Evidence for isosorbide dinitrate (ISDN) promoting effect on prostacyclin release by the lung and prostacyclin implication in ISDN-induced inhibition of platelet aggregation in humans

Platelets and Cardiovascular Disease

Platelets play an important, but often under-recognized role in cardiovascular disease. For example, the normal response of the platelet can be altered, either by increased pro-aggregatory stimuli or by diminished anti-aggregatory substances to produce conditions of increased platelet activation/aggregation and occur in active cardiovascular disease states both on a chronic (e.g. stable angina pectoris) and acute basis (e.g. acute myocardial infarction). In addition, platelet hyperaggregability is also associated with the risk factors for coronary artery disease (e.g. smoking, hypertension, and hypercholesterolaemia). Finally, the utility of an increasing range of anti-platelet therapies in the management of the above disease states further emphasizes the pivotal role platelets play in the pathogenesis of cardiovascular disease. This paper provides a comprehensive overview of the normal physiologic role of platelets in maintain homeostasis, the pathophysiologic processes that contribute to platelet dysfunction in cardiovascular disease and the associated role and benefits of anti-platelet therapies.

Inhibition of platelet aggregation in baboons: therapeutic implications for xenotransplantation

Activation of endothelial cells and platelet sequestration play major roles in rejection of xenografts. The histopathology of both hyperacute and acute vascular or delayed rejection of vascularized discordant xenografts is characterized by interstitial hemorrhage and intravascular thrombosis. Agents that prevent platelet activation and consequent microthrombus formation have proven beneficial in xenograft rejection but do not fully preclude vascular thrombosis. Recently, several new anti-platelet therapies have undergone extensive clinical testing for atherosclerotic thrombotic vascular disorders; other putative therapies are undergoing pre-clinical evaluation. We have investigated the effect of several of these novel agents on platelet aggregation in baboons in order to screen for future potential in xenograft rejection models.

METHODS

Drugs tested in these experiments were aurintricarboxylic acid (ATA, von Willebrand Factor-GPIb inhibitor), fucoidin (a selectin-inhibitor), 1-benzylimidazole (1-BI, thromboxane synthase antagonist), prostacyclin (PGI2, endothelial stabilizer), heparin (thrombin antagonist), nitroprusside sodium or nicotinamide (NPN or NA, both NO-donors), and eptifibatide (EFT, GPIIb/IIIa receptor antagonist). These were infused intravenously to nine baboons. Coagulation parameters and platelet counts were monitored and baboons were observed for adverse side-effects. The efficacy of these agents in inhibiting platelet aggregation was assayed in a platelet aggregometer.

RESULTS

Treatment with ATA and fucoidin resulted in complete inhibition of platelet aggregation but also in major perturbation of coagulation parameters. 1-BI and PGI2 had no effect when administered alone, but in combination resulted in moderate inhibition of aggregation without disturbance in PT or PTT. NPN and NA had no substantive effects on platelet aggregation. Heparin resulted in specific inhibition of thrombin-induced platelet aggregation and, as anticipated, was associated with moderate prolongation of PTT. Importantly, EFT caused complete inhibition of platelet aggregation without changes in coagulation. Platelet counts, fibrinogen levels, and fibrinogen degradation products remained within the normal ranges in all experiments.

CONCLUSIONS

Although excellent inhibition of platelet activation was obtained with ATA and fucoidin, clinical use may be precluded by concomitant disturbances of coagulation. Combinations of heparin and EFT may prove beneficial in preventing the thrombotic disorders associated with xenograft rejection while maintaining adequate hemostatic responses. These agents are to be evaluated in our pig-to-primate xenotransplantation models.

Inhibition of platelet aggregation by transdermal glyceryl trinitrate

OBJECTIVE

To determine the optimum conditions for the demonstration of an antiplatelet effect of nitric oxide and to use these conditions to elucidate the effects of a transdermal glyceryl trinitrate patch on platelet aggregation in normal volunteers.

METHODS

An open prospective crossover study. The effects of nitric oxide on platelet aggregation in whole blood and platelet rich plasma as stimulated by adenosine diphosphate and U46619 was assessed in the presence and absence of iloprost and MB22948. Optimum conditions for the demonstration of an antiplatelet effect of nitric oxide were then applied to whole blood from normal volunteers in the presence and absence of a transdermal glyceryl trinitrate patch.

SETTING

University hospital.

SUBJECTS

Eight normal volunteers.

MAIN OUTCOME MEASURES

Platelet aggregation in the presence and absence of transdermal glyceryl trinitrate.

RESULTS

The optimum conditions for the demonstration of an antiplatelet effect of nitric oxide in whole blood were collecting blood into a tube containing MB22948 and citrate and stimulating platelet aggregation with adenosine diphosphate in the presence or absence of iloprost. Using this method a significant effect of transdermal glyceryl trinitrate on platelet aggregation was shown (P < 0.03) in the presence and absence of iloprost.

CONCLUSIONS

These results are consistent with an inhibitory effect on platelet aggregation of nitric oxide liberated by transdermal glyceryl trinitrate. Optimum test conditions are needed to show this effect. The clinical significance of the antiplatelet effect of transdermal glyceryl trinitrate is unknown.

The metabolism of glyceryl trinitrate to nitric oxide in the macrophage cell line J774 and its induction by Escherichia coli lipopolysaccharide

The metabolism of glyceryl trinitrate (GTN) to nitric oxide (NO) was studied in the mouse macrophage cell line J774 and in the human monocytic cell line U937 in the absence or presence of Escherichia coli lipopolysaccharide (LPS). Two bioassay systems were used: inhibition of platelet aggregation and measurement of cGMP after stimulation by NO of guanylate cyclase in J774 cells. In addition, NO produced from GTN by cells or by cellular fractions was measured as nitrite (NO2-) one of its breakdown products. J774 cells (1.25 x 10(5) cells) treated with indomethacin (10 microM) enhanced the platelet inhibitory activity of GTN (22-352 microM) but not that of sodium nitroprusside (4 microM). This effect was abrogated by co-incubation with oxyhaemoglobin (oxyHb, 10 microM) indicating release of NO from GTN. U937 cells (up to 60 x 10(5)) did not metabolize GTN to NO. LPS (0.5 micrograms/mL for 18 hr) enhanced at least 2-fold the capacity of J774 cells but not that of U937 cells to form NO from GTN and this enhancement was attenuated when cycloheximide (10 micrograms/mL) was incubated together with LPS. In the absence of LPS stimulation, cycloheximide had no effect. Furthermore, when incubated with GTN (200 microM), J774 cells treated with LPS released more NO from GTN as indicated by a 3-fold greater increase in their level of cGMP which was prevented by oxyHb (10 microM). Incubation of J774 cells with GTN (75-600 microM) for 30 min led to a concentration-dependent increase in NO2- which was substantially reduced when the cells were boiled. The microsomal fraction was more potent than the cytosol in producing NO2- from GTN (1.2-2.4 mM). Release of NO2- from GTN by J774 cells was not affected by treating the cells with the NO synthase inhibitor, NG-monomethyl-L-arginine (MeArg, 300 microM). In J774 cells made tolerant to GTN, potentiation of the anti-platelet effects of GTN (11-352 microM) and release of NO2- from GTN was reduced. Thus, J774 cells but not U937 cells convert GTN to NO. This enzymic pathway (present mainly in the microsomal fraction of the J774 cells) is induced by LPS and is not regulated by endogenous NO released from L-Arg by the enzyme NO synthase. Furthermore, when compared to normal cells, tolerant J774 cells metabolize GTN to NO less effectively as assessed by a reduced capacity to potentiate the anti-platelet effect of GTN and to release NO2-.(ABSTRACT TRUNCATED AT 400 WORDS)

Modification of platelet function by isosorbide dinitrate in patients with coronary artery disease

The effect of a four weeks oral treatment with 100 mg isosorbide dinitrate (ISDN) daily on platelet function was evaluated in 40 patients (aged 40-65 years) with proven coronary artery disease. Isosorbide dinitrate decreased platelet reactivity to ADP (p less than 0.001), increased platelet sensitivity to PGI2 (p less than 0.01) while the production of TXB2 from exogenous arachidonic acid substrate and from endogenous substrate were both significantly reduced. Circulating platelet aggregates as measured by the Wu-test were markedly reduced (p less than 0.001) but there was little change in the plasma concentration of the platelet proteins beta-thromboglobulin and platelet factor 4. Overall, platelet activation correlated with smoking, hypertension and a family history of coronary artery disease. The reduced platelet activation seen during treatment with isosorbide dinitrate may contribute to the therapeutic benefit seen with this drug in patients with coronary artery disease.

Metabolism of glyceryl trinitrate to nitric oxide by endothelial cells and smooth muscle cells and its induction by Escherichia coli lipopolysaccharide

Here, we demonstrate that the metabolism of glyceryl trinitrate (GTN) to nitric oxide (NO) occurs not only in bovine aortic smooth muscle cells (SMCs) but also in endothelial cells (ECs) and that this biotransformation is enhanced by pretreatment with Escherichia coli lipopolysaccharide (LPS). Two bioassay systems were used: inhibition of platelet aggregation and measurement of cGMP after stimulation by NO of guanylate cyclase in SMCs or ECs. In addition, NO produced from GTN by cells was measured as nitrite (NO2-), one of its breakdown products. Indomethacin (10 microM)-treated SMCs or ECs enhanced the platelet inhibitory activity of GTN. This effect was abrogated by coincubation with oxyhemoglobin (oxyHb; 10 microM), indicating release of NO from GTN. LPS (0.5 microgram/ml; 18 h) enhanced at least 2- to 3-fold the capacity of SMCs or ECs to form NO from GTN, and this enhancement was attenuated when cycloheximide (10 micrograms/ml) was incubated together with LPS. Furthermore, when incubated with GTN (200 microM) SMCs or ECs treated with LPS (0.5 microgram/ml; 18 h) released more NO from GTN than nontreated cells as indicated by a much higher (8- to 9-fold) increase in the levels of cGMP. Exposure of SMCs to GTN (600 microM) for 30 min led to an increase in the levels of NO2- dependent on cell numbers, which was enhanced when SMCs were treated with LPS. Incubation of nontreated or LPS-treated cells with NG-monomethyl-L-arginine (300 microM; 60 min) did not influence the metabolism of GTN to NO. SMCs failed to enhance the antiplatelet activity of sodium nitroprusside. Anesthetized rats treated with an intraperitoneal injection of LPS (20 mg/kg) 18 h beforehand showed enhanced hypotensive responses to GTN (0.25-1 mg/kg). These effects were blocked by methylene blue (10 mg/kg) but not by indomethacin (3 mg/kg). LPS did not alter the hypotensive responses induced by phentolamine, verapamil, or SIN-1. Thus, both in vitro and in vivo, LPS induces the enzyme(s) metabolizing GTN to NO.

The antiplatelet effects of organic nitrates and related nitroso compounds in vitro and in vivo and their relevance to cardiovascular disorders

Organic nitrates, cornerstones of antianginal therapy, are believed to exert their principal anti-ischemic benefit by relaxing vascular smooth muscle. Recent evidence suggests that these compounds and related nitro(so) vasodilators are also potent platelet inhibitors. In view of the well recognized role of thrombotic events mediated by platelets in acute coronary syndromes, the antiplatelet effect of nitrates may also be of mechanistic importance in the treatment of these disorders. This review details the biochemical mechanism by which nitro(so) compounds inhibit platelet function and summarizes the in vitro and in vivo evidence that supports their antithrombotic effects.

Preferential venoconstriction by cyclooxygenase inhibition in vivo without attenuation of nitroglycerin venodilation

Because prostacyclin is a rather potent venodilator in vivo, we analyzed the effect of cyclooxygenase inhibition on venous tone in 14 anesthetized dogs during ganglionic and beta-adrenergic blockade and atraumatic conditions. Effective vascular stiffness (a reciprocal of effective vascular compliance) as a variable of integrated venous tone was 0.30 +/- 0.01 mm Hg.kg/ml (n = 35) and was augmented up to twofold by diclofenac (1, 3, and 10 mg/kg i.v.), ibuprofen (6 and 60 mg/kg), or indomethacin (5 mg/kg) parallel to augmentations in central venous pressure, while the rise in arterial pressure was less than half of the increase induced by equivenoconstrictor dosages of norepinephrine. After preconstriction by indomethacin or diclofenac, nitroglycerin (1.5 micrograms/kg/min) lowered effective vascular stiffness (by 24 +/- 2% or 23 +/- 5%, respectively), similarly as during preconstriction by norepinephrine (by 24 +/- 4%). Long-term cyclooxygenase inhibition (diclofenac 2 x 1 mg/kg/day for 4 days) did not modify arterial pressure, heart rate, or hematocrit levels in conscious dogs at rest, but it lowered plasma volume to 52.5 +/- 1.9 ml/kg (sham treatment: 59.1 +/- 1.6 ml/kg, p less than 0.05, n = 4). In conclusion, venoconstriction by clinical dosages of cyclooxygenase inhibitors does not interfere with the venodilator action of nitroglycerin and is compensated chronically by adjustments of plasma volume.

Altered intraplatelet arachidonic acid metabolism during the acute state of unstable angina

Thromboxane A2 (TxA2) generation and 1-14C arachidonic acid (AA) metabolism by platelets (stimulated with thrombin) were studied in vitro in 16 patients with unstable angina both during the acute and chronic inactive phase of the angina. Eight patients with stable effort angina and 21 controls were also investigated. In acute unstable angina 1-14C AA metabolism was significantly increased through cyclooxygenase pathway resulting in a higher selective TxA2 generation than in stable effort angina and in controls (p less than 0.01). No differences were found between patients with stable effort angina and controls. The alterations in AA metabolism were no longer found when patients reverted to the inactive phase of angina. TxA2 generation by platelets was independent of the number of the daily ischemic attacks (r = 0.17, ns) in patients with unstable angina. Present results indicate that an altered intraplatelet AA metabolism leading to the increased TxA2 synthesis occurs simultaneously with the conversion of angina from the chronic to the acute phase.

Antithrombogenic endothelial cell defense. Basal characteristics in cultured endothelial cells and modulation by short-term and long-term exposure to isosorbide nitrates

The antithrombogenic endothelial cell defense (ATECD) describes the properties that enable the endothelium to prevent circulating blood platelets from adhering to, or aggregating on, the vascular wall. ATECD was investigated in an experimental model in which bovine passage 0 cultured endothelial cells (EC) were incubated with aggregating platelets and autologous plasma in a computer-operated aggregometer-like device. A maximal platelet aggregation required 150 X 10(-6) M adenosine diphosphate (ADP) to be present in ECs. A 5-minute coincubation for ECs and platelets was found to be adequate in evaluating the maximal ATECD value. By increasing the EC number in the aggregation suspension, platelet aggregation was progressively inhibited through a sigmoid curve (50% inhibition of aggregation required 2 X 10(4) EC). Pharmacologic modulations of ATECD by isosorbide dinitrate (ISDN) + 2-isosorbide mononitrate (2-ISMN) + 5-isosorbide mononitrate (5-ISMN) were investigated under experimental conditions reflecting either an acute nitrate effect (platelet + control ECs + drug + ADP) or a chronic effect (platelet + 5-day nitrate-treated ECs + ADP). Under acute circumstances, ISDN antiplatelet activities were profoundly magnified by ECs. Aggregation was fully arrested with 5 X 10(-5) M ISDN and an EC number of 2 X 10(4), whereas the same ISDN concentration alone induced 30% inhibition of control aggregation. In contrast, there were no significant changes in platelet aggregation whether incubation was done in the presence or absence of 2-ISMN or 5-ISMN, ISDN metabolites. Long-term exposure of ECs to isosorbide nitrates (ISN) resulted in increased acquired EC changes in ATECD. 5-ISMN was a poor antiplatelet agent but was capable of counteracting ISDN effects on ATECD.(ABSTRACT TRUNCATED AT 250 WORDS)