Roles of platelet-activating factor, thromboxane A2, ADP and thrombin in thrombogenesis in the guinea pig

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Prediction of the therapeutic index of marketed anti-coagulants and anti-platelet agents by guinea pig models of thrombosis and hemostasis

INTRODUCTION

Animal models of thrombosis and hemostasis are critical for target validation in pharmaceutical research. Guinea pig haemostatic mechanisms, such as the platelet thrombin receptor repertoire, resemble those of humans. Measuring the performance characteristics of marketed antithrombotic drugs in guinea pig models is a key to predicting therapeutic indices of new agents. The goal of the current study was to benchmark representative marketed drugs in thrombosis and hemostasis models in guinea pigs.

METHODS

Effects of the cyclooxygenase inhibitor, aspirin, the P2Y(12) antagonist, clopidogrel, the glycoprotein IIb/IIIa inhibitor, tirofiban, and the direct thrombin inhibitors, argatroban and hirudin, were evaluated in this study. Antithrombotic agents were tested in FeCl(3)-induced carotid artery thrombosis and arterio-venous shunt thrombosis models. Haemostatic effects of drugs were evaluated in cuticle and renal bleeding models. Ex vivo measurements of platelet function and coagulation inhibition were performed to benchmark preclinical doses of each agent to those used clinically.

RESULTS

The overall rank-order of potency in thrombosis models based on per cent of vessels occluded, average carotid blood flow, and thrombus weight was aspirin=argatroban=tirofiban<hirudin=clopidogrel. In bleeding models, the rank order was: aspirin<clopidogrel=argatroban=tirofiban<hirudin.

CONCLUSION

This characterization of representative drugs from two important classes of anti-coagulant and anti-platelet agents in efficacy and bleeding models in guinea pigs provides a reference point for evaluation of new antithrombotic agents.

Restoration of middle cerebral artery thrombosis by novel glycoprotein IIb/IIIa antagonist FK419 in guinea pig

We compared the antithrombotic efficacy of FK419 [(S)-2-acetylamino-3-[(R)-[1-[3-(piperidin-4-yl)propionyl]piperidin-3-ylcarbonyl]amino] propionic acid trihydrate], a novel nonpeptide glycoprotein IIb/IIIa antagonist, with recombinant tissue plasminogen activator (rt-PA) and other antithrombotic agents (aspirin, ozagrel, argatroban and heparin). FK419 not only inhibited ADP- and collagen-induced guinea pig platelet aggregation, but also induced disaggregation for ADP-induced aggregated platelets in vitro. In the photochemically induced middle cerebral artery thrombosis model in guinea pigs, FK419 dose-dependently shortened the time to first reperfusion and the total middle cerebral artery occlusion time and reduced ischemic brain damage and ameliorated neurological deficits measured 24 h after middle cerebral artery occlusion. Rt-PA similarly improved the middle cerebral artery patency, brain damage and neurological deficits. Neither aspirin, ozagrel, argatroban nor heparin restored the middle cerebral artery blood flow and improved the brain damage or neurological deficits. These results demonstrated that novel glycoprotein IIb/IIIa antagonist FK419 could disperse thrombus and ameliorated ischemic brain damage, suggesting that FK419 would be an attractive intervention for stroke patients.

Antithrombotic Effects of FK419, a Novel Nonpeptide Platelet GPIIb/IIIa Antagonist, in a Guinea Pig Photochemically Induced Middle Cerebral Artery Thrombosis Model: Comparison with Ozagrel and Argatroban

Platelet activation and subsequent aggregation play a key role in the pathogenesis of ischemic brain damage. Recent studies revealed that enhanced platelet activation is also observed after ischemia, suggesting that secondary thrombus formation might participate in the development of cerebral infarction. The binding of platelet glycoprotein GPIIb/IIIa (integrin alpha(IIb)beta3) to fibrinogen is the final common pathway in platelet aggregation. Therefore, GPIIb/IIIa antagonists might be useful in acute ischemic stroke as well as in the secondary prevention of ischemic stroke. In the present study, we evaluated the effect of three compounds, FK419 ((S)-2-acetylamino-3-[(R)-[1-[3-(piperidin-4-yl) propionyl] piperidin-3-ylcarbonyl] amino] propionic acid trihydrate), a novel nonpeptide GPIIb/IIIa antagonist, ozagrel, a selective thromboxane A(2) synthase inhibitor, and argatroban, a thrombin inhibitor, on middle cerebral artery (MCA) patency and ischemic brain damage using photochemically induced MCA thrombosis model in guinea pigs. FK419, ozagrel, or argatroban was administered 5 min after the termination of photoirradiation. FK419 dose-dependently improved MCA patency by decreasing the total occlusion time, time to continuous reperfusion, and the number of cyclic flow reductions, at doses that inhibited ADP-induced platelet aggregation ex vivo. In contrast, ozagrel only improved total occlusion time, and argatroban showed no improvement in MCA patency. FK419 also reduced ischemic brain damage in a dose-dependent fashion, whereas ozagrel and argatroban did not. Finally, FK419 ameliorated neurological deficits, whereas ozagrel and argatroban did not. These results indicate that FK419, a GPIIb/IIIa antagonist, ameliorates ischemic brain damage by improving MCA patency after occlusion and that FK419 is a promising candidate for the treatment of acute ischemic stroke.

Inhibitory effects of total flavones of Hippophae Rhamnoides L on thrombosis in mouse femoral artery and in vitro platelet aggregation

Total flavones of Hippophae Rhamnoides L (TFH) are extracted from Sea buckthorn, a Chinese herbal medicine. Sea buckthorn has antioxidant, anti-ulcerogenic and hepato-protective actions, and its berry oil is reported to suppress platelet aggregation. Though it is frequently used for patients with thrombosis, the likely mechanism(s) and effects of TFH on thrombogenesis remain unclear. Thus, we have investigated the effect in-vivo of TFH on thrombogenesis and in vitro on platelet aggregation, comparing them to those of aspirin. We measured thrombotic occlusion time in a mouse femoral artery thrombosis model by the photochemical reaction between intravenously injected rose bengal and green light irradiation. In vitro platelet aggregation in whole blood was measured by single platelet counting. Thrombotic occlusion time was 8.5 +/- 0.6 min in the control group. TFH at a dose of 300 micro g/kg, intravenously administered 15 min before the rose bengal injection, significantly prolonged it to 11.6 +/- 1.0 min (P < 0.05), a similar effect on in-vivo thrombogenesis to that of aspirin. TFH at a concentration of 3.0 micro g/ml significantly (P < 0.01) inhibited in vitro platelet aggregation induced by collagen (2 micro g/ml) in a concentration dependent manner, in contrast TFH did not affect aggregation induced by arachidonic acid (80 micro M) and ADP (0.3 micro M). The results of the present study, in which TFH prevented in-vivo thrombogenesis, probably due to inhibition of platelet aggregation, suggest a possible clinical approach for the prevention of thrombosis.

Genistein, an isoflavone included in soy, inhibits thrombotic vessel occlusion in the mouse femoral artery and in vitro platelet aggregation

Diet can be the most important factor that influences risks for cardiovascular diseases. Genistein included in soy is one candidate that may benefit the cardiovascular system. Here, we investigated the inhibitory effects of genistein on thrombotic vessel occlusion in the mouse femoral artery using a photochemical reaction, and in vitro platelet aggregation in whole blood measured by single platelet counting. Genistein (10 mg/kg), intravenously administered 10 min before the rose bengal injection, significantly prolonged the thrombotic occlusion time from 6.1+/-0.4 to 8.4+/-0.8 min (P<0.05). Genistein at doses higher than 30 microM significantly (P<0.01) inhibited in vitro platelet aggregation induced by collagen (1 and 3 microg/ml). When 10 mg/kg genistein was intravenously administered, ex vivo platelet aggregation induced by collagen (1 and 3 microg/ml) was significantly suppressed (P<0.01). In conclusion, genistein prevented in vivo thrombogenesis and suppressed in vitro platelet aggregation. These results suggest that dietary supplementation of soy may prevent the progression of thrombosis and atherosclerosis.

tPA, but not uPA, significantly affects antithrombotic therapy by a glycoprotein IIb/IIIa antagonist, but not by a factor Xa inhibitor

To define the interaction of fibrinolytic components with platelets or coagulation factors on thrombus formation, we investigated mouse deficient in tissue plasminogen activator (tPA -/-) or urokinase plasminogen activator (uPA -/-) and in their wild-type control (tPA +/+, uPA +/+). A thrombus was induced in the murine carotid artery using photochemical reaction. Blood flow was monitored and the time needed before the vessel became completely obstructed was within 12 min in all types of mice. When DX-9065a, a selective factor Xa inhibitor, or GR144053, a platelet glycoprotein (GP) complex IIb/IIIa antagonist was applied, the time required to occlusion was prolonged in a dose-dependent manner in all types of mice. When a factor Xa inhibitor was injected in tPA -/- mice, the estimated ED50 was not changed. However, when GR144053 was injected in tPA -/- mice, the most significant changes were observed: the estimated ED51 was 19.6 times higher than the one in tPA +/+ mice. Platelet aggregation, hemostasis tests, and bleeding times were not significantly different among the different types of mice. In conclusion, the antithrombotic effect of platelet inhibition by a GPIIb/IIIa antagonist, is severely affected by the absence or presence of tPA production. On the contrary, the inhibition of factor Xa shows a stable antithrombotic effect with or without tPA. Thus the lack of tPA, but not of uPA, significantly affects antithrombotic efficacy.

Antiaggregatory, antithrombotic effects of MS-180, a novel platelet glycoprotein IIb/IIIa receptor antagonist

The antiaggregatory and antithrombotic effects of (S)-(-)-ethyl[6-[4-(morpholinoformimidoyl)benzamido]-3,4-dihydro-2 H-1-benzo-pyran-3-yl]acetate hydrochloride (MS-180), a novel platelet glycoprotein IIb/IIIa receptor antagonist, were investigated. Ma-HCl, (S)-(-)-[6-[4-(Morpholinoformimidoyl)benzamido]-3,4-dihydro-2H-1-b enzopyran-3-yl]acetic acid hydrochloride, the hydrochloride salt of Ma (active metabolite), inhibited the binding of fibrinogen to immobilized human glycoprotein IIb/III receptor with an IC50 value of 0.12+/-0.03 nM without affecting binding to either fibronectin or vitronectin receptors. In anesthetized guinea pigs, intraduodenal administration of MS-180 caused dose-dependent inhibition of both ADP- and collagen-induced ex vivo platelet aggregation. At the same dosages, occluded thrombus formation and platelet release reactions were also markedly suppressed. In anesthetized dogs, the bleeding time was prolonged slightly even when submaximal inhibition (< 90%) of ex vivo platelet aggregation was achieved following i.v. administration of Ma-HCl. Aspirin (100 mg/kg) prolonged the bleeding time to the same extent as MS-180 (1 mg/kg), although it suppressed only collagen-induced platelet aggregation. Therefore, MS-180 may be clinically useful for the treatment of thrombotic diseases.

A new thromboxane receptor antagonist, Z-335, ameliorates experimental thrombosis without prolonging the rat tail bleeding time

We investigated the antithrombotic activity of Z-335, a new thromboxane A2 receptor antagonist, using experimental thrombosis models, and also tested its effect on the rat tail bleeding time. Z-335 (0.1, 0.3, and 1 mg/kg, p.o.) dose-dependently prevented the occurrence of arachidonic acid-induced pulmonary thromboembolism in mice. During photochemically induced thrombosis in the femoral artery of guinea pigs, Z-335 (0.3, 1, and 3 mg/kg, i.v.) dose-dependently prolonged the time required to form thrombi. Moreover, Z-335 (10 mg/kg/day, p.o.) strongly suppressed lauric acid-induced hind limb injury in rats. Z-335 (0.3, 3, 30, and 300 mg/kg, p.o.) did not prolong the tail bleeding time in rats. These results strongly suggest that Z-335 ameliorates experimental thrombosis without prolonging the rat tail bleeding time, and may therefore be a useful antithrombotic drug.

Antithrombotic effects in a rat model of aspirin-insensitive arterial thrombosis of desethyl KBT-3022, the main active metabolite of a new antiplatelet agent, KBT-3022

The antithrombotic effect of desethyl KBT-3022, which is the main active metabolite of the new antiplatelet agent, KBT-3022 (ethyl 2-[4,5-bis(4-methoxyphenyl)thiazol-2-yl] pyrrol-1-ylacetate; a cyclooxygenase inhibitor), was determined using a photochemically induced arterial thrombosis model in the rat femoral artery. Pretreatment with desethyl KBT-3022 (0.1, 0.3 and 1 mg/kg, i.v.) prolonged the time required to achieve thrombotic occlusion in the femoral artery and inhibited collagen-induced platelet aggregation in whole blood ex vivo, each in a dose-dependent manner. In all 6 rats used, particularly at the highest dose (1 mg/kg, i.v.) tested, cyclic variations in blood flow were hardly ever observed and complete cessation of blood flow did not occur during the 30-min observation time. BM-13505 (1, 3 and 10 mg/kg, i.v.), a thromboxane A2 receptor antagonist, also prolonged the time to occlusion, but cyclic variations in blood flow did occur. On the other hand, aspirin (10 and 30 mg/kg, i.v.) had little effect in terms of preventing thrombosis, although it inhibited collagen-induced platelet aggregation to the same extent as did desethyl KBT-3022. Desethyl KBT-3022 inhibited the thrombin-induced aggregation of washed platelets in a concentration-dependent manner (1-40 microM), whereas aspirin and BM-13505 did not. These findings suggest that the potent antithrombotic effect of desethyl KBT-3022 may be attributable in part to its additional ability to inhibit thrombin-induced platelet aggregation. Accordingly, thromboxane A2 and thrombin may be important thrombotic mediators in this rat model.

Inhibitory effect of prostaglandin E1 on intimal thickening following photochemically induced endothelial injury in the rat femoral artery

The inhibitory effect of prostaglandin E1, which has an anti-platelet action and a vasodilating action via intracellular cyclic AMP elevation, was studied on intimal thickening in the rat femoral artery. A segment of the femoral artery was occluded by a platelet and fibrin-rich thrombus due to photochemical reaction between systemically administered Rose Bengal and transluminal green light which causes endothelial injury followed by platelet adhesion and aggregation at the site of photochemical reaction. Three weeks after endothelial injury, intimal thickening occurred at the irradiated site. Prostaglandin E1 (0.3 microgram/kg per min), administered as a continuous infusion 10 min before photochemical reaction significantly (P < 0.05) prolonged the time to occlusion of the femoral artery. In a separate experiment, prostaglandin E1 (0.3 microgram/kg per min) administered as a continuous infusion for 7 days just after endothelial injury significantly (P < 0.05) inhibited intimal thickening compared with a control group. In cultured rat-derived vascular smooth muscle cells, prostaglandin E1 produced concentration-dependent inhibition of migration and proliferation, stimulated by platelet-derived growth factor. These results suggest that prostaglandin E1 may be effective in preventing vascular restenosis after vascular surgery and angioplasty.

Characterization of in vitro and in vivo platelet responses to thrombin and thrombin receptor-activating peptides in guinea pigs

Guinea pig platelets are similar to human platelets in their responsiveness to thrombin receptor-activating peptides and other agonists. Therefore, guinea pigs anesthetized with Inactin (90 mg/kg i.p.) were used to assess in vivo activities of thrombin and thrombin receptor-activating peptides (TRAPs) using 111 In-labeled platelets and a microcomputer-based system. The aggregatory responses are expressed as percent change for a 20 min period over basal radioactivity (AUC). Reversible accumulation of platelets occurred in the pulmonary microcirculation in response to stimuli. Human thrombin (50 and 100 U/kg i.v.) caused a dose-related platelet accumulation. Responses of similar magnitude were induced by SFLLRN (TRAP-(1-6)) and Ala-Phe(p-F)-Arg-Cha-HArg-Tyr-NH2 (high-affinity thrombin receptor-activating peptide, 0.03, 0.1 and 0.3 mg/kg i.v.). High-affinity thrombin receptor-activating peptide, a new synthetic oligopeptide agonist, is about 3-fold more potent than TRAP-(1-6), a wild-type sequence. Similarly, high-affinity thrombin receptor-activating peptide is about 4 times more potent than TRAP-(1-6) in the radioligand binding study using platelet membrane. By comparison, high-affinity thrombin receptor-activating peptide manifested an aggregatory activity (EC60 = 1.2 microM) about 15 times more potent than that of TRAP-(1-6)(EC60 = 18.6 microM) in washed guinea pig platelets. The intrapulmonary platelet aggregation in response to thrombin, TRAP-(1-6) and high-affinity thrombin receptor-activating peptide was characterized by long duration (approximately 30 min); a reduction in response (18-54%) tended to occur with repeated challenges, presumably due to desensitization and consumption. The response to thrombin (100 U/kg) was greatly inhibited by (D)-Phe-Pro-Arg-chloromethyl ketone (PPACK), a potent thrombin inhibitor (250 micrograms/kg + 6 micrograms/kg per min i.v. x 30): AUC, 150 +/- 552 vs. 7171 +/- 1052 in the control period (n = 8, P < 0.05). The response to high-affinity thrombin receptor-activating peptide (0.03 mg/kg), which acts on thrombin receptor directly, was not affected by PPACK. It is concluded that guinea pigs are an appropriate preparation for evaluation of in vivo activity of thrombin inhibitors as well as thrombin receptor agonists and antagonists.

A comparative study of the antithrombotic effect of aurintricarboxylic acid on arterial thrombosis in rats and guinea pigs

1. The antithrombotic effect of aurintricarboxylic acid (ATA) which inhibits binding of von Willebrand factor (vWF) to platelet glycoprotein lb (GPlb) receptor was evaluated in photochemically-induced thrombosis models in the femoral artery of rats and guinea-pigs. 2. ATA at a dose of 10 mg kg-1 significantly prolonged the time required for thrombotic occlusion of the artery in rats. The antithrombotic efficacy was associated with a significant inhibition of platelet retention and ex vivo botrocetin-induced platelet aggregation. 3. On the other hand, in guinea-pigs, ATA at the same dose inhibited the platelet retention and the platelet aggregation, but did not prevent thromboocclusion. 4. ATA inhibited aggregation of washed platelets from rats or guinea-pigs in response to botrocetin and thrombin in a dose-dependent manner (1-30 microM), and to the same extent. 5. ATA moderately increased activated partial thromboplastin time and bleeding time in both species. 6. These results indicate that vWF may play a role in the development of occlusive arterial thrombosis in the rat, but not in the guinea-pig. 7. The antithrombotic activity of ATA may partly arise from its inhibitory effect on thrombin, in addition to that on the vWF-GPlb pathway

A chemiluminescent detection of superoxide radical produced by adherent leucocytes to the subendothelium following thrombolysis: studies with a photochemically induced thrombosis model in the guinea pig femoral artery

Reocclusion following thrombolysis is a major limitation of thrombolytic therapy with recombinant tissue-type plasminogen activator (rt-PA) because denuded vessel wall exposed to blood following thrombolysis is a favourable surface for platelet and leucocyte deposition. We have applied a chemiluminescence technique to detect superoxide radical (0(-2)) produced by leucocytes adherent to the femoral artery 24 h after photochemically induced thrombogenesis in the guinea pig in vivo and subsequent thrombolysis by rt-PA. Intravenous administration of MCLA, a specific chemiluminescence reagent for detecting O(-2), markedly increased photon emission. the photon emission was markedly potentiated by phorbol myristate acetate and was suppressed by superoxide dismutase. Reocclusion 24 h after rt-PA induced thrombolysis was observed in 10 of 16 animals. Histological observations revealed extensive polymorphonuclear leucocytes adherent to the vessel wall at the site of thrombogenesis and thrombolysis. A higher level of 0(-2) could be detected from the arteries in which thrombolysis was induced compared with those without thrombolysis. Further, the level 0(-2) detected was greater in reoccluded arteries compared with those in which reflow was established. These observations suggest that 0(-2) is produced by adherent leucocytes at the site of thrombolysis and that leucocytes are involved in reocclusion after thrombolysis.

Antithrombotic activity of a symmetrical triglyceride with eicosapentaenoic acid and gamma-linolenic acid in guinea pig mesenteric microvasculature

The antithrombotic effect of a synthetic symmetrical triglyceride having eicosapentaenoic acid (EPA) at positions 1 and 3, and gamma-linolenic acid (GLA) at position 2 was investigated. Administration of the triglyceride significantly increased thrombus formation time and thrombotic occlusion time induced by light irradiation and a fluorescent dye in guinea pigs after 14 days administration compared to that of soybean oil. The antithrombotic effect of the triglyceride was similar to that of EPA ethyl ester. Administration of the triglyceride increased GLA, dihomo-gamma-linolenic acid (DGLA) and EPA contents in plasma and the liver, and the ratio of DGLA to arachidonic acid. These results might be responsible for this antithrombotic effect.

An experimental myocardial infarction model in the rat and its properties

The photochemical reaction between rose bengal and light (540 nm) produces thrombotic occlusion in rat coronary artery. We have now developed an experimental myocardial infarction (MI) model by photochemically induced thrombosis (PIT) in rats and investigated the mechanisms responsible for the induction of MI. PIT in the coronary artery induced myocardial ischemia, which was determined by tissue oxygen tension (tpO2), and resulted in MI. Pretreatment with a thromboxane (TX) A2-receptor antagonist, vapiprost, prevented a decrease in myocardial tpO2 and markedly reduced the MI area, although vapiprost inhibited collagen-induced platelet aggregation by 30% ex vivo. An ADP-induced platelet aggregation inhibitor, clopidogrel, also reduced the MI area. In contrast to vapiprost, clopidogrel inhibited collagen-induced platelet aggregation by 90% ex vivo. Pretreatment with a 5-HT2-receptor antagonist, ketanserin, which did not inhibit collagen-induced platelet aggregation ex vivo, prevented the decrease in myocardial tpO2 and reduced the MI area. These results suggest that TXA2, 5-HT and ADP play a role in the induction of MI and that platelet aggregation and other factors induce ischemia in this model.

Enhancement of thrombolytic efficacy of tissue-type plasminogen activator by adjuvants in the guinea pig thrombosis model

Reocclusion following thrombolysis is a major limitation of thrombolytic therapy with recombinant tissue-type plasminogen activator (rt-PA). We investigated the effects of vapiprost ((1R-(1 alpha(Z),2 beta,3 beta,5 alpha))-7-(5-((1,1'-biphenyl)-4-yl-methoxy)- 3-hydroxy-2-(1-piperidinyl)cyclopentyl)-4-heptenoic acid, a thromboxane A2 receptor antagonist); argatroban ((2R,4R)-4-methyl-1-[N2-(3-methyl-1,2,3,4-tetrahydro-8-quinolinyl)sulfon yl] - L-arginyl)]-2-piperidine-carboxylic acid, a specific thrombin inhibitor) and MK-886 (3-[1-(4-chlorobenzyl)-3-t-butyl-thio-5-isopropylindol-2-yl]-2,2- dimethylpropanoic acid, a specific leukotriene biosynthesis inhibitor) on the thrombolytic efficacy of rt-PA. The guinea pig femoral artery was thrombotically occluded by photochemical reaction between rose bengal and green light. Thirty min after the occlusion, rt-PA was administered and the time (T1) for reopening of the vessel and the frequency of reocclusion (Fro) 24 h after thrombolysis were monitored. With rt-PA alone, T1 was 28 +/- 7 min (n = 10) and Fro was 70%. T1 was reduced to 9 and 20 min by a combination of rt-PA with vapiprost and argatroban respectively. Fro was reduced by all three adjuvants. Histological observations revealed extensive adherence of polymorphonuclear leucocytes to the damaged endothelium at the site of thrombolysis. It is concluded that thromboxane A2, thrombin and leucocytes are involved in reocclusion after thrombolysis.

Enhancement of thrombotic arterial occlusion following cholesterol feeding in the guinea-pig: a role for thromboxane A2

We have developed a photochemical model to induce thrombotic occlusion of the guinea-pig femoral artery. Using this model, we investigated the effect of cholesterol feeding on arterial occlusion time in the guinea-pig. Animals were divided into two groups, one on standard diet and the other on standard diet containing 0.5% cholesterol for 3 weeks. The time for femoral artery occlusion was significantly shorter (p < 0.05) in cholesterol fed animals as compared to the control group. In vitro collagen-, U-46619- (a thromboxane A2 adenosine diphosphate analogue) and (ADP)-induced platelet aggregation responses in whole blood in cholesterol-fed animals were increased 13-, 10- and 4-fold, respectively. U-46619- and collagen-induced washed platelet aggregation responses were also significantly enhanced by cholesterol feeding (p < 0.01). Further, TXA2 generation by collagen-stimulated washed platelets in cholesterol-fed animals increased similar to the platelet aggregation responses. However, platelet-activating factor (PAF)-induced platelet aggregation in whole blood was relatively unaffected by cholesterol feeding. 11-dehydro TXB2 levels in plasma were increased significantly by cholesterol feeding. Our observations suggest that increased plasma TXA2 level and platelet aggregation response to TXA2 and stimulated TXA2 synthesis in platelets play a role in enhanced arterial occlusion in cholesterol fed guinea-pigs.

Experimental intimal thickening studies using the photochemically induced thrombosis model in the guinea-pig femoral artery

We have used the photochemically induced thrombosis model to study intimal thickening in the guinea-pig femoral artery. The femoral artery was occluded by a combination of rose bengal and green light which caused endothelial cell damage followed by formation of platelet rich thrombus at the site of endothelial damage. Thrombolysis was then achieved by administration of tissue-type plasminogen activator. Intimal thickening in the femoral arteries was histologically measured at 0, 1, 3, 6 and 9 weeks after the treatment. The neointimal areas gradually increased until 3 weeks and then remained unchanged up to 9 weeks. Cells in the neointima were identified as smooth muscle cells by immunohistochemical staining with an actin-specific antibody, HHF35. Cell proliferation in the media begun within 48 h after the thrombolysis and bromodeoxyuridine labeled cells appeared to migrate to the intima within 1 week after the thrombolysis. Administration of an angiotensin converting enzyme inhibitor, cilazapril (30 mg/kg/day, p.o.) for 3 weeks, suppressed intimal thickening and decreased the medial area in the femoral artery. These observations suggest that in this model cell proliferation and migration characteristics of pathological intimal thickening occur and this model is useful for investigating the effect of pharmacological preparations on intimal thickening.

Inhibitory effect of KW-3635, a new thromboxane A2-receptor antagonist, on arterial thrombosis in guinea pigs

The antithrombotic effects of the thromboxane (TX) A2-receptor antagonist and aspirin were determined using a photochemically-induced arterial thrombosis model in the femoral arteries of guinea pigs. KW-3635 (sodium (E)-11-[2-(5,6-dimethyl-1-benzimidazolyl)ethylidene]-6,11- dihydrodibenz[b,e]-oxepine-2-carboxylate monohydrate) and BM-13505, both of which are TXA2-receptor antagonists, and aspirin inhibited the thrombus formation at the doses that inhibited the ex vivo platelet aggregation induced by sodium arachidonate (100 microM) or collagen (3 micrograms/ml). These results support the notion that TXA2 is an important mediator in the present model of arterial thrombogenesis.