Body Glucose Clearance Research Articles

A 7-day high-fat, high-calorie diet induces fibre-specific increases in intramuscular triglyceride and perilipin protein expression in human skeletal muscle.

We have recently shown that a high-fat, high-calorie (HFHC) diet decreases whole body glucose clearance without impairing skeletal muscle insulin signalling, in healthy lean individuals. These diets are also known to increase skeletal muscle IMTG stores, but the effect on lipid metabolites leading to skeletal muscle insulin resistance has not been investigated. This study measured the effect of 7days' HFHC diet on (1) skeletal muscle concentration of lipid metabolites, and (2) potential changes in the perilipin (PLIN) content of the lipid droplets storing intramuscular triglyceride (IMTG). The HFHC diet increased PLIN3 protein expression and redistributed PLIN2 to lipid droplet stores in type I fibres. The HFHC diet increased IMTG content in type I fibres, while lipid metabolite concentrations remained the same. The data suggest that the increases in IMTG stores assists in reducing the accumulation of lipid metabolites known to contribute to skeletal muscle insulin resistance. A high-fat, high-calorie (HFHC) diet reduces whole body glucose clearance without impairing skeletal muscle insulin signalling in healthy lean individuals. HFHC diets also increase skeletal muscle lipid stores. However, unlike certain lipid metabolites, intramuscular triglyceride (IMTG) stored within lipid droplets (LDs) does not directly contribute to skeletal muscle insulin resistance. Increased expression of perilipin (PLIN) proteins and colocalisation to LDs has been shown to assist in IMTG storage. We aimed to test the hypothesis that 7days on a HFHC diet increases IMTG content while minimising accumulation of lipid metabolites known to disrupt skeletal muscle insulin signalling in sedentary and obese individuals. We also aimed to identify changes in expression and subcellular distribution of proteins involved in IMTG storage. Muscle biopsies were obtained from the m. vastus lateralis of 13 (11 males, 2 females) healthy lean individuals (age: 23±2.5years; body mass index: 24.5±2.4kgm-2 ), following an overnight fast, before and after consuming a high-fat (64% energy), high-calorie (+47% kcal) diet for 7days. After the HFHC diet, IMTG content increased in type I fibres only (+101%; P<0.001), whereas there was no change in the concentration of either total diacylglycerol (P=0.123) or total ceramides (P=0.150). Of the PLINs investigated, only PLIN3 content increased (+50%; P<0.01) solely in type I fibres. LDs labelled with PLIN2 increased (+80%; P<0.01), also in type I fibres only. We propose that these adaptations of LDs support IMTG storage and minimise accumulation of lipid metabolites to protect skeletal muscle insulin signalling following 7days' HFHC diet.

Insulin receptor signaling in the subfornical organ protects against the development of metabolic syndrome

Metabolic syndrome encompasses a combination of conditions including obesity, diabetes, dyslipidemia, and hypertension. Brain insulin resistance has emerged as a contributor to the development of metabolic syndrome, although the neural regions involved remain unclear. While most investigations have focused on insulin action in the hypothalamus, recent evidence suggests that the insulin receptor (IR) gene is also expressed in the subfornical organ (SFO); a circumventricular organ well known for cardiovascular/fluid regulation and recently recognized as a metabolic nucleus. We therefore hypothesized that IR signaling in the SFO is involved in metabolic regulation. We first examined protein levels of SFO IR in male C57Bl/6 mice (n=3) using immunohistochemistry, and observed that insulin receptor expressing cells are rich in the medial to caudal SFO (Figure), while robust IR‐immunoreactivity (ir) on fibers is detected in the rostral SFO. Co‐immunohistochemistry further revealed heterogeneous cellular expression of the SFO IR, with 11.9 ± 2.2% of IR‐ir detected on astrocytes (GFAP), 57.2 ± 2.6% on endothelial cells (TIE2), and 18.3 ± 0.8% on neurons (NeuN). Interestingly, neuronal expression of IR in the SFO was restricted to glutamatergic cells (CAMKII co‐localization), but absent in GABAergic cells (GAD67‐mCherry reporter mice). To test the functional role of SFO IR, we next utilized mice harboring a conditional allele of the IR gene (IRfl/fl), and selectively knocked down the SFO IR via SFO‐targeted delivery of an adeno‐associated virus encoding Cre‐recombinase (AAV‐Cre‐eGFP; n=4), or control vector (AAV‐eGFP; n=3). Both groups remained on normal chow, and metabolic parameters were continuously monitored using indirect calorimetry for 12 weeks. Selective removal of SFO IR did not influence food and water intake, but resulted in a greater increase in body weight (e.g. 12 weeks: 27.9 ± 1.5 vs. 31.4 ± 1.4 g, AAV‐eGFP vs. AAV‐CreeGFP, ANOVA interaction p=0.0005). This was associated with a significantly lower energy expenditure (e.g. 12 week average: 12.5 ± 0.6 vs. 11.7 ± 0.2 kcal/hr/kg, AAV‐eGFP vs. AAV‐Cre‐eGFP, ANOVA interaction p=0.013) and a slight reduction in ambulatory activity in AAV‐Cre‐eGFP mice relative to controls. Examination of regional adipose tissue also revealed an ~40% increase in overall adiposity following ablation of SFO IR (total adipose: 1.4 ± 0.4 vs. 2.2 ± 0.3 g, AAV‐eGFP vs. AAV‐Cre‐eGFP, p=0.1). Glucose and insulin tolerance testing in a separate cohort indicated that whole body glucose clearance and insulin sensitivity were comparable between groups. SFO‐specific reductions in IR were confirmed by anatomical and molecular verification. These data demonstrate that ablation of SFO IRs under normal diet conditions results in a deleterious metabolic state. Moreover, these findings indicate a tonic metabolic regulatory role for SFO IR, and suggest that impairments in IR signaling in the SFO may contribute to a development of metabolic syndrome.Support or Funding InformationHL116776This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The effect of rowing on endothelial function and insulin action in healthy controls and in patients with type 2 diabetes

Patients with type 2 diabetes (T2DM) have an increased risk for cardiovascular disease. We examined the effects of 8 weeks of home-based rowing training (heart rate corresponding to 65-70% of VO(2 peak) ) on endothelial function and glucose clearance (local and systemic effects) in male subjects with T2DM (n=9) and matched controls (n=8). Before and after training (30 min every other day), all subjects underwent sequential graded brachial artery infusions of non-insulin vasodilators (acetylcholine; sodium nitroprusside; adenosine). Forearm blood flow was improved by training in controls (without and with insulin: P=0.003 and 0.05, respectively) but not in subjects with T2DM. Likewise, whole body glucose clearance increased in response to training in controls (P=0.05) but not in T2DM. However, in both groups, the capacity for local forearm glucose extraction (controls: P=0.001; T2DM: P=0.002) and clearance (controls: P<0.001; T2DM: P=0.01) were positively affected by exercise. While the subjects with T2DM did not respond to the same degree as controls to 8 weeks of home-based exercise, there are clear benefits as illustrated by improvements in local glucose disposal. Training of higher intensity or duration may be required in order to elicit a response similar to controls.

Insulin-Like Growth Factor I Deficiency Prolongs Survival and Antagonizes Paraquat–Induced Cardiomyocyte Dysfunction: Role of Oxidative Stress

Interruption of insulin-like growth factor I (IGF-1) signaling has been demonstrated to prolong life span although the underlying mechanism has not been elucidated. The aim of this study was to examine the influence of severe IGF-1 deficiency on survival rate, cardiomyocyte viability, contractile function, and intracellular Ca(2+) property in response to challenge with the pro-oxidant paraquat. C57 negative and liver IGF-1 deficient (LID) transgenic mice were administrated paraquat (75 mg/kg) and survival was monitored. LID mice displayed a significantly improved survival than did C57 mice evaluated by the Kaplan-Meier curve. MTT assay revealed that in vitro IGF-1 treatment significantly sensitized paraquat-induced cell death in both C57 and LID groups, with significantly better cell viability in LID cardiomyocytes. Compared to C57 mouse cardiomyocytes, LID myocytes displayed reduced peak shortening (PS), decreased maximal velocity of shortening/relengthening (+/- dL/dt), prolonged time-to-90% relengthening (TR(90)), and comparable tolerance to high stimulus frequency and intracellular Ca(2+) homeostasis. Paraquat treatment for 48 hours reduced PS, +/- dL/dt, tolerance to high stimulus frequency, resting and rise in intracellular Ca(2+), and prolonged TR(90), all of which were nullified or masked by IGF-1 deficiency. Paraquat increased reactive oxygen species and carbonyl production upregulated the Ca(2+) regulating protein SERCA2a, and downregulated Na(+) -Ca(2+) exchanger, the effects of which were nullified or masked by IGF-1 deficiency. Although LID mice displayed reduced whole body glucose clearance, cardiomyocytes from LID mice exhibited dramatically enhanced insulin-stimulated phosphorylation of insulin receptor and Akt. These data demonstrated that IGF-1 deficiency may antagonize or mask the paraquat-induced decrease in survival, cardiomyocyte dysfunction, oxidative stress, and change in Ca(2+) regulating proteins.

Control of muscle glucose uptake: test of the rate‐limiting step paradigm in conscious, unrestrained mice

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

NO-1886 inhibits size of adipocytes, suppresses plasma levels of tumor necrosis factor-α and free fatty acids, improves glucose metabolism in high-fat/high-sucrose-fed miniature pigs

The synthetic compound NO-1886 is a lipoprotein lipase activator that has been proven to be highly effective in lowering plasma triglycerides and elevating high-density lipoprotein cholesterol. Recently, we found that NO-1886 also had a plasma glucose-reducing action in high-fat/high-sucrose diet-induced diabetic rabbits. In the current study, we investigated the effects of NO-1886 on the morphology of adipocytes, plasma levels of tumor necrosis factor-α (TNF-α) and free fatty acids (FFA) in miniature pigs fed a high-fat/high-sucrose diet. Our results showed that feeding a high-fat/high-sucrose diet to miniature pigs increased the size of adipocytes, and the plasma levels of TNF-α, FFA, and glucose. This diet also induced insulin resistance and impaired the acute insulin response to glucose loading. Supplementing 1% NO-1886 to the high-fat/high-sucrose diet inhibited adipocyte enlargement, and suppressed plasma levels of TNF-α, FFA, and glucose. The decrease in plasma TNF-α and FFA was simultaneous with the decrease in plasma glucose. We also found an increased whole body glucose clearance and an increased acute insulin response to intravenous glucose loading by NO-1886 supplementation. These data suggest that NO-1886 improves the glucose metabolism in high-fat/high-sucrose diet-induced diabetic minipigs by decreasing fat deposit, and suppressing plasma TNF-α and FFA levels. Therefore, NO-1886 is potentially beneficial for the treatment of insulin-resistant syndrome.

GLUT4, AMP kinase, but not the insulin receptor, are required for hepatoportal glucose sensor-stimulated muscle glucose utilization.

Recent evidence suggests the existence of a hepatoportal vein glucose sensor, whose activation leads to enhanced glucose use in skeletal muscle, heart, and brown adipose tissue. The mechanism leading to this increase in whole body glucose clearance is not known, but previous data suggest that it is insulin independent. Here, we sought to further determine the portal sensor signaling pathway by selectively evaluating its dependence on muscle GLUT4, insulin receptor, and the evolutionarily conserved sensor of metabolic stress, AMP-activated protein kinase (AMPK). We demonstrate that the increase in muscle glucose use was suppressed in mice lacking the expression of GLUT4 in the organ muscle. In contrast, glucose use was stimulated normally in mice with muscle-specific inactivation of the insulin receptor gene, confirming independence from insulin-signaling pathways. Most importantly, the muscle glucose use in response to activation of the hepatoportal vein glucose sensor was completely dependent on the activity of AMPK, because enhanced hexose disposal was prevented by expression of a dominant negative AMPK in muscle. These data demonstrate that the portal sensor induces glucose use and development of hypoglycemia independently of insulin action, but by a mechanism that requires activation of the AMPK and the presence of GLUT4.

Heterogeneous metabolic adaptation of C57BL/6J mice to high-fat diet.

C57BL/6J mice were fed a high-fat, carbohydrate-free diet (HFD) for 9 mo. Approximately 50% of the mice became obese and diabetic (ObD), approximately 10% lean and diabetic (LD), approximately 10% lean and nondiabetic (LnD), and approximately 30% displayed intermediate phenotype. All of the HFD mice were insulin resistant. In the fasted state, whole body glucose clearance was reduced in ObD mice, unchanged in the LD mice, and increased in the LnD mice compared with the normal-chow mice. Because fasted ObD mice were hyperinsulinemic and the lean mice slightly insulinopenic, there was no correlation between insulin levels and increased glucose utilization. In vivo, tissue glucose uptake assessed by 2-[(14)C]deoxyglucose accumulation was reduced in most muscles in the ObD mice but increased in the LnD mice compared with the values of the control mice. In the LD mice, the glucose uptake rates were reduced in extensor digitorum longus (EDL) and total hindlimb but increased in soleus, diaphragm, and heart. When assessed in vitro, glucose utilization rates in the absence and presence of insulin were similar in diaphragm, soleus, and EDL muscles isolated from all groups of mice. Thus, in genetically homogenous mice, HFD feeding lead to different metabolic adaptations. Whereas all of the mice became insulin resistant, this was associated, in obese mice, with decreased glucose clearance and hyperinsulinemia and, in lean mice, with increased glucose clearance in the presence of mild insulinopenia. Therefore, increased glucose clearance in lean mice could not be explained by increased insulin level, indicating that other in vivo mechanisms are triggered to control muscle glucose utilization. These adaptive mechanisms could participate in the protection against development of obesity.

Free fatty acids inhibit the glucose-stimulated increase of intramuscular glucose-6-phosphate concentration in humans.

To test Randle's hypothesis we examined whether free fatty acids (FFAs) affect glucose-stimulated glucose transport/phosphorylation and allosteric mediators of muscle glucose metabolism under conditions of fasting peripheral insulinemia. Seven healthy men were studied during somatostatin-glucose-insulin clamp tests [plasma insulin, 50 pmol/L; plasma glucose, 5 mmol/L (0-180 min), 10 mmol/L (180-300 min)] in the presence of low (0.05 mmol/L) and increased (2.6 mmol/L) plasma FFA concentrations. (31)P and (1)H nuclear magnetic resonance spectroscopy was used to determine intracellular concentrations of glucose-6-phosphate (G6P), inorganic phosphate, phosphocreatine, ADP, pH, and intramyocellular lipids. Rates of glucose turnover were measured using D-[6,6-(2)H(2)]glucose. Plasma FFA elevation reduced rates of glucose uptake at the end of the euglycemic period (R(d 150-180 min): 8.6 +/- 0.5 vs. 12.6 +/- 1.6 micromol/kg.min, P < 0.05) and during hyperglycemia (R(d 270-300 min): 9.9 +/- 0.6 vs. 22.3 +/- 1.7 micromol/kg.min, P < 0.01). Similarly, intramuscular G6P was lower at the end of both euglycemic (G6P(167-180 min): -22 +/- 7 vs. +24 +/- 7 micromol/L, P < 0.05) and hyperglycemic periods (G6P(287-300 min): -7 +/- 9 vs. +28 +/- 7 micromol/L, P < 0.05). Changes in intracellular inorganic phosphate exhibited a similar pattern, whereas FFA did not affect phosphocreatine, ADP, pH, and intramyocellular lipid contents. In conclusion, the lack of an increase in muscular G6P along with reduction of whole body glucose clearance indicates that FFA might directly inhibit glucose transport/phosphorylation in skeletal muscle.

Hyperlipidaemia is associated with increased insulin-mediated glucose metabolism, reduced fatty acid metabolism and normal blood pressure in transgenic mice overexpressing human apolipoprotein C1.

Insulin resistance for glucose metabolism is associated with hyperlipidaemia and high blood pressure. In this study we investigated the effect of primary hyperlipidaemia on basal and insulin-mediated glucose and on non-esterified fatty acid (NEFA) metabolism and mean arterial pressure in hyperlipidaemic transgenic mice overexpressing apolipoprotein C1 (APOC1). Previous studies have shown that APOC1 transgenic mice develop hyperlipidaemia primarily because of an impaired hepatic uptake of very low density lipoprotein (VLDL). Basal and hyperinsulinaemic (6 mU.kg-1.min-1), euglycaemic (7 mmol/l) clamps with 3(-)3H-glucose or 9,10(-)3H-palmitic acid infusions and in situ freeze clamped tissue collection were carried out. The APOC1 mice showed increased basal plasma cholesterol, triglyceride, NEFA and decreased glucose concentrations compared with wild-type mice (7.0 +/- 1.2 vs 1.6 +/- 0.1, 9.1 +/- 2.3 vs 0.6 +/- 0.1, 1.9 +/- 0.2 vs 0.9 +/- 0.1 and 7.0 +/- 1.0 vs 10.0 +/- 1.1 mmol/l, respectively, p < 0.05). Basal whole body glucose clearance was increased twofold in APOC1 mice compared with wild-type mice (18 +/- 2 vs 10 +/- 1 ml.kg-1.min-1, p < 0.05). Insulin-mediated whole body glucose uptake, glycolysis (generation of 3H2O) and glucose storage increased in APOC1 mice compared with wild-type mice (339 +/- 28 vs 200 +/- 11; 183 +/- 39 vs 128 +/- 17 and 156 +/- 44 vs 72 +/- 17 mumol.kg-1.min-1, p < 0.05, respectively), corresponding with a twofold to threefold increase in skeletal muscle glycogenesis and de novo lipogenesis from 3-(3)H-glucose in skeletal muscle and adipose tissue (p < 0.05). Basal whole body NEFA clearance was decreased threefold in APOC1 mice compared with wild-type mice (98 +/- 21 vs 314 +/- 88 ml.kg-1.min-1, p < 0.05). Insulin-mediated whole body NEFA uptake, NEFA oxidation (generation of 3H2O) and NEFA storage were lower in APOC1 mice than in wild-type mice (15 +/- 3 vs 33 +/- 6; 3 +/- 2 vs 11 +/- 4 and 12 +/- 2 vs 22 +/- 4 mumol.kg-1.min-1, p < 0.05) in the face of higher plasma NEFA concentrations (1.3 +/- 0.3 vs 0.5 +/- 0.1 mmol/l, p < 0.05), respectively. Mean arterial pressure and heart rate were similar in APOC1 vs wild-type mice (82 +/- 4 vs 85 +/- 3 mm Hg and 459 +/- 14 vs 484 +/- 11 beats.min-1). 1) Hyperlipidaemic APOC1 mice show reduced NEFA and increased glucose metabolism under both basal and insulin-mediated conditions, suggesting an intrinsic defect in NEFA metabolism. Primary hyperlipidaemia alone in APOC1 mice does not lead to insulin resistance for glucose metabolism and high blood pressure.

Training-induced enhancement of insulin action in human skeletal muscle: the influence of aging.

Age-induced reduction of whole body insulin action has been attributed to decreased insulin action in skeletal muscle. Physical training improves insulin action, but the effect has never been investigated specifically in aged human skeletal muscle. Seven young men [age: 23 +/- 1 yr (mean +/- SE; range, 21-24 yr); weight: 70 +/- 1 kg; body fat: 8 +/- 1%] and eight aged men [59 +/- 1 yr (range, 58-64 yr); 83 +/- 2 kg; 20 +/- 2%] performed one-legged bicycle training on a modified ergometer cycle for 10 weeks, 6 days/week, at 70% of VO2 peak. Glucose clearance rates in whole body and leg were measured 16 hr after training by a hyperinsulinemic (28, 88, and 480 mU.min-1.min-2), isoglycemic clamp combined with leg balance technique. Peak oxygen uptake during the bicycle test was always lower (p < .05) in aged vs. young subjects. Furthermore, VO2 peak was higher after training in trained (T) vs. untrained (UT) (p < .05) legs. Whole body glucose clearance rate was lower in aged vs. young subjects (p < .05) when expressed per kg body weight, but similar when expressed relative to fat free mass. Leg blood flow was always lower in aged vs. young men (p < .05). At basal and during insulin infusion, leg blood flow in young men did not differ significantly in T vs. UT legs (maximum insulin: 81 +/- 7 vs. 71 +/- 5 ml.min-1.kg leg-1), while in aged subjects it increased (p < .05) with training (maximum insulin: 57 +/- 5 vs. 48 +/- 5 ml.min-1.kg leg-1). Leg glucose extraction was always higher in aged vs. young men during the two last clamp steps (p < .05). Furthermore, leg glucose extraction was increased by training in young (p < .05) but not significantly in aged subjects. Leg glucose clearance rates increased (p < .05) with training and was similar in aged men (T: 1 +/- 1, 8 +/- 1, 21 +/- 2, and 24 +/- 2; UT: 1 +/- 1, 6 +/- 1, 14 +/- 2, and 20 +/- 2 ml.min-1.kg leg-1) and young men (T: 1 +/- 1, 12 +/- 3, 23 +/- 3, and 26 +/- 3; UT: 1 +/- 1, 8 +/- 2, 17 +/- 2, and 21 +/- 2 ml.min-1.kg leg-1). Therefore, insulin action in muscle is not reduced by aging. At high insulin concentrations, the leg blood flow is lower, whereas glucose extraction is higher in aged compared with young men. Training increases overall insulin action on glucose clearance in skeletal muscle identically in aged and young subjects.

Impact of early growth retardation on glucoregulatory control and insulin action in mature rats.

Glucoregulation and insulin action were examined in adult female progeny of dams fed an 8% protein diet during gestation and lactation. Progeny were provided with standard (20% protein) diet at weaning (24 days), at which time body weight was reduced by 21%. Basal (postabsorptive) endogenous glucose production (Ra) and glucose utilization by oxidative muscles was decreased in offspring that had experienced early growth retardation. The glucose disappearance rate after intravenous glucose and the glucose infusion rate required to maintain euglycemia during insulin infusion were both increased (by 35 and 24%, respectively) by early growth retardation. Insulin secretion was unimpaired. During euglycemic-hyperinsulinemic clamp, Ra was suppressed further, and there was a twofold greater increment in the stimulation of whole body glucose clearance rate in the offspring that experienced early growth retardation. Insulin-stimulated glucose utilization by peripheral tissues was unaffected.

Fasting hyperglycemia in non-insulin-dependent diabetes mellitus: Contributions of excessive hepatic glucose production and impaired tissue glucose uptake

The factors responsible for fasting hyperglycemia were investigated in 77 normal weight non-insulin-dependent diabetic (NIDD) and 72 age-, sex-, and weight-matched control individuals. In diabetic subjects with mild fasting hyperglycemia (<140 mg/dL) hepatic glucose production (1.85 ± 0.03 mg/kg · min) was similar to controls (1.84 ± 0.02); the major factor responsible for the elevated basal glucose level in the diabetic group was a decreased efficiency in the tissue uptake of glucose, as reflected by a 30% decline in the rate of glucose clearance (1.56 ± 0.03 v 2.00 ± 0.03 mL/kg · min, P < .001). In contrast, in diabetic subjects with fasting plasma glucose concentrations above 140 mg/dL, basal hepatic glucose production was significantly elevated (2.42 ± 0.08 mg/kg · min, P < .001) and correlated closely with the increase in fasting plasma glucose concentration ( r = .796, P < .001). The basal rate of whole body glucose clearance reached a plateau value at fasting glucose levels of 160 to 180 mg/dL and did not contribute to the further rise in fasting plasma glucose concentrations above 160 to 180 mg/dL. Decreased efficiency of tissue glucose uptake is responsible the development of fasting hyperglycemia in patients with mild NIDDM (fasting plasma glucose < 140 mg/dL). As the diabetic state worsens, an increase in basal hepatic glucose production is the major factor responsible for the progressive rise in fasting glucose levels.

Regulation of splanchnic and peripheral glucose uptake by insulin and hyperglycemia in man.

We investigated the effects of hyperinsulinemia and hyperglycemia on peripheral glucose uptake, hepatic glucose production, and splanchnic glucose uptake in man. Euglycemic and hyperglycemic clamp studies were carried out in 37 healthy subjects in combination with hepatic vein catheterization and [3H-3]glgcose infusion. In the basal state, hepatic glucose production ([3H-3]glucose) exceeded net splanchnic glucose output (catheter) in every subject (2.3 ± 0.04 versus 1.7 ± 0.07 mg/min · kg, P &amp;lt; 0.001), indicating uptake of glucose by the splanchnic region at a rate of 0.6 ± 0.05 mg/ min · kg. In agreement with this estimate, [3H-3]glucose concentration was consistently lower in hepatic venous than in arterial blood, by 3.0 ± 0.2% (P &amp;lt; 0.001). When plasma insulin levels were raised to 37 ± 2, 53 ± 2, 101 ± 2, 428 ± 37, and 1189 ± 14 μU/ml, with maintenance of euglycemia, total glucose uptake rose to 2.9 ± 0.4, 3.9 ± 1.0, 5.1 ± 0.4, 9.9 ±1.1, and 11.8 ± 1.3 mg/min · kg, respectively. The whole body glucose clearance rose significantly above baseline at each hyperinsulinemic plateau (P &amp;lt; 0.05 or less). Hepatic glucose production fell by 68% (P &amp;lt; 0.01) at the lowest hyperinsulinemic level, by 87% at insulin levels of 53 ± 2 μU/ml, and by over 95% with each higher insulin dose. Splanchnic glucose uptake was not significantly increased over basal values at any insulin concentration. When plasma glucose levels were raised to 137 ± 3 and 224 ± 2 mg/dl peripheral plasma insulin levels rose to 20 ± 4 and 55 ± 5 μU/ml, respectively. Total glucose uptake was enhanced (2.5 ± 0.4 and 5.3 ± 1.0 mg/min · kg, P &amp;lt; 0.05 and P &amp;lt; 0.01, respectively). Suppression of hepatic glucose production was &amp;lt;90% at the lower hyperglycemic level, and virtually complete at the higher one. Splanchnic glucose uptake was not changed by mild hyperglycemia (0.5 ± 0.05 mg/min · kg), but rose significantly (1.3 ± 0.3 mg/ min · kg, P &amp;lt; 0.01) with further hyperglycemia. The latter effect resulted primarily from increased glucose delivery to the splanchnic area, since the splanchnic glucose extraction ratio (4.0 ± 0.3%) was not different from baseline (3.0 ± 0.3%). When hyperglycemia (224 ± 1 mg/dl) was combined with a somatostatin infusion, thereby reducing plasma insulin from 15 ± 3 to 10 ± 1 μU/ml (P &amp;lt; 0.01), both total glucose uptake (2.8 ± 0.03 mg/min · kg) and clearance (1.3 ± 0.01 mg/min · kg) were significantly (P &amp;lt; 0.01) lower than in the hyperglycemic studies in which insulin secretion was not blocked. Hepatic glucose production, however, was effectively suppressed (by 74%, P &amp;lt; 0.001), whereas splanchnic glucose uptake was only slightly increased above baseline. Replacement of insulin (via an exogenous infusion at a rate of 0.3 mU/min · kg) restored total glucose uptake, splanchnic glucose uptake, and suppression of hepatic glucose production to the levels seen with hyperglycemia without somatostatin. When hyperglycemia (216 ± 2 mg/dl) was combined with somatostatin and glucagon replacement (no insulin), hepatic glucose production was still suppressed by 47 ± 1% to 1.18 ± 0.01 mg/kg · min (P &amp;lt; 0.001 versus hyperglycemia + SRIF without glucagon replacement). The results indicate that both hyperglycemia and hypoglucagonemia contribute to the decline in hepatic glucose production following somatostatin infusion. In conclusion, hyperinsulinemia alone stimulates glucose uptake by peripheral but not splanchnic tissues. The dose-response characteristics of stimulation of peripheral glucose uptake and inhibition of hepatic glucose production by insulin are very different, the half-maxima being ∼120 and ∼50 μU/ml, respectively. Hyperglycemia enhances glucose uptake by both peripheral and splanchnic tissues, but this action requires an intact endogenous insulin response. In contrast, hyperglycemia can suppress endogenous glucose production even in the presence of low insulin levels.

Interrelationship of glucagon, insulin and glucose. The insulinogenic effect of glucagon.

Glucagon in doses of 0.25 or 1.0 mg. was injected rapidly, intravenously, into healthy normal subjects. There was a striking rise in arterial immunoreactive insulin levels within one minute of the start ot the injection before any change in arterial glucose concentration occurred Plasma insulin levels reached their peak within ten minutes and began to fall thereafter. In contrast, blood glucose levels were highest twenty to thirty minutes after the injection. A second glucagon injection during the hyperglycemic phase produced a larger rise in plasma insulin than the first. During constant infusions of smaller amounts of glucagon, the insulinogenic effect of glucagon was usually only apparent if blood glucose levels were raised. The effect of consecutive forty-minute intravenous infusions of glucagon (10μg./min.), glucose (0.5 gm./min.) and a glucagon-glucose (5μg. and 250 mg./min., respectively) on plasma insulin and arterio-venous blood glucose differences in six healthy normal subjects was studied. Plasma insulin levels were two to five times higher during glucose infusions alone, despite similar peak arterial blood glucose levels during the infusions. Similar results were obtained in other subjects during separate infusions of glucose and glucagon. Total body glucose clearance, judged by the rate of fall in arterial blood glucose levels, was faster after glucagon- than after glucose-induced hyperglycemia. Forearm A-V blood glucose differences were not obviously greater during glucagon, in spite of the higher plasma insulin levels, than during glucose infusions. Possible reasons for the failure of forearm A-V blood glucose differences to reflect accurately total peripheral glucose assimilation under these experimental conditions are discussed, and it is suggested that the co-existence of hyperglucagonemia and hyperinsulinemia may tend to favor incorporation of glucose into adipose tissue rather than muscle. Plasma glucagon levels were measured by immunoassay and the half-life was about ten minutes. The results provide evidence of a direct effect of glucagon upon insulin secretion. The suggestion is made that the insulinogenic effect of glucagon is physiological importance and is due to accelerated intra-β-cell glycogenolysis.