Training-induced vascular adaptations to ischemic muscle

Peripheral arterial insufficiency is a progressive degenerative disease associated with an increased morbidity and mortality. It decreases exercise tolerance and often presents with symptoms of intermittent claudication. Enhanced physical activity is one of the most effective means of improving the life of affected patients. While this occurs for a variety of reasons, vascular remodeling can be an important means for improved oxygen exchange and blood flow delivery. Relevant exercise-induced signals stimulate angiogenesis, within the active muscle (e.g. hypoxia), and arteriogenesis (enlargement of pre-existing vessels via increased shear stress) to increase oxygen exchange and blood flow capacity, respectively. Evidence from pre-clinical studies shows that the increase in collateral blood flow observed with exercise progresses over time of training, is accompanied by significant enlargement of isolated collateral vessels, and enhances the responses observed with angiogenic growth factors (e.g. VEGF, FGF-2). Thus, enhanced physical activity can be an effective means of enlarging the structure and function of the collateral circuit. Interestingly, disrupting normal NO production (via L-NAME) eliminates this increase in collateral blood flow induced by training, but does not disturb the increase in muscle capillarity within the active muscle. Similarly, inhibiting VEGF receptor kinase activity eliminates the increase in collateral-dependent blood flow, and lessens, but does not eliminate, angiogenesis within the calf muscle. These findings illustrate distinctions between the processes influencing angiogenesis and arteriogenesis. Further, sympathetic modulation of the collateral circuit does not eliminate the increase in collateral circuit conductance induced by exercise training. These findings indicate that structural enlargement of the collateral vessels is essential to realize the increase in collateral-dependent blood flow capacity caused by exercise training. This raises the potential that meaningful vascular remodeling can occur in patients with intermittent claudication who actively participate in exercise training.

Arteriogenesis: role of nitric oxide

Arteriogenesis is an important process for adapting the pre-existing circuit of vessels into functional collateral conduits for delivery of oxygen enriched blood to tissue distal to occlusion of a large, peripheral conduit artery. Recent evidence has shown that arteriogenesis is regulated by nitric oxide (NO), angiogenic factors and shear stress. NO significantly impacts vasomotor tone to enhance conductance of the newly recruited collateral arteries, and this effect is augmented by exercise training prior to arterial occlusion. NO-mediated increases in vascular conductance allows for greater collateral dependent blood flow to the tissue distal to occlusion. NO production is also critical to the efficacy of therapeutic arteriogenesis achieved by delivery of exogenous angiogenic growth factors (VEGF, FGF-2) or by exercise training. The critical role of NO in therapeutic arteriogenesis is independent of NO-mediated changes in vascular conductance and implies a central role in arteriogenic signaling events. Maintenance, or improvement, of NO production and signaling, such as with regular exercise, may improve endothelial cell function and thus may help preserve the arteriogenic potential of preexisting collateral networks.

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.

Caveolae and the organization of carbohydrate metabolism in vascular smooth muscle

We have previously found that glycolysis and gluconeogenesis occur in separate "compartments" of the VSM cell. These compartments may result from spatial separation of glycolytic and gluconeogenic enzymes (Lloyd and Hardin [1999] Am J Physiol Cell Physiol. 277:C1250-C1262). We have also found that an intact plasma membrane is essential for compartmentation to exist (Lloyd and Hardin [2000] Am J Physiol Cell Physiol. 278:C803-C811), suggesting that glycolysis and gluconeogenesis may be associated with distinct plasma membrane microdomains. Caveolae are one such microdomain, in which proteins of related function colocalize. Thus, we hypothesized that membrane-associated glycolysis occurs in association with caveolae, while gluconeogenesis is localized to non-caveolae domains. To test this hypothesis, we disrupted caveolae in vascular smooth muscle (VSM) of pig cerebral microvessels (PCMV) with beta methyl-cyclodextrin (CD) and examined the metabolism of [2-(13)C]glucose (a glycolytic substrate) and [1-(13)C]fructose 1,6-bisphosphate (FBP, a gluconeogenic substrate in PCMV) using (13)C nuclear magnetic resonance spectroscopy. Caveolar disruption reduced flux of [2-(13)C]glucose to [2-(13)C]lactate, suggesting that caveolar disruption partially disrupted the glycolytic pathway. Caveolae disruption may also have resulted in a breakdown of compartmentation, since conversion of [1-(13)C]FBP to [3-(13)C]lactate was increased by CD treatment. Alternatively, the increased [3-(13)C]lactate production may reflect changes in FBP uptake, since conversion of [1-(13)C]FBP to [3-(13)C]glucose was also elevated in CD-treated cells. Thus, a link between caveolar organization and metabolic organization may exist.

Examining glucose transport in single vascular smooth muscle cells with a fluorescent glucose analog

Changes in vascular smooth muscle glucose transport are thought to contribute to the pathogenesis of vascular disease in conditions such as diabetes, yet no single-cell assay for glucose uptake by VSM exists. Therefore, we examined the uptake of the fluorescent glucose analog 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxyglucose (2-NBDG) in isolated pig vascular smooth muscle cells (VSMC) using digital imaging microscopy. Uptake of 2-NBDG by VSMC was inhibited by D-glucose but not by L-glucose, suggesting that 2-NBDG enters VSMC via glucose transporters. Uptake of 2-NBDG was linear in the presence of 10 mM D-glucose (n=6, R2=0.9408) but not in its absence (n=4, R2=0.9993), indicating that 2-NBDG is not metabolized and accumulates within the cells. 2-NBDG fluorescence in VSMC was often non-uniform and appeared to represent binding of 2-NBDG to some cytoplasmic component. The present study demonstrates that 2-NBDG is a useful tool for examining vascular smooth muscle glucose uptake at the single cell level.

Sorting of metabolic pathway flux by the plasma membrane in cerebrovascular smooth muscle cells

We used beta-escin-permeabilized pig cerebral microvessels (PCMV) to study the organization of carbohydrate metabolism in the cytoplasm of vascular smooth muscle (VSM) cells. We have previously demonstrated (Lloyd PG and Hardin CD. Am J Physiol Cell Physiol 277: C1250-C1262, 1999) that intact PCMV metabolize the glycolytic intermediate [1-(13)C]fructose 1,6-bisphosphate (FBP) to [1-(13)C]glucose with negligible production of [3-(13)C]lactate, while simultaneously metabolizing [2-(13)C]glucose to [2-(13)C]lactate. Thus gluconeogenic and glycolytic intermediates do not mix freely in intact VSM cells (compartmentation). Permeabilized PCMV retained the ability to metabolize [2-(13)C]glucose to [2-(13)C]lactate and to metabolize [1-(13)C]FBP to [1-(13)C]glucose. The continued existence of glycolytic and gluconeogenic activity in permeabilized cells suggests that the intermediates of these pathways are channeled (directly transferred) between enzymes. Both glycolytic and gluconeogenic flux in permeabilized PCMV were sensitive to the presence of exogenous ATP and NAD. It was most interesting that a major product of [1-(13)C]FBP metabolism in permeabilized PCMV was [3-(13)C]lactate, in direct contrast to our previous findings in intact PCMV. Thus disruption of the plasma membrane altered the distribution of substrates between the glycolytic and gluconeogenic pathways. These data suggest that organization of the plasma membrane into distinct microdomains plays an important role in sorting intermediates between the glycolytic and gluconeogenic pathways in intact cells.

Role of microtubules in the regulation of metabolism in isolated cerebral microvessels

We used (13)C-labeled substrates and nuclear magnetic resonance spectroscopy to examine carbohydrate metabolism in vascular smooth muscle of freshly isolated pig cerebral microvessels (PCMV). PCMV utilized [2-(13)C]glucose mainly for glycolysis, producing [2-(13)C]lactate. Simultaneously, PCMV utilized the glycolytic intermediate [1-(13)C]fructose 1,6-bisphosphate (FBP) mainly for gluconeogenesis, producing [1-(13)C]glucose with only minor [3-(13)C]lactate production. The dissimilarity in metabolism of [2-(13)C]FBP derived from [2-(13)C]glucose breakdown and metabolism of exogenous [1-(13)C]FBP demonstrates that carbohydrate metabolism is compartmented in PCMV. Because glycolytic enzymes interact with microtubules, we disrupted microtubules with vinblastine. Vinblastine treatment significantly decreased [2-(13)C]lactate peak intensity (87.8 +/- 3.7% of control). The microtubule-stabilizing agent taxol also reduced [2-(13)C]lactate peak intensity (90.0 +/- 2. 4% of control). Treatment with both agents further decreased [2-(13)C]lactate production (73.3 +/- 4.0% of control). Neither vinblastine, taxol, or the combined drugs affected [1-(13)C]glucose peak intensity (gluconeogenesis) or disrupted the compartmentation of carbohydrate metabolism. The similar effects of taxol and vinblastine, drugs that have opposite effects on microtubule assembly, suggest that they produce their effects on glycolytic rate by competing with glycolytic enzymes for binding, not by affecting the overall assembly state of the microtubule network. Glycolysis, but not gluconeogenesis, may be regulated in part by glycolytic enzyme-microtubule interactions.