Mechanism of inhibition of neointimal formation by the angiotensin-converting enzyme inhibitor cilazapril. A study in balloon catheter-injured rat carotid arteries

Oral administration of the thrombin receptor antagonist E5555 (atopaxar) attenuates intimal thickening following balloon injury in rats

Thrombin is a powerful agonist for a variety of cellular responses including platelet aggregation and vascular smooth muscle cell (SMC) proliferation. These actions are mediated by a thrombin receptor known as protease-activated receptor-1 (PAR-1). Recently we discovered that 1-(3-tert-butyl-4-methoxy-5-morpholinophenyl)-2-(5,6-diethoxy-7-fluoro-1-imino-1,3-dihydro-2H-isoindol-2-yl)ethanone hydrobromide (E5555, atopaxar) is a potent and selective thrombin receptor antagonist. This study characterized the pharmacological effects of E5555 on SMC proliferation in vitro and in a rat model of intimal thickening after balloon injury in vivo. E5555 selectively inhibited rat aortic SMC proliferation induced by thrombin and thrombin receptor-activating peptide (TRAP) with half maximal inhibitory concentration (IC(50)) values of 0.16 and 0.038 μM, respectively. E5555 did not inhibit rat SMC proliferation induced by basic fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF) at concentrations up to 1μM. In addition, E5555 inhibited human aortic SMC proliferation induced by thrombin at concentrations of 0.3 and 3units/ml with IC(50) values of 0.028 and 0.079 μM, respectively, whereas it did not affect bFGF-induced proliferation at concentrations up to 1μM. Repeated oral administration of 30 mg/kg E5555 (once daily for 16 days) significantly reduced neointimal formation in the balloon-injured rat arterial model. These results suggested that a PAR-1 antagonist could be effective for treating restenosis following vascular intervention in addition to preventing thrombus formation. E5555 could thus have therapeutic potential for restenosis and chronic atherothrombotic disease.

Jak2 Tyrosine Kinase: A Potential Therapeutic Target for AT1 Receptor Mediated Cardiovascular Disease

Patients with hypertension often manifest a dysregulated renin-angiotensin-aldosterone system (RAAS). Most of the available treatment approaches for hypertension are targeted towards the RAAS including direct renin inhibition, ACE inhibition, angiotensin II type 1 receptor (AT₁-R) blockade, and aldosterone receptor antagonism. The Jak2 signaling pathway is intricately coupled to the AT₁-R signaling processes involved in hypertension. Here, we review the involvement of Jak2 in the pathogenesis of hypertension, and its potential as a therapeutic target for treatment of AT₁-R mediated cardiovascular disease. Jak2 may provide a rational therapeutic approach for patients whose blood pressure is not controlled by standard therapies.

Statin therapy is not associated with improved vascular access outcomes

BACKGROUND AND OBJECTIVES

Neointimal hyperplasia is the major cause of vascular access failure in hemodialysis patients. Statins reduce neointimal hyperplasia in experimental models, which may reduce access failure. The study presented here evaluated whether vascular access outcomes are superior in patients receiving statin therapy than in those not on statins.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A prospective computerized vascular access database was retrospectively queried to determine the access outcomes of 601 patients receiving an upper-arm fistula or graft at a single large dialysis center.

RESULTS

Primary fistula failure was observed in 37% of patients on statin therapy versus 38% not on statin therapy. Primary graft failure occurred in 20% of patients on statin therapy versus 14% not on statin therapy. A multiple variable logistic regression analysis including statin use, diabetes, coronary artery disease, peripheral artery disease, sex, and age found that only sex predicted primary fistula failure and graft failure. After excluding primary failures, cumulative fistula survival was similar for patients with or without statin therapy (hazard ratio [HR] 1.26; 95% confidence interval [CI] 0.76 to 2.16). Likewise, cumulative graft survival was similar for statin therapy versus no statin therapy (HR 0.88; 95% CI 0.59 to 1.32). Using a multivariable survival analysis model to predict cumulative fistula survival, only age predicted fistula failure (HR 1.21 per decade; 95% CI 1.02 to 1.44). None of the variables in this model predicted cumulative graft survival.

CONCLUSIONS

Statin therapy is not associated with improved fistula or graft outcomes in patients with chronic kidney disease.

c-Jun regulates shear- and injury-inducible Egr-1 expression, vein graft stenosis after autologous end-to-side transplantation in rabbits, and intimal hyperplasia in human saphenous veins

Coronary artery bypass graft failure represents an unsolved problem in interventional cardiology and heart surgery. Late occlusion of autologous saphenous vein bypass grafts is a consequence of neointima formation underpinned by smooth muscle cell (SMC) migration and proliferation. Poor long term patency and the lack of pharmacologic agents that prevent graft failure necessitate effective alternative therapies. Our objective here was to evaluate the effect of targeted inhibition of the bZIP transcription factor c-Jun on intimal hyperplasia in human saphenous veins and vein graft stenosis after autologous end-to-side transplantation. DNAzymes targeting c-Jun attenuated intimal hyperplasia in human saphenous vein explants. Adenovirus-forced c-Jun expression stimulated SMC proliferation, proliferating cell nuclear antigen, and MMP-2 expression. c-Jun DNAzymes abrogated Adeno-c-Jun-inducible SMC growth and wound repair and reduced intimal thickening in jugular veins of New Zealand white rabbits 4 weeks after autologous end-to-side transplantation to carotid arteries. Conversely, in a DNAzyme-free setting, Adeno-c-Jun potentiated neointima formation in the veins compared with Adeno-LacZ. Inducible c-Jun expression is ERK1/2- and JNK-dependent but p38-independent. Injury- and shear-inducible c-Jun controls early growth response-1. These data demonstrate that strategies targeting c-Jun may be useful for the prevention of vein graft stenosis. Control of one important shear-responsive transcription factor by another indicates the existence of transcriptional amplification mechanisms that magnify the vascular response to cell injury or stress through inducible transcriptional networks.

Quinaprilat-eluting stents do not attenuate intimal thickening following stenting in porcine coronary arteries

OBJECTIVE

Vascular injury increases angiotensin-converting enzyme (ACE) activity in the vessel wall, and experimental evidence suggests that high-dose oral ACE inhibition reduces intimal hyperplasia following balloon angioplasty. Local drug delivery can achieve high local concentrations which may be especially efficacious in inhibiting tissue growth following stent implantation. The aim of this study was to evaluate the angiographic and histomorphologic effects of quinaprilat-eluting stents in normal porcine coronary arteries.

METHODS

Ten pigs received phosphorylcholine-coated stents in each of the three major coronary arteries: one loaded with 780 microg quinaprilat, one with the solvent and one non-loaded control. Quantitative angiography was performed before and after stenting and at 4 weeks follow-up. At this time point the stented arteries were also analyzed using histology and morphometry.

RESULTS

Repeated measures ANOVA yielded significantly smaller angiographic lumen in both quinaprilat and solvent groups: 2.62+/-0.31 and 2.65+/-0.31 mm, respectively versus control: 2.70+/-0.32 mm at follow-up, p<0.05. Histology confirmed this finding with an increment in intimal area (2.5+/-0.86 mm(2)) and thickness (0.57+/-0.29 mm) in the quinaprilat group; versus solvent (1.98+/-0.57 mm(2) 0.4+/-0.26 mm) and controls (1.92+/-0.50mm(2) and 0.41+/-0.18 mm).

CONCLUSION

Quinaprilat-eluting stents do not attenuate neointimal thickening following implantation in normal porcine coronary arteries, but rather show a tendency towards the opposite.

Vascular Access in Hemodialysis: Issues, Management, and Emerging Concepts

This article (1) identifies the types of hemodialysis access, (2) summarizes the clinical standard of care for dialysis access grafts and fistulae, (3) describes the pathology and pathogenesis of venous stenosis in dialysis access grafts and fistulae, (4) tabulates avail-able therapies for hemodialysis vascular access dysfunction and speculates on the rea-sons for the lack of effective therapies, and (5) discusses the development and application of novel therapeutic interventions for this difficult clinical problem. The possibility that dialysis access grafts and fistulae could be the ideal clinical model for testing novel local therapies to block neointimal hyperplasia is discussed.

ACE inhibitors and angiotensin II receptor antagonists

The biological actions of angiotensin II (ANG), the most prominent hormone of the renin-angiotensin-aldosterone system (RAAS), may promote the development of atherosclerosis in many ways. ANG aggravates hypertension, metabolic syndrome, and endothelial dysfunction, and thereby constitutes a major risk factor for cardiovascular disease. The formation of atherosclerotic lesions involves local uptake, synthesis and oxidation of lipids, inflammation, as well as cellular migration and proliferation--mechanisms that may all be enhanced by ANG via its AT1 receptor. ANG may also increase the risk of acute thrombosis by destabilizing atherosclerotic plaques and enhancing the activity of thrombocytes and coagulation. After myocardial infarction, ANG promotes myocardial remodeling and fibrosis, and its many pathological mechanisms deteriorate the prognosis of these high-risk patients in particular. Therapeutically, inhibitors of the angiotensin I-converting enzyme (ACEI) and AT1 receptor blockers (ARB) are available to suppress the generation and cellular signaling of ANG, respectively. Despite major differences in the efficacy of ANG suppression and the modulation of other hormones and receptors, both classes of drugs are generally effective in attenuating numerous pathomechanisms of ANG in vitro, and in diminishing the development of atherosclerotic lesions and restenosis after angioplasty in various animal models. In clinical therapy, ACEI and ACE are well-tolerated antihypertensive drugs that also improve the prognosis of heart failure patients. After myocardial infarction and in stable coronary heart disease, ACEI have been shown to reduce mortality in a manner independent of hemodynamic alterations. However, there is little evidence that inhibitors of the RAAS may be effective against arterial restenosis, and a possible benefit of these substances compared to other antihypertensive drugs in the primary prevention of coronary heart disease in hypertensive patients is still a matter of debate, possibly depending on the specific substance and condition being investigated. As such, the general clinical efficacy of ACEI and ARB may be due to a positive influence on hemodynamic load, vascular function, myocardial remodeling, and neuro-humoral regulation, rather than to a direct attenuation of the atherosclerotic process. Further therapeutic advances may be achieved by identifying optimum drugs, patient populations, and treatment protocols.

L-ascorbic acid stimulates expression of smooth muscle-specific markers in smooth muscle cells both in vitro and in vivo

The dedifferentiation of vascular smooth muscle cells (VSMCs) plays a critical role in the progression of atherosclerosis and restenosis after angioplasty. Thus, factors that stimulate smooth muscle cell differentiation should be useful for therapy for these diseases. Previously, we found that l-ascorbic acid (L-Asc) induces the expression of smooth muscle-specific genes in a pluripotent bone marrow stromal cell line, TBR-B. This finding suggests that l-Asc stimulates the differentiation of smooth muscle cells. In the present study, we investigated the effects of l-Asc and its derivatives on the differentiation state of VSMCs in vitro and in vivo. l-Asc and its long-lasting derivatives stimulated the production of smooth muscle-specific myosin heavy chain-1 (SM1) and calponin 1 in a dose-dependent manner in rat cultured VSMCs, and the elevated production of SM1 and calponin 1 was maintained for at least 2 weeks. Moreover, oral administration of 3 g/kg of l-Asc to the balloon-injured rats induced a higher expression of SM1 and calponin 1 in the injured arteries compared with that from administration of the delivery vehicle alone. These data demonstrated new biologic activity, such as the stimulation of VSMC differentiation, of l-Asc and its long-lasting derivatives. In addition, these compounds may serve as useful tools for analysis of the differentiation of VSMCs and for therapy for vascular diseases.

Hemodialysis vascular access dysfunction from basic biology to clinical intervention

Hemodialysis vascular access dysfunction is a major cause of morbidity and hospitalization in the hemodialysis population at a cost of over 1 billion dollars per annum. Venous stenosis and thrombosis as a result of venous neointimal hyperplasia are the major causes of hemodialysis vascular access dysfunction. Despite the magnitude of the clinical problem, there are currently no effective therapies for this condition. We believe that this could be because of an inadequate understanding of the pathogenesis of this condition. At a histological level, venous neointimal hyperplasia (both in human specimens and in a pig model) is characterized by the presence of smooth muscle cells/myofibroblasts, microvessel formation (angiogenesis), and the accumulation of extracellular matrix components, all of which could be potential targets for therapeutic intervention. In particular, polytetrafluoroethylene dialysis access grafts could be the ideal clinical model for testing out novel local therapies to block neointimal hyperplasia. The current review describes the lesion of venous neointimal hyperplasia in human samples and in a pig model and suggests possible future directions for the development of effective local therapies for this condition.

Heat shock protein (HSP) 47 and collagen are upregulated during neointimal formation in the balloon-injured rat carotid artery

Heat shock protein (HSP) 47, a collagen-specific molecular chaperone, is thought to be essential for the proper processing and secretion of procollagen molecules. We investigated the time course and localization of HSP47 and collagen expression after balloon catheter angioplasty in the rat carotid artery, based on the premise that accumulation of extracellular matrix components is a main feature of intimal hyperplasia in humans and in laboratory animals. Low levels of HSP47 expression were evident in uninjured carotid arteries. Northern blot analysis revealed that HSP47 mRNA expression was markedly stimulated 1--3 days after the induced injury and a high level was maintained for 7 days, followed by a gradual decline for up to 21 days after the injury. These changes in HSP47 expression paralleled changes in alpha 1(I) collagen expression. Immunohistochemical staining revealed colocalization of HSP47 and collagen in smooth muscle cells (SMCs) of the media and intima. In situ hybridization analysis showed that activated SMCs, which proliferated and migrated into the intima, expressed high levels of HSP47. In cultured human aortic SMCs, similar upregulation of HSP47 and alpha1(I) collagen by TGF-beta was noted. These results show that SMCs activated after balloon injury express high levels of HSP47 and collagen during cell proliferation and migration, hence an overproduction of collagen and development of intimal thickening. Thus, HSP47 plays a role in the formation and progression of neointima after angioplasty.

Effects of probucol on rat carotid artery responses to balloon catheter injury

Balloon catheter injury to the rat common carotid artery has been widely used for testing potential therapies for post-angioplasty restenosis. However, the model has become somewhat discredited because a number of drugs that inhibit intimal thickening, measured 14 days after balloon catheter injury, have been found to be ineffective in clinical trials. Probucol has recently been shown to reduce the incidence of post-angioplasty restenosis in a number of small clinical trials, making it possible to reassess the validity of the rat balloon injury model. The effects of probucol on the underlying causes of intimal thickening in balloon-injured rat carotid arteries were quantified. Probucol inhibited medial smooth muscle cell proliferation by 23% on day 4 after injury (P=0.009), and by 65% on day 10 after injury (P=0.026). Smooth muscle cell migration was reduced by 64% (P=0.008) in probucol-treated animals. In marked contrast, intimal smooth muscle cell proliferation was significantly increased by 41% (P=0.024) by probucol. There was no significant effect on intimal thickening or smooth muscle cell death. These data suggest that drugs that inhibit both medial smooth muscle cell proliferation and migration in the rat balloon injury model may prove useful in the treatment of post-angioplasty restenosis.

The multiple actions of angiotensin II in atherosclerosis

Angiotensin II (Ang II), the effector peptide of the renin-angiotensin system, has been implied in the pathogenesis of atherosclerosis on various levels. There is abundant experimental evidence that pharmacological antagonism of Ang II formation by angiotensin converting enzyme inhibition or blockade of the cellular effects of Ang II by angiotensin type 1 receptor blockade inhibits formation and progression of atherosclerotic lesions. Angiotensin promotes generation of oxidative stress in the vasculature, which appears to be a key mediator of Ang II-induced endothelial dysfunction, endothelial cell apoptosis, and lipoprotein peroxidation. Ang II also induces cellular adhesion molecules, chemotactic and proinflammatory cytokines, all of which participate in the induction of an inflammatory response in the vessel wall. In addition, Ang II triggers responses in vascular smooth muscle cells that lead to proliferation, migration, and a phenotypic modulation resulting in production of growth factors and extracellular matrix. While all of these effects contribute to neointima formation and development of atherosclerotic lesions, Ang II may also be involved in acute complications of atherosclerosis by promoting plaque rupture and a hyperthrombotic state. Accordingly, Ang II appears to have a central role in the pathophysiology of atherosclerosis.

Angiotensin I-converting enzyme activity and vascular sensitivity to angiotensin I in rat injured carotid artery

We used a vasoreactivity assay to examine the functional significance of angiotensin I-converting enzyme overexpression in smooth muscle cells after vascular injury. Rat carotid arteries isolated at days 2 to 14 after in vivo endothelial denudation were compared with the contralateral freshly denuded (control) vessels. Arterial rings were constricted ex vivo with angiotensin I in the absence or presence of the angiotensin I-converting enzyme inhibitors captopril (300 nM and 3 microM) or perindoprilate (1 nM). Angiotensin I-converting enzyme activity was determined by cleavage of the chromogenic substrate Hip-His-Leu. Angiotensin I-converting enzyme activity in injured arteries was increased (2-fold) at day 7 only after vascular injury. Contractions to angiotensin I were unaffected after injury. Inhibition by captopril and perindoprilate of angiotensin I-induced contractions was significantly less potent in injured arteries at day 7 as compared to control vessels. Mechanical removal of neointimal smooth muscle cells normalized the inhibition by captopril in injured arteries at day 7. Captopril did not affect angiotensin II-induced contractions. Thus, upregulation of angiotensin I-converting enzyme after arterial injury confers resistance to angiotensin I-converting enzyme inhibitors.

Protective roles of endogenous carbon monoxide in neointimal development elicited by arterial injury

We reported that carbon monoxide (CO) generated through heme oxygenase (HO) inhibits mitogen-induced proliferation of vascular smooth muscle cells (VSMCs). We report that balloon injury induces HO-1, the stress-inducible isozyme of HO, in VSMCs and inhibits neointimal formation through the action of endogenous CO. Northern blot analysis and immunohistochemistry revealed that HO-1 is markedly induced in the media as early as 1 day after injury, whereas only a little expression was detected in the intact carotid artery. The neointimal proliferative changes were augmented or inhibited by the HO inhibitors or inducer, respectively, and effects of these interventions were not altered by suppression of endogenous nitric oxide (NO), if any. To elucidate the mechanisms by which HO controls the proliferative changes, effects of alterations in the HO reaction were examined by determining angiotensin II-elicited VSMC proliferation in vitro: the HO inducer attenuated and its inhibitor restored the proliferative response to angiotensin II (1 nM and 100 nM). Hemoglobin, a reagent trapping both NO and CO, but not met-hemoglobin, which can capture NO but not CO, augmented the proliferative response. These data suggest that endogenous CO serves as a protective factor that limits the excessive VSMC proliferation associated with vascular diseases.

Local administration of ramiprilat is less effective than oral ramipril in preventing restenosis after balloon angioplasty in an animal model

PURPOSE

To test the hypothesis that local administration of angiotensin converting enzyme (ACE) inhibitor via a microporous balloon catheter would be more effective than oral administration of ACE inhibitor in preventing neointima formation after balloon angioplasty.

MATERIALS AND METHODS

Neointima formation was induced by balloon denudation followed by 0.5% cholesterol diet in 29 New Zealand White rabbits. Directly after denudation, local administration of 1.8 mg of ramiprilat (n = 7) or saline (n = 7) with a microporous balloon catheter at a pressure of 3 atm was performed. Both groups additionally received ramipril orally (1 mg/d). Seven animals were treated exclusively with oral ramipril. The control group was fed a 0.5% cholesterol diet and given no medication (n = 8). Six weeks after intervention, the animals were killed and morphometric and immunohistologic analyses were performed.

RESULTS

Oral administration of ramipril resulted in a significant reduction of placque area (-66%, P < .05). Oral and local administration of the ACE inhibitor was followed by a nonsignificant reduction of the neointimal area (-17%). Local administration of saline combined with oral ramipril failed to prevent neointimal formation (reduction of 6%, NS).

CONCLUSION

Oral administration of ramipril resulted in a significant reduction of neointimal proliferation in New Zealand White rabbits. The possible benefit of an additional administration of local ramiprilat was diminished by an excessive neointimal hyperplasia, which was most likely caused by the inherent vessel trauma with use of the microporous balloon catheter.

Mechanisms of neointima formation--lessons from experimental models

The lamina intima of an artery is the region between the endothelial cell surface and the internal elastic lamina, which forms the luminal margin of the media. In humans the intima of atherosclerosis-prone arteries becomes thicker due to accumulation of smooth muscle cells, which originate from the media. The introduction of percutaneous transluminal coronary angioplasty (PTCA) boosted scientific interest in intimal thickening, because restenosis remains an unresolved problem of this intervention. In order to unravel the mechanisms of intimal thickening there is a need for appropriate animal models. A brief overview of these models is given together with factors that control proliferation and/or migration. Despite intensive research on neointima formation, an effective therapy for restenosis has not emerged to date. This may be due to the fact that other processes, such as acute elastic recoil and chronic constrictive remodeling may contribute to lumen narrowing as well. Other limitations of neointima models are related to species and anatomical differences. Most studies are performed in arteries that are either lesion-free, or contain relatively mild plaques, in contrast to the complicated, stenotic lesions that are the substrate for human PTCA. Other differences are the severity of the injury and incorporation of a mural fibrin-rich thrombus. Nevertheless, studies based on superficial injury, like the frequently used balloon denudation model, are useful. There are similarities with angioplasty, such as endothelial cell damage and proliferation of medial and intimal smooth muscle cells. The use of techniques such as differential display, gene transfer and application of antisense oligonucleotides may provide new therapeutic approaches to reduce neointima formation.

Discovery and main pharmacological properties of mibefradil (Ro 40-5967), the first selective T-type calcium channel blocker

PROPERTIES OF MIBEFRADIL: Mibefradil is a novel calcium channel antagonist with structural and pharmacological characteristics clearly distinct from those of classical calcium antagonists. It is a potent vasodilator with a high selectivity for the coronary vasculature over the peripheral vasculature and the myocardium. Most importantly, this compound can relax vascular muscle and slow the heart rate without reducing cardiac contractility. In addition, it does not stimulate neurohormonal reflexes and it exhibits a good pharmacological profile characterized by a long duration of action.

MECHANISM OF ACTION

The mechanism of action of mibefradil is characterized by the selective blockade of transient, low-voltage-activated (T-type) calcium channels over long-lasting, high-voltage-activated (L-type) calcium channels, which is probably responsible for many of its unique properties. CLINICAL USE OF MIBEFRADIL: Although calcium antagonists are mainly used for the treatment of hypertension and angina pectoris, there is strong preclinical evidence that mibefradil may also be beneficial in the treatment of congestive heart failure.

Kinins and the events influenced by an angiotensin-converting enzyme inhibitor during neointima formation in the rat carotid artery

OBJECTIVE

In balloon-injured rat carotid arteries, angiotensin-converting enzyme inhibitors (ACEI) decrease neointima formation, and a kinin receptor antagonist partially reverses this inhibitory effect. We studied which of the events leading to neointima formation are involved in the effects of ACEI and kinins.

METHODS

We administered 5 mg/kg per day ramipril, either alone or combined with the kinin receptor antagonist icatibant (Hoe 140), on the days each wave occurred and studied the effects on neointima formation 14 days after balloon injury. Ramipril alone or combined with icatibant had no effect on neointima formation when administered from 2 days before to 3 or 5 days after balloon injury. In contrast, ramipril inhibited neointima formation when administered from day 7 to day 14. Treatment with icatibant had a small effect, which was sufficient to abolish the effects of ramipril (control 0.11 +/- 0.01 mm2, ramipril 0.08 +/- 0.01 mm2; P < 0.05; ramipril plus icatibant 0.09 +/- 0.01 mm2; NS, ramipril plus icatibant versus control). Thus ramipril was not effective when treatment was stopped after 3 or 5 days, but was mildly effective when treatment was administered during the second week. The effect on migration was studied by counting the number of neointimal cells in rats treated from 2 days before to 4 days after injury. Ramipril decreased the number of cells by 93% compared with controls (control 65.0 +/- 13.5 cells/slice, ramipril 4.7 +/- 2.0 cells/slice; P < 0.001), and this effect was blunted significantly by icatibant (19.5 +/- 5.7 cells/slice; P < 0.009, versus ramipril; P < 0.007, versus controls). The influence of treatment on the rate of proliferation (the 5'-bromo-2'-deoxyuridine index) was studied in the media 3 days, and in the neointima 7 and 10 days after balloon injury. Although proliferation peaked in the neointima after 7 days, there were no differences among the groups at any time. Thus neither ramipril nor icatibant affected the rate of proliferation at the times sampled. Ramipril increased cell density (cells/mm2) in the neointima, and this effect was abolished by cotreatment with icatibant (P < 0.05).

CONCLUSION

The ACEI needs to be present throughout the experimental period to be most effective. ACEI act on neointima formation in part by inhibiting migration; thus, because ramipril was mildly effective when administered from 7 to 10 days after injury, it is likely that vascular smooth muscle cell migration also occurs continuously. Kinins help mediate roughly 30% of the effect of ACEI on migration. In addition, ACEI, through kinins, affect a process that increases the density of the cells in the neointima, perhaps by decreasing extracellular matrix deposition.

Prevention of neointima formation by mibefradil after vascular injury in rats: comparison with ACE inhibition

Cilazapril, an angiotensin-converting enzyme inhibitor, and mibefradil, a selective T-type voltage-operated calcium channel blocker, have been shown to prevent neointima formation after vascular injury. The goal of the present study was to evaluate the mechanism of action of both drugs. For this purpose, the influence of the renin angiotensin system on the effects of mibefradil (30 mg/kg po) and cilazapril (10 mg/kg po) on neointima formation after carotid injury were evaluated in normotensive rats (normal renin angiotensin system) and DOCA hypertensive rats (suppressed renin angiotensin system). In addition, in order to differentiate an effect on cell migration or cell proliferation, both drugs were given either before or after the smooth muscle migration phase. Finally, cilazapril and mibefradil were given in combination. In normotensive rats, mibefradil and cilazapril decreased neointima formation, resulting in neointima/media ratios of 38% (p < 0.05) and 53% (p < 0.01), respectively. However, in DOCA hypertensive rats, mibefradil was active, with a reduction of the neointima/media ratio by 63% (p < 0.001), whereas cilazapril reduced it only slightly (19%) and not significantly. In addition, cilazapril was active only when treatment started before the migration phase (63%, reduction in neointima/media ratio, p < 0.001) but not when started thereafter (13% reduction in neointima/media ratio, n.s.). In contrast, treatment with mibefradil was also active when started after the migration phase (51% reduction in neointima/ media ratio, p < 0.001 when treatment started 1 day before balloon injury and 41%, p < 0.01 when treatment started 5 days after balloon injury). The combination of both drugs was additive (67% reduction in neointima/media ratio, p < 0.001 vs. control). These experiments clearly show that mibefradil and cilazapril have a different mechanism of action after vascular injury. Mibefradil most likely prevents the proliferation of smooth muscle cells. In contrast, cilazapril most likely inhibits the migration of smooth muscle cells. These two different mechanisms of action explain why the effects of both drugs are additive.