Unchanged gene expression of glycogen synthase in muscle from patients with NIDDM following sulphonylurea-induced improvement of glycaemic control

Summary We have previously shown that the mRNA expression of muscle glycogen synthase is decreased in non-insulin-dependent diabetic (NIDDM) patients; the objective of the present protocol was to examine whether the gene expression of muscle glycogen synthase in NIDDM is affected by chronic sulphonylurea treatment. Ten obese patients with NIDDM were studied before and after 8 weeks of treatment with a weight-maintaining diet in combination with the sulphonylurea gliclazide. Gliclazide treatment was associated with significant reductions in HbA_1c (p=0.01) and fasting plasma glucose (p=0.005) as well as enhanced beta-cell responses to an oral glucose load. During euglycaemic, hyperinsulinaemic clamp (2 mU · kg^–1 · min^–1) in combination with indirect calorimetry, a 35% (p=0.005) increase in whole-body insulin-stimulated glucose disposal rate, predominantly due to an increased non-oxidative glucose metabolism (p=0.02) was demonstrated in the gliclazide-treated patients when compared to pre-treatment values. In biopsies obtained from vastus lateralis muscle during insulin infusion, the half-maximal activation of glycogen synthase was achieved at a significantly lower concentration of the allosteric activator glucose 6-phosphate (p=0.01). However, despite significant increases in both insulin-stimulated non-oxidative glucose metabolism and muscle glycogen synthase activation in gliclazide-treated patients no changes were found in levels of glycogen synthase mRNA or immunoreactive protein in muscle. In conclusion, improved blood glucose control in gliclazide-treated obese NIDDM patients has no impact on the gene expression of muscle glycogen synthase.Abbreviations NIDDM Non-insulin-dependent diabetes mellitus - GS glycogen synthase - G6P glucose-6-phosphate - OGTT oral glucose tolerance test - AUC area under the curve - HGO hepatic glucose output - PCR polymerase chain reaction - bp base pair     

Post-marathon Paradox in IDDM: Unchanged Insulin Sensitivity in Spite of Glycogen Depletion

Acute physical exercise usually enhances insulin sensitivity. We examined the effect of a competitive 42 km marathon run on glucose uptake and lipid oxidation in 7 runners with insulin-dependent diabetes mellitus (IDDM), aged 36 ± 3 yr, BMI 23.9 ± 0.5 kg m⁻², VO_{2 max} 46 ± 1 ml kg⁻¹ min⁻¹, HbA_1c 7.7 ± 0.3 %, duration of diabetes 16 ± 5 yr, runtime 3 h 47 ± 8 min. On the marathon day, they reduced pre-race insulin doses by 26 ± 8 %, and ingested 130 ± 33 g carbohydrate before, 91 ± 26 g during, and 115 ± 20 g after the race. During the run, blood glucose concentration fell from 14.4 ± 2.0 to 7.4 ± 3.0 mmol l⁻¹ (p < 0.05) and serum insulin from 51 ± 8 to 33 ± 8 pmol l⁻¹ (p < 0.05). Serum NEFA increased by 4-fold (p < 0.05), but fell to the normal level by next morning. Muscle glycogen content was 56 % lower (p < 0.05) and glycogen synthase fractional activity 40 % greater (p < 0.05) in the morning after the marathon as compared to the resting control day. In spite of glycogen depletion, whole body glucose disposal (euglycaemic insulin clamp) was unchanged, while glucose oxidation (indirect calorimetry) was decreased by 49 % (p < 0.05) and lipid oxidation increased by 41 % (p < 0.01). There was an inverse correlation between the rates of lipid oxidation and glucose uptake after the marathon (r = −0.75; p < 0.05). In conclusion: after successfully managed marathon running in patients with IDDM, insulin sensitivity was not increased in spite of low glycogen content and enhanced glycogen synthase activity after marathon, probably because of increased lipid oxidation. © 1997 by John Wiley & Sons, Ltd.     

Relationship of skeletal muscle glucose 6-phosphate to glucose disposal rate and glycogen synthase activity in insulin-resistant and non-insulin-dependent diabetic rhesus monkeys

Summary Reduced insulin action on skeletal muscle glycogen synthase activity and reduced whole-body insulin-mediated glucose disposal rates in insulin-resistant subjects may be associated with an alteration in muscle glucose transport (or phosphorylation) or with a defect distal to glucose 6-phosphate. To examine this issue we determined the glucose 6-phosphate concentration and glycogen synthase activity in muscle samples obtained under basal and euglycaemic hyperinsulinaemic clamp conditions in 27 rhesus monkeys (Macaca mulatta). They ranged from metabolically normal (n =11) to insulin-resistant (n =8) to overtly diabetic (non-insulin-dependent) (n =8). The glucose 6-phosphate measured under insulin-stimulated conditions was inversely correlated to insulin-stimulated glycogen synthase independent activity (r = –0.54, p<0.005), the change in glycogen synthase independent activity (insulin-stimulated minus basal) (r = –0.58, p<0.002) and to whole-body insulin-mediated glucose disposal rate (r = –0.60, p<0.002). The insulin-resistant and diabetic monkeys had significantly higher insulin-stimulated glucose 6-phosphate concentrations (0.57±0.11 and 0.62±0.11 nmol/mg dry weight, respectively) compared to the normal monkeys (0.29±0.05 nmol/mg dry weight) (p's <0.05). We conclude that under euglycaemic/hyperinsulinaemic conditions, a defect distal to glucose 6-phosphate is a major contributor to reduced whole-body insulin-mediated glucose disposal rates and to reduced insulin action on glycogen synthase in insulin-resistant and diabetic monkeys. [Diabetologia (1994) 37: 127–133] Received: 28 May 1993 and in revised form: 13 August 1993     

Partial rescue of in vivo insulin signalling in skeletal muscle by impaired insulin clearance in heterozygous carriers of a mutation in the insulin receptor gene

Aims/hypothesis Recently we reported the coexistence of postprandial hypoglycaemia and moderate insulin resistance in heterozygous carriers of the Arg1174Gln mutation in the insulin receptor gene (INSR). Controlled studies of in vivo insulin signalling in humans with mutant INSR are unavailable, and therefore the cellular mechanisms underlying insulin resistance in Arg1174Gln carriers remain to be clarified.Subjects, materials and methods We studied glucose metabolism and insulin signalling in skeletal muscle from six Arg1174Gln carriers and matched control subjects during a euglycaemic–hyperinsulinaemic clamp.Results Impaired clearance of exogenous insulin caused four-fold higher clamp insulin levels in Arg1174Gln carriers compared with control subjects (p<0.05). In Arg1174Gln carriers insulin increased glucose disposal and non-oxidative glucose metabolism (p<0.05), but to a lower extent than in controls (p<0.05). Insulin increased Akt phosphorylation at Ser473 and Thr308, inhibited glycogen synthase kinase-3α activity, reduced phosphorylation of glycogen synthase at sites 3a+3b, and increased glycogen synthase activity in Arg1174Gln carriers (all p<0.05). In the insulin-stimulated state, Akt phosphorylation at Thr308 and glycogen synthase activity were reduced in Arg1174Gln carriers compared with controls (p<0.05), whereas glycogen synthase kinase-3α activity and phosphorylation of glycogen synthase at sites 3a+3b were similar in the two groups.Conclusions/interpretation In vivo insulin signalling in skeletal muscle of patients harbouring the Arg1174Gln mutation is surprisingly intact, with modest impairments in insulin-stimulated activity of Akt and glycogen synthase explaining the moderate degree of insulin resistance. Our data suggest that impaired insulin clearance in part rescues in vivo insulin signalling in muscle in these carriers of a mutant INSR, probably by increasing insulin action on the non-mutated insulin receptors.     

Muscle enzyme activity and insulin sensitivity in Type 1 (insulin-dependent) diabetes mellitus

Summary The mechanisms of insulin insensitivity in diabetes are poorly understood. We have therefore assessed the relationship between glucose disposal during a euglycaemic clamp, muscle glycogen formation, and the activities of insulin regulated enzymes within skeletal muscle in five Type 1(insulin-dependent) diabetic patients, both on conventional injection therapy (HbA₁ 11.0±1.0 (SD) %) and after 6 weeks continuous subcutaneous insulin infusion (HbA₁ 7.6±1.4%,p < 0.01). On both regimens, overnight euglycaemia before the clamp was maintained with an intravenous insulin infusion. The increase in clamp glucose requirements (insulin 0.1 U kg^–1·h^–1) between injection therapy and continuous subcutaneous insulin infusion was significant (6.2±0.9 (SE) to 7.0 ± 0.9 mg·kg^–1·min^–1,p<0.05), but small compared to differences between subjects. Glucose requirement remained lower than in control subjects (10.4 ± 0.7 mg·kg^–1·min^–1,p < 0.05). The increase in muscle glycogen with the clamp was slightly higher on continuous subcutaneous insulin infusion (9.5 ± 2.5 mg/g protein) than on injection therapy (8.5 ± 2.4 mg/g,p < 0.05), but less than in control subjects (17.9 ± 2.1 mg/g,p < 0.05). The expressed activity of glycogen synthase and pyruvate dehydrogenase increased significantly between fasting and the end of the clamps in the patients (p < 0.001 and < 0.005), but was not significantly different between the two treatment regimens. Expressed glycogen synthase activity at the end of the clamp was lower on both treatments than in control subjects (p < 0.05). Both enzyme activities were, however, highly correlated with glucose requirement between patients, (r=0.89–0.94,p<0.05-0.02), and glycogen synthase was similarly correlated in the control subjects (r = 0.84,p < 0.05). Patients had significantly different enzyme activities, glucose requirement, and glycogen stored by analysis of variance (p < 0.05-0.01). Correlation of each enzyme activity between subjects on the two treatment regimens was also high (r=0.94–0.98,p < 0.02–0.01). At the end of the clamp the enzyme activities were themselves closely related (injectionsr = 0.99,p < 0.001; infusionr = 0.88,p < 0.05), and glycogen synthase activity predicted muscle glycogen deposition (r=0.94–0.97,p < 0.02–0.01). We suggest that: (1) preceding metabolic control has a relatively small influence on whole body insulin sensitivity measured immediately after careful overnight control; (2) insulin sensitivity derived from glucose clamp data is strongly related to skeletal muscle glycogen deposition and skeletal muscle enzyme activities.     

Intracellular Glucose and Fat Metabolism in Identical Twins Discordant for Non-insulin-dependent Diabetes Mellitus (NIDDM): Acquired Versus Genetic Metabolic Defects?

Intracellular glucose and lipid metabolism was studied in 12 identical twin pairs discordant for non-insulin-dependent (Type 2) diabetes mellitus (NIDDM) and 13 control subjects without family history of diabetes during low (baseline) and high plasma insulin concentrations, using the hyperinsulinaemic clamp technique combined with indirect calorimetry, tritiated water glycolytic flux rates and biopsy skeletal muscle glycogen synthase activity determinations. Baseline and insulin stimulated rates of lipid oxidation were elevated—and glucose oxidation decreased—in the NIDDM twins compared with the non-diabetic co-twins and controls (all p < 0.05). Baseline and insulin stimulated rates of glucose and lipid oxidation were similar in non-diabetic twins and controls. Exogenous glycolytic flux was decreased in NIDDM twins compared with both their non-diabetic co-twins and controls during clamp insulin measurements (p < 0.02), but similar in all study groups during baseline measurements. Insulin stimulated glucose disposal, exogenous glucose storage (glucose disposal–exogenous glycolytic flux) and skeletal muscle glycogen synthase activity were all significantly decreased in NIDDM twins compared with both their non-diabetic co-twins and controls. Furthermore, glucose disposal and glucose storage were decreased in the non-diabetic twins (n = 12) compared with controls (p < 0.05 both). However, insulin stimulated fractional skeletal muscle glycogen synthase activity was not significantly decreased in non-diabetic twins compared with controls. In conclusion: (1) the glucose fatty acid cycle plays a major role in the secondary—but not the primary—abnormalities of glucose metabolism in NIDDM; (2) insulin resistance in non-diabetic identical co-twins of NIDDM patients is restricted exclusively to the pathway of exogenous glucose storage; (3) however, the decreased glucose storage is not explained solely by an impairment of insulin stimulated skeletal muscle glycogen synthase activity; and finally (4) the impairment of skeletal muscle glycogen synthase activity in NIDDM has an apparent non-genetic component and can be escaped (or postponed) in individuals (twins) with a 100 % genetic predisposition to NIDDM.     

Morphometric documentation of abnormal intramyocellular fat storage and reduced glycogen in obese patients with Type II diabetes

Aims/hypothesis. Insulin resistance of skeletal muscle has been associated with increased lipid availability. This study aimed to estimate volume fractions of intramyocellular triglyceride droplets and glycogen granules in skeletal muscle using electron microscopy and furthermore, relate these findings to insulin sensitivity and the level of circulating lipids. Methods. We compared 11 obese patients with Type II (non-insulin-dependent) diabetes mellitus and 11 obese normoglycaemic subjects matched for age and sex. Glucose metabolism was determined using the euglycaemic hyperinsulinaemic clamp technique (40 mU · m^–2· min^–1) coupled with indirect calorimetry and tritiated glucose. On the second day, using an automatic procedure, a fasting muscle biopsy was carried out and processed for electron microscopy. Volume fractions of intramyocellular structures were estimated by pointcounting on photographic pictures in a blinded manner. Results. Insulin-stimulated total glucose disposal rate was lower in the Type II diabetic subjects compared with the obese normoglycaemic subjects (4.96 ± 049 vs 10.35 ± 0.89 mg · min^–1· kg ffm^–1, p < 0.001) as was glucose storage (2.03 ± 0.50 vs 6.59 ± 0.83, p < 0.001). The electron microscopy study revealed that the diabetic subjects had higher intramyocellular amounts of triglyceride (1.43 ± 0.21 vs 0.39 ± 0.07 %, p < 0.001) and lower amounts of glycogen (3.53 ± 0.33 vs 6.94 ± 0.54 %, p < 0.001). Mitochondrial volume was identical indicating equal aerobic capacity. The fractional intramyocellular lipid volume was found to be positively associated with fasting NEFA (r = 0.63, p = < 0.05 and r = 0.79, p = < 0.05) and triglyceride (r = 0.74, p = 0.01 and r = 0.62, p < 0.05) in the obese diabetic and normoglycaemic cohorts respectively. Intramyocellular lipid content was negatively correlated to insulin sensitivity (r = –0.71, p < 0.02) in the obese diabetic group whereas no significant association was found in the obese normoglycaemic group. Conclusion/interpretation. This study shows that fat accumulates intramyocellulary while glycogen stores are simultaneously reduced in obese subjects with Type II (non-insulin-dependent) diabetes mellitus. Quantitatively, a major component of the excessive lipid accumulation could be secondary in origin, related to the diabetic state in itself, although a contribution from the altered insulin action cascade of obesity and diabetes cannot be excluded. In both groups significant positive relations were found between circulating and intramyocellular lipid. [Diabetologia (2001) 44: 824–833] Received: 6 December 2000 and in revised form: 16 March 2001     

Peroxisome Proliferator-Activated Receptor (PPAR) γ and Retinoid X Receptor (RXR) agonists have complementary effects on glucose and lipid metabolism in human skeletal muscle

Aims/hypothesis. To determine the independent and potentially synergistic effects of agonists for PPARγ and RXR on glucose and lipid metabolism, as well as gene expression, in human skeletal muscle cell cultures. Methods. Fully differentiated myotubes from non-diabetic subjects and subjects with Type II (non-insulin-dependent) diabetes mellitus were chronically (2 days) treated with LG100 268 (4 μmol/l), an RXR agonist, or troglitazone (4.6 μmol/l), a PPARγ agonist or both, to determine the effects on glucose uptake, activity of glycogen synthase and palmitate oxidation. Results. The combination of both agents increased glucose uptake (60 ± 9 % compared to control subjects) but not either agent alone (16 ± 9 and 26 ± 6 % for LG100 268 and troglitazone, p < 0.01, respectively). The agent LG100 268 alone had little effect on the activity of glycogen synthase but the effect of troglitazone increased with LG100 268 (p < 0.05). With chronic exposure, LG100 268 upregulated palmitate oxidation (53 ± 12 % increase, p < 0.005), in a way similar to troglitazone (68 ± 23 %, p < 0.005). Synergism was observed when both agonists were combined (146 ± 38 %, p < 0.005 vs either agent alone). Treatment with either agent led to about a twofold increase in the expression of fatty acid transporter (FAT/CD36). Troglitazone upregulated PPARγ protein expression, whereas LG100 268 had no effect. Furthermore, neither LG100 268 nor troglitazone had any effect on the protein expression of RXR isoforms or PPARα. Conclusion/interpretation. Co-activation of PPARγ and RXR results in additive or synergistic effects on glucose and lipid metabolism in skeletal muscle, but unlike troglitazone, LG100 268 does not alter expression of its own receptor. [Diabetologia (2001) 44: 444–452] Received: 5 October 2000 and in revised form: 8 December 2000     

Hyperglycaemia compensates for the defects in insulin-mediated glucose metabolism and in the activation of glycogen synthase in the skeletal muscle of patients with Type 2 (non-insulin-dependent) diabetes mellitus

Summary Insulin resistance and a defective insulin activation of the enzyme glycogen synthase in skeletal muscle during euglycaemia may have important pathophysiological implications in Type 2 (non-insulin-dependent) diabetes mellitus. Hyperglycaemia may serve to compensate for these defects in Type 2 diabetes by increasing glucose disposal through a mass action effect. In the present study, rates of whole-body glucose oxidation and glucose storage were measured during fasting hyperglycaemia and isoglycaemic insulin infusion (40 mU·m^–2min^–1, 3 h) in 12 patients with Type 2 diabetes. Eleven control subjects were studied during euglycaemia. Biopsies were taken from the vastus lateralis muscle. Fasting and insulin-stimulated glucose oxidation, glucose storage and muscle glycogen synthase activation were all fully compensated (normalized) during hyperglycaemia in the diabetic patients. The insulin-stimulated increase in muscle glycogen content was the same in the diabetic patients and in the control subjects. Besides hyperglycaemia, the diabetic patients had elevated muscle free glucose and glucose 6-phosphate concentrations. A positive correlation was demonstrated between intracellular free glucose concentration and muscle glycogen synthase fractional velocity insulin activation (0.1 mmol/l glucose 6-phosphate: r=0.65, p<0.02 and 0.0 mmol/l glucose 6-phosphate: r= 0.91, p<0.0001). In conclusion, this study indicates an important role for hyperglycaemia and elevated muscle free glucose and glucose 6-phosphate concentrations in compensating (normalizing) intracellular glucose metabolism and skeletal muscle glycogen synthase activation in Type 2 diabetes.     

In vivo insulin action and muscle glycogen synthase activity in Type 2 (non-insulin-dependent) diabetes mellitus: effects of diet treatment

Summary Insulin resistant glucose metabolism is a key element in the pathogenesis of Type 2 (non-insulin-dependent) diabetes mellitus. Insulin resistance may be of both primary (genetic) and secondary (metabolic) origin. Before and after diet-induced improvement of glycaemic control seven obese patients with newly-diagnosed Type 2 diabetes were studied with the euglycaemic clamp technique in combination with indirect calorimetry and forearm glucose balance. Muscle biopsies were obtained in the basal state and again after 3 h of hyperinsulinaemia (200 mU/l) for studies of insulin receptor and glycogen synthase activities. Similar studies were performed in seven matched control subjects. Insulin-stimulated glucose utilization improved from 110±11 to 183±23 mg·m^–2·min^–1 (p<0.03); control subjects: 219+23 mg·m^–2·min^–1 (p=NS, vs post-diet Type 2 diabetes). Nonoxidative glucose disposal increased from 74±17 to 138+19 mg·m^–2·min^–1 (p<0.03), control subjects: 159±22 mg· m^–2·m^–1 (p=NS, vs post-diet Type 2 diabetic patients). Forearm blood glucose uptake during hyperinsulinaemia increased from 1.58±0.54 to 3.35±0.23 mol·l^–1·min^–1 (p<0.05), control subjects: 2.99±0.86 mol·l^–1·min^–1 (p=NS, vs post-diet Type 2 diabetes). After diet therapy the increase in insulin sensitivity correlated with reductions in fasting plasma glucose levels (r=0.97, p<0.001), reductions in serum fructosamine (r=0.77, p<0.05), and weight loss (r=0.78, p<0.05). Values of muscle glycogen synthase sensitivity to glucose 6-phosphate (A_0.5 for glucose 6-phosphate) were similar in the basal state. However, insulin stimulation of glycogen synthase was more pronounced after diet treatment (A_0.5: 0.43±0.06 (before) vs 0.30±0.04 mmol/l (after); p<0.03; control subjects: 0.22±0.03 mmol/l). Muscle insulin receptor binding and kinase activity were similar before and after diet treatment and comparable to values in the control group. The data suggest that impaired insulin stimulation of in vivo glucose turn-over and muscle glycogen synthase activity tend to be restored during successful diet treatment of patients with Type 2 diabetes.     

Insulin Infusion Normalizes Cardiovascular Responses and Plasma Noradrenaline after Oral Glucose in Type 1 (Insulin-dependent) Diabetes

ABSTRACT. We examined whether the abnormal regulation of the cardiovascular system and plasma noradrenaline observed after oral glucose in insulin-dependent diabetic patients could be normalized by intravenous infusion of insulin. Eight patients with type 1 (insulin-dependent) diabetes were examined after an oral glucose load with and without simultaneous infusion of insulin. Insulin infusion increased plasma insulin from 0.07 to 0.31 nmol/1. In the control experiment (glucose only), mean heart rate and mean arterial systolic blood pressure remained unchanged and plasma noradrenaline (NA) decreased (p < 0.05). After oral glucose plus intravenous insulin, mean heart rate increased by 11 % and mean systolic blood pressure by 5 % (p < 0.05, p < 0.01), whereas plasma NA did not change significantly. The present study indicates that physiologic increments in plasma insulin concentration are of importance in the regulation of the cardiovascular system and plasma NA following an oral glucose load.     

Inhibition of adipose tissue lipolysis increases intramuscular lipid use in type 2 diabetic patients

Aims/hypothesis In the present study, we investigated the consequences of adipose tissue lipolytic inhibition on skeletal muscle substrate use in type 2 diabetic patients.Materials and methods We studied ten type 2 diabetic patients under the following conditions: (1) at rest; (2) during 60 min of cycling exercise at 50% of maximal workload capacity and subsequent recovery. Studies were done under normal, fasting conditions (control trial: CON) and following administration of a nicotinic acid analogue (low plasma non-esterified fatty acid trial: LFA). Continuous [U-¹³C]palmitate and [6,6 -²H₂]glucose infusions were applied to quantify plasma NEFA and glucose oxidation rates, and to estimate intramuscular triacylglycerol (IMTG) and glycogen use. Muscle biopsies were collected before and after exercise to determine net changes in lipid and glycogen content specific to muscle fibre type.Results Following administration of the nicotinic acid analogue (Acipimox), the plasma NEFA rate of appearance was effectively reduced, resulting in lower NEFA concentrations in the LFA trial (p<0.001). Plasma NEFA oxidation rates were substantially reduced at rest, during exercise and subsequent recovery in the LFA trial. The lower plasma NEFA oxidation rates were compensated by an increase in IMTG and endogenous carbohydrate use (p<0.05). Plasma glucose disposal rates did not differ between trials. In accordance with the tracer data, a greater net decline in type I muscle fibre lipid content was observed following exercise in the LFA trial (p<0.05).Conclusions/interpretation This study shows that plasma NEFA availability regulates IMTG use, and that adipose tissue lipolytic inhibition, in combination with exercise, could be an effective means of augmenting intramuscular lipid and glycogen use in type 2 diabetic patients in an overnight fasted state.     

Reduced glycogen synthase activity in skeletal muscle from obese patients with and without Type 2 (non-insulin-dependent) diabetes mellitus

Summary In order to evaluate the importance of a defect in insulin mediated non-oxidative glucose metabolism and glycogen synthase activity in skeletal muscles in obese subjects with and without Type 2 (non-insulin-dependent) diabetes mellitus we studied: 10 lean and 10 obese control subjects and 12 obese diabetic patients using the euglycaemic hyperinsulinaemic clamp technique (basal, 20 mU · (m²)^–1 · min^–1, 80mU·(m²)^–1·min^–1) in combination with indirect calorimetry. Muscle biopsies were taken from m. vastus lateralis at each insulin level. We found that non-oxidative glucose metabolism could be stimulated by insulin in all three groups (p<0.01). The values obtained at the highest insulin levels (around 140 U/ml) were lower in both obese groups compared to the lean control subjects (118±21, 185±31, 249±14 mg·(m²)^–1·min^–1 (p< 0.01)). Insulin stimulation of the glycogen synthase activity at a glucose-6-phosphate concentration of 0.1 mmol/l was absent in both obese groups, while activities increased significantly in the lean control subjects (19.6±4.2% to 45.6±6.8%, p< 0.01). Glycogen synthase activities at the highest insulin concentrations only differed significantly between lean control subjects and obese diabetic patients (45±7% and 31±5%, p< 0.05). We conclude that insulin resistance in peripheral tissues in obese subjects with and without Type 2 diabetes may be partly explained by a reduced insulin mediated non-oxidative glucose metabolism and that this abnormality might be due to an absent insulin stimulation of glycogen synthase in skeletal muscles. This enzyme defect is correlated to obesity itself.     

Fetal Growth and Insulin Resistance in Adult Life: Relationship Between Glycogen Synthase Activity in Adult Skeletal Muscle and Birthweight

Growth retardation during fetal life is associated with insulin insensitivity and an increased prevalence of impaired glucose tolerance in adult life. Because insulin-mediated stimulation of glycogen synthase may be an important rate-limiting step for insulin action at the cellular level, we have sought to determine whether impaired activation of muscle glycogen synthase is linked with early growth retardation. Postprandial glycogen synthase activity was therefore measured in muscle biopsies from a group of 27 women with normal glucose tolerance aged around 50 who were born in Preston, Lancashire, whose birthweight and body size at birth were recorded. Glycogen synthase activity measured at 0.1 mmol 1⁻¹ glucose-6-phosphate correlated with insulin sensitivity as measured by a short insulin tolerance test (r = 0.42, p < 0.05) and the waist to hip ratio (r = −0.48, p < 0.01), but not body mass index, body fat percentage or age. Within the group of women with normal glucose tolerance there was no relationship between intra-uterine growth as evidenced by birthweight or body size at birth and the response to insulin of skeletal muscle glycogen synthase in adult life. Thus we found no evidence for a direct link between fetal growth and insulin sensitivity in this pathway.     

Increased plasma leptin in gestational diabetes

Aims/hypothesis. Insulin resistance as well as marked changes in body weight and energy metabolism are associated with pregnancy. Its impact on plasma leptin is not known and was determined in this longitudinal study in both diabetic and normal pregnancy. Methods. At 28 gestational weeks plasma concentrations of leptin and B-cell hormones were measured at fasting and after an oral glucose load (OGTT:75 g) in women with gestational diabetes and pregnant women with normal glucose tolerance and compared with women who were not pregnant (C). Results. Plasma leptin (ng/ml) was higher (p < 0.001) in women with gestational diabetes (24.9 ± 1.6) than in women with normal glucose tolerance (18.2 ± 1.5) and increased in both groups when compared with the non-pregnant women (8.2 ± 1.3; p < 0.0005). No change in plasma leptin concentrations was induced by OGTT in any group. Basal insulin release was higher (p < 0.05) in women with gestational diabetes compared with the pregnant women with normal glucose tolerance. Marked insulin resistance was confirmed by a 20 % lower (p < 0.05) insulin sensitivity in subgroup analysis and a decrease of almost 40 % in fasting glucose/insulin ratio (p < 0.005) in women with gestational diabetes. Leptin correlated in women with gestational diabetes with basal plasma concentrations of glucose (p < 0.02), insulin (p < 0.004) and proinsulin (p < 0.01) as well as with BMI (p < 0.001) and overall pregnancy induced maternal weight gain (p < 0.009). With normalisation of blood glucose 8 weeks after delivery in women with gestational diabetes their plasma leptin decreased (p < 0.0005) to 17.3 ± 1.9 ng/ml but did not completely normalize (p < 0.05 vs non-pregnant women). Conclusion/interpretation. Our data show that women with gestational diabetes without any change in plasma leptin upon oral glucose loading have increased plasma leptin concentrations during and after pregnancy, a clear association of plasma leptin with the respective concentration of glucose and insulin resistance as well as with changes in body weight, and a failure to normalize spontaneously BMI to the same extent as pregnant women with normal glucose tolerance when compared with matched control subjects. [Diabetologia (2001) 44: 164–172] Received: 11 July 2000 and in revised form: 10 October 2000     

Non-esterified fatty acids regulate lipid and glucose oxidation and glycogen synthesis in healthy man

Summary We examined the interrelationship of lipid and glucose metabolism in the basal state and during insulin stimulus in 19 healthy men (27±2 years, body mass index 23.6±0.6 kg/m²). In each subject, we performed a 4-h euglycaemic (5.3±0.1 mmol/l) hyperinsulinaemic (647±21 pmol/l) insulin clamp with indirect calorimetry in the basal state and during insulin infusion, and muscle biopsies before and at the end of the clamp. In the basal state, serum non-esterified fatty acid levels correlated directly with lipid oxidation (r =0.56, p<0.05) and indirectly with glucose oxidation (r = –0.80, p<0.001). Lipid and glucose oxidation rates were inversely related in the basal state (r = –0.47, p<0.05) and during insulin infusion (r = –0.65, p<0.01). Basal lipid oxidation and glycogen synthase total activity correlated inversely (r = –0.54, p<0.05). Lipid oxidation both in the basal state (r = –0.61, p<0.01) and during insulin infusion (r = –0.62, p<0.05) was inversely related to muscle glycogen content after the insulin clamp. Fasting plasma triglyceride concentration correlated directly to fasting insulin (r =0.55, p<0.05) and C-peptide (r =0.50, p<0.03) concentrations and inversely to non-oxidative glucose disposal rate at the end of clamp (r = –0.54, p<0.05). In conclusion: 1) Serum non-esterified fatty acid concentration enhances lipid and reduces glucose oxidation. 2) Lipid oxidation is inversely related to total glycogen synthase activity. 3) Lipid oxidation both in the basal state and during insulin stimulus correlates inversely with muscle glycogen content after insulin infusion. 4) Even in normotriglyceridaemic subjects, plasma triglycerides reduce insulin-stimulated non-oxidative glucose disposal. These data suggest that serum non-esterified fatty acids in physiologic concentrations have an important role in the regulation of lipid and glucose oxidation as well as glucose storage as glycogen. [Diabetologia (1994) 37: 202–209] Received: 2 June 1993 and in revised form: 27 August 1993     

Inhibition of muscle glycogen synthase activity and non-oxidative glucose disposal during hypoglycaemia in normal man

Summary The purpose of the present study was to evaluate the role of muscle glycogen synthase activity in the reduction of glucose uptake during hypoglycaemia. Six healthy young men were examined twice; during 120 min of hyperinsulinaemic (1.5 mU · kg^–1 · min^–1) euglycaemia followed by: 1) 240 min of graded hypoglycaemia (plasma glucose nadir 2.8 mmol/l) or 2) 240 min of euglycaemia. At 350–360 min a muscle biopsy was taken and indirect calorimetry was performed at 210–240 and 330–350 min. Hypoglycaemia was associated with markedly increased levels of adrenaline, growth hormone and glucagon and also with less hyperinsulinaemia. During hypoglycaemia the fractional velocity for glycogen synthase was markedly reduced; from 29.8±2.3 to 6.4±0.9%, p<0.05. Total glucose disposal was decreased during hypoglycaemia (5.58±0.55 vs 11.01±0.75 mg · kg^–1 · min^–1 (euglycaemia); p<0.05); this was primarily due to a reduction of non-oxidative glucose disposal (2.43±0.41 vs 7.15±0.7 mg · kg^–1 · min^–1 (euglycaemia); p<0.05), whereas oxidative glucose disposal was only suppressed to a minor degree. In conclusion hypoglycaemia virtually abolishes the effect of insulin on muscle glycogen synthase activity. This is in keeping with the finding of a marked reduction of non-oxidative glucose metabolism.Abbreviations HGP Hepatic glucose production - Rd glucose disposal - GH growth hormone - 3-OHB 3-hydroxybutyrate - G 6-P glucose 6-phosphate - NEFA non-esterified fatty acids - PP-1 phosphatase-1 - Ra rate of appearance - Rd-nonox non-oxidative glucose disposal - Rd-ox oxidative glucose disposal - A0.5 half-maximal activity     

Oxidative stress could precede endothelial dysfunction and insulin resistance in Indian Mauritians with impaired glucose metabolism

Aims/hypothesis. To measure oxidative stress, endothelial dysfunction and insulin resistance in Indian Mauritians at different stages of development of Type II (non-insulin-dependent) diabetes mellitus. Methods. Plasma total 8-epi-PGF₂α, an indicator of oxidative stress, was determined in age-matched subjects with normal glucose metabolism (n = 39), impaired glucose tolerance (n = 14), newly diagnosed diabetes (n = 8) and established diabetes (n = 14). Plasma glucose and insulin were measured at baseline and 2 h following an oral glucose tolerance test. Endothelial function was assessed by non-invasive digital pulse wave photoplethysmography. Results. Plasma 8-epi-PGF₂α increased in subjects with impaired glucose tolerance (p < 0.05) compared with control subjects, and was even higher in newly diagnosed diabetic patients (p < 0.01) and established (p < 0.01) diabetic patients. A tendency towards reduced endothelial function in subjects with impaired glucose tolerance became significant in patients with newly diagnosed and established diabetes (p < 0.01), and was correlated with 8-epi-PGF₂α (r = 0.36, p < 0.01). Insulin resistance (homeostasis model assessment) did not change in subjects with impaired glucose tolerance compared with control subjects, but increased in newly diagnosed (p < 0.01) and established (p < 0.001) diabetic subjects. The 8-epi-PGF₂α was correlated with fasting glucose (r = 0.50, p < 0.001), triglycerides (r = 0.40, p < 0.001) and insulin resistance (r = 0.35, p < 0.001). Conclusion/interpretation. Oxidant stress is an early event in the evolution of Type II diabetes and could precede the development of endothelial dysfunction and insulin resistance. [Diabetologia (2001) 44: 706–712 Received: 1 November 2001 and in revised form: 14 February 2001     

Glucose tolerance and insulin secretion in children of mothers with pregestational IDDM or gestational diabetes

Summary The offspring of mothers with diabetes mellitus during pregnancy are presumed to develop altered glucose homeostasis. We analysed metabolic parameters at birth and glucose tolerance and insulin secretion during oral glucose tolerance tests at 1–9 years of age in 129 children born to mothers with pregestational insulin-dependent diabetes (IDDM) and 69 infants of gestational diabetic mothers. Newborns of IDDM mothers displayed higher insulin (p < 0.001), glucose (p < 0.05), and insulin/glucose ratios (p < 0.002) than newborns of gestational diabetic mothers. During childhood, frequencies of impaired glucose tolerance (IGT) rose in infants of IDDM mothers from 9.4 % at 1–4 years to 17.4 % at 5–9 years of age, while in children of gestational diabetic mothers an increase from 11.1 % up to 20.0 % was observed. Offspring of gestational diabetic mothers displayed higher stimulated blood glucose (p < 0.025) than infants of IDDM mothers, while children of IDDM mothers showed higher stimulated insulin (p < 0.025), accompanied by increased fasting and stimulated insulin/glucose ratios (p < 0.05 and p < 0.02, respectively). Stimulated insulin in childhood was positively correlated to insulin at birth (p < 0.05). Furthermore, insulin/glucose ratio in childhood showed a positive correlation to insulin (p < 0.01) and insulin/glucose ratio at birth (p < 0.005). In conclusion, a pathogenetic role of fetal and neonatal hyperinsulinism for the development of IGT in both groups of infants of diabetic mothers is suggested, in particular for early induction of insulin resistance in the offspring of mothers with pregestational IDDM. [Diabetologia (1997) 40: 1097–1100] Received: 24 February 1997 and in revised form: 7 May 1997     

Multiple defects of both hepatic and peripheral intracellular glucose processing contribute to the hyperglycaemia of NIDDM

Summary Non-insulin-dependent diabetic (NIDDM) patients were studied during a modified euglycaemic state when fasting hyperglycaemia was normalized by a prior (–210 to –150 min) — and later withdrawn (–150–0 min) — intravenous insulin infusion. Glucose metabolism was assessed in NIDDM patients (n=10) and matched control subjects (n=10) using tritiated glucose turnover rates, indirect calorimetry and skeletal muscle glycogen synthase activity determinations. Total and non-oxidative exogenous glycolytic flux rates were measured using appearance rates of tritiated water. A+180 min euglycaemic hyperinsulinaemic (40 mU·m^–2·min^–1) clamp was performed to determine the insulin responsiveness of the various metabolic pathways. Plasma glucose concentration increased spontaneously during baseline measurements in the NIDDM patients (–120 to 0 min: 4.8±0.3 to 7.0±0.3 mmol/l; p<0.01), and was primarily due to an elevated rate of hepatic glucose production (3.16±0.13 vs 2.51±0.16 mg·kg FFM^–1·min^–1; p<0.01). In the NIDDM subjects baseline glucose oxidation was decreased (0.92±0.17 vs 1.33±0.14 mg·kg FFM^–1·min^–1; p<0.01) in the presence of a normal rate of total exogenous glycolytic flux and skeletal muscle glycogen synthase activity. The simultaneous finding of an increased lipid oxidation rate (1.95±0.13 vs 1.61±0.07 mg·kg FFM^–1·min^–1; p=0.05) and increased plasma lactate concentrations (0.86±0.05 vs 0.66±0.03 mmol/l; p=0.01) are consistent with a role for both the glucose-fatty acid cycle and the Cori cycle in the maintenance and development of fasting hyperglycaemia in NIDDM during decompensation. Insulin resistance was demonstrated during the hyperinsulinaemic clamp in the NIDDM patients with a decrease in the major peripheral pathways of intracellular glucose metabolism (oxidation, storage and muscle glycogen synthase activity), but not in the pathway of non-oxidative glycolytic flux which was not completely suppressed during insulin infusion in the NIDDM patients (0.55±0.15 mg·kg FFM^–1·min^–1; p<0.05 vs 0; control subjects: 0.17±0.29; NS vs 0). Thus, these data also indicate that the defect(s) of peripheral (skeletal muscle) glucose processing in NIDDM goes beyond the site of glucose transport across the cell membrane.Abbreviations NIDDM Non-insulin-dependent diabetes mellitus - FFM fat free mass - HGP hepatic glucose production - R_d peripheral glucose disposal (uptake) rate - G6P glucose 6-phosphate - UDPG uridine diphosphate glucose - FV fractional velocity