Effect of verapamil on accumulation of free and esterified cholesterol in the thoracic aorta of cholesterol-fed rabbits

Verapamil increases the apolipoprotein-mediated release of cellular cholesterol by induction of ABCA1 expression via Liver X receptor-independent mechanism

OBJECTIVE

Release of cellular cholesterol and phospholipid mediated by helical apolipoprotein and ATP-binding cassette transporter (ABC) A1 is a major source of plasma HDL. We investigated the effect of calcium channel blockers on this reaction.

METHODS AND RESULTS

Expression of ABCA1, apoA-I-mediated cellular lipid release, and HDL production were enhanced in cAMP analogue-treated RAW264 cells by verapamil, and similar effects were also observed with other calcium channel blockers. The verapamil treatment resulted in rapid increase in ABCA1 protein and its mRNA, but not the ABCG1 mRNA, another target gene product of the nuclear receptor liver X receptor (LXR). By using the cells transfected with a mouse ABCA1 promoter-luciferase construct (-1238 to +57bp), verapamil was shown to enhance the transcriptional activity. However, it did not increase transcription of LXR response element-driven luciferase vector.

CONCLUSIONS

The data demonstrated that verapamil increases ABCA1 expression through LXR-independent mechanism and thereby increases apoA-I-mediated cellular lipid release and production of HDL.

Verapamil reduces intimal hyperplasia in a sheep carotid artery patch graft model

BACKGROUND

The current study investigated the effect of verapamil on the development of intimal hyperplasia (IH) using a sheep model.

METHODS

A gelatin-sealed Dacron patch graft was implanted into the left common carotid artery in 40 adult Merino sheep. Sheep were then randomly allocated to four groups. Group 1 were controls given no treatment (n = 10). Groups 2-4 were treated with intravenous verapamil 0.5 mg/kg per day in two divided doses for different lengths of time. Group 2 (n = 10) received treatment for 1 week; group 3 (n = 10) received treatment for 2 weeks and group 4 (n = 10) received treatment for 4 weeks. All sheep were killed at 4 weeks and the grafted segments of artery were harvested for IH assessment by an image analysis system.

RESULTS

The IH index from the three groups treated with verapamil was significantly less than that of the control (group 1,0.287+/-0.077: group 2,0.205+/-0.064, P<0.05; group 3, 0.193+/-0.059, P<0.01; group 4,0.171+/-0.046, P<0.01).

CONCLUSION

The present study suggests that verapamil inhibits the development of IH even when treatment is given for only 1 week.

Carotid artery wall alterations as intermediate end points

Carotid artery wall alterations can now be measured in a quantitative, well reproducible way by B-mode ultrasound. The thickness of the intima-media complex (IMT) is commonly measured at three levels along the carotid tree. Well standardized protocols to use in randomized prospective trials are available. It is argued that IMT measurements can be used as intermediate end points of antihypertensive treatment trials as 1) IMT alterations are frequent in hypertension and closely correlated to systolic and pulse pressures; 2) IMT alterations are predictive of an increased risk of cardiovascular events; 3) changes in IMT occur during antihypertensive treatment; 4) at least one study has shown differential effects of different antihypertensive agents on IMT (VHAS: verapamil better than chlorthalidone); 5) in this study a greater effect on IMT has been accompanied by a reduced cardiovascular event rates. Larger studies are desirable to confirm these conclusions.

The Verapamil in Hypertension and Atherosclerosis Study (VHAS): results of long-term randomized treatment with either verapamil or chlorthalidone on carotid intima-media thickness

BACKGROUND

It is unclear whether the carotid intima-media thickness can be influenced by antihypertensive treatment and whether some antihypertensive agents, such as calcium antagonists, may have a greater effect on this parameter than others, such as diuretics. The present paper reports the principal results of the ultrasound substudy of the randomized, prospective, controlled, Verapamil in Hypertension and Atherosclerosis Study (VHAS).

DESIGN AND METHODS

In 498 hypertensive patients in eight Italian centres, randomized to either verapamil (240 mg once a day) or chlorthalidone (25 mg once a day), a B-mode ultrasound scan was performed according to a standardized procedure at baseline and after 3, 12, 24, 36 and 48 months of treatment. The maximum intima-media thicknesses of the far walls of common, bifurcation and internal carotid arteries were measured bilaterally, and the following indices calculated: the mean thickness at the six measured sites, the mean thickness at the common and bifurcation sites and the single maximum thickness. The primary endpoint for treatment efficacy was the slope of the change over 4 years (rate of change, mm/year), corrected by using the initial mean over the six sites (baseline + 3 months) as a covariate (mm/year per mm). The patients were also classified into three strata according to their baseline single maximum thickness: those with normal carotid arteries (single maximum ( 1 mm), those with thickened carotid arteries (single maximum > 1 and < or = 1.5 mm and those with carotid plaques (single maximum > 1.5 mm).

RESULTS

Among the 456 patients with satisfactory baseline ultrasound readings, 33% were classified with normal carotid arteries, 27% with thickened carotid arteries and 40% with plaques. In the intention-to-treat population (377 patients with ultrasound measurements taken on at least three different occasions over a period of at least 2 years), the rate of change in the mean thickness at the six sites measured was rather small (0.015 mm/year), but significantly (P < 0.05) smaller in patients with plaques (0.003 mm/year) than in patients with thickened or with normal carotids (0.023 and 0.025 mm/year, respectively). When related to initial values, the rate of change in the mean thickness at the six sites had a negative slope (-0.059 mm/year per mm, P < 0.01). Although rates of change in the carotid intima-media thickness in unstratified patients were not different in those treated with verapamil or with chlorthalidone, when changes in the mean thickness of six sites were related to the initial value, the slope of this relationship was significantly different in the two treatment groups (verapamil -0.082 versus chlorthalidone -0.037 mm/year per mm, P < 0.02). The blood pressure-lowering effect of the two randomized treatments was similar. Taking fatal and nonfatal, major and minor cardiovascular events together, there were 19 events in the verapamil group and 35 in the chlorthalidone group, with a significantly (P < 0.01) greater incidence in patients with plaques, and among patients with plaques in those who were randomized to chlorthalidone (P < 0.05).

CONCLUSIONS

In accord with evidence from animal models of atherosclerosis, the calcium antagonist verapamil was more effective than the diuretic chlorthalidone in promoting regression of thicker carotid lesions. Changes in the carotid intima-media thickness were small in both groups, and the differences between the changes under the two treatments were consequently small, but the observation that these small differences in carotid wall changes were paralleled by differences in the incidence of cardiovascular events (better intima-media thickness regression with verapamil paralleled by a lower cardiovascular event rate) suggests that even small effects on carotid plaques may have clinical and prognostic relevance.

Effect of verapamil on intimal thickening and vascular reactivity in the collared carotid artery of the rabbit

1. Intimal thickening is a common site for atherosclerosis. Therefore, we investigated whether the calcium entry blocker verapamil (10 mg kg-1 body weight day-1, s.c.) can retard intimal thickening and changes in vascular reactivity induced by a non-occlusive, silicone collar positioned around the left carotid artery of rabbits. The contralateral carotid artery was sham-operated and served as a control. 2. Verapamil and placebo (saline 0.1 ml kg-1, day-1, s.c.) treatments were initiated 7 days before placing the collar and lasted 3 weeks. Thereafter, segments were cut from collared and sham-treated arteries for histology and isometric tension recording. 3. The intima/media (I/M ratio increased after 14 days of collar treatment, but intimal thickening was not inhibited by verapamil (I/M ratio placebo 0.31 +/- 0.07, verapamil 0.32 +/- 0.09). 4. The collar decreased the capacity to develop force, as indicated by the response to a supramaximal concentration of KCl, decreased the sensitivity (pD2) to acetylcholine (ACh) and phenylephrine (Phe), but increased the sensitivity to 5-hydroxytryamine (5-HT). 5. Although verapamil did not affect intimal thickening, it normalized the hypersensitivity to 5-HT in collared arteries. 6. The contraction to the supramaximal concentration of KCl was not affected by verapamil. Verapamil decreased the Emax of ACh, but this was only seen in collar-treated arteries. Verapamil also decreased the sensitivity to ACh and Phe, in both sham- and collar-treated arteries. 7. We conclude that verapamil, without preventing thickening of the intima, can modify collar-induced changes in vascular reactivity.

Effects of efonidipine hydrochloride on cholesterol esterification mediated by beta-very low density lipoprotein in J774 macrophages

The effects of efonidipine hydrochloride (efonidipine), a dihydropyridine calcium antagonist, on the cholesterol ester metabolism induced by beta-migrating very low density lipoprotein (beta-VLDL) in J774 macrophages were studied. The cholesteryl ester content in the macrophages was increased by incubation with beta-VLDL, and the increase was inhibited by efonidipine. Oleic acid incorporation into cellular cholesteryl ester was increased by beta-VLDL in J774 macrophages. The incorporation at an early phase of beta-VLDL induction (0-3 hr) was inhibited by efonidipine. This inhibitory effect of efonidipine was greater at an early phase of beta-VLDL induction (0-3 hr) than at a late phase of the induction (8-11 hr). Pretreatment of the cells with efonidipine enhanced the inhibitory effect. Efonidipine also inhibited beta-VLDL degradation but not the binding and association in macrophages without pretreatment. beta-VLDL binding and association to macrophages were decreased by pretreatment of the cells with efonidipine. beta-VLDL metabolism was also decreased by dibutyryl cyclic AMP pretreatment. The decrease of beta-VLDL metabolism by efonidipine was prevented by co-treatment with efonidipine and HA1004, a protein kinase A inhibitor. Furthermore, efonidipine increased the intracellular cyclic AMP content in J774 macrophages. These findings suggest that efonidipine suppresses cholesterol ester deposition in atherosclerotic foam cells by inhibiting the modified lipoprotein metabolism and cholesterol esterification mainly through elevation of the cellular cyclic AMP level.

Effect of verapamil and nifedipine on cholesteryl ester metabolism and low-density lipoprotein oxidation in macrophages

Using mouse macrophage cultures, the effects of verapamil and nifedipine on cholesteryl ester and low-density lipoprotein (LDL) metabolism were studied with special reference to the following parameters: (a) incorporation of [14C]oleate into cholesteryl esters (ChE), (b) contents of total and free cholesterol (FCh), (c) liberation of [14C]oleate from ChE and incorporation of [3H]FCh into ChE, (d) excretion of [3H]Ch from the cells, and (e) LDL oxidation. Verapamil and nifedipine (10-100 microM) were shown to decrease in a dose-dependent manner the incorporation of [14C]oleate into ChE and to increase the concentration of FCh but had no appreciable effect on the concentration of total cholesterol in macrophages cultured in the presence of acetylated LDL. The drugs stimulated the liberation of [14C]oleate from cellular ChE. The pharmacological concentrations (25-75 microM) of verapamil and nifedipine increased the excretion of [3H]FCh from ChE of macrophages in the presence of serum and high-density lipoproteins. The same concentrations of the drugs inhibited both LDL-derived malonyldialdehyde-like products and nitroblue tetrazolium dye reduction in a dose-dependent fashion. The results obtained suggest that verapamil and nifedipine exert their macrophage-mediated antiatherosclerotic effect via reduction of LDL oxidative modification, reduction of intracellular ChE synthesis, stimulation of ChE hydrolysis and cholesterol excretion from the cells.

Calcium channel blockers and coronary atherosclerosis: from the rabbit to the real world

Many calcium channel blockers have been shown to retard the development of atherosclerosis in cholesterol-fed rabbits. The mechanisms that may contribute to this effect include stimulation of cholesteryl ester hydrolase activity in smooth muscle cells, amelioration of hypercholesterolemic-induced endothelial dysfunction, or inhibition of smooth muscle cell proliferation and migration. The effect of calcium channel blockers on the evolution of coronary atherosclerosis in humans has been assessed in three clinical trials. In the Montreal Heart Institute trial, nicardipine did not influence the overall rate of progression and regression; however, patients treated with nicardipine experienced significantly less progression of minimal lesions, defined as stenoses of less than or equal to 20% severity. In the International Nifedipine Trial on Antiatherosclerotic Therapy (INTACT), nifedipine had no effect on overall progression and regression but, by one method of analysis, reduced the rate of appearance of new coronary lesions. In a preliminary report, diltiazem prevented the development of coronary atherosclerosis in heart transplant recipients. These studies indicate that calcium channel blockers retard the development of early atherosclerosis not only in animal models but also in human coronary arteries. Other studies recently completed or now under way will help to clarify the clinical role of calcium channel blockers in antiatherosclerotic therapy.

Effects of nicorandil on cell proliferation and cholesteryl ester accumulation in arterial smooth muscle cells in culture

Ca2+ regulates a variety of cellular mechanisms in vascular cells as well as in platelets. Nicorandil interacts with the intracellular Ca(2+)-activated processes in vascular smooth muscle cells, while Ca2+ channel blockers such as verapamil and diltiazem block voltage-dependent Ca2+ channels. The effects of nicorandil are due to the hyperpolarization of the membrane, interference with mobilization of Ca2+ from the intracellular storage sites, and blockade of receptor-operated Ca2+ channels. In the present study, the effects of nicorandil on cell proliferation and cholesteryl ester accumulation in rat arterial smooth muscle cells in culture were compared to Ca2+ channel blockers. Smooth muscle cells were prepared from rat thoracic aorta, and the rate of proliferation was determined by measuring the cell number and by [3H]-thymidine incorporation into cellular DNA. The effect of nicorandil on cholesteryl ester content in smooth muscle cells was determined by thin-layer chromatography of the cell extracts. Nicorandil at concentrations of 10(-6) to 10(-4) M, as well as Ca2+ channel blockers (verapamil and diltiazem) inhibited the proliferation and DNA synthesis of cultured smooth muscle cells. The acute inhibitory effects on cell proliferation were observed significantly 16 hours after the addition of the three agents in serum-stimulated cells. These effects were dose dependent, both in acute and in chronic treatment with the three agents. Addition of 10(-5) M nicorandil to medium supplemented with 10% serum resulted in a decrease of the net cholesteryl ester content by 18 +/- 1%, while cellular free cholesterol content was the same as control. Similar results were also obtained in the presence of verapamil and diltiazem.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effect of clentiazem (TA-3090), a new calcium antagonist, on balloon catheter-induced intimal thickening of rabbit aorta

Male Japanese white rabbits were fed a restricted amount (100 g/head/day) of an atherogenic diet containing 0.2% cholesterol and 6% peanut oil during an 8-week experimental period. Atherosclerotic lesions, characterized by intimal thickening with lipid deposition, were produced by de-endothelialization of the rabbit aorta with a 4 F balloon catheter halfway through the experiment. Clentiazem (TA-3090), a new calcium antagonist, was administered at an oral dose of 30 mg/kg/day for 4 weeks starting on the day of deendothelialization. Clentiazem significantly depressed the intimal thickening without any effect on serum lipid levels. Clentiazem (1, 3, and 10 microM) significantly and dose-dependently inhibited the in vitro proliferation of smooth muscle cells that had been explanted from the neointima of the deendothelialized aorta. At a higher concentration, this drug markedly inhibited collagen-induced aggregation of rabbit platelets. Diltiazem also showed similar effects, but the effects of clentiazem were more potent than those of diltiazem. These results suggest that clentiazem exhibits an antiatherogenic effect, at least partly through prevention of smooth muscle cell proliferation in atheromatous lesions, in addition to its hypotensive action.

A preliminary study of diltiazem in the prevention of coronary artery disease in heart-transplant recipients

BACKGROUND

Accelerated coronary artery disease is a major cause of late morbidity and mortality among heart-transplant recipients. Because calcium-channel blockers can suppress diet-induced atherosclerosis in laboratory animals, we assessed the efficacy of diltiazem in preventing coronary artery disease in transplanted hearts.

METHODS

Consecutive eligible cardiac-transplant recipients were randomly assigned to receive diltiazem (n = 52) or no calcium-channel blocker (n = 54). Coronary angiograms obtained early after cardiac transplantation and annually thereafter were used for the visual assessment of the extent of coronary artery disease. The average diameters of identical coronary artery segments were measured on the angiograms obtained at base line and at the first and second follow-up examinations.

RESULTS

In the 57 patients who had all three angiograms, the average coronary artery diameter (+/- SD) 0.27 decreased in the group that received no calcium-channel blocker from 2.41 +/- 0.27 mm at base line to 2.19 +/- 0.28 mm at one year, and to 2.22 +/- 0.26 mm at two years (P < 0.001 for both years). The average diameter in the diltiazem group changed little from the base-line value of 2.32 +/- 0.22 mm (2.32 +/- 0.27 mm at one year and 2.36 +/- 0.22 mm at two years). The average change in the diameter of the segment differed significantly between the two treatment groups (P < 0.001), and the estimated effect of treatment changed only negligibly after adjustment for other relevant clinical variables. New angiographic evidence of coronary artery disease developed in 14 patients not given calcium-channel blockers, as compared with 5 diltiazem-treated patients (P = 0.082). Coronary stenoses greater than 50 percent of the luminal diameter developed in seven patients not given calcium-channel blockers, as compared with two patients given diltiazem; death due to coronary artery disease or retransplantation occurred in five patients in the group that did not receive calcium-channel blockers and none of those who received diltiazem.

CONCLUSIONS

Our preliminary results suggest that diltiazem can prevent the usual reduction in the diameter of the coronary artery in cardiac-transplant recipients, but further follow-up will be required to determine whether diltiazem can decrease the long-term incidence of symptomatic coronary artery disease.

Effect of verapamil on atherosclerosis

Whether antihypertensive agents exert an antiatherosclerotic effect by blood pressure reduction or independently of their antihypertensive effect is clinically relevant. Animal studies have generally shown that the calcium antagonist verapamil has a preventive rather than a therapeutic antiatherosclerotic effect, which is independent of its antihypertensive effect. However, doses used in animal studies were much higher than those administered to humans and, in animals, the time of administration of verapamil coincided with the application of atherogenic stimulus. Human studies have given controversial results. Verapamil appears to effectively reduce the restenosis rate after coronary angioplasty. However, in patients with coronary stenosis who were undergoing bypass surgery, results were conflicting: a retrospective study provided positive results, while a prospective study gave negative results. An ongoing study investigating the effect of verapamil on the carotid arteries of hypertensive patients could help clarify the relationship between blood pressure reduction and the progression, regression or development of carotid lesions.

Concept of an antiatherosclerotic efficacy of calcium entry blockers. INTACT Investigators

Animal experiments suggest an inhibitory effect of calcium entry blockers on arterial calcinosis and the formation of atherosclerotic plaques. Experiments with isolated tissues suggest various mechanisms for an antiatherosclerotic effect of calcium entry blockers. INTACT, the International Nifedipine Trial on Antiatherosclerotic Therapy, is the first study investigating, with a prospective, placebo-controlled, randomized, double-blind design, the influence of a calcium entry blocker (nifedipine 80 mg/day) on the progression of coronary atherosclerosis in patients with proven coronary artery disease. Study endpoints were changes of established coronary stenoses (diameter reduction greater than or equal to 20%), as well as the formation of new stenoses as documented by coronary angiography. Standardized coronary angiograms were taken before and after a treatment period of 3 years. The angiograms were quantitatively analyzed with the computer-assisted edge detection system CAAS. Of the 425 patients included in the study, 282 patients (134 on nifedipine and 148 on placebo) revealed no protocol violations. In the inclusion angiograms of these patients, 893 coronary stenoses were detected which were not significantly influenced in their development by nifedipine. However, 196 entirely new coronary lesions, 185 stenoses and 11 occlusions, were found in the follow-up angiograms. There were 78 lesions in 54 patients (40%) on nifedipine (0.58 new lesions/patient) and 118 lesions in 73 patients (49%; n.s.) on placebo (0.8 new lesions/patient; p = 0.031). In two other studies on the inhibiting effect of dihydropyridine calcium entry blockers on the progression of coronary artery disease in man defining angiographic endpoints, the drugs were also shown to reduce the number of newly formed significant coronary lesions. If further trials in man confirm a protective role of calcium entry blockers against the formation of atherosclerotic coronary lesions, a new strategy in the prevention of coronary artery disease has to be considered.

The antiatherogenic effects of antihypertensive drugs: experimental and clinical evidence

The question whether some classes of antihypertensive drugs have an antiatherogenic action independent of the antihypertensive one has been investigated through a large series of experimental studies; on the other hand, clinical evidence is still rather scanty. Most experimental investigations have shown a significant antiatherogenic action of the antihypertensive compounds, but only when the drug is administered simultaneously with the atherogenic stimulus (mostly cholesterol feeding). When the drug is administered weeks or months after the beginning of atherosclerosis (as in the Watanabe heritable hyperlipemic rabbit), no antiatherogenic effect has been shown, with a single exception. Although the most numerous data available are on calcium antagonists, almost all antihypertensive classes tested have shown some favorable effect on some experimental model. The few clinical studies completed so far as on coronary patients; they have provided arteriographic evidence that various calcium-antagonists have little effect on well-developed lesions and display a beneficial action on new or early lesions only. Ultrasound studies are now underway to compare the effects of calcium-antagonists and diuretics on asymptomatic carotid atherosclerotic lesions in hypertensive patients.

A controlled clinical trial to assess the effect of a calcium channel blocker on the progression of coronary atherosclerosis

To determine whether calcium channel blockers influence the progression of coronary atherosclerosis, 383 patients age 65 years or less with 5-75% stenoses in at least four coronary artery segments were selected at random within 1 month of coronary arteriography to participate in double-blind therapy with a placebo or nicardipine 30 mg three times daily. Coronary events (5 deaths, 22 myocardial infarctions, and 28 unstable anginas) occurred in 28 of 192 nicardipine patients and 23 of 191 placebo patients (p = NS). At 24 months coronary arteriography was repeated in 335 patients. Progression, defined as a 10% or more worsening in diameter stenosis, measured quantitatively, was found in 147 of 1,153 lesions (12.7%) in 168 nicardipine patients and in 170 of 1,170 lesions (14.5%) in 167 placebo patients (p = NS). Ninety-two nicardipine patients (55%) and 95 placebo patients (57%) had progression at one or more sites (p = NS). Regression, that is, an improvement by 10% or more in diameter stenosis, was seen in 140 of 2,323 lesions (6.0%) overall, with no significant intergroup difference. Among the 217 patients with 411 stenoses of 20% or less in the first study, such minimal lesions progressed in only 15 of 99 nicardipine patients compared with 32 of 118 placebo patients (15% versus 27%, p = 0.046). In this subgroup, 16 of 178 minimal lesions in nicardipine patients and 38 of 233 minimal lesions in placebo patients progressed (p = 0.038). By stepwise logistic-regression analysis, baseline systolic blood pressure (p = 0.04) and the change in systolic blood pressure between baseline and 6 months (p = 0.002) correlated with progression of minimal lesions. This suggested blood pressure reduction may account for the beneficial action of nicardipine. These results suggested nicardipine has no effect on advanced coronary atherosclerosis but may retard the progression of minimal lesions.

Nifedipine reduces atherogenesis in cholesterol-fed heterozygous WHHL rabbits

We have developed a new model to study the interaction between diet and genetics in atherogenesis, the cholesterol-fed heterozygous WHHL rabbit. To determine the effects of calcium blockers on atherosclerosis in this model, two groups of heterozygous WHHL rabbits were fed 0.25% cholesterol and 2% peanut oil with (n = 6) and without (n = 6) oral nifedipine (40 mg/kg/day) for 16 weeks. Body weights, serum cholesterol, triglycerides and calcium, and blood pressures were not significantly different between the 2 groups during the study period. Heterozygous WHHL rabbits in the nifedipine group had less aortic surface area with sudanophilic lesions (23 +/- 15% vs. 62 +/- 18%, P less than 0.01) and fewer segments of coronary arteries with lesions (19 +/- 9% vs. 35 +/- 8%, P less than 0.02). Total aortic cholesterol, phospholipid, and calcium were also reduced in nifedipine-treated rabbits compared with untreated animals. We conclude that nifedipine reduced atherosclerosis in this model. Although the mechanism is unknown, it is apparent that nifedipine acts independently of changes in plasma lipids and blood pressure.

Antiatherogenic effects of calcium-channel blockers: possible mechanisms of action

Calcium-channel blockers (Ca blockers), such as nifedipine, verapamil, diltiazem, flunarizine, and their respective derivatives, have been reported to suppress the formation of arterial lesions in animals fed atherogenic diets. The fact that structurally unrelated Ca blockers exert similar antiatherogenic effects may suggest that the drugs act by a calcium-channel-dependent mechanism. However, in cell culture experiments in which putative antiatherosclerotic effects were observed only in the presence of a very high drug concentration (greater than 10 microM), calcium-channel-independent mechanisms are likely. It does not appear that Ca blockers act predominantly by altering coronary risk factors such as arterial pressure or hypercholesterolemia. On the other hand, current evidence is accumulating that Ca blockers may act by suppressing chemotaxis and the proliferation of cells involved in lesion formation. Recent reports indicate that relatively low concentrations (less than 1 microM) of nifedipine may promote the release of cholesterol from fat-laden smooth cells and macrophages. Controlled clinical trials are needed to determine whether Ca blockers have utility in the prevention of the progression of atherosclerosis in humans.

Quantitative analysis of antiatherosclerotic effect of nifedipine in cholesterol-fed rabbits

Reports concerning the effect of slow calcium-channel blockers on experimental atherosclerosis are controversial. We examined the antiatherosclerotic effect of nifedipine (40 mg/day for 16 weeks) on aorta of rabbits on diets containing 0.3%, 0.5%, and 1.0% cholesterol. There were no significant differences in levels of serum lipids with or without nifedipine in the same cholesterol-fed rabbits. The results obtained show that nifedipine suppressed the extent of lipid deposition and surface involvement (S.I.) in aorta in 0.3% cholesterol-fed rabbits, whereas nifedipine only tended to suppress S.I. in 0.5% cholesterol-fed rabbits and had no effect in 1.0% cholesterol-fed rabbits. The log dose-response relationship of S.I. was obtained by plotting the concentration of cholesterol in the feed or the "integrated value" of the total serum cholesterol (TC), i.e., the cumulative sum of the serum TC values obtained at each week. The log dose-response curve was shifted in parallel with the right in nifedipine groups. The Lineweaver-Burk plot constructed from the dose-response curve had the same points crossing the ordinate with or without nifedipine. These results suggested that nifedipine suppressed S.I. in a competitive manner with cholesterol on the specific binding site of lipid deposition. Electron-microscopic findings also demonstrated that fat droplets in smooth muscle cells, extracellular matrix containing collagen, and elastic fibers decreased in nifedipine-treated rabbits.

Reduction of intimal hyperplasia and enhanced reactivity of experimental vein bypass grafts with verapamil treatment

Recent studies have shown that calcium antagonists exert an antiatherogenic effect in animals fed cholesterol. Vein graft intimal hyperplasia is believed to be an early event in atherosclerotic lesion formation, which is a significant cause of graft failure. Altered vasoreactivity has also been postulated in the etiology of vein graft failure. Therefore this study examined the effect of verapamil treatment on the development of intimal hyperplasia and the vasoreactivity of experimental vein bypass grafts. The right external jugular vein was grafted into the right carotid artery of 30 male New Zealand white rabbits fed normal rabbit chow. The left external jugular vein was used as the control vein. Fifteen animals received verapamil (1.25 mg/day for 28 days) via the femoral vein by means of an osmotic pump. In 15 control animals the pump contained saline. Plasma verapamil concentration was 50.9 +/- 13.2 ng/mL (x +/- SEM), a dose that showed no effect on either blood pressure, total serum cholesterol, or in vitro platelet aggregation to ADP. Fourteen of fifteen grafts were patent in each group, for a patency rate of 93%. Histologic examination using computer morphometry showed significant reduction of intimal hyperplasia at the proximal, middle, and distal graft segments (p less than 0.05). In addition in vitro isometric tension studies of the vein grafts and control veins showed that verapamil causes enhanced reactivity of both vein grafts and control veins in response to norepinephrine and histamine (p less than 0.05). Reactivity of vein grafts to serotonin was unaltered. While none of the normal veins in the control group responded to serotonin, normal veins treated with verapamil contracted readily in response to serotonin. Endothelial-dependent relaxation to acetylcholine was absent in both control and verapamil-treated vein grafts, while normal veins from both groups responded to the same extent to acetylcholine. Because we could not demonstrate any difference in platelet or endothelium function between untreated and verapamil-treated animals, we examined the direct effect of verapamil on smooth muscle. Verapamil significantly inhibited [3H]-thymidine incorporation into DNA in vascular smooth muscle cells in culture in a dose-dependent manner. Verapamil treatment significantly reduces intimal hyperplasia in experimental vein grafts and inhibits smooth muscle cell proliferation in culture. Furthermore the enhanced reactivity to norepinephrine and histamine in the verapamil-treated vessels has no detrimental effect on the patency rate at 4 weeks. Thus by inhibiting intimal hyperplasia, calcium antagonists may improve the long-term patency of vein bypass grafts.

In vitro inhibition of arterial myocyte growth and stimulation of low density lipoprotein metabolism by SIM 6080, a new calcium antagonist

We have investigated the in vitro effect of the new calcium antagonist SIM 6080 on proliferation of rat aortic smooth muscle cells and on LDL receptor-mediated catabolism in human fibroblasts. Verapamil was used as the reference compound. SIM 6080 inhibited the proliferation of rat aortic myocytes in concentrations ranging between 1 and 20 microM. The inhibition, evaluated as cell number and nuclear incorporation of [3H]thymidine, was dose and time dependent; the cell doubling time increased with drug concentrations up to 69 h versus 20 h for controls. Similar results on both LDL pathway and smooth muscle cell proliferation were achieved with verapamil, but higher concentrations were needed. The specific uptake and degradation of 125I-LDL was evaluated in human fibroblasts after 48 h incubation with SIM 6080 (1-10 microM). The compound dose dependently enhanced the receptor-mediated 125I-LDL uptake, with a fourfold increase as a maximal effect (10 microM); LDL degradation was less sensitive to the drug. The present results provide evidence that the new calcium antagonist SIM 6080 interferes in vitro with processes involved in atherogenesis.

Noninvasive assessment of lipid disposition in treated and untreated atherosclerotic rabbits

In an effort to visualize whole body cholesteryl ester (CE) deposition using the nuclear medicine imaging technique of gamma camera scintigraphy, 125I-cholesteryl iopanoate (125I-CI), a nonhydrolyzable CE analogue, was used as a marker for CE deposition in atherosclerotic New Zealand white rabbits. Groups of animals were fed either a cholesterol-enriched diet (2%, w/w) or the same diet supplemented with the hypolipidemic drugs colestipol (1%, w/w) and/or clofibrate (0.3%, w/w). Injections of 125I-CI were administered biweekly. At the end of 15 weeks, animals were scintigraphically scanned and sacrificed for tissue analysis. The results demonstrated that while drug treatment had no significant effect on plasma lipid levels, it substantially lessened atherosclerotic involvement in the thoracic-abdominal aorta. These differences in aortic lipid accumulation were reflected in the whole-body scans which showed a reduction in tissue accumulation of 125I-CI in the drug-treated groups. Gamma camera scintigraphy thus represents a rapid means of visualizing tissue CE accumulation which could facilitate the evaluation of lipid-lowering drug efficacy and possible antiatherosclerotic effect.

Effects of calcium antagonists on lipids and atherosclerosis

The arterial accumulation of cholesterol and calcium is a hallmark of atherosclerosis. Calcium antagonists (CAs) lessen the severity of experimentally induced atherosclerosis in cholesterol-fed animals. The reduction of aortic cholesterol is one of the most striking findings. This effect is achieved without a reduction of plasma lipid or blood pressure, and is probably related to an interference of CAs with lipid metabolism in the arterial wall. To what extent these properties of CAs are due to their ability to block calcium channels still remains to be addressed. This report briefly discusses the available in vivo and in vitro evidence for the antiatherosclerotic properties of CAs, and outlines the possible mechanisms by which these compounds affect cellular lipid metabolism.

Antiatherogenic properties of calcium antagonists. State of the art

Atherosclerosis is an arterial disease characterized by localized accumulation of collagen, elastin, lipids, and calcium at sites associated with macrophage infiltration and altered smooth muscle metabolism. Studies in several types of animal models, especially cholesterol-fed rabbits, have shown that calcium competitors, calcium chelators, anticalcifying agents, and calcium antagonists can reduce the accumulation of atherogenic lesion components and decrease the progression of lesions. Although there are some conflicting data in the animal model studies, it is now apparent that several classes of calcium antagonists inhibit the progression of early arterial lesions induced by cholesterol-feeding in animals. The dihydropyridine class of calcium antagonists may be more potent as anti-atherosclerotic agents than the other classes. Mechanisms involving regulation of endothelial cell, smooth muscle cell, and macrophage metabolism may be responsible for the effects of calcium antagonists on early lesion progression. Recent studies in cell culture-model systems suggest that calcium antagonists may significantly alter activities that regulate lipoprotein-derived cholesterol accumulation by arterial wall cells. Some of these activities are independent of calcium flux across voltage-operated calcium channels. Thus, calcium antagonists may reduce the progression of atherogenic lesions by a combination of decreasing calcium accumulation within arterial wall cells and by altering calcium channel-independent metabolic activities, which affect lesion development.

Reduced atherogenesis in cholesterol-fed diabetic rabbits. Giant lipoproteins do not enter the arterial wall

In cholesterol-fed rabbits, alloxan-diabetes has an anti-atherogenic effect, which is associated with severe elevation of plasma triglyceride concentrations. To study this effect, we measured lipoprotein sizes and aortic permeability coefficients for cholesteryl ester and for albumin in hypertriglyceridemic diabetic cholesterol-fed rabbits and in normotriglyceridemic cholesterol-fed rabbits. With the same high cholesterol concentration in plasma, hypertriglyceridemic diabetic rabbits had 70% of plasma cholesterol in very large lipoproteins (diameter greater than 75 nm), whereas normotriglyceridemic rabbits had only about 10% of plasma cholesterol in these giant lipoproteins. The aortic permeability coefficients for cholesteryl ester in hypertriglyceridemic diabetic cholesterol-fed rabbits was only 10% to 50% of that in normotriglyceridemic cholesterol-fed rabbits. Aortic permeability coefficients for albumin did not differ significantly between the hypertriglyceridemic and normotriglyceridemic rabbits. The results suggest that the large size of a major fraction of plasma lipoproteins in the hypertriglyceridemic diabetic cholesterol-fed rabbits is responsible for the relatively low aortic permeability coefficient for cholesteryl ester from plasma and hence for reduced atherogenesis in these animals.

Dietary cholesterol lowers liver copper in rabbits

Atherosclerosis and hypercholesterolemia have been produced in rabbits since 1913 by feeding them cholesterol. These experiments have a great influence on current thinking about the etiology and possible prevention of ischemic heart disease. Male, New Zealand White rabbits were fed 0.5% dietary cholesterol. Cholesterol and copper in plasma increased sixty-fold and 50%, respectively. Liver copper decreased 74% and hematocrit decreased 26%. Iron was unchanged in heart and liver, but was increased in kidney. Zinc was decreased in heart, but was unchanged in liver or kidney. Changes in organ iron and zinc were smaller than the decrease in liver copper. Similar experiments with higher doses of dietary cholesterol may have resulted in copper deficiency. It may be appropriate to revise interpretations of data from these experiments and to reformulate hypothesis based on the data. Results are consonant with the theoretical implication of copper metabolism and copper deficiency in the etiology and pathogenesis of ischemic heart disease.

Calcium antagonists as antiatherogenic agents

Pharmacological manipulations of calcium metabolism in animals fed high-fat diets may slow the progression of atherosclerosis. Agents with calcium-chelating (diphosphonic acid and thiophene carboxylic acid derivatives) or calcium-channel-blocking properties (dihydropyridine derivatives, verapamil and its derivatives, diltiazem) have been demonstrated to suppress atherogenesis in rabbits and monkeys. The antiatherosclerotic effects appear to occur in the absence of hypolipidemic effects. The mechanisms of action of the structurally unrelated drugs remain to be elucidated.

Inhibitory effect of calcium antagonists on balloon catheter-induced arterial smooth muscle cell proliferation and lesion size

Calcium antagonists inhibit atherogenesis in the cholesterol-fed rabbit without producing hypolipidaemia, suggesting a direct action on the arterial wall. In this study, the effects of several calcium antagonists on the myoproliferative response to balloon catheter injury of the aorta have been investigated in normolipidaemic rats and rabbits. The incorporation of [3H]thymidine into rat aortic DNA 48 h after balloon injury was markedly reduced by twice daily oral administration of nifedipine, verapamil, diltiazem or lanthanum. DNA synthesis in other proliferating tissues was unaffected. Twice daily oral administration of prazosin or minoxidil, antihypertensive agents that are not calcium antagonists, also selectively reduced arterial DNA synthesis. In balloon catheterised rabbits twice daily oral administration of nifedipine (10 mg/kg) caused a 39% reduction in the cross-sectional area of the neo-intima 14 days after injury. These results show that nifedipine and other antihypertensive agents inhibit smooth muscle cell proliferation.

Long-term effects of verapamil on aortic smooth muscle cells cultured in the presence of hypercholesterolemic serum

Smooth muscle cells derived from rabbit and bovine aorta were cultured for up to 5 weeks in the presence of d less than 1.019 g/ml fraction of hypercholesterolemic rabbit serum. When this fraction was added to serum containing culture medium, there was a significant increase in DNA, protein, and cholesteryl ester per dish. Addition of 50 microM verapamil markedly reduced the stimulatory effect of the d less than 1.019 g/ml fraction on both DNA and protein content per dish. The effect of verapamil on cholesteryl ester content was more complex: there was an increase within the first week, but later the net accumulation of cholesteryl ester per dish was lower than in untreated dishes. The recovery of less DNA in verapamil-treated dishes was not due to increased cell loss, as evidenced by retention of a residualizing marker, 3H-cholesteryl linoleyl ether. Moreover, verapamil did reduce incorporation of 3H-thymidine into DNA. In verapamil-treated dishes, there was flattening and a cobblestone appearance of the cells. A hypothesis is proposed to explain the inhibitory effect of verapamil on the development of atheroma formation in cholesterol-fed rabbits: Assuming that macrophages play an active role in cholesteryl ester removal from atheroma, verapamil, which reduces lysosomal cholesteryl ester hydrolysis in macrophages, would permit the lipid-laden macrophage to remove more cholesteryl ester per cell from the arterial wall. In addition, the presently reported results support the possibility that verapamil may impede the development of atheroma formation by reduction of smooth muscle cell proliferation.