Acute bout of exercise induced prolonged muscle glucose transporter-4 translocation and delayed counter-regulatory hormone response in type 1 diabetes

Previous studies have demonstrated that an acute bout of aerobic exercise induces a subsequent delayed onset of hypoglycemia among patients with type 1 diabetes. However, the mechanisms of exercise-induced hypoglycemia in type 1 diabetes are still unclear. Streptozotocin (STZ) was injected to 6-week-old male Wistar rats, and three days after STZ injection, animals were randomly assigned into 2 groups: STZ with insulin only (STZ) and STZ with insulin and exercise (STZ+EX). Normal Wistar rats with exercise were used as control (CON+EX). Insulin was intraperitoneally injected (0.5 U/kg) to both STZ groups (-0.5 h), and a bout of aerobic exercise (15 m/min for 30 min) was conducted at euglycemic conditions (0 h). Blood was collected at 0, 1, 3, and 5 h after exercise from the carotid artery. While the blood glucose level was stable during the post-exercise period (0-5 h) in the STZ and CON+EX groups, it decreased significantly only in the STZ+EX group at 3 h. Plasma glucagon, adrenalin, and noradrenalin levels significantly increased at 1 h in the STZ group, whereas significant hormonal responses were observed at 5 h in the STZ+EX group. In skeletal muscle glucose metabolism-related pathway, the level of glucose transporter-4 (GLUT-4) translocation was significantly higher at 1 h in the CON and STZ groups. However, in the STZ+EX group, these activations were maintained by 5 h, indicating a sustained glucose metabolism in the STZ+EX group. A single bout of aerobic exercise induced a delayed onset of hypoglycemia in STZ-treated rats. A prolonged enhancement of GLUT-4 translocation and delayed counter-regulatory hormone responses may have contributed to the induction of hypoglycemia.

Muscle mitochondrial ATP synthesis and glucose transport/phosphorylation in type 2 diabetes

BACKGROUND

Muscular insulin resistance is frequently characterized by blunted increases in glucose-6-phosphate (G-6-P) reflecting impaired glucose transport/phosphorylation. These abnormalities likely relate to excessive intramyocellular lipids and mitochondrial dysfunction. We hypothesized that alterations in insulin action and mitochondrial function should be present even in nonobese patients with well-controlled type 2 diabetes mellitus (T2DM).

METHODS AND FINDINGS

We measured G-6-P, ATP synthetic flux (i.e., synthesis) and lipid contents of skeletal muscle with (31)P/(1)H magnetic resonance spectroscopy in ten patients with T2DM and in two control groups: ten sex-, age-, and body mass-matched elderly people; and 11 younger healthy individuals. Although insulin sensitivity was lower in patients with T2DM, muscle lipid contents were comparable and hyperinsulinemia increased G-6-P by 50% (95% confidence interval [CI] 39%-99%) in all groups. Patients with diabetes had 27% lower fasting ATP synthetic flux compared to younger controls (p = 0.031). Insulin stimulation increased ATP synthetic flux only in controls (younger: 26%, 95% CI 13%-42%; older: 11%, 95% CI 2%-25%), but failed to increase even during hyperglycemic hyperinsulinemia in patients with T2DM. Fasting free fatty acids and waist-to-hip ratios explained 44% of basal ATP synthetic flux. Insulin sensitivity explained 30% of insulin-stimulated ATP synthetic flux.

CONCLUSIONS

Patients with well-controlled T2DM feature slightly lower flux through muscle ATP synthesis, which occurs independently of glucose transport /phosphorylation and lipid deposition but is determined by lipid availability and insulin sensitivity. Furthermore, the reduction in insulin-stimulated glucose disposal despite normal glucose transport/phosphorylation suggests further abnormalities mainly in glycogen synthesis in these patients.

Effects of hyperglycemia on hepatic gluconeogenic flux during glycogen phosphorylase inhibition in the conscious dog

The aim of these studies was to investigate the effect of hyperglycemia with or without hyperinsulinemia on hepatic gluconeogenic flux, with the hypothesis that inhibition would be greatest with combined hyperglycemia/hyperinsulinemia. A glycogen phosphorylase inhibitor (BAY R3401) was used to inhibit glycogen breakdown in the conscious overnight-fasted dog, and the effects of a twofold rise in plasma glucose level (HI group) accompanied by 1) euinsulinemia (HG group) or 2) a fourfold rise in plasma insulin were assessed over a 5-h experimental period. Hormone levels were controlled using somatostatin with portal insulin and glucagon infusion. In the HG group, net hepatic glucose uptake and net hepatic lactate output substantially increased. There was little or no effect on the net hepatic uptake of gluconeogenic precursors other than lactate (amino acids and glycerol) or on the net hepatic uptake of free fatty acids compared with the control group. Consequently, whereas hyperglycemia had little effect on gluconeogenic flux to glucose 6-phosphate (G-6-P), net hepatic gluconeogenic flux was reduced because of increased hepatic glycolytic flux during hyperglycemia. Net hepatic glycogen synthesis was increased by hyperglycemia. The effect of hyperglycemia on gluconeogenic flux to G-6-P and net hepatic gluconeogenic flux was similar. We conclude that, in the absence of appreciable glycogen breakdown, the increase in glycolytic flux that accompanies hyperglycemia results in decreased net carbon flux to G-6-P but no effect on gluconeogenic flux to G-6-P.

Profiling of pentose phosphate pathway intermediates in blood spots by tandem mass spectrometry: application to transaldolase deficiency

BACKGROUND

Recently, several patients with abnormal polyol profiles in body fluids have been reported, but the origins of these polyols are unknown. We hypothesized that they are derived from sugar phosphate intermediates of the pentose phosphate pathway (PPP), and we developed a semiquantitative method for profiling of pentose phosphate pathway intermediates.

METHODS

Sugar phosphates in blood spots were simultaneously analyzed by liquid chromatography-tandem mass spectrometry using an ion-pair-loaded C(18) HPLC column. The tandem mass spectrometer was operated in the multiple-reaction monitoring mode. Enzymatically prepared D-[(13)C(6)]glucose 6-phosphate was used as internal standard. The method was used to study sugar phosphates abnormalities in a patient affected with a deficiency of transaldolase (TALDO1; EC 2.2.1.2).

RESULTS

In control blood spots, dihydroxyacetone phosphate, pentulose 5-phosphates, pentose 5-phosphates, hexose 6-phosphates, and sedoheptulose 7-phosphate were detected. Detection limits ranged from approximately 100 to approximately 500 nmol/L. Glyceraldehyde 3-phosphate and erythrose 4-phosphate were undetectable. Intra- and interassay imprecision (CVs) were 10-17% and 12-21%, respectively. In blood from the TALDO1-deficient patient, sedoheptulose 7-phosphate was increased.

CONCLUSIONS

The new method allows investigation of patients in whom a defect in the PPP is suspected. Measurements of sugar phosphate intermediates of the PPP may provide new insights into metabolic defects underlying the accumulating polyols.

A convenient diagnostic function test of peripheral blood neutrophils in glycogen storage disease type Ib

Neutrophils from patients suffering from glycogen storage disease type Ib (GSD-Ib) show several defects. one of which is a decreased rate of glucose utilization. In this study, we established experimental conditions to show the stimulation of the neutrophil respiratory burst by extracellular glucose. With phorbol-myristate-acetate as stimulus of the burst, the activity of the NADPH oxidase in GSD-Ib neutrophils hardly increased on addition of glucose. In control and GSD-type Ia neutrophils, a clear increase was observed. The lack of response to extracellular glucose in GSD-Ib neutrophils is correlated with the inability to raise intracellular glucose-6-P levels on glucose addition, thereby limiting the activity of the generation of NADPH in the hexose-monophosphate shunt. Our study shows the usefulness of this test for the diagnosis of neutrophil function abnormality in GSD-Ib patients.

Mediated, amperometric biosensor for glucose-6-phosphate monitoring based on entrapped glucose-6-phosphate dehydrogenase, Mg2+ ions, tetracyanoquinodimethane, and nicotinamide adenine dinucleotide phosphate in carbon paste

In this study, an amperometric carbon paste biosensor is developed for glucose-6-phosphate (G6P) monitoring which is based on entrapped Mg2+ ions, G6P dehydrogenase, NADP+ polyethylenimine (PEI) and the electroactive mediator, tetracyanoquinodimethane (TCNQ). The calibration line had a slope of 1.55 x 10(-5) A. M-1 with a correlation coefficient of 0.9965. The limit of detection (defined as three times the standard deviation of the response of the electrode to blank phosphate buffer injections (noise)) of the G6P biosensor was 5.0 x 10(-5) M. The application of this biosensor for monitoring G6P in human blood using the standard addition method is also demonstrated. A two-parameter empirical equation which adequately describes the deactivation of the biosensor steady-state response with time is also proposed.

Glucose production, utilization, and cycling in response to moderate exercise in obese subjects with type 2 diabetes and mild hyperglycemia

The glucoregulatory and hormonal responses to moderate-intensity exercise (50% VO2max for 45 min) were examined in subjects with type 2 diabetes and mild hyperglycemia. We studied seven obese subjects with type 2 diabetes and seven lean and seven obese control subjects (fasting plasma glucose levels, 7.5 +/- 0.5, 4.8 +/- 0.1, and 5.2 +/- 0.1 mmol/l, respectively). Glucose production, utilization, and cycling (flux between glucose and glucose-6-phosphate [G-6-P]) were measured with [6-(3)H]glucose and [2-(3)H]glucose using the constant specific-activity method. Insulin levels decreased normally during exercise in diabetic subjects. Plasma glucose levels decreased in diabetic subjects, but remained constant in control subjects. Basal glucose production was not different among groups and increased similarly during exercise. The decrease in plasma glucose in diabetic subjects was due to greater glucose utilization (867 +/- 83 vs. 726 +/- 143 micromol x m(-2) x min(-1); P < 0.05). This was a consequence of the mass effect of hyperglycemia, since glucose metabolic clearance increased similarly in all groups. Glucose cycling, expressed as a percentage of total glucose output (i.e., flux through G-6-P) was elevated at rest (P < 0.01), but decreased during exercise (P < 0.01). The catecholamine response to exercise was blunted in diabetic subjects, presumably indicating autonomic dysfunction. In conclusion, during moderate-intensity exercise in obese diabetic subjects with mild hyperglycemia, 1) insulin secretory responses were normally regulated; 2) glucose homeostasis was different from that in nondiabetic subjects because glucose levels decreased during exercise; 3) the decrease in plasma glucose was due to greater-than-normal rates of glucose utilization, which were sustained by hyperglycemia; and 4) elevated basal rates of glucose cycling decreased during exercise, presumably because exercise simultaneously lowered plasma glucose, was associated with a blunted catecholamine response, and accentuated an underlying defect in hepatic glucokinase activity in type 2 diabetes.