Inhibition of coronary artery thrombosis by SIN-1, a donor of nitric oxide

Inactivation of platelet glycoprotein IIb/IIIa receptor by nitric oxide donor 3-morpholino-sydnonimine

Nitric oxide (NO) is known to modulate platelet adhesion and aggregation, which are both mediated by fibrinogen receptor glycoprotein (GP)IIb/IIIa. To investigate effects of NO on GPIIb/IIIa activation and inactivation, platelets were exposed to NO donor 3-morpholino-sydnonimine (SIN-1) before and after stimulation with different agonists: thromboxane analog U-46619, epinephrine, adenosine diphosphate, human a-thrombin, and phorbol-12-myristate-13-acetate (0.02 micromol/l). (1) Flow cytometry analysis of SIN-1-pre-incubated samples using PAC-1 monoclonal antibody revealed an inhibition of receptor activation by 80.9 +/- 1.2, 71.3 +/- 1.8, 56 +/- 4.9, 87 +/- 3.4, and 56 +/- 5% (mean +/- SEM, relative to baseline). (2) Administration of SIN-1 after stimulation reversed receptor activation by 55 +/- 5.2, 56 +/- 2.0, 53 +/- 5.4, 42 +/- 4.3, and 44 +/- 5%, respectively. With 0.1 micromol/l phorbol-12-myristate-13-acetate, GPIIb/IIIa activation was irreversible. (3) SIN-1 effects could completely be blocked by equimolar addition of guanylyl cyclase inhibitor 1H(1,2,4)oxadiazolo(4,3-alpha)quinoxalin-1-on. (4) Spontaneous receptor closure after activation with human alpha-thrombin and adenosine diphosphate was not due to platelet-derived NO; SIN-1, however accelerated spontaneous receptor inactivation. (5) SIN-1-inactivated receptors still responded to stimulation. In conclusion, SIN-1 or NO modulates GPIIb/IIIa conformational change in vitro via guanosine 3',5'-monophosphate-dependent pathways. Whereas spontaneous receptor inactivation may be enhanced by exogenous NO, platelet-derived NO is not involved in receptor inactivation.

Nitric oxide donors

Nitric oxide (NO) donors are pharmacologically active substances that release NO in vivo or in vitro. NO has a variety of functions such as the release of prostanoids, inhibition of platelet aggregation, effect on angiogenesis, and production of oxygen free radicals. This report discusses the chemical and pharmacological characteristics of NO donors, their effect on platelet function and cyclooxygenase, their cardiac action including myocardial infarction, and release of superoxide anions. This review stresses NO tolerance and the effect of NO donors on angiogenesis in myocardial infarction and in solid tumors.

Endogenous nitric oxide protects against thromboembolism in venules but not in arterioles

Because nitric oxide (NO) inhibits aggregation and adhesion of blood platelets, NO may play a role in platelet-vessel wall interactions. Therefore, the purpose of this study was to investigate the involvement of endogenous NO in thromboembolic processes, as induced by wall puncture, in rabbit mesenteric arterioles and venules (diameters 20 to 43 microm). In venules, inhibition of NO synthase by superfusion of the mesentery with N omega-nitro-L-arginine (L-NA; 0.1 mmol/L) significantly increased the duration of embolization (from 50 seconds to 511 seconds) and the number of emboli produced (from 2 to 11 emboli per vessel), while the median period of time needed to produce an embolus was not influenced. On the contrary, in arterioles, L-NA had no significant effect on embolization (duration of embolization: 426 seconds in the control and 382 seconds in the L-NA group, with 20 and 12 emboli per vessel, respectively). Addition to the L-NA superfusate of L-arginine (L-ARG; 1 mmol/L), the active precursor for endogenous NO synthesis, resulted in a complete reversal of the L-NA effects in venules, while addition of the inactive D-arginine (D-ARG; 1 mmol/L) had no effect. Addition of L-ARG and D-ARG had no significant effect in arterioles. Addition to the L-NA superfusate of the exogenous NO donor sodium nitroprusside (0.1 micromol/L) also resulted in reversal of the L-NA effects in venules, while in arterioles, it slightly but significantly decreased embolization duration. The differences in effect of L-NA on embolization between arterioles and venules were not caused by differences in fluid dynamic conditions. It is concluded that the role of endogenous NO in inhibiting thromboembolic processes is more important in venules than in arterioles.

The antiadhesive and antithrombotic effects of the nitric oxide donor SIN-1 are combined with a decreased vasoconstriction in a porcine model of balloon angioplasty

Nitric oxide has been reported to modulate platelet and neutrophil interactions with the arterial wall. In this study, we investigated the effects of the nitric oxide donor 3-morpholino-sydnonimine (SIN-1) on platelet and neutrophil adhesion, and the vasomotor response, in a porcine model of angioplasty. Carotid arterial injury was produced by balloon dilation in control (n = 10) and treated (SIN-1; 10 micrograms/kg + 1 microgram/kg/min, i.v.) (n = 8) pigs. At the site of deep arterial injury, the average platelet adhesion was 53.6 +/- 11.3 x 10(6)/cm2 in the control animals and was significantly inhibited by more than 70%, to 15.1 +/- 4.1 x 10(6)/cm2 (P < .01), by SIN-1. Neutrophil adhesion was also decreased by SIN-1, from 255.9 +/- 29.7 to 101.8 +/- 19.7 x 10(3)cm2 (P < .001). Mural thrombosis was found in 12 (71%) of the 17 injured arteries in the control group but in only 2 (17%) of the 12 injured arteries in the SIN-1-treated group (P < .05). Concomitantly, SIN-1 reduced platelet and neutrophil adhesion to the site of endothelial injury distally. The internal diameter of the carotid arteries was similar between the two groups before dilation but was 40% greater at the site of endothelial injury distally in SIN-1-treated animals after dilation (P < .05), as compared with controls. Accordingly, postangioplasty vasoconstriction was significantly attenuated from 46.3 +/- 2.9% in control pigs to 32.5 +/- 4.8% (P < .05) in SIN-1-treated animals. The beneficial effects of SIN-1 were associated with inhibition of neutrophil-mediated whole blood aggregation and of neutrophil-endothelium interactions. The potent antiadhesive and antithrombotic properties of SIN-1 in vivo were confirmed in ex vivo superfusion experiments. These results indicate that administration of a nitric oxide donor may be effective in preventing the acute pathophysiological responses to arterial injury by angioplasty.

Nitroglycerin inhibits experimental thrombosis and reocclusion after thrombolysis

Nitroglycerin inhibits platelet aggregation in vitro, but its effect on thrombosis and platelet function in vivo is controversial. This study assessed the effect of nitroglycerin on primary thrombus formation in response to vessel wall injury and secondary thrombus formation, or rethrombosis, after lysis of an existing thrombus. In the first protocol the right carotid artery was instrumented with a flow probe, stenosis, an anodal electrode, and a proximal infusion line. A 300 microA anodal current was used to induce endothelial injury and subsequent thrombotic occlusion of the vessel. Anisoylated plasminogen streptokinase activator complex (APSAC; 0.05 U/kg intraarterially) was injected proximal to the thrombus 30 minutes after occlusion. After APSAC, nitroglycerin (1 microgram/kg/min intraarterially, n = 7) or vehicle (n = 6) was infused proximal to the thrombus for 3 hours. Reocclusion occurred in two of seven nitroglycerin-treated dogs and six of six vehicle-treated dogs (p < 0.05). In the second protocol both carotid arteries were instrumented as described previously. Anodal current (300 microA, 180 minutes) was applied to the right carotid (n = 12) artery to determine control times to occlusion. The left carotid artery served as the test vessel, receiving either nitroglycerin (1 microgram/kg/min intraarterially, n = 6) or trimethaphan (0.05 mg/kg/hr intraarterially, n = 6). Trimethaphan was used to produce controlled hypotension to match the approximately 10% decrease in mean arterial blood pressure that was observed during nitroglycerin infusion. Control arteries and those treated with trimethaphan formed occlusive thrombi in all instances. Nitroglycerin infusion resulted in a lower incidence of occlusion (1 of 6; p < 0.05 vs control value) and inhibited ex vivo platelet aggregation to adenosine diphosphate and arachidonic acid (p < 0.05). Local infusion of nitroglycerin reduced the formation of primary thrombi, independent of the hypotensive effect of the drug, and exerted systemic effects on platelet aggregation. Furthermore, platelet inhibition with nitroglycerin reduced the incidence of secondary thrombus formation (rethrombosis) after thrombolysis. The results suggest that a potential benefit of nitroglycerin therapy may be derived from its ability to inhibit thrombotic events in patients with unstable angina or myocardial infarction.

SIN-1 reduces platelet adhesion and platelet thrombus formation in a porcine model of balloon angioplasty

BACKGROUND

Nitric oxide inhibits platelet adhesion and platelet aggregation in vivo. In this study, we investigated the effects of the nitric oxide donor SIN-1 on platelet adhesion and platelet-thrombus formation following experimental angioplasty.

METHODS AND RESULTS

Bilateral carotid angioplasty was performed in 20 anesthetized pigs. Animals received either SIN-1 (3-morpholino-sydnonimine; 10 micrograms/kg/min; n = 8) or placebo (n = 8) before and during angioplasty. An additional control group of pigs received trimetaphan (n = 4), which induced hemodynamic changes similar to those that followed treatment with SIN-1. Platelet deposition was quantified by the injection of autologous 111In-labeled platelets. SIN-1 reduced platelet deposition after deep arterial injury compared with placebo (mean +/- SEM, 10.870 +/- 2.415 versus 40.326 +/- 9.889 platelets x 10(6)/cm2, p < 0.05). SIN-1 reduced platelet adhesion after superficial injury compared with both placebo and trimetaphan (2.231 +/- 0.333 versus 5.278 +/- 0.606 versus 5.022 +/- 1.136 platelets x 10(6)/cm2, respectively; p < 0.005). Scanning electron microscopy confirmed that platelets were deposited in the form of an adherent monolayer following superficial endothelial denudation and were reduced in number following treatment with SIN-1. The effects of SIN-1 on platelet function were associated with a significant increase in platelet cyclic GMP concentration from baseline (3.15 +/- 0.88 versus 1.58 +/- 0.73 pmol/10(9) platelets, p < 0.005).

CONCLUSIONS

SIN-1 reduces platelet adhesion and platelet-thrombus formation following experimental angioplasty. The antiadhesive effects of SIN-1 are independent of changes in systemic hemodynamics. These results imply that the administration of a nitric oxide donor may prove effective in modifying the pathophysiological response to angioplasty injury.

Coronary cyclic flow variations "precondition" ischemic myocardium

BACKGROUND

Repeated brief episodes of myocardial ischemia performed by mechanical clamping of a coronary artery "precondition" the heart and reduce infarct size after a subsequent sustained ischemia. It is not known, however, whether spontaneous episodes of transient ischemia caused by formation of platelet thrombi, which may occur in unstable angina, have a similar cardioprotective effect.

METHODS AND RESULTS

Therefore, our objective was to determine whether brief spontaneous thrombotic episodes of ischemia/reperfusion could limit infarct size and preserve contractile function following 60 minutes (protocol 1) or 90 minutes (protocol 2) of sustained ischemia and 4-4.5 hours of reperfusion in the canine model. Before the sustained coronary occlusion, dogs underwent a 30-minute "treatment" period consisting of: no intervention (control group), four repeated episodes of 3-minute mechanical occlusion plus 5-minute reperfusion (preconditioned group), or coronary artery stenosis and endothelial injury, resulting in a mean of four spontaneous episodes of cyclic flow variations (CFV group) caused by formation and dislodgment of platelet thrombi. In protocol 1 (60-minute sustained ischemia plus 4.5-hour reperfusion), infarct size was significantly smaller in both the preconditioned and CFV groups compared with controls (3.5 +/- 1.4%,* 3.4 +/- 2.1%,* and 9.9 +/- 2.7% of the myocardium at risk, respectively; *p less than 0.05 versus control). In contrast, neither preconditioning nor CFV preserved contractile function: Segment shortening during sustained occlusion was equally depressed at -15% to -20% of baseline values among the three groups and equally stunned at +12% to +18% of baseline during the 4.5 hours of reflow. In protocol 2 (90-minute sustained ischemia plus 4-hour reperfusion), only CFV continued to exert a cardioprotective effect: Infarct size averaged 15.0 +/- 4.1%, 7.4 +/- 2.5%,* and 16.5 +/- 4.4% of the region at risk in the preconditioned, CFV, and control groups, respectively (*p less than 0.05 versus control). Contractile function, however, was similar among all three groups both during 90 minutes of sustained occlusion and throughout 4 hours of reperfusion.

CONCLUSIONS

We therefore conclude that repeated coronary thrombus formation preconditions the ischemic myocardium: In fact, in contrast to mechanical preconditioning, cardioprotection provided by CFV persisted following 90 minutes of sustained coronary occlusion. However, preconditioning by thrombotic or mechanical occlusion neither preserved myocardial contractile function during sustained coronary occlusion nor prevented stunning after reperfusion. These data raise the possibility that clinical episodes of unstable angina prior to acute myocardial infarction may precondition the ischemic myocardium.