Inhibition of platelet aggregation by amiloride

Sodium-glucose cotransporter 2 inhibitors antagonize lipotoxicity in human myeloid angiogenic cells and ADP-dependent activation in human platelets: potential relevance to prevention of cardiovascular events

BACKGROUND

The clear evidence of cardiovascular benefits in cardiovascular outcome trials of sodium-glucose cotransporter 2 inhibitors (SGLT2i) in type 2 diabetes might suggest an effect on atherosclerotic plaque vulnerability and/or thrombosis, in which myeloid angiogenic cells (MAC) and platelets (PLT) are implicated. We tested the effects of SGLT2i on inflammation and oxidant stress in a model of stearic acid (SA)-induced lipotoxicity in MAC and on PLT activation. The possible involvement of the Na⁺/H⁺ exchanger (NHE) was also explored.

METHOD

MAC and PLT were isolated from peripheral blood of healthy subjects and incubated with/without SGLT2i [empagliflozin (EMPA) and dapagliflozin (DAPA) 1-100 μM] to assess their effects on SA (100 μM)-induced readouts of inflammation, oxidant stress and apoptosis in MAC and on expression of PLT activation markers by flow-cytometry after ADP-stimulation. Potential NHE involvement was tested with amiloride (aspecific NHE inhibitor) or cariporide (NHE1 inhibitor). Differences among culture conditions were identified using one-way ANOVA or Friedman test.

RESULTS

NHE isoforms (1,5-9), but not SGLT2 expression, were expressed in MAC and PLT. EMPA and DAPA (100 μM) significantly reduced SA-induced inflammation (IL1β, TNFα, MCP1), oxidant stress (SOD2, TXN, HO1), but not apoptosis in MAC. EMPA and DAPA (both 1 μM) reduced PLT activation (CD62p and PAC1 expression). SGLT2i effects were mimicked by amiloride, and only partially by cariporide, in MAC, and by both inhibitors in PLT.

CONCLUSIONS

EMPA and DAPA ameliorated lipotoxic damage in stearate-treated MAC, and reduced ADP-stimulated PLT activation, potentially via NHE-inhibition, thereby pointing to plaque stabilization and/or thrombosis inhibition as potential mechanism(s) involved in SGLT2i-mediated cardiovascular protection.

Potential Protective Role of Blood Pressure-Lowering Drugs on the Balance between Hemostasis and Fibrinolysis in Hypertensive Patients at Rest and During Exercise

In patients with hypertension, the triad represented by endothelial dysfunction, platelet hyperactivity, and altered fibrinolytic function disturbs the equilibrium between hemostasis and fibrinolysis and translates into a hypercoagulable state, which underlies the risk of thrombotic complications. This article reviews the scientific evidence regarding some biological effects of antihypertensive drugs, which can protect patients from the adverse consequences of hypertensive disease, improving endothelial function, enhancing antioxidant activity, and restoring equilibrium between hemostatic and fibrinolytic factors. These protective effects appear not to be mediated through blood pressure reduction and are not shared by all molecules of the same pharmacological class.

Effect on ex vivo platelet aggregation and in vivo cyclic flow with Na+/H+ exchange inhibition: Gumina, NHE-1 inhibition and platelet aggregation

BIIB 513 and EMD 85131 are selective inhibitors of the Na+/H+ exchanger-1 (NHE-1) that are benzoylguanidine derivatives of the clinically employed diuretic amiloride. Prior studies have suggested a role for NHE-1 activity in platelet activation and aggregation using amiloride or its non- benzoylguanidines derivatives. However, the concentrations employed in these prior studies were at levels known to exert effects on other ion transport systems besides the NHE-1. Therefore, the purpose of this study was to examine the effects of more selective NHE-1 inhibitors, BIIB 513 and EMD 85131, on platelet aggregation and in vivo cyclic flow following arterial injury. BIIB 513 and EMD 85131 effects on ex vivo canine and human platelet aggregation in response to various agents was monitored via platelet aggregation. For analysis of in vivo thrombus formation, a femoral artery crush injury model was employed and a flow meter was used to monitor the effect of BIIB 513 on cyclic blood flow. Treatment of either canine or human platelets with up to 1 mM of BIIB 513 had no effect on aggregation induced by platelet activating factor (PAF), thrombin receptor activator peptide (TRAP), or adenosine diphosphate (ADP). Additionally, the structurally related compound EMD 85131 at up to 1 mM failed to inhibit TRAP induced platelet aggregation. In vivo administration of up to 9 mg/kg of BIIB 513 intravenously failed to affect cyclic flow in a canine model of femoral artery injury. These data demonstrate that the specific and selective NHE-1 inhibitors BIIB 513 or EMD 85131 have no effect on ex vivo platelet aggregation or in vivo cyclic flow following arterial injury.

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.

Aspirin or amiloride for cerebral perfusion defects in cocaine dependence

Cocaine dependent (CD) patients have regional cerebral blood flow (rCBF) deficits that may be related to occlusion of blood vessels by vasoconstriction and abnormal platelet aggregation. This study determined whether aspirin, which reverses platelet aggregation, or amiloride, a vasodilator, significantly reversed this rCBF hypoperfusion. This 1-month randomized trial compared clusters of voxels with significant hypoperfusion in recently abstinent CD patients after aspirin (325 mg daily), amiloride (10 mg daily) or placebo treatment. Forty-nine primary CD patients and 18 non-drug abusing controls were compared using single photon emission computed tomography (SPECT) neuroimaging with 99mTc-hexamethyl-propyleneamine-oxime and statistical parametric mapping (SPM). Platelet aggregation to adenosine diphosphate (ADP) was examined after treatment to determine whether rCBF improvement was related to decreased platelet aggregation. Following treatment, areas of hypoperfusion were improved with amiloride, unchanged with aspirin, and worsened with placebo in comparison to baseline levels. Platelet aggregation after ADP showed no significant change during the month, but reduced rCBF significantly improved after 1-month treatment with amiloride compared with placebo and cocaine abstinence alone.

Neurobiological adaptations to psychostimulants and opiates as a basis of treatment development

Abuse of illicit substances, in particular psychostimulants and opiates, is a worldwide public health issue. Chronic use of cocaine and amphetamine causes common neurobiological adaptations that may guide new treatment development. These include perturbations in dopamine and serotonin neurotransmission, leading to trials of antidepressants, and serotonin and dopamine augmentation strategies. The detection of cerebral perfusion abnormalities caused by psychostimulants has led to examination of antiplatelet and excitatory amino acid (EAA) antagonist therapies. Further, development of cocaine vaccines allows for testing of peripheral blockade approaches to cocaine addiction. New approaches to behavioral treatments for cocaine dependence are also reviewed. For opiate dependence, understanding of heroin's effects on mu and kappa opiate receptors has led to investigations of the partial mu agonist buprenorphine in opiate maintenance. Evidence for hyper-excitability of locus coeruleus (LC) noradrenergic neurons and EEA inputs to the LC guides trials of new alpha 2-adrenergic agonists and EEA antagonists to alleviate opiate withdrawal. Finally, clinical experience with withdrawal from methadone and LAAM has led to trials of antagonist-accelerated opiate withdrawal. Improved treatment of psychostimulant and opiate addiction is critically needed, and likely to have wide-reaching impact in health care and society.

Prevention of monocyte adhesion and inflammatory cytokine production during blood platelet storage: an in vitro model with implications for transfusion practice

A novel platelet additive solution [ThromboSoltrade mark (TS)] was designed to allow extended refrigerated platelet storage. It has been shown to preserve platelet function and prevent cytokine accumulation in platelet concentrates stored for up to 9 days. It consists of amiloride, adenosine, sodium nitroprusside, dipyridamole, quinacrine, and ticlopidine. We hypothesized that the cytokine inhibition may be due to prevention of monocyte (MC) adhesion and activation on the surfaces of platelet storage bag plastic polymers. In an in vitro model, we incubated purified peripheral blood MCs on discs of polyolefin and polyvinylchloride from platelet storage bags, and on polystyrene, in the presence of TS for up to 7 days. We found that after incubation with TS, adherent MC numbers were decreased by >80-95% compared with controls on all surfaces examined. Levels of cytokines [interleukin (IL)-1beta, IL-1RA, IL-6, IL-8, and tumor necrosis factor-alpha] were low in wells with TS but rose progressively in the controls during incubation. Amiloride alone had similar effects on adhesion and cytokine release as the complete TS preparation. Removing amiloride from TS abrogated these effects. These findings suggest an important role for TS and amiloride in monocyte function, and have implications for the development of agents designed for prolonged platelet storage.

Sodium-hydrogen exchange and platelet function

On stimulation of platelets with agonists, for example, thrombin, a rapid rise in intracellular pH is observed. This alkalinization is mediated by an increase in transport activity of the Na(+)/H(+) exchanger isoform NHE1. In addition to this Na(+)/H(+) exchange mechanism, platelets express bicarbonate/chloride exchangers, which also contribute to pH(i) homeostasis. The main functions of NHE1 in platelets include pH(i) control, volume regulation, and participation in cell signaling. The isoform NHE1 is highly sensitive toward inhibition by EIPA, Hoe694, and Hoe642. The regulation of NHE1 activity is complex and is not completely understood. It includes the MAP kinase cascade, the Ca/calmodulin system, several heterotrimeric G proteins (Galpha12, Galpha13, Galphaq, and Galphai), small G proteins (ras, cdc42, rhoA), and downstream kinases (e.g., p160ROCK). Volume challenges stimulate tyrosine phosphorylation of cytoplasmic proteins, which ultimately activate NHE1. Thrombin, thromboxane, platelet-activating factor, angiotensin II, endothelin, phorbol ester, and Ca(2+) ionophors stimulate NHE1 activity in platelets. Blockade of platelet NHE1 can inhibit platelet activation. With the development of highly specific NHE1 inhibitors, detailed investigation of the relationships between NHE1 activity and platelet activation now becomes feasible.

ADP-induced platelet activation

Platelet activation is central to the pathogenesis of hemostasis and arterial thrombosis. Platelet aggregation plays a major role in acute coronary artery diseases, myocardial infarction, unstable angina, and stroke. ADP is the first known and an important agonist for platelet aggregation. ADP not only causes primary aggregation of platelets but is also responsible for the secondary aggregation induced by ADP and other agonists. ADP also induces platelet shape change, secretion from storage granules, influx and intracellular mobilization of Ca2+, and inhibition of stimulated adenylyl cyclase activity. The ADP-receptor protein mediating ADP-induced platelet responses has neither been purified nor cloned. Therefore, signal transduction mechanisms underlying ADP-induced platelet responses either remain uncertain or less well understood. Recent contributions from chemists, biochemists, cell biologists, pharmacologists, molecular biologists, and clinical investigators have added considerably to and enhanced our knowledge of ADP-induced platelet responses. Although considerable efforts have been directed toward identifying and cloning the ADP-receptor, these have not been completely successful or without controversy. Considerable progress has been made toward understanding the mechanisms of ADP-induced platelet responses but disagreements persist. New drugs that do not mimic ADP have been found to inhibit fairly selectively ADP-induced platelet activation ex vivo. Drugs that mimic ADP and selectively act at the platelet ADP-receptor have been designed, synthesized, and evaluated for their therapeutic efficacy to block selectively ADP-induced platelet responses. This review examines in detail the developments that have taken place to identify the ADP-receptor protein and to better understand mechanisms underlying ADP-induced platelet responses to develop strategies for designing innovative drugs that block ADP-induced platelet responses by acting selectively at the ADP-receptor and/or by selectively interfering with components of ADP-induced platelet activation mechanisms.

Pharmacotherapy of cerebral ischemia in cocaine dependence

This paper will examine how cerebral perfusion deficits in cocaine abusers may be a target for pharmacotherapy. The review covers five areas: (1) cerebral ischemia and neuropsychological impairment in cocaine abusers, (2) neuroimaging evidence for cerebral perfusion defects in cocaine abusers, (3) mechanisms for cocaine induced cerebral thrombosis, (4) neurotoxicity from cerebral ischemia and excitatory amino acids, (5) glutamate antagonists as potential treatment agents for cocaine induced neurotoxicity. Several pharmacotherapies are suggested including antiplatelet agents and excitatory amino acid (EAA) antagonists such as lamotrigine. Clinical trials in cocaine abusers with cerebral perfusion defects are indicated and EAA antagonists hold promise as they are developed for stroke treatment.

A possible role for extracellular bicarbonate in U-46619-induced rat platelet aggregation

U-46619, a thromboxane A2 agonist, has been believed not to induce aggregation of rat platelets. However, we have found that U-46619 evoked rat platelet aggregation when the cells were suspended in HCO3-containing medium, whereas it was inactive in medium lacking HCO3-. 4,4'-Diisothiocyanostilbene-2,2'-disulfonate (DIDS), selective inhibitor of the HCO3/Cl- exchanger, inhibited the aggregation of rat platelets induced by U-46619 as well as by collagen and ADP. Thrombin-induced aggregation was also inhibited by DIDS, although to a much lesser extent. Several inhibitors of anion transporters, such as phloretin, pyridoxal phosphate and ethacrynic acid, suppressed U-46619-induced aggregation. These observations indicate that HCO3-influx via an anion exchanger may be involved in the signal transduction pathway of U-46619 in rat platelets.

Effects of L- and D-arginine and some related esters on the cytosolic mechanisms of alpha-thrombin-induced human platelet activation

1. In Fura-2 preloaded human platelets, the increase in cytosolic calcium induced by alpha-thrombin was reduced by some L- and D-arginine ester compounds the IC50 (microM) values of which were 7.4 for TAEE, 56.9 for BAEE, 77.6 for TAME, 560 for T(d)AME, 656.3 for L-ArgOMe and 2206.7 for D-ArgOMe. alpha-tosyl-L-Arginine, L- and D-arginine were inactive. 2. The inhibitory activity of the L-arginine esters was not modified when platelets were pretreated with 100 microM N omega-monomethyl-L-arginine. 3. The L-arginine esters did not increase cyclic GMP content in platelets either in the presence or absence of indomethacin and apyrase at rest and after alpha-thrombin stimulation. 4. The kinetic parameters of platelet Na+/H+ antiporter (amiloride-inhibitable, evaluated after cytosolic nigericin-induced acidification) were modified by L- and D-arginine esters, while the native amino acids were ineffective. 5. The inhibitory effects of the L- and D-arginine esters on platelet activation appear to be mainly due to their inhibitory effect on Na+/H+ antiporter.

Modulation of Na+/H+ exchange and intracellular pH by protein kinase C and protein phosphatase in blood platelets

Phosphorylation of the Na+/H+ exchanger in human platelets is apparently controlled by the balancing activities of protein kinase C (PKC) and protein phosphatase (PP). To explore cellular expressions of these activities, we have examined the impact of modulation of PKC and PP on Na+/H+ exchange activity, its pHi set point and intracellular pH (pHi). These parameters were followed spectrofluorimetrically in BCECF-loaded platelets. Phorbol 12-myristate 13-acetate (PMA) and dihexanoylglycerol (DHG), which stimulate PKC, and okadaic acid, which inhibits PP 1 and 2A, elevate the measured parameters in concert, while staurosporine, which inhibits protein kinases, had opposite effects. The stimulatory and inhibitory effects are similarly very rapid, being discerned within seconds. It is concluded that: (a) phosphorylation of the Na+/H+ exchanger is the common origin of the diverse effects of PMA, DHG, okadaic acid and staurosporine, (b) Na+/H+ exchange properties are tightly regulated by phosphorylation and dephosphorylation, and (c) the exchanger plays a major role in pHi regulation in platelets.

Thrombin-induced platelet aggregation is affected by external Na+ independently of the Na+/H+ exchange

Thrombin affects blood platelets by activation of Na+/H+ exchange and induction of aggregation, but the relationship between these effects is under debate. The present study attempts to clarify whether the activation of the exchanger activity is required for platelet aggregation. In apparent support of such a requirement, thrombin-induced aggregation is higher in Na+ medium than in N-methylglucamine+ medium and is inhibited by sphingosine, an inhibitor of protein kinase C known to regulate the Na+/H+ exchanger. However, the inhibition of aggregation by sphingosine occurs in both Na+-containing and Na+-free media, the aggregation is identical in Na+ and K+-containing media, and is not inhibited by 5-N-(3-aminophenyl)amiloride, at a concentration 10-fold higher than its Ki for platelet Na+/H+ exchange. Furthermore, at low concentration (0.005 U/ml) thrombin induces aggregation but does not activate the exchange. It is concluded that the activation of Na+/H+ exchange is not required for thrombin-induced platelet aggregation and that the apparent augmentation of aggregation by Na+ is due to an inhibitory effect of N-methylglucamine+.

Na+/H+ exchange and aggregation of human platelets activated by ADP: the exchange is not required for aggregation

Isolated human blood platelets, loaded with the pH-sensitive fluorescence dye 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein show cytoplasmic alkalinization upon stimulation with thrombin but acidification with ADP stimulation. In both cases a Na+/H+ exchange is activated. This can be revealed by the sensitivity of the induced pH changes to amiloride and to 5-N-(3-aminophenyl)amiloride (APA), known inhibitors of the Na+/H+ exchanger, and by a dependence on sodium in the external medium. ADP-induced platelet aggregation is not affected by omission of sodium from the external medium. Furthermore, aggregation is barely inhibited (less than 10%) by amiloride or APA at concentrations up to 50 microM while the Ki values in affecting the Na+/H+ exchange are 5.9 and 1.6 microM for amiloride and APA, respectively. Platelet aggregation is inhibited by amiloride or APA at concentrations higher than 50 microM, but this inhibition is apparently due to a secondary effect of the agents. It is concluded that platelet aggregation induced by ADP is not dependent on activation of Na+/H+ exchange.