Biomedical subjects

G I Shulman

Publications and source records attributed to G I Shulman.

At least 91 records · Page 5Linked to original sources

PubMed1997

13C and 31P NMR studies on the effects of increased plasma free fatty acids on intramuscular glucose metabolism in the awake rat.

The effects of increased plasma free fatty acids (FFA) on insulin-dependent whole body glucose disposal, skeletal muscle glycolysis, glycogen synthesis, pyruvate versus FFA/ketone oxidation, and glucose 6-phosphate (Glu-6-P) were investigated in the awake rat. A control group (glycerol-infused) and high plasma FFA group (Liposyn-infused) were clamped at euglycemia (approximately 6 mM)-hyperinsulinemia (10 milliunits/kg/min) throughout the experiment (180-240 min). In the initial experiment, 13C NMR was used to observe [1-13C]glucose incorporation into [1-13C]glycogen in the rat hindlimb for glycogen synthesis calculations and into [3-13C]lactate and [3-13C]alanine for glycolytic flux calculations. These experiments were followed by 31P NMR measurements of Glu-6-P changes under identical conditions of the initial experiment. Plasma FFA concentrations were 2.25 +/- 0.36 and 0.20 +/- 0.03 mM in the high plasma FFA and control groups respectively (p < 0.0005). Glucose infusion rates (Ginf) decreased significantly in the Liposyn-infused rats (29.5 +/- 0.7 and 27.2 +/- 1.2 mg/kg/min for control and high plasma FFA group, respectively, at 15 min to 30.7 +/- 2.3 and 17.7 +/- 1.3 mg/kg/min, respectively, at the end of the experiment, p < 0.002). Glycogen synthesis rates were 163 +/- 32 and 104 +/- 17 nmol/g/min, and glycolytic rates were 57.9 +/- 8.0 and 19. 5 +/- 3.6 nmol/g/min (p < 0.002) in the control and high plasma FFA groups, respectively. The relative flux of pyruvate versus free fatty acids and ketones entering the tricarboxylic acid cycle was greater in the control (57 +/- 9%) versus high plasma FFA group (25 +/- 4%) (p < 0.005) as assessed by [4-13C]glutamate/[3-13C]lactate steady state isotopic enrichment measurements. Finally, Glu-6-P concentrations increased by 29.8 +/- 7.0 and 52.8 +/- 12.3% (p < 0. 05) in the control and high plasma FFA groups, respectively, above their basal concentrations by 180 min. In conclusion, we have demonstrated the ability to use in vivo NMR to elucidate the metabolic fate of glucose within skeletal muscle of an awake rat during a euglycemic-hyperinsulinemic clamp and increased levels of plasma FFA. These data suggest that increased concentrations of plasma FFA inhibit insulin-stimulated muscle glucose metabolism in the rat through inhibition of glycolysis.

Alanine↗

PubMed1997

Low-flow ischemia leads to translocation of canine heart GLUT-4 and GLUT-1 glucose transporters to the sarcolemma in vivo.

BACKGROUND: Myocardial ischemia increases heart glucose utilization in vivo. However, whether low-flow ischemia leads to the translocation of glucose transporter (GLUT)-4 and/or GLUT-1 to the sarcolemma in vivo is unknown. METHODS AND RESULTS: In a canine model, we evaluated myocardial glucose metabolism in vivo and the distribution of GLUT-4 and GLUT-1 by use of immunoblotting of sarcolemma and intracellular membranes and immunofluorescence localization with confocal microscopy. In vivo glucose extraction increased fivefold (P < .001) and was associated with net lactate release in the ischemic region. Ischemia led to an increase in the sarcolemma content of both GLUT-4 (15 +/- 2% to 30 +/- 3%, P < .02) and GLUT-1 (41 +/- 4% to 58 +/- 3%, P < .03) compared with the nonischemic region and to a parallel decrease in their intracellular contents. Immunofluorescence demonstrated the presence of both GLUT-4 and GLUT-1 on cardiac myocytes. GLUT-1 had a more prominent cell surface pattern than GLUT-4, which was primarily intracellular in the nonischemic region. However, significant GLUT-4 surface labeling was found in the ischemic region. CONCLUSIONS: Translocation of the insulin-responsive GLUT-4 transporter from an intracellular storage pool to the sarcolemma occurs in vivo during acute low-flow ischemia. GLUT-1 is also present in an intracellular storage pool from which it undergoes translocation to the sarcolemma in response to ischemia. These results indicate that both GLUT-1 and GLUT-4 are important in ischemia-mediated myocardial glucose uptake in vivo.

Animals↗

PubMed1997

Local ventromedial hypothalamus glucose perfusion blocks counterregulation during systemic hypoglycemia in awake rats.

The ventromedial hypothalamic nucleus (VMH) is necessary for the integrated hormonal response to hypoglycemia. To determine the role of the VMH as a glucose sensor, we performed experiments designed to specifically prevent glucopenia in the VMH, while producing hypoglycemia elsewhere. We used awake chronically catheterized rats, in which local VMH glucose perfusion (100 mM or 15 mM of D-glucose) was combined with a sequential euglycemic-hypoglycemic clamp. In two control groups the VMH was perfused either with (a) an iso-osmotic solution lacking glucose, or with (b) nonmetabolizable L-glucose (100 mM). During systemic hypoglycemia glucagon and catecholamine concentrations promptly increased in the control animals perfused with either 100 mM L-glucose or the iso-osmotic solution lacking glucose. In contrast, glucagon, epinephrine and norepinephrine release was inhibited in the animals in which the VMH was perfused with D-glucose; hormonal secretion was partially suppressed by the VMH perfusion with 15 mM D-glucose and suppressed by approximately 85% when the VMH was perfused with 100 mM D-glucose, as compared with the control groups. We conclude that the VMH must sense hypoglycemia for full activation of catecholamine and glucagon secretion and that it is a key glucose sensor for hypoglycemic counterregulation.

Animals↗

PubMed1997

In vivo NMR investigation of intramuscular glucose metabolism in conscious rats.

In vivo 13C nuclear magnetic resonance (NMR) spectroscopy was used to determine quantitatively the flux of muscle glycolysis, glycogen synthesis, pyruvate dehydrogenase, and pyruvate carboxylation in the hindlimb of conscious rats. 13C NMR spectroscopy was used to observe [1-13C]glucose label precursor incorporation into intramuscular [1-13C]glycogen, [3-13C]lactate, and [3-13C]alanine during a hyperglycemic (approximately 11 mM)-hyperinsulinemic (10 mU.kg-1.min-1) clamp. The glycogen synthesis rate was calculated to be 224 +/- 23 nmol.g-1.min-1. The kinetic data obtained from the label turnover in the intramuscular C-3 lactate and C-3 alanine metabolite pools, as well as in plasma C-3 lactate and C-3 alanine, were combined with a steady-state rate analysis to determine the glycolytic flux (67.4 +/- 10.1 nmol.g-1.min-1). Steady-state isotopomer analysis of glutamate and pyruvate in skeletal muscle tissue extracts was used to determine the anaplerotic contribution of substrate via pyruvate carboxylation (Vpc). The pyruvate dehydrogenase flux (Vpdh) was calculated after a steady-state flux correction for Vpc. Calculated values of Vpc and Vpdh were 24.8 +/- 4.3 and 110.0 +/- 18.7 nmol.g-1.min-1, respectively. In addition, [2-13C]acetate was used in a separate study to determine that pyruvate carboxylation was the major pathway for anaplerosis in skeletal muscle under conditions of hyperglycemia-hyperinsulinemia.

Alanine↗

PubMed1997

Effects of insulin-like growth factor I on glucose metabolism in rats with liver cirrhosis.

To determine the effect of insulin-like growth factor I (IGF-I) on glucose metabolism in cirrhosis, a 2-h euglycemic clamp with IGF-I (0.65 nmol.kg-1.min-1) or insulin (12 pmol.kg-1.min-1) was performed in awake rats with carbon tetrachloride-induced liver cirrhosis. Rates of [3-3H]glucose-determined whole body glucose turnover were similar in the fasting state in cirrhotic and control rats (36.4 +/- 2.6 and 37.7 +/- 2.8 mumol.kg-1.min-1, respectively). In the control group, IGF-I and insulin had similar effects on turnover (81.6 +/- 27.0 and 76.1 +/- 9.9 mumol.kg-1.min-1), muscle glycogen synthesis (47.5 +/- 12.3 and 37.5 +/- 2.5 nmol.g muscle-1.min-1), and suppression of endogenous glucose production (EGP; -54 +/- 14 and -60 +/- 12%). Cirrhotic rats were markedly insulin resistant, reflected by a 43% reduction of turnover (43.8 +/- 9.4 mumol.g muscle-1.min-1; P = 0.03), a 73% reduction in muscle glycogen synthesis (10.2 +/- 3.4 nmol.g muscle-1.min-1; P < 0.0001), and a diminished suppression of EGP (-32 +/- 17% vs. control: -56 +/- 14%; P < 0.05). In contrast, during the IGF-I clamps, turnover increased threefold in the cirrhotic rats (P = 0.001), rates of muscle glycogen synthesis were 7.4 times higher than during the insulin stimulation (P < 0.0001), and EGP was suppressed by 80 +/- 12% (P < 0.05). In conclusion, insulin resistance in cirrhotic rats is mostly due to defects in insulin-stimulated muscle glycogen synthesis, and the ability of IGF-I to stimulate muscle glycogen synthesis as well as suppress EGP is maintained in cirrhotic rats. These findings suggest that alterations in both hepatic and peripheral glucose metabolism in patients with cirrhosis might be amenable to IGF-I therapy.

Animals↗

PubMed1997

The effect of leptin is enhanced by microinjection into the ventromedial hypothalamus.

To determine whether changes in food intake produced by leptin involve targeting the hormone to distinct central nervous system regions, guide cannulas were positioned stereotaxically into three brain regions--the ventromedial hypothalamus (VMH) (bilaterally, n = 6), the dorsal raphe nucleus (n = 3), and the lateral ventricle (n = 3)--of nonobese male rats (400-500 g). Daily food intake and body weight changes were measured during twice-daily injections of saline (0.1 microl) followed by recombinant human leptin (0.05 microg) for 3 days via the brain cannulas. VMH-injected rats also were followed during a postleptin saline recovery interval. This small dose of leptin did not change food intake or body weight from that during the preceding saline injection period in ventricle-injected or dorsal raphe-injected rats. In sharp contrast, VMH-injected rats ate much less food (56 +/- 8% basal) and lost 9 +/- 3 g/day or 5% of their body weight during 3 days of leptin administration. VMH-injected animals fully recovered from leptin-induced effects within 3 days. We conclude that small doses of leptin that do not effect eating behavior when delivered to the ventricle or the dorsal raphe (another brain region believed to regulate feeding), suppress food intake when injected into the VMH. These data suggest that the VMH or a brain region in close proximity to it is a key target for the biological actions of leptin.

Animals↗

PubMed1997

Metabolic defects in lean nondiabetic offspring of NIDDM parents: a cross-sectional study.

First-degree relatives of NIDDM patients have an approximately 40% lifetime risk of developing diabetes, and insulin resistance is the best predictor. However, insulin resistance is altered by many other factors, including age, diet, exercise, and medications. To investigate the metabolic and endocrine alterations associated with insulin resistance when all the above confounding factors are excluded, we examined the first phase of insulin secretion and insulin sensitivity in 49 white normoglycemic (4.99 +/- 0.51 vs. 4.95 +/- 0.41 mmol/l) nonexercising lean (BMI, 24 +/- 3 vs. 23 +/- 2 kg/m2; 105 +/- 3 vs. 104 +/- 3% of ideal body weight) offspring of NIDDM patients. These subjects were compared with 29 matched healthy control subjects by means of an intravenous glucose bolus (0.3 g/kg body wt), immediately followed by a euglycemic-hyperinsulinemic (approximately 420 pmol/l) clamp, along with lipid and amino acid profiles. The offspring showed fasting hyperinsulinemia (40.6 +/- 15.8 vs. 30.9 +/- 13.6 pmol/l; P = 0.005) and higher free fatty acid (FFA) levels (582 +/- 189 vs. 470 +/- 140 micromol; P = 0.007), whereas triglycerides, total cholesterol, and HDL and LDL cholesterol levels were comparable with those of control subjects. Alanine (320 +/- 70 vs. 361 +/- 73 micromol/l; P = 0.017), serine (P = 0.05), and glutamine and glycine (P = 0.02) were lower in the offspring than in the control subjects, whereas branched-chain amino acids (343 +/- 54 vs. 357 +/- 54 micromol/l; P = 0.28) were not different. Insulin sensitivity was lower (4.86 +/- 1.65 vs. 6.17 +/ 1.56 mg x kg(-1) x min(-1); P = 0.001), and an inverse correlation with fasting FFAs in the offspring (adjusted R2 = 0.21, P = 0.0005), but not in control subjects (adjusted R2 = 0.03, P = 0.368), was found. Because insulin sensitivity in the offspring appeared to be a mixture of three distributions, they were subdivided into three subgroups: very low, low, and normal insulin sensitivity (20, 47, and 33%, respectively). The same alterations in amino acid and FFA metabolism were observed in the very low and low subgroups but not in the normal subgroup. The first phase of insulin secretion appeared to compensate significantly for insulin resistance in the low subgroup versus the normal subgroup and controls, but was inappropriately low in the subgroup with very low insulin sensitivity considering its degree of insulin resistance. In conclusion, lean insulin-resistant offspring of NIDDM parents showed 1) trimodal distribution of insulin sensitivity, 2) high fasting plasma FFA concentrations, 3) an inverse correlation between insulin sensitivity and FFA concentration, 4) low plasma gluconeogenic amino acid concentrations, and 5) defective insulin secretion when related to insulin sensitivity in the subgroup of very resistant offspring. These results suggest that, in this white population, insulin sensitivity may be determined by a single major gene and that alterations in FFA metabolism may play a role in the pathogenesis of NIDDM.

Adult↗

PubMed1996

Glucose plus insulin regulate fat oxidation by controlling the rate of fatty acid entry into the mitochondria.

We tested the hypothesis that glucose plus insulin determine the rate of fat oxidation in humans by controlling the rate of fatty acid entrance into the mitochondria. We gave constant infusions of [1-13C]oleate, a long-chain fatty acid, and [1-14C]octanoate, a medium-chain fatty acid, for 3 h in seven volunteers (basal). Immediately after the basal period, a hyperinsulinemic (insulin infusion = 120 mU x m(-2) min(-1)), hyperglycemic (plasma glucose = 140 mg/dl) clamp was started and continued for 5 h. During the last 3 h of the clamp, the infusions of [1-13C]oleate and [1-14C]octanoate were repeated. Intracellular acylcarnitine concentrations were measured in muscle biopsies obtained before and after the clamp. Plasma oleate enrichment and FFA concentration were kept constant by means of variable infusions of lipids and heparin. Oleate, but not octanoate, requires carnitine binding to gain access to the mitochondrial matrix; hence, if glucose and/or insulin limit long-chain fatty acid entrance into the mitochondria, then, during the clamp, long-chain acylcarnitine formation should be decreased, causing a decrease in oleate, but not octanoate, oxidation. Oleate oxidation decreased from the basal value of 0.7+/-0.1 to 0.4+/-0.1 micromol x kg(-1) x min(-1) (P < 0.05). In contrast, octanoate oxidation remained unchanged. Long-chain acylcarnitine concentration decreased from 855+/-271 in the basal state to 376+/-83 nmol/gram dry weight during the clamp (P < 0.05). We conclude that glucose and/or insulin determine fatty acid oxidation by controlling the rate of long-chain fatty acid entrance into the mitochondria.

Adult↗

PubMed1996

Increased glucose transport-phosphorylation and muscle glycogen synthesis after exercise training in insulin-resistant subjects.

BACKGROUND: Insulin resistance in the offspring of parents with non-insulin-dependent diabetes mellitus (NIDDM) is the best predictor of development of the disease and probably plays an important part in its pathogenesis. We studied the mechanism and degree to which exercise training improves insulin sensitivity in these subjects. METHODS: Ten adult children of parents with NIDDM and eight normal subjects were studied before starting an aerobic exercise-training program, after one session of exercise, and after six weeks of exercise. Insulin sensitivity was measured by the hyperglycemic-hyperinsulinemic clamp technique combined with indirect calorimetry, and the rate of glycogen synthesis in muscle and the intramuscular glucose-6-phosphate concentration were measured by carbon-13 and phosphorus-31 nuclear magnetic resonance spectroscopy, respectively. RESULTS: During the base-line study, the mean (+/-SE) rate of muscle glycogen synthesis was 63 +/- 9 percent lower in the offspring of diabetic parents than in the normal subjects (P < 0.001). The mean value increased 69 +/- 10 percent (P = 0.04) and 62 +/- 11 percent (P = 0.04) after the first exercise session and 102 +/- 11 percent (P = 0.02) and 97 +/- 9 percent (P = 0.008) after six weeks of exercise training in the offspring and the normal subjects, respectively. The increment in glucose-6-phosphate during hyperglycemic-hyperinsulinemic clamping was lower in the offspring than in the normal subjects (0.039 +/- 0.013 vs. 0.089 +/- 0.009 mmol per liter, P = 0.005), reflecting reduced glucose transport-phosphorylation, but this increment was normal in the offspring after the first exercise session and after exercise training. Basal and stimulated insulin secretion was higher in the offspring than the normal subjects and was not altered by the exercise training program. CONCLUSIONS: Exercise increases insulin sensitivity in both normal subjects and the insulin-resistant offspring of diabetic parents because of a twofold increase in insulin-stimulated glycogen synthesis in muscle, due to an increase in insulin-stimulated glucose transport-phosphorylation.

Adult↗

PubMed1996

Impaired hepatic glycogen synthesis in glucokinase-deficient (MODY-2) subjects.

All glucokinase gene mutations identified to date have been localized to exons that are common to the pancreatic and hepatic isoforms of the enzyme. While impaired insulin secretion has been observed in glucokinase-deficient subjects the consequences of this mutation on hepatic glucose metabolism remain unknown. To examine this question hepatic glycogen concentration was measured in seven glucokinase-deficient subjects with normal glycosylated hemoglobin and 12 control subjects using 13C nuclear magnetic spectroscopy during a day in which three isocaloric mixed meals were ingested. The relative fluxes of the direct and indirect pathways of hepatic glycogen synthesis were also assessed using [1-13C]glucose in combination with acetaminophen to noninvasively sample the hepatic UDP-glucose pool. Average fasting hepatic glycogen content was similar in glucokinase-deficient and control subjects (279+/-20 vs 284+/-14 mM; mean+/-SEM), and increased in both groups after the meals with a continuous pattern throughout the day. However, the net increment in hepatic glycogen content after each meal was 30-60% lower in glucokinase-deficient than in the control subjects (breakfast, 46% lower, P < 0.02; lunch, 62% lower, P = 0.002; dinner; 30% lower, P = 0.04). The net increment over basal values 4 h after dinner was 105 +/-18 mM in glucokinase-deficient and 148+/-11 mM in control subjects (P = 0.04). In the 4 h after breakfast, flux through the gluconeogenic pathway relative to the direct pathway of hepatic glycogen synthesis was higher in glucokinase-deficient than in control subjects (50+/-2% vs 34+/-5%; P = 0.038). In conclusion glucokinase-deficient subjects have decreased net accumulation of hepatic glycogen and relatively augmented hepatic gluconeogenesis after meals. These results suggest that in addition to the altered beta cell function, abnormalities in liver glycogen metabolism play an important role in the pathogenesis of hyperglycemia in patients with glucokinase-deficient maturity onset diabetes of young.

Adult↗

PubMed1996

Glucose metabolism distal to a critical coronary stenosis in a canine model of low-flow myocardial ischemia.

Myocardial regions perfused through a coronary stenosis may cease contracting, but remain viable. Clinical observations suggest that increased glucose utilization may be an adaptive mechanism in such "hibernating" regions. In this study, we used a combination of 13C-NMR spectroscopy, GC-MS analysis, and tissue biochemical measurements to track glucose through intracellular metabolism in intact dogs infused with [1-13C]glucose during a 3-4-h period of acute ischemic hibernation. During low-flow ischemia [3-13C]alanine enrichment was higher, relative to plasma [1-13C]glucose enrichment, in ischemic than in nonischemic regions of the heart, suggesting a greater contribution of exogenous glucose to glycolytic flux in the ischemic region (approximately 72 vs. approximately 28%, P < 0.01). Both the fraction of glycogen synthase present in the physiologically active glucose-6-phosphate-independent form (46 +/- 10 vs. 9 +/- 6%, P < 0.01) and the rate of incorporation of circulating glucose into glycogen (94 +/- 25 vs. 20 +/- 15 nmol/gram/min, P < 0.01) were also greater in ischemic regions. Measurement of steady state [4-13C)glutamate/[3-13C]alanine enrichment ratios demonstrated that glucose-derived pyruvate supported 26-36% of total tricarboxylic acid cycle flux in all regions, however, indicating no preference for glucose over fat as an oxidative substrate in the ischemic myocardium. Thus during sustained regional low-flow ischemia in vivo, the ischemic myocardium increases its utilization of exogenous glucose as a substrate. Upregulation is restricted to cytosolic utilization pathways, however (glycolysis and glycogen synthesis), and fat continues to be the major source of mitochondrial oxidative substrate.

Alanine↗

PubMed1996

Mechanism of free fatty acid-induced insulin resistance in humans.

To examine the mechanism by which lipids cause insulin resistance in humans, skeletal muscle glycogen and glucose-6-phosphate concentrations were measured every 15 min by simultaneous 13C and 31P nuclear magnetic resonance spectroscopy in nine healthy subjects in the presence of low (0.18 +/- 0.02 mM [mean +/- SEM]; control) or high (1.93 +/- 0.04 mM; lipid infusion) plasma free fatty acid levels under euglycemic (approximately 5.2 mM) hyperinsulinemic (approximately 400 pM) clamp conditions for 6 h. During the initial 3.5 h of the clamp the rate of whole-body glucose uptake was not affected by lipid infusion, but it then decreased continuously to be approximately 46% of control values after 6 h (P < 0.00001). Augmented lipid oxidation was accompanied by a approximately 40% reduction of oxidative glucose metabolism starting during the third hour of lipid infusion (P < 0.05). Rates of muscle glycogen synthesis were similar during the first 3 h of lipid and control infusion, but thereafter decreased to approximately 50% of control values (4.0 +/- 1.0 vs. 9.3 +/- 1.6 mumol/[kg.min], P < 0.05). Reduction of muscle glycogen synthesis by elevated plasma free fatty acids was preceded by a fall of muscle glucose-6-phosphate concentrations starting at approximately 1.5 h (195 +/- 25 vs. control: 237 +/- 26 mM; P < 0.01). Therefore in contrast to the originally postulated mechanism in which free fatty acids were thought to inhibit insulin-stimulated glucose uptake in muscle through initial inhibition of pyruvate dehydrogenase these results demonstrate that free fatty acids induce insulin resistance in humans by initial inhibition of glucose transport/phosphorylation which is then followed by an approximately 50% reduction in both the rate of muscle glycogen synthesis and glucose oxidation.

Adult↗

PubMed1996

NMR studies of muscle glycogen synthesis in insulin-resistant offspring of parents with non-insulin-dependent diabetes mellitus immediately after glycogen-depleting exercise.

To examine the impact of insulin resistance on the insulin-dependent and insulin-independent portions of muscle glycogen synthesis during recovery from exercise, we studied eight young, lean, normoglycemic insulin-resistant (IR) offspring of individuals with non-insulin-dependent diabetes mellitus and eight age-weight matched control (CON) subjects after plantar flexion exercise that lowered muscle glycogen to approximately 25% of resting concentration. After approximately 20 min of exercise, intramuscular glucose 6-phosphate and glycogen were simultaneously monitored with 31P and 13C NMR spectroscopies. The postexercise rate of glycogen resynthesis was nonlinear. Glycogen synthesis rates during the initial insulin independent portion (0-1 hr of recovery) were similar in the two groups (IR, 15.5 +/- 1.3 mM/hr and CON, 15.8 +/- 1.7 mM/hr); however, over the next 4 hr, insulin-dependent glycogen synthesis was significantly reduced in the IR group [IR, 0.1 +/- 0.5 mM/hr and CON, 2.9 +/- 0.2 mM/hr; (P < or = 0.001)]. After exercise there was an initial rise in glucose 6-phosphate concentrations that returned to baseline after the first hour of recovery in both groups. In summary, we found that following muscle glycogen-depleting exercise, IR offspring of parents with non-insulin-dependent diabetes mellitus had (i) normal rates of muscle glycogen synthesis during the insulin-independent phase of recovery from exercise and (ii) severely diminished rates of muscle glycogen synthesis during the subsequent recovery period (2-5 hr), which has previously been shown to be insulin-dependent in normal CON subjects. These data provide evidence that exercise and insulin stimulate muscle glycogen synthesis in humans by different mechanisms and that in the IR subjects the early response to stimulation by exercise is normal.

Adult↗

PubMed1996

The roles of insulin and glucagon in the regulation of hepatic glycogen synthesis and turnover in humans.

To determine the respective roles of insulin and glucagon for hepatic glycogen synthesis and turnover, hyperglycemic clamps were performed with somatostatin [0.1 micrograms/(kg.min)] in healthy young men under conditions of: (I) basal fasting) portal vein insulinemia-hypoglucagonemia, (II) basal portal vein insulinemia-basal glucagonemia, and (III) basal peripheral insulinemia-hypoglucagonemia. Synthetic rates, pathway (direct versus indirect) contributions, and percent turnover of hepatic glycogen were assessed by in vivo 13C nuclear magnetic resonance spectroscopy during [1-13C]glucose infusion followed by a natural abundance glucose chase in conjunction with acetaminophen to noninvasively sample the hepatic UDP-glucose pool. In the presence of hyperglycemia (10.4 +/- 0.1 mM) and basal portal vein insulinemia (192 +/- 6 pM), suppression of glucagon secretion (plasma glucagon, I:31 +/- 4, II: 63 +/- 8 pg/ml) doubled the hepatic accumulation of glycogen (Vsyn) compared with conditions of basal glucagonemia [I: 0.40 +/- 0.06, II: 0.19 +/- 0.03 mumol/(liter.min): P < 0.0025]. Glycogen turnover was markedly reduced (I: 19 +/- 7%, II: 69 +/- 12%; P < 0.005), so that net rate of glycogen synthesis increased approximately fivefold (P < 0.001) by inhibition of glucagon secretion. The relative contribution of gluconeogenesis (indirect pathway) to glycogen synthesis was lower during hypoglucagonemia (42 +/- 6%) than during basal glucagonemia (54 +/- 5%; P < 0.005). Under conditions of basal peripheral insulinemia (54 +/- 2 pM) and hypoglucagonemia (III) there was negligible hepatic glycogen synthesis and turnover. In conclusion, small changes in portal vein concentrations of insulin and glucagon independently affect hepatic glycogen synthesis and turnover. Inhibition of glucagon secretion under conditions of hyperglycemia and basal concentrations of insulin results in: (a) twofold increase in rate of hepatic glycogen synthesis, (b) reduction of glycogen turnover by approximately 73%, and (c) augmented percent contribution of the direct pathway to glycogen synthesis compared with conditions of basal glucagonemia.

Adult↗

PubMed1996

Direct assessment of liver glycogen storage by 13C nuclear magnetic resonance spectroscopy and regulation of glucose homeostasis after a mixed meal in normal subjects.

Despite extensive recent studies, understanding of the normal postprandial processes underlying immediate storage of substrate and maintenance of glucose homeostasis in humans after a mixed meal has been incomplete. The present study applied 13C nuclear magnetic resonance spectroscopy to measure sequential changes in hepatic glycogen concentration, a novel tracer approach to measure postprandial suppression of hepatic glucose output, and acetaminophen to trace the pathways of hepatic glycogen synthesis to elucidate the homeostatic adaptation to the fed state in healthy human subjects. After the liquid mixed meal, liver glycogen concentration rose from 207 +/- 22 to 316 +/- 19 mmol/liter at an average rate of 0.34 mmol/liter per min and peaked at 318 +/- 31 min, falling rapidly thereafter (0.26 mmol/liter per min). The mean increment at peak represented net glycogen synthesis of 28.3 +/- 3.7 g (approximately 19% of meal carbohydrate content). The contribution of the direct pathway to overall glycogen synthesis was 46 +/- 5 and 68 +/- 8% between 2 and 4 and 4 and 6 h, respectively. Hepatic glucose output was completely suppressed within 30 min of the meal. It increased steadily from 60 to 255 min from 0.31 +/- 32 to 0.49 +/- 18 mg/kg per min then rapidly returned towards basal levels (1.90 +/- 0.04 mg/kg per min). This pattern of change mirrored precisely the plasma glucagon/insulin ratio. These data provide for the first time a comprehensive picture of normal carbohydrate metabolism in humans after ingestion of a mixed meal.

Adult↗

PubMed1996

Noninvasive assessment of hepatic triglyceride content in humans with 13C nuclear magnetic resonance spectroscopy.

Hepatic lipid content was assessed noninvasively in 15 patients with hepatic steatosis by 13C nuclear magnetic resonance (NMR) spectroscopy, and compared in a double-blind fashion with histological grading and morphometric quantitation of fat in liver biopsies taken within 2 weeks of the study. The lipid content in the liver biopsies was expressed as the volume fraction of total parenchyma occupied by fat. Hepatic triglyceride content was determined by comparing the 13C NMR signal intensity in vivo with the signal intensity obtained from a lipid phantom of known concentrations. There was an approximately 30-fold increase in the 13C NMR signals of the saturated carbons (methyl/methylene [CH2]n) region of hepatic triglycerides from patients with grade 4 steatosis compared with those with grade 0, yielding a good dynamic range for measuring hepatic triglyceride content. The correlation coefficient between the morphometric and 13C NMR techniques was 0.89 (P < .01). These studies demonstrate that 13C NMR spectroscopy can be used to noninvasively assess hepatic triglyceride content in humans. This method may be clinically useful for diagnosis and follow-up of patients with hepatic steatosis.

Adult↗

PubMed1996

Time course of the defective alpha-cell response to hypoglycemia in diabetic BB rats.

Although it is understood that patients with insulin-dependent diabetes mellitus (IDDM) lose the ability to release glucagon during a hypoglycemic challenge, the relationship of this defect to the disease onset and loss of beta-cell function is not well defined. To address this issue, we measured the counterregulatory response in three groups of BB/wor rats during sequential 90-minute euglycemic (7 mmol/L) and hypoglycemic (3 mmol/L) insulin clamps (180 pmol/kg.min). Group 1 (n = 8) consisted of nondiabetic BB rats (aged 84 +/- 3 days), and groups 2 and 3 were rats studied 1 day (n = 7) or 7 days (n = 6) after diabetes onset. Plasma glucagon concentrations were similar in all groups during euglycemia (244 +/- 47 ng/L for nondiabetic, 308 +/- 38 for 1 day of diabetes, and 277 +/- 30 for 7 days of diabetes). Moreover, after 1 day of diabetes, the increase in plasma glucagon during hypoglycemia was similar to that seen in controls (to 581 +/- 94 and 650 +/- 118 ng/L, respectively) even though insulin production by the pancreas was virtually absent. However, after 7 days of diabetes, plasma glucagon only increased to 339 +/- 59 ng/L during hypoglycemia (P = nonsignificant v basal), despite normal pancreatic glucagon content (11.5 +/- 1.2 v 10.8 +/- 0.6 micrograms/g in nondiabetic controls). In conclusion, the hypoglycemia-associated defect in glucagon release occurs early in the course of diabetes in BB rats and is not associated with decreased baseline plasma or pancreatic glucagon levels. This impairment, although not immediately linked to the decrease in pancreatic insulin content, occurs soon afterward, implying that the two events are related.

Animals↗

PubMed1996

Contribution of net hepatic glycogenolysis to glucose production during the early postprandial period.

Relative contributions of net hepatic glycogenolysis and gluconeogenesis to glucose production during the first 12 h of a fast were studied in 13 healthy volunteers by noninvasively measuring hepatic glycogen content using 13C nuclear magnetic resonance spectroscopy. Rates of net hepatic glycogenolysis were calculated by multiplying the change in liver glycogen content with liver volume determined by magnetic resonance imaging. Rates of gluconeogenesis were calculated as the difference between rates of glucose production determined with an infusion of [6,6-2H]-glucose and net hepatic glycogenolysis. At 6 P.M. a liquid mixed meal (1,000 kcal; 60% as glucose) was given, to which [2-2H]glucose was added to trace glucose absorption. Hepatic glycogen content was measured between 11 P.M. and 1 A.M. and between 3 and 6 A.M. At 11 P.M. the concentration was 470 mM and it decreased linearly during the night. The mean liver volume was 1.47 +/- 0.06 liters. Net hepatic glycogenolysis (5.8 +/- 0.8 mumol.kg body wt-1.min-1) accounted for, on average, 45 +/- 6% and gluconeogenesis for 55 +/- 6% of the rate of whole body glucose production (12.6 +/- 0.6 mumol.kg body wt-1.min-1). In conclusion, this study shows that, even early in the phase of the postabsorptive period when liver glycogen stores are maximal, gluconeogenesis contributes approximately 50% to hepatic glucose production.

Adult↗