Peripheral adaptations in trained aged rats with femoral artery stenosis

Leptomeningeal anastomoses: Mechanisms of pial collateral remodeling in ischemic stroke

Arterial collateralization, as determined by leptomeningeal anastomoses or pial collateral vessels, is a well‐established vital player in cerebral blood flow restoration and neurological recovery from ischemic stroke. A secondary network of cerebral collateral circulation apart from the Circle of Willis, exist as remnants of arteriole development that connect the distal arteries in the pia mater. Recent interest lies in understanding the cellular and molecular adaptations that control the growth and remodeling, or arteriogenesis, of these pre‐existing collateral vessels. New findings from both animal models and human studies of ischemic stroke suggest a multi‐factorial and complex, temporospatial interplay of endothelium, immune and vessel‐associated cell interactions may work in concert to facilitate or thwart arteriogenesis. These valuable reports may provide critical insight into potential predictors of the pial collateral response in patients with large vessel occlusion and may aid in therapeutics to enhance collateral function and improve recovery from stroke.

This article is categorized under:

Neurological Diseases > Molecular and Cellular Physiology

Assessing the hemodynamic contribution of capillaries, arterioles, and collateral arteries to vascular adaptations in arterial insufficiency

OBJECTIVE

There is currently a lack of clarity regarding which vascular segments contribute most significantly to flow compensation following a major arterial occlusion. This study uses hemodynamic principles and computational modeling to demonstrate the relative contributions of capillaries, arterioles, and collateral arteries at rest or exercise following an abrupt, total, and sustained femoral arterial occlusion.

METHODS

The vascular network of the simulated rat hindlimb is based on robust measurements of blood flow and pressure in healthy rats from exercise and training studies. The sensitivity of calf blood flow to acute or chronic vascular adaptations in distinct vessel segments is assessed.

RESULTS

The model demonstrates that decreasing the distal microcirculation resistance has almost no effect on flow compensation, while decreasing collateral arterial resistance is necessary to restore resting calf flow following occlusion. Full restoration of non-occluded flow is predicted under resting conditions given all chronic adaptations, but only 75% of non-occluded flow is restored under exercise conditions.

CONCLUSION

This computational method establishes the hemodynamic significance of acute and chronic adaptations in the microvasculature and collateral arteries under rest and exercise conditions. Regardless of the metabolic level being simulated, this study consistently shows the dominating significance of collateral vessels following an occlusion.

Exercise training and peripheral arterial disease

Peripheral arterial disease (PAD) is a common vascular disease that reduces blood flow capacity to the legs of patients. PAD leads to exercise intolerance that can progress in severity to greatly limit mobility, and in advanced cases leads to frank ischemia with pain at rest. It is estimated that 12 to 15 million people in the United States are diagnosed with PAD, with a much larger population that is undiagnosed. The presence of PAD predicts a 50% to 1500% increase in morbidity and mortality, depending on severity. Treatment of patients with PAD is limited to modification of cardiovascular disease risk factors, pharmacological intervention, surgery, and exercise therapy. Extended exercise programs that involve walking approximately five times per week, at a significant intensity that requires frequent rest periods, are most significant. Preclinical studies and virtually all clinical trials demonstrate the benefits of exercise therapy, including improved walking tolerance, modified inflammatory/hemostatic markers, enhanced vasoresponsiveness, adaptations within the limb (angiogenesis, arteriogenesis, and mitochondrial synthesis) that enhance oxygen delivery and metabolic responses, potentially delayed progression of the disease, enhanced quality of life indices, and extended longevity. A synthesis is provided as to how these adaptations can develop in the context of our current state of knowledge and events known to be orchestrated by exercise. The benefits are so compelling that exercise prescription should be an essential option presented to patients with PAD in the absence of contraindications. Obviously, selecting for a lifestyle pattern that includes enhanced physical activity prior to the advance of PAD limitations is the most desirable and beneficial.

Effect of cilostazol and pentoxifylline on gait biomechanics in rats with ischemic left hindlimb

OBJECTIVE

The purpose of this study was to determine the impact of pharmacologic treatment with cilostazol and pentoxifylline on gait biomechanics of ischemic rat hindlimbs compared with nonischemic controls.

METHODS

An experimental study was designed using 30 Wistar rats divided into five groups (n = 6): control (C); ischemia (I) - animals submitted to left common iliac artery interruption without pharmacologic treatment; pentoxifylline (Pen) - rats submitted to procedure and treated with pentoxifylline 3 mg/kg twice a day for 6 weeks; cilostazol (Cil) - animals submitted to procedure and treated with cilostazol 30 mg/kg twice a day for 6 weeks; and sham (S) - animals submitted to procedure without artery interruption. Gait analysis was performed using a computed treadmill. Time, number, and duration of each hindlimb contact were obtained. The total number of contacts (TNC) and the total duration of contacts (TDC) were compared between left and right hindlimb and among groups. Left hindlimb ischemic incapacitation index (LHII) was defined by the formula: LHII = (1-TNCleft x TDCleft / TNCright x TDCright) x 100.

RESULTS

Left hindlimb TNC values were twofold lower in I, Pen, and Cil groups than in C and S groups (P < .01). In I, Pen, and Cil groups, TNC values for the left hindlimb were half of the right hindlimb ones (P < .01). Left hindlimb TDC values were lower in I and Pen groups than the other groups (P < .01). Cil group presented twofold increased values, not different from C and S groups (P = 0.16). Right hindlimb TNC values were greater for I group (P < .01). LHII was around zero in C and S groups and 82 in both I and Pen groups (P < .01). Cil group presented a LHII of 42; higher than C and S groups, but lower than I and Pen groups (P < .01).

CONCLUSIONS

Cilostazol at a dose of 30 mg/kg twice a day promoted improvement in gait performance in rats submitted to chronic hindlimb ischemia. Pentoxifylline at a dose of 3 mg/kg twice a day did not show this effect.

Blockade of the TP receptor attenuates the exercise pressor reflex in decerebrated rats with chronic femoral artery occlusion

Cyclooxygenase metabolites stimulate or sensitize group III and IV muscle afferents, which comprise the sensory arm of the exercise pressor reflex. The thromboxane (TP) receptor binds several of these metabolites, whose concentrations in the muscle interstitium are increased by exercise under freely perfused conditions and even more so under ischemic conditions, which occur in peripheral artery disease. We showed that the exercise pressor reflex is greater in rats with simulated peripheral artery disease than in rats with freely perfused limbs. These findings prompted us to test the hypothesis that the TP receptor contributes to the exaggerated exercise pressor reflex occurring in a rat model of peripheral artery disease. We compared the cardiovascular responses to static contraction and stretch before and after femoral arterial injections of daltroban (80 μg), a TP receptor antagonist. We performed these experiments in decerebrate rats whose femoral arteries were ligated 72 h before the experiment (a model of simulated peripheral artery disease) and in control rats whose hindlimbs were freely perfused. Daltroban reduced the pressor response to static contraction in both freely perfused (n = 6; before: Δ12 ± 2 mmHg, after: Δ6 ± 2 mmHg, P = 0.024) and 72-h-ligated rats (n = 10; before: Δ25 ± 3 mmHg, after: Δ7 ± 4 mmHg, P = 0.001). Likewise, daltroban reduced the pressor response to stretch in the freely perfused group (n = 9; before: Δ30 ± 3 mmHg, after: Δ17 ± 3 mmHg, P < 0.0001) and in the ligated group (n = 11; before: Δ37 ± 5 mmHg, after: Δ23 ± 3 mmHg, P = 0.016). Intravenous injections of daltroban had no effect on the pressor response to contraction. We conclude that the TP receptor contributes to the pressor responses evoked by contraction and stretch in both freely perfused rats and rats with simulated peripheral artery disease.

Tissue capillary supply--it's quality not quantity that counts!

This article explores how common misunderstandings about the microcirculation - that capillary supply varies directly with O(2) demand, that local capillary supply in muscle is determined by fibre type and that it is appropriate to model capillary distribution as either random or in a fixed geometric pattern - arise from quantifying capillarity by simple measures of quantitative extent, rather than the more functionally relevant qualitative distribution. We show that the latter approach reveals exquisite control of angiogenesis that determines the location of new vessels with astonishing accuracy, motivating a reappraisal of the physiological remodelling process and a new approach to computational investigations into peripheral O(2) transport.

Effect of stimulation frequency on angiogenesis and gene expression in ischemic skeletal muscle of rabbit

To examine the comparative effects of different frequencies of electrical stimulation (ES) on angiogenesis and gene expression, New Zealand white rabbits with femoral artery ligation in one hindlimb and electrode implantation on the sciatic nerve of the same hindlimb were randomly assigned to 4 groups: control group, 1 Hz group, 10 Hz group, and 40 Hz group. The ES procedure involved 5 min stimulation, followed by 5 min rest, repeated 8 times daily for 4 consecutive weeks. The resting blood flow (RBF) was measured via the microspheres technique. Collateralization was evaluated by immunohistochemistry. Angiogenic factors were analyzed by real-time RT-PCR. Both RBF and capillary density were significantly increased in the 10 Hz and 40 Hz groups, but were not changed in the 1 Hz group. Vascular endothelial growth factor (VEGF) mRNA was highest in the 40 Hz group. Hypoxia-inducible factor 1alpha (HIF-1alpha) mRNA was significantly elevated only in the 40 Hz group. VEGF receptor fetal liver kinase 1 (Flk-1) mRNA was upregulated equally in the 10 Hz and 40 Hz groups, but fibroblast growth factor 2 (FGF-2) mRNA did not change in any group. Our results suggest that the optimal frequency of ES for angiogenesis is within the 10-40 Hz range.

Effect of ischaemic exercise training of a normal limb on angiogenesis of a pathological ischaemic limb in rabbits

The present study was designed to test the hypothesis that local exercise of a normal limb can promote angiogenesis in a pathological ischaemic limb. New Zealand White rabbits underwent left femoral artery ligation (Lig) and electrode implantation on the right sciatic nerve. The rabbits were randomly assigned to four groups: (i) Lig-N group, which did not receive ES (electrical stimulation); (ii) Lig-High group, which received high-intensity ES (2.5 mA, 40Hz for 1 ms) on the right hindlimb; (iii) Lig-Low group, which received low-intensity ES (0.3 mA, 40Hz for 1 ms) on the right hindlimb; (iv) Double-Lig-High group, which underwent femoral artery ligation on both hindlimbs and received high-intensity ES (2.5 mA, 40Hz for 1 ms) on the right hindlimb. The ES procedure included 5 min of stimulation, followed by 5 min of rest, and was repeated eight times a day for 4 weeks. Collateral circulation was examined grossly by angiography, resting blood flow was measured using the microspheres technique, and capillary supply was evaluated by immunohistochemistry. VEGF (vascular endothelial growth factor) mRNA and protein were analysed by real-time RT (reverse transcription)–PCR and Western blotting respectively. Collateral blood flow in all of the major muscles of the left hindlimb in the Lig-High group was highest among the four groups (P<0.01). Capillary supply (P<0.001), VEGF mRNA (P<0.01) and VEGF protein (P<0.01) in the gastrocnemius muscle increased remarkably in the Lig-High group; no statistically significant difference was observed among the other three groups. In conclusion, angiogenesis associated with an up-regulation of VEGF expression in pathological ischaemic limb may be facilitated by 4 weeks of physiological ischaemic exercise training in a normal limb.

Contribution of nerve growth factor to augmented TRPV1 responses of muscle sensory neurons by femoral artery occlusion

In rats, hindlimb muscle ischemia induced by femoral artery occlusion augments the sympathetic nervous response to stimulation of transient receptor potential vanilloid type 1 (TRPV1) by injection of capsaicin into the arterial blood supply of the hindlimb muscles. The enhanced sympathetic response is due to alterations in TRPV1 receptor expression and its responsiveness in sensory neurons. The underlying mechanism by which TRPV1 receptor responses are increased after muscle vascular insufficiency/ischemia is unclear. In this report we tested the hypothesis that muscle ischemia elevates nerve growth factor (NGF) levels in primary afferent neurons, thereby increasing TRPV1 responsiveness. Muscle vascular insufficiency induced by the femoral artery ligation significantly increased NGF in the dorsal root ganglion (DRG) compared with sham controls. Furthermore, when NGF was infused in the hindlimb muscles of healthy rats (72 h using an osmotic minipump), the magnitude of the DRG neuron response to capsaicin was augmented (5.4 +/- 0.54 nA with NGF infusion vs. 3.0 +/- 0.17 nA in control; P < 0.05). With the addition of NGF in the culture dish containing the DRG neurons, the magnitude of the DRG neuron response to capsaicin was greater (6.4 +/- 0.27 nA; P < 0.05 vs. control) than that seen in control (2.9 +/- 0.16 nA). Note that this NGF effect was seen in isolectin B(4)-negative DRG neurons, a group of thin fiber nerves that contain neuropeptides and depend on NGF for survival. These data suggest that NGF affects a selective subpopulation of the afferent neurons in mediating augmented TRPV1 responses after femoral artery occlusion.

Inhibition of angiogenesis by high salt diet is associated with impaired muscle performance following chronic muscle stimulation

OBJECTIVE

High dietary salt has been demonstrated to inhibit angiogenesis in skeletal muscle. The purpose of this study was to determine whether high salt impairs steady state muscle performance following a chronic stimulation protocol.

METHODS

Sprague-Dawley rats were placed on a control diet (CD, 0.4% NaCl) or high salt diet (HSD, 4.0% NaCl) prior to implantation of an electrical muscle stimulator. In chronically stimulated animals, hind limb muscles were stimulated to contract eight hours daily for seven days. Sham animals received a stimulator that was never activated.

RESULTS

Following chronic stimulation, tibialis anterior (TA) muscles of animals on CD demonstrated an 84.6% increase in force of contraction at the end of an acute stimulation bout relative to sham animals fed CD. Decreased muscle fatigue was associated with an increase in capillaries per TA fiber (C:F). Chronic stimulation in HSD rats induced a smaller improvement (52.2%) in final force compared to HSD sham rats. This impairment of muscle performance in high salt-fed rats correlated with inhibited angiogenesis. Infusion of angiotensin II in HSD animals restored angiogenesis and muscle fatigue to CD levels.

CONCLUSIONS

This study suggests that angiogenic inhibition by high salt is associated with impaired skeletal muscle performance following chronic stimulation.

Femoral artery occlusion augments TRPV1-mediated sympathetic responsiveness

Muscle metabolic by-products stimulate thin fiber muscle afferent nerves and evoke reflex increases in blood pressure and sympathetic nerve activity. Previous studies reported that chemically sensitive transient receptor potential vanilloid type 1 (TRPV1) channels present on sensory muscle afferent neurons have an important impact on sympathetically mediated cardiovascular responses. The reflex-mediated reduction in blood flow to skeletal muscle leads to limited exercise capacity in patients with peripheral arterial occlusive disease. Thus, in this study, we tested the hypothesis that the expression of enhanced TRPV1 receptor and its responsiveness in primary afferent neurons innervating muscles initiate exaggerated reflex sympathetic responses after vascular insufficiency to the muscle. Muscle vascular insufficiency was induced by the femoral artery ligation in rats for 24 h. Our data show that 1) the ligation surgery leads to the upregulation of TRPV1 expression in the dorsal root ganglion; 2) the magnitude of the dorsal root ganglion neuron TRPV1 response induced by capsaicin is greater in vascular insufficiency (4.0 +/- 0.31 nA, P < 0.05 vs. sham-operated control) than that in sham-operated control (2.9 +/- 0.23 nA); and 3) renal sympathetic nerve activity and mean arterial pressure responses to capsaicin (0.5 microg/kg body wt) are also enhanced by vascular insufficiency (54 +/- 11%, 9 +/- 2 mmHg in sham-operated controls vs. 98 +/- 13%, 33 +/- 5 mmHg after vascular insufficiency, P < 0.05). In conclusion, sympathetic nerve responses to the activation of metabolite-sensitive TRPV1 receptors are augmented in rats with the femoral artery occlusion compared with sham-operated control animals, due to alterations in the expression of TRPV1 receptor and its responsiveness in sensory neurons.

Effects of a moderate-intensity static magnetic field on VEGF-A stimulated endothelial capillary tubule formation in vitro

Effects of a moderate-intensity static magnetic field (SMF) on the early-stage development of endothelial capillary tubule formation were examined during the initial cell growth periods using co-cultured human umbilical vein endothelial cells and human diploid fibroblasts. The co-cultured cells within a well (16 mm in diameter) were exposed to SMF intensity up to 120 mT (Bmax) with the maximum spatial gradient of 21 mT/mm using a disc-shaped permanent magnet (16 mm in diameter and 2.5 mm in height) for up to 10 days. Control exposure was performed without magnet. Some vascular endothelial cells were treated with vascular endothelial growth factor (VEGF)-A (10 ng/ml) to promote the tubule formation every 2-3 days. Four experimental protocols were performed: (1) non-exposure (control); (2) SMF exposure alone; (3) non-exposure with VEGF-A; (4) SMF exposure with VEGF-A. Photomicrographs of tubule cells immunostained with an anti-platelet-endothelial cell adhesion molecule-1 (PECAM-1 [CD31[) antibody as a pan-endothelial marker, were analyzed after culture at 37 degrees C for 4, 7, and 10 days. The mean values of the area density and the length of tubules (related mainly to arteriogenesis) as well as the number of bifurcations (related mainly to angiogenesis) were determined as parameters of tubule formation and were compared between the groups. After a 10 day incubation, in the peripheral part of the culture wells, SMF alone significantly promoted the tubule formation in terms of the area density and the length of tubules, compared with control group. In the central part of the wells, however, SMF did not cause any significant changes in the parameters of tubule formation. After a 7 day incubation, VEGF-A significantly promoted all the parameters of tubule formation in any part of the wells, compared with control group. With regard to the synergistic effects of SMF and VEGF-A on tubule formation, after a 10 day incubation, SMF significantly promoted the VEGF-A-increased area density and length of tubules in the peripheral part of the wells, compared with the VEGF-A treatment alone. However, SMF did not induce any significant changes in the VEGF-A-increased number of bifurcations in any part of the wells. The tubule cells observed in the wells had elongated, spindle-like shapes, and the direction of cell elongation was random, irrespective of the presence and direction of SMF. These findings suggest that the application of SMF to intact or VEGF-A-stimulated vascular endothelial cells leads mainly to promote or enhance arteriogenesis in the peripheral part of the wells, where the spatial gradient increases relative to the central part. The effects of SMF on the VEGF-A-enhanced tubule formation appear to be synergistic or additive in arteriogenesis but not in angiogenesis.

Angiogenesis during exercise and training

In this review the factors involved in angiogenesis are discussed in their various roles in initiating angiogenesis and inducing changes in the extracellular matrix to facilitate sprouting angiogenesis which is a major part of the angiogenesis seen in exercise and exercise training. A key role in angiogenesis is played by vascular endothelial growth factor (VEGF). The regulation of blood vessel growth to match the needs of the tissue depends on the control of VEGF production through changes in the stability of its mRNA and in its rate of transcription. The detailed studies describing its characteristics and its upregulation in acute exercise are presented along with a brief overview of the changes in the extracellular matrix that facilitate sprouting angiogenesis that occurs in response to exercise and training. Although the mechanisms involved in the growth and remodeling of arterioles and larger vessels are less detailed some recent studies have provided new insights. These are presented here to show a relationship between capillary development and arteriolar growth or remodeling in exercise training that raises questions to be addressed in future studies.

Muscle reflex control of sympathetic nerve activity in heart failure: the role of exercise conditioning

Muscle reflex control of sympathetic nerve activity has been an area of considerable investigation. During exercise, the capacity of the peripheral vasculature to dilate far exceeds the maximal attainable levels of cardiac output. The activation of sympathetic nervous system and engagement of the myogenic reflex serve as the controlling influence between the heart and the muscle vasculature to maintain blood pressure (BP). Two basic theories of neural control have evolved. The first termed "central command", suggests that a volitional signal emanating from central motor areas leads to increased sympathetic activation during exercise. According to the second theory the stimulation of mechanical and chemical afferents in exercising muscle lead to engagement of the "exercise pressor reflex". Some earlier studies suggested that group III muscle afferent fibers are predominantly mechanically sensitive whereas unmyelinated group IV muscle afferents respond to chemical stimuli. In recent years new evidence is emerging which challenges the concept of functional differentiation of muscle afferents as well as the classic description of muscle "mechano" and "metabo" receptors. Studies measuring concentrations of interstitial substances during exercise suggest that K(+) and phosphate, but not H(+) and lactate, may be important muscle afferent stimulants. The role of adenosine as a muscle afferent stimulant remains an area of debate. There is strong evidence that sympathetic vasoconstriction due to muscle reflex engagement plays an important role in restricting blood flow to the exercising muscle. In heart failure (HF), exercise leads to premature fatigue and accumulation of muscle metabolites resulting in a greater degree of muscle reflex engagement and in the process further decreasing the muscle blood flow. Conditioning leads to an increased ability of the muscle to maintain aerobic metabolism, lower interstitial accumulation of metabolites, less muscle reflex engagement and a smaller sympathetic response. Beneficial effects of physical conditioning may be mediated by a direct reduction of muscle metaboreflex activity or via reduction of metabolic signals activating these receptors. In this review, we will discuss concepts of flow and reflex engagement in normal human subjects and then contrast these findings with those seen in heart failure (HF). We will then examine the effects of exercise conditioning on these parameters in normal subjects and those with congestive heart failure (CHF).

Age is no barrier to muscle structural, biochemical and angiogenic adaptations to training up to 24 months in female rats

Ageing is associated with reduced transport and utilization of O(2), diminishing exercise tolerance. Reductions may occur in cardiac output (delivery), and skeletal muscle oxidative capacity (utilization). To determine the reversibility of the declines in the muscular determinants of these limitations, skeletal muscle morphological, angiogenic and biochemical responses to acute exercise and endurance training were investigated in female Fischer 344 rats (n = 42; seven groups of six rats) aged 6 (Y) and 24 (O) months compared with resting untrained controls (Y(C), O(C)). Treadmill training lasted 8 weeks (10 deg incline, 1 h per day, 5 days per week). Two groups ran at maximum tolerated speeds (Y(TR), O(TR)), while an additional Y group (Y(TM)) trained at O(TR) speed. There was no effect of age on vascular endothelial growth factor gene expression in gastrocnemius muscles after acute exercise. Similarly, age did not impair the effects of training, with increases (P < 0.05; +/-s.e.m.) occurring in all of the following: 1 h exercise running speed (Y(TR) 92 +/- 4% versus O(TR) 140 +/- 25%); citrate synthase (Y(TR) 37 +/- 8% versus O(TR) 97 +/- 33%) and beta-hydroxyacyl-CoA-dehydrogenase (Y(TR) 31 +/- 7%, versus O(TR) 72 +/- 24%) activities; and capillary-to-fibre ratio (Y(TR) 5.2 +/- 0.2% versus O(TR) 8.1 +/- 0.2%). However, Y(TM) muscle was unchanged in each measure compared with Y(C). In conclusion, these muscular responses to training were (1) not reduced by ageing, but (2) dependent on relative and not absolute work rate, since, at the same speed, O(TR) rats showed greater changes than Y(TM). Therefore, increases in exercise tolerance and muscle adaptations are not impaired in female rats up to 24 months of age, and require a smaller absolute exercise stimulus (than young) to be manifest.

VEGF receptor antagonism blocks arteriogenesis, but only partially inhibits angiogenesis, in skeletal muscle of exercise-trained rats

Both collateral vessel enlargement (arteriogenesis) and capillary growth (angiogenesis) in skeletal muscle occur in response to exercise training. Vascular endothelial growth factor (VEGF) is implicated in both processes. Thus we examined the effect of a VEGF receptor (VEGF-R) inhibitor (ZD4190, AstraZeneca) on collateral-dependent blood flow in vivo and collateral artery size ex vivo (indicators of arteriogenesis) and capillary contacts per fiber (CCF; an index of angiogenesis) in skeletal muscle of both sedentary and exercise-trained rats 14 days after bilateral occlusion of the femoral arteries. Total daily treadmill run time increased appreciably from ∼70 to ∼100 min (at 15–20 m/min, twice per day) and produced a large (∼75%, P < 0.01) increase in calf muscle blood flow and a greater size of the collateral artery (wall cross-sectional area). ZD4190, which previously has been shown to inhibit the activity of VEGF-R2 and -R1 tyrosine kinase in vitro (IC50 = 30 and 700 nM, respectively), completely blocked the increase in collateral-dependent blood flow and inhibited collateral vessel enlargement. Thus exercise-stimulated collateral arteriogenesis appears to be completely dependent on VEGF-R signaling. Interestingly, enhanced mRNA expression of the VEGF family ligand placental growth factor (2- to 3.5-fold), VEGF-R1 (∼2-fold), and endothelial nitric oxide synthase (2- to 3.5-fold) in an isolated collateral artery implicates these factors as important in arteriogenesis. Training of ischemic muscle also induced angiogenesis, as shown by an increase (∼25%, P < 0.01) in CCF in white gastrocnemius muscle. VEGF-R inhibition only partially blocked ( P < 0.01) but did not eliminate the increase ( P < 0.01) in capillarity. Our findings indicate that VEGF-R tyrosine kinase activity is essential for collateral arteriogenesis and important for the angiogenesis induced in ischemic muscle by exercise training; however, other angiogenic stimuli are also important for angiogenesis in flow-limited active muscle.

What makes vessels grow with exercise training?

Exercise and muscle contractions create a powerful stimulus for structural remodeling of the vasculature. An increase in flow velocity through a vessel increases shear stress, a major stimulus for enlargement of conduit vessels. This leads to an endothelial-dependent, nitric oxide-dependent enlargement of the vessel. Increased flow within muscle, in the absence of contractions, leads to an enhanced capillarity by intussusceptive angiogenesis, a process of capillary splitting by intraluminal longitudinal divide. In contrast, sprouting angiogenesis requires extensive endothelial cell proliferation, with degradation of the extracellular matrix to permit migration and tube formation. This occurs during muscle adaptations to chronic contractions and/or muscle overload. The angiogenic growth factor VEGF appears to be an important element in angiogenesis. Recent advances in research have identified hemodynamic and mechanical stimuli that upregulate angiogenic processes, demonstrated a complexity of potent growth factors and interactions with their corresponding receptors, detected an interaction of cellular signaling events, and identified important tissue reorganization processes that must be coordinated to effect vascular remodeling. It is likely that much of this information is applicable to the vascular remodeling that occurs in response to exercise and/or muscle contractions.

Time course of changes in collateral blood flow and isolated vessel size and gene expression after femoral artery occlusion in rats

The objectives of this study were to assess the time course of enlargement and gene expression of a collateral vessel that enlarges following occlusion of the femoral artery and to relate these responses to the increases in collateral-dependent blood flow to the calf muscles in vivo. We employed exercise training to stimulate collateral vessel development. Rats were exercise trained or kept sedentary for various times of up to 25 days postbilateral occlusion (n=approximately 9/time point). Collateral blood flow to the calf muscles, determined with microspheres, increased modestly over the first few days to approximately 40 ml.min(-1).100 g(-1) in sedentary animals; the increase continued over time to approximately 80 ml.min(-1).100 g(-1) in the trained animals. Diameters of the isolated collateral vessels increased progressively over time, whereas an increased vessel compliance observed at low pressures was similar across time. These responses were greater in the trained animals. The time course of upregulation of vascular endothelial growth factor and placental growth factor, and particularly endothelial nitric oxide synthase and fms-like tyrosine kinase 1, mRNAs in the isolated collateral vessel implicates these factors as integral to the arteriogenic process. Collateral vessel enlargement and increased compliance at low pressures contribute to the enlarged circuit available for collateral blood flow. However, modulation of the functioning collateral vessel diameter, by smooth muscle tone, must occur to account for the observed increases in collateral blood flow measured in vivo.

Angiogenic growth factor expression in rat skeletal muscle in response to exercise training

Angiogenesis occurs in skeletal muscle in response to exercise training. To gain insight into the regulation of this process, we evaluated the mRNA expression of factors implicated in angiogenesis over the course of a training program. We studied sedentary control (n = 17) rats and both sedentary (n = 18) and exercise-trained (n = 48) rats with bilateral femoral artery ligation. Training consisted of treadmill exercise (4 times/day, 1-24 days). Basal mRNA expression in sedentary control muscle was inversely related to muscle vascularity. Angiogenesis was histologically evident in trained white gastrocnemius muscle by day 12. Training produced initial three- to sixfold increases in VEGF, VEGF receptors (KDR and Flt), the angiopoietin receptor (Tie-2), and endothelial nitric oxide synthase mRNA, which dissipated before the increase in capillarity, and a substantial (30- to 50-fold) but transient upregulation of monocyte chemoattractant protein 1 mRNA. These results emphasize the importance of early events in regulating angiogenesis. However, we observed a sustained elevation of the angiopoietin 2-to-angiopoietin 1 ratio, suggesting continued vascular destabilization. The response to exercise was (in general) tempered in high-oxidative muscles. These findings place importance on cellular events coupled to the onset of angiogenesis.

Arteriogenesis and angiogenesis in rat ischemic hindlimb: role of nitric oxide

Nitric oxide (NO) has been implicated in both collateral expansion (arteriogenesis) and capillary growth (angiogenesis). Exercise training increases collateral-dependent blood flow to tissues at risk of ischemia and enhances capillarity in active skeletal muscle. Exercise also acutely elevates NO. Thus we assessed the role of NO in training-induced arteriogenesis and angiogenesis. These studies utilized a rat model of peripheral vascular disease (bilateral femoral artery ligation). Untreated rats (control) and rats treated with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 65-70 mg x kg(-1) x day(-1), via drinking water) were divided into sedentary or exercise-trained subgroups. After approximately 3 wk, L-NAME treatment had elevated preexercise mean arterial pressure approximately 39-58%, confirming NO synthesis inhibition. The training program (treadmill exercise twice per day, 20-25 m/min, 15% grade, approximately 18 days) increased collateral-dependent blood flow to the distal hindlimb, with the greatest increase (approximately 59%) in the calf (P < 0.001). This increase was inhibited by L-NAME. In contrast, the training-induced increase in muscle capillarity was not blocked by L-NAME. Thus arteriogenesis and angiogenesis appear to differ in their requirement for NO.

Prior exercise training increases collateral-dependent blood flow in rats after acute femoral artery occlusion

We evaluated whether prior training would improve collateral blood flow (BF) to the calf muscles after acute-onset occlusion of the femoral artery. Exercise training was performed in the absence of any vascular occlusion. Adult male Sprague-Dawley rats ( approximately 325 g) were kept sedentary (n = 14), limited to cage activity, or exercise trained (n = 14) for 6 wk by treadmill running. Early in the day of measurement, animals were surgically prepared for BF determination, and the femoral arteries were occluded bilaterally. Four to five hours later, collateral BF was determined twice during treadmill running with the use of (141)Ce and (85)Sr microspheres: first, at a demanding speed and, second, after a brief rest and at a higher speed. The absence of any further increase in BF at the higher speed indicated that maximal collateral BF was measured. Prior training increased calf muscle BF by approximately 70% compared with sedentary animals; however, absolute BF remained below values previously observed in animals with a well-developed collateral vascular tree. Thus prior training appeared to optimize the use of the existing collateral circuit. This implies that altered vasoresponsiveness induced in normal nonoccluded vessels with exercise training serves to improve collateral BF to the periphery.

Effects of forearm bier block with bretylium on the hemodynamic and metabolic responses to handgrip

We tested the hypothesis that a reduction in sympathetic tone to exercising forearm muscle would increase blood flow, reduce muscle acidosis, and attenuate reflex responses. Subjects performed a progressive, four-stage rhythmic handgrip protocol before and after forearm bier block with bretylium as forearm blood flow (Doppler) and metabolic (venous effluent metabolite concentration and (31)P-NMR indexes) and autonomic reflex responses (heart rate, blood pressure, and sympathetic nerve traffic) were measured. Bretylium inhibits the release of norepinephrine at the neurovascular junction. Bier block increased blood flow as well as oxygen consumption in the exercising forearm (P < 0.03 and P < 0.02, respectively). However, despite this increase in flow, venous K(+) release and H(+) release were both increased during exercise (P < 0.002 for both indexes). Additionally, minimal muscle pH measured during the first minute of recovery with NMR was lower after bier block (6.41 +/- 0.08 vs. 6.20 +/- 0.06; P < 0.036, simple effects). Meanwhile, reflex effects were unaffected by the bretylium bier block. The results support the conclusion that sympathetic stimulation to muscle during exercise not only limits muscle blood flow but also appears to limit anaerobiosis and H(+) release, presumably through a preferential recruitment of oxidative fibers.

bFGF increases collateral blood flow in aged rats with femoral artery ligation

We tested the hypothesis that aged animals are as responsive as the young adult animals in expanding collateral vasculature under a similar treatment of basic fibroblast growth factor (bFGF). Two age groups of male Fischer 344 rats (11 mo old; n = 32, 23 mo old; n = 43) weighing approximately 385 g were subdivided into normal, acute ligation [femoral artery (FA) ligated 3 days before blood flow (BF) measurement] or ligated groups for 16 days and received recombinant human bFGF intra-arterial infusion at doses of 0, 0.5, 5, and 50 microg x kg(-1) x day(-1). BF was determined with (85)Sr- and (141)Ce-labeled microspheres during treadmill running at 15 and 20 m/min at 15% grade. Blood pressure (BP) values were approximately 149 and approximately 163 mmHg (p < 0.05); heart rates were approximately 496 and approximately 512 beats/min in the aged and young adult groups during running, respectively. Maximal collateral BF values were confirmed by no additional BF increase in the calf muscle at the higher speed. Ligation of the FA for 3 days reduced the BF reserve to the calf muscle by approximately 90%. Calf muscle BF was modestly greater (10 ml x min(-1) x 100 g(-1)) by 16 days in the carrier group. bFGF infusion expanded collateral BF in a dose-dependent manner with an increase of 33 and 42 ml x min(-1) x 100 g(-1) (P < 0.001) in the 5 and 50 microg x kg(-1) x day(-1) bFGF groups, respectively. Aged animals showed similar BF improvements as observed with the adult groups in response to ligation surgery and bFGF treatment. Our data indicate that the aged rats (approximately 23 mo old) remain responsive to exogenous bFGF induced in developing collateral-dependent BF as the young adult (approximately 11 mo old) controls. This suggests that the influence of bFGF in expanding collateral BF should not be preempted in the aged group, the population most affected by peripheral arterial insufficiency.

Exercise training enhances basic fibroblast growth factor-induced collateral blood flow

This study evaluated whether daily exercise would enhance the peripheral collateral vessel development found in response to exogenous basic fibroblast growth factor (bFGF) infusion. After bilateral femoral occlusion, male Sprague-Dawley rats (approximately 325 g) received intra-arterial infusions of either bFGF (1 microg/day; n = 15) or carrier solution (n = 13) via osmotic pumps for 2 wk. Subgroups of each treatment were kept sedentary (cage activity) or trained by walking at 20 m/min at 15% grade, two times a day, 5 days/wk for 4 wk. Training markedly increased citrate synthase activity in the active muscle (P < 0.001). Muscle function and blood flows (85Sr microsphere) were evaluated using an isolated hindquarter perfused at 100 mmHg via the abdominal aorta. The significant increase in blood flow to the entire hindlimb in the sedentary animals, caused by bFGF infusion (P < 0.05), was further increased (P < 0.01) in the bFGF-trained group. The quantitatively largest increases in blood flows were observed in the collateral-dependent tissues of the distal hindlimb. Blood flows to the entire calf muscle group increased approximately 140% in carrier-trained (P < 0.001), approximately 180% in bFGF sedentary (P < 0.001), and approximately 240% in the bFGF-trained (P < 0.001) groups compared with the carrier sedentary group. The increases in collateral blood flow were functionally important, as improvements in calf muscle performance correlated with measured blood flows. Our results demonstrate that exogenous bFGF administration in combination with a moderate-intensity exercise program greatly increases collateral-dependent blood flow and improves muscle performance. That physical activity enriched the bFGF response is consistent with the hypothesis that hemodynamic factors are important contributors to collateral vessel enlargement.

Regional blood flows in the goat latissimus dorsi muscle before and after chronic stimulation

Latissimus dorsi muscle (LDM) regional blood flows were determined in anesthetized goats by using colored microspheres under noncontracting and contracting conditions, either before or after 8-10 wk of chronic muscle stimulation. Surgical dissection of the LDM, leaving only the thoracodorsal artery to supply the muscle, did not alter regional noncontracting blood flows but significantly reduced the normal hyperemic response to muscle contraction in muscle regions (posterior-medial) furthest from the entrance of the thoracodorsal artery. Eight to 10 wk after acute muscle dissection, posterior-medial hyperemic flows were restored. Chronic stimulation of the LDM for 8-10 wk, in either dissected or nondissected muscles, did not alter regional blood flows in noncontracting muscle; however, it significantly reduced hyperemic flows in all muscle regions, although capillary density was increased and the muscle was transformed into a predominantly type I fiber type. These results, coupled with data from previous experiments, suggest that the muscle damage observed in the posterior-medial regions of the LDM after surgical dissection and chronic stimulation may be related to reduced hyperemic flow responses caused by surgical isolation of the muscle.

Heparin increases exercise-induced collateral blood flow in rats with femoral artery ligation

The potential for heparin to enhance the training-induced increase in collateral-dependent blood flow to the distal hind-limb muscles was evaluated after bilateral femoral artery ligation in adult male rats (approximately 350 g). Rats received either saline (n = 34) or heparin (n = 36) injections and were kept sedentary (limited to cage activity) or exercised on a treadmill 5 days per week up a 15% incline by one of two protocols: (1) exercise at a constant moderate speed (20 m/min) for approximately 6 wks or (2) exercise at a progressively increased speed for 7 to 8 weeks (started at 20 m/min, increased at 15 minutes to 25 m/min, and then increased at 30 minutes to 30 m/min). Heparin- and saline-treated rats, exercised by the moderate-speed protocol, were run for the same time each day. Collateral-dependent blood flow to the distal limb tissue was determined by using 15-microns 85Sr-labeled microspheres in an isolated hindquarter preparation perfused in the descending aorta at 100 mm Hg. For comparison with the above groups, sedentary animals with acute femoral artery ligation and without femoral obstruction were included. Exercise tolerance increased from approximately 7 minutes initially to 30 to 40 minutes per bout; tolerance was greater in the heparin-injected rats than in the saline-injected rats (P < .05). Muscle performance of the gastrocnemius-plantaris-soleus muscle group (GPS) during isometric contractions in situ improved with training, was further increased by heparin administration (P < .001), and generally scaled with recovery of blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)