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

The influence of alloxan-induced diabetes on Müller cell contraction-promoting activities in vitreous

PURPOSE

Previous studies from this laboratory revealed that vitreous insulin-like growth factor biological activity increases in diabetes and that this change can precede the onset of proliferative diabetic retinopathy. The goal of this study was to characterize this phenomenon in an animal model of alloxan-induced diabetes.

METHODS

Swine made diabetic with intravenous alloxan were euthanized at times varying from 0 to 90 days. Vitreous samples from normal and diabetic swine were evaluated for changes in Müller cell contraction-promoting activity, the presence of insulin-like growth factor binding protein (IGFBP), and carbonic anhydrase-I and -II. Ocular tissues from these animals were also evaluated for changes in contraction-promoting growth factors and IGFBP message levels.

RESULTS

Alloxan-induced diabetes is associated with significant increases in vitreous Müller cell contraction-promoting activity that are present in as few as 30 days and are sustained for at least 90 days. Biochemical studies revealed that the increases cannot be attributed to loss of growth factor-attenuating IGFBPs, changes in local expression of contraction-promoting growth factors, or vitreous hemorrhage.

CONCLUSIONS

The previously reported increases in Müller cell contraction-promoting activity detected in human diabetic vitreous are present in diabetic swine within 30 days of chemical induction. The increase does not appear to be attributable to loss of growth factor control, increases in local growth factor expression, or vitreous hemorrhage, suggesting that other mechanisms are involved. It is the authors' speculation that diabetes induces blood-vitreous barrier changes that allow a different subset of plasma proteins to enter vitreous fluids.

Metabolic control analysis of the Warburg-effect in proliferating vascular smooth muscle cells

The accumulation and proliferation of vascular smooth muscle cells (VSMC) within the vessel wall is an important pathogenic feature in the development of atherosclerosis. Glucose metabolism has been implicated to play an important role in this cellular mechanism. To further elucidate the role of glucose metabolism in atherogenesis, glycolysis and its regulation have been investigated in proliferating VSMC. Platelet derived growth factor (PDGF BB)-induced proliferation of VSMCs significantly stimulated glucose flux through glycolysis. Further evaluating the enzymatic regulation of this pathway, the analysis of flux:metabolite co-responses revealed that anaerobic glycolytic flux is controlled at different sites of gycolysis in proliferating VSMCs, being consistent with the concept of multisite modulation. These findings indicate that regulation of glycolytic flux in proliferating VSMCs differs from traditional concepts of metabolic control of the Embden-Meyerhof pathway.

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.

IGF-I promotes a shift in metabolic flux in vascular smooth muscle cells

13C-nuclear magnetic resonance (NMR) spectroscopy was used to test our hypothesis that insulin-like growth factor I (IGF-I) stimulates glucose flux into both nonoxidative and oxidative pathways in vascular smooth muscle cells (VSMC). Rat VSMC were exposed to uniformly labeled [13C]glucose ([U-13C]glucose; 5.5 mM) and [3-13C]pyruvate (1 mM) in the presence and absence of IGF-I (100 ng/ml). IGF-I increased glucose flux through glycolysis and the tricarboxylic acid (TCA) cycle as well as total anaplerotic flux into the TCA cycle. Previous work in our laboratory identified an increase in GLUT1 content and glucose metabolism in neointimal VSMC that was sufficient to promote proliferation and inhibit apoptosis. To test whether IGF-I could potentiate the GLUT1-induced increased flux in the neointima, we utilized VSMC harboring constitutive overexpression of GLUT1. Indeed, IGF-I markedly potentiated the GLUT1-induced increase in glucose flux through glycolysis and the TCA cycle. Taken together, these findings demonstrate that upregulation of glucose transport through either IGF-I or increased GLUT1 content stimulates glucose flux through both nonoxidative and oxidative pathways in VSMC.