Insulin-stimulated Glycolysis Research Articles

Gender characteristics of the parameters of carbohydrate metabolism and pancreatic hormones in permanent residents of the Arctic region, taking into account age

BACKGROUND: An increase with age in the blood glucose level of permanent residents of the Arctic region is a risk factor for metabolic changes, including diabetes mellitus (DM), the frequency and prevalence of which is often accompanied by an increase in body weight and obesity. However, at the moment, there is no clear idea of the age range in the structure of metabolic changes in northerners, and the influence of gender on the content of glucose, its metabolites, pancreatic hormones and the nature of the relationship between them in adults requires clarification.AIMS: To assess the values of the parameters of carbohydrate metabolism, pancreatic hormones, body mass index and the nature of the relationships between them in residents of the Arctic region who were born and permanently residing in the Nenets (NAO), Yamalo-Nenets Autonomous Okrug (Yamalo-Nenets Autonomous Okrug (Yamalo-Nenets Autonomous Okrug) and Mezen District of the Arkhangelsk Region, taking into account gender and age.MATERIALS AND METHODS: In the blood serum healthy residents of the NAO, YaNAO and the Mezensky district of the Arkhangelsk region of mature age, born and permanently residing in the Arctic region, randomly selected after a medical examination and questioning, which depending on gender and age were divided into groups: women 21–35, 36–45, 46–55 years old and men 22–35, 36–45, 46–60 years old. The blood serum was determined by the content of glucose, lactate, pyruvate, insulin, proinsulin, C-peptide was determined, measured height and weight.RESULTS: The study involved 1146 people of mature age (women 21–35 (221), 36–45 (222) and 46–55 (371), men 22–35 (105), 36–45 (84), 46–60 (143) years). Regardless of gender, the content of glucose (Glu) significantly increases with age, the peak of which occurs in women aged 46–55 years and in men aged 46–60 years. Age-related changes in the content of lactate (Lac) and pyruvate (Pir) were not found, but in men aged 36–45 years, the level of Lac was higher than in women, and the content of Pir, on the contrary, in all age groups, regardless of gender, was shifted to the lower limit of normal and below it. Against the background of an increase in Glu in women aged 46–55 years the level of insulin was significantly increased, and in men aged 46–60 years, the level of proinsulin was significantly increased. The value of the HOMA index, regardless of gender, exceeded the norm, and the value of the Caro index was below the critical level, which indicates hidden disorders of carbohydrate metabolism.CONCLUSIONS: In permanent residents of the Arctic region (NAO, YaNAO, Mezensk district of the Arkhangelsk region) of mature age, sex differences were established in the content of glucose, lactate, pyruvate and in the regulation of insulin-stimulated glycolysis by pancreatic hormones, as well as an excess of the BMI norm in 50,9, 67,0, 75,0% of women and 52,1, 66,6, 63,7% males.

Altered Skeletal Muscle Fiber Composition and Size Precede Whole-Body Insulin Resistance in Young Men with Low Birth Weight

Low birth weight (LBW), a surrogate marker of an adverse fetal milieu, is linked to muscle insulin resistance, impaired insulin-stimulated glycolysis, and future risk of type 2 diabetes. Skeletal muscle mass, fiber composition, and capillary density are important determinants of muscle function and metabolism, and alterations have been implicated in the pathogenesis of insulin resistance. The aim of this study was to investigate whether an adverse fetal environment (LBW) induces permanent changes in skeletal muscle morphology, which may contribute to the dysmetabolic phenotype associated with LBW. Vastus lateralis muscle was obtained by percutaneous biopsy from 20 healthy 19-yr-old men with birth weights at 10th percentile or lower for gestational age (LBW) and 20 normal birth weight controls, matched for body fat, physical fitness, and whole-body glucose disposal. Myofibrillar ATPase staining was used to classify muscle fibers as type I, IIa, and IIx (formerly type IIb), and double immunostaining was performed to stain capillaries (LBW, n=8; normal birth weight, n=12). LBW was associated with increased proportion of type IIx fibers (+66%; P=0.03), at the expense of decreased type IIa fibers (-22%; P=0.003). No significant change was observed in proportion of type I fibers (+16%; P=0.11). In addition, mean area of type IIa fibers was increased (+29%; P=0.01) and tended to be increased for type I fibers as well (+17%; P=0.08). Capillary density was not significantly different between groups. Alterations in fiber composition and size may contribute to development of type 2 diabetes in individuals with LBW.

Fatty Acids Attenuate Insulin Regulation of 5′-AMP–Activated Protein Kinase and Insulin Cardioprotection After Ischemia

The cardioprotective effect of insulin during ischemia-reperfusion has been associated with stimulation of glucose uptake and glycolysis. Although fatty acids and 5'-AMP activated protein kinase (AMPK) are regulators of glucose metabolism, it is unknown what effect insulin has on postischemic function and AMPK activity in the presence of high levels of fatty acid. Isolated ejecting mouse hearts were perfused with Krebs-Henseleit solution containing 5 mmol x L(-1) glucose and 0, 0.2, or 1.2 mmol x L(-1) palmitate, with or without 100 microU/mL insulin. During aerobic perfusion in the absence of palmitate, insulin stimulated glycolysis by 73% and glucose oxidation by 54%, while inhibiting AMPK activity by 43%. In the presence of 0.2 or 1.2 mmol x L(-1) palmitate, insulin stimulated glycolysis by 111% and 105% and glucose oxidation by 72% and 274% but no longer inhibited AMPK activity. During reperfusion of hearts in the absence of palmitate, insulin increased recovery of cardiac power by 47%. This was associated with a 97% increase in glycolysis and a 160% increase in glucose oxidation. However, in the presence of 1.2 mmol x L(-1) palmitate, insulin now decreased recovery of cardiac power by 42%. During reperfusion, glucose oxidation was inhibited by high fat, but insulin-stimulated glycolysis remained high, resulting in increased proton production. In the absence of fatty acids, insulin blunted the ischemia-induced activation of AMPK, but this effect was lost in the presence of fatty acids. We demonstrate that the cardioprotective effect of insulin and its ability to inhibit AMPK activity are lost in the presence of high concentrations of fatty acids.

Young, low-birth-weight men are not more susceptible to the diabetogenic effects of a prolonged free fatty acid exposure than matched controls

Low birth weight (LBW) is associated with increased risk of developing type 2 diabetes later in life. Progression from normal to impaired glucose tolerance and overt diabetes may depend, to some extent, on elevation of plasma free fatty acids (FFAs). We undertook this study to elucidate whether a prolonged physiological lipid load could unmask or augment existing metabolic defects in otherwise healthy young LBW subjects. Forty 19-year-old men (LBW [n = 20], controls [normal birth weight, NBW] [n = 20]) without a family history of diabetes underwent an intravenous glucose tolerance test (0.3 g kg −1), followed by 2-step hyperinsulinemic-euglycemic clamps (2 × 120 minutes: 10 and 40 mU m −2 min −1) in combination with [3- 3H]-glucose and indirect calorimetry. The tests were preceded, in randomized order, by a 24-hour continuous intralipid (20%, 0.4 mg mL −1 h −1) or saline infusion. Estimates of cellular glucose metabolism were obtained and a disposition index calculated. Clamp FFA concentrations were 4- to ten-fold higher during lipid infusion. Both groups experienced a similar decrease in insulin-stimulated glucose disposal in response to lipid infusion (∼15%; P < .05), which was mainly accounted for by reduced glucose oxidation (∼30%; P < .001). Glycolysis, glucose storage, and glucose production were not significantly altered by lipid infusion. Nevertheless, the LBW group had significantly lower insulin-stimulated glycolysis during lipid infusion (∼27%; P < .05) than the NBW group. An appropriate increase in insulin secretion matched the decline in insulin sensitivity in both groups. A 24-hour low-grade intralipid infusion has similar effects on whole-body glucose metabolism and first-phase insulin secretion in 19-year-old, healthy, lean, LBW men with normal glucose tolerance and in NBW controls. We reproduced our previous finding of lower insulin-stimulated glycolysis in this population.

Lactate induces insulin resistance in skeletal muscle by suppressing glycolysis and impairing insulin signaling.

Elevation of plasma lactate levels induces peripheral insulin resistance, but the underlying mechanisms are unclear. We examined whether lactate infusion in rats suppresses glycolysis preceding insulin resistance and whether lactate-induced insulin resistance is accompanied by altered insulin signaling and/or insulin-stimulated glucose transport in skeletal muscle. Hyperinsulinemic euglycemic clamps were conducted for 6 h in conscious, overnight-fasted rats with or without lactate infusion (120 micromol x kg(-1) x min(-1)) during the final 3.5 h. Lactate infusion increased plasma lactate levels about fourfold. The elevation of plasma lactate had rapid effects to suppress insulin-stimulated glycolysis, which clearly preceded its effect to decrease insulin-stimulated glucose uptake. Both submaximal and maximal insulin-stimulated glucose transport decreased 25-30% (P < 0.05) in soleus but not in epitrochlearis muscles of lactate-infused rats. Lactate infusion did not alter insulin's ability to phosphorylate the insulin receptor, the insulin receptor substrate (IRS)-1, or IRS-2 but decreased insulin's ability to stimulate IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase activities and Akt/protein kinase B activity by 47, 75, and 55%, respectively (P < 0.05 for all). In conclusion, elevation of plasma lactate suppressed glycolysis before its effect on insulin-stimulated glucose uptake, consistent with the hypothesis that suppression of glucose metabolism could precede and cause insulin resistance. In addition, lactate-induced insulin resistance was associated with impaired insulin signaling and decreased insulin-stimulated glucose transport in skeletal muscle.

Plasma Free Fatty Acids Decrease Insulin-Stimulated Skeletal Muscle Glucose Uptake by Suppressing Glycolysis in Conscious Rats

The effects of elevated plasma free fatty acid (FFA) levels on insulin -stimulated whole-body and skeletal muscle glucose transport, glucose uptake, glycolysis, and glycogen synthesis were studied in conscious rats during hyperinsulinemic-euglycemic clamps with (n = 26) or without (n = 23) Intralipid and heparin infusion. Whole-body and skeletal muscle glucose uptake, glycolysis, and glycogen synthesis were estimated using D-[3-3H]glucose and 2-[14C]deoxyglucose (study 1), and glucose transport activity was assessed by analyzing plasma kinetics of L-[14C]glucose and 3-O-[3H]-methylglucose (study 2). Plasma FFA levels decreased during the clamps without intralipid but increased above basal during the clamps with Intralipid infusion (P < 0.01 for both). Elevated plasma FFA levels decreased insulin-stimulated whole-body glucose uptake by approximately 15% and approximately 20% during physiological and maximal insulin clamps, respectively (P < 0.01). Similarly, insulin-stimulated glucose uptake was also decreased in individual skeletal muscles with Intralipid infusion (P < 0.05). The most profound effect of elevated plasma FFA levels was a 30-50% suppression of insulin-stimulated glycolysis in whole body and individual skeletal muscles in both clamps. In contrast, physiological insulin-stimulated glycogen synthesis was increased with elevated plasma FFA levels in whole body and individual skeletal muscles (P < 0.05). Glucose-6-phosphate (G-6-P) levels were increased in soleus and extensor digitorum longus (EDL) muscles with Intralipid infusion in both clamps (P < 0.05). Intralipid infusion did not alter the time profiles of plasma L-glucose and 3-O-methylglucose after an intravenous injection during maximal insulin clamps, and compartmental analysis indicated no significant effect of elevated FFA levels on glucose transport activity in insulin-sensitive tissues (P > 0.05). Thus, elevated plasma FFA decreased insulin-stimulated glucose uptake in skeletal muscle by suppressing glycolysis and increasing G-6-P levels. These findings suggest that the classic glucose-fatty acid cycle was the predominant mechanism underlying the inhibitory effect of FFA on skeletal muscle glucose uptake.

Pancreastatin inhibits insulin-stimulated glycogen synthesis but not glycolysis in rat hepatocytes

The effect of rat pancreastatin on glycogen synthesis and glycolysis rate was studied in insulin-stimulated rat hepatocytes. We have determined the incorporation of [U- 14C]glucose into glycogen as a measurement of the rate of glycogen synthesis; and the production of lactate as a measurement of the rate of glycolysis. Rat pancreastatin by itself did not affect either the rate of glycogen synthesis or glycolysis in rat hepatocytes from 6 h fasted rats. However, pancreastatin inhibited about 45% the insulin-stimulated glycogen synthesis whereas it enhanced the rate of glycolysis of insulin-stimulated hepatocytes about 25%. These effects were found to be dependent on pancreastatin concentration from 10 −11 M to 10 −7 M. Maximal effect was achieved at 10 −8 M and the half-maximal effect was observed at 0.3 nM. Pancreastatin decreased the rate of glycogen synthesis in a wide range of insulin concentrations (10 −12–10 −8 M). However, the effect on insulin-stimulated glycolysis was only observed at high concentrations of pancreastatin and insulin. These results suggest a role of pancreastatin in the possible mechanisms involved in insulin resistance.

Actions of Novel Antidiabetic Agent Englitazone in Hyperglycemic Hyperinsulinemic ob/ob Mice

The effects of CP 68722 (racemic englitazone) were examined in ob/ob mice, in adipocytes and soleus muscles from ob/ob mice, and in 3T3-L1 adipocytes. Administration of englitazone at 5-50 mg.kg-1.day-1 lowered plasma glucose and insulin dose dependently without producing frank hypoglycemia in either the diabetic or nondiabetic lean animals. The glucose-lowering effect in ob/ob mice preceded the reduction in hyperinsulinemia. On cessation of drug, plasma insulin returned to untreated levels within 48 h, whereas plasma glucose rose slowly over 5 days. Englitazone (50 mg/kg) for 11 days lowered plasma glucose (22.2 +/- 1.4 to 14.0 +/- 1.9 mM), insulin (7.57 +/- 0.67 to 1.64 +/- 0.60 nM), nonesterified fatty acids (1813 +/- 86 to 914 +/- 88 microM), glycerol (9.20 +/- 0.98 to 4.94 +/- 0.03 mM), triglycerides (1.99 +/- 0.25 to 1.03 +/- 0.11 g/L), and cholesterol (6.27 +/- 0.96 to 3.87 +/- 0.57 mM), but no effects were observed 3 h after a single dose. Basal and insulin-stimulated lipogenesis were enhanced in adipocytes from ob/ob mice treated with 50 mg/kg englitazone for 11 days compared with lipogenesis in cells from vehicle-treated controls. Treatment of ob/ob mice with 50 mg/kg englitazone reversed the defects in insulin-stimulated glycolysis (from [3-3H]glucose) and glycogenesis and basal glucose oxidation (from [1-14C]glucose) in isolated soleus muscles. Englitazone (30 microM) stimulated 2-deoxy-D-glucose transport in 3T3-L1 adipocytes from 0.37 +/- 0.03 to 0.65 +/- 0.06 and 1.53 nmol.min-1.mg-1 protein at 24 and 48 h, respectively. Thus, englitazone has 1) insulinomimetic and insulin-enhancing actions in vitro and 2) glucose-, insulin-, triglyceride-, and cholesterol-lowering properties in an animal model of non-insulin-dependent diabetes mellitus (NIDDM) in which sulfonylureas have little or no effect. Thus, this new agent may have beneficial effects including a reduced risk of hypoglycemia in patients with NIDDM.

Effects of insulin on skeletal muscle glucose storage, oxidation, and glycolysis in humans

The effects of physiological hyperinsulinemia (approximately 75 mU/l) on glucose storage, oxidation, and glycolysis in skeletal muscle were assessed with euglycemic clamps performed in seven healthy volunteers, in conjunction with leg balance for glucose, lactate, alanine, O2, and CO2. Infusion of insulin increased leg glucose uptake, storage, and oxidation but did not alter net release of lactate and alanine. The respiratory quotient (RQ) across the leg increased from a basal value of 0.74 +/- 0.02 to 0.99 +/- 0.02 during hyperinsulinemia. Under conditions of insulin stimulation, 49 +/- 5% of leg glucose uptake was stored, 37 +/- 4% was oxidized, and 14 +/- 2% was released as lactate and alanine. We conclude that during physiological hyperinsulinemia and euglycemia 1) skeletal muscle lipid oxidation is nearly entirely suppressed and glucose becomes the primary oxidative substrate of muscle, 2) glucose storage and oxidation are the major pathways of skeletal muscle glucose metabolism and are quantitatively similar at physiological insulin levels, and 3) the majority of insulin-stimulated glycolysis is oxidized, with only a small portion released as lactate or alanine.

Resistance of protein and glucose metabolism to insulin in denervated rat muscle.

Denervated (1-10 days) rat epitrochlearis muscles were isolated, and basal and insulin-stimulated protein and glucose metabolism were studied. Although basal rates of glycolysis and glucose transport were increased in 1-10-day-denervated muscles, basal glycogen-synthesis rates were unaltered and glycogen concentrations were decreased. Basal rates of protein degradation and synthesis were increased in 1-10-day-denervated muscles. The increase in degradation was greater than that in synthesis, resulting in muscle atrophy. Increased rates of proteolysis and glycolysis were accompanied by elevated release rates of leucine, alanine, glutamate, pyruvate and lactate from 3-10-day-denervated muscles. ATP and phosphocreatine were decreased in 3-10-day-denervated muscles. Insulin resistance of glycogen synthesis occurred in 1-10-day denervated muscles. Insulin-stimulated glycolysis and glucose transport were inhibited by day 3 of denervation, and recovered by day 10. Inhibition of insulin-stimulated protein synthesis was observed only in 3-day-denervated muscles, whereas regulation by insulin of net proteolysis was unaffected in 1-10-day-denervated muscles. Thus the results demonstrate enhanced glycolysis, proteolysis and protein synthesis, and decreased energy stores, in denervated muscle. They further suggest a defect in insulin's action on protein synthesis in denervated muscles as well as on glucose metabolism. However, the lack of concurrent changes in all insulin-sensitive pathways and the absence of insulin-resistance for proteolysis suggest multiple and specific cellular defects in insulin's action in denervated muscle.

Permissive action of triiodothyronine on the long-term increase of glycolysis by insulin in cultured rat hepatocytes.

The long-term influence of triiodothyronine (T3) and insulin on glycolysis, some glycolytic/gluconeogenic enzymes and insulin responsiveness and sensitivity was investigated in rat hepatocytes cultured for 48 h without T3, with 10 microM T3, with 10nM insulin and with insulin plus T3. From 48 h-51 h basal glycolysis ([14C]lactate formation from [14C]glucose) was measured in the absence and short-term insulin-stimulated glycolysis in the presence of 100 nM insulin. 1) T3 addition for 48 h had no significant influence on basal or on insulin-stimulated glycolysis. 2) Insulin addition for 48 h increased basal glycolysis to 300%, and insulin-stimulated glycolysis to 160%. 3) T3 plus insulin addition for 48 h elevated basal glycolysis to 560% and insulin-stimulated glycolysis to 230%. 4) The 48-h treatment with T3 did not change glucokinase (GK) and pyruvate kinase (PK) activity, yet it increased phosphoenol-pyruvate carboxykinase (PEPCK) activity to 150%. 5) The 48-h treatment with insulin as well as T3 plus insulin enhanced GK to 200% and PK to 140% and decreased PEPCK to 65%. 6) The long-term effect of T3 on glycolysis was maximal at initial concentrations of 100 nM. 7) The long-term treatment with T3 did not alter the short-term responsiveness or sensitivity of glycolysis for insulin, neither in cells from euthyroid nor from hypothyroid rats. The present results allow the conclusion that T3 had a permissive effect on the long-term increase of glycolysis by insulin, and that T3 exerted this function by altering neither the cellular content of key enzymes nor the short-term insulin responsiveness and sensitivity.

Activation of glycolysis by insulin with a sequential increase of the 6-phosphofructo-2-kinase activity, fructose-2,6-bisphosphate level and pyruvate kinase activity in cultured rat hepatocytes.

The involvement of 6-phosphofructo-2-kinase, fructose 2,6-bisphosphate [Fru(2,6)P2] and pyruvate kinase in the insulin-dependent short-term activation of glycolysis was studied in primary cultures of rat hepatocytes. The short-term influence of insulin on these parameters was dependent on the insulin concentration used for the long-term culture. Cells were cultured either with 10 nM or 0.1 nM insulin for 48 h, and are referred to as 'insulin cells' and 'control cells', respectively. Insulin cells exhibited a high level of Fru(2,6)P2. Addition of insulin to insulin cells led to an immediate stimulation of glycolysis (two-fold) and activation of pyruvate kinase. The concentration of Fru(2,6)P2 and activity of 6-phosphofructo-2-kinase remained constant. Control cells exhibited a very low level of Fru(2,6)P2 and low activity of 6-phosphofructo-2-kinase directly after the medium change. However, both parameters increased during a 1-2-h incubation in the absence of insulin. Although the level of Fru(2,6)P2 thus changed up to tenfold the glycolytic rate remained at a constant value. Addition of insulin to control cells led to a 5-8-fold stimulation of glycolysis but only after a 30-90-min lag phase. During this lag period insulin strongly increased sequentially the 6-phosphofructo-2-kinase, the level of Fru(2,6)P2 and the pyruvate kinase activity. The activation of the latter enzyme slightly preceded the onset of the insulin-stimulated glycolysis. Addition of insulin to control cells, which were preincubated for 3 h in the absence of insulin and in which the Fru(2,6)P2 level had risen insulin-independently, led to an immediate increase in glycolysis without a lag phase. It is concluded that in this insulin-sensitive cell system: the changes of glycolytic flux did not correlate with changes in the level of total Fru(2,6)P2 either in insulin or in control cells; an increase in the Fru(2,6)P2 concentration was not obligatory for the insulin-dependent stimulation of glycolysis in insulin cells; activation of pyruvate kinase and thus glycolysis by insulin did not proceed unless the Fru(2,6)P2 level had been elevated above a threshold level. The lack of correlation between total Fru(2,6)P2 levels and the glycolytic flux and the apparent existence of a threshold concentration for Fru(2,6)P2 suggest a permissive action for this effector in enzyme interconversion.

Impaired muscle glucose metabolism in acute renal failure.

Renal failure is associated with peripheral insulin resistance and consequent carbohydrate intolerance. This report investigates carbohydrate metabolism in vitro in epitrochlearis and hemidiaphragm muscles taken from acutely uraemic and sham-operated rats. Muscles from acutely uraemic rats (compared to those from sham-operated rats) incubated with 5 mM glucose showed increased rates of basal and insulin-stimulated glycolysis and glycogen turnover, but pyruvate dehydrogenase and tricarboxylic acid-cycle flux was not increased in uraemia. Glycolysis (but not glycogen turnover) in muscles from acutely uraemic rats tended to show decreased responsiveness to stimulation by insulin. It is concluded that acute uraemia is associated with a defect(s) in muscle that produces intrinsic insulin resistance and results in diversion of glucose (both in basal and insulin-stimulated states) from glycogen synthesis into glycolysis.

Effects of exercise on insulin binding and glucose metabolism in muscle.

To elucidate the mechanism of enhanced insulin sensitivity by muscle after exercise, we studied insulin binding, 2-deoxy-D-[1-14C]glucose (2-DOG) uptake and [5-3H]glucose utilization in glycolysis and glycogenesis in soleus and extensor digitorum longus (EDL) muscles of mice after 60 min of treadmill exercise. In the soleus, glycogenesis was increased after exercise (P less than 0.05) and remained sensitive to the action of insulin. Postexercise insulin-stimulated glycolysis was also increased in the soleus (P less than 0.05). In the EDL, glycogenesis was increased after exercise (P less than 0.05). However, this was already maximal in the absence of insulin and was not further stimulated by insulin (0.1-4 nM). The disposal of glucose occurred primarily via the glycolytic pathway (greater than 60%) in the soleus and EDL at rest and after exercise. The uptake of 2-DOG uptake was not altered in the soleus after exercise (4 h incubation at 18 degrees C). However, with 1-h incubations at 37 degrees C, a marked increase in 2-DOG uptake after exercise was observed in the soleus (P less than 0.05) in the absence (0 nM) and presence of insulin (0.2-4 nM) (P less than 0.05). A similar postexercise increase in 2-DOG uptake occurred in EDL. Despite the marked increase in glucose uptake and metabolism, no changes in insulin binding were apparent in either EDL or soleus at 37 degrees C or 18 degrees C. This study shows that the postexercise increase of glucose disposal does not appear to be directly attributable to increments in insulin binding to slow-twitch and fast-twitch muscles.(ABSTRACT TRUNCATED AT 250 WORDS)