Kinetics of in vivo muscle insulin-mediated glucose uptake in human obesity

The kinetics of in vivo insulin-mediated glucose uptake in human obesity have not been previously studied. To examine this, we used the glucose-clamp technique to measure whole-body and leg muscle glucose uptake in seven lean and six obese men during hyperinsulinemia (approximately 2000 pM) at four glucose levels (approximately 4.5, approximately 8.3, approximately 13.5, and approximately 23.5 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 x blood flow). With this approach, we found that rates of whole-body and leg glucose uptake were significantly lower in obese than in lean subjects at each glucose plateau. Leg blood flow rates increased from 4.3 +/- 0.4 to 6.5 +/- 0.8 dl/min over the range of glucose in lean subjects (P less than 0.05) but remained unchanged in obese subjects. The apparent maximal capacity (Vmax), based on whole-body and leg glucose uptake, was reduced in obese compared with lean subjects, but the apparent Km was similar in the lean and obese subjects (6-9 mM, NS). To assess the affinity of muscle for glucose extraction independent of changes in muscle plasma flow, we determined the mean half-maximal effective glucose concentration (EG50) and found it was similar in the lean and obese subjects (6.0 +/- 0.3 vs. 6.0 +/- 0.8 mM, NS). We conclude that 1) the kinetics of in vivo insulin-mediated glucose uptake in skeletal muscle in human obesity are characterized by reduced Vmax but normal Km; 2) the EG50 for insulin-mediated glucose extraction in skeletal muscle was 6 mM in both lean and obese subjects, consistent with a Km characteristic of the glucose-transport system; 3) obese subjects were unable to generate increases in blood flow in response to hyperglycemia under hyperinsulinemic conditions, and this contributed significantly to lower rates of leg and whole-body glucose uptake.     

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)     

Direct evidence for insulin-induced capillary recruitment in skin of healthy subjects during physiological hyperinsulinemia

It has been proposed that insulin-mediated changes in muscle perfusion modulate insulin-mediated glucose uptake. However, the putative effects of insulin on the microcirculation that permit such modulation have not been studied in humans. We examined the effects of systemic hyperinsulinemia on skin microvascular function in eight healthy nondiabetic subjects. In addition, the effects of locally administered insulin on skin blood flow were assessed in 10 healthy subjects. During a hyperinsulinemic clamp, we measured leg blood flow with venous occlusion plethysmography, skin capillary density with capillaroscopy, endothelium-(in)dependent vasodilatation of skin microcirculation with iontophoresis of acetylcholine and sodium nitroprusside combined with laser Doppler fluxmetry, and skin vasomotion by Fourier analysis of microcirculatory blood flow. To exclude nonspecific changes in the hemodynamic variables, a time-volume control study was performed. Insulin iontophoresis was used to study the local effects of insulin on skin blood flow. Compared to the control study, systemic hyperinsulinemia caused an increase in leg blood flow (-0.54 +/- 0.93 vs. 1.97 +/- 1.1 ml. min(-1). dl(-1); P < 0.01), an increase in the number of perfused capillaries in the resting state (-3.7 +/- 3.0 vs. 3.4 +/- 1.4 per mm(2); P < 0.001) and during postocclusive reactive hyperemia (-0.8 +/- 2.2 vs. 5.1 +/- 3.7 per mm(2); P < 0.001), an augmentation of the vasodilatation caused by acetylcholine (722 +/- 206 vs. 989 +/- 495%; P < 0.05) and sodium nitroprusside (618 +/- 159 vs. 788 +/- 276%; P < 0.05), and a change in vasomotion by increasing the relative contribution of the 0.01- to 0.02-Hz and 0.4- to 1.6-Hz spectral components (P < 0.05). Compared to the control substance, locally administered insulin caused a rapid increase ( approximately 13.5 min) in skin microcirculatory blood flow (34.4 +/- 42.5 vs. 82.8 +/- 85.7%; P < 0.05). In conclusion, systemic hyperinsulinemia in skin 1) induces recruitment of capillaries, 2) augments nitric oxide-mediated vasodilatation, and 3) influences vasomotion. In addition, locally administered insulin 4) induces a rapid increase in total skin blood flow, independent of systemic effects.     

Acute vasoconstriction-induced insulin resistance in rat muscle in vivo

Insulin-mediated changes in blood flow are associated with altered blood flow distribution and increased capillary recruitment in skeletal muscle. Studies in perfused rat hindlimb have shown that muscle metabolism can be regulated by vasoactive agents that control blood flow distribution within the hindlimb. In the present study, the effects of a vasoconstrictive agent that has no direct effect on skeletal muscle metabolism but that alters perfusion distribution in rat hindlimb was investigated in vivo to determine its effects on insulin-mediated vascular action and glucose uptake. We measured the effects of alpha-methylserotonin (alpha-met5HT) on mean arterial blood pressure, heart rate, femoral blood flow, hindlimb vascular resistance, and glucose uptake in control and euglycemic insulin-clamped (10 mU x min(-1) x kg(-1)) anesthetized rats. Blood flow distribution within the hindlimb muscles was assessed by measuring the metabolism of 1-methylxanthine (1-MX), an exogenously added substrate for capillary xanthine oxidase. Alpha-met5HT (20 microg x min(-1) x kg(-1)) infusion alone increased mean arterial blood pressure by 25% and increased hindlimb vascular resistance but caused no change in femoral blood flow. These changes were associated with decreased hindlimb 1-MX metabolism indicating less capillary flow. Insulin infusion caused decreased hindlimb vascular resistance that was associated with increased femoral blood flow and 1-MX metabolism. Treatment with alpha-met5HT infusion commenced before insulin infusion prevented the increase in femoral blood flow and inhibited the stimulation of 1-MX metabolism. Alpha-met5HT infusion had no effect on hindlimb glucose uptake but markedly inhibited the insulin stimulation of glucose uptake (P < 0.05) and was associated with decreased glucose infusion rates to maintain euglycemia (P < 0.05). A significant correlation (P < 0.05) was observed between 1-MX metabolism and hindlimb glucose uptake but not between femoral blood flow and glucose uptake. The results indicate that in vivo, certain types of vasoconstriction in muscle such as elicited by 5HT2 agonists, which prevent normal insulin recruitment of capillary flow, cause impaired muscle glucose uptake. Moreover, if vasoconstriction of this kind results from stress-induced increase in sympathetic outflow, then this may provide a clue as to the link between hypertension and insulin resistance that is often observed in humans.     

Delayed transcapillary delivery of insulin to muscle interstitial fluid after oral glucose load in obese subjects

Obese subjects exhibit a delay in insulin action and delivery of insulin to muscle interstitial fluid during glucose/insulin infusion. The aim of the present study was to follow the distribution of insulin to skeletal muscle after an oral glucose load in obese subjects. We conducted an oral glucose tolerance test (OGTT) in 10 lean and 10 obese subjects (BMI 23 +/- 0.6 vs. 33 +/- 1.2 kg/m(2); P < 0.001). Insulin measurements in muscle interstitial fluid were combined with forearm arteriovenous catheterization and blood flow measurements. In the obese group, interstitial insulin was significantly (35-55%) lower than plasma insulin (P < 0.05) during the 1st h after the OGTT, whereas in lean subjects, no significant difference was found between interstitial and plasma insulin levels during the same time period. The permeability surface area product for glucose, representing capillary recruitment, increased in the lean group (P < 0.05) but not in the obese group (NS). Obese subjects had a significantly higher plasma insulin level at 90-120 min after oral glucose (398 +/- 57 vs. 224 +/- 37 pmol/l in control subjects; P < 0.05). The significant gradient between plasma insulin and muscle interstitial insulin during the first hour after OGTT suggests a slow delivery of insulin in obese subjects. The hindered transcapillary transport of insulin may be attributable to a defect in insulin-mediated capillary recruitment.     

Intracellular defects in glucose metabolism in obese patients with NIDDM.

Skeletal muscle insulin resistance in obese patients with non-insulin-dependent diabetes mellitus (NIDDM) is characterized by decreased glucose uptake. Although reduced glycogen synthesis is thought to be the predominant cause for this deficit, studies supporting this notion often have been conducted at supraphysiological insulin concentrations in which glucose storage is the overwhelming pathway of glucose disposal. However, at lower, more physiological insulin concentrations, decreased muscle glucose oxidation could play a significant role. This study was undertaken to determine whether, under euglycemic conditions, insulin resistance for leg muscle glucose uptake in NIDDM patients is due primarily to decreased glucose storage or to oxidation. The leg balance technique and leg indirect calorimetry were used under steady-state euglycemic conditions to estimate muscle glucose uptake, storage, and oxidation in eight moderately obese NIDDM patients and eight matched-control subjects. Leg muscle biopsies also were performed to determine whether alterations in muscle pyruvate dehydrogenase or glycogen synthase activities could explain defects in glucose oxidation or storage. At insulin concentrations of approximately 500-600 pM, leg glucose uptake, oxidation, and storage in the NIDDM group (2.03 +/- 0.42, 1.00 +/- 0.13, 0.66 +/- 0.36 mumol.min-1.100 ml-1) were significantly lower (P less than 0.05) than rates in control subjects (5.14 +/- 0.64, 1.92 +/- 0.17, 2.80 +/- 0.54). Pyruvate dehydrogenase and glycogen synthase activities were also decreased, consistent with the in vivo metabolic defects. The average deficit in leg glucose uptake in NIDDM was 3.11 +/- 0.42 mumol.min-1.100 ml-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Exercise training improves insulin-mediated capillary recruitment in association with glucose uptake in rat hindlimb.

Exercise training is considered to be beneficial in the treatment and prevention of insulin insensitivity, and much of the effect occurs in muscle. We have recently shown that capillary recruitment by insulin in vivo is associated with and may facilitate insulin action to increase muscle glucose uptake. In the present study, we examined the effect of 14 days of voluntary exercise training on euglycemic-hyperinsulinemic clamped (10 mU. min(-1). kg(-1) for 2 h), anesthetized rats. Whole-body glucose infusion rate (GIR), hindleg glucose uptake, femoral blood flow (FBF), vascular resistance, and capillary recruitment, as measured by metabolism of infused 1-methylxanthine (1-MX), were assessed. In sedentary animals, insulin caused a significant (P < 0.05) increase in FBF (1.6-fold) and capillary recruitment (1.7-fold) but a significant decrease in vascular resistance. In addition, hindleg glucose uptake was increased (4.3-fold). Exercise training increased insulin-mediated GIR (24%), hindleg glucose uptake (93%), and capillary recruitment (62%) relative to sedentary animals. Neither capillary density nor total xanthine-oxidase activity in skeletal muscle were increased as a result of the training regimen used. We concluded that exercise training improves insulin-mediated increases in capillary recruitment in combination with augmented muscle glucose uptake. Increased insulin-mediated glucose uptake may in part result from the improved hemodynamic control attributable to exercise training.     

Acute impairment of insulin-mediated capillary recruitment and glucose uptake in rat skeletal muscle in vivo by TNF-alpha

The vascular actions of insulin may contribute to the increase in glucose uptake by skeletal muscle. We have recently shown that when capillary recruitment by insulin is blocked in vivo, an acute state of insulin resistance is induced. Another agent that may have vascular effects is the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha), which has been reported to play an important role in the insulin resistance of obesity, type 2 diabetes, and sepsis in both animals and humans. Thus, in the present study, we have investigated the effect of an intravenous 3-h TNF treatment (0.5 microg x h(1) x kg(-1)) in control and euglycemic-hyperinsulinemic-clamped (10 mU x min(-1) x kg(-1) for 2 h) anesthetized rats. Hind-leg glucose uptake, muscle uptake of 2-deoxyglucose (2-DG), femoral blood flow (FBF), vascular resistance (VR), and capillary recruitment as measured by metabolism of infused 1-methylxanthine (1-MX) were assessed. Insulin alone caused a significant (P < 0.05) increase in FBF (1.7-fold) and capillary recruitment (2.5-fold), with a significant decrease in VR. In addition, hind-leg glucose uptake was increased (fourfold), as was 2-DG uptake in the soleus and plantaris muscles. TNF completely prevented the insulin-mediated changes in FBF, VR, and capillary recruitment and significantly reduced (P < 0.05) the insulin-mediated increase in total hind-leg glucose uptake (by 61%) and muscle 2-DG uptake (by at least 50%). TNF alone had no significant effect on any of these variables. It is concluded that acute administration in vivo of TNF completely blocks the hemodynamic actions of insulin on rat skeletal muscle vasculature and blocks approximately half of the glucose uptake by muscle. It remains to be determined whether these two effects are interdependent.     

Nitric oxide synthase inhibition reduces glucose uptake during exercise in individuals with type 2 diabetes more than in control subjects

Nitric oxide (NO) synthase inhibition reduces leg glucose uptake during cycling without reducing leg blood flow (LBF) in young, healthy individuals. This study sought to determine the role of NO in glucose uptake during exercise in individuals with type 2 diabetes. Nine men with type 2 diabetes and nine control subjects matched for age, sex, peak pulmonary oxygen uptake (VO(2) peak), and weight completed two 25-min bouts of cycling exercise at 60 +/- 2% VO(2) peak, separated by 90 min. N(G)-monomethyl-L-arginine (L-NMMA) (total dose 6 mg/kg) or placebo was administered into the femoral artery for the final 15 min of exercise in a counterbalanced, blinded, crossover design. LBF was measured by thermodilution in the femoral vein, and leg glucose uptake was calculated as the product of LBF and femoral arteriovenous glucose difference. During exercise with placebo, glucose uptake was not different between control subjects and individuals with diabetes; however, LBF was lower and arterial plasma glucose and insulin levels were higher in individuals with diabetes. L-NMMA had no effect on LBF or arterial plasma glucose and insulin concentrations during exercise in both groups. L-NMMA significantly reduced leg glucose uptake in both groups, with a significantly greater reduction (P = 0.04) in the diabetic group (75 +/- 13%, 5 min after L-NMMA) compared with the control group (34 +/- 14%, 5 min after L-NMMA). These data suggest a greater reliance on NO for glucose uptake during exercise in individuals with type 2 diabetes compared with control subjects.     

Endothelin limits insulin action in obese/insulin-resistant humans

The normal action of insulin to vasodilate and redistribute blood flow in support of skeletal muscle metabolism is impaired in insulin-resistant states. Increased endogenous endothelin contributes to endothelial dysfunction in obesity and diabetes. Here, we test the hypothesis that increased endogenous endothelin action also contributes to skeletal muscle insulin resistance via impairments in insulin-stimulated vasodilation. We studied nine lean and seven obese humans, measuring the metabolic and hemodynamic effects of insulin (300 mU . m(-2) . min(-1)) alone and during femoral artery infusion of BQ123 (an antagonist of type A endothelin receptors, 1 micromol/min). Endothelin antagonism augmented skeletal muscle responses to insulin in obese subjects through changes in both leg blood flow (LBF) and glucose extraction. Insulin-stimulated LBF was significantly increased in obese subjects only. These changes, combined with differential effects on glucose extraction, resulted in augmented insulin-stimulated leg glucose uptake in obese subjects (54.7 +/- 5.7 vs. 107.4 +/- 18.9 mg/min with BQ123), with no change in lean subjects (103.7 +/- 11.4 vs. 88.9 +/- 16.3, P = 0.04 comparing BQ123 across groups). BQ123 allowed augmented leg glucose extraction in obese subjects even in the face of NOS antagonism. These findings suggest that increased endogenous endothelin action contributes to insulin resistance in skeletal muscle of obese humans, likely through both vascular and tissue effects.     

Independent association of type 2 diabetes and coronary artery disease with myocardial insulin resistance

Clustering of classical cardiovascular risk factors is insufficient to account for the excess coronary artery disease (CAD) of patients with diabetes, and chronic hyperglycemia and insulin resistance (IR) are obvious culprits. Whole-body and skeletal muscle IR is characteristic of patients with type 2 diabetes, but whether it extends to the normally contracting cardiac muscle is controversial. We investigated whether type 2 diabetes is associated with myocardial IR independent of CAD in a case-control series (n = 55) of male nondiabetic and diabetic (type 2 and type 1) patients with or without angiographically documented CAD. Baseline blood flow ((15)O-water) and insulin-stimulated glucose uptake ((18)F-fluoro-deoxyglucose) during euglycemic (5.6 mmol/l), physiological hyperinsulinemia (40 mU x min(-1) x m(-2) insulin clamp) were measured by positron emission tomography in skeletal muscle and normally contracting myocardium. Skeletal muscle glucose uptake was reduced in association with both CAD and type 2 diabetes. In regions with normal baseline perfusion, insulin-mediated myocardial glucose uptake was reduced in non-CAD type 2 diabetic (0.36 +/- 0.14 micro mol x min(-1). g(-1)) and nondiabetic CAD patients (0.44 +/- 0.15 micro mol x min(- 1) x g(-1)) in comparison with healthy control subjects (0.61 +/- 0.08) or with non-CAD type 1 diabetic patients (0.80 +/- 0.13; P < 0.001 for both CAD and diabetes). Neither basal skeletal muscle nor basal myocardial blood flow differed across groups; both skeletal muscle and myocardial IR were directly related to whole-body IR. We conclude that type 2 diabetes is specifically associated with myocardial IR that is independent of and nonadditive with angiographic CAD and proportional to skeletal muscle and whole-body IR.     

Microvascular recruitment is an early insulin effect that regulates skeletal muscle glucose uptake in vivo

Insulin increases glucose disposal into muscle. In addition, in vivo insulin elicits distinct nitric oxide synthase-dependent vascular responses to increase total skeletal muscle blood flow and to recruit muscle capillaries (by relaxing resistance and terminal arterioles, respectively). In the current study, we compared the temporal sequence of vascular and metabolic responses to a 30-min physiological infusion of insulin (3 mU. min(-1). kg(-1), euglycemic clamp) or saline in rat skeletal muscle in vivo. We used contrast-enhanced ultrasound to continuously quantify microvascular volume. Insulin recruited microvasculature within 5-10 min (P < 0.05), and this preceded both activation of insulin-signaling pathways and increases in glucose disposal in muscle, as well as changes in total leg blood flow. Moreover, l-NAME (N(omega)-nitro-l-arginine-methyl ester), a specific inhibitor of nitric oxide synthase, blocked this early microvascular recruitment (P < 0.05) and at least partially inhibited early increases in muscle glucose uptake (P < 0.05). We conclude that insulin rapidly recruits skeletal muscle capillaries in vivo by a nitric oxide-dependent action, and the increase in capillary recruitment may contribute to the subsequent glucose uptake.     

Physiologic hyperinsulinemia enhances human skeletal muscle perfusion by capillary recruitment.

Despite intensive study, the relation between insulin's action on blood flow and glucose metabolism remains unclear. Insulin-induced changes in microvascular perfusion, independent from effects on total blood flow, could be an important variable contributing to insulin's metabolic action. We hypothesized that modest, physiologic increments in plasma insulin concentration alter microvascular perfusion in human skeletal muscle and that these changes can be assessed using contrast-enhanced ultrasound (CEU), a validated method for quantifying flow by measurement of microvascular blood volume (MBV) and microvascular flow velocity (MFV). In the first protocol, 10 healthy, fasting adults received insulin (0.05 mU. kg(-1). min(-1)) via a brachial artery for 4 h under euglycemic conditions. At baseline and after insulin infusion, MBV and MFV were measured by CEU during continuous intravenous infusion of albumin microbubbles with intermittent harmonic ultrasound imaging of the forearm deep flexor muscles. In the second protocol, 17 healthy, fasting adults received a 4-h infusion of either insulin (0.1 mU. kg(-1). min(-1), n = 9) or saline (n = 8) via a brachial artery. Microvascular volume was assessed in these subjects by an alternate CEU technique using an intra-arterial bolus injection of albumin microbubbles at baseline and after the 4-h infusion. With both protocols, muscle glucose uptake, plasma insulin concentration, and total blood flow to the forearm were measured at each stage. In protocol 2 subjects, tissue extraction of 1-methylxanthine (1-MX) was measured as an index of perfused capillary volume. Caffeine, which produces 1-MX as a metabolite, was administered to these subjects before the study to raise plasma 1-MX levels. In protocol 1 subjects, insulin increased muscle glucose uptake (180%, P < 0.05) and MBV (54%, P < 0.01) and decreased MFV (-42%, P = 0.07) in the absence of significant changes in total forearm blood flow. In protocol 2 subjects, insulin increased glucose uptake (220%, P < 0.01) and microvascular volume (45%, P < 0.05) with an associated moderate increase in total forearm blood flow (P < 0.05). Using forearm 1-MX extraction, we observed a trend, though not significant, toward increasing capillary volume in the insulin-treated subjects. In conclusion, modest physiologic increments in plasma insulin concentration increased microvascular blood volume, indicating altered microvascular perfusion consistent with a mechanism of capillary recruitment. The increases in microvascular (capillary) volume (despite unchanged total blood flow) indicate that the relation between insulin's vascular and metabolic actions cannot be fully understood using measurements of bulk blood flow alone.     

Resistance to exercise-induced increase in glucose uptake during hyperinsulinemia in insulin-resistant skeletal muscle of patients with type 1 diabetes

Insulin and exercise have been shown to activate glucose transport at least in part via different signaling pathways. However, it is unknown whether insulin resistance is associated with a defect in the ability of an acute bout of exercise to enhance muscle glucose uptake in vivo. We compared the abilities of insulin and isometric exercise to stimulate muscle blood flow and glucose uptake in 12 men with type 1 diabetes (age 24 +/- 1 years, BMI 23.0 +/- 0.4 kg/m(2)) and in 11 age- and weight-matched nondiabetic men (age 25 +/- 1 years, BMI 22.3 +/- 0.6 kg/m(2)) during euglycemic hyperinsulinemia (1 mU. kg(-1). min(-1) insulin infusion for 150 min). One-legged exercise was performed at an intensity of 10% of maximal isometric force for 105 min (range 45-150). Rates of muscle blood flow, oxygen consumption, and glucose uptake were quantitated simultaneously in both legs using [(15)O]water, [(15)O]oxygen, [(18)F]-2-fluoro-2-deoxy-D-glucose, and positron emission tomography. Resting rates of oxygen consumption were similar during hyperinsulinemia between the groups (2.4 +/- 0.3 vs. 2.0 +/- 0.5 ml. kg(-1) muscle. min(-1); normal subjects versus patients with type 1 diabetes, NS), and exercise increased oxygen consumption similarly in both groups (25.3 +/- 4.3 vs. 20.1 +/- 3.0 ml. kg(-1) muscle. min(-1), respectively, NS). Rates of insulin-stimulated muscle blood flow and the increments in muscle blood flow induced by exercise were also similar in normal subjects (129 +/- 14 ml. kg(-1). min(-1)) and in patients with type 1 diabetes (115 +/- 12 ml. kg(-1). min(-1)). The patients with type 1 diabetes exhibited resistance to both insulin stimulation of glucose uptake (34 +/- 6 vs. 76 +/- 9 micromol. kg(-1) muscle. min(-1), P < 0.001) and also to the exercise-induced increment in glucose uptake (82 +/- 15 vs. 162 +/- 29 micromol. kg(-1) muscle. min(-1), P < 0.05). We conclude that the ability of exercise to increase insulin-stimulated glucose uptake in vivo is blunted in patients with insulin-resistant type 1 diabetes compared with normal subjects. This could be caused by either separate or common defects in exercise- and insulin-stimulated pathways.     

Insulin action on glucose and branched-chain amino acid metabolism in cancer cachexia: differential effects of insulin.

In addition to the maintenance of glucose homeostasis, insulin plays a major role in the regulation of branched-chain amino acid (BCAA) metabolism. We investigated insulin action on glucose turnover rates, arterial BCAA concentrations, and forearm BCAA flux in cancer cachexia. Six weight-losing patients with localized gastrointestinal malignancy and five age-matched control subjects underwent sequential 120-minute euglycemic insulin infusions. Steady-state insulin concentrations were 50 +/- 5, 97 +/- 14, and 435 +/- 14 microU/ml in patients and 44 +/- 4, 95 +/- 6, and 495 +/- 42 microU/ml in control subjects at the insulin infusion rates of 0.5, 1.0, and 4.0 mU/kg.min, respectively. During the 0.5 and 1.0 mU/kg.min insulin infusions, a primed, continuous infusion of D-[3-3H] glucose was used to quantify endogenous glucose production. Total body glucose uptake was decreased in patients with cancer compared with control subjects at the 0.5 mU/kg.min (2.9 +/- 0.4 vs 3.6 +/- 1.2 mg/kg.min), 1.0 mU/kg.min (5.3 +/- 0.3 vs 8.7 +/- 0.8 mg/kg.min; p less than 0.05), and 4.0 mU/kg.min (10.9 +/- 0.9 vs 13.7 +/- 1.1 mg/kg.min) insulin infusion rates, consistent with a state of insulin resistance. Progressive euglycemic insulin infusion induced a marked, comparable insulin-dependent decrease in arterial plasma BCAA concentrations in both patients with cancer and control subjects. There was no change in postabsorptive forearm BCAA flux with progressive hyperinsulinemia. Insulin-induced branched-chain hypoaminoacidemia is unimpaired in this group of patients manifesting resistance to insulin action on glucose metabolism, thereby providing evidence of a differential resistance to insulin action on glucose metabolism versus insulin action on BCAA concentrations in cancer cachexia. Peripheral BCAA flux is not affected by systemic insulin infusion, suggesting that skeletal muscle is not a major site of BCAA disposal during insulin-mediated hypoaminoacidemia.     

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)     

Interactions of impaired glucose transport and phosphorylation in skeletal muscle insulin resistance: a dose-response assessment using positron emission tomography

It has been postulated that glucose transport is the principal site of skeletal muscle insulin resistance in obesity and type 2 diabetes, though a distribution of control between glucose transport and phosphorylation has also been proposed. The current study examined whether the respective contributions of transport and phosphorylation to insulin resistance are modulated across a dose range of insulin stimulation. Rate constants for transport and phosphorylation in skeletal muscle were estimated using dynamic positron emission tomography (PET) imaging of 2-deoxy-2[18F]fluoro-D-glucose ([18F]FDG) during insulin infusions at three rates (0, 40, and 120 mU/m2 per min) in lean glucose-tolerant, obese glucose-tolerant, and obese type 2 diabetic subjects. Parallel studies of arteriovenous fractional extraction across the leg of [18F]FDG and [2-3H] glucose were performed to measure the "lumped constant" (LC) (i.e., the analog effect) for [18F]FDG to determine whether this value is affected by insulin dose or insulin resistance. The value of the LC was similar across insulin doses and groups. Leg glucose uptake (LGU) also provided a measure of skeletal muscle glucose metabolism independent of PET. [18F]FDG uptake determined by PET imaging strongly correlated with LGU across groups and across insulin doses (r = 0.81, P < 0.001). Likewise, LGU correlated with PET parameters of glucose transport (r = 0.67, P < 0.001) and glucose phosphorylation (r = 0.86, P < 0.001). Glucose transport increased in response to insulin in the lean and obese groups (P < 0.05), but did not increase significantly in the type 2 diabetic group. A dose-responsive pattern of stimulation of glucose phosphorylation was observed in all groups of subjects (P < 0.05); however, glucose phosphorylation was lower in both the obese and type 2 diabetic groups compared with the lean group at the moderate insulin dose (P < 0.05). These findings indicate an important interaction between transport and phosphorylation in the insulin resistance of obesity and type 2 diabetes.     

Skeletal muscle microvascular recruitment by physiological hyperinsulinemia precedes increases in total blood flow.

Supraphysiological doses of insulin enhance total limb blood flow and recruit capillaries in skeletal muscle. Whether these processes change in response to physiological hyperinsulinemia is uncertain. To examine this, we infused either saline (n = 6) or insulin (euglycemic clamp, 3.0 mU x min(-1) x kg(-1), n = 9) into anesthetized rats for 120 min. Femoral artery flow was monitored continuously using a Doppler flow probe, and muscle microvascular recruitment was assessed by metabolism of infused 1-methylxanthine (1-MX) and by contrast-enhanced ultrasound (CEU). Insulin infusion raised plasma insulin concentrations by approximately 10-fold. Compared with saline, physiological hyperinsulinemia increased femoral artery flow (1.02 +/- 0.10 vs. 0.68 +/- 0.09 ml/min; P < 0.05), microvascular recruitment (measured by 1-MX metabolism [6.6 +/- 0.5 vs. 4.5 +/- 0.48 nmol/min; P < 0.05] as well as by CEU [167.0 +/- 39.8 vs. 28.2 +/- 13.8%; P < 0.01]), and microvascular flow velocity (beta, 0.14 +/- 0.02 vs. 0.09 +/- 0.02 s(-1)). Subsequently, we studied the time dependency of insulin's vascular action in a second group (n = 5) of animals. Using CEU, microvascular volume was measured at 0, 30, and 90 min of insulin infusion. Insulin augmented microvascular perfusion within 30 min (52.8 +/- 14.8%), and this persisted at 90 min (64.6 +/- 9.9%). Microvascular recruitment occurred without changes to femoral artery flow or beta. We conclude that insulin increases tissue perfusion by recruiting microvascular beds, and at physiological concentrations this precedes increases in total muscle blood flow by 60-90 min.     

Free fatty acid elevation impairs insulin-mediated vasodilation and nitric oxide production

The effect and time course of free fatty acid (FFA) elevation on insulin-mediated vasodilation (IMV) and the relationship of FFA elevation to changes in insulin-mediated glucose uptake was studied. Two groups of lean insulin-sensitive subjects underwent euglycemic-hyperinsulinemic (40 mU x m(-2) x min(-1)) clamp studies with and without superimposed FFA elevation on 2 occasions approximately 4 weeks apart. Groups differed only by duration of FFA elevation, either short (2-4 h, n = 12) or long (8 h, n = 7). On both occasions, rates of whole-body glucose uptake were measured, and changes in leg blood flow (LBF) and femoral vein nitric oxide nitrite plus nitrate (NOx) flux in response to the clamps were determined. Short FFA infusion did not have any significant effect on the parameters of interest. In contrast, long FFA infusion decreased rates of whole-body glucose uptake from 47.7 +/-2.8 to 32.2 +/- 0.6 micromol x kg(-1) x min(-1) (P < 0.01), insulin-mediated increases in LBF from 66 +/- 8 to 37 +/- 7% (P < 0.05), and insulin-induced increases in NOx flux from 25 +/- 9 to 5 +/- 9% (P < 0.05). Importantly, throughout all groups, FFA-induced changes in whole-body glucose uptake correlated significantly with FFA-induced changes in insulin-mediated increases in LBF (r = 0.706, P < 0.001), which indicates coupling of metabolic and vascular effects. In a different protocol, short FFA elevation blunted the LBF response to NG-monomethyl-L-arginine (L-NMMA), which is an inhibitor of NO synthase. LBF in response to L-NMMA decreased by 17.3 +/- 2.4 and 9.0 +/- 1.4% in the groups without and with FFA elevation, respectively (P < 0.05), which indicates that FFA elevation interferes with shear stress-induced NO production. Thus, impairment of shear stress-induced vasodilation and IMV by FFA elevation occurs with different time courses, and impairment of IMV occurs only if glucose metabolism is concomitantly reduced. These findings suggest that NO production in response to the different stimuli may be mediated via different signaling pathways. FFA-induced reduction in NO production may contribute to the higher incidence of hypertension and macrovascular disease in insulin-resistant patients.     

Effect of physical training on insulin action in obesity

We evaluated insulin secretion and insulin action with the hyperglycemic (125 mg/dl above basal) and euglycemic insulin (40 mU.m-2.min-1) clamps in seven moderately obese subjects before and after a 6-wk exercise training program. Thirty-nine normal-weight, age-matched subjects served as controls. Both fasting plasma insulin concentration and glucose-stimulated (hyperglycemic clamp) insulin secretion were significantly (P less than .001) increased in the obese subjects. After the training program fasting insulin levels decreased by 26% (P less than .01). Insulin secretion in response to hyperglycemia decreased by a similar percentage (P less than .01). Nonetheless, total-body glucose metabolism increased significantly (P less than .05) during the hyperglycemic clamp. With the euglycemic insulin clamp, obese subjects were shown to be significantly (P less than .001) insulin resistant compared with controls. The decrease in total-body glucose uptake resulted from defects in both peripheral glucose disposal and suppression of hepatic glucose production. After the 6-wk training program, insulin-mediated total-body glucose metabolism increased due to significant improvements in peripheral glucose uptake (P less than .01) and more effective suppression of hepatic glucose production (P less than .05). These results indicate that a moderate-intensity physical conditioning program is capable of ameliorating the insulin resistance and reducing glucose-stimulated hyperinsulinemia observed in obese subjects with normal glucose tolerance.