Impaired activation of glycogen synthase in people at increased risk for developing NIDDM.

To study whether impaired activation of muscle glycogen synthase represents an early defect in the pathogenesis of insulin resistance in non-insulin-dependent diabetes mellitus (NIDDM), we quantitated rates of nonoxidative glucose metabolism and measured activities of glycogen synthase and phosphorylase and concentrations of free glucose and glucose-6-phosphate in muscle biopsies, obtained before and after a euglycemic insulin clamp, in 16 NIDDM patients, 18 first-degree relatives of NIDDM patients, and 16 nondiabetic control subjects. Insulin-stimulated glucose storage (20.1 +/- 1.5 and 11.6 +/- 1.7 vs. 27.9 +/- 1.7 mumol.kg-1 lean body mass [LBM].min-1, P less than 0.01-0.001 [3.6 +/- 0.3 and 2.1 +/- 0.3 vs. 5.0 +/- 0.3 mg.kg-1 LBM.min-1] and glycogen synthase activity, measured at 0.1 mM glucose-6-phosphate concentration (11.3 +/- 1.3 and 11.6 +/- 1.3 vs. 18.3 +/- 2.0 nmol.min-1.mg-1 protein, P less than 0.01), were impaired in relatives and diabetic subjects compared with control subjects. Glycogen synthase activity correlated with the rate of glucose storage (r = 0.53, P less than 0.001). Glycogen phosphorylase fractional activity did not differ among the groups. Apart from increased intramuscular basal glucose concentrations in NIDDM patients, no consistent differences were observed in free glucose and glucose-6-phosphate concentrations between the groups. We conclude that impaired activation of muscle glycogen synthase by insulin is observed in patients with a genetic risk of developing NIDDM and may represent an early defect in the pathogenesis of NIDDM.     

Impaired insulin-mediated skeletal muscle blood flow in patients with NIDDM.

Patients with non-insulin-dependent diabetes mellitus (NIDDM) exhibit decreased rates of skeletal muscle insulin-mediated glucose uptake (IMGU). Because IMGU is equal to the product of the arteriovenous glucose difference (AVG delta) across and blood flow (F) into muscle (IMGU = AVG delta x F), reduced tissue permeability (AVG delta) and/or glucose and insulin delivery (F) can potentially lead to decreased IMGU. The components of skeletal muscle IMGU were studied in six obese NIDDM subjects (103 +/- 9 kg) and compared with those previously determined in six lean (weight 68 +/- 3 kg), and six obese (94 +/- 3 kg) with normal glucose tolerance. The insulin dose-response curves for whole body and leg muscle IMGU were constructed using the combined euglycemic clamp and leg balance techniques during sequential insulin infusions (range of serum insulin 130-80,000 pmol/L). In lean, obese, and NIDDM subjects, whole body IMGU, femoral AVG delta, and leg IMGU increased in a dose-dependent fashion over the range of insulin with an ED50 of 400-500 pmol/L in lean, 1000-1200 pmol/L in obese, and 4000-7000 pmol/L in NIDDM subjects (P less than 0.01 lean vs. obese and NIDDM). In lean and obese subjects, maximally effective insulin concentrations increased leg blood flow approximately 2-fold from basal with an ED50 of 266 pmol/L and 957 pmol/L, respectively (P less than 0.01 lean vs. obese). In contrast, leg F did not increase from the basal value in NIDDM subjects (2.7 +/- 0.1 vs. 3.5 +/- 0.5 dl/min, NS).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of insulin and amino acids on leg protein turnover in IDDM patients

To determine whether the responses of muscle protein metabolism to insulin and amino acids in patients with insulin-dependent diabetes mellitus (IDDM) were different from those in nondiabetic subjects, leg tissue kinetics of [15N]phenylalanine and [1-13C]leucine and its metabolites were measured in eight insulin-withdrawn IDDM patients and eight nondiabetic subjects during basal insulinemia and during infusion of insulin (0.29 nmol.min-1.m-2). The diabetic patients were studied in the absence of amino acids, and both groups were studied during infusion of a mixed-amino acid solution (AA). In the diabetic patients, insulin alone and combined with additional AA reduced leg tissue phenylalanine release by 42 and 41%, respectively (both P less than 0.05), but uptake was unchanged. Leg tissue leucine oxidation was unchanged by insulin alone but was increased (P = 0.012) fourfold during insulin infusion with additional AA. In the nondiabetic subjects, insulin with AA infusion increased leg tissue phenylalanine uptake (45.7 +/- 7.5 to 73.1 +/- 7.3 nmol.min-1.100 g-1, P less than 0.01). Insulin-stimulated glucose uptake in the diabetic patients (1.60 +/- 0.28 mumol.min-1.100 g-1, P = 0.04). These results suggest that, in IDDM patients, 1) infusion of insulin fails to stimulate muscle protein synthesis even when combined with a substantially increased provision of AA, and 2) compared with nondiabetic subjects, muscle protein synthesis as well as glucose uptake exhibit blunted responses to insulin.     

Enhanced peripheral and splanchnic insulin sensitivity in NIDDM men after single bout of exercise

We studied glucose metabolism in non-insulin-dependent diabetic (NIDDM) men with and without glycogen-depleting cycle exercise 12 h beforehand and have compared the results to our previous data in lean and obese subjects. Rates of total glucose utilization, glucose oxidation, nonoxidative glucose disposal (NOGD), glucose metabolic clearance rate (MCR), and endogenous glucose production (EGP) were determined with a "two-level" insulin-clamp technique (100-min infusions at 40 and 400 mU X m-2 X min-1) combined with indirect calorimetry and D-3-[3H]glucose infusion. Muscle biopsy specimens from vastus lateralis were analyzed for glycogen content and glycogen synthase activity before and after insulin infusions. After exercise, NIDDM subjects had muscle glycogen concentrations comparable with those of lean and obese subjects. The activation of glycogen synthase both by prior exercise and insulin infusion was similar to lean controls. After exercise, total glucose disposal was significantly increased during the 40-mU X m-2 X min-1 infusion (P less than .05), but the increase observed during the 400-mU X m-2 X min-1 infusion was not significant. These increases after exercise were the result of significantly higher NOGD during both levels of insulin infusion. The MCR of glucose during both insulin infusions was reduced in NIDDM compared with lean subjects but was very similar to that in obese nondiabetics. Basal EGP was significantly reduced on the morning after exercise (4.03 +/- 0.27 vs. 3.21 +/- 0.21 mg x kg-1 fat-free mass x min-1) (P less than .05) and associated with significant reductions of fasting plasma glucose (197 +/- 12 vs. 164 +/- 9 mg/dl).(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo insulin resistance induced by amylin primarily through inhibition of insulin-stimulated glycogen synthesis in skeletal muscle

We examined the in vivo mechanisms of amylin-induced resistance in concious rats (n = 18). During 180-min euglycemic insulin-clamp (21.5 pmol.kg-1.min-1) studies, amylin (50, 200, or 500 pmol.kg-1.min-1; plasma concentration from 3 x 10(-10) to 9 x 10(-9) M) infusion determined a 19-27% reduction in glucose uptake (117.8 +/- 7.0 vs. 145.8 +/- 11.0, 107.1 +/- 9.2 vs. 145.1 +/- 6.7, and 105.0 +/- 7.2 vs. 144.4 +/- 7.0 mumol.kg-1.min-1 at 50, 200, or 500 pmol.kg-1.min-1, respectively, P less than 0.01) versus insulin alone, whereas 10-pmol.kg-1.min-1 amylin infusion (plasma concn 5 x 10(-11) M) failed to affect insulin-mediated glucose disposal. After amylin infusion, the contribution of whole-body glycolysis to overall glucose disposal increased from 43-48 to 62-79%, whereas muscle glycogen synthesis decreased significantly at all peptide concentrations greater than 3 x 10(-10) M, completely accounting for the decrease in glucose uptake. Skeletal muscle glucose-6-phosphate concentration rose from 0.219 +/- 0.038 mumol/g (insulin alone) to 0.350 +/- 0.018, 0.440 +/- 0.020, and 0.505 +/- 0.035 mumol/g (insulin plus amylin at 50, 200, or 500 pmol.kg-1.min-1, P less than 0.01). Suppression of hepatic glucose production by insulin was unaffected by a 50-pmol.kg-1.min-1 amylin infusion (18.5 +/- 4.3 vs. 21.7 +/- 2.9 mumol.kg-1.min-1), whereas it was slightly but significantly impaired by amylin infusion at 200 pmol.kg-1.min-1 (17.8 +/- 3.9 vs. 24.7 +/- 4.5 mumol.kg-1.min-1, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Simultaneous insulinlike growth factor I and insulin resistance in obese Zucker rats.

It has recently been shown that the ability of insulinlike growth factor I (IGF-I) to stimulate glucose uptake and to lower circulating amino acid levels is retained in insulin-resistant diabetic BB rats. To examine in vivo effects of IGF-I in obese Zucker rats (another model of insulin resistance) 6 obese and 6 lean rats received euglycemic IGF-I infusions (0.65 nmol.kg-1.min-1). IGF-I-stimulated glucose uptake in obese rats was 50% lower than lean control rats (45.0 +/- 2.8 vs. 92.2 +/- 6.1 mumol.kg-1.min-1, respectively), even though the rise in circulating IGF-I levels was greater in the obese group during IGF-I infusion. In addition, branched chain amino acid concentrations that declined by 45% in lean controls were not suppressed significantly in obese rats (392 +/- 33 basal vs. 327 +/- 29 microM at 90 min). Equivalent results were observed during euglycemic insulin clamps (12 pmol.kg-1.min-1) in 7 obese and 11 lean rats. These studies demonstrate that obese Zucker rats are resistant to the effects of IGF-I and insulin on glucose and amino acid metabolism.     

Effect of insulin on oxidation of intracellularly and extracellularly derived glucose in patients with NIDDM. Evidence for primary defect in glucose transport and/or phosphorylation but not oxidation

Insulin-stimulated glucose oxidation is decreased in patients with non-insulin-dependent diabetes mellitus (NIDDM). It is not known whether this decrease is due to a primary defect in the oxidative pathway or is secondary to impaired glucose transport and/or phosphorylation. To address this issue, glucose oxidation was measured under steady-state conditions at low (approximately 270 pmol) and high (approximately 17 mumol) insulin concentrations in seven patients with NIDDM and seven healthy nondiabetic subjects matched for sex, age, and obesity. Glucose oxidation was measured simultaneously by indirect calorimetry and the isotopedilution technique. Although glucose oxidation and nonoxidative storage were lower (P less than 0.05) in diabetic than nondiabetic subjects during the low- and high-dose insulin infusions, oxidation of intracellularly derived glucose, estimated by subtracting the rate of oxidation measured isotopically (i.e., glucose oxidation derived from the extracellular space) from that measured by indirect calorimetry (i.e., total glucose oxidation), did not differ in diabetic and nondiabetic subjects during the low-dose insulin infusion (3.3 +/- 0.1 vs. 3.0 +/- 0.1 mumol.kg-1.min-1). Both techniques provided identical estimates of glucose oxidation during the high-dose insulin infusion. Impaired oxidation of extracellularly but not intracellularly derived glucose strongly suggests that the cause of decreased glucose oxidation in patients with NIDDM is secondary to impaired glucose transport and/or phosphorylation rather than a primary abnormality in the oxidative pathway.     

Metformin normalizes nonoxidative glucose metabolism in insulin-resistant normoglycemic first-degree relatives of patients with NIDDM.

Many first-degree relatives of patients with non-insulin-dependent diabetes mellitus (NIDDM) are characterized by insulin resistance. Because metformin improves peripheral insulin sensitivity, we examined the acute effect of metformin and placebo on glucose and lipid metabolism in nine insulin-resistant first-degree relatives of NIDDM patients with the euglycemic insulin-clamp technique combined with indirect calorimetry and infusion of [3-3H]glucose. Either placebo or 500 mg metformin was taken in random order twice the day before and once 1 h before the clamp. Nine healthy individuals without family history of diabetes served as control subjects. Basal plasma glucose was normal and did not differ between the metformin and the placebo study (4.8 +/- 0.2 vs. 5.0 +/- 0.2 mM) and neither did basal hepatic glucose production (10.59 +/- 0.54 vs. 10.21 +/- 0.80 mumol.kg-1.min-1). Insulin-stimulated glucose disposal was significantly increased by 25% after metformin compared with placebo (26.67 +/- 2.87 vs. 21.31 +/- 1.73 mumol.kg-1.min-1, P less than 0.05). The enhancement in glucose utilization was primarily due to normalization of nonoxidative glucose disposal (from 8.02 +/- 1.35 to 15.07 +/- 2.69 mumol.kg-1.min-1, P less than 0.01, vs. 15.65 +/- 2.72 mumol.kg-1.min-1 in control subjects). In contrast, glucose oxidation during the clamp was slightly lower after metformin compared with both placebo (11.59 +/- 0.83 vs. 13.30 +/- 1.00 mumol.kg-1.min-1, P = 0.06) and healthy control subjects (15.68 +/- 1.38 mumol.kg-1.min-1, P less than 0.05). We conclude that acutely administered metformin improves peripheral insulin sensitivity in insulin-resistant normoglycemic individuals primarily by stimulating the nonoxidative pathway of glucose metabolism.     

Mechanisms of hyperglycemia-induced insulin resistance in whole body and skeletal muscle of type I diabetic patients.

To examine the mechanisms of hyperglycemia-induced insulin resistance, eight insulin-dependent (type I) diabetic men were studied twice, after 24 h of hyperglycemia (mean blood glucose 20.0 +/- 0.3 mM, i.v. glucose) and after 24 h of normoglycemia (7.1 +/- 0.4 mM, saline) while receiving identical diets and insulin doses. Whole-body and forearm glucose uptake were determined during a 300-min insulin infusion (serum free insulin 359 +/- 22 and 373 +/- 29 pM, after hyper- and normoglycemia, respectively). Muscle biopsies were taken before and at the end of the 300-min insulin infusion. Plasma glucose levels were maintained constant during the 300-min period by keeping glucose for 150 min at 16.7 +/- 0.1 mM after 24-h hyperglycemia and increasing it to 16.5 +/- 0.1 mM after normoglycemia and by allowing it thereafter to decrease in both studies to normoglycemia. During the normoglycemic period (240-300 min), total glucose uptake (25.0 +/- 2.8 vs. 33.8 +/- 3.9 mumol.kg-1 body wt.min-1, P less than 0.05) was 26% lower, forearm glucose uptake (11 +/- 4 vs. 18 +/- 3 mumol.kg-1 forearm.min-1, P less than 0.05) was 35% lower, and nonoxidative glucose disposal (8.9 +/- 2.2 vs. 19.4 +/- 3.3 mumol.kg-1 body wt-1min-1, P less than 0.01) was 54% lower after 24 h of hyper- and normoglycemia, respectively. Glucose oxidation rates were similar. Basal muscle glycogen content was similar after 24 h of hyperglycemia (234 +/- 23 mmol/kg dry muscle) and normoglycemia (238 +/- 22 mmol/kg dry muscle). Insulin increased muscle glycogen to 273 +/- 22 mmol/kg dry muscle after 24 h of hyperglycemia and to 296 +/- 33 mmol/kg dry muscle after normoglycemia (P less than 0.05 vs. 0 min for both). Muscle ATP, free glucose, glucose-6-phosphate, and fructose-6-phosphate concentrations were similar after both 24-h treatment periods and did not change in response to insulin. We conclude that a marked decrease in whole-body, muscle, and nonoxidative glucose disposal can be induced by hyperglycemia alone.     

Contribution of impaired muscle glucose clearance to reduced postabsorptive systemic glucose clearance in NIDDM

The reduced postabsorptive rates of systemic glucose clearance in non-insulin-dependent diabetes mellitus (NIDDM) are thought to be the consequence of insulin resistance in peripheral tissues. Although the peripheral tissues involved have not been identified, it is generally assumed to be primarily muscle, the major site of insulin-mediated glucose disposal. To test this hypothesis, we measured postabsorptive systemic and forearm glucose utilization and clearance in 15 volunteers with NIDDM and 15 age- and weight-matched nondiabetic volunteers. Although systemic glucose utilization was increased in NIDDM subjects (14.5 +/- 0.5 vs. 11.2 +/- 0.2 mumol.kg-1.min-1, P less than 0.001), systemic glucose clearance was reduced 1.40 +/- 0.06 vs. 2.13 +/- 0.05 ml.kg-1.min-1, P less than 0.01). Although forearm glucose utilization was increased in NIDDM subjects (0.663 +/- 0.058 vs. 0.411 +/- 0.019 mumol.dl-1.min-1, P less than 0.001), forearm glucose dl-1 clearance was reduced (0.628 +/- 0.044 vs. 0.774 +/- 0.037 ml.L-1.min-1, P less than 0.01). However, extrapolation of forearm data to total-body muscle indicated that impaired clearance reduced muscle glucose disposal by only 61 +/- 21 mumol/min, whereas impaired systemic clearance reduced systemic glucose disposal by 662 +/- 82 mumol/min. Thus, impaired muscle glucose clearance accounted for less than 10% of the reduced systemic glucose clearance in NIDDM subjects. Therefore, we conclude that muscle insulin resistance plays only a minor role in the reduced systemic glucose clearance found in NIDDM in the postabsorptive state and propose that reduced brain glucose clearance is largely responsible.     

Kinetics of insulin-mediated and non-insulin-mediated glucose uptake in humans

The kinetics of insulin-mediated glucose uptake (IMGU) and non-insulin-mediated glucose uptake (NIMGU) in humans have not been well defined. We used the glucose-clamp technique to measure rates of whole-body and leg muscle glucose uptake in six healthy lean men during hyperinsulinemia (approximately 460 pM) to study IMGU and during somatostatin-induced insulinopenia to study NIMGU at four glucose levels (4.5, 9, 12, and 21 mM). To measure leg glucose uptake, the femoral artery and vein were catheterized, and blood flow was measured by thermodilution (leg glucose uptake = arteriovenous glucose difference [A-VG] x blood flow). With this approach, we found that, during hyperinsulinemia, both whole-body and leg glucose uptake increased in a curvilinear fashion at every glucose level, the highest glucose uptake values obtained being 139 +/- 17 mumol.kg-1.min-1 and 3656 +/- 931 mumol.min-1.leg-1, respectively. Leg blood flow increased twofold from 6.0 +/- 1.7 to 11.7 +/- 3.1 dl/min (P less than 0.01) over the range of glucose and was correlated with whole-body glucose uptake (r = 0.55, P less than 0.005). Leg muscle glucose extraction, independent of changes in blood flow, which is reflected by the A-VG, saturated over the range of glucose (1.28 +/- 0.12, 2.22 +/- 0.30, 2.92 +/- 0.42, 3.02 +/- 0.41 mM, NS between last 2 values) with a half-maximal effective glucose concentration (EG50) of 5.3 +/- 0.4 mM.(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolic effects of IGF-I in diabetic rats

Insulinlike growth factor I (IGF-I) stimulates glucose utilization (GU) in nondiabetic rats. We compared the effects of IGF-I and insulin on glucose metabolism in control (fed plasma glucose 7.7 +/- 0.1 mM, n = 30) and partially (90%) pancreatectomized diabetic (plasma glucose 18.4 +/- 0.8 mM, n = 30) awake unstressed rats. IGF-I was infused at 0.65 or 1.96 nmol.kg-1.min-1 and insulin at 22 or 29 pmol.kg-1.min-1 in combination with [3-3H]glucose while euglycemia was maintained by a variable glucose infusion. In controls, GU during the 0.65- and 1.96-nmol.kg-1.min-1 IGF-I infusions (127 +/- 7 and 168 +/- 4 mumol.kg-1.min-1, respectively) was similar to rates observed during the 22- and 29-pmol.kg-1.min-1 insulin infusions (121 +/- 2 and 156 +/- 5 mumol.kg-1.min-1). Whole-body glycolytic rate (3H2O generation) and muscle glycogen synthetic rate were identical during insulin and IGF-I infusions. In diabetic rats, GU was reduced by 30% versus control rats (P less than 0.01) during both the low-dose (88 +/- 7 vs. 121 +/- 7 mumol.kg-1.min-1) and higher-dose (109 +/- 4 vs. 156 +/- 5 mumol.kg-1.min-1) insulin clamps. The defect in insulin action involved both muscle glycogen synthesis and glycolysis. In diabetic rats, IGF-I elicited rates of GU similar to controls (115 +/- 10 and 164 +/- 12 mumol.kg-1.min-1 during the 0.65- and 1.96-nmol.kg-1.min-1 infusions, respectively) and corrected the intracellular defects in glycogen synthesis and glycolysis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gene expression of GLUT4 in skeletal muscle from insulin-resistant patients with obesity, IGT, GDM, and NIDDM.

In obesity, impaired glucose tolerance (IGT), non-insulin-dependent diabetes mellitus (NIDDM), and gestational diabetes mellitus (GDM), defects in glucose transport system activity, contribute to insulin resistance in target tissues. In adipocytes from obese and NIDDM patients, we found that pretranslational suppression of the insulin-responsive GLUT4 glucose transporter isoform is a major cause of cellular insulin resistance; however, whether this process is operative in skeletal muscle is not clear. To address this issue, we performed percutaneous biopsies of the vastus lateralis in lean and obese control subjects and in obese patients with IGT and NIDDM and open biopsies of the rectus abdominis at cesarian section in lean and obese gravidas and gravidas with GDM. GLUT4 was measured in total postnuclear membrane fractions from both muscles by immunoblot analyses. The maximally insulin-stimulated rate of in vivo glucose disposal, assessed with euglycemic glucose clamps, decreased 26% in obesity and 74% in NIDDM, reflecting diminished glucose uptake by muscle. However, in vastus lateralis, relative amounts of GLUT4 per milligram membrane protein were similar (NS) among lean (1.0 +/- 0.2) and obese (1.5 +/- 0.3) subjects and patients with IGT (1.4 +/- 0.2) and NIDDM (1.2 +/- 0.2). GLUT4 content was also unchanged when levels were normalized per wet weight, per total protein, and per DNA as an index of cell number. Levels of GLUT4 mRNA were similarly not affected by obesity, IGT, or NIDDM whether normalized per RNA or for the amount of an unrelated constitutive mRNA species. Because muscle fibers (types I and II) exhibit different capacities for insulin-mediated glucose uptake, we tested whether a change in fiber composition could cause insulin resistance without altering overall levels of GLUT4. However, we found that quantities of fiber-specific isoenzymes (phopholamban and types I and II Ca(2+)-ATPase) were similar in all subject groups. In rectus abdominis, GLUT4 content was similar in the lean, obese, and GDM gravidas whether normalized per milligram membrane protein (relative levels were 1.0 +/- 0.2, 1.3 +/- 0.1, and 1.0 +/- 0.2, respectively) or per wet weight, total protein, and DNA. We conclude that in human disease states characterized by insulin resistance, i.e., obesity, IGT, NIDDM, and GDM, GLUT4 gene expression is normal in vastus lateralis or rectus abdominis. To the extent that these muscles are representative of total muscle mass, insulin resistance in skeletal muscle may involve impaired GLUT4 function or translocation and not transporter depletion as observed in adipose tissue.     

Operation of Randle's cycle in patients with NIDDM

It has been suggested that the insulin resistance of non-insulin-dependent diabetes mellitus (NIDDM) may be caused by substrate competition between glucose and free fatty acids (FFAs) (Randle's cycle). We measured substrate oxidation and energy metabolism in 10 nonobese untreated NIDDM patients with fasting glucose levels of 7-8 mM with indirect calorimetry in the basal state and during an isoglycemic-hyperinsulinemic (approximately 100 mU/L) clamp without (control) and with a concomitant infusion (approximately 0.35 mmol/min) of Intralipid, a triglyceride emulsion. In the control study, fasting rates of total glucose turnover [( 3-3H]glucose) and glucose and lipid oxidation (9.4 +/- 1.4, 7.3 +/- 1.3, and 3.0 +/- 0.4 mumol.kg-1.min-1, respectively) were comparable with those of nondiabetic individuals. After insulin administration, lipid oxidation was normally suppressed (to 1.3 +/- 0.3 mumol.kg-1.min-1, P less than 0.01), as were the circulating levels of FFA, glycerol, and beta-hydroxybutyrate, whereas glucose oxidation doubled (14.1 +/- 1.8 mumol.kg-1.min-1, P less than 0.01). Because glycemia was clamped at 7.5 mM, endogenous glucose production (EGP) was completely suppressed, and total glucose disposal was stimulated (to 25.7 +/- 5.2 mumol.kg-1.min-1, P less than 0.01 vs. baseline), but glucose clearance (3.6 +/- 0.8 ml.kg-1.min-1) was 30% reduced compared with normal. With concomitant lipid infusion, FFA, glycerol, and beta-hydroxybutyrate all rose during the clamp; correspondingly, lipid oxidation was maintained at fasting rates (3.6 +/- 0.2 mumol.kg-1.min-1, P less than 0.01 vs. control).(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolic effects of hyperglycemia and hyperinsulinemia on fate of intracellular glucose in NIDDM

Hyperglycemia in non-insulin-dependent diabetes mellitus (NIDDM) stimulates peripheral glucose uptake, which tends to compensate for impaired insulin-mediated glucose uptake. The metabolic fate of glucose and suppression of fat oxidation may differ, however, when glucose uptake is stimulated primarily by insulin or hyperglycemia. To address this issue, three hyperinsulinemic glucose-clamp studies were performed in combination with indirect calorimetry in seven nonobese subjects with NIDDM. In the first two experiments, when glucose uptake was matched at approximately 8 mg.kg-1 fat-free mass (FFM).min-1 with primarily hyperinsulinemia (1350 +/- 445 pM) or hyperglycemia (20.8 +/- 1.8 mM), identical rates of glucose oxidation (3.21 +/- 0.29 and 3.10 +/- 0.23 mg.kg-1 FFM.min-1, NS) and nonoxidative glucose metabolism (5.19 +/- 0.75 and 5.46 +/- 0.61 mg.kg-1 FFM.min-1, NS) were achieved. When glucose uptake was increased further to 11.11 +/- 0.36 mg.kg-1 FFM.min-1 with less insulin (625 +/- 70 pM) and hyperglycemia, glucose oxidation (3.85 +/- 0.26 mg.kg-1 FFM.min-1) and nonoxidative glucose metabolism (7.26 +/- 0.51 mg.kg-1 FFM.min-1) rose significantly (both P less than 0.05 from matched studies at lower rates of glucose uptake). During all glucose-clamp studies, free fatty acids were comparably suppressed by 40-46% (all P less than 0.005 vs. basal values), whereas fat oxidation was suppressed by 70-80% (all P less than 0.005 vs. basal values). A strong negative correlation was observed between rates of glucose and fat oxidation (r = -0.88, P less than 0.001) when all studies were combined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lilly lecture 1989. Toward physiological understanding of glucose tolerance. Minimal-model approach

Glucose tolerance depends on a complex interaction among insulin secretion from the beta-cells, clearance of the hormone, and the actions of insulin to accelerate glucose disappearance and inhibit endogenous glucose production. An additional factor, less well recognized, is the ability of glucose per se, independent of changes in insulin, to increase glucose uptake and suppress endogenous output (glucose effectiveness). These factors can be measured in the intact organism with physiologically based minimal models of glucose utilization and insulin kinetics. With the glucose minimal model, insulin sensitivity (SI) and glucose effectiveness (SG) are measured by computer analysis of the frequently sampled intravenous glucose tolerance test. The test involves intravenous injection of glucose followed by tolbutamide or insulin and frequent blood sampling. SI varied from a high of 7.6 x 10(-4) min-1.microU-1.ml-1 in young Whites to 2.3 x 10(-4) min-1.microU-1.ml-1 in obese nondiabetic subjects; in all of the nondiabetic subjects, SG was normal. In subjects with non-insulin-dependent diabetes mellitus (NIDDM), not only was SI reduced 90% below normal (0.61 +/- 0.16 x 10(-4) min-1.microU-1.ml-1), but in this group alone, SG was reduced (from 0.026 +/- 0.008 to 0.014 +/- 0.002 min-1); thus, defects in SI and SG are synergistic in causing glucose intolerance in NIDDM. One assumption of the minimal model is that the time delay in insulin action on glucose utilization in vivo is due to sluggish insulin transport across the capillary endothelium. This was tested by comparing insulin concentrations in plasma with those in lymph (representing interstitial fluid) during euglycemic-hyperinsulinemic glucose clamps. Lymph insulin was lower than plasma insulin at basal (12 vs. 18 microU/ml) and at steady state, indicating significant loss of insulin from the interstitial space, presumably due to cellular uptake of the insulin-receptor complex. Additionally, during clamps, lymph insulin changed more slowly than plasma insulin, but the rate of glucose utilization followed a time course identical with that of lymph (r = .96) rather than plasma (r = .71). Thus, lymph insulin, which may be reflective of interstitial fluid, is the signal to which insulin-sensitive tissues are responding. These studies support the concept that, at physiological insulin levels, the time for insulin to cross the capillary endothelium is the process that determines the rate of insulin action in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)     

Role of lipid oxidation in pathogenesis of insulin resistance of obesity and type II diabetes

Increased lipid oxidation is generally observed in subjects with obesity and diabetes and has been suggested to be responsible for the insulin resistance associated with these conditions. We measured, by continuous indirect calorimetry, lipid and glucose oxidation and nonoxidative glucose disposal in 82 obese subjects during a 100-g oral glucose tolerance test (OGTT) and in 26 during a euglycemic insulin (40 mU.min-1.m-2) clamp. The obese subjects were subdivided into those with normal glucose tolerance (group A), those with impaired glucose tolerance (group B), and those with overt diabetes (group C). Forty-five healthy nonobese subjects were subdivided into a young and an older control group, which were age-matched to the nondiabetic obese (groups A and B) and diabetic obese (group C) subjects, respectively. In the postabsorptive state, as well as in response to insulin stimulation (both OGTT and insulin clamp), lipid oxidation was significantly increased in all three obese groups in comparison with either young or older controls. Basal glucose oxidation was significantly decreased in obese nondiabetic and obese glucose--intolerant subjects (groups A and B) compared with age-matched controls. During the OGTT and during the insulin clamp, insulin-stimulated glucose oxidation was decreased in all three obese groups. In contrast, nonoxidative glucose disposal was markedly inhibited in nondiabetic and diabetic obese patients during the euglycemic insulin clamp but not during the OGTT. After glucose ingestion, nonoxidative glucose uptake was normal in nondiabetic obese and glucose-intolerant obese subjects and decreased in diabetic obese subjects. Statistical analysis revealed that lipid and glucose oxidation were strongly and inversely related in the basal state, during euglycemic insulin clamp, and during OGTT. The negative correlation between lipid oxidation and nonoxidative glucose uptake, although significant, was much weaker. Fasting and post-OGTT hyperglycemia were the strongest (negative) correlates of nonoxidative glucose disposal in both single and multiple regression models. We conclude that 1) reduced glucose oxidation and reduced nonoxidative glucose disposal partake of the insulin resistance of nondiabetic obese and diabetic obese individuals; 2) hyperglycemia provides a compensatory mechanism for the defect in nonoxidative glucose disposal in nondiabetic obese subjects; however, this compensation is characteristically lost when overt diabetes ensues; and 3) increased lipid oxidation may contribute, in part, to the defects in glucose oxidation and nonoxidative glucose uptake in obesity.     

Impact of insulin deficiency on glucose fluxes and muscle glucose metabolism during exercise.

Exercise in the insulin-deficient diabetic state is characterized by a further increase in elevated circulating glucose and NEFA levels and by excessive counterregulatory hormone levels. The aim of this study was to distinguish the direct glucoregulatory effects of insulinopenia during exercise from the indirect effects that result from the metabolic and hormonal environment that accompanies insulin deficiency. For this purpose, dogs underwent 90 min of treadmill exercise during SRIF infusion with (SRIF + INS, n = 8) or without (SRIF - INS, n = 6) intraportal insulin replacement. Glucagon was not replaced, thus allowing assessment of the direct effect of insulinopenia at the liver independent of the potentiation of glucagon action. Glucose was infused to maintain euglycemia. Hepatic glucose production (Ra); glucose utilization (Rd); and LGlcU, LGlcE, and LGlcO were assessed with tracers ([3H]glucose, [14C]glucose) and arteriovenous differences. With exercise, insulin fell from 66 +/- 6 to 42 +/- 6 pM in the SRIF + INS group, and was undetectable in the SRIF - INS group. Plasma glucose was 6.33 +/- 0.38 and 6.26 +/- 0.30 mM at rest in the SRIF + INS and SRIF - INS groups, respectively, and was unchanged with exercise. Ra rose from 7.5 +/- 2.3 to 16.5 +/- 2.2 mumol.kg-1.min-1 and 9.1 +/- 2.0 to 31.4 +/- 3.9 mumol.kg-1.min-1 with exercise in the SRIF + INS and SRIF - INS groups, whereas Rd rose from 19.5 +/- 2.0 to 46.8 +/- 3.9 mumol.kg-1.min-1 and 15.1 +/- 1.8 to 29.9 +/- 3.3 mumol.kg-1.min-1. LGlcU rose from 36 +/- 9 to 112 +/- 25 mumol/min and 15 +/- 4 to 59 +/- 13 mumol/min and LGlcO rose from 5 +/- 2 to 61 +/- 12 mumol/min and 5 +/- 3 to 32 +/- 9 mumol/min with exercise in the SRIF+INS and SRIF-INS groups, respectively. Arterial levels and limb balances of NEFAs and glycerol were similar in the two groups. In summary, during exercise: 1) marked insulinopenia attenuates the increases in muscle glucose uptake and oxidation by approximately 50%, independent of changes in circulating metabolic substrate levels; 2) substantial increases in muscle glucose uptake and oxidation are, however, still present even in the absence of detectable insulin levels; and 3) insulinopenia facilitates the increase in Ra, independent of the potentiation of basal glucagon action. In conclusion, marked insulinopenia contributes directly to the exacerbation of glucoregulation during exercise in the diabetic state by limiting the rises in glucose uptake and metabolism and by enhancing hepatic glucose production.     

Effects of insulin on hemodynamics and metabolism in human forearm

We investigated the vascular response (blood flow and resting vascular resistance) and the metabolic response (exchange of metabolites and respiratory gases) to local insulin administration in the forearms of healthy young volunteers with the use of the perfused-forearm technique. In the postabsorptive state, the deep tissues of the forearm (mostly skeletal muscle) took up glucose (mean +/- SE 1.09 +/- 0.17 mumol.min-1.dl-1 forearm vol), beta-hydroxybutyrate (0.267 +/- 0.130 mumol.min-1.dl-1), and O2 (9.96 +/- 1.02 mumol.min-1.dl-1) and released lactate (0.284 +/- 0.098 mumol.min-1.dl-1), glycerol (0.029 +/- 0.012 mumol.min-1.dl-1), citrate (0.091 +/- 0.030 mumol.min-1.dl-1), alanine (0.184 +/- 0.044 mumol.min-1.dl-1), CO2 (7.36 +/- 0.97 mumol.min-1.dl-1), and protons (12.1 +/- 1.4 pmol.min-1.dl-1). Forearm blood flow (by venous occlusion plethysmography) was 2.95 +/- 0.18 ml.min-1.dl-1, and intra-arterial systolic/diastolic blood pressure was 116 +/- 3/76 +/- 2 mmHg. Local indirect calorimetry indicated dominance of fat as the oxidative substrate (RQ 0.76 +/- 0.09) and an energy expenditure rate of 1.03 +/- 0.11 cal.min-1.dl-1 forearm vol. One hundred minutes of intra-arterial insulin infusion (deep venous plasma insulin concn of 125 +/- 11 microU/ml) had no detectable effect on forearm blood flow, resting forearm vascular resistance, heart rate, or blood pressure. Local hyperinsulinemia significantly stimulated glucose uptake (to 4.79 +/- 0.61 mumol.min-1.dl-1 forearm vol, P less than 0.001), lactate and pyruvate release (to 0.710 +/- 0.093 and 0.032 +/- 0.016 mumol.min-1.dl-1 forearm vol, respectively; P less than 0.01 for both), potassium uptake (0.76 +/- 0.22 mueq.min-1.dl-1, P less than 0.001), and free fatty acid uptake (0.123 +/- 0.041 mumol.min-1.dl-1 forearm vol, P less than 0.05); glycerol balance switched to a net uptake (P less than 0.001), alanine release was restrained by 33% (P less than 0.05), and beta-hydroxybutyrate and citrate release were unchanged. Despite these metabolic changes, local rates of substrate oxidation and energy expenditure were not altered by insulin. In contrast, forearm proton release was significantly stimulated by insulin (to 14.8 +/- 1.4 pmol.min-1.dl-1, P less than 0.02). Proton release was also found to be directly related to resting forearm vascular resistance independent of the effect of insulin (multiple r = 0.64, P less than 0.001).(ABSTRACT TRUNCATED AT 400 WORDS)     

Predominant role of gluconeogenesis in increased hepatic glucose production in NIDDM

Excessive hepatic glucose output is an important factor in the fasting hyperglycemia of non-insulin-dependent diabetes mellitus (NIDDM). To determine the relative contributions of gluconeogenesis and glycogenolysis in a quantitative manner, we applied a new isotopic approach, using infusions of [6-3H]glucose and [2-14C]acetate to trace overall hepatic glucose output and phosphoenolpyruvate gluconeogenesis in 14 postabsorptive NIDDM subjects and in 9 nondiabetic volunteers of similar age and weight. Overall hepatic glucose output was increased nearly twofold in the NIDDM subjects (22.7 +/- 1.0 vs. 12.0 +/- 0.6 mumol.kg-1.min-1 in the nondiabetic volunteers, P less than .001); phosphoenolpyruvate gluconeogenesis was increased more than threefold in the NIDDM subjects (12.7 +/- 1.4 vs. 3.6 +/- 0.4 mumol.kg-1.min-1 in the nondiabetic subjects, P less than .001) and was accompanied by increased plasma lactate, alanine, and glucagon concentrations (all P less than .05). The increased phosphoenolpyruvate gluconeogenesis accounted for 89 +/- 6% of the increase in overall hepatic glucose output in the NIDDM subjects and was significantly correlated with the fasting plasma glucose concentrations (r = .67, P less than .01). Glycogenolysis, calculated as the difference between overall hepatic glucose output and phosphoenolpyruvate gluconeogenesis, was not significantly different in the NIDDM subjects (9.9 +/- 0.06 mumol.kg-1.min-1) and the nondiabetic volunteers (8.4 +/- 0.3 mumol.kg-1.min-1). We conclude that increased gluconeogenesis is the predominant mechanism responsible for increased hepatic glucose output in NIDDM.