Australian elapids

The venom from Australian elapid snakes contains a complex mixture of polypeptide toxins that adversely affect multiple homeostatic systems within their prey in a highly specific and targeted manner. Included in these toxin families are the recently described venom natriuretic peptides, which display similar structure and vasoactive functions to mammalian natriuretic peptides. This paper describes the identification and detailed comparative analysis of the cDNA transcripts coding for the mature natriuretic peptide from a total of nine Australian elapid snake species. Multiple isoforms were identified in a number of species and represent the first description of a natriuretic peptide from the venom gland for most of these snakes. Two distinct natriuretic peptide isoforms were selected from the common brown snake (Pseudonaja textilis), PtNP-a, and the mulga (Pseudechis australis), PaNP-c, for recombinant protein expression and functional analysis. Only one of these peptides, PtNP-a, displayed cGMP stimulation indicative of normal natriuretic peptide activity. Interestingly, both recombinant peptides demonstrated a dose-dependent inhibition of angiotensin converting enzyme (ACE) activity, which is predictive of the vasoactive effects of the toxin. The natriuretic peptides, however, did not possess any coagulopathic activity, nor did they inhibit or potentiate thrombin, adenosine diphosphate or arachidonic acid induced platelet aggregation. The data presented in this study represent a significant resource for understanding the role of various natriuretic peptides isoforms during the envenomation process by Australian elapid snakes.

The effect of nitric oxide-donating vasodilators on monocyte chemotaxis and intracellular cGMP concentrations in vitro

The effects of sodium nitroprusside (SNP) and 3-morpholino sydnonimine (SIN-1), isosorbide dinitrate (ISDN) and glyceryl trinitrate (GTN), and molsidomine (the inactive precursur of SIN-1) on monocyte chemotaxis and cyclic GMP (cGMP) concentration were studied. SNP and SIN-1 inhibited monocyte N-formyl-methionyl-leucyl-phenylalanine-stimulated migration and increased cGMP concentrations in a dose-dependent (> 10(-5) mol.l-1) and time-dependent manner. Furthermore, 8-bromo cGMP inhibited monocyte chemotaxis in a dose-dependent fashion. In contrast, ISDN, GTN and molsidomine did not alter monocyte migration or cGMP concentration. These results support earlier observations that nitric oxide inhibits monocyte function in vitro via a cGMP-mediated mechanism. The differential effects of the spontaneous and thiol-dependent NO-donating nitrovasodilators on monocyte function suggests that monocytes, like platelets, are not able to directly metabolise ISDN and GTN. If similar observations can be made in vivo, it is possible that certain nitrovasodilators might be used therapeutically to inhibit monocyte function, for example during atherogenesis.

Particulate guanylate cyclase and adenylate cyclase activities after activation with various agents in rabbit platelets. An ultracytochemical study

The cytochemical localization of particulate guanylate cyclase and adenylate cyclase activities in rabbit platelets were studied after stimulation with various agents, at the electron microscope level. In the presence of platelet aggregating agents such as thrombin and ADP, the particulate reaction product of guanylate cyclase activity was detectable on plasma membrane and on membranes of the open canalicular system. In contrast, samples incubated with platelet-activating factor showed no activation of the cyclase activity. Atrial natriuretic factor stimulated the particulate guanylate cyclase. The ultracytochemical localization of this activated cyclase was the same as that of thrombin- or ADP-stimulated guanylate cyclase. Adenylate cyclase activity was studied in platelets incubated with prostaglandin E1 plus or minus insulin. The enzyme reaction product was found at the same sites where guanylate cyclase was detected. Therefore guanylate and adenylate cyclase activities do not seem to be preferentially localised in platelet membranes.

Effect of atrial natriuretic factor on platelet function in whole blood ex-vivo in man

Atrial natriuretic factor (ANF) binding sites have been shown to be present on human platelet membranes. We investigated the effect of an infusion of ANF 5 pmol.kg-1.min-1 on platelet aggregation in whole blood ex-vivo in 8 normal volunteers. Spontaneous platelet aggregation, collagen (0.6-2 micrograms.m.-1)-induced or ADP (0.5-2.0 microM)-induced aggregation was not affected by ANF. Plasma aldosterone was however significantly attenuated by ANF. These results show that a pharmacological dose of ANF does not affect platelet aggregation in man. These results suggest that the high plasma levels of ANF normally achieved in chronic heart failure or acute myocardial infarction are unlikely to contribute to the platelet hyperractivity, often observed in these conditions.

Atrial natriuretic peptide involvement in human platelet aggregability in vitro

Atrial natriuretic peptide (ANP) counteracts the destabilization and aggregation of platelets which is induced by vasopressin, angiotensin, and adrenaline.

Platelet receptors for atrial natriuretic peptide in man

The aim of our study was to characterize the receptor binding of human alpha-atrial natriuretic peptide (ANP) to human blood cells. Whereas no receptors were detected on red cells, on mononuclear cells and on granulocytes, we found ANP-receptors on human platelets. The binding studies were performed by incubating 40 X 10(6) platelets with 125I-ANP and with the competing ligand, when used, in a total incubation volume of 1 ml. Centrifugation was used to separate bound from free hormone. Specific binding of 125I-ANP was rapid, saturable and reversible. A steady state was achieved within 90 minutes. Scatchard analysis of saturation and competition experiments demonstrated the existence of one class of high affinity binding sites for ANP with a Kd of 8-16pM and 10-26 receptors per cell. The Kd obtained in our binding studies was in the range of physiological ANP concentrations in human plasma (8-20pM). Although characterization of platelet ANP receptors has the inherent disadvantage that there are only few of them, they could be a useful model to investigate the ANP receptor-status under different physiological and pathological conditions in man.