Impaired insulin-stimulated muscle glycogen synthase activation in vivo in man is related to low fasting glycogen synthase phosphatase activity

Insulin action at a molecular level - 100 years of progress

The discovery of insulin 100 years ago and its application to the treatment of human disease in the years since have marked a major turning point in the history of medicine. The availability of purified insulin allowed for the establishment of its physiological role in the regulation of blood glucose and ketones, the determination of its amino acid sequence, and the solving of its structure. Over the last 50 years, the function of insulin has been applied into the discovery of the insulin receptor and its signaling cascade to reveal the role of impaired insulin signaling-or resistance-in the progression of type 2 diabetes. It has also become clear that insulin signaling can impact not only classical insulin-sensitive tissues, but all tissues of the body, and that in many of these tissues the insulin signaling cascade regulates unexpected physiological functions. Despite these remarkable advances, much remains to be learned about both insulin signaling and how to use this molecular knowledge to advance the treatment of type 2 diabetes and other insulin-resistant states.

Skeletal muscle insulin resistance in endocrine disease

We summarize the existing literature data concerning the involvement of skeletal muscle (SM) in whole body glucose homeostasis and the contribution of SM insulin resistance (IR) to the metabolic derangements observed in several endocrine disorders, including polycystic ovary syndrome (PCOS), adrenal disorders and thyroid function abnormalities. IR in PCOS is associated with a unique postbinding defect in insulin receptor signaling in general and in SM in particular, due to a complex interaction between genetic and environmental factors. Adrenal hormone excess is also associated with disrupted insulin action in peripheral tissues, such as SM. Furthermore, both hyper- and hypothyroidism are thought to be insulin resistant states, due to insulin receptor and postreceptor defects. Further studies are definitely needed in order to unravel the underlying pathogenetic mechanisms. In summary, the principal mechanisms involved in muscle IR in the endocrine diseases reviewed herein include abnormal phosphorylation of insulin signaling proteins, altered muscle fiber composition, reduced transcapillary insulin delivery, decreased glycogen synthesis, and impaired mitochondrial oxidative metabolism.

Muscle glucose transport and phosphorylation in type 2 diabetic, obese nondiabetic, and genetically predisposed individuals

Our objectives were to quantitate insulin-stimulated inward glucose transport and glucose phosphorylation in forearm muscle in lean and obese nondiabetic subjects, in lean and obese type 2 diabetic (T2DM) subjects, and in normal glucose-tolerant, insulin-resistant offspring of two T2DM parents. Subjects received a euglycemic insulin (40 mU.m(-2).min(-1)) clamp with brachial artery/deep forearm vein catheterization. After 120 min of hyperinsulinemia, a bolus of d-mannitol/3-O-methyl-d-[(14)C]glucose/d-[3-(3)H]glucose (triple-tracer technique) was given into brachial artery and deep vein samples obtained every 12-30 s for 15 min. Insulin-stimulated forearm glucose uptake (FGU) and whole body glucose metabolism (M) were reduced by 40-50% in obese nondiabetic, lean T2DM, and obese T2DM subjects (all P < 0.01); in offspring, the reduction in FGU and M was approximately 30% (P < 0.05). Inward glucose transport and glucose phosphorylation were decreased by approximately 40-50% (P < 0.01) in obese nondiabetic and T2DM groups and closely paralleled the decrease in FGU. The intracellular glucose concentration in the space accessible to glucose was significantly greater in obese nondiabetic, lean T2DM, obese T2DM, and offspring compared with lean controls. We conclude that 1) obese nondiabetic, lean T2DM, and offspring manifest moderate-to-severe muscle insulin resistance (FGU and M) and decreased insulin-stimulated glucose transport and glucose phosphorylation in forearm muscle; these defects in insulin action are not further reduced by the combination of obesity plus T2DM; and 2) the increase in intracelullar glucose concentration under hyperinsulinemic euglycemic conditions in obese and T2DM groups suggests that the defect in glucose phosphorylation exceeds the defect in glucose transport.

Chronic calorie restriction alters glycogen metabolism in rhesus monkeys

Chronic caloric restriction (CR) prevents the development of obesity and maintains health, slows aging processes, and prevents or substantially delays the development of non-insulin-dependent diabetes. Because changes in energy metabolism could be involved in all of these positive effects of CR, we examined glycogen synthase (GS) and glycogen phosphorylase (GP) activities and glucose 6-phosphate (G6P) and glycogen concentrations in skeletal muscle samples before and during a euglycemic hyperinsulinemic clamp in 6 older aged monkeys in which CR had been continued for 10.4 +/- 2.1 years. Basal GS activity (fractional velocity and independent) was significantly higher in the CR monkeys than has been previously shown in normal, hyperinsulinemic and diabetic monkeys. The normal effect of insulin to activate GS was absent in the CR group due to the paradoxical finding in some of these monkeys of a reduction in GS activity by insulin. Insulin also had the unexpected effect of increasing the independent activity of GP above basal activity (p<0.05). There was an inverse relationship between the change (insulin-stimulated minus basal) in GS fractional velocity and GP activity ratio (r=-0.91, p<0.005). The basal independent activities of GS and GP were also inversely correlated (r=-0.79, p<0.05). The insulin-stimulated concentration of G6P tended to be higher than the basal concentration (p<0.06) and was significantly higher than that previously shown in normal monkeys (p<0.05). We suggest that long-term calorie restriction (1) results in alterations in glycogen metabolism that may be important to the anti-diabetogenic and antiaging effects of CR and (2) unmasks early defects which may indicate the likelihood of ultimately developing obesity and diabetes.

Male preponderance in early diagnosed type 2 diabetes is associated with the ARE insertion/deletion polymorphism in the PPP1R3A locus

BACKGROUND

The ARE insertion/deletion polymorphism of PPP1R3A has been associated with variation in glycaemic parameters and prevalence of diabetes. We have investigated its role in age of diagnosis, body weight and glycaemic control in 1,950 individuals with type 2 diabetes in Tayside, Scotland, and compared the ARE2 allele frequencies with 1,014 local schoolchildren.

RESULTS

Men homozygous for the rarer allele (ARE2) were younger at diagnosis than ARE1 homozygotes (p = 0.008). Conversely, women ARE2 homozygotes were diagnosed later than ARE1 homozygotes (p = 0.036). Thus, men possessing the rarer (ARE2) allele were diagnosed with type 2 diabetes earlier than women (p < 0.000001). In contrast, there was no difference in age of diagnosis by gender in those individuals carrying only the common ARE1 variant. Furthermore, although there was no difference in the frequency between the children and the type 2 diabetic population overall, marked differences in allele frequencies were noted by gender and age-of diagnosis. The ARE2 allele frequency in early diagnosed males (diagnosed earlier than the first quartile of the overall ages at diagnosis) was higher than that found in both later diagnosed males and healthy children (p = 0.021 and p = 0.03 respectively). By contrast, the frequency in early diagnosed females was significantly lower than later diagnosed females and that found in children (p = 0.021 and p = 0.037). Comparison of the male to female ratios at different ages-diagnosed confirms a known phenomenon that men are much more prone to early type 2 diabetes than women. When this feature was examined by the common ARE 1/1 genotype we found that the male to female ratio remained at unity with all ages of diagnosis, however, carriers of the ARE2 variant displayed a marked preponderance of early male diagnosis (p = 0.003).

CONCLUSION

The ARE2 allele of PPP1R3A is associated with a male preponderance to early diagnosed type 2 diabetes. Susceptibility to type 2 diabetes in later life is not modulated by the ARE2 allele in either sex.

Differential regulation of protein dynamics in splanchnic and skeletal muscle beds by insulin and amino acids in healthy human subjects

To determine the in vivo effect of amino acids (AAs) alone or in combination with insulin on splanchnic and muscle protein dynamics, we infused stable isotope tracers of AAs in 36 healthy subjects and sampled from femoral artery and vein and hepatic vein. The subjects were randomized into six groups and were studied at baseline and during infusions of saline (group 1), insulin (0.5 mU. kg(-1). min(-1)) (group 2), insulin plus replacement of AAs (group 3) insulin plus high-dose AAs (group 4), or somatostatin and baseline replacement doses of insulin, glucagon and GH plus high dose of AAs (group 5) or saline (group 6). Insulin reduced muscle release of AAs mainly by inhibition of protein breakdown. Insulin also enhanced AA-induced muscle protein synthesis (PS) and reduced leucine transamination. The main effect of AAs on muscle was the enhancement of PS. Insulin had no effect on protein dynamics or leucine transamination in splanchnic bed. However, AAs reduced protein breakdown and increased synthesis in splanchnic bed in a dose-dependent manner. AAs also enhanced leucine transamination in both splanchnic and muscle beds. Thus insulin's anabolic effect was mostly on muscle, whereas AAs acted on muscle as well as on splanchnic bed. Insulin achieved anabolic effect in muscle by inhibition of protein breakdown, enhancing AA-induced PS, and reducing leucine transamination. AAs largely determined protein anabolism in splanchnic bed by stimulating PS and decreasing protein breakdown.

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.

Decreased insulin action in skeletal muscle from patients with McArdle's disease

Insulin action is decreased by high muscle glycogen concentrations in skeletal muscle. Patients with McArdle's disease have chronic high muscle glycogen levels and might therefore be at risk of developing insulin resistance. In this study, six patients with McArdle's disease and six matched control subjects were subjected to an oral glucose tolerance test and a euglycemic-hyperinsulinemic clamp. The muscle glycogen concentration was 103 +/- 45% higher in McArdle patients than in controls. Four of six McArdle patients, but none of the controls, had impaired glucose tolerance. The insulin-stimulated glucose utilization and the insulin-stimulated increase in glycogen synthase activity during the clamp were significantly lower in the patients than in controls (51.3 +/- 6.0 vs. 72.6 +/- 13.1 micromol x min(-1) x kg lean body mass(-1), P < 0.05, and 53 +/- 15 vs. 79 +/- 9%, P < 0.05, n = 6, respectively). The difference in insulin-stimulated glycogen synthase activity between the pairs was significantly correlated (r = 0.96, P < 0.002) with the difference in muscle glycogen level. The insulin-stimulated increase in Akt phosphorylation was smaller in the McArdle patients than in controls (45 +/- 13 vs. 76 +/- 13%, P < 0.05, respectively), whereas basal and insulin-stimulated glycogen synthase kinase 3alpha and protein phosphatase-1 activities were similar in the two groups. Furthermore, the ability of insulin to decrease and increase fat and carbohydrate oxidation, respectively, was blunted in the patients. In conclusion, these data show that patients with McArdle's glycogen storage disease are insulin resistant in terms of glucose uptake, glycogen synthase activation, and alterations in fuel oxidation. The data further suggest that skeletal muscle glycogen levels play an important role in the regulation of insulin-stimulated glycogen synthase activity.

Free fatty acids induce peripheral insulin resistance without increasing muscle hexosamine pathway product levels in rats

To evaluate the role of the hexosamine biosynthesis pathway (HBP) in fat-induced insulin resistance, we examined whether fat-induced insulin resistance is additive to that induced by increased HBP flux via glucosamine infusion and, if so, whether such additive effects correlate with muscle HBP product levels. Prolonged hyperinsulinemic (approximately 550 pmol/l) euglycemic clamps were conducted in conscious overnight-fasted rats. After the initial 150 min to attain steady-state insulin action, rats received an additional infusion of saline, Intralipid, glucosamine, or Intralipid and glucosamine (n = 8 or 9 for each) for 330 min. At the conclusion of clamps, skeletal muscles (soleus, extensor digitorum longus, and tibialis anterior) were taken for the measurement of HBP product levels. Intralipid and glucosamine infusions decreased insulin-stimulated glucose uptake (Rd) by 38 and 28%, respectively. When the infusions were combined, insulin-stimulated Rd decreased 47%, significantly more than with Intralipid or glucosamine alone (P < 0.05). The glucosamine-induced insulin resistance was associated with four- to fivefold increases in muscle HBP product levels. In contrast, the Intralipid-induced insulin resistance was accompanied by absolutely no increase in HBP product levels in all of the muscles examined. Also, when infused with glucosamine, Intralipid decreased insulin action below that with glucosamine alone without changing HBP product levels. In a separate study, short-term (50 and 180 min) Intralipid infusion also failed to increase muscle HBP product levels. In conclusion, increased availability of plasma free fatty acids induces peripheral insulin resistance without increasing HBP product levels in skeletal muscle.

Acetyl-L-carnitine infusion increases glucose disposal in type 2 diabetic patients

Little information is available in the literature on the effect of L-carnitine to improve glucose disposal in healthy control subjects and type 2 diabetic patients. No data are reported on the pharmacological properties of acetyl-L-carnitine (ALC) in type 2 diabetes mellitus. The present study evaluates glucose uptake and oxidation rates with either ALC or placebo administration in 18 type 2 diabetic patients. On different days, each patient received both a primed-constant infusion of ALC (5 mg/kg body weight [BW] priming bolus and either 0.025, 0.1, or 1.0 mg/kg BW/min constant infusion) and a comparable placebo formulation. During the infusion period, continuous indirect calorimetric monitoring and a euglycemic-hyperinsulinemic clamp (EHC) study were performed. The total end-clamp glucose tissue uptake (M value) was significantly increased by the administration of ALC (from 3.8 to 5.2 mg/kg/min, P = .006), and the dose dependence of this effect reached borderline statistical significance (P = .037). The increase in the M/I ratio was also highly significant after ALC administration (from 3.9 to 5.8 x 10(-2) mg/kg/min/(microUI/mL, P < .001), while no statistically significant effect was attributable to the different dosages. The increase in the M value was related to increased glucose storage (highly significant effect of ALC) rather than increased glucose oxidation (no statistical significance). In conclusion, the effect of ALC on glucose disposal has no relationship to the amount administered. This could be due to an effect of ALC on the enzymes involved in both the glycolytic and gluconeogenetic pathways, and a possible reversibility of glycogen synthase inhibition in diabetic subjects.

Paradoxical phosphorylation of skeletal muscle glycogen synthase by in vivo insulin in very lean young adult rhesus monkeys

Calorie restriction (CR) has previously been shown to unexpectedly induce a reversal of in vivo insulin action (phosphorylation instead of dephosphorylation) on skeletal muscle glycogen synthase (GS) in four out of six long-term calorie-restricted (CR) monkeys. The purpose of the present study was to determine whether this increase in Ka (concentration of glucose 6-phosphate [G6P] at which GS activity is half-maximal) during insulin is also present in very lean (VL) young adult monkeys maintained on a controlled feeding regimen. Muscle samples from 10 VL monkeys (10 +/- 2% body fat; 7 years old) were obtained before and during a euglycemic hyperinsulinemic clamp and the Ka was determined and compared to the Ka of two other groups of monkeys, one matched in age but fully ad libitum (AL)-fed (n = 9.8 +/- 1 years old, 20 +/- 3% body fat, p = 0.01 vs. VL monkeys), and the other our previously described weight-clamped long-term CR monkeys (n = 6.20 +/- 1 years old, 21 +/- 2% body fat, p = 0.01 vs. VL monkeys). All of the AL monkeys had the expected decrease in Ka with insulin; however, similar to the 4 out of 6 CR monkeys, 7 out of 10 VL monkeys had an increase in Ka with insulin. The 11 monkeys with an increase in Ka (+Ka) (7 VL + 4 CR) were compared to the 14 monkeys with a decrease in Ka with insulin (-Ka) (3 VL + 2 CR + 9 AL). The +Ka monkeys had lower basal Ka (p = 0.0001), higher basal GS fractional activity (p = 0.0003), lower basal G6P content (p = 0.002), lower glycogen phosphorylase fractional activity (p = 0.01), and lower whole-body insulin-mediated glucose disposal rate (p < 0.05) than the -Ka monkeys. We conclude that the condition of steady-state restrained calorie intake (as in the CR monkeys and in the controlled feeding VL monkeys) produces the paradoxical action of in vivo insulin to phosphorylate muscle GS, and raises the possibility that the presence of the unusual response to insulin may serve as a marker in calorie-restrained individuals for the genotype of obesity, insulin resistance and/or Type 2 diabetes.

A type 2 diabetes-associated polymorphic ARE motif affecting expression of PPP1R3 is involved in RNA-protein interactions

We have previously described a polymorphism in the 3' untranslated region (UTR) of the PPP1R3 gene that encodes the muscle-specific glycogen-targeting regulatory PP1 subunit. This polymorphism alters the distance between two putative mRNA-destabilizing ATTTA (AUUUA) motifs and is distinguished by a 10-nucleotide (allele ARE1) vs a 2-nucleotide interval (allele ARE2). ARE2 is associated with insulin resistance as well as increased prevalence of type 2 diabetes in the Pima Indians, and correlates with reduced expression of this subunit in vivo, causing a 10-fold half-life reduction of reporter mRNA in NIH3T3 cells. Gel shift assays, Northwestern blotting, and RNA-protein UV crosslinking revealed three proteins (43, 80, and 139 kDa) binding to the polymorphic ARE region in these cells. The interactions are sequence specific, and can be suppressed by an unlabeled competitor in a dose-dependent manner. The less stable ARE2 allele shows at least 2-fold higher relative protein binding, indicating that the polymorphic ARE region has a mRNA-destabilizing role. We suggest that the increased protein binding to ARE2 contributes to a faster degradation of PPP1R3 mRNA carrying this allele, and the resulting lower concentration of the protein contributes to insulin resistance, thus increasing the risk for development of type 2 diabetes.

Acanthosis nigricans

Acanthosis nigricans is a lesion affecting localized areas of the skin in persons with obesity and/or hyperinsulinemia. Roughening of the skin correlates with histological papilomatosis and the apparent darkening is due to hyperkeratosis. Biochemical mechanisms for developing this hyperplastic lesion are unclear, but likely involve local cutaneous growth factors. Cross sectional surveys of unselected populations have demonstrated that young children have low prevalences of obesity and acanthosis nigricans, but the prevalences of both increase with increasing age until plateaus are reached after the age of ten. Nearly 40% of Native American teenagers have acanthosis nigricans, whereas about 13% of African American, 6% of Hispanic, and less than 1% of white, non-Hispanic children aged 10-19 have clinically apparent acanthosis nigricans. We conclude that the presence of this skin lesion is a clinical surrogate of laboratory-documented hyperinsulinemia. Acanthosis nigricans identifies a subgroup within an ethnic group who have the highest insulin concentration, the most severe insulin resistance, and thus the highest risk for the development of type 2 diabetes.

L-carnitine improves glucose disposal in type 2 diabetic patients

OBJECTIVE

Aim of the present study is to evaluate the effects of L-carnitine on insulin-mediated glucose uptake and oxidation in type II diabetic patients and compare the results with those in healthy controls.

DESIGN

Fifteen type II diabetic patients and 20 healthy volunteers underwent a short-term (2 hours) euglycemic hyperinsulinemic clamp with simultaneous constant infusion of L-carnitine (0.28 micromole/kg bw/minute) or saline solution. Respiratory gas exchange was measured by an open-circuit ventilated hood system. Plasma glucose, insulin, non-esterified fatty acids (NEFA) and lactate levels were analyzed. Nitrogen urinary excretion was calculated to evaluate protein oxidation.

RESULTS

Whole body glucose uptake was significantly (p<0.001) higher with L-carnitine than with saline solution in the two groups investigated (48.66+/-4.73 without carnitine and 52.75+/-5.19 micromoles/kg(ffm)/minute with carnitine in healthy controls, and 35.90+/-5.00 vs. 38.90+/-5.16 micromoles/kg(ffm)/minute in diabetic patients). Glucose oxidation significantly increased only in the diabetic group (17.61+/-3.33 vs. 16.45+/-2.95 micromoles/kg(ffm)/minute, p<0.001). On the contrary, glucose storage increased in both groups (controls: 26.36+/-3.25 vs. 22.79+/-3.46 micromoles/kg(ffm)/minute, p<0.001; diabetics: 21.28+/-3.18 vs. 19.66+/-3.04 micromoles/kg(ffm)/minute, p<0.001). In type II diabetic patients, plasma lactate significantly decreased during L-carnitine infusion compared to saline, going from the basal period to the end-clamp period (0.028+/-0.0191 without carnitine and 0.0759+/-0.0329 with carnitine, p<0.0003).

CONCLUSIONS

L-carnitine constant infusion improves insulin sensitivity in insulin resistant diabetic patients; a significant effect on whole body insulin-mediated glucose uptake is also observed in normal subjects. In diabetics, glucose, taken up by the tissues, appears to be promptly utilized as fuel since glucose oxidation is increased during L-carnitine administration. The significantly reduced plasma levels of lactate suggest that this effect might be exerted through the activation of pyruvate dehydrogenase, whose activity is depressed in the insulin resistant status.

Defects in insulin secretion and insulin action in non-insulin-dependent diabetes mellitus are inherited. Metabolic studies on offspring of diabetic probands

No studies are available that have compared early defects in glucose metabolism in the offspring of insulin-deficient and insulin-resistant probands with non-insulin-dependent diabetes mellitus (NIDDM). To investigate this issue, we evaluated insulin secretion capacity with oral and intravenous glucose tolerance tests and with the hyperglycemic clamp, and insulin action with the euglycemic insulin clamp in 20 offspring of NIDDM patients with low fasting C-peptide (+/-450 pmol/liter), reflecting deficient insulin secretion (IS-group), 18 offspring of NIDDM patients with high fasting C-peptide (>/= 880 pmol/liter), reflecting insulin resistance (IR-group), and 14 healthy control subjects without a family history of NIDDM. The frequency of impaired glucose tolerance was 45.0% in the IS-group and 50% in the IR-group. The IS-group had lower insulin-glucose response at 30 min in the oral glucose tolerance test (85.2+/-10.0 pmol insulin per mmol glucose) than the control group (136.4+/-23.1 pmol insulin per mmol glucose; P < 0.05) and the IR-group (115.6+/-11.8 pmol insulin per mmol glucose; P = 0.05). Furthermore, the acute insulin response during the first 10 min of an intravenous glucose tolerance test was lower in the IS-group than in the IR-group. Maximal insulin secretion capacity evaluated by C-peptide levels during the hyperglycemic clamp did not differ between the groups. The IR-group had lower rates of whole body glucose uptake (60.1+/-4.6 micromol per lean body mass per minute) than did the control group (84.2+/-5.0 micromol per lean body mass per minute; P < 0.001) or the IS-group (82.6+/-5.9 micromol per lean body mass per minute; P < 0.01) and this was due to reduced glucose nonoxidation. To conclude, both impaired insulin secretion and insulin action seem to be inherited and could represent the primary defects in glucose metabolism in the offspring of NIDDM probands.

New variants in the glycogen synthase gene (Gln71His, Met416Val) in patients with NIDDM from eastern Finland

Impaired glycogen synthesis after insulin stimulation accounts for most of the insulin resistance in patients with non-insulin-dependent diabetes mellitus (NIDDM). The glycogen synthase gene (GYS1), which encodes the rate-limiting enzyme for glycogen synthesis, is a promising candidate gene for NIDDM. Therefore, we screened all 16 exons of this gene by single-strand conformation polymorphism analysis in 40 patients with NIDDM (age 67 +/- 2 years, body mass index 28.2 +/- 0.6 kg/m2) from Taipalsaari, eastern Finland. The Gly464Ser variant (exon 11) and a silent polymorphism TTC342TTT (exon 7) have been reported previously. In addition, we found a new variant Gln71His (exon 2) and a new amino acid polymorphism Met416Val (exon 10). An additional sample of 65 patients with NIDDM and 82 normoglycaemic men (age 54 +/- 1 years, body mass index 26.3 +/- 1.4 kg/m2) were screened. The allele frequency of the TTC342TTT silent substitution was 0.29 in both NIDDM and normoglycaemic subjects. The Gln71His and Gly464Ser variants were found in 1 (1%) and 3 (3%) subjects, respectively, of the 105 NIDDM patients and in none of the 82 normoglycaemic men. The Met416Val polymorphism was found in 16 (15%) of the 105 NIDDM patients and in 14 (17%) of the 82 control subjects (all heterozygous). The Met416Val polymorphism was not associated with insulin resistance in two groups of normoglycaemic subjects. In conclusion, the new Gln71His and Met416Val substitutions and other variants of the glycogen synthase gene are unlikely to make a major contribution to insulin resistance and NIDDM in diabetic patients from eastern Finland.

Dodecanedioic acid infusion induces a sparing effect on whole-body glucose uptake, mainly in non-insulin-dependent diabetes mellitus

Even-numbered dicarboxylic acids (DA) have been proposed as an alternative fuel substrate in parenteral nutrition. In particular, dodecanedioic acid (C12) shows a rapid plasma clearance from tissues, a very low urinary excretion compared with other DA and a high oxidation rate. The aim of the present study was to investigate the effect of C12 infusion on insulin-stimulated glucose uptake in patients with non-insulin-dependent diabetes mellitus (NIDDM) compared with healthy volunteers. A primed-constant infusion of C12 (0.39 mmol/min) was administered over 240 min, and at 120 min a 2 h euglycaemic hyperinsulinaemic clamp was performed. Blood specimens were sampled every 30 min and fractioned urines were collected over 24 h. The levels of C12 were measured by HPLC. Indirect calorimetry was performed continuously during the entire session. Body composition was assessed in all subjects studied to obtain fat-free mass (FFM) values. Whole-body glucose uptake decreased significantly during C12 infusion in both groups, although this effect was much more evident (P < 0.01) in NIDDM patients (52.4 (SD 15.8) % decrease compared with saline) than in controls (25.9 (SD 12.1) % decrease). The M value (mumol/kgFFM per min) was reduced by C12 to lower levels in NIDDM patients than in normal controls (12.6 (SD 3.9) v. 25.9 (SD 4.5), P < 0.01). Urinary excretion of C12 over 24 h was significantly lower in NIDDM patients than in controls (4.26 (SD 0.30) mmol v. 5.43 (SD 0.48), P < 0.01), corresponding to less than 3% of the administered dose. The infusion of C12 decreased non-protein RQ significantly in both groups of patients. In conclusion, this study shows, for the first time, that C12 significantly reduces glucose uptake in both normal controls and NIDDM patients, although this sparing effect on glucose uptake is much more pronounced in diabetic patients. These data suggest that C12 decreases glucose uptake and oxidation, mainly through a mechanism of substrate competition. Thus, it might be a useful alternative substrate in enteral or parenteral nutrition, sparing glucose utilization and increasing glycogen stores, in those clinical conditions, like NIDDM, where reduced insulin-induced glucose uptake and oxidation are observed.

Relationship of glycogen synthase and glycogen phosphorylase to protein phosphatase 2C and cAMP-dependent protein kinase in liver of obese rhesus monkeys

The regulation of glycogen synthase (GS) and glycogen phosphorylase (GP) activity by phosphorylation/ dephosphorylation has been proposed to be via changes in activities of several different protein (serine/threonine) phosphatases and kinases, including protein phosphatase (PP) 1/2A, PP2C, and cAMP-dependent protein kinase (PKA). In order to determine whether PP1/2A, PP2C, and/or PKA activities are related to GS and/or GP activities, these enzymes were measured in freeze-clamped liver biopsies obtained under basal fasting conditions from 16 obese monkeys. Four monkeys were normoglycemic and normoinsulinemic, five were hyperinsulinemic, and seven had type 2 diabetes (NIDDM). Liver glycogen and glucose 6-phosphate (G6P) contents were also determine. Basal enzyme activities and basal substrate concentrations were not significantly different between the three group of obese monkeys; however, there were several significant linear relationships observed when the monkeys were treated as one group. Therefore, multiple regression was used to determine the correlation between key variables. GS fractional activity was correlated to GP fractional activity (p < 0.05) and to PP2C activity (p = 0.005) (adjusted R2, 53%). GP independent activity was correlated to GS independent activity (p < 0.07) and to PKA fractional activity (p = 0.005) (adjusted R2, 64%). PP2C activity was correlated to GS fractional activity (p < 0.0005) and to PP1/2A activity (p < 0.0001) (adjusted R2, 83%). PKA fractional activity was correlated to GP total activity (p < 0.0005) and to age (p = 0.001) (adjusted R2, 82%). G6P content was correlated to glycogen content (p < 0.05) and to PP2C activity (p = 0.0005) (adjusted R2, 73%). In conclusion, PP2C and PKA are involved in the regulation of GS and GP activity in the basal state in liver of obese monkeys with a wide range of glucose tolerance.

Prolonged suppression of glucose metabolism causes insulin resistance in rat skeletal muscle

To determine whether an impairment of intracellular glucose metabolism causes insulin resistance, we examined the effects of suppression of glycolysis or glycogen synthesis on whole body and skeletal muscle insulin-stimulated glucose uptake during 450-min hyperinsulinemic euglycemic clamps in conscious rats. After the initial 150 min to attain steady-state insulin action, animals received an additional infusion of saline, Intralipid and heparin (to suppress glycolysis), or amylin (to suppress glycogen synthesis) for up to 300 min. Insulin-stimulated whole body glucose fluxes were constant with saline infusion (n = 7). In contrast, Intralipid infusion (n = 7) suppressed glycolysis by approximately 32%, and amylin infusion (n = 7) suppressed glycogen synthesis by approximately 45% within 30 min after the start of the infusions (P < 0.05). The suppression of metabolic fluxes increased muscle glucose 6-phosphate levels (P < 0.05), but this did not immediately affect insulin-stimulated glucose uptake due to compensatory increases in other metabolic fluxes. Insulin-stimulated whole body glucose uptake started to decrease at approximately 60 min and was significantly decreased by approximately 30% at the end of clamps (P < 0.05). Similar patterns of changes in insulin-stimulated glucose fluxes were observed in individual skeletal muscles. Thus the suppression of intracellular glucose metabolism caused decreases in insulin-stimulated glucose uptake through a cellular adaptive mechanism in response to a prolonged elevation of glucose 6-phosphate rather than the classic mechanism involving glucose 6-phosphate inhibition of hexokinase.

Expression of the gene encoding glycogen phosphorylase is elevated in diabetic rat skeletal muscle and is regulated by insulin and cyclic AMP

Glycogen phosphorylase regulates the breakdown of glycogen into glucose, but as previous studies have demonstrated, the control of glycogen metabolism becomes deregulated in diabetes mellitus. Messenger RNA levels encoding several different proteins are altered in skeletal muscle biopsies of patients with insulin-dependent and non-insulin-dependent diabetes. The possible alteration of expression of the gene encoding the skeletal muscle isoform of glycogen phosphorylase during diabetes has not previously been investigated. We examined the effect of streptozotocin-induced diabetes and insulin treatment on glycogen phosphorylase mRNA in rat skeletal muscle; glycogen phosphorylase mRNA levels were elevated in diabetic rat muscle tissue, but were partially suppressed in diabetic rat muscle following insulin treatment. To distinguish between the effects of insulin and counter-regulatory hormones on glycogen phosphorylase mRNA levels, we employed differentiating rat L6 myoblasts in culture. Insulin stimulated the accumulation of glycogen phosphorylase mRNA as determined by Northern blot analysis. Moreover, insulin and dibutyryl cAMP stimulated expression of a transiently transfected chloramphenicol acetyl transferase reporter gene under the control of the muscle glycogen phosphorylase promoter in differentiating myotubes in culture, suggesting that the effects of insulin and counter-regulatory hormones on glycogen phosphorylase mRNA are at the level of transcription. These results suggest that insulin and epinephrine may participate in the induction of the glycogen phosphorylase gene during myogenesis; moreover, activation of this gene in muscle tissue may be a contributing factor in impaired glycogen storage during uncontrolled diabetes.

Phosphonate inhibitors of protein-tyrosine and serine/threonine phosphatases

In all, 15 aryl-containing phosphonates have been synthesized and tested for their effect on protein-tyrosine phosphatase (PTPase) activity. Two compounds, (naphth-2-yl) difluoromethylphosphonic acid (12) and (napthy-1-yl) difluoromethylphosphonic acid (13) have been found to inhibit dephosphorylation of [32P]insulin receptors by PTP-1B, a protein tyrosine phosphatase (PTPase), with IC50 values of 40-50 microM. Compound 12 competitively inhibited insulin-receptor dephosphorylation by PTP-1B. Compound 12 also inhibited PTP-1B-catalysed dephosphorylation of a synthetic tyrosine phosphorylated substrate poly(Glu80-Tyr20) at the same potency, indicating that 12 acted via interaction with the PTPase. Additionally, 12 inhibited insulin-receptor PTPase(s) and epridermal-growth-factor-receptor PTPase(s) present in solubilized membranes from CHO (Chinese-hamster ovary)/HIRc and A431 cells respectively. IC50 values of 40-50 microM were obtained in all cases with compound 12. Of note is the fact that these compounds did not have any effect on insulin-receptor autophosphorylation. Nine out of the 15 compounds potently inhibited serine/threonine phosphatase PP-2A activity without any effect on serine/threonine phosphatase PP-1 when tested at a concentration as high as 675 microM. The most potent compounds acting toward PP-2A had IC50 values of 45-50 microM. These PP-2A inhibitors could be useful tools for studying serine/threonine-phosphatase-mediated signal transduction. Two compounds, 12 and 13, inhibited both tyrosine phosphatase PTP-1B and serine/threonine phosphatase PP-2A with similar potency; IC50 values being 40-50 microM in both cases. Details of the synthesis of compounds 10, 11 and 13 are given in Supplementary Publication SUP 50177 (6 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1995) 305, 9.

Defects in liver and muscle glycogen metabolism in neonatal and adult New Zealand obese mice

Impaired glycogen synthesis is present in subjects at risk for developing non-insulin-dependent diabetes mellitus (NIDDM), suggesting that it is a primary defect in NIDDM. To examine whether defects in glycogen metabolism are present at birth in an animal model of NIDDM, glycogen synthase (GS), glycogen phosphorylase (GP), and total glycogen content were measured in liver and quadriceps muscle of 1-day- and 20-week-old insulin-resistant New Zealand Obese (NZO) mice and control (NZC) mice. In livers of both neonatal and adult NZO mice, active GS was reduced by 54% and 36%, respectively, as compared with that in NZC mice (P < .03). Total liver GS activity was the same in neonates, but was 65% higher in adult NZO as compared with NZC mice (P < .02). Liver glycogen was 28% lower at birth in NZO mice (P < .03), but was 49% higher at 20 weeks of age. Active and total GP were the same in NZO and NZC animals, despite hyperinsulinemia in 20-week-old NZO mice. In muscle, active GS was reduced by 41% in both 1-day- and 20-week-old NZO mice (P < .02). Total GS was also lower in NZC mice at 1 day of age (P < .01), but not at 20 weeks. No differences were detected in GP activity or in total glycogen content in muscle. Therefore, reduced GS activity is an early defect present at birth in the insulin-resistant NZO mouse in both liver and muscle. However, it is not the sole determinant of the amount of glycogen deposited in tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased muscle glucose transport/phosphorylation is an early defect in the pathogenesis of non-insulin-dependent diabetes mellitus

Recent studies have demonstrated that reduced insulin-stimulated muscle glycogen synthesis is the major cause of insulin resistance in patients with non-insulin-dependent diabetes mellitus (NIDDM). This reduced rate has been assigned to a defect in either glucose transport or hexokinase activity. However it is unknown whether this is a primary or acquired defect in the pathogenesis of NIDDM. To examine this question, we measured the rate of muscle glycogen synthesis and the muscle glucose 6-phosphate (G6P) concentration using 13C and 31P NMR spectroscopy as well as oxidative and nonoxidative glucose metabolism in six lean, normoglycemic offspring of parents with NIDDM and seven age/weight-matched control subjects under hyperglycemic (approximately 11 mM)-hyperinsulinemic (approximately 480 pM) clamp conditions. The offspring of parents with NIDDM had a 50% reduction in total glucose metabolism, primarily due to a decrease in the nonoxidative component. The rate of muscle glycogen synthesis was reduced by 70% (P < 0.005) and muscle G6P concentration was reduced by 40% (P < 0.003), which suggests impaired muscle glucose transport/hexokinase activity. These changes were similar to those previously observed in subjects with fully developed NIDDM. When the control subjects were studied at similar insulin levels (approximately 440 pM) but euglycemic plasma glucose concentration (approximately 5 mM), both the rate of glycogen synthesis and the G6P concentration were reduced to values similar to the offspring of parents with NIDDM. We conclude that insulin-resistant offspring of parents with NIDDM have reduced nonoxidative glucose metabolism and muscle glycogen synthesis secondary to a defect in muscle glucose transport/hexokinase activity prior to the onset of overt hyperglycemia. The presence of this defect in these subjects suggests that it may be the primary factor in the pathogenesis of NIDDM.

Insulin stimulation of glycogen synthesis and glycogen synthase activity is blocked by wortmannin and rapamycin in 3T3-L1 adipocytes: evidence for the involvement of phosphoinositide 3-kinase and p70 ribosomal protein-S6 kinase

We have investigated the involvement of phosphoinositide (PI) 3-kinase and p70 ribosomal protein-S6 kinase (p70s6k) in mediating insulin stimulation of glycogen synthesis in 3T3-L1 adipocytes using specific inhibitors. Wortmannin inhibited PI 3-kinase activity (IC50 approximately 10 nM), inhibition being complete at 100 nm. Wortmannin (100 nM) completely blocked the ability of insulin to activate glycogen synthase in 3T3-L1 adipocytes and the ability of insulin to stimulate glucose incorporation into glycogen in 3T3-L1 fibroblasts. Rapamycin, which blocks insulin-stimulated activation of p70s6k, decreased insulin activation of glycogen synthase in a dose-dependent manner (IC50 approximately 0.8 ng/ml), with a maximum approx. 75% inhibition of insulin's stimulatory effect. Rapamycin inhibited insulin-stimulated glucose incorporation into glycogen to a similar extent and with similar dose-dependency, while having no effect on insulin-stimulated glucose transport. We conclude that PI 3-kinase and p70s6k are involved in the signalling pathways by which insulin stimulates glycogen synthase in 3T3-L1 adipocytes.

Effects of puberty and diabetes on metabolism of insulin-sensitive fuels

Insulin's ability to stimulate glucose metabolism is reduced during normal puberty; these changes are exaggerated in adolescents with insulin-dependent diabetes mellitus (IDDM). Because the effects of puberty and IDDM on the other actions of insulin have not been established, we studied leucine kinetics (using [1-13C]leucine) and fat metabolism during euglycemic hyperinsulinemia (20 mU.m2.min-1) for 3 h in eight healthy and nine IDDM (HbA1 14 +/- 2%) adolescents and six healthy young adult controls. IDDM subjects received overnight low-dose insulin infusion to normalize fasting glucose. Basal and steady-state insulin values (approximately 240 pM) during the study were similar in all three groups. Insulin-stimulated glucose metabolism was reduced by 40% in healthy adolescents vs. adults (P < 0.05) and by an additional 40% in poorly controlled IDDM (P < 0.05 vs, normal adolescents). Although basal glucose and lipid oxidation rates (measured by indirect calorimetry) were similar in all three groups, when insulin was infused, glucose oxidation increased and lipid oxidation decreased only in the two nondiabetic groups. Similarly, insulin significantly reduced plasma free fatty acid levels only in the nondiabetics. Basal leucine flux (an index of protein degradation) was similar in healthy controls but was markedly increased in IDDM adolescents. Despite similar increments in plasma insulin during the clamp, leucine flux remained higher in IDDM adolescents than in healthy controls. Basal leucine oxidation rates were also increased in IDDM subjects compared with nondiabetic groups and declined to a lesser extent during insulin infusion. We conclude that insulin resistance of puberty is selective for glucose metabolism, sparing amino acid/protein metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

Human rsk isoforms: cloning and characterization of tissue-specific expression

Serine-threonine protein kinases in the ribosomal S6 kinase (rsk or p90rsk) family have been implicated as signaling intermediates in the cellular response to several growth factors. To investigate the molecular diversity of human p90rsk isoforms, mixed degenerate oligonucleotide polymerase chain reaction was used to isolate partial rsk cDNAs (1.1 kb). Three closely related human rsk cDNAs were obtained (HU-1, HU-2, HU-3). These cDNAs are encoded by separate genes based on DNA sequence diversity and distinct patterns seen with genomic Southern blots. Northern analysis revealed different sized mRNA transcripts for each isoform. A full-length HU-1 cDNA (3.1 kb) was subsequently isolated from a HeLa cell library. 5'-cDNA clones for HU-2 and HU-3 were isolated using the "rapid amplification of cDNA ends" strategy. Experiments using human x hamster somatic cell hybrids localized the HU-1 gene to human chromosome 3; HU-2 is on chromosome 6; and HU-3 is on the X chromosome. The tissue distribution of human rsk mRNAs was determined using ribonuclease protection assays. HU-3 mRNA was present in multiple RNA samples. HU-2 was expressed in fibroblast > muscle > lymphocyte = placenta > liver. HU-1 was expressed in Epstein-Barr virus lymphocyte > > muscle = liver > fat = placenta. These results indicate that the multiplicity of p90rsk isoforms is increased to at least three for humans and that marked tissue-/cell-specific differences in p90rsk isoform expression are present.

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

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.

The impact of metformin therapy on hepatic glucose production and skeletal muscle glycogen synthase activity in overweight type II diabetic patients

The effect of metformin therapy on glucose metabolism was examined in eight overweight newly presenting untreated type II diabetic patients (five males, three females). Patients were treated for 12 weeks with either metformin (850 mg x 3) or matching placebo using a double-blind crossover study design; patients were studied at presentation and at the end of each treatment period. Insulin action was assessed by measuring activation of skeletal muscle glycogen synthase (GS) before and during a 4-hour hyperinsulinemic euglycemic clamp (100 mU.kg-1 x h-1). Metformin therapy was associated with a significant decrease in fasting blood glucose (6.8 +/- 0.6 v 8.3 +/- 0.9 mmol.L-1, P < .01) and glycosylated hemoglobin ([HbA1] 7.7% +/- 0.4% v 8.5% +/- 0.5%, P < .01) levels. Fasting hepatic glucose production (HGP) was also significantly decreased following metformin therapy (1.98 +/- 0.13 v 2.41 +/- 0.20 mg.kg-1 x min-1, P < .02), whereas fasting insulin and C-peptide concentrations remained unaltered. The decrease in basal HGP correlated closely with the decrease in fasting blood glucose concentration (r = .92, P < .001). Insulin-stimulated glucose uptake was assessed using the hyperinsulinemic euglycemic clamp technique and was increased post-metformin (3.8 +/- 0.6 v 3.1 +/- 0.7 mg.kg-1 x min-1, P < .05). This was primarily the result of increased nonoxidative glucose metabolism (1.1 +/- 0.6 v 0.4 +/- 0.6 mg.kg-1 x min-1, P < .05); oxidative glucose metabolism did not change. Metformin had no measurable effect on insulin activation of skeletal muscle GS, the rate-limiting enzyme controlling muscle glucose storage.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathogenesis of type 2 (non-insulin-dependent) diabetes mellitus: candidates for a signal transmitter defect causing insulin resistance of the skeletal muscle

Insulin resistance of skeletal muscle, liver and fat combined with an abnormality of insulin secretion characterizes Type 2 (non-insulin-dependent) diabetes mellitus. There is increasing evidence that the insulin resistance of the skeletal muscle plays a key role early in the development of Type 2 diabetes. As a consequence recent research efforts have focussed on the characterization of insulin signal transduction elements in the muscle which are candidates for a localization of a defect causing insulin resistance i.e. the insulin receptor, phosphatases related to insulin action, glycogen synthase and the glucose transporters. In this review we attempt to summarize present knowledge about abnormalities of these systems in skeletal muscle of Type 2 diabetic and pre-diabetic individuals. We try to classify abnormalities as secondary events or as candidates for putative primary molecular defects which might initiate the development of insulin resistance as early as in the "pre-diabetic" state.

Abnormal regulation of ribosomal protein S6 kinase by insulin in skeletal muscle of insulin-resistant humans

Insulin resistance in Pima Indians appears to result from a post-receptor impairment of insulin signal transduction that affects only some responses to insulin. To identify the primary lesion responsible for insulin resistance, we investigated the influence of insulin on ribosomal protein S6 kinase activities in skeletal muscle of insulin-sensitive and insulin-resistant nondiabetic Pima Indians during a 2-h hyperinsulinemic, euglycemic clamp. In sensitive subjects, S6 kinase activity was transiently activated fivefold over basal activity by 45 min of insulin infusion. Although basal activities in the two groups were similar, the response to insulin was delayed and restricted to about threefold over basal in subjects resistant to insulin. Two major S6 kinase activities in extracts of human muscle were resolved by chromatography on Mono Q. Peak 1, which accounted for basal activity owes to an enzyme antigenically related to the 90-kD S6 kinase II, a member of the rsk gene family. The major insulin-stimulated S6 kinase eluted as peak 2 and is antigenically related to a 70-kD S6 kinase. Our results show that insulin resistance impairs signaling to the 70-kD S6 kinase.

Assessment of proinsulin's effects on intermediary metabolism using the forearm technique in normal man

We have compared the effects of human proinsulin and insulin on forearm metabolism. Seven normal, non-obese subjects were infused with 386 pmol/kg per hour of proinsulin and 180 pmol/kg per hour of insulin using the euglycaemic clamp technique. Glucose appearance and utilization rates were quantified using a primed continuous infusion of [6',6'-2H2]glucose. Mean blood glucose was 4.1 +/- 0.1 and 4.1 +/- 0.2 mmol/l during proinsulin and insulin infusions respectively. Basal insulin concentrations increased from 0.02 +/- 0.01 to 0.25 +/- 0.03 nmol/l. The proinsulin infusion was chosen to give steady-state levels approximately 20-fold higher on a molar basis than those of insulin, based on previous findings that proinsulin has only 5% the biological potency of insulin. Basal proinsulin concentrations increased from 0.003 to 5.4 +/- 0.3 nmol/l. Hepatic glucose production was suppressed similarly during the last hour of each hormone infusion: 0.07 +/- 0.16 (proinsulin, P), and 0.01 +/- 0.13 (insulin, I) mg/kg per minute. Glucose disposal, however, was significantly increased during the final hour of the insulin infusion: 4.7 +/- 0.4 (I) and 3.4 +/- 0.2 (P) mg/kg per minute (P = 0.025). Net forearm glucose uptake (FGU) increased by a greater amount during insulin compared with proinsulin infusion: 1.44 +/- 0.02 (I) and 0.71 +/- 0.01 (P) mumol/100 ml forearm per minute (P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple metabolic effects of CGRP in conscious rats: role of glycogen synthase and phosphorylase

Calcitonin gene-related peptide (CGRP) is a neuropeptide that is released at the neuromuscular junction in response to nerve excitation. To examine the relationship between plasma CGRP concentration and intracellular glucose metabolism in conscious rats, we performed insulin (22 pmol.kg-1.min-1) clamp studies combined with the infusion of 0, 20, 50, 100, 200, and 500 pmol.kg-1.min-1 CGRP (plasma concentrations ranging from 2 x 10(-11) to 5 x 10(-9) M). CGRP antagonized insulin's suppression of hepatic glucose production at plasma concentrations (approximately 10(-10) M) that are only two- to fivefold its basal portal concentration. Insulin-mediated glucose disposal was decreased by 20-32% when CGRP was infused at 50 pmol.kg-1.min-1 (plasma concentration 3 x 10(-10) M) or more. The impairment in insulin-stimulated glycogen synthesis in skeletal muscle accounted for all of the CGRP-induced decrease in glucose disposal, while whole body glycolysis was increased despite the reduction in total glucose uptake. The muscle glucose 6-phosphate concentration progressively increased during the CGRP infusions. CGRP inhibited insulin-stimulated glycogen synthase in skeletal muscle with a 50% effective dose of 1.9 +/- 0.36 x 10(-10) M. This effect on glycogen synthase was due to a reduction in enzyme affinity for UDP-glucose, with no changes in the maximal velocity. In vitro CGRP stimulated both hepatic and skeletal muscle adenylate cyclase in a dose-dependent manner. These data suggest that 1) CGRP is a potent antagonist of insulin at the level of muscle glycogen synthesis and hepatic glucose production; 2) inhibition of glycogen synthase is its major biochemical action in skeletal muscle; and 3) these effects are present at concentrations of the peptide that may be in the physiological range for portal vein and skeletal muscle. These data underscore the potential role of CGRP in the physiological modulation of intracellular glucose metabolism.

Correlations of glycogen synthase and phosphorylase activities with glycogen concentration in human muscle biopsies. Evidence for a double-feedback mechanism regulating glycogen synthesis and breakdown

The purpose of this study was to verify in man the relationships of muscle glycogen synthase and phosphorylase activities with glycogen concentration that were reported in animal studies. The upper level of glycogen concentration in muscle is known to be tightly controlled, and glycogen concentration was reported to have an inhibitory effect on synthase activity and a stimulatory effect on phosphorylase activity. Glycogen synthase and phosphorylase activity and glycogen concentration were measured in muscle biopsies in a group of nine normal subjects after stimulating an increase of their muscle glycogen concentration through either an intravenous glucose-insulin infusion to stimulate glycogen synthesis, or an Intralipid (Vitrum, Stockholm, Sweden) infusion in the basal state to inhibit glycogen mobilization by favoring lipid oxidation at the expense of glucose oxidation. Phosphorylase activity increased from 71.3 +/- 21.0 to 152.8 +/- 20.0 nmol/min/mg protein (P < .005) after the glucose-insulin infusion. Phosphorylase activity was positively correlated with glycogen concentration (P = .005 and P = .0001) after the glucose-insulin and Intralipid infusions, respectively. Insulin-stimulated glycogen synthase activity was significantly negatively correlated with glycogen concentration at the end of the Intralipid infusion (P < .005). In conclusion, by demonstrating a negative correlation of glycogen concentration with glycogen synthase and a positive correlation with phosphorylase, this study might confirm in man the double-feedback mechanism by which changes in glycogen concentration regulate glycogen synthase and phosphorylase activities. It suggests that this mechanism might play an important role in the regulation of glucose storage.

Role of glucose and insulin resistance in development of type 2 diabetes mellitus: results of a 25-year follow-up study

Type 2 diabetes mellitus is characterised by resistance of peripheral tissues to insulin and a relative deficiency of insulin secretion. To find out which is the earliest or primary determinant of disease, we used a minimum model of glucose disposal and insulin secretion based on intravenous glucose tolerance tests to estimate insulin sensitivity (SI), glucose effectiveness (ie, insulin-independent glucose removal rate, SG), and first-phase and second-phase beta-cell responsiveness in normoglycaemic offspring of couples who both had type 2 diabetes. 155 subjects from 86 families were followed-up for 6-25 years. More than 10 years before the development of diabetes, subjects who developed the disease had lower values of both SI (mean 3.2 [SD 2.4] vs 8.1 [6.7] 10(-3) I min-1 pmol-1 insulin; p < 0.0001) and SG (1.6 [0.9] vs 2.3 [1.2] 10(-2) min-1, p < 0.0001) than did those who remained normoglycaemic). For the subjects with both SI and SG below the group median, the cumulative incidence of type 2 diabetes during the 25 years was 76% (95% confidence interval 54-99). By contrast, no subject with both SI and SG above the median developed the disease. Subjects with low SI/high SG or high SI/low SG had intermediate risks. Insulin secretion, especially first phase, tended to be increased rather than decreased in this prediabetic phase and was appropriate for the level of insulin resistance. The development of type 2 diabetes is preceded by and predicted by defects in both insulin-dependent and insulin-independent glucose uptake; the defects are detectable when the patients are normoglycaemic and in most cases more than a decade before diagnosis of disease.

31P nuclear magnetic resonance measurements of muscle glucose-6-phosphate. Evidence for reduced insulin-dependent muscle glucose transport or phosphorylation activity in non-insulin-dependent diabetes mellitus

To assess the rate-limiting step in muscle glycogen synthesis in non-insulin-dependent diabetes mellitus (NIDDM), the concentration of glucose-6-phosphate (G6P) was measured by 31P nuclear magnetic resonance (NMR) during a hyperglycemic-hyperinsulinemic clamp. Six subjects with NIDDM and six age weight-matched controls were studied at similar steady-state plasma concentrations of insulin (approximately 450 pmol/liter) and glucose (11 mmol/liter). The concentration of G6P in the gastrocnemius muscle was measured by 31P NMR. Whole-body oxidative and nonoxidative glucose metabolism was determined by the insulin-glucose clamp technique in conjunction with indirect calorimetry. Nonoxidative glucose metabolism which under these conditions is a measure of muscle glycogen synthesis (1990. N. Engl. J. Med. 322:223-228), was 31 +/- 7 mumol/(kg body wt-min) in the normal subjects and 13 +/- 3 mumol/(kg body wt-min) in the NIDDM subjects (P less than 0.05). The concentration of G6P was higher (0.24 +/- 0.02 mmol/kg muscle) in the normal subjects than in the NIDDM subjects (0.17 +/- 0.02, P less than 0.01). Increasing insulin concentrations to insulin 8,500 pmol/liter in four NIDDM subjects restored the glucose uptake rate and G6P concentrations to normal levels. In conclusion, the lower concentration of G6P in the diabetic subjects despite a decreased rate of nonoxidative glucose metabolism is consistent with a defect in muscle glucose transport or phosphorylation reducing the rate of muscle glycogen synthesis.

Defective insulin response of phosphorylase phosphatase in insulin-resistant humans

Insulin-stimulated glycogen synthase activity in human muscle is reduced in insulin-resistant subjects. Insulin regulation of human muscle glycogen synthase may require activation of a type-1 protein phosphatase (PP-1). We investigated the change of phosphorylase phosphatase and glycogen synthase activities in muscle biopsies obtained during a 2-h hyperinsulinemic euglycemic clamp in 12 insulin-sensitive (group S) and 8 insulin-resistant (group R) subjects. Fasting phosphorylase phosphatase activity was lower in group R than in group S, and did not increase significantly with insulin infusion in group R until 20 min. In group S, phosphorylase phosphatase was significantly stimulated by 10 min, remaining significantly higher than in group R at all time points. The insulin-mediated changes in phosphatase activities were not decreased by 3 nM okadaic acid but were completely inhibited by 1 microM okadaic acid, thereby verifying that insulin-stimulated phosphorylase phosphatase is accounted for by a PP-1. Subcellular fractionation demonstrated reduced fasting PP-1 activities in both the glycogen and cytosolic fractions of muscle obtained from subjects in group R compared to those in group S. These results suggest that insulin activation of PP-1 could contribute to the stimulation of glycogen synthase by this hormone in human muscle. Lower fasting PP-1 activity in cytosol and glycogen fractions plus lower insulin-stimulated PP-1 activity could explain, in part, reduced insulin-stimulated glycogen synthase in skeletal muscle of insulin-resistant subjects.

Deficiency in phosphorylase phosphatase activity despite elevated protein phosphatase type-1 catalytic subunit in skeletal muscle from insulin-resistant subjects

Glycogen synthase is activated by protein phosphatase type-1 (PP-1). The spontaneous PP-1 activity accounts for only a small fraction of total PP-1 activity, which can be exposed by trypsin digestion of inhibitor proteins in the presence of Mn2+. We determined total PP-1 activity in muscle biopsies from insulin-sensitive and -resistant nondiabetic Pima Indians. Inhibitor-2 sensitive PP-1 represented 90% of total phosphatase activity. Spontaneous and total PP-1 activities were reduced in insulin resistant subjects (P less than 0.05-0.01), suggesting that the reduced PP-1 activity is not the result of inhibition by trypsin-labile phosphatase regulatory subunits. This difference was further investigated by Western blots using two different antibodies. An antibody raised against the rabbit muscle PP-1 catalytic subunit was used to analyze muscle extracts concentrated by DEAE-Sepharose adsorption. An antibody raised against a peptide derived from the COOH-terminal end of the PP-1 catalytic subunit was used to analyze crude muscle extracts. Both antibodies recognized a PP-1 catalytic subunit of approximately 33 kD, which unexpectedly was more abundant in insulin-resistant subjects (P less than 0.05-0.01). The increase in the tissue PP-1 protein content may be a response to compensate for the impairment in the enzyme activity.

Protein phosphatase-1 and -2A, kinase FA, and casein kinase II in skeletal muscle of streptozotocin diabetic rats

Protein phosphatase-1 (PP-1) and -2A (PP-2A), two regulatory subunits of PP-1, the glycogen-binding subunit G and inhibitor-2 (I-2), kinase FA, and casein kinase II (CK-II) were investigated in skeletal muscle of diabetic rats 2 days after streptozotocin injection. FA and CK-II activate PP-1 in vitro and might be involved in the activation of PP-1 by insulin. Following muscle fractionation we found that (1) diabetes decreased both basal and trypsin-stimulated PP-1 activities; the decrease was more significant in the glycogen-bound and microsomal fractions than in the cytosol (cytosolic PP-1 decreased as specific activity but not as activity/g of muscle); also PP-2A was lower in diabetic cytosols; (2) less G was immunoprecipitated from diabetic glycogen-bound fractions compared to controls, while I-2 was not significantly changed; (3) diabetes decreased also FA (assayed as PP-1 activator) and CK-II (assayed using a synthetic peptide as substrate); (4) diabetes did not have any effect on phosphorylase (a + b) activity in the glycogen-bound fraction. Altogether the data show that acute diabetes decreased PP-1, one of its regulatory subunits and two potentially physiological regulators of PP-1, in addition to PP-2A. This may indicate that insulin is responsible for the long-term regulation of the same enzymes that are also under acute insulin control.

Genetics of non-insulin dependent diabetes mellitus

It is probable that NIDDM has a multifactorial origin in which environmental factors hasten the progression of the disease in genetically predisposed individuals. The importance of the genetic contribution to NIDDM has been established by the study of certain inbred populations, the almost 100% concordance of disease in monozygotic twins and by familial clustering. However, progress in identifying specific genetic factors involved in NIDDM has been slow and no consistent evidence has emerged supporting a major aetiological role for any of the genes so far studied. This may be due in part to methodological problems encountered in the identification of such disease susceptibility genes.

Effects of hyperinsulinemia and hyperglycemia on insulin receptor function and glycogen synthase activation in skeletal muscle of normal man

Insulin receptor function, glycogen synthase activity, and activation by phosphatases were studied in biopsies of human skeletal muscle under conditions of hyperglycemia and/or hyperinsulinemia for 150 minutes. Twenty-one healthy volunteers underwent either (A) a hyperinsulinemic, euglycemic clamp (serum insulin, 160.0 +/- 7.7 mU/L; plasma glucose, 4.9 +/- 0.1 mmol/L; n = 9), (B) a hyperglycemic clamp during normoinsulinemia (serum insulin, 18.1 +/- 3.3 mU/L; plasma glucose, 12.9 +/- 0.2 mmol/L; n = 6), or (C) a combined hyperinsulinemic, hyperglycemic clamp (serum insulin, 158.3 +/- 15.0 mU/L; plasma glucose, 11.4 +/- 0.8 mmol/L; n = 6). During all studies, the endogenous insulin secretion was inhibited with somatostatin. Insulin binding and kinase activity of insulin receptors solubilized from vastus lateralis muscle biopsies were unaffected by hyperglycemia and/or hyperinsulinemia. Hyperinsulinemia activated the muscle glycogen synthase with a decrease in the half-maximal activation constant (A0.5) for glucose-6-phosphate (G6P) from 0.53 +/- 0.04 to 0.21 +/- 0.02 mmol/L (study A, P less than .02) and from 0.53 +/- 0.06 to 0.19 +/- 0.05 mmol/L (study C, P less than .03). In addition, the rate of glycogen synthase activation by phosphatases increased from 0.078 +/- 0.017 to 0.134 +/- 0.029 U/min/mg protein (study A, P less than .03) and from 0.082 +/- 0.013 to 0.145 +/- 0.033 U/min/mg protein (study C, P = .05). Hyperglycemia during normoinsulinemia did not affect A0.5 or phosphatase activity. In conclusion, (1) hyperinsulinemia for 2 1/2 hours increases glycogen synthase activity and activation by phosphatases independently on the glycemia; and (2) insulin receptor binding and basal and insulin-stimulated receptor kinase activity are not modified during short-term hyperinsulinemia and/or hyperglycemia.

Effects of growth hormone on fuel utilization and muscle glycogen synthase activity in normal humans

To examine the insulin antagonistic effects of growth hormone (GH), seven healthy subjects underwent, in random order, two 5-h euglycemic clamp studies with moderate hyperinsulinemia. A GH infusion (45 ng.kg-1.min-1) was given throughout one of the studies. GH inhibited the insulin-stimulated glucose disposal by 27% from 4.4 +/- 0.7 to 3.3 +/- 0.4 mg.kg-1.min-1 (P less than 0.02) and raised the nonprotein energy expenditures (NPEE) from 18.7 +/- 0.5 to 20.5 +/- 0.3 kcal.kg-1.24 h-1 (P less than 0.03). Lipid oxidation contributed 71.7 +/- 5.6% of NPEE during the GH infusion as compared with 48.7 +/- 5.2% during the control clamp (P less than 0.02). In skeletal muscle biopsies, insulin binding to wheat germ agglutinin-purified insulin receptors and insulin receptor kinase activity were unaffected by GH infusion. Glycogen synthase activation by insulin was inhibited by 41% during the GH clamp (fractional velocity 14.1 +/- 2.5 vs. 8.3 +/- 1.4%, P less than 0.03). In conclusion, GH 1) increases energy expenditures and inhibits glucose oxidation in favor of an increased lipid oxidation, and 2) inhibits insulin-mediated activation of the glycogen synthase in skeletal muscle biopsies by a mechanism distal to insulin receptor binding and kinase activity.

The effect of sulphonylurea therapy on skeletal muscle glycogen synthase activity and insulin secretion in newly presenting type 2 (non-insulin-dependent) diabetic patients

Ten newly presenting, Type 2 (non-insulin-dependent), Caucasian diabetic patients were studied before and after 8 weeks treatment with the sulphonylurea gliclazide, and in parallel 13 similar patients were studied before and after 8 weeks treatment with diet alone. Eight non-diabetic subjects were also studied. Insulin action was assessed by measuring activation of skeletal muscle glycogen synthase (GS) prior to and during a 4-h hyperinsulinaemic euglycaemic clamp (100 mU kg-1 h-1). Fasting plasma glucose (+/- SE) and glycosylated haemoglobin decreased to a greater extent in the gliclazide treated patients (fall of 6.2 +/- 0.7 vs 2.1 +/- 0.5 mmol l-1, p less than 0.005 and 4.7 +/- 0.5 vs 2.1 +/- 0.5%, p less than 0.005). This was accompanied by an increase in fasting serum insulin concentrations in the gliclazide treated patients (7.0 +/- 1.3 to 10.1 +/- 1.1 mU l-1, p less than 0.005), but no change in the diet treated patients. Fractional GS activity did not increase during the clamp at presentation in either treatment group (change +2.9 +/- 1.8 and -1.5 +/- 1.9%, respectively) whereas it increased markedly in the control subjects (+16.4 +/- 3.4%, both p less than 0.001). After 8-week treatment there was a significant increase in GS activity during the clamp in the patients receiving gliclazide (+6.9 +/- 2.7%, p less than 0.05), but no change in GS activity in the patients on diet alone (+0.5 +/- 1.4%). The difference in post-treatment muscle insulin action was significant (p less than 0.05). There was no correlation between the degree of improvement in metabolic control and the improvement in response of GS to insulin in the gliclazide treated patients (r = -0.06), suggesting a possible direct drug effect on skeletal muscle. Glucose requirement during the clamp at presentation was markedly lower in both treatment groups than in the non-diabetic subjects (gliclazide 2.1 +/- 0.3, diet 2.0 +/- 0.6 vs 7.8 +/- 0.4 mg kg-1 min-1, both p less than 0.001), and despite a marked improvement in both groups after treatment (4.3 +/- 0.4 and 3.1 +/- 0.5 mg kg-1 min-1, both p less than 0.001) remained lower than in the non-diabetic subjects (p less than 0.001).(ABSTRACT TRUNCATED AT 400 WORDS)

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

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, 80 mU.(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 less than 0.01). The values obtained at the highest insulin levels (around 140 microU/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 less than 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 less than 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 less than 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.

Impaired activation of skeletal muscle glycogen synthase in non-insulin-dependent diabetes mellitus is unrelated to the degree of obesity

Twenty-five newly presenting, untreated, white, non-insulin-dependent diabetic (NIDDM) subjects were studied within 72 hours of diagnosis. They were allocated to three groups according to their body mass index [BMI] (lean BMI less than 25.0, n = 9; overweight BMI 25.0 to 30.0, n = 6; obese BMI greater than .30.0 kg/m2, n = 10). All three groups exhibited equivalent hyperglycemia. Eleven normal control subjects were also studied. The degree of activation of skeletal muscle glycogen synthase (GS) was used as an intracellular marker of insulin action, before and during a 240-minute insulin infusion (100 mU/kg/h). Fractional GS activity did not increase in the lean (change, -0.9 +/- 3.3%), the overweight (-1.9 +/- 2.7%), or the obese (+2.2 +/- 1.6%) NIDDM subjects during the insulin infusion and was markedly decreased compared with the control subjects (change, +14.6 +/- 2.4%, all P less than .001). Glucose requirement was also significantly decreased in all three NIDDM groups (103 +/- 23 v 81 +/- 14 v 53 +/- 14 mg/m2/min, respectively) compared with the control subjects (319 +/- 18 mg/m2/min, all P less than .001). There was a significant negative correlation with BMI (r = -.51, P less than .01), but the difference in glucose requirement between the lean and obese NIDDM groups was not significant. Muscle GS activity at the end of the euglycemic clamp correlated with glucose requirement (r = .53, P less than .001), and a similar correlation was observed between the insulin-induced change in muscle GS activity from basal and glucose requirement (r = .47, P less than .005).(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of skeletal muscle casein kinase II by insulin is not diminished in subjects with insulin resistance

Insulin resistance, which may precede the development of non-insulin-dependent diabetes mellitus in Pima Indians, appears to result from a postreceptor defect in signal transduction in skeletal muscle. To identify the putative postreceptor lesion responsible for insulin resistance in Pima Indians, we investigated the influence of insulin on the activity of casein kinase II (CKII) in skeletal muscle of seven insulin-sensitive, four insulin-resistant, nondiabetic, and five insulin-resistant diabetic Pima Indians during a 2 h hyperinsulinemic, euglycemic clamp. In sensitive subjects, CKII was transiently activated reaching a maximum over basal activity (42%) at 45 min before declining. CKII was also stimulated in resistant (19%) and diabetic (34%) subjects. Basal CKII activity in resistant subjects was 40% higher than in either sensitive or diabetic subjects, although the concentration of CKII protein, as determined by Western blotting, was equal among the three groups. Basal CKII activity was correlated with fasting plasma insulin concentrations, suggesting that the higher activity in resistant subjects resulted from insulin action. Extracts of muscle obtained from all three groups either before or after insulin administration were treated with immobilized alkaline phosphatase, which reduced and equalized CKII activity. These results suggest that insulin stimulates CKII activity in human skeletal muscle by a mechanism involving phosphorylation of either CKII or of an effector molecule, and support the idea that elevated basal activity in resistant subjects results from insulin action. It appears that the ability of insulin to activate CKII in skeletal muscle is not impaired in insulin-resistant Pima Indians, and that the biochemical lesion responsible for insulin resistance occurs either downstream from CKII or in a different pathway of insulin action.

Defective insulin response of cyclic adenosine monophosphate-dependent protein kinase in insulin-resistant humans

Insulin-stimulated glycogen synthase activity in human muscle correlates with insulin-mediated glucose disposal and is reduced in insulin-resistant subjects. Inhibition of the cyclic AMP-dependent protein kinase (A-kinase) is considered as a possible mechanism of insulin action for glycogen synthase activation. In this study, we investigated the time course of insulin action on human muscle A-kinase activity during a 2-h insulin infusion in 13 insulin-sensitive (group S) and 7 insulin-resistant subjects (group R). Muscle biopsies were obtained from quadriceps femoris muscle at times 0, 10, 20, 40, and 120 min. Insulin infusion resulted in significant inhibition of A-kinase activity at 20 and/or 40 min using 0.2, 0.6, and 1.0 microM cyclic AMP in group S. A-kinase activities both before and after insulin administration were lower in group S than in group R using 0.6 microM cyclic AMP. The decrease in apparent affinity for cyclic AMP during insulin infusion was larger for group S compared with group R. Glycogen synthase activity increased significantly after insulin infusion in both groups and was higher in group S compared with group R. The data suggest that a defective response of A-kinase to insulin in insulin-resistant subjects could contribute to their reduced insulin stimulation of skeletal muscle glycogen synthase.

Multiple defects in muscle glycogen synthase activity contribute to reduced glycogen synthesis in non-insulin dependent diabetes mellitus

To define the mechanisms of impaired muscle glycogen synthase and reduced glycogen formation in non-insulin dependent diabetes mellitus (NIDDM), glycogen synthase activity was kinetically analyzed during the basal state and three glucose clamp studies (insulin approximately equal to 300, 700, and 33,400 pmol/liter) in eight matched nonobese NIDDM and eight control subjects. Muscle glycogen content was measured in the basal state and following clamps at insulin levels of 33,400 pmol/liter. NIDDM subjects had glucose uptake matched to controls in each clamp by raising serum glucose to 15-20 mmol/liter. The insulin concentration required to half-maximally activate glycogen synthase (ED50) was approximately fourfold greater for NIDDM than control subjects (1,004 +/- 264 vs. 257 +/- 110 pmol/liter, P less than 0.02) but the maximal insulin effect was similar. Total glycogen synthase activity was reduced approximately 38% and glycogen content was approximately 30% lower in NIDDM. A positive correlation was present between glycogen content and glycogen synthase activity (r = 0.51, P less than 0.01). In summary, defects in muscle glycogen synthase activity and reduced glycogen content are present in NIDDM. NIDDM subjects also have less total glycogen synthase activity consistent with reduced functional mass of the enzyme. These findings and the correlation between glycogen synthase activity and glycogen content support the theory that multiple defects in glycogen synthase activity combine to cause reduced glycogen formation in NIDDM.

Skeletal muscle glycogen synthase activity in subjects with non-insulin-dependent diabetes mellitus after glyburide therapy

Sulfonylureas are used in the treatment of non-insulin-dependent diabetes mellitus (NIDDM) largely because of their ability to enhance insulin secretion and possibly to potentiate insulin action. In this study, we investigated the effects of chronic glyburide treatment on glycogen synthase activity determined in skeletal muscle biopsies taken during euglycemic hyperinsulinemic clamps in nine Pima Indians with NIDDM. Insulin was infused at the rate of 40 mU/m2/min (low dose) followed by 400 mU/m2/min (high dose). Compared with the fasting value, the mean glycogen synthase activity assayed at low glucose-6-phosphate (G6P) concentration (active glycogen synthase) showed no significant changes during insulin infusion before glyburide treatment. After glyburide treatment, the mean active glycogen synthase increased by 39% (P less than .05) above the fasting value during the high-dose insulin infusion. Total glycogen synthase activity assayed at high G6P concentration did not change after glyburide treatment. Changes of insulin-stimulated active glycogen synthase associated with glyburide treatment correlated with changes in total body glucose disposal rates (r = .70, P less than .05) during euglycemic clamps. We conclude that glyburide treatment of subjects with NIDDM is associated with an increase in insulin action in vivo and concomitantly with improved insulin action on skeletal muscle glycogen synthase.