Comparison of the effects of molsidomine, nitroglycerin and isosorbide dinitrate on experimentally induced coronary artery thrombosis in the dog

Exogenous nitric oxide inhibits platelet activation in whole blood

We examined the effect of NO on collagen-induced whole blood aggregation and platelet activation in whole blood by using impedance aggregometry and flow cytometry. For the extracellular generation of NO, we chose sodium nitroprusside dihydrate (SNP), and as intracellular generators of NO, L-arginine and isosorbide dinitrate (ISDN). The latter two significantly inhibited whole blood aggregation, whereas SNP had no such effect. The inhibitory effect of ISDN was diminished by addition of methylene blue (MB) or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl 3-oxide (carboxyl-PTIO), and the inhibitory effect of L-arginine was diminished by addition of N(G)-monomethyl-L-arginine monoacetate (L-NMMA). Although the addition of ISDN increased the cyclic guanosine monophosphate (cGMP) level in whole blood and in the suspension of platelets and white blood cells (PLTs + WBCs), no increase was found in platelet-rich plasma (PRP). The P-selectin expression on the platelet surface in whole blood was reduced by ISDN and L-arginine. These findings suggest that the intracellular generation of NO inhibits whole blood aggregation, and this mechanism may play an important role in its antithrombotic effect in whole blood.

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.

Synergistic platelet antiaggregatory effects of the adenylate cyclase activator iloprost and the guanylate cyclase activating agent SIN-1 in vivo

The aim of our study was to evaluate the platelet antiaggregatory and hemodynamic effects of the stable prostacyclin analog iloprost and the NO-donor SIN-1, an active metabolite of molsidomine. The number of circulating platelets was determined in anesthetized male Wistar rats as a measure of in vivo platelet aggregation. Platelet count decreased from 648 +/- 25 to 476 +/- 15 x 10(3) platelets/microliter and from 578 +/- 36 to 411 +/- 40 (mean +/- SEM) in response to two repetitive injections of collagen (70 micrograms/kg body weight). Treatment with SIN-1 bolus injections (0.3 or 1 mg/kg bw) and/or continuous i.v. infusion of iloprost (0.2 or 0.4 micrograms/kg bw/min) was initiated 15 min before the second collagen bolus. As a single agent, SIN-1 did not influence platelet count. Iloprost at 0.2 micrograms/kg/min reduced platelet aggregation (PA) by 15.5%, and at 0.4 micrograms/kg/min by 27.1% (p = n.s.). When iloprost (0.2 micrograms/kg) and SIN-1 (0.3 mg/kg) were administered simultaneously, PA was suppressed by 56.2%, iloprost at 0.4 micrograms/kg/min and SIN-1 at 0.3 mg/kg were even more effective and inhibited PA synergistically by 64.9% (p < 0.05). A statistically significant decrease in mean arterial blood pressure (MABP) was seen in response to all SIN-1 and iloprost groups. However, the vasorelaxant effect of both agents given simultaneously was not synergistic but less than additive. In conclusion, iloprost and SIN 1 exert synergistic platelet inhibitory effects in vivo. In contrast, the decrease in MABP is less then additive. These results are of major interest for the therapeutic regimen with NO-donors (nitrovasodilators).

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.

Molsidomine: alternative approaches to treat myocardial ischemia

The long-acting antianginal drug molsidomine has been shown experimentally to reduce myocardial infarct size when administered prior to or after cardiac insult. This is due to several drug actions. Dilation of postcapillary capacitance vessels diminishes venous return, preload, heart dimensions, and myocardial oxygen consumption. Relaxation of stenosed conductive coronary arteries increases the perfusion of myocardial areas at risk of infarction due to enhanced collateral circulation. Increased regional blood supply nourishes predominantly subendocardial cardiac muscles as a result of reduction of extravascular coronary pressure, and resistance. The stable heart rate and cardiac contractility favor improved heart performance. The inhibition of platelet aggregation in vivo by molsidomine or its active metabolites, SIN-1 and SIN-1A, is linked to the stimulation of prostacyclin synthesis, inhibition of thromboxane release with induction of thrombosis and vasoconstriction, and enhanced concentrations of cyclic guanosine monophosphate. Dilation of coronary arteries after intracoronary administration of SIN-1, with inhibition of platelet aggregation by restrained release of adenosine diphosphate and stabilization of platelet membranes, facilitates the recanalization of stenosed arteries and reduces coronary muscle tone at the site of thrombosis. Activation of the human fibrinolytic system and drug-induced release of a plasminogen activator favor dysaggregatory effects. The drug's inhibiting actions on lipoxygenase products of arachidonate (e.g., 12-hydroperoxy-eicosatetraenoic acid and leukotrienes) may shift prostaglandin catabolism to cyclooxygenase products (e.g., prostacyclin) that protect against the expansion of ischemia and the induction of coronary spasm. Experimentally, the hemodynamic effectiveness of molsidomine can be antagonized by catecholamines (afterload effects) and dihydroergotamine (preload and afterload effects) respectively. Further clinical investigations will clarify the application of these mechanisms for the therapeutic success of the drug in human myocardial infarction.

Molsidomine on cardiovascular leukotriene D4 actions

Anaesthetized open-chest dogs were used to study the effects of intracoronary leukotriene D4 (LTD4; 0.5 micrograms/kg) on haemodynamics, the electrocardiogram (ECG), coronary blood flow in the left circumflex artery and coronary resistance in the absence or presence of the antianginal drug molsidomine (500 micrograms/kg i.v.). LTD4 increased left ventricular (LV) filling pressure from 6.5 +/- 3.8 to 14.7 +/- 3.2 mm Hg (P less than 0.01), decreased LV dP/dtmax from 2,500 +/- 200 to 1,240 +/- 205 mm Hg/s (P less than 0.05), elevated the S-T segment of the ECG from 0.3 +/- 0.2 to 2.4 +/- 0.6 mV (P less than 0.01), and coronary resistance from 3.4 +/- 0.9 to 33.7 +/- 4.8 mm Hg X min X ml-1 (P less than 0.001). Coronary artery blood flow fell from 33.8 +/- 2.7 to 3 +/- 3 ml/min (P less than 0.001). Molsidomine treatment 15 min prior to repeated intracoronary LTD4 application attenuated vasoconstrictor response to LTD4 and the subsequent elevation in total coronary vascular resistance. The negative inotropic actions of the eicosanoid were counteracted by molsidomine. Filling pressure decreased and no temporary signs of ischaemia were noted in the ECG. Inhibition of the cyclooxygenase enzyme activity by i.v. indomethacin (5 mg/kg) had no effects on the LTD4-induced haemodynamic alterations, the ECG, and coronary flow and resistance. The antagonistic molsidomine actions on haemodynamic and electrocardiographic LTD4 effects were not influenced by previous indomethacin treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

The activity of molsidomine in experimental models of ischemic cardiac disease

The cardioprotective and antithrombotic activity of molsidomine, a novel therapeutic agent for the treatment of coronary heart disease, was investigated in a series of animal models of myocardial ischemia. Molsidomine given to dogs with marked ST segment elevation (epicardial electrogram), induced by a reduction of left descending coronary artery (LAD) flow to 20% to 30% of the original value, resulted within 40 minutes in complete normalization of ECG changes. In another animal model molsidomine given either before or after occlusion of the LAD significantly reduced infarct size. All these molsidomine effects were accompanied by a marked lowering of preload, resulting in a reduction of extravascular coronary artery resistance and in increased blood flow toward the ischemic zones. In a model of myocardial ischemia and reperfusion in the anesthetized dog, molsidomine had a marked protective effect against the incidence of spontaneous ventricular fibrillation. This effect could also be attributed to the anti-ischemic activity of molsidomine, which would reduce the disparity between the refractory periods in normal and ischemic areas and thus increase ventricular stability. The antithrombotic activity of molsidomine was investigated in dogs in which the left circumflex coronary artery was electrically stimulated, a procedure that leads to thrombotic occlusion. Molsidomine in a dose-dependent manner prevented coronary thrombotic occlusion and reduced thrombus wet weight and the size of the resulting infarction. These effects of molsidomine were not related to its hemodynamic activity, since nitroglycerin and isosorbide dinitrate had similar hemodynamic effects but did not prevent coronary thrombosis. The antithrombotic activity of molsidomine is probably related to its ability to lower coronary venous blood thromboxane levels.