In JoVE (1)

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Articles by Deanna P. Bracy in JoVE

 JoVE Medicine

Hyperinsulinemic-euglycemic Clamps in Conscious, Unrestrained Mice

1Diabetes and Obesity Research Center, Sanford-Burnham Medical Research Institute at Lake Nona, 2Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, 3Vanderbilt Mouse Metabolic Phenotyping Center, Vanderbilt University School of Medicine, 4Department of Pediatrics and Cellular and Integrative Physiology, Indiana University School of Medicine

JoVE 3188

Other articles by Deanna P. Bracy on PubMed

Fiber Type-specific Determinants of Vmax for Insulin-stimulated Muscle Glucose Uptake in Vivo

American Journal of Physiology. Endocrinology and Metabolism. Mar, 2003  |  Pubmed ID: 12556351

The aim of this study was to determine barriers limiting muscle glucose uptake (MGU) during increased glucose flux created by raising blood glucose in the presence of fixed insulin. The determinants of the maximal velocity (V(max)) of MGU in muscles of different fiber types were defined. Conscious rats were studied during a 4 mU x kg(-1) x min(-1) insulin clamp with plasma glucose at 2.5, 5.5, and 8.5 mM. [U-(14)C]mannitol and 3-O-methyl-[(3)H]glucose ([(3)H]MG) were infused to steady-state levels (t = -180 to 0 min). These isotope infusions were continued from 0 to 40 min with the addition of a 2-deoxy-[(3)H]glucose ([(3)H]DG) infusion. Muscles were excised at t = 40 min. Glucose metabolic index (R(g)) was calculated from muscle-phosphorylated [(3)H]DG. [U-(14)C]mannitol was used to determine extracellular (EC) H(2)O. Glucose at the outer ([G](om)) and inner ([G](im)) sarcolemmal surfaces was determined by the ratio of [(3)H]MG in intracellular to EC H(2)O and muscle glucose. R(g) was comparable at the two higher glucose concentrations, suggesting that rates of uptake near V(max) were reached. In summary, by defining the relationship of arterial glucose to [G](om) and [G](im) in the presence of fixed hyperinsulinemia, it is concluded that 1) V(max) for MGU is limited by extracellular and intracellular barriers in type I fibers, as the sarcolemma is freely permeable to glucose; 2) V(max) is limited in muscles with predominantly type IIb fibers by extracellular resistance and transport resistance; and 3) limits to R(g) are determined by resistance at multiple steps and are better defined by distributed control rather than by a single rate-limiting step.

Hexokinase II Partial Knockout Impairs Exercise-stimulated Glucose Uptake in Oxidative Muscles of Mice

American Journal of Physiology. Endocrinology and Metabolism. Nov, 2003  |  Pubmed ID: 12865258

Muscle glucose uptake (MGU) is distributively controlled by three serial steps: delivery of glucose to the muscle membrane, transport across the muscle membrane, and intracellular phosphorylation to glucose 6-phosphate by hexokinase (HK). During states of high glucose fluxes such as moderate exercise, the HK activity is of increased importance, since augmented muscle perfusion increases glucose delivery, and increased GLUT4 at the cell membrane increases glucose transport. Because HK II overexpression augments exercise-stimulated MGU, it was hypothesized that a reduction in HK II activity would impair exercise-stimulated MGU and that the magnitude of this impairment would be greatest in tissues with the largest glucose requirement. To this end, mice with a HK II partial knockout (HK+/-) were compared with their wild-type control (WT) littermates during either sedentary or moderate exercise periods. Rg, an index of glucose metabolism, was measured using 2-deoxy-[3H]glucose. No differences in glucose metabolism were detected between sedentary groups. The increase in Rg due to exercise was impaired in the highly oxidative heart and soleus muscles of HK+/- compared with WT mice (7 +/- 10 vs. 29 +/- 9 and 8 +/- 3 vs. 25 +/- 7 micromol. 100 g-1. min-1, respectively). However, the increase in Rg due to exercise was not altered in gastrocnemius and superficial vastus lateralis muscles in HK+/- and WT mice (8 +/- 2 vs. 12 +/- 3 and 5 +/- 2 vs. 8 +/- 2 micromol. 100 g-1. min-1, respectively). In conclusion, MGU is impaired by reductions in HK activity during exercise, a physiological condition characterized by high glucose flux. This impairment is critically dependent on the tissue's glucose metabolic rate and correlates with tissue oxidative capacity.

Quinides of Roasted Coffee Enhance Insulin Action in Conscious Rats

The Journal of Nutrition. Nov, 2003  |  Pubmed ID: 14608069

Consumption of large amounts of coffee has been shown to decrease the incidence of type 2 diabetes. However, the specific compounds and mechanisms responsible for this effect are not known. The aim of this study was to determine the effects of a decaffeinated coffee extract and a synthetic quinide, representative of those found in roasted coffee, 3,4-diferuloyl-1,5-quinolactone, on insulin-stimulated glucose disposal and muscle glucose uptake. Experiments were performed on conscious rats during hyperinsulinemic, euglycemic clamps receiving gastric infusions of saline, a decaffeinated coffee extract (DECAF) (220 mg/kg), or 3,4-diferuloyl-1,5-quinide (DIFEQ) (110 mg/kg). Following treatment, rats received an intravenous bolus of deoxy-[2-3H] glucose to assess muscle glucose uptake (Rg, micromol x 100 g(-1) x min(-1)). Glucose infusions [mg/(kg x min)] required to maintain euglycemia during the tracer period were higher with DIFEQ (14.6 +/- 0.7) than with saline (10.8 +/- 0.7) and DECAF (11.5 +/- 1.1). Despite increased glucose requirements, Rg in skeletal (soleus, gastrocnemius, superficial vastus lateralis) and cardiac muscle were unchanged. DECAF or DIFEQ did not affect heart rate, blood pressure, plasma nonesterified fatty acids or liver aminotransferase activity. These results demonstrate that DIFEQ increases whole-body glucose disposal independently of skeletal muscle Rg.

Distributed Control of Glucose Uptake by Working Muscles of Conscious Mice: Roles of Transport and Phosphorylation

American Journal of Physiology. Endocrinology and Metabolism. Jan, 2004  |  Pubmed ID: 13129858

Muscle glucose uptake (MGU) is determined by glucose delivery, transport, and phosphorylation. C57Bl/6J mice overexpressing GLUT4, hexokinase II (HK II), or both were used to determine the barriers to MGU. A carotid artery and jugular vein were catheterized for arterial blood sampling and venous infusions. Experiments were conducted in conscious mice approximately 7 days after surgery. 2-Deoxy-[3H]glucose was administered during rest or treadmill exercise to calculate glucose concentration-dependent (Rg) and -independent (Kg) indexes of MGU. Compared with wild-type controls, GLUT4-overexpressing mice had lowered fasting glycemia (165 +/- 6 vs. 115 +/- 6 mg/dl) and increased Rg by 230 and 166% in the gastrocnemius and superficial vastus lateralis (SVL) muscles under sedentary conditions. GLUT4 overexpression was not able to augment exercise-stimulated Rg or Kg. Whereas HK II overexpression had no effect on fasting glycemia (170 +/- 6 mg/dl) or sedentary Rg, it increased exercise-stimulated Rg by 82, 60, and 169% in soleus, gastrocnemius, and SVL muscles, respectively. Combined GLUT4 and HK II overexpression lowered fasting glycemia (106 +/- 6 mg/dl), increased nonesterified fatty acids, and increased sedentary Rg. Combined GLUT4 and HK II overexpression did not enhance exercise-stimulated Rg compared with HK II-overexpressing mice because of the reduced glucose concentration. GLUT4 combined with HK II overexpression resulted in a marked increase in exercise-stimulated Kg. In conclusion, control of MGU shifts from membrane transport at rest to phosphorylation during exercise. Glucose transport is not normally a significant barrier during exercise. However, when the phosphorylation barrier is lowered by HK II overexpression, glucose transport becomes a key site of control for regulating MGU during exercise.

Hexokinase II Overexpression Improves Exercise-stimulated but Not Insulin-stimulated Muscle Glucose Uptake in High-fat-fed C57BL/6J Mice

Diabetes. Feb, 2004  |  Pubmed ID: 14747279

The aim of the present study was to determine the specific sites of impairment to muscle glucose uptake (MGU) in the insulin-resistant high-fat-fed, conscious C57BL/6J mouse. Wild type (WT) and hexokinase II overexpressing (HK(Tg)) mice were fed either a standard diet or high-fat diet and studied at 4 months of age. A carotid artery and jugular veins had catheters chronically implanted for sampling and infusions, respectively, and mice were allowed to recovery for at least 5 days. Mice were fasted for 5 h and underwent a hyperinsulinemic-euglycemic clamp or saline infusion for 120 min. Separate groups of mice were studied during 30-min sedentary or treadmill exercise periods. A bolus of 2-deoxy[(3)H]glucose was administered 25 min before the end of each study for determination of R(g), an index of tissue-specific glucose uptake. Fasting blood glucose was increased in high-fat compared with standard diet-fed WT (194 +/- 4 vs. 171 +/- 4 mg/dl) but not HK(Tg) (179 +/- 5 vs. 171 +/- 3 mg/dl) mice. High-fat feeding created hyperinsulinemia in both WT and HK(Tg) mice (58 +/- 8 and 77 +/- 15 micro U/ml) compared with standard diet-fed mice (21 +/- 2 and 20 +/- 1 micro U/ml). R(g) was not affected by genotype or diet during either saline infusion or sedentary conditions. HK II overexpression augmented insulin-stimulated R(g) in standard diet-fed but not high-fat-fed mice. Exercise-stimulated R(g) was impaired by high-fat feeding in WT mice, but this impairment was largely rectified in HK(Tg) mice. In conclusion, high-fat feeding impairs both insulin- and exercise-stimulated MGU, but only exercise-stimulated MGU was corrected by HK II overexpression.

AMP Kinase-induced Skeletal Muscle Glucose but Not Long-chain Fatty Acid Uptake is Dependent on Nitric Oxide

Diabetes. Jun, 2004  |  Pubmed ID: 15161745

The purpose of this study was to examine the effects of AMP kinase (AMPK) activation on in vivo glucose and long-chain fatty acid (LCFA) uptake in skeletal muscle and to examine the nitric oxide (NO) dependence of any putative effects. Catheters were chronically implanted in the carotid artery and jugular vein of male Sprague-Dawley rats. After 4 days of recovery, rats were given either water or water containing 1 mg/ml nitro-l-arginine methylester (l-NAME) for 2.5 days. After an overnight fast, rats underwent one of five protocols: saline, 5-aminoimidazole-4-carboxamide-1-B-d-ribofuranoside (AICAR) (10 mg. kg(-1). min(-1)), l-NAME, AICAR + l-NAME, or AICAR + Intralipid (20%, 0.02 ml. kg(-1). min(-1)). Glucose was clamped at approximately 6.5 mmol/l in all groups, and an intravenous bolus of 2-deoxy[(3)H]glucose and [(125)I]-15-(p-iodophenyl)-3-R,S-methylpentadecanoic acid was administered to obtain indexes of glucose (K(g)) and LCFA (K(f)) uptake and clearance. At 150 min, soleus, gastrocnemius, and superficial vastus lateralis were excised for tracer determination. Both K(g) and K(f) increased with AICAR in all muscles studied. K(g) decreased with increasing muscle composition of type 1 slow-twitch fibers, whereas K(f) increased. In addition, AICAR-induced increases in K(g) but not K(f) were abolished by l-NAME in the majority of muscles examined. This shows that the mechanisms by which AMPK stimulates glucose and LCFA uptake are distinct.

AMPK Stimulation Increases LCFA but Not Glucose Clearance in Cardiac Muscle in Vivo

American Journal of Physiology. Endocrinology and Metabolism. Nov, 2004  |  Pubmed ID: 15265760

AMP-activated protein kinase (AMPK) independently increases glucose and long-chain fatty acid (LCFA) utilization in isolated cardiac muscle preparations. Recent studies indicate this may be due to AMPK-induced phosphorylation and activation of nitric oxide synthase (NOS). Given this, the aim of the present study was to assess the effects of AMPK stimulation by 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR; 10 on glucose and LCFA utilization in cardiac muscle and to determine the NOS dependence of any observed effects. Catheters were chronically implanted in a carotid artery and jugular vein of Sprague-Dawley rats. After 4 days of recovery, conscious, unrestrained rats were given either water or water containing 1 mg/ml nitro-L-arginine methyl ester (L-NAME) for 2.5 days. After an overnight fast, rats underwent one of four protocols: saline, AICAR, AICAR + L-NAME, or AICAR + Intralipid (20%, 0.02 Glucose was clamped at approximately 6.5 mM in all groups, and an intravenous bolus of 2-deoxy-[(3)H]glucose and [(125)I]-15-(p-iodophenyl)-3-R,S-methylpentadecanoic acid was administered to obtain indexes of glucose and LCFA uptake and clearance. Despite AMPK activation, as evidenced by acetyl-CoA carboxylase (Ser(221)) and AMPK phosphorylation (Thr(172)), AICAR increased cardiac LCFA but not glucose clearance. L-NAME + AICAR established that this effect was not due to NOS activation, and AICAR + Intralipid showed that increased cardiac LCFA clearance was not LCFA-concentration dependent. These results demonstrate that, in vivo, AMPK stimulation increases LCFA but not glucose clearance by a NOS-independent mechanism.

Regulation of Insulin-stimulated Muscle Glucose Uptake in the Conscious Mouse: Role of Glucose Transport is Dependent on Glucose Phosphorylation Capacity

Endocrinology. Nov, 2004  |  Pubmed ID: 15284204

Previous work suggests that normal GLUT4 content is sufficient for increases in muscle glucose uptake (MGU) during hyperinsulinemia, because glucose phosphorylation is the more formidable barrier to insulin-stimulated MGU. It was hypothesized that a partial ablation of GLUT4 would not impair insulin-stimulated MGU when glucose phosphorylation capacity is normal but would do so when glucose phosphorylation capacity is increased. Thus, chow-fed C57BL/6J mice with a GLUT4 partial knockout (GLUT4(+/-)), hexokinase II overexpression (HK(Tg)), or both (HK(Tg) + GLUT4(+/-)) and wild-type littermates were studied. Carotid artery and jugular vein catheters were implanted for sampling and infusions at 4 months of age. After a 5-d recovery, 5-h fasted mice (n = 8-11/group) underwent a 120-min saline infusion or insulin clamp (4 mU/kg.min insulin with glucose maintained at 165 mg/dl) and received a 2-deoxy[(3)H]glucose bolus to provide an index of MGU (R(g)) for the soleus, gastrocnemius, and superficial vastus lateralis. Basal R(g) from all muscles studied from saline-infused mice were not changed by any of the genetic modifications. HK(Tg) mice had augmented insulin-stimulated R(g) in all muscles studied compared with remaining genotypes. Insulin-stimulated R(g) was not impaired in any of the muscles studied from GLUT4(+/-) mice. However, the enhanced insulin-stimulated R(g) created by HK overexpression was ablated in HK(Tg) + GLUT4(+/-) mice. Thus, a 50% reduction of normal GLUT4 content in the presence of normal HK activity does not impair insulin-stimulated MGU. However, when the glucose phosphorylation barrier is lowered by HK overexpression, GLUT4 availability becomes a limitation to insulin-stimulated MGU.

Control of Exercise-stimulated Muscle Glucose Uptake by GLUT4 is Dependent on Glucose Phosphorylation Capacity in the Conscious Mouse

The Journal of Biological Chemistry. Dec, 2004  |  Pubmed ID: 15456776

Previous work suggests that normal GLUT4 content is sufficient for increases in muscle glucose uptake (MGU) during exercise because GLUT4 overexpression does not increase exercise-stimulated MGU. Instead of glucose transport, glucose phosphorylation is a primary limitation of exercise-stimulated MGU. It was hypothesized that a partial ablation of GLUT4 would not impair exercise-stimulated MGU when glucose phosphorylation capacity is normal but would do so when glucose phosphorylation capacity was increased. Thus, C57BL/6J mice with hexokinase II (HKII) overexpression (HK(Tg)), a GLUT4 partial knock-out (G4(+/-)), or both (HK(Tg) + G4(+/-)) and wild-type (WT) littermates were implanted with carotid artery and jugular vein catheters for sampling and infusions at 4 months of age. After a 7-day recovery, 5-h fasted mice remained sedentary or ran on a treadmill at 0.6 mph for 30 min (n = 9-12 per group) and received a bolus of 2-deoxy[3H]glucose to provide an index of MGU (Rg). Arterial blood glucose and plasma insulin concentrations were similar in WT, G4(+/-), HKTg, and HKTg + G4(+/-) mice. Sedentary Rg values were the same in all genotypes in all muscles studied, confirming that glucose transport is a significant barrier to basal glucose uptake. Gastrocnemius and soleus Rg were greater in exercising compared with sedentary mice in all genotypes. During exercise, G4(+/-) mice had a marked increase in blood glucose that was corrected by the addition of HK II overexpression. Exercise Rg (micromol/100g/min) was not different between WT and G4(+/-) mice in the gastrocnemius (24 +/- 5 versus 21 +/- 2) or the soleus (54 +/- 6 versus 70 +/- 7). In contrast, the enhanced exercise Rg observed in HKTg mice compared with that in WT mice was absent in HKTg + G4(+/-) mice in both the gastrocnemius (39 +/- 7 versus 22 +/- 6) and the soleus (98 +/- 13 versus 65 +/- 13). Thus, glucose transport is not a significant barrier to exercise-stimulated MGU despite a 50% reduction in GLUT4 content when glucose phosphorylation capacity is normal. However, when glucose phosphorylation capacity is increased by HK II overexpression, GLUT4 availability becomes a marked limitation to exercise-stimulated MGU.

Heart-type Fatty Acid-binding Protein Reciprocally Regulates Glucose and Fatty Acid Utilization During Exercise

American Journal of Physiology. Endocrinology and Metabolism. Feb, 2005  |  Pubmed ID: 15454399

The role of heart-type cytosolic fatty acid-binding protein (H-FABP) in mediating whole body and muscle-specific long-chain fatty acid (LCFA) and glucose utilization was examined using exercise as a phenotyping tool. Catheters were chronically implanted in a carotid artery and jugular vein of wild-type (WT, n = 8), heterozygous (H-FABP(+/-), n = 8), and null (H-FABP(-/-), n = 7) chow-fed C57BL/6J mice, and mice were allowed to recover for 7 days. After a 5-h fast, conscious, unrestrained mice were studied during 30 min of treadmill exercise (0.6 mph). A bolus of [(125)I]-15-(p-iodophenyl)-3-R,S-methylpentadecanoic acid and 2-deoxy-[(3)H]glucose was administered to obtain rates of whole body metabolic clearance (MCR) and indexes of muscle LCFA (R(f)) and glucose (R(g)) utilization. Fasting, nonesterified fatty acids (mM) were elevated in H-FABP(-/-) mice (2.2 +/- 0.9 vs. 1.3 +/- 0.1 and 1.3 +/- 0.2 for WT and H-FABP(+/-)). During exercise, blood glucose (mM) increased in WT (11.7 +/- 0.8) and H-FABP(+/-) (12.6 +/- 0.9) mice, whereas H-FABP(-/-) mice developed overt hypoglycemia (4.8 +/- 0.8). Examination of tissue-specific and whole body glucose and LCFA utilization demonstrated a dependency on H-FABP with exercise in all tissues examined. Reductions in H-FABP led to decreasing exercise-stimulated R(f) and increasing R(g) with the most pronounced effects in heart and soleus muscle. Similar results were seen for MCR with decreasing LCFA and increasing glucose clearance with declining levels of H-FABP. These results show that, in vivo, H-FABP has reciprocal effects on glucose and LCFA utilization and whole body fuel homeostasis when metabolic demands are elevated by exercise.

Control of Muscle Glucose Uptake: Test of the Rate-limiting Step Paradigm in Conscious, Unrestrained Mice

The Journal of Physiology. Feb, 2005  |  Pubmed ID: 15576451

The aim of this study was to test whether in fact glucose transport is rate-limiting in control of muscle glucose uptake (MGU) under physiological hyperinsulinaemic conditions in the conscious, unrestrained mouse. C57Bl/6J mice overexpressing GLUT4 (GLUT4(Tg)), hexokinase II (HK(Tg)), or both (GLUT4(Tg) + HK(Tg)), were compared to wild-type (WT) littermates. Catheters were implanted into a carotid artery and jugular vein for sampling and infusions at 4 month of age. After a 5-day recovery period, conscious mice underwent one of two protocols (n = 8-14/group) after a 5-h fast. Saline or insulin (4 mU kg(-1) min(-1)) was infused for 120 min. All mice received a bolus of 2-deoxy[(3)H]glucose (2-(3)HDG) at 95 min. Glucose was clamped at approximately 165 mg dl(-1) during insulin infusion and insulin levels reached approximately 80 microU ml(-1). The rate of disappearance of 2-(3)HDG from the blood provided an index of whole body glucose clearance. Gastrocnemius, superficial vastus lateralis and soleus muscles were excised at 120 min to determine 2-(3)HDG-6-phosphate levels and calculate an index of MGU (R(g)). Results show that whole body and tissue-specific indices of glucose utilization were: (1) augmented by GLUT4 overexpression, but not HKII overexpression, in the basal state; (2) enhanced by HKII overexpression in the presence of physiological hyperinsulinaemia; and (3) largely unaffected by GLUT4 overexpression during insulin clamps whether alone or combined with HKII overexpression. Therefore, while glucose transport is the primary barrier to MGU under basal conditions, glucose phosphorylation becomes a more important barrier during physiological hyperinsulinaemia in all muscles. The control of MGU is distributed rather than confined to a single rate-limiting step such as glucose transport as glucose transport and phosphorylation can both become barriers to skeletal muscle glucose influx.

Hexokinase II Protein Content is a Determinant of Exercise Endurance Capacity in the Mouse

The Journal of Physiology. Jul, 2005  |  Pubmed ID: 15878951

Hexokinase (HK) II content is elevated in fatigue resistant muscle fibres and exercise trained muscle. The aim of this study was to determine if exercise capacity is dependent on muscle HK protein content. C57Bl/6 mice with a 50% HK knockout (HK+/-), no genetic manipulation (wild-type, WT) and an approximately 3-fold HK overexpression (HKTg) were tested. Mice (n = 12/group) completed both a maximal oxygen consumption test(VO2max) test and an endurance capacity test (run at approximately 75% VO2max) on an enclosed treadmill equipped to measure gas exchange. Arterial and venous catheters were surgically implanted into separate groups of mice (n = 9-11/group) in order to measure an index of muscle glucose uptake Rg during 30 min of treadmill exercise. Maximum work rate (0.95 +/- 0.05, 1.00 +/- 0.04 and 1.06 +/- 0.07 kg m min-1), (137 +/- 3, 141 +/- 4 and 141 +/- 5 ml kg-1 min-1) and maximal respiratory exchange ratio (1.04 +/- 0.02, 1.00 +/- 0.03 and 1.04 +/- 0.04) were similar in HK+/-, WT and HKTg, respectively. Exercise endurance capacity (measured as time to exhaustion) increased as HK content increased (55 +/- 11, 77 +/- 5 and 98 +/- 9 min) and this was related to Rg measured in mice during 30 min of exercise (13 +/- 2, 24 +/- 5 and 42 +/- 5 micromol (100 g)-1 min-1). Muscle glycogen in sedentary HK+/-mice and HK+/- mice following 30 min of exercise were significantly lower than in HKTg and WT mice. However, the net exercise-induced muscle glycogen breakdown was equal in the three genotypes. In summary, HK protein content within the range studied (a) was not associated with a difference in the capacity to perform maximal intensity exercise, (b) was a powerful determinant of the ability to sustain moderate intensity exercise, as reducing HK content impaired endurance and increasing HK content enhanced endurance, and (c) although directly related to exercise endurance, was not a determinant of net muscle glycogen usage during exercise. In conclusion, adaptations that increase HK protein content and/or functional activity such as regular exercise contribute to increased muscular endurance.

Partial Gene Deletion of Heart-type Fatty Acid-binding Protein Limits the Severity of Dietary-induced Insulin Resistance

Diabetes. Nov, 2005  |  Pubmed ID: 16249436

The aim of this study was to determine the contribution of heart-type fatty acid-binding protein (H-FABP) to glucose and long-chain fatty acid (LCFA) utilization in dietary-induced insulin resistance. We tested the hypothesis that H-FABP facilitates increases in LCFA flux present in glucose-intolerant states and that a partial reduction in the amount of this protein would compensate for all or part of the impairment. Transgenic H-FABP heterozygotes (HET) and wild-type (WT) littermates were studied following chow diet (CHD) or high-fat diet (HFD) for 12 weeks. Catheters were surgically implanted in the carotid artery and jugular vein for sampling and infusions, respectively. Following 5 days of recovery, mice received either a saline infusion or underwent a euglycemic insulin clamp (4 mU x kg(-1) x min(-1)) for 120 min. At 90 min, a bolus of 2-deoxyglucose and [125I]-15-(rho-iodophenyl)-3-R,S-methylpentadecanoic acid were administered to obtain indexes of glucose and LCFA utilization. At 120 min, skeletal muscles were excised for tracer determination. All HFD mice were obese and hyperinsulinemic; however, only HFD-WT mice were hyperglycemic. Glucose infusion rates during insulin clamps were 49 +/- 4, 59 +/- 4, 16 +/- 4, and 33 +/- 4 mg x kg(-1) x min(-1) for CHD-WT, CHD-HET, HFD-WT, and HFD-HET mice, respectively, showing that HET limited the severity of whole-body insulin resistance with HFD. Insulin-stimulated muscle glucose utilization was attenuated in HFD-WT but unaffected in HFD-HET mice. Conversely, rates of LCFA clearance were increased with HFD feeding in HFD-WT but not in HFD-HET mice. In conclusion, a partial reduction in H-FABP protein normalizes fasting glucose levels and improves whole-body insulin sensitivity in HFD-fed mice despite obesity.

Considerations in the Design of Hyperinsulinemic-euglycemic Clamps in the Conscious Mouse

Diabetes. Feb, 2006  |  Pubmed ID: 16443772

Despite increased use of the hyperinsulinemic-euglycemic clamp to study insulin action in mice, the effects of experimental parameters on the results obtained have not been addressed. In our studies, we determined the influences of sampling sites, fasting duration, and insulin delivery on results obtained from clamps in conscious mice. Carotid artery and jugular vein catheters were implanted in C57BL/6J mice (n = 6-10/group) fed a normal diet for sampling and infusions. After a 5-day recovery period, mice underwent a 120-min clamp (2.5-mU . kg(-1) . min(-1) insulin infusion; approximately 120-130 mg/dl glucose) while receiving [3-(3)H]glucose to determine glucose appearance (endoR(a)) and disappearance (R(d)). Sampling large volumes (approximately 100 mul) from the cut tail resulted in elevated catecholamines and basal glucose compared with artery sampling. Catecholamines were not elevated when taking small samples ( approximately 5 mul) from the cut tail. Overnight (18-h) fasting resulted in greater loss of total body, lean, and fat masses and hepatic glycogen but resulted in enhanced insulin sensitivity compared with 5-h fasting. Compared with a 16-mU/kg insulin prime, a 300-mU/kg prime resulted in hepatic insulin resistance and slower acquisition of steady-state glucose infusion rates (GIR) after a 5-h fast. The steady-state GIR was expedited after the 300-mU/kg prime in 18-h-fasted mice. The GIR and R(d) rose with increasing insulin infusions (0.8, 2.5, 4, and 20 mU . kg(-1) . min(-1)), but endoR(a) was fully suppressed with doses higher than 0.8 mU . kg(-1) . min(-1). Thus, common variations in experimental factors yield different results and should be considered in designing and interpreting clamps.

Chronic Treatment with Sildenafil Improves Energy Balance and Insulin Action in High Fat-fed Conscious Mice

Diabetes. Apr, 2007  |  Pubmed ID: 17229936

Stimulation of nitric oxide-cGMP signaling results in vascular relaxation and increased muscle glucose uptake. We show that chronically inhibiting cGMP hydrolysis with the phosphodiesterase-5 inhibitor sildenafil improves energy balance and enhances in vivo insulin action in a mouse model of diet-induced insulin resistance. High-fat-fed mice treated with sildenafil plus L-arginine or sildenafil alone for 12 weeks had reduced weight and fat mass due to increased energy expenditure. However, uncoupling protein-1 levels were not increased in sildenafil-treated mice. Chronic treatment with sildenafil plus L-arginine or sildenafil alone increased arterial cGMP levels but did not adversely affect blood pressure or cardiac morphology. Sildenafil treatment, with or without l-arginine, resulted in lower fasting insulin and glucose levels and enhanced rates of glucose infusion, disappearance, and muscle glucose uptake during a hyperinsulinemic (4 mU x kg(-1) x min(-1))-euglycemic clamp in conscious mice. These effects occurred without an increase in activation of muscle insulin signaling. An acute treatment of high fat-fed mice with sildenafil plus l-arginine did not improve insulin action. These results show that phosphodiesterase-5 is a potential target for therapies aimed at preventing diet-induced energy imbalance and insulin resistance.

Glucose Kinetics and Exercise Tolerance in Mice Lacking the GLUT4 Glucose Transporter

The Journal of Physiology. Jul, 2007  |  Pubmed ID: 17495042

The absence of GLUT4 severely impairs basal glucose uptake in vivo, but does not alter glucose homeostasis or circulating insulin. Glucose uptake in isolated contracting skeletal muscle (MGU) is also impaired by the absence of GLUT4, and onset of muscle fatigue is hastened. Whether the body can compensate and preserve glucose homeostasis during exercise, as it does in the basal state, is unknown. One aim was to test the effectiveness of glucoregulatory compensation for the absence of GLUT4 in vivo. The absence of GLUT4 was also used to further define the role of hexokinase (HK) II, which catalyses glucose phosphorylation after it is transported in the cell. HK II increases MGU during exercise, as well as exercise endurance. In the absence of GLUT4, HK II expression will not affect MGU. A second aim was to test whether, in the absence of GLUT4, HK II retains its ability to increase exercise endurance. Wild-type (WT), GLUT4 null (GLUT4(-/-)), and GLUT4 null overexpressing HK II (GLUT4(-/-)HK(Tg)) mice were studied using a catheterized mouse model that allows blood sampling and isotope infusions during treadmill exercise. The impaired capacity of working muscle to take up glucose in GLUT4(-/-) is partially offset by an exaggerated increase in the glucagon: insulin ratio, increased liver glucose production, hyperglycaemia, and a greater capillary density in order to increase the delivery of glucose to the exercising muscle of GLUT4(-/-). Hearts of GLUT4(-/-) also exhibited a compensatory increase in HK II expression and a paradoxical increase in glucose uptake. Exercise tolerance was reduced in GLUT4(-/-) compared to WT. As expected, MGU in GLUT4(-/-)HK(Tg) was the same as in GLUT4(-/-). However, HK II overexpression retained its ability to increase exercise endurance. In conclusion, unlike the basal state where glucose homeostasis is preserved, hyperglycaemia results during exercise in GLUT4(-/-) due to a robust stimulation of liver glucose release in the face of severe impairments in MGU. Finally, studies in GLUT4(-/-)HK(Tg) show that HK II improves exercise tolerance, independent of its effects on MGU.

Phosphorylation Barriers to Skeletal and Cardiac Muscle Glucose Uptakes in High-fat Fed Mice: Studies in Mice with a 50% Reduction of Hexokinase II

Diabetes. Oct, 2007  |  Pubmed ID: 17639019

Muscle glucose uptake (MGU) is regulated by glucose delivery to, transport into, and phosphorylation within muscle. The aim of this study was to determine the role of limitations in glucose phosphorylation in the control of MGU during either physiological insulin stimulation (4 mU x kg(-1) x min(-1)) or exercise with chow or high-fat feeding.

Insulin Action in the Double Incretin Receptor Knockout Mouse

Diabetes. Feb, 2008  |  Pubmed ID: 17977951

The incretins glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide have been postulated to play a role in regulating insulin action, although the mechanisms behind this relationship remain obscure. We used the hyperinsulinemic-euglycemic clamp to determine sites where insulin action may be modulated in double incretin receptor knockout (DIRKO) mice, which lack endogenous incretin action.

Metabolic Implications of Reduced Heart-type Fatty Acid Binding Protein in Insulin Resistant Cardiac Muscle

Biochimica Et Biophysica Acta. Oct, 2008  |  Pubmed ID: 18692568

Insulin resistance is characterized by elevated rates of cardiac fatty acid utilization resulting in reduced efficiency and cardiomyopathy. One potential therapeutic approach is to limit the uptake and oxidation of fatty acids. The aims of this study were to determine whether a quantitative reduction in heart-type fatty acid binding protein (FABP3) normalizes cardiac substrate utilization without altering cardiac function. Transgenic (FABP3(+/-)) and wild-type (WT) littermates were studied following low fat (LF) or high fat (HF) diets, with HF resulting in obese, insulin-resistant mice. Cardiovascular function (systolic blood pressure, % fractional shortening) and heart dimension were measured at weaning and every month afterward for 3 mo. During this period cardiovascular function was the same independent of genotype and diet. Catheters were surgically implanted in the carotid artery and jugular vein for sampling and infusions in mice at 4 mo of age. Following 5 d recovery, mice underwent either a saline infusion or a hyperinsulinemic-euglycemic clamp (4 mU kg(-1) min(-1)). Indices of long chain fatty acid and glucose utilization (R(f), R(g); mumol g wet weight(-1) min(-1)) were obtained using 2-deoxy[(3)H]glucose and [(125)I]-15-rho-iodophenyl)-3-R,S-methylpentadecanoic acid. FABP3(+/-) had enhanced cardiac R(g) compared with WT during saline infusion in both LF and HF. FABP3(+/-) abrogated the HF-induced decrement in insulin-stimulated cardiac R(g). On a HF diet, FABP(+/-) but not WT had an increased reliance on fatty acids (R(f)) during insulin stimulation. In conclusion, cardiac insulin resistance and glucose uptake is largely corrected by a reduction in FABP3 in vivo without contemporaneous deleterious effects on cardiac function.

The Glucagon-like Peptide-1 Receptor Regulates Endogenous Glucose Production and Muscle Glucose Uptake Independent of Its Incretin Action

Endocrinology. Mar, 2009  |  Pubmed ID: 19008308

Glucagon-like peptide-1 (GLP-1) diminishes postmeal glucose excursions by enhancing insulin secretion via activation of the beta-cell GLP-1 receptor (Glp1r). GLP-1 may also control glucose levels through mechanisms that are independent of this incretin effect. The hyperinsulinemic-euglycemic clamp (insulin clamp) and exercise were used to examine the incretin-independent glucoregulatory properties of the Glp1r because both perturbations stimulate glucose flux independent of insulin secretion. Chow-fed mice with a functional disruption of the Glp1r (Glp1r(-/-)) were compared with wild-type littermates (Glp1r(+/+)). Studies were performed on 5-h-fasted mice implanted with arterial and venous catheters for sampling and infusions, respectively. During insulin clamps, [3-(3)H]glucose and 2[(14)C]deoxyglucose were used to determine whole-body glucose turnover and glucose metabolic index (R(g)), an indicator of glucose uptake. R(g) in sedentary and treadmill exercised mice was determined using 2[(3)H]deoxyglucose. Glp1r(-/-) mice exhibited increased glucose disappearance, muscle R(g), and muscle glycogen levels during insulin clamps. This was not associated with enhanced muscle insulin signaling. Glp1r(-/-) mice exhibited impaired suppression of endogenous glucose production and hepatic glycogen accumulation during insulin clamps. This was associated with impaired liver insulin signaling. Glp1r(-/-) mice became significantly hyperglycemic during exercise. Muscle R(g) was normal in exercised Glp1r(-/-) mice, suggesting that hyperglycemia resulted from an added drive to stimulate glucose production. Muscle AMP-activated protein kinase phosphorylation was higher in exercised Glp1r(-/-) mice. This was associated with increased relative exercise intensity and decreased exercise endurance. In conclusion, these results show that the endogenous Glp1r regulates hepatic and muscle glucose flux independent of its ability to enhance insulin secretion.

Skeletal Muscle AMP-activated Protein Kinase is Essential for the Metabolic Response to Exercise in Vivo

The Journal of Biological Chemistry. Sep, 2009  |  Pubmed ID: 19525228

AMP-activated protein kinase (AMPK) has been postulated as a super-metabolic regulator, thought to exert numerous effects on skeletal muscle function, metabolism, and enzymatic signaling. Despite these assertions, little is known regarding the direct role(s) of AMPK in vivo, and results obtained in vitro or in situ are conflicting. Using a chronically catheterized mouse model (carotid artery and jugular vein), we show that AMPK regulates skeletal muscle metabolism in vivo at several levels, with the result that a deficit in AMPK activity markedly impairs exercise tolerance. Compared with wild-type littermates at the same relative exercise capacity, vascular glucose delivery and skeletal muscle glucose uptake were impaired; skeletal muscle ATP degradation was accelerated, and arterial lactate concentrations were increased in mice expressing a kinase-dead AMPKalpha2 subunit (alpha2-KD) in skeletal muscle. Nitric-oxide synthase (NOS) activity was significantly impaired at rest and in response to exercise in alpha2-KD mice; expression of neuronal NOS (NOSmicro) was also reduced. Moreover, complex I and IV activities of the electron transport chain were impaired 32 +/- 8 and 50 +/- 7%, respectively, in skeletal muscle of alpha2-KD mice (p < 0.05 versus wild type), indicative of impaired mitochondrial function. Thus, AMPK regulates neuronal NOSmicro expression, NOS activity, and mitochondrial function in skeletal muscle. In addition, these results clarify the role of AMPK in the control of muscle glucose uptake during exercise. Collectively, these findings demonstrate that AMPK is central to substrate metabolism in vivo, which has important implications for exercise tolerance in health and certain disease states characterized by impaired AMPK activation in skeletal muscle.

Endothelial Nitric Oxide Synthase is Central to Skeletal Muscle Metabolic Regulation and Enzymatic Signaling During Exercise in Vivo

American Journal of Physiology. Regulatory, Integrative and Comparative Physiology. May, 2010  |  Pubmed ID: 20200137

Endothelial nitric oxide synthase (eNOS) is associated with a number of physiological functions involved in the regulation of metabolism; however, the functional role of eNOS is poorly understood. We tested the hypothesis that eNOS is critical to muscle cell signaling and fuel usage during exercise in vivo, using 16-wk-old catheterized (carotid artery and jugular vein) C57BL/6J mice with wild-type (WT), partial (+/-), or no expression (-/-) of eNOS. Quantitative reductions in eNOS expression ( approximately 40%) elicited many of the phenotypic effects observed in enos(-/-) mice under fasted, sedentary conditions, with expression of oxidative phosphorylation complexes I to V and ATP levels being decreased, and total NOS activity and Ca(2+)/CaM kinase II Thr(286) phosphorylation being increased in skeletal muscle. Despite these alterations, exercise tolerance was markedly impaired in enos(-/-) mice during an acute 30-min bout of exercise. An eNOS-dependent effect was observed with regard to AMP-activated protein kinase signaling and muscle perfusion. Muscle glucose and long-chain fatty acid uptake, and hepatic and skeletal muscle glycogenolysis during the exercise bout was markedly accelerated in enos(-/-) mice compared with enos(+/-) and WT mice. Correspondingly, enos(-/-) mice exhibited hypoglycemia during exercise. Thus, the ablation of eNOS alters a number of physiological processes that result in impaired exercise capacity in vivo. The finding that a partial reduction in eNOS expression is sufficient to induce many of the changes associated with ablation of eNOS has implications for chronic metabolic diseases, such as obesity and insulin resistance, which are associated with reduced eNOS expression.

Glucagon-like Peptide-1 Receptor Knockout Mice Are Protected from High-fat Diet-induced Insulin Resistance

Endocrinology. Oct, 2010  |  Pubmed ID: 20685876

Glucagon-like peptide-1 augments nutrient-stimulated insulin secretion. Chow-fed mice lacking the glucagon-like peptide-1 receptor (Glp1r) exhibit enhanced insulin-stimulated muscle glucose uptake but impaired suppression of endogenous glucose appearance (endoRa). This proposes a novel role for the Glp1r to regulate the balance of glucose disposal in muscle and liver by modulating insulin action. Whether this is maintained in an insulin-resistant state is unknown. The present studies tested the hypothesis that disruption of Glp1r expression overcomes high-fat (HF) diet-induced muscle insulin resistance and exacerbates HF diet-induced hepatic insulin resistance. Mice with a functional disruption of the Glp1r (Glp1r-/-) were compared with wild-type littermates (Glp1r+/+) after 12 wk on a regular chow diet or a HF diet. Arterial and venous catheters were implanted for sampling and infusions. Hyperinsulinemic-euglycemic clamps were performed on weight-matched male mice. [3-(3)H]glucose was used to determine glucose turnover, and 2[14C]deoxyglucose was used to measure the glucose metabolic index, an indicator of glucose uptake. Glp1r-/- mice exhibited increased glucose disappearance and muscle glucose metabolic index on either diet. This was associated with enhanced activation of muscle Akt and AMP-activated protein kinase and reduced muscle triglycerides in HF-fed Glp1r-/- mice. Chow-fed Glp1r-/- mice exhibited impaired suppression of endoRa and hepatic insulin signaling. In contrast, HF-fed Glp1r-/- mice exhibited improved suppression of endoRa and hepatic Akt activation. This was associated with decreased hepatic triglycerides and impaired activation of sterol regulatory element-binding protein-1. These results show that mice lacking the Glp1r are protected from HF diet-induced muscle and hepatic insulin resistance independent of effects on total fat mass.

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