Objective—: Aging is a major risk factor for increased ischemic tissue injury. Whether collateral rarefaction and impaired remodeling contribute to this is unknown. We quantified the number and diameter of native collaterals and their remodeling in 3-, 16-, 24-, and 31-month-old mice. Methods and Results—: Aging caused an “age-dose-dependent” greater drop in perfusion immediately after femoral artery ligation, followed by a diminished recovery of flow and increase in tissue injury. These effects were associated with a decline in collateral number, diameter, and remodeling. Angiogenesis was also impaired. Mechanistically, these changes were not accompanied by reduced recruitment of T cells or macrophages to remodeling collaterals. However, endothelial nitric oxide synthase signaling was dysfunctional, as indicated by increased protein nitrosylation and less phosphorylated endothelial nitric oxide synthase and vasodilator-stimulated phosphoprotein in collateral wall cells. The cerebral circulation exhibited a similar age-dose-dependent loss of collateral number and diameter and increased tortuosity, resulting in an increase in collateral resistance and infarct volume (eg, 6- and 3-fold, respectively, in 24-month-old mice) after artery occlusion. This was not associated with rarefaction of similarly sized arterioles. Collateral remodeling was also reduced. Conclusion—: Our findings demonstrate that aging causes rarefaction and insufficiency of the collateral circulation in multiple tissues, resulting in more severe ischemic tissue injury.

Severity of stroke varies widely among individuals. Whether differences in the extent of the native (preexisting) pial collateral circulation exist and contribute to this variability is unknown. We addressed these questions and probed for potential genetic contributions using morphometric analysis of the collateral circulation in 15 inbred mouse strains recently shown to exhibit wide differences in infarct volume. Morphometrics were determined in the unligated left hemisphere (for native collaterals) and ligated right hemisphere (for remodeled collaterals) 6 days after permanent middle cerebral artery (MCA) occlusion. Variation among strains in native collateral number, diameter, MCA, anterior cerebral artery (ACA), and posterior cerebral artery (PCA) tree territories were, respectively: 56-fold, 3-fold, 42%, 56%, and 61%. Collateral length ( P<0.001) and the number of penetrating arterioles branching from them also varied ( P<0.05). Infarct volume correlated inversely with collateral number ( P<0.0001), diameter ( P<0.0001), and penetrating arteriole number ( P<0.05) and directly with MCA territory ( P<0.05). Relative collateral conductance and MCA territory, when factored together, strongly predicted infarct volume ( P<0.0001). Outward remodeling of collaterals in the ligated hemisphere varied ∼3-fold. These data show that the extent of the native pial collateral circulation and collateral remodeling after obstruction vary widely with genetic background, and suggest that this variability, due to natural polymorphisms, is a major contributor to variability in infarct volume.

It is well known that the protective capacity of the collateral circulation falls short in many individuals with ischemic disease of the heart, brain, and lower extremities. In the past 15 years, opportunities created by molecular and genetic tools, together with disappointing outcomes in many angiogenic trials, have led to a significant increase in the number of studies that focus on: understanding the basic biology of the collateral circulation; identifying the mechanisms that limit the collateral circulation’s capacity in many individuals; devising methods to measure collateral extent, which has been found to vary widely among individuals; and developing treatments to increase collateral blood flow in obstructive disease. Unfortunately, accompanying this increase in reports has been a proliferation of vague terms used to describe the disposition and behavior of this unique circulation, as well as the increasing misuse of well-ensconced ones by new (and old) students of collateral circulation. With this in mind, we provide a brief glossary of readily understandable terms to denote the formation, adaptive growth, and maladaptive rarefaction of collateral circulation. We also propose terminology for several newly discovered processes that occur in the collateral circulation. Finally, we include terms used to describe vessels that are sometimes confused with collaterals, as well as terms describing processes active in the general arterial–venous circulation when ischemic conditions engage the collateral circulation. We hope this brief review will help unify the terminology used in collateral research.

The density of native (preexisting) collaterals and their capacity to enlarge into large conduit arteries in ischemia (arteriogenesis) are major determinants of the severity of tissue injury in occlusive disease. Mechanisms directing arteriogenesis remain unclear. Moreover, nothing is known about how native collaterals form in healthy tissue. Evidence suggests vascular endothelial growth factor (VEGF), which is important in embryonic vascular patterning and ischemic angiogenesis, may contribute to native collateral formation and arteriogenesis. Therefore, we examined mice heterozygous for VEGF receptor-1 (VEGFR-1 ^+/− ), VEGF receptor-2 (VEGFR-2 ^+/− ), and overexpressing (VEGF ^hi/+ ) and underexpressing VEGF-A (VEGF ^lo/+ ). Recovery from hindlimb ischemia was followed for 21 days after femoral artery ligation. All statements below are P <0.05. Compared to wild-type mice, VEGFR-2 ^+/− showed similar: ischemic scores, recovery of hindlimb perfusion, pericollateral leukocytes, collateral enlargement, and angiogenesis. In contrast, VEGFR-1 ^+/− showed impaired: perfusion recovery, pericollateral leukocytes, collateral enlargement, worse ischemic scores, and comparable angiogenesis. Compared to wild-type mice, VEGF ^lo/+ had 2-fold lower perfusion immediately after ligation (suggesting fewer native collaterals which was confirmed by angiography) and blunted recovery of perfusion. VEGF ^hi/+ mice had 3-fold greater perfusion immediately after ligation, more native collaterals, and improved recovery of perfusion. These differences were confirmed in the cerebral pial cortical circulation where, compared to VEGF ^hi/+ mice, VEGF ^lo/+ formed fewer collaterals during the perinatal period when adult density was established, and had 2-fold larger infarctions after middle cerebral artery ligation. Our findings indicate VEGF and VEGFR-1 are determinants of arteriogenesis. Moreover, we describe the first signaling molecule, VEGF-A, that specifies formation of native collaterals in healthy tissues.

Rationale : The collateral circulation is tissue- and life-saving in obstructive arterial disease. Disappointing outcomes in clinical trials aimed at augmenting collateral growth highlight the need for greater understanding of collateral biology. Objective : The role of endothelial nitric oxide synthase (eNOS) in forming native (preexisting) collaterals and remodeling in obstructive disease are unknown or controversial issues, respectively. Methods and Results : We compared the native collateral circulation in healthy tissue and collateral remodeling after femoral artery ligation (FAL) in wild-type and eNOS-knockout (KO) mice. Perfusion after FAL fell further in adult eNOS-KOs, in association with fewer native collaterals in hindlimb (confirmed in brain). This was not attributable to impaired collateral formation in the embryo-neonate, but rather from collateral loss during growth to adulthood. Compared to wild-type, eNOS-KOs evidenced reduced collateral remodeling, angiogenesis, and flow-mediated dilation of the arterial bed supplying the collaterals, resulting in lower perfusion and greater ischemic injury at all time points over 21 days following FAL. To probe the mechanism for impaired remodeling, we performed genome-wide expression profiling of isolated, remodeling hindlimb collaterals 24 hour after FAL. Upregulation of genes encoding cytokines/chemokines, inflammatory, stress response, and cell cycle proteins was evident in wild-type mice. In contrast, expression was lower in 40 of 44 cell cycle genes in eNOS-KO mice, in association with impaired proliferation of vascular wall cells. Conclusions : Our findings suggest a novel role for eNOS in maintaining native collateral density during natural growth to adulthood and in collateral remodeling in obstructive disease, the latter through regulation of cell proliferation.