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Cardiovascular Research 2005 65(2):513-523; doi:10.1016/j.cardiores.2004.10.032
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Copyright © 2004, European Society of Cardiology

Preconditioning of arteriogenesis

Dimitri Scholz* and Wolfgang Schaper1

Department of Exp. Cardiology, Max-Planck-Institute, Benekestrasse 2, D-61231 Bad Nauheim, Germany

* Corresponding author. Current address: Gazes Cardiac Research Institute, Medical University of South Carolina, 114 Doughty Str., Charleston SC 29403, USA. Tel.: +1 843 789 6838; fax: +1 843 876 5068. Email address: scholzd{at}musc.edu w.schaper{at}kerckhoff.mpg.de

Received 22 January 2004; revised 15 October 2004; accepted 19 October 2004


   Abstract
Top
 Abstract
1. Introduction
 2. Material and methods
 3. Results
 4. Discussion
 References
 
Objective: Last decade, the shear stress caused by increased blood flow in collateral circulation after occlusion of main artery was recognized as a trigger of vascular remodeling and collateral growth. The goal of this study was to differentiate whether the on-going increased blood flow is necessary for the vascular remodeling or the remodeling, once in progress, develops independently of flow.

Methods: Femoral artery occlusion was performed in C57B1/6 mice. After 1–3 days, the ligature was removed and normal limb perfusion was re-established, monitored by laser Doppler Imaging (LDI). Two weeks after the first occlusion, both femoral arteries were re-occluded to compare collateral growth on the "naive" and "preconditioned" sides. After perfusion fixation, ultrastructural studies and morphometry of the collateral vessels were performed.

Results: Blood flow fell after occlusion to about 15% of control levels and recovered to about 40% by day 3. The reperfusion normalized sustainable blood flow. After the second occlusion, blood flow on both sides fell again to about 15% but recovered to 70% in the "preconditioned" compared to 40% in the "naive" side during the following 3 days. 5-Bromo-2'-desoxy-uridine (BrdU) administered during reperfusion was detected mainly in the neointima that, in many cases, had markedly narrowed the lumen. Two to three days after re-occlusion, a statistically significant lumen enlargement on the "preconditioned" side was observed, while neointima disappeared.

Conclusion: Cellular proliferation and remodeling of collateral arteries were induced by short period of increased blood flow (occlusion of the femoral artery) but realized mostly during the low blood flow (reperfusion of the femoral artery). The neointima developing as a result of this remodeling can be recruited as a functional part of the arterial wall if the collateral perfusion increases as a result of repetitive occlusion of the femoral artery. The "medialization" of the neointima might cause the observed quicker gain of collateral lumen diameter and conductance, saving distal muscle tissue from the ischemia.

KEYWORDS Collateral artery; Angiogenesis; Neointima; Reperfusion


   1. Introduction
Top
 Abstract
 1. Introduction
2. Material and methods
 3. Results
 4. Discussion
 References
 
Collateral arteries are known to maintain blood flow after the occlusion of a conduit artery under experimental conditions and in patients where the complete occlusion of one coronary artery is compatible with survival if adequate collaterals exist [1–4]. We had previously studied the growth of collateral arteries (arteriogenesis) in experimental models in the dog and pig heart [5,6] and in the hindlimb of rabbits [7,8] and mice [9,10].

The hypothesis about the participation of changing blood flow in histogenesis is more than 100 years old. Thoma [11] observed a relationship between blood flow velocity and arterial diameter in chicken embryos. Murray [12] postulated that the whole circulatory system and adaptation to changes in the milieu (internal and external) is organized to minimize the mechanical and metabolical burden of vascular tissue. More recently, Glagov et al. [13] have shown that blood vessels react to the change of shear stress with the growth and/or remodeling to keep the shear stress possibly constant. Increased shear stress induces the remodeling of the cytoskeleton in endothelial cells [14,15]. Shear stress responsive elements (SSRE) in the promoter region of some endothelial genes regulate their expression by interaction with transcription factors such as nuclear factor-kB (NF-kB), early growth response-1 (EGR-1) and activator protein-1 (AP-1) [16–19]. Shear forces tend to cut endothelial cells from their substrate. The cell-to-substrate contacts (in vitro) corresponding to the focal adhesion points in vivo are points of force interaction and become phosphorylated when exposed to shear stress [8]. The activation of mechanoreceptors by shear stress causes the synthesis of adhesion molecules and attraction of blood monocytes, which activate arteriogenesis [8,20].

Shear stress should exceed some thresholds to activate endothelial cells and start arteriogenesis [8,21]. If the shear stress decreases below this threshold (e.g., because the vessel has grown), the activation should disappear, and the arteriogenesis might switch to its last maturation phase [8] or regress again [22,23]. The reduction of shear stress because of the vessel growth could explain the negative feedback and the end of arteriogenesis before achieving the optimal result [8,9]. Alternatively, it is possible that the endothelial cells react to the rate of change of shear stress rather than to its absolute level. In both cases, shear stress should be relevant only at the very beginning as a trigger of arteriogenesis and would play less or no role in advanced stages. Our previous data suggested that mechanical receptors on the albuminal membrane of human umbilical vein endothelial cells (HUVEC) were activated by 30 min of shear stress but deactivated after 14 h of continuous shear stress [8].

In the last decade, the shear stress caused by increased blood flow in collateral circulation after occlusion of main vessel was established in the literature as a trigger of vascular remodeling and collateral growth [19,24–26]. This was directly shown by our previous studies [8,9]. The goal of the present study was to differentiate whether the ongoing increased blood flow is necessary for the vascular remodeling or the remodeling, once in progress, develops flow-independent. For this purpose, the femoral artery in mice was occluded and reopened after 1, 2 or 3 days of occlusion, and the development of collateral arteries during following 2 weeks of low collateral-flow was estimated physiologically and morphometrically.


   2. Material and methods
Top
 Abstract
 1. Introduction
 2. Material and methods
3. Results
 4. Discussion
 References
 
2.1. Animals and surgery
Seventy three C57Bl/6 mice (Charles River, Sulzfeld, Germany) were used for this study with permission of the State of Hessen, Regierungspräsidium Darmstadt, according to section 8 of the German "Law for the protection of animals," which conforms with the Guide for the Care and Use of Laboratory Animals (NIH, 1996). The operation technique was described previously [9]. Briefly, under anesthesia with a mixture of ketamine and xylazine, the right femoral artery was occluded proximally to the origins of the arteria poplitea (Fig. 4A).


Figure 4
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  		Fig. 4 (A) Representative angiogram at day 3 after the second occlusion. Collateral arteries are filled with contrast medium. Ligatures are labeled with arrows. Left on the picture: "preconditioned" side: 3 days of occlusion+11 days of reperfusion+3 days of occlusion. Right on the picture: "naive" side: 3 days of occlusion. Arrowheads label the sampling sites. (B–E) Epon cross-sections (0.5 µm thick) stained with toluidine blue, representative for four experimental groups: control (B), 3 days of occlusion, naïve side (C), after reperfusion (D) and after second occlusion (E). Arrow on panel (E) emphasizes a remnant of neointima. Panel (F) shows morphometrical results for collateral diameter and panel (G) for vessel wall area. *Statistically significant difference compared to control and first occlusion groups (p<0.05).

 
To reduce collateral blood flow, the femoral artery was opened after 1, 2 or 3 days of occlusion. The operation silk was removed, and the limb perfusion reestablishing was monitored by Laser Doppler Imaging (LDI). Usually, a 100% blood flow recovery was established immediately after removal of the ligature. Mice with incomplete limb reperfusion (seven mice or 10%) were excluded from the study because their collateral blood flow could remain increased. From remaining 12 animals per group, the half was used for morphometrical study after the end of 2-week reperfusion (see below). In the other half, both femoral arteries were occluded (i.e., for reperfused artery, the second time) to directly compare the rate of "naïve" and "preconditioned" arteriogenesis. One, two or three days thereafter, mice were euthanized, and the hindlimbs perfusion was fixed for further structural studies. All surgeries were done using the OPMI 1-FR Zeiss stereomicroscope.

2.2. Laser Doppler Imaging (LDI)
A red Laser Doppler imager (MLDI 5063, Moor Instruments, Devon, UK) was used to estimate relative blood flow as described in Ref. [9]. LDI measures the movement of blood cells, which is broadly used [27–29] for blood flow quantification under assumption of a constant haematocrit. Blood flow measurements in mouse feet were performed under anesthesia before, immediately after occlusion and at days 1, 2 or 3 thereafter (depending on the experimental group), after removal of the silk ligature, at days 7 and 14, after the second occlusion and 1, 2 or 3 days thereafter. Mice were kept 5 min in the climatized chamber at 37 °C before and during the entire LDI measurements. For each mouse and each time point, the LDI values for the occluded side were expressed as a percentage from corresponding pre-values (after background subtraction). Because of bilateral occlusion, we could not use the ratio occluded to nonoccluded side, like in previous studies [9,10,30]. However, numerous experiments have shown that unilateral occlusion does not influence the LDI value on the nonoccluded side, and therefore the current method of calculation would be acceptable.

2.3. Perfusion fixation and cell proliferation test
5-Bromo-2'-desoxy-uridine (BrdU; Sigma-Aldrich, Taufkirchen, Germany) was administered to 18 mice (six mice per group; for 1, 2 or 3 days of preconditioning) with the drinking water in a concentration of 0.8 mg/ml during the reperfusion of the femoral artery. This was started 24 h after the removal of ligature silk to ensure that dividing cells activated during first occlusion phase had completed their mitotic cycle. We would like to label only cells entering the cycle during the reperfusion. The average BrdU consumption was calculated as 203 ± 9 mg/kg/day. BrdU incorporated in proliferating cells was visualized by specific antibodies raised against BrdU. For this purpose, mice after 1, 2 and 3 days of occlusion and 2 weeks of reperfusion were euthanized, and the descending thoracic aortas were anterogradely cannulated to ensure the lower limbs were perfusion fixated. The circulatory system was rinsed of blood for 2 min with 10 mM Tris–HCl buffer (pH 7.4) containing 140 mM Na+, 2 mM Ca++ and 0.1% adenosine (Fluka, Steinheim, Germany) under the pressure of 1000 mm H2O. This was followed by freshly prepared 1% formaldehyde for 10 min and then by contrast medium [9] to fill collateral arteries under microscopical control.

The 8-µm thick cryosections of the upper leg musculature with collateral vessels were permeabilized by a mixture containing 70 ml ethanol+30 ml of 50 mM Glycin buffer pH 2.0. After enzymatic digestion with trypsin (10 min at 37 °C), sections were incubated with 4 M HCl (20 min at RT) and covered with anti-BrdU-FITC kit (Sigma-Aldrich).

2.4. Ultrastructural and morphometrical study
The number and arrangement of mice collateral arteries are constant [9,44]. Two out of six collateral arteries are situated superficially, embedded each in a separate muscle strain belonging to the adductor muscle. These collaterals connect the arteria profunda femoris with the arteria saphena and supply the lower leg with blood after femoral occlusion. They could be viewed in vivo even before occlusion (Fig. 4A) and were chosen therefore for histological, ultrastructural and morphometrical studies. About 1 mm long pieces of midzone of collateral arteries were dissected together with the surrounding muscle tissue, postfixed in 3% glutaraldehyde then in 1% OsO4, dehydrated in alcohols and embedded in Epon. Cross-sections (0.5 µm thick) were cut with the Ultracut microtome (Reichert-Jung, Bensheim, Germany), stained with toluidine blue, analyzed and photographed with the Leica DAM (Leica, Bensheim, Germany) microscope equipped with the Leica CM 200 digital camera. The diameter of lumen and the wall thickness were measured interactively using the NIH Image software. Morphometrical analysis was performed according to previously described principles [31], adapted to semithin sections [8,9]. Ultrathin sections about 70 nm thick were contrasted with uranyl acetate and lead citrate and analyzed with a Philips CM 10 electron microscope.

2.5. Statistics
Results were analyzed statistically using the one-way ANOVA test. Differences were taken to be statistically significant if p<0.05.


   3. Results
Top
 Abstract
 1. Introduction
 2. Material and methods
 3. Results
4. Discussion
 References
 
3.1. Preconditioning accelerated blood flow recovery
After occlusion of the right femoral artery, the distal blood flow assessed by Laser Doppler Imaging (LDI) perfusion fell to about 15% of the nonoccluded control side and slightly recovered during the next 3 days (Fig. 1). After removal of the ligature silk at days 1, 2 or 3, a complete and sustained reperfusion occurred immediately. After the second, double-sided occlusion on day 14, blood flow in both feet fell again to about 15% but recovered much faster in the "preconditioned" compared to the "naïve" side. The almost complete recovery was observed in four of six mice after 3 days of preconditioning and one of six mice after 2 days of preconditioning.


Figure 1
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  		Fig. 1 Blood flow monitoring by Laser Doppler Imaging (LDI). (A) LDI examples for mice feet (and tail) representing experimental groups at shown time points. Red color corresponds to the high perfusion value, the blue color to the low one, with yellow in between. Before surgery (pre), both feet showed similar perfusion. After occlusion of the right femoral artery (occ-R), the perfusion in right foot dropped, while that in the left foot remained unchanged. One day (upper line), two days (middle line) or three days (bottom line) after occlusion, some recovery of blood flow occurred. The removal of femoral ligature (reflow) caused an immediate, full and sustained (checked at 7 days and 14 days) recovery. Second double-sided occlusion of the femoral artery (occ-RL) caused the same flow reduction in both feet as the first occlusion, but the preconditioned right side demonstrated better recovery 1, 2 or 3 days thereafter after the second occlusion than the "naive" side (i.e., occluded at day 14). (B) Quantification of LDI measurements separately for right ("preconditioned;" R) and left ("naive;" L) feet expressed as a percentage from corresponding values for same foot before occlusion (pre). The endpoint difference became statistically significant (p<0.05) in case of 2 or 3 days of "preconditioning."

 
3.2. Collateral arteries continued to grow during the femoral reperfusion
To prove that the cellular proliferation really took place during the arterial remodeling, we administered BrdU to animals in vivo and detected its incorporation into the nuclei of endothelial and smooth muscle cells of growing collateral arteries by immunolabeling (Fig. 2). Moreover, because BrdU was administered 24 h after reperfusion of the femoral artery, all labeled cells were consider to start S-phase during the reperfusion of femoral artery (and low perfusion of collateral arteries). Therefore, we can assume that this proliferation took place while the femoral artery was reperfused, and the collaterals most probably were not.


Figure 2
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  		Fig. 2 Proliferation of all cell types of the vessel wall of collateral arteries and development of the neointima during the collateral low-perfusion phase. Immune labeling for BrdU (green), nuclei was stained red with 7-AAD. BrdU 5-bromo-2'-desoxy-uridine (BrdU, Sigma-Aldrich) was administrated with drinking water in concentration 0.8 mg/ml beginning 24 h after the reperfusion. Representative cross-sections through the collateral arteries after 2 days (A) or 3 days (B–F) of occlusion of femoral artery followed by reperfusion. Positively labelled cells localized in tunica media (A, B), endothelium (D), neointima (C, E, F) and tunica adventitia (D, F). Remnants of the lamina elastica interna, bordering the neointima, can be recognized by autofluorescence as a thin punctured line (arrowheads on C, D). The symmetrical (C, D) or asymmetrical (E, F) neointima partially closing the lumen (E, F).

 
BrdU was localized in collateral arteries in all cell types: endothelial, smooth muscle cells (both medial and neointimal) and adventitional fibroblasts (Fig. 2). In mice with a longer preconditioning period (3 days), numerous BrdU-positive cells were found in the collateral arteries, while only few positive cells were noticed in collaterals after a short preconditioning period (1 day, Fig. 2A).

3.3. Neointima was formed after reperfusion and medialized after second occlusion
The neointima closed the arterial lumen almost completely (Fig. 2E,F). The neointima formation during reperfusion was so efficient that reocclusion caused the same degree of underperfusion as the first occlusion or the occlusion of the contralateral "naive" femoral occlusion (Fig. 1).

After the second occlusion, neointima seemed to disappear (Fig. 3A). However, under larger magnification of electron microscope, some layers of smooth muscle cells could be recognized between endothelial cells and lamina elastica interna (Fig. 3B). These cells were incorporated into the arterial wall like an interior part of the tunica media. This remodeling and the associated lumen expansion may explain the quick recovery of blood flow after the second occlusion (Fig. 1) as well as prevention of skeletal muscle necrosis (see below). This finding supports our previous assumption that the neointima is not always a sign of vascular degeneration but can also serve as an SMC incubator [32].


Figure 3
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  		Fig. 3 "Medialization" of the neointima. Collateral artery after 3 days of occlusion followed by 14 days of reperfusion and again 3 days of occlusion of the femoral artery. The arterial wall contains 2–3 layers of almost normal-looking smooth muscle cells. Electron micrograph composed from about 20 separate pictures. Insert: larger magnification from the framed part. The lamina elastica interna (arrows) is partially dissolved and separated from the endothelial layer by about two layers of smooth muscle cells, which therefore can be defined as the neointima.

 
3.4. Collateral growth
One to three days after occlusion of the femoral artery, the collateral arteries did not yet significantly increase their diameter nor their wall thickness compared to preocclusion values (Fig. 4). After 2 following weeks of reperfusion, however, the wall thickness increased dramatically, whereas the diameter was only nonsignificant. This growth explains the discrepancy between active cellular proliferation by BrdU (Fig. 2) and no gain of conductance by LDI (Fig. 1).

However, after the second occlusion, the lumen diameter was significantly enlarged (Fig. 4F), which corresponds well to our conductance data (Fig. 1).

3.5. Survival of the lower limb musculature
After acute occlusion of the femoral artery, muscle fibers in the M. gastrocnemius were severely damaged and become replaced by proliferating satellite cells beginning from day 3 (Fig. 5, left column). In contrast, the second period of ischemia (preconditioned side) did not cause significant damages of muscle tissue (Fig. 5, right column). Its morphology differed from the control only by a slightly increased amount of fat cells and rows of (rather normal-looking) nuclei in muscle fibers, suggesting recent mitotic activity.


Figure 5
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  		Fig. 5 Structural alteration of lower limb musculature. Naive side: 1 day after occlusion of the femoral artery: loss of nuclei and destroyed myofibrils. 3 days: proliferation of satellite cells. Anaphase chromosomes are prominent. 5 days: satellite cells continue to proliferate, old contractile material disappears, old nucleus is still present as an apoptotic body (arrow). Preconditioned side: 1 day occlusion followed by 14 days of reperfusion and 1 day of occlusion. 1*14*1: new muscle fibers develop, nuclei move toward their definite position on the periphery. 3*14*3: new muscle fibers look compact and almost normal, fat cells are prominent. Longitudinal section on the bottom shows the arrangement of dividing nuclei.

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Material and methods
 3. Results
 4. Discussion
References
 
In the present study, two important phenomena were shown: (1) The occlusion of the femoral artery in mice initiates the collateral growth but is not inevitable for the ongoing of the growth. Collateral arteries continued to grow after the femoral artery was reperfused under low collateral blood flow. The collateral growth during this phase was detected by in vivo BrdU incorporation and morphometry of arteries on histological section. The collateral growth explains the quick functional recovery after the second occlusion of the femoral artery. (2) The thick arterial wall represents neointima developed as a result of low perfusion of collateral arteries when the ongoing cell growth can close the lumen. The neointimal smooth muscle cells may be recruited into the media if perfusion increases (as a result of second occlusion of the femoral artery) faster than can be explained by proliferation. This is the first report showing a beneficial "incubator" role of the neointima on lumen size of remodeled arteries, which we called "medialization" of the neointima. The neointima served as an art of SMC incubator, which can be recruited into vessel wall, supporting our previous assumptions [32].

The neointima formation during reperfusion was so efficient that the second occlusion caused similar degree of underperfusion as the first occlusion ("naive" side). However, the resumption of function is much faster in the "preconditioned" side, which was shown by blood flow measurements (by LDI), morphometrically by increased vessel diameter and by an effective preventing of a second bout of skeletal muscle necrosis. In previous studies, we have shown that the recovery of peripheral blood flow is completely dependent of collateral vessels [9]. The new findings can be explained therefore by assuming the continued collateral growth during the "reflow" phase, which is masked because new cells are located in the neointima.

4.1. Collateral destiny
The literature data about what happens with collateral arteries after revascularisation of the main artery are rather rare and controversial. Some authors described that nonfunctional coronary collaterals in patients may regress [22,23,33], others report that collaterals may remain for a long time on standby even if they are no longer used [34–37]. Recently, Werner et al. [4] hypothesized that larger collaterals have better potential for functional recovery. In experimental studies in dogs [38], the gradual occlusion of the circumflex coronary artery forced the development of the collateral arteries preventing the infarct. After removal of the occlusion, collaterals become nonfunctional. However, if the second occlusion was performed up to 90 days following release, the collateral blood flow increased during the first 30–90 min thereafter "many times higher than those for normal dogs." The authors concluded that the collaterals "remain ready to resupply the affected myocardium within 1 h or less compared to the days required for comparable increases following the first occlusion." The present study confirms those results and proposes a cellular mechanism explaining the phenomenon. Watanabe et al. [39] demonstrated similar results: rapid but not instantaneous regression and recovery of mature collateral function in response to requirements of myocardium dependent on collateral vessels. The regression could be partially prevented by nitroglycerine, which makes the vascular tone responsible for physiological regression and recovery. Our results show that the collateral arteries may even grow while nonfunctioning.

After the reocclusion of the femoral artery, the collaterals regained their functionality quickly by rearrangement/migration of smooth muscle cells multiplied during the previous phase.

4.2. Neointima
Neointima formation is usually assumed to be a hallmark of damage to the arterial wall and an expression of overshoot repair [40]. It was described in the context of pathophysiological situations: intimal injury [41], atherosclerosis [42], stent implantation [43], etc. The activation of endothelial cells by shear stress followed by infiltration of monocytes triggers arteriogenesis [8,9,44,45]. Neointima formation in arteriogenesis was described in larger animals such as rabbit [8] or dog [46], but it remained unclear whether this was a pathological sign or a pathophysiological adaptation. Collateral development in mice following femoral occlusion does not produce a neointima. Reperfusion of an occluded femoral artery is so far the only method to produce an intima in collateral vessels. This provides a clean basis for the study of mechanisms of intima formation. In all species, the neointima consists to 100% of smooth muscle cells and was speculated to be an "incubator" for new smooth muscle cells in arteriogenesis [47]. Marano et al. induced reversible neointimal formation in rabbit carotid artery by reducing blood flow (and shear stress) with a soft collar [48]. In the present study, the neointima developed after releasing the femoral artery occlusion and decreased after second occlusion. We hypothesize it was the abrupt reduction of shear stress that caused the neointima formation in collateral arteries.

In rabbits, many collateral arteries are found at the beginning of growth, while only few achieve mature size [49], we named this selection "pruning" but, until now, failed to demonstrate its mechanism. The present study suggests that the neointima may serve as a cell reservoir that can be relatively quickly recruited if the need arises. However, longstanding deprivation of the physical stimulus will, on the long run, lead to complete occlusion and obliteration of the vessel. More studies using different experimental models and time schedules are necessary to elucidate the circumstances under which the neointima formation could be beneficial.

4.3. Preconditioning of lower limb musculature?
The speed of resumption of collateral function after second of the conduit artery can also be demonstrated by the absence of ischemia/necrosis of the skeletal muscles of the lower limb. Deep ischemia and cell damage occur in the lower limb musculature during the first 12 h after occlusion of the femoral artery [30,45,50]. Recently, we have shown that the skeletal muscle content of high-energy phosphates and glycogen decreased below 10% of control, nucleoside and lactate increased threefold, muscle fibers were attacked by leukocytes and became damaged [30]. In the present study, the acute occlusion caused similar muscle damage. In contrast to the acute occlusion, the second occlusion after the period of reflow caused less cell damage. Although the collateral flow (at first) decreased to the same low level, no necrosis was found. We suggest that the protective effect could be explained by rapid remodeling of the collateral vessels. Additionally, ischemic skeletal muscle shows functional adaptation to hypoxia by selective gene expression [50]. Our methods did not allow differentiating whether collateral development or adaptation participated more in tissue survival.

4.4. How to induce arteriogenesis before cell damage in lower limb occurs?
This question is crucial for patients suffering from ischemia. In the present study, we have shown that the 24- to 48-h occlusion is necessary to initiate collateral growth. On the other hand, between 6 and 12 h after occlusion, the ischemic injury in the M. gastrocnemius became irreversible [30]. This is rather discouraging, especially if we consider that the myocardium is even far less tolerant to ischemia than the skeletal muscle. We hope, however, that the multiple short-time occlusion "training" of collateral arteries [51] mimicking the real situation in patients would induce arteriogenesis without the irreversible ischemic tissue damage. Further experiments in this direction could elucidate this.

4.5. Conclusion
Cellular proliferation in and remodeling of collateral arteries were induced by short period of increased blood flow (occlusion of the femoral artery) but realized mostly during the low blood flow (reperfusion of the femoral artery). The neointima developing as a result of this remodeling can be recruited as a functional part of the arterial wall if the collateral perfusion increases as a result of repetitive occlusion of the femoral artery. The "medialization" of the neointima might cause the observed quicker gain of collateral lumen diameter and conductance, saving distal muscle tissue from the ischemia.


   Notes
 
1 Tel.: +49 6032 705401; fax: +49 6032 705419. Back

Time for primary review 31 days


   References
Top
 Abstract
 1. Introduction
 2. Material and methods
 3. Results
 4. Discussion
 References
 

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