Vascular smooth muscle glycogen metabolism studied by 13C-NMR

Metabolism of vascular smooth muscle cells in vascular diseases

Vascular smooth muscle cells (VSMCs) are the fundamental component of the medial layer of arteries and are essential for arterial physiology and pathology. It is becoming increasingly clear that VSMCs can alter their metabolism to fulfill the bioenergetic and biosynthetic requirements. During vascular injury, VSMCs switch from a quiescent "contractile" phenotype to a highly migratory and proliferative "synthetic" phenotype. Recent studies have found that the phenotype switching of VSMCs is driven by a metabolic switch. Metabolic pathways, including aerobic glycolysis, fatty acid oxidation, and amino acid metabolism, have distinct, indispensable roles in normal and dysfunctional vasculature. VSMCs metabolism is also related to the metabolism of endothelial cells. In the present review, we present a brief overview of VSMCs metabolism and how it regulates the progression of several vascular diseases, including atherosclerosis, systemic hypertension, diabetes, pulmonary hypertension, vascular calcification, and aneurysms, and the effect of the risk factors for vascular disease (aging, cigarette smoking, and excessive alcohol drinking) on VSMC metabolism to clarify the role of VSMCs metabolism in the key pathological process.

Glycogen turnover forms lactate during exercise

The power needs of muscle are supplied by rapid anaerobic glycogenolytic ATP generation. Lactate is built up by the mismatch between steady state energy needs and short-term power demands and the increased concentration drives the lactate shuttle. In this model, contributions to fatigue should be looked for in the flux of glucose through glycogen rather than in the concentrations of fuel.

Mitochondrial Oxidative Substrate Selection in Porcine Bladder Smooth Muscle

PURPOSE

Alterations in bladder smooth muscle (BSM) metabolism due to alterations in plasma lipid levels may be important with the increasingly high fat diets eaten by most Americans. To determine the susceptibility of BSM to lipotoxicity we examined the normal pattern of mitochondrial substrate selection in BSM and the ability of BSM to respond to changes in metabolic substrate provision.

MATERIALS AND METHODS

BSM strips were incubated in 5 mM 1-13C-glucose and 0 to 5 mM 1,2-13C-acetate. The pattern of substrate use measured by 13C-nuclear magnetic resonance using BSM extracts. BSM was also cultured for 4 days to elicit changes in cell phenotype.

RESULTS

At physiological levels of glucose and acetate about 50% of the substrate used by mitochondria was glucose. When acetate concentration was changed from physiological levels (0.1 mM) to pathophysiological levels (0.5 mM), BSM was able to increase the use of acetate, while sparing the use of glucose and intracellular substrates, likely lipids. Above 0.5 mM acetate BSM was unable to further use acetate. With increasing acetate use anaplerosis increased, consistent with a depletion of tricarboxylic acid cycle intermediates. After 4 days of organ culture BSM mitochondria used significantly more unlabeled intracellular substrates and less 13C labeled glucose than control bladder, consistent with metabolic adaptation to increase lipid use, such as what occurs with hyperlipidemia.

CONCLUSIONS

We conclude that BSM has modest plasticity of the pattern of mitochondrial substrate selection and excess lipid provision may be able to induce lipotoxicity in BSM, resulting in impaired detrusor function.

Nuclear magnetic resonance spectroscopy and imaging in animal research

Nuclear magnetic resonance (NMR) spectroscopy and imaging can be used to investigate, noninvasively, a wide range of biological processes in systems as diverse as protein solutions, single cells, isolated perfused organs, and tissues in vivo. It is also possible to combine different NMR techniques enabling metabolic, anatomical, and physiological information to be obtained in the same experiment. This review provides a simple overview of the basic principles of NMR and outlines both the advantages and disadvantages of NMR spectroscopy and imaging. A few examples of potential applications of NMR spectroscopy and imaging are presented, which demonstrate the range of questions that can be asked using these techniques. The potential impact of using NMR techniques in a biomedical research program on the total number of animals required for specific investigations, as well as the number of animals used in research, are discussed. The article concludes with a personal perspective on the impact of continuing improvements in NMR technology for future applications in animal research.

Alterations in the oxidative metabolic profile in vascular smooth muscle from hyperlipidemic and diabetic swine

High cholesterol, especially LDL cholesterol, has been associated with the development of atherosclerotic plaques in arteries. To investigate the changes in cellular substrate metabolism early in the atherogenic process, Sinclair miniature swine were treated for 12 weeks with either a control diet, a high fat diet, or a high fat diet with the addition of alloxan to induce diabetes. The fractional entry into the TCA cycle of 1,2-(13)C-acetate (5 mM), 1-(13)C-glucose (5 mM), and unlabeled, endogenous lipids was determined in control, hyperlipidemic, and diabetic/hyperlipidemic pigs using 13C-isotopomer analysis of glutamate. The diabetic state of the pigs was validated by plasma glucose measurements made after 10 weeks of alloxan treatment for control (65 +/- 6 mg/dL), hyperlipidemic (63 +/- 5 mg/dL), and diabetic/hyperlipidemic (333 +/- 52 mg/dL) pigs. Plasma glucose values did not correlate with the percentage of glucose entry into the TCA cycle (R2 = 0.0819, n = 10). Alterations in the pattern of substrate oxidation were better correlated with changes in plasma lipids (cholesterol and triglycerides) than with changes in plasma glucose. Plasma total cholesterol and total triglyceride levels significantly correlated with changes in acetate metabolism (R2 = 0.7768 and R2 = 0.4787, respectively) and with changes in glucose metabolism (R2 = 0.6067 and R2 = 0.4506, respectively). We conclude that alterations in lipid profile, especially those that were observed in the diabetic milieu, are associated with early changes in vascular smooth muscle oxidative metabolism. These changes in oxidative metabolism may precede alterations in smooth muscle phenotype and, therefore, may play an important role in the early pathogenesis of atherosclerosis.

Influence of glycogen storage on vascular smooth muscle metabolism

The role of glycogen as an oxidative substrate for vascular smooth muscle (VSM) remains controversial. To elucidate the importance of glycogen as an oxidative substrate and the influence of glycogen flux on VSM substrate selection, we systematically altered glycogen levels and measured metabolism of glucose, acetate, and glycogen. Hog carotid arteries with glycogen contents ranging from 1 to 11 micromol/g were isometrically contracted in physiological salt solution containing 5 mM [1-(13)C]glucose and 1 mM [1, 2-(13)C]acetate at 37 degrees C for 6 h. [1-(13)C]glucose, [1, 2-(13)C]acetate, and glycogen oxidation were simultaneously measured with the use of a (13)C-labeled isotopomer analysis of glutamate. Although oxidation of glycogen increased with the glycogen content of the tissue, glycogen oxidation contributed only approximately 10% of the substrate oxidized by VSM. Whereas [1-(13)C]glucose flux, [3-(13)C]lactate production from [1-(13)C]glucose, and [1, 2-(13)C]acetate oxidation were not regulated by glycogen content, [1-(13)C]glucose oxidation was significantly affected by the glycogen content of VSM. However, [1-(13)C]glucose remained the primary ( approximately 40-50%) contributor to substrate oxidation. Therefore, we conclude that glucose is the predominate substrate oxidized by VSM, and glycogen oxidation contributes minimally to substrate oxidation.

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.

Pattern of substrate utilization in vascular smooth muscle using 13C isotopomer analysis of glutamate

Although vascular smooth muscle (VSM) derives the majority of its energy from oxidative phosphorylation, controversy exists concerning which substrates are utilized by the tricarboxylic acid (TCA) cycle. We used 13C isotopomer analysis of glutamate to directly measure the entry of exogenous [13C]glucose and acetate and unlabeled endogenous sources into the TCA cycle via acetyl-CoA. Hog carotid artery segments denuded of endothelium were superfused with 5 mM [1-13C]glucose and 0-5 mM [1,2-13C]acetate at 37 degreesC for 3-12 h. We found that both resting and contracting VSM preferentially utilize [1,2-13C]acetate compared with [1-13C]glucose and unlabeled substrates. The entry of glucose into the TCA cycle (30-60% of total entry via acetyl-CoA) exhibited little change despite alterations in contractile state or acetate concentrations ranging from 0 to 5 mM. We conclude that glucose and nonglucose substrates are important oxidative substrates for resting and contracting VSM. These are the first direct measurements of relative substrate entry into the TCA cycle of VSM during activation and may provide a useful method to measure alterations in VSM metabolism under physiological and pathophysiological conditions.