Tej M. Singh

Adaptive Remodeling of Internal Elastic Lamina and Endothelial Lining During Flow-Induced Arterial Enlargement

Gaps in the internal elastic lamina (IEL) have been observed in arteries exposed to high blood flow. To characterize the nature and consequences of this change, blood flow was increased in the carotid arteries of 56 adult, male, Japanese white rabbits by creating an arteriovenous fistula between the common carotid artery and the external jugular vein. The common carotid artery proximal to the arteriovenous fistula was studied at intervals from 1 hour to 8 weeks after exposure to high flow. In the controls, the IEL showed only the usual, small, physiological holes, 2 to 10 microm in diameter. At 3 days, some of the holes in the IEL had become enlarged, but they could not be detected by scanning electron microscopy, despite manifest endothelial cell proliferation. At 4 days, gaps in the IEL appeared as small, luminal surface depressions, 15 to 50 microm wide. At 7 days, the gaps in the IEL had enlarged and formed circumferential, luminal depressions occupying 15+/-5% of the lumen surface. Endothelial cell proliferation persisted in the gaps while proliferative activity decreased where the IEL remained intact. At 4 weeks, as the artery became elongated and dilated, the gaps in the IEL widened as intercommunicating circumferential and longitudinal luminal depressions occupying 64+/-5% of the lumen surface. At 8 weeks, the rate of elongation and dilatation of the artery slowed and the widening of the gaps in the IEL diminished. Endothelial cells covered the gaps throughout. We conclude that flow-induced arterial dilatation is accompanied by an adaptive remodeling of the intima. The gaps in the IEL permit an increase in lumen surface area while endothelial cell proliferation assures a continuous cell lining throughout.

Successful transplantation of adult-sized kidneys into infants requires maintenance of high aortic blood flow.

BACKGROUND Nationally, results of renal transplantation in infants are inferior to those in older children and adults. Within the infant group, best results are obtained with adult-sized kidneys (ASKs) rather than size-compatible pediatric kidneys. However, transplantation of ASKs into infants has an increased risk of acute tubular necrosis and graft loss from vascular thrombosis and primary nonfunction. The aim of this study was to define and understand the hemodynamic changes induced by ASK transplantation, so that outcomes of transplantation in infants can be improved. METHODS Nine hemodynamically stable and optimally hydrated infants were studied under a controlled sedation with cine phase-contrast magnetic resonance at three time periods: before transplantation, 8-12 days after transplantation, and 4-6 months after transplantation. Cross-sectional images of both the infant aorta and the adult transplant renal artery were obtained and blood flow was quantitated. Renal volumes were also obtained, and expected renal artery blood flow based on early posttransplant volume was calculated. In addition, renal artery blood flow was determined in 10 in situ native adult kidneys prior to donor nephrectomy. Supplemental nasogastric or gastrostomy tube feeding was carried out during the blood flow study period to optimize intravascular volume. RESULTS Mean infant aortic blood flows were 331+/-148 ml/min before transplantation, 761+/-272 ml/ min at 8-12 days after transplantation (P=0.0006 with pretransplant flow), and 665+/-138 ml/min at 4-6 months after transplantation (P=0.0001 with pretransplant flow). Mean transplanted renal artery flows were 385+/-158 ml/min at 8-12 days and 296+/-113 ml/min at 4-6 months after transplantation. Transplanted renal artery flows were less than prenephrectomy in situ donor renal artery blood flow (618+/-130 ml/min; P=0.02 and P=0.0003) and expected normal renal artery blood flow (666+/-87 ml/min; P=0.003 and P=0.001) at both 8-12 days and 4-6 months after transplantation. A 26% reduction in renal volume (P=0.003) occurred between the two postoperative time periods, and this paralleled the decrease in posttransplant renal artery flow. One-year graft and patient survival in the nine infants was 100%. The mean serum creatinine levels at 3, 6, and 12 months were 0.43+/-0.10, 0.48+/-0.15, and 0.49+/-0.16 mg/dl. CONCLUSIONS This study is the first to quantitatively document the blood flow changes occurring after ASK transplantation in infants. There was a greater than two-fold increase in aortic blood flow after ASK transplantation, and this increase was sustained for at least 4 months and appeared to be driven by the blood flow demand of the ASK. However, actual posttransplant renal artery blood flow was significantly less than normal renal artery flow. Our study suggests that aggressive intravascular volume maintenance may be necessary to achieve and maintain optimum aortic blood flow, so as not to further compromise posttransplant renal artery flow and to avoid low-flow states that could induce acute tubular necrosis, vascular thrombosis, or primary nonfunction.

High flow drives vascular endothelial cell proliferation during flow-induced arterial remodeling associated with the expression of vascular endothelial growth factor.

Endothelial cell activation and proliferation are the essential steps in flow-induced arterial remodeling. We investigated endothelial cell turnover in the early stages of high-flow in the rabbit common carotid arteries using an arteriovenous fistula (AVF) model by kinetic investigation of cell proliferation and cell molecular analysis. BrdU was administrated to label endothelial cells (ECs) in DNA synthetic phase (S-phase) of the cell mitotic cycle. Pulse labeling revealed that ECs entered S-phase at 1.5 days of AVF (0.93 +/- 0.19%). Endothelial cell labeling index (EC-LI) peaked at 2 days of AVF (8.90 +/- 0.87%) with a high index of endothelial cell mitosis (EC-MI, 1.67 +/- 0.47%). Endothelial cell density increased remarkably at 3 days of AVF with a significant decrease in EC-LI (54%) and EC-MI (60%). Study of kinetics of EC proliferation revealed that endothelial cells took 16-24 h to finish one cycle of cell mitosis. Tracking investigation of pulse BrdU-labeled endothelial cells at 1.5 days showed that more than 66% of endothelial cells were BrdU-labeled 1.5 days after labeling. VEGF, integrin alphanubeta3, PECAM-1, and VE-cadherin were upregulated significantly preceding endothelial cell proliferation and kept at high levels during endothelial cell proliferation. These data suggest that endothelial cell proliferation is the initial step in flow-induced arterial remodeling. Hemodynamic forces may drive endothelial cell downstream migration. Expression of VEGF and cell junction molecules contribute to flow-induced arterial remodeling.

Sequential Increases and Decreases in Blood Flow Stimulates Progressive Intimal Thickening

OBJECTIVES To assess the effect of chronic, repetitive increases and decreases in blood flow on an artery. MATERIALS AND METHODS Arteriovenous fistulae were created in Japanese male rabbits between the left common carotid artery and the corresponding external jugular vein. Animals were placed into either control groups or one of six cycle groups consisting of flow variations (0.5 cycles, 1.0 cycle, 1.5 cycles, 2.0 cycles, 2.5 cycles and 3.0 cycles). Each complete cycle consisted of 4 weeks of increased flow followed by 6 weeks of normalised flow by fistula ligation. RESULTS Arteries exposed to increased flow for 4 weeks (0.5 cycles) had a significant increase in lumen diameter without intimal thickening. After 6 weeks of normalised flow (1.0 cycle), shear stress became subnormal (0.42 +/- 0.17 N/m2), intimal thickening developed. In subsequent cycles, intimal thickening continued to develop with each point of flow normalisation and reduction in shear stress. Histologic and ultrastructural analysis revealed endothelial cells preservation at all time points, with individual strata of smooth muscle cell proliferation in the intima corresponding to the cycle numbers. CONCLUSION Progressive intimal thickening occurred in the previously flow-induced remodelled artery when shear stress was reduced to subnormal levels with preserved endothelium, but was inhibited by high flow periods.

Role of fibrinopeptide B in early atherosclerotic lesion formation

The development of atherosclerotic lesions involves many cell types, including macrophages. Fibrinopeptide B (FPB) has been shown to be a potent chemotactic agent for macrophages, which are abundant as intimal foam cells in atherosclerotic lesions, especially in cholesterol-fed rabbits. We hypothesize that intimal low-density lipoproteins also cause fibrinogen in the intima to release FPB and that FPB attracts macrophages in response to the high lipid levels associated with lesion development. To test our hypothesis, we used an atherosclerotic model. Silk sutures containing either FPB, fibrinopeptide A (FPA), lipopolysaccharide (LPS), or saline control were prepared. One suture of each type was placed in the adventitia of the femoral artery of a rabbit. Animals were killed at 1 or 2 weeks. Only vessels exposed to either FPB or LPS showed significant intimal thickening in the region adjacent to the suture site. Semi-thin electron microscopic sections indicated that the intimal wall was highly cellular and that many cells contained lipid vacuoles after 2 weeks. These sections also showed that the endothelium remained intact and that no injury to the media of the artery had occurred. Electron microscopy of the tissue samples showed the proliferation of smooth muscle cells and deposition of extracellular matrix in the 2-week animals, whereas foam cells were present in the 1-week animals. We conclude that FPB does indeed attract macrophages to the intima and that these macrophages may become foam cells. The model we have developed can be used to study possible mechanisms for the entry of macrophages into the intima during early lesion development and to further understand the complex interactions of FPB, fibrinogen, and lipids in atherosclerotic lesion development.

Endovascular Repair of Traumatic Aortic Pseudoaneurysm With Associated Celiacomesenteric Trunk

Purpose: To report stent-graft repair of a traumatic aortic pseudoaneurysm in proximity to a celiacomesenteric trunk. Case Report: An 18-year-old woman suffered a large gunshot wound to the right flank. At laparotomy, only a large, nonexpanding right retroperitoneal hematoma was found, which was thought to represent significant penetrating trauma to the kidney mass. The patient was monitored in the intensive care unit. One week later, computed tomography revealed a partially infarcted right kidney and a 2.3-cm supraceliac aortic pseudoaneurysm, with adjacent bullet fragments. An angiogram confirmed the pseudoaneurysm and showed it to be 7 mm from the celiacomesenteric trunk. Endovascular repair was undertaken with a 16times55-mm AneuRx stent-graft, which was successfully placed across the aortic pseudoaneurysm without covering the celiacomesenteric trunk. Imaging at 12 months revealed no endoleak and full pseudoaneurysm exclusion. Conclusions: This operative approach is appropriate for the individual patient who has suitable anatomy and a clinical course that requires immediate repair of an aortic injury to prevent further complications and delays in ancillary treatments.

The effect of hypercholesterolemia on early atherosclerotic lesions initiated by fibrinopeptide B

Hypercholesterolemia and thrombosis have been implicated as factors in the development of atherosclerosis. Fibrinopeptide B (FPB) is a short chain peptide cleaved from fibrinogen during the production of fibrin. FPB is a known chemoattractant and has been shown to produce experimental atherosclerotic lesions in association with hypercholesterolemia. The present study was designed to examine the role of hypercholesterolemia in this process and to study the time course of the development of these lesions. Twelve New Zealand White rabbits were placed on an atherogenic diet and had suture carrying either FPB, fibrinopeptide A (FPA), or saline (controls) implanted in the adventitia of the femoral arteries and were sacrificed at 14 days. An equal number of animals were left on a standard diet and underwent similar treatment. Eleven animals were treated as the hypercholesterolemic group but were sacrificed at 2, 4, and 7 days. The thickness of the intima was measured adjacent to the suture in the animals sacrificed at 14 days, and the hypercholesterolemic FPB sites were thicker (12.23 mu +/- 6.60) than either hypercholesterolemic FPA (6.06 mu +/- 3.72), saline (4.94 mu +/- 1.42), or the normocholesterolemic FPB (5.99 mu +/- 4.61), FPA (3.89 mu +/- 2.20), or saline (3.97 mu +/- 1.83) (P less than 0.05 for all groups). Transmission electron microscopy of the hypercholesterolemic FPB group showed evidence of macrophages, actively secreting smooth muscle cells with newly deposited elastin, and foam cells by 7 days. We conclude that FPB attracts or stimulates macrophages and smooth muscle cells and that the resultant cellular and extracellular proliferation favors early atherosclerotic lesion formation in the presence of hypercholesterolemia.