Significance of quiescent smooth muscle migration in the injured rat carotid artery

Angiotensin II induces smooth muscle cell proliferation in the normal and injured rat arterial wall

The present study was undertaken to explore the possibility that neointimal smooth muscle cells, the characteristic cells of restenosis and atherosclerosis, are selectively stimulated to replicate by a hypertensive stimulus. Angiotensin II (AII) was infused by osmotic minipumps for 2 weeks in 4.5-month-old rats. Group A received AII (200 ng/min) 2 weeks after a balloon catheter-induced injury of the thoracic aorta and left common carotid artery. Group B received only AII, group C only balloon denudation, and group D neither balloon injury nor AII. During the AII or Ringer's solution infusion, all animals received [3H]thymidine via a second minipump to measure DNA synthesis. AII increased the systolic pressure by more than 40 mm Hg. AII significantly increased DNA synthesis in the media of the carotid artery from 0.2 +/- 0.2% in group C to 2.5 +/- 1.5% in group A (mean +/- SD, n = 5 or 6). DNA synthesis in the neointima of the carotid artery significantly increased with AII from 4.8 +/- 4.2% in group C to 19.8 +/- 13.9% in group A. Cross-sectional area of the neointima almost doubled during AII infusion, and it increased approximately 25% in the media. Comparable results were obtained in the aorta. In a second experiment, AII was infused (125 ng/min) for 2 weeks in 11-week-old rats. Concomitantly, [3H]thymidine was given. Control rats received Ringer's solution and [3H]thymidine in their pumps. Blood pressures were elevated to the same extent as in the older animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Restenosis after percutaneous transluminal coronary angioplasty: pathologic observations in 20 patients

Histopathologic examination was performed in 20 patients undergoing antemortem coronary angioplasty. Thirty-four lesions were dilated and the interval between coronary angioplasty and death ranged from several hours to 4 years. Intimal proliferation of smooth muscle cells, as a major cause of restenosis, was observed in 83% to 100% of 28 lesions examined 11 days to 2 years after coronary angioplasty. In 20 lesions examined within 6 months, proliferating smooth muscle cells were predominantly of the synthetic type and there was abundant extracellular matrix substance chiefly composed of proteoglycans. In eight lesions examined between 6 months and 2 years, contractile type smooth muscle cells were dominant and extracellular matrix was composed chiefly of collagen. In three lesions examined after 2 years, evidence of antemortem coronary angioplasty was hardly identifiable and these lesions were almost indistinguishable from conventional atherosclerotic plaque. These temporal changes in histologic pattern provide a pathologic background for clinical reports that restenosis is predominantly found within 6 months after coronary angioplasty. Morphometric analysis revealed that the extent of intimal proliferation was significantly greater in lesions with evidence of medial or adventitial tears than in lesions with no or only intimal tears.

Secretion of a potent new migration factor for smooth muscle cells (SMC) by cultured SMC

Migration of smooth muscle cells (SMC) in the arterial wall is important in the formation of intimal thickening. In this work, cultured SMC from the rat and rabbit aortic media at 2nd to 12th passages were found to secrete a potent migration factor for SMC which was named SMC-derived migration factor (SDMF). This factor stimulated the migration of SMC dose-dependently and its maximum activity was 2-8 times that of PDGF. Checker board analysis showed that SDMF was chemotactic, but not chemokinetic. In further studies, SDMF was found to be inactivated at 100 degrees C for 10 min or by trypsinization, but not inactivated by mercaptoethanol. This factor was not dialyzable. Molecular weight was approximately 500 kDa by a gel filtration. The activity was not inhibited by an anti-PDGF antibody or a fibronectin antiserum. These data suggest that SDMF is a potent migration factor for SMC and that SDMF is distinct from PDGF, fibronectin or other known migration factors. This autocrine system of secretion of SDMF by SMC and its induction of SMC migration may contribute to intimal thickening of the arterial wall in atherosclerosis.

1,25 (OH)2 vitamin D3 attenuates the loss of resistance artery contractile function associated with incubation in culture media

Contractile properties of rat mesenteric resistance arteries were studied immediately after isolation or after 48-hr incubation in culture medium [Dulbecco's modified Eagle's/Ham's F-12 (1:1) with insulin, transferrin and antibiotics]. Incubation in culture medium depressed active stress generating capacity, increased sensitivity to norepinephrine and ablated endothelium-dependent relaxation. The decrease in stress generation results from the loss of a releasable pool of intracellular Ca2+; the enhanced sensitivity is associated with decreased neuronal amine pump function. Addition of 300 pg/ml 1,25 (OH)2 vitamin D3 to the culture medium afforded nearly complete protection against the loss of stress generating capacity and partially preserved endothelial function. It is concluded that 1,25 (OH)2 vitamin D3 partially prevents phenotypic modulation of the vascular myocyte induced by culture conditions.

Cinematographic analysis of vascular smooth muscle cell interactions with extracellular matrix

The interactions of vascular smooth muscle cells with growth modulators and extracellular matrix molecules may play a role in the proliferation and migration of these cells after vascular injury and during the development of atherosclerosis. Time-lapse cinematographic techniques have been used to study cell division and migration of bovine carotid artery smooth muscle cells in response to matrix molecules consisting of solubilized basement membrane (Matrigel) and type I collagen. When cells were grown adjacent to Matrigel, both migration and cell proliferation were increased and interdivision time was shortened. Cells grown in Matrigel or in type I collagen had markedly reduced migration rates but interdivision time was not altered. Further, diffusible components of the Matrigel were found to stimulate proliferation of the smooth muscle cells.

Restenosis after directional coronary atherectomy: differences between primary atheromatous and restenosis lesions and influence of subintimal tissue resection

Rates of restenosis were evaluated in 70 patients (74 lesions) after successful directional coronary atherectomy. The extent of vascular tissue resection was correlated with restenosis rates for coronary (n = 59) and vein bypass graft (n = 15) lesions. After 6 months, the overall restenosis rate was 50% (37 of 74 lesions); it was 42% (15 of 36 lesions) when intima alone was resected, 50% (7 of 14 lesions) when media was resected and 63% (15 of 24 lesions) when adventitia was resected. Subintimal tissue resection increased the restenosis rate for vein grafts (43% with intimal resection versus 100% with subintimal resection, p = 0.01) but not for coronary arteries (50% versus 48%). There was no overall difference in restenosis rates after atherectomy between primary lesions and restenosis lesions that occurred after balloon angioplasty (46% versus 54%). Among postballoon angioplasty restenosis lesions, a higher rate of restenosis after atherectomy was found with subintimal than with intimal resection (78% versus 32%, p = 0.01). Tissues from patients undergoing a second atherectomy for restenosis after initial atherectomy (n = 8) demonstrated neointimal hyperplasia that appeared histologically identical to restenotic tissue developing after balloon angioplasty (n = 37). These data suggest that the cellular response to directional coronary atherectomy is characterized by neointimal proliferation similar to that which may develop after balloon angioplasty. The extent of fibrous hyperplasia appears to be related to the depth of tissue resection in vein graft lesions and coronary artery restenosis lesions that occur after balloon angioplasty but not in primary atheromatous coronary artery lesions.

Myosin isoform expression and smooth muscle cell heterogeneity in normal and atherosclerotic rabbit aorta

Two monoclonal antimyosin antibodies, Western blotting experiments, and immunofluorescence procedures were used to investigate myosin isoform expression in normal and atherosclerotic aortas of adult rabbits. The SM-E7 antibody reacted with the two myosin heavy chain (MHC) isoforms of smooth muscle (SM) type (SM-MHC-1 and SM-MHC-2) expressed in the adult rabbit aorta. The NM-G2 antibody recognized an epitope shared by the nonmuscle (NM) myosin heavy chains (NM-MHC) present in fibroblasts, macrophages, lymphocytes, and platelets. Two smooth muscle cell (SMC) populations were identified in the medial layer of normal adult aorta, namely cells that contained SM myosin exclusively and cells that showed the coexistence of SM and NM myosin isoforms. The size of the cell population with double myosin isoform content increased markedly during experimental atherogenesis and represented by far the predominant SMC phenotype in the atherosclerotic plaque. Western blotting analysis performed on crude extracts from the atherosclerotic plaque showed the presence of SM-MHC-1 and NM-MHC isoforms in this tissue. Co-expression of SM and NM myosin at the molecular and the cellular level were found in aortic tissue during the early stages of development. These results indicate that in experimental atherosclerosis, the accumulation in the plaque of SMC with an "immature" pattern of myosin isoform expression is accompanied by similar modifications in the differentiation pattern of SMC of the underlying media.

PDGF ligand and receptor gene expression during repair of arterial injury

Smooth muscle cells (SMC) in rat carotid artery leave the quiescent state and proliferate after balloon catheter injury, but the signals for mitogenesis are not known. In this study, the possibility that cells within damaged arteries produce a growth factor that could act locally to stimulate SMC replication and repair was examined. We found that the genes for PDGF-A and -B (ligand) and PDGF receptor (alpha and beta subunits) were expressed in normal and injured carotid arteries and were independently regulated during repair of carotid injury. Two phases of PDGF ligand and receptor gene expression were observed: (a) In the early stage, a large decrease in PDGF beta-receptor mRNA levels preceded 10- to 12-fold increases in PDGF-A transcript abundance in the first 6 h after wounding. No change in PDGF alpha-receptor or PDGF-B gene expression was found at these times. (b) In the chronic phase, 2 wk after injury, neointimal tissue had lower levels of PDGF alpha-receptor mRNA (threefold) and higher levels of PDGF beta-receptor mRNA (three- to fivefold) than did restored media. Moreover, in situ hybridization studies identified a subpopulation of neointimal SMC localized at or near the luminal surface with a different pattern of gene expression than the underlying carotid SMC. Luminal SMC were strongly positive for PDGF-A and PDGF beta-receptor transcripts, while showing little or no hybridization for PDGF-B or PDGF alpha-receptor. Immunohistochemical studies showed strongly positive staining for PDGF-A in SMC along the luminal surface. These data show that changes in PDGF ligand and receptor expression occur at specific times and locations in injured carotid artery and suggest that these changes may play a role in regulating arterial wound repair.

Blood vessel structure and function: a brief update on recent advances

This article briefly reviews recent advances in knowledge of the histology and function of blood vessels. It focuses upon the multifunctional roles of endothelium and smooth muscle cells. Particular reference is made to the synthesis of a number of factors now known to be involved in maintenance of the integrity of the vessel wall and the initiation of arterial disease. The cells of the vascular wall are much more versatile and dynamic than previously thought.

Time course of smooth muscle cell proliferation in the intima and media of arteries following experimental angioplasty

Smooth muscle cell (SMC) proliferation is known to be an important factor for the development of restenosis after percutaneous transluminal coronary angioplasty. To determine the time course of intimal and medial SMC proliferation and morphological changes after experimental angioplasty, an intimal atheroma was produced with repeated weak electrical stimulations in the right carotid artery of 45 male New Zealand White rabbits. Angioplasty was subsequently performed in 35 rabbits, and the proliferative responses were analyzed with histomorphological and immunohistological criteria at 3, 7, 14, 21, 28, and 42 days after intervention. A hemodynamic relevant stenosis after angioplasty was found in eight (23%) of 35 dilated arteries. In five rabbits the stenosis was due to a mural thrombus, and in three animals restenosis was caused by intimal SMC proliferation. In all dilated arteries the intimal wall thickness increased from 13 +/- 5 intimal cell layers (after electrical stimulation) to 33 +/- 14 cell layers during 28 days after angioplasty (p less than 0.05). Later than 4 weeks after angioplasty, no additional increase of intimal thickening occurred. Application of bromodeoxyuridine 18 and 12 hours before excision of the vessels allowed determination of the percent of cells undergoing DNA synthesis in the intima and media using monoclonal antibody against bromodeoxyuridine. SMCs were identified by alpha-actin staining. Immunohistological quantification of intimal SMC proliferation showed a maximum of cells undergoing DNA synthesis within the first 7 days after angioplasty (p less than 0.01). In contrast, medial proliferation of SMCs was delayed and showed a small but significant increase 21 days after dilatation (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Arterial injury-induced smooth muscle cell proliferation in rats is accompanied by increase in polyamine synthesis and level

Proliferation of smooth muscle cells (SMC), enhancement of polyamine biosynthesis and increase in polyamine level in response to deendothelialization in the rat aorta were studied. [3H]Thymidine incorporation into SMC in aortas denuded with a balloon catheter began 25 h after injury, and maximal incorporation occurred 33-37 h after injury. Afterwards, [3H]thymidine incorporation declined, approaching the baseline level, but was slightly higher than that of sham-operated controls until 14 days after injury. Intimal thickening started 7 days after injury, and peaked at 21 days. Prior to these proliferative changes in aortic SMC, a rapid and transient increase in ornithine decarboxylase (ODC) activity was observed within 8 h after injury. There was no significant difference in ODC activity between injured and intact aortas after 4 days. The levels of polyamines, putrescine, spermidine, and spermine increased and were maximal at 48 h after injury, 8.1, 3.4 and 1.4 times the control levels, respectively. Increased levels of polyamines, in particular spermidine, continued until 7 days after injury. These results suggest that the enhancement of polyamine synthesis and the increased polyamine content of the aorta play important roles in the proliferation of SMC and in the development of intimal thickening, particularly in the initial proliferative response of medial SMC after deendothelialization.

Effects of blood coagulation factor XIII on the development of experimental cerebral aneurysms in rats

Pathological and experimental studies have shown that cerebral aneurysms develop in part as a result of injury to the blood vessel wall. One of the peculiar aspects of aneurysm development is a defective proliferative or healing response to such injury. To examine this phenomenon, blood coagulation Factor XIII, which is known to enhance the healing process of wounds in general, was given to rats to induce experimental cerebral aneurysms. The rats were subjected to ligation of one common carotid artery and induction of hypertension, and were fed beta-aminoproprionitrile. Two weeks thereafter, Factor XIII was injected intravenously daily for 5 days (10 U/100 gm body weight/day). Twelve days after the start of Factor XIII injections, the rats were sacrificed and examined under light and electron microscopy. In seven of 12 bifurcations which developed small aneurysms, prominent intimal thickening was observed in the aneurysm lumen. In the most advanced cases, the aneurysm lumen was completely filled with proliferated smooth-muscle cells and collagen. In five of nine bifurcations that showed no aneurysm development, apparent intimal thickening was found at the site where aneurysms might be expected to grow. In the group of rats studied for induction of cerebral aneurysms but not given Factor XIII, none of 11 bifurcations with or without aneurysms showed such intimal thickening. The results indicated that the proliferative response at the sites of aneurysm development was modified by exogenous Factor XIII.

Integrin receptors on aortic smooth muscle cells mediate adhesion to fibronectin, laminin, and collagen

Extracellular matrix receptors on vascular smooth muscle cells help in anchoring the cells during contraction and in promoting cellular migration after vessel injury. We found that rat aortic smooth muscle cells attach to surfaces coated with fibronectin, laminin, and collagen types I and IV. Cell attachment to these substrates appears to be mediated by members of the beta 1 integrin family of extracellular matrix receptors. Antibodies to the beta 1 subunit not only demonstrated the presence of integrin complexes in focal adhesion plaques but also blocked cell adhesion to the different substrates. Ligand-affinity chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis isolated a series of receptor complexes that were recognized by antisera to beta 1 integrin receptors. Each of the receptors appeared to be a heterodimer in which one of several alpha subunits shared a common 120-kDa (nonreduced) beta 1 subunit protein. The rat aortic smooth muscle cells had one alpha subunit (150 kDa nonreduced, 140 kDa reduced) that bound exclusively to fibronectin. There was a second alpha subunit (150 kDa nonreduced, 160 kDa reduced) that bound exclusively to collagen type I. In addition, there was a third alpha subunit (185 kDa nonreduced, 200 kDa reduced) that was promiscuous and bound to collagen types I and IV as well as to laminin; the 185-kDa alpha subunit appeared to bind to collagen more efficiently than it did to laminin. Thus, smooth muscle cells express multiple integrin receptors with different ligand specificities that appear to mediate cell interactions with the extracellular matrix.

Syndromes of accelerated atherosclerosis: role of vascular injury and smooth muscle cell proliferation

Vascular injury represents a critical initiating event in the pathogenesis of various vascular diseases, including atherosclerosis. This review discusses 1) the current understanding and a new pathologic classification of vascular injury; 2) the resultant cellular pathophysiologic responses, specifically, lipid accumulation, platelet aggregation, thrombus formation and smooth muscle cell proliferation; 3) the role of vascular injury in the pathogenesis of spontaneous and accelerated atherosclerosis; and 4) emerging therapeutic approaches in preventing these vascular diseases. The process of type I vascular injury (nondenuding functional injury) followed by lipid accumulation, monocyte and platelet adhesion, smooth muscle cell proliferation and resultant plaque formation represents the prevalent view of the early stages of spontaneous atherogenesis. The syndromes of accelerated atherosclerosis (namely, heart transplant atherosclerosis, coronary vein graft disease and restenosis after percutaneous transluminal coronary angioplasty) appear to share etiologic mechanisms with spontaneous atherosclerosis by means of the "response to injury" hypothesis. However, type II and type III vascular injury (denuding endothelial and intimal injury with or without medial damage) followed by thrombus and its organization by smooth muscle cell proliferation and subsequent fibrosis appear to be responsible for the vascular process. This accelerated and premature occlusive process accounts for significant morbidity and mortality in patients with these conditions. Better understanding of the nature of vascular injury and its pathophysiologic responses in these clinical situations may aid in developing therapeutic strategies for preventing these vascular diseases.

Effect of hyperlipidemic serum and irradiation on wound healing in primary quiescent cultures of vascular cells

After 8 weeks in culture, outgrowths from explants of aortic media of rhesus monkeys and New Zealand rabbits result in circular colonies of mature smooth muscle cells, quiescent in 10% serum. Such cultures were wounded by cutting out a 1.5-mm-wide strip. Migration of cells into the wound area was measured daily, and proliferation was assessed by [3H]thymidine incorporation. Migration began within 24 hr and at 7 days the defect was filled by proliferates of migrated cells. The cumulative labeling index was highest in the cells in the wound gap but was also increased in the remaining part of the culture. Wounding thus stimulated the uninjured portion of these primary cultures to proliferate, while in subcultures of these cells increase in [3H]thymidine incorporation was confined to the wound area. While hyperlipidemic serum has been shown to induce proliferation in unwounded cultures, it did not enhance cell replication elicited by wounding but reduced cell density and labeling index in the wound gap. Irradiation prior to wounding reduced cell proliferation to control values, while migration of cells was not significantly affected. In irradiated cultures, the inhibitory action of hyperlipidemic serum on cell migration became evident. Such quiescent cultures thus allow us to separate the effects of a specific injury on the proliferative and migratory responses of vascular smooth muscle.

Acute and long-term effects of tissue culture on contractile reactivity in renal arteries of the rat

To evaluate long-term effects of contractile and mitogenic stimuli on the contractile reactivity of arterial smooth muscle, we measured the incorporation of the thymidine analogue 5-bromo-2'-deoxyuridine (BrdUrd) and mechanical responses in arterial segments that had been maintained in tissue culture. The experiments were performed on renal arteries that had been isolated from adult rats, chemically sympathectomized, mechanically denuded from endothelium and mounted under distension. Exposure of arterial segments for up to 3 weeks to culture medium supplemented with fetal calf serum resulted in the following consecutive changes: a strong acute contraction, selective pharmacological changes that included decreased contractile responses to phenylephrine and vasopressin and increased relaxing responses to isoproterenol, increased incorporation of BrdUrd, a progressive fall in contractile responses to all vasoconstrictor stimuli, and an increase in excitability. Serum-free medium resulted in a much smaller acute arterial contraction, induced less incorporation of BrdUrd, accelerated the occurrence of hyperexcitability, but did not affect early pharmacological changes or the subsequent fall in overall arterial contractility with tissue culture. Dialysis of the serum or addition of ketanserin abolished the contractile effect of serum but did not affect the incorporation of BrdUrd or the loss of contractility with tissue culture. Addition of serotonin to serum-free culture medium mimicked the contractile response to serum but not the stimulation of BrdUrd incorporation. These data indicate that tissue culture alters the properties of the arterial wall, that contraction does not underlie the proliferative response of arterial smooth muscle to serum-derived mitogens in vitro, and that stimulation of DNA synthesis does in itself not lead to selective changes in arterial contractility.

Inhibitors of angiotensin-converting enzyme prevent myointimal proliferation after vascular injury

The role of a local angiotensin system in the vascular response to arterial injury was investigated by administering the angiotensin-converting enzyme (CE) inhibitor cilazapril to normotensive rats in which the left carotid artery was subjected to endothelial denudation and injury by balloon catheterization. In control animals, by 14 days after balloon injury, the processes of smooth muscle cell (SMC) proliferation, migration of SMCs from the media to the intima, and synthesis of extracellular matrix produced marked thickening of the intima, with reduction of the cross-sectional area of the lumen. However, in animals that received continuous treatment with the CE inhibitor, neointima formation was decreased (by about 80 percent), and lumen integrity was preserved. Thus, the angiotensin-converting enzyme may participate in modulating the proliferative response of the vascular wall after arterial injury, and inhibition of this enzyme may have therapeutic applications to prevent the proliferative lesions that occur after coronary angioplasty and vascular surgery.

Effect of heparin on adaptation of vein grafts to arterial circulation

We studied the effect of heparin on wall thickening in balloon-injured carotid arteries and vein grafts in rabbits. Heparin (0.3 mg/kg of body weight/hour) reduced intimal cross-sectional area in balloon-injured carotid arteries at 2 weeks (0.20 +/- 0.05 mm2 vs. 0.05 +/- 0.02 mm2, p = 0.02). Autoradiography after a single pulse of tritiated thymidine revealed no labeling in the few intimal cells present in heparin-treated animals, whereas control smooth muscle cells (SMC) had a 10% labeling index. Heparin did not affect medial proliferation, suggesting that the decrease in intimal thickening was largely due to inhibition of SMC migration into the intima. Heparin caused a slight reduction in intimal cross-sectional area at 2 weeks in vein grafts (0.17 +/- 0.03 mm2 vs. 0.09 +/- 0.02 mm2, p = 0.03) but no significant reduction in wall thickness at any other time and no reduction in SMC proliferation rate (thymidine labeling index). DNA content per surface area or dry weight was the same in control and heparin-treated vein grafts at 4 weeks, implying that SMC content and the amount of matrix made by individual SMC was not affected. These data suggest that either SMC in veins are less susceptible to heparin than SMC in arteries, or the mechanism of thickening is substantially different. Heparin may not block all forms of SMC proliferation and may only be a weak inhibitor in processes that primarily are in response to changes in pressure.

Restenosis after coronary angioplasty. Potential biologic determinants and role of intimal hyperplasia

Restenosis after successful PTCA remains a major problem limiting the efficacy of the procedure. The pathophysiologic mechanism of restenosis has been enigmatic so far, but accumulated evidence strongly suggests that intimal hyperplasia is the major mechanism. Based on current understanding of the process of intimal hyperplasia, one unifying concept may be that there are at least two major local biologic determinants influencing this process, lesion characteristics and regional flow dynamics. Lesion characteristics include the plaque structure and the quantity of smooth muscle. These may provide the anatomic substrate that determines the extent of injury and the degree of smooth muscle cell proliferation. The amount of smooth muscle cells in the stenotic lesion activated by injury to undergo proliferation may determine the eventual bulk of the restenotic lesion. In addition, low wall shear stress could promote intimal hyperplasia and cause structural change of vessels to decrease the lumen, whereas high wall shear stress exerts the opposite effects. Intimal hyperplasia after balloon injury is a complex process involving platelets, growth factors, endothelial cells, smooth muscle cells, mechanical injury, wall shear stress, and probably other unknown factors. Platelets not only contribute growth factors such as PDGF but also cause organized thrombus. Different growth factors may be involved in initiating smooth muscle cell proliferation and may come from many different sources, including smooth muscle cells, endothelial cells, and macrophages. Intact confluent endothelial cells may produce heparin sulfates and inhibit intimal proliferation; however, regenerating endothelial cells may have the opposite effect. Thus, the proliferative potential of smooth muscle cells, endothelial recovery, extent of injury, wall shear stress, and other unknown factors may all influence this process. Based on these concepts concerning the biology of restenosis, some research directions concerning potential forms of therapy are proposed.

Histopathologic phenomena at the site of percutaneous transluminal coronary angioplasty: the problem of restenosis

Seventeen postangioplasty cases were morphologically studied at postmortem. Four of the eleven, early and intermediate cases (few hours to 1 month from angioplasty to death), revealed intraluminal thrombi, although in only two cases were those thrombi occlusive. Almost all of the nine early cases (eight of nine) exhibited intimal disruptions. Except for two of these cases in which circumferential and/or longitudinal dissections were present, the remainder of the intimal cracks were superficial and of limited extent. Limited dissection between intima and media is not considered a serious or detrimental local event. The early cases showed an aneurysmal dilatation of the plaque-free segment of the arterial wall in eccentric plaques. This finding was interpreted as the result of uneven distribution of the dilating force (circumferential stress) on the aterial wall. Late cases (survival over 1 month) revealed characteristic medial and intimal lesions indicative of the initial dilatation injury. It is hypothesized that intrinsic arterial wall changes (medial disruption) at the plaque-free segment and the resulting altered arterial geometry at the site of dilatation have a significant hemodynamic effect on the vascular conduit and may enhance and sustain the myoproliferative intimal response.

Balloon catheter injury to rabbit carotid artery. I. Changes in smooth muscle phenotype

Stereology was used to investigate the changes in ultrastructure of smooth muscle cells during the formation of an experimental intimal thickening induced by injury with an inflated balloon catheter. The volume density of myofilaments in the cell cytoplasm was measured in smooth muscle cell-lined areas (which are freely permeable to Evans blue dye and, hence, stain blue) and in re-endothelialized areas (which remain white after injection of Evans blue) of the rabbit carotid artery. Two weeks after injury, the volume densities of myofilaments in the intimal smooth muscle cells in both white and blue areas were significantly less than that for control medial smooth muscle (67.9% +/- 3.6%; mean +/- SE), being 38.8% +/- 1.0% and 35.9 +/- 3.3%, respectively. By 6 weeks after injury, the volume density had increased significantly in both white (55.1% +/- 3.4%) and blue areas (53.5% +/- 3.0%), and these values did not change significantly by 18 weeks. The volume density of myofilaments in the luminal (lining) smooth muscle cells in the blue areas was significantly less than that of control medial cells and remained low (26.7% +/- 2.1%) up to 18 weeks after injury. The initial balloon-induced injury caused considerable damage to the smooth muscle cells in the media, and the remaining medial cells underwent similar changes in ultrastructure to the cells in the neointima. At 2 weeks, the cells had a low volume density of myofilaments (44.9% +/- 2.4%), which returned to a level not significantly different from the control artery by 6 weeks after injury. There were no differences in the estimates of the volume density of myofilaments between the inner and outer media of the injured arteries. These findings suggest that, after injury produced by a balloon catheter, the smooth muscle cells in both the media and the resultant intimal thickening undergo a reversible change in ultrastructure.

Effect of 18- to 24-hour heparin administration for prevention of restenosis after uncomplicated coronary angioplasty

Smooth muscle cell proliferation induced by intimal denudation similar to that produced by coronary angioplasty has been decreased by early and brief heparin administration in animal models. Heparin might also decrease the incidence of thrombus-induced postdenudation arterial obstruction, albiet at risk of bleeding. To evaluate the risks and benefits of heparin after coronary angioplasty, 416 patients (469 vessel sites) with successful coronary angioplasty without large dissection were randomized to heparin (titrated to partial thromboplastin time 1.5 to 2.5 times normal) or dextrose administered for 18 to 24 hours. Patients also had received heparin during angioplasty (usually 10,000 to 15,000 units), and they received aspirin for a period of 6 months. Heparin and dextrose groups were not different in the percentage of patients with male sex (74% in the heparin group and 75% in the dextrose group, p = NS), postangioplasty tear or dissection (30% in the heparin group and 30% in the dextrose group, p = NS), or postangioplasty percent stenosis greater than 35% (31% in the heparin group and 30% in the dextrose group, p = NS). Partial thromboplastin time during treatment in the heparin group was 56 +/- 22 seconds and in the dextrose group 27 +/- 9 seconds (p less than 0.001). Late angiographic follow-up (180 +/- 81 days) was achieved in 58.4% in the heparin group and 64.5% in the control group. Of patients with late angiographic follow-up, 41.2% and 36.7% randomized to heparin and dextrose, respectively, had documented restenosis (greater than or equal to 50% diameter stenosis at one or more the sites dilated) (p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of vascular smooth muscle cell phenotype in long-term culture

Studies of bovine carotid artery smooth muscle cells, during long-term in vitro subcultivation (up to 100 population doublings), have revealed phenotypic heterogeneity among cells, as characterized by differences in proliferative behavior, cell morphology, and contractile-cytoskeletal protein profiles. In vivo, smooth muscle cells were spindle-shaped and expressed desmin and alpha-smooth muscle actin (50% of total actin) as their predominant cytoskeletal and contractile proteins. Within 24 h of culture, vimentin rather than desmin was the predominant intermediate filament protein, with little change in alpha-actin content. Upon initial subcultivation, all cells were flattened and fibroblastic in appearance with a concomitant fivefold reduction in alpha-actin content, whereas the beta and gamma nonmuscle actins predominated. In three out of four cell lines studied, fluctuations in proliferative activity were observed during the life span of the culture. These spontaneous fluctuations in proliferation were accompanied by coordinated changes in morphology and contractile-cytoskeletal protein profiles. During periods of enhanced proliferation a significant proportion of cells reverted to their original spindle-shaped morphology with a simultaneous increase in alpha-actin content (20 to 30% of total actin). These results suggest that in long-term culture smooth muscle cells undergo spontaneous modulations in cell phenotype and may serve as a useful model for studying the regulation of intracellular protein expression.

Arterial smooth muscle cells in vivo: relationship between actin isoform expression and mitogenesis and their modulation by heparin

Quiescent smooth muscle cells (SMC) in normal artery express a pattern of actin isoforms with alpha-smooth muscle (alpha SM) predominance that switches to beta predominance when the cells are proliferating. We have examined the relationship between the change in actin isoforms and entry of SMC into the growth cycle in an in vivo model of SMC proliferation (balloon injured rat carotid artery). alpha SM actin mRNA declined and cytoplasmic (beta + gamma) actin mRNAs increased in early G0/G1 (between 1 and 8 h after injury). In vivo synthesis and in vitro translation experiments demonstrated that functional alpha SM mRNA is decreased 24 h after injury and is proportional to the amount of mRNA present. At 36 h after injury, SMC prepared by enzymatic digestion were sorted into G0/G1 and S/G2 populations; only the SMC committed to proliferate (S/G2 fraction) showed a relative slight decrease in alpha SM actin and, more importantly, a large decrease in alpha SM actin mRNA. A switch from alpha SM predominance to beta predominance was present in the whole SMC population 5 d after injury. To determine if the change in actin isoforms was associated with proliferation, we inhibited SMC proliferation by approximately 80% with heparin, which has previously been shown to block SMC in late G0/G1 and to reduce the growth fraction. The switch in actin mRNAs and synthesis at 24 h was not prevented; however, alpha SM mRNA and protein were reinduced at 5 d in the heparin-treated animals compared to saline-treated controls. These results suggest that in vivo the synthesis of actin isoforms in arterial SMC depends on the mRNA levels and changes after injury in early G0/G1 whether or not the cells subsequently proliferate. The early changes in actin isoforms are not prevented by heparin, but they are eventually reversed if the SMC are kept in the resting state by the heparin treatment.

Identification of macrophages in intimal thickening of rat carotid arteries by cytochemical localization of purine nucleoside phosphorylase

Complete desquamation of the endothelium of the rat carotid artery by balloon catheter stripping resulted within 2 weeks in the formation of a large intimal thickening. After an enzyme cytochemical technique was applied to localize cytosolic purine nucleoside phosphorylase (PNP), light microscopical evaluation indicated that this intimal thickening in normocholesterolemic rats was composed of 5.8% to 11.8% (mean 8.8%) PNP-positive cells. At the electron microscopic level, all these PNP-positive cells were identified as macrophages by the absence of a basement membrane and plasmalemmal vesicles and by the occurrence of specific intracytoplasmic granules. The nearly nonreactive intimal cells were classified as modified smooth muscle cells. Additional evidence of the macrophage nature of the PNP-stained intimal cells was obtained by differential immunogold labeling of these cells with a monoclonal antibody against rat macrophages. Moreover, in hypercholesterolemic rats, only the cells stained for PNP transformed into foam cells (between 8.5% and 11.4% of all nucleated intimal cells; mean 9.6%). This study shows that PNP cytochemistry discriminates macrophages from modified smooth muscle cells in the rat carotid intimal thickening. It further suggests that the intimal thickening in normocholesterolemic rats originates not only from migration and proliferation of smooth muscle cells, but also from a considerable number of leukocyte-derived macrophages. Whether the latter cells are actively involved in the establishment of the intimal thickening as has been suggested in dietary hypercholesterolemia, remains to be verified.

Cardiovascular complications of chronic catheterization of the jugular vein in the dog

Cardiovascular changes associated with indwelling catheters were evaluated in 51 adult beagle dogs catheterized for 4 to 9 weeks. Pathologic changes consistent with traumatic injury were in the vena cava and endocardium of the right atrium of 88% of cannulated dogs. Lesions were characterized by surface denudation and diffuse intimal thickening due to myointimal hyperplasia and deposition of extracellular matrix. Affected intima was lined by hyperplastic, poorly differentiated endothelial cells and contained round to oval cells with characteristics of smooth muscle cells. After 9 weeks, thickened intima was vascularized and composed of spindle-shaped cells and fibrillar stroma. Intimal sclerosis and localized proliferative papillary projections in the vena cava cranial to areas of myointimal hyperplasia occurred infrequently. Traumatic lesions, regardless of location or severity, did not extend below the internal elastic membrane. Inflammatory cellular responses, when present, were minimal. The location, distribution, and morphogenesis of catheter-related cardiovascular lesions distinguishes them from those induced by chemical toxicity or pharmacotoxicity.

Gamma-interferon regulates vascular smooth muscle proliferation and Ia antigen expression in vivo and in vitro

A significant fraction of the arterial smooth muscle cells in atherosclerotic plaques and injury-induced intimal thickenings express class II major histocompatibility complex (Ia) antigens. This might be the consequence of gamma-interferon secretion by T lymphocytes also present in these lesions. We have therefore analyzed the effects of gamma-interferon on cultured rat aortic smooth muscle cells. Recombinant gamma-interferon inhibited smooth muscle proliferation in vitro in a dose-response relation; inhibition was detectable down to a concentration of 1 unit/ml. In similar concentrations, gamma-interferon also induced Ia expression by the cells. This suggested that Ia antigens might be selectively expressed by nonproliferating smooth muscle cells. In vivo, there was a strong negative correlation between Ia expression and 3H-thymidine labeling of smooth muscle cells in intimal thickenings induced by balloon catheter injury. In rats receiving continuous infusions of 3H-thymidine for two weeks after injury, Ia-positive 3H-positive cells had undergone fewer rounds of replication than Ia-negative ones. This indicates that Ia-expression both in vivo and in vitro is associated with a reduced proliferative capacity. These results suggest that gamma-interferon, a secretory product of activated T lymphocytes, acts as a natural regulator of smooth muscle cell growth and Ia expression in injury-induced intimal thickenings and atherosclerotic plaques.

The effect of intravenously injected beta very low density lipoprotein on small and large arterial injuries

A series of daily injections of beta very low density lipoprotein (beta-VLDL) was administered over 4-5 days to rabbits whose arteries contained either experimental circumferential lesions or areas of intimal thickening. The circumferential lesions were similar to those that occur spontaneously and were produced by the application of longitudinal tension. The intimal thickening was produced by denuding the endothelium with a balloon catheter. Over the period of injection of beta-VLDL the plasma cholesterol levels rose in a pulse-like manner from 60 to 100 mg/dl. Following cessation of injections the cholesterol levels initially rose further and then decreased to normal levels within 4 weeks. Injections of beta-VLDL, commencing 1-2 days after production of the circumferential lesions, resulted in an increase in the number of mononuclear leukocytes (primarily macrophages) and in a moderate accumulation of lipid by these cells and the medial smooth muscle cells. If the injections were started 14 days postinjury there was some accumulation of lipid in the large lesions but none in small lesions. There was no lipid accumulation in any lesions if the beta-VLDL was administered 3 months postinjury or if the animals were injected 2 days after injury and examined 3 months later. A very slight accumulation of lipid occurred in the intimal thickening, or neo-intima, following a series of beta-VLDL injections given to rabbits 2 or 6 weeks after balloon catheter injury. The series of injections produced a significant increase in the number of mononuclear leukocyte profiles per area of the neo-intima, suggesting an increased infiltration of these cells into the injured artery. These results suggest that a small transient increase in the plasma concentration of cholesterol-carrying lipoproteins may lead to increased infiltration of mononuclear leukocytes into areas of intimal thickening or areas of "spontaneously occurring" injury.

Evaluation of medial hypertrophy in resistance vessels of spontaneously hypertensive rats

The role of smooth muscle cell hypertrophy, hyperploidy, and hyperplasia in medial hypertrophy of mesenteric resistance vessels of 107- to 111-day-old spontaneously hypertensive rats (SHR) was examined using a combination of morphometric, biochemical, and immunological techniques. Mesenteric arteries were classified on the basis of branching order for comparative purposes. Branch level I vessels were those that directly enter the jejunal wall, while Branches II to IV represented more proximal vessels; Branch IV vessels were those that branch from the superior mesenteric artery. Medial hypertrophy was assessed in perfusion-fixed vessels by morphometric evaluation of medial cross-sectional area and smooth muscle content. Medial cross-sectional area and smooth muscle content were significantly increased in larger (Branches III and IV) but not smaller (Branches I and II) mesenteric resistance vessels of SHR compared with control normotensive Wistar-Kyoto rats (WKY). Smooth muscle cell hypertrophy and hyperploidy were evaluated in isolated cells obtained by enzymatic dissociation of mesenteric resistance vessels. Approximately 80% of the cells in these preparations were identified as smooth muscle cells using a smooth muscle-specific isoactin antibody. Feulgen-DNA microdensitometric evaluation of isolated cells showed that polyploid cells were present in mesenteric resistance vessels but at very low frequencies, and no differences were apparent between SHR and WKY. Likewise, no differences in cellular protein content or relative smooth muscle cell size (i.e., area profile) were observed between cells obtained from SHR and WKY vessels. These results demonstrate that the increase in medial smooth muscle content observed in larger mesenteric resistance vessels of SHR cannot be accounted for by smooth muscle hypertrophy and hyperploidy, inferring that hyperplasia must be present. Results indicate that studies of the initiating mechanisms for medial smooth muscle hypertrophy in SHR resistance vessels, at least relatively early in hypertension, should focus on examination of factors that induce true cellular proliferation rather than hypertrophy and hyperploidy.

The arterial smooth muscle cell in systemic hypertension

Both the pathogenesis and vascular complications of hypertension appear to involve change in vascular smooth muscle cell (SMC) structure and function. Recent data on vascular SMC biology are reviewed. Specific questions that should be addressed by future research on SMC polyploidy in hypertension; SMC differentiation, growth and function; SMC hypertrophy and hyperplasia in hypertension; and hypertension, vascular aging and atherogenesis are posed.

Smooth muscle cell migration and proliferation: atherogenic mechanisms in hypertension

The proliferative and migratory behavior of explanted rat aortic smooth muscle cells (SMC) was investigated in cells obtained from either 24-week-old normotensive Wistar-Kyoto (WKY) or age-matched spontaneously hypertensive (SHR) rats. Time lapse video analysis of primary SMC growth in the presence of 10% serum revealed that interdivision times of cells from SHR were significantly shorter than those from WKY. Differences in the proliferative capacity of these cells were still present after two subcultivations, as analyzed by conventional growth curves. In contrast to the proliferative behavior, no differences in the migratory characteristics of SMC could be detected in a migration assay analyzing the SMC outgrowth of standardized aortic explants under low serum conditions (0.1% fetal bovine serum). It has been shown that another model of hypertension, the 4 week DOC/salt hypertensive rat results in a different reaction of SMC. Therefore, it can be considered that the extent of the potentially atherogenic alterations of SMC function in hypertension is dependent on the type, duration and the rate of increase of hypertension.

Heparin regulates smooth muscle S phase entry in the injured rat carotid artery

Smooth muscle cell (SMC) proliferation in injured arteries is inhibited by heparin, but the mechanism of inhibition is unknown. In particular, it is not clear whether heparin prevents exit of quiescent SMC from the resting state, inhibits progression through the prereplicative (G1) sequence, or acts during DNA synthesis itself. In this study, induction of ornithine decarboxylase (ODC) activity was used as a marker of SMC entry into the cell cycle in an attempt to localize the site of heparin action during the initial hours after rat carotid injury. Rapid and transient induction of ODC activity was observed that reached a maximum (twenty-three-fold) 6 hours after wounding. Heparin failed to prevent ODC induction but greatly reduced frequencies of [3H]thymidine-labelled SMC nuclei 33 hours after injury. Moreover, heparin infusion could be delayed for up to 18 hours after the injury event with no significant loss of antiproliferative effect. Further delays resulted in marked loss of growth inhibition. The results of these studies show that SMC rapidly and synchronously leave the resting state after injury and suggest that heparin acts late in the prereplicative (G1) sequence or early in S phase to inhibit SMC proliferation in damaged arteries.

Restenosis after percutaneous transluminal coronary angioplasty: the Emory University Hospital experience

Restenosis after coronary angioplasty, a gradual encroachment of the dilated arterial lumen by overgrowth of neointimal cells, occurs in a minority (25% to 30%) of patients. Clinical and anatomic descriptors of those patients who develop restenosis have been identified and suggest a complex, multifactorial etiology. Optimal initial enlargement of the diameter probably reduces the chance of restenosis on geometric grounds alone. The independent predictive value of a low final translesional pressure gradient indicates that adequate blood flow may reduce platelet adhesion and thrombus formation. The importance of the latter factor remains uncertain. Individual proliferative responses to the intimal and medial injury caused by balloon dilatation appear to be modulated by both lesion-specific and patient factors. Lesion-specific factors appear most important and relate to vessel site, tortuosity and branching. These factors are also thought to influence native atherogenesis but the relation between restenosis and atherogenesis remains obscure. Patient risk factors for coronary artery disease also appear to influence restenosis. Procedural factors and risk factor modification may partially modify the restenosis response; however, prevention of restenosis will depend on finding agents that block either stimulation or proliferation of smooth muscle cells.

Role of platelets and thrombosis in mechanisms of acute occlusion and restenosis after angioplasty

The vascular disruption produced by angioplasty initiates platelet deposition through the processes of platelet adhesion and recruitment of circulating platelets to form an enlarging mural platelet thrombus. Thrombin produced by simultaneous activation of the coagulation cascade by subendothelial connective tissue structures enhances platelet deposition and stabilizes the forming thrombus with enmeshing fibrin. Platelet recruitment involves the expression of the glycoprotein IIb/IIIa receptor for fibrinogen and other cytoadhesive proteins including fibronectin, thrombospondin and von Willebrand factor. Platelet deposition and thrombus formation caused by angioplasty appear to be important in the development of 2 complications: acute thrombotic occlusion and restenosis. Experimental mechanical vascular injury produces a predictable, although rather variable, amount of vascular narrowing due to transient smooth muscle cell proliferative intimal lesion formation. This intimal thickening by proliferating smooth muscle cells is in part mediated by platelet mitogens, particularly platelet-derived growth factor, which are released into the damaged vessel from platelets at the time of angioplasty. Platelet-derived growth factor may also be released from other associated vascular and blood cells in response to mechanical injury, e.g., endothelium, monocyte/macrophage and smooth muscle cells themselves. The actual mitogens, and their cells of origin, that mediate restenosis after therapeutic angioplasty remain to be established. Various oral antiplatelet agents have been shown to reduce arterial thrombotic occlusion in a number of controlled clinical trials, e.g., aspirin in transient ischemic attacks and unstable angina, aspirin and dipyridamole in saphenous vein coronary artery bypass and progression of peripheral vascular disease and dipyridamole in artificial heart valves. Acute arterial thrombosis may require more potent, immediate and transient intervention, e.g., monoclonal antibody to platelet receptor expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Common mechanisms of proliferation of smooth muscle in atherosclerosis and hypertension

At least two exogenous sources of agents able to control vascular smooth muscle proliferation can be identified. Platelets contain and release mitogens as well as a factor, TGF-beta, that inhibits cell growth on plastic surfaces while stimulating it when cells are grown in suspension in soft agar. Macrophages release mitogens, including PGDF, and macrophage invasion is characteristic of early experimental lesions in fat-fed animals. Finally, it is at least possible that endothelial cell production of mitogens may represent a response to some as yet undefined external injury. The vessel wall also offers sources of growth control endogenous to the smooth muscle cell layers. The vessel wall contains heparan sulfate able to inhibit cell growth of smooth muscle cells, which by themselves can synthesize PDGF. This provides possible positive and negative control of replication intrinsic to the smooth muscle cells themselves. The role of these intrinsic or extrinsic factors in the smooth muscle proliferation of hypertension and atherosclerosis remains hypothetical. It is intriguing to implicate platelets and/or macrophages in the denuding injuries seen in small hypertensive vessels and in advancing atherosclerotic plaques. At least for the latter case, however, there seem to be other critical factors. Simple denudation and thrombosis, for example, are not sufficient to stimulate smooth muscle growth, and the kinetics of proliferation after balloon denudation imply the presence of some other event required to initiate smooth muscle proliferation. Similarly, smooth muscle replication in large vessels of hypertensive animals occurs without loss of endothelial continuity. This implies that replication in response to hypertension depends on factors intrinsic to the vessel wall. Benditt's observation of monoclonality also implies some intrinsic mechanism allowing cells to grow in a focal manner. It is intriguing to consider the possibility that this commitment process could require the release of cells from the intrinsic inhibitory effects of heparan sulfate located around the cells or the synthesis of growth factors secreted by the smooth muscle cells themselves. If we add the hypothesis that only some cells are capable of such a response, we would expect the sort of oligodense phenomenon demonstrated by Benditt. Proof of such a hypothesis, however, will have to await development of methods to explore these mechanisms directly in the vessel wall responding to injury.

Cytocontractile structures and proteins of smooth muscle cells during the formation of experimental lesions

The time course of structural changes in vascular smooth muscle cells (SMC) was investigated during the formation of an experimental lesion in response to balloon injury. We compared the filamentous organization, evaluated by quantitative electron microscopy, with the cellular content of two representative cytocontractile proteins (myosin and tropomyosin) as assessed by immunofluorescence. We found that the changes peak between 7 and 14 days after injury and that they are visible both in the neointima and to a lesser extent in the inner media. While virtually all SMC are of a filament-rich phenotype in the undisturbed media, after balloon injury SMC migrated into the intima and about 90% of these latter cells were either of a organelle-rich or an intermediate phenotype, with the remaining 10% being of the filament-rich phenotype. In the inner media about 40% of cells were either of organelle-rich or intermediate phenotype. In contrast to these profound organizational changes of responding SMC, histochemistry revealed only a slight and probably transient decrease of the cellular content of myosin and tropomyosin at that time point. Twenty-eight days after injury the discrepancies between the content and the organization of cytocontractile proteins became more apparent. While virtually all SMC showed a homogeneous intensive staining with both antibodies, indistinguishable from the media SMC, the organization of cytoplasmic filaments had not totally recovered. Even though this morphological study does not permit conclusions to be drawn on the contractile function of the cells, it shows that both the organization and the content of cytocontractile protein have to be analyzed and compared for SMC changes to be evaluated during the formation of an experimental lesion.

A review of the proliferative behaviour, morphology and phenotypes of vascular smooth muscle

This article reviews the proliferative, structural and synthetic behaviour of vascular smooth muscle cells under a variety of conditions. It shows how some experimental procedures produce dramatic increases in smooth muscle cell proliferation and, in many cases, subsequent cell migration to the intimal layer. Possible control mechanisms influencing changes in such activity are discussed. The morphology, histogenesis and differentiation of vascular smooth muscle is reviewed, with particular emphasis on the differentiation of such cells into contractile or synthetic phenotypes. The significance of the synthetic phenotype is discussed in relation to the synthesis of intra- and extracellular components.

Injury and repair of smaller muscular and elastic arteries. A light microscopical study on the different healing patterns of rabbit femoral and carotid arteries following dilatation injuries by a balloon catheter

26 rabbits of the Danish country strain were subjected to mechanical dilatation injury of the left femoral and carotid arteries with Fogarty's embolectomy catheters F2 and F3 respectively. The rabbits were killed 2, 7, 14 and 28 days after the dilatation injury and the arteries examined histologically. Initially both of the arteries exhibited necrosis of the media and infiltration of the vessel wall with neutrophils and mononuclear cells. From day 7, intimal thickening was observed in both types of arteries, progressing in thickness during the later stages. However, thrombosis occurred in the majority of the carotid arteries, whereas this was only infrequently seen in the femoral arteries. In all of the dilated arteries, the elastic laminas were stretched or fragmented and never regained their normal appearance. In the carotid artery, giant cells accumulated around the fragmented elastin and calcified areas, located primarily at the intima-medial border. These changes were never observed in the femoral artery. At the twenty-eight days stage, proliferation of the smooth muscle cells more or less led to restitution of the media in the femoral artery, whereas the carotid artery showed medial restitution only to a lesser extent. The similarities between the injured carotid artery and human temporal arteritis, and the utility of the model as an animal model for the study of temporal arteritis are underlined.

Strain and site dependence of polyploidization of cultured rat smooth muscle

Smooth muscle cell (SMC) growth may play an important role in the pathogenesis of vascular diseases such as atherosclerosis and hypertension. Recent studies have demonstrated that, under different growth stimuli in vivo, SMC may respond by proliferation of diploid cells, polyploidization to the tetraploid (or even octaploid) state, or both. In this study, we used flow cytometry to evaluate the intrinsic tendencies of aortic SMC and nonarterial cells from rats of different strains, ages, and blood pressures to polyploidize in response to in vitro growth stimulation. Significant strain-related differences in polyploidization of aortic SMC were found (P less than 0.001): highest in WKY (normotensive inbred rat related to SHR), intermediate in SHR (genetically hypertensive rat), and lowest in Sprague-Dawley and Fischer (normotensive outbred and inbred rats). Animal age had less or no effect on the degree of polyploidization. Nonarterial cells (venous SMC and lung cells) from WKY and SHR remained essentially diploid, suggesting tissue specificity of in vitro polyploidization. Studies of the growth kinetics of uncloned and clonal populations of aortic SMC revealed decreased proliferation as the ploidy increased in WKY, SHR, and Sprague-Dawley. These findings suggest that genetic strain factors as well as cell type/site of origin significantly influence in vitro polyploidization, whereas animal age and blood pressure do not. The findings also emphasize the need to consider ploidy changes when evaluating in vitro SMC growth kinetics. Further studies will improve understanding of SMC growth regulation and the functional significance of vascular polyploidy.

Kinetics of cellular proliferation after arterial injury. IV. Heparin inhibits rat smooth muscle mitogenesis and migration

Heparin inhibits the development of intimal thickening after carotid injury in the rat; however, the specific cellular events responsible for this effect have not been defined. In this study, smooth muscle cell growth fraction and migration into the intima were quantitatively measured in heparin-treated animals. All rats were subjected to left carotid balloon injury and received continuous intraperitoneal infusion of tritiated thymidine; they were given either heparin or lactated Ringer's solution intravenously for periods of time up to 7 days. Both smooth muscle cell growth fraction and migration of nondividing smooth muscle cells were markedly reduced in heparin-treated rats. If heparin was administered for only the first 3 days after carotid injury, both smooth muscle growth fraction and migration were reduced at 7 days; on the other hand, heparin had no effect on growth fraction and migration if given from day 4 to day 7. Finally, heparin given for a period of 1 week produced marked reduction in smooth muscle cell accumulation in injured arteries at 2 and 4 weeks. These results suggest that the effect of heparin on injury-induced intimal thickening might be due to inhibition of both smooth muscle cell entry into the growth fraction and migration of medial smooth muscle cells into the intima. These effects are long lasting, and are not reversed even if heparin is stopped after a short course of administration.

Replication of smooth muscle cells in vascular disease

Smooth muscle proliferation has been recognized as central to the pathology of both major forms of vascular disease: atherosclerosis and hypertension. Recent advances in our knowledge of mechanisms of control of proliferation suggest that events occurring in adult animals may recapitulate portions of the developmental biology of the smooth muscle cell. This review attempts to consider the current state of knowledge of the mechanisms controlling smooth muscle proliferation in these two diseases, to put that knowledge into the context of what is known about smooth muscle biology, and to offer two hypotheses on the possible roles of smooth muscle developmental biology in manifestations of atherosclerosis and hypertension in adult humans.