Peripheral glucose uptake and skeletal muscle GLUT4 content in man: effect of insulin and free fatty acids

Oxidative Stress, Intrauterine Growth Restriction, and Developmental Programming of Type 2 Diabetes

Intrauterine growth restriction (IUGR) leads to reduced birth weight and the development of metabolic diseases such as Type 2 diabetes in adulthood. Mitochondria dysfunction and oxidative stress are commonly found in key tissues (pancreatic islets, liver, and skeletal muscle) of IUGR individuals. In this review, we explore the role of oxidative stress in IUGR-associated diabetes etiology.

Altered PI3-kinase/Akt signalling in skeletal muscle of young men with low birth weight

BACKGROUND

Low birth weight (LBW) is associated with increased future risk of insulin resistance and type 2 diabetes mellitus. The underlying molecular mechanisms remain poorly understood. We have previously shown that young LBW men have reduced skeletal muscle expression of PI3K p85alpha regulatory subunit and p110beta catalytic subunit, PKCzeta and GLUT4 in the fasting state. The aim of this study was to determine whether insulin activation of the PI3K/Akt and MAPK signalling pathways is altered in skeletal muscle of young adult men with LBW.

METHODS

Vastus lateralis muscle biopsies were obtained from 20 healthy 19-yr old men with BW< or = 10th percentile for gestational age (LBW) and 20 normal birth weight controls (NBW), matched for physical fitness and whole-body glucose disposal, prior to (fasting state) and following a 4-hr hyperinsulinemic euglycemic clamp (insulin stimulated state). Expression and phosphorylation of selected proteins was determined by Western blotting.

PRINCIPAL FINDINGS

Insulin stimulated expression of aPKCzeta (p<0.001) and Akt1 (p<0.001) was decreased in muscle of LBW men when compared to insulin stimulated controls. LBW was associated with increased insulin stimulated levels of IRS1 (p<0.05), PI3K p85alpha (p<0.001) and p110beta (p<0.05) subunits, while there was no significant change in these proteins in insulin stimulated control muscle. In addition LBW had reduced insulin stimulated phospho-Akt (Ser 473) (p<0.01), indicative of reduced Akt signalling. Insulin stimulated expression/phosphorylation of all the MAPK proteins studied [p38 MAPK, phospho-p38 MAPK (Thr180/Tyr182), phospho-ERK (Thr 202/Tyr204), JNK1, JNK2 and phospho-JNK (Thr 183/Tyr185)] was not different between groups.

CONCLUSIONS

We conclude that altered insulin activation of the PI3K/Akt but not the MAPK pathway precedes and may contribute to development of whole-body insulin resistance and type 2 diabetes in men with LBW.

Chronic free fatty acid infusion in rats results in insulin resistance but no alteration in insulin-responsive glucose transporter levels in skeletal muscle

To investigate the mechanism by which free fatty acids (FFA) affect glucose uptake, we studied the effect of chronic elevation (24 h) of plasma FFA in rats on whole body glucose disposal and glucose utilization index (GUI) in the basal state and under a euglycemic hyperinsulinemic clamp in relation to the amount of insulin-responsive glucose transporter (IRGT, i.e., GLUTU) protein in different muscles (oxidative and glycolytic) and adipose tissue. Infusion of intralipid in the basal state led to a approximately 40% increase in whole body glucose uptake and a approximately 250% increase in GUI in adipose tissue as compared to control rats. There was no change in the amount of IRGT protein in any of the muscle types whereas in fat depots it was either unchanged or decreased. Under moderate of supraphysiological hyperinsulinemia, increment of whole body glucose disposal was significantly lower in intralipid perfused rats when compared to controls (approximately 110 microU/mL: 0.7 +/- 0.1 vs. 1.3 +/- 0.1 mg/min, P < 0.02; approximately 1000 microU/mL: 3.0 +/- 0.2 vs. 3.9 +/- 0.4 mg/min, P < 0.02). Under moderate hyperinsulinemia stimulation, GUI was significantly reduced in different muscles and adipose tissue as compared to controls. We conclude that peripheral insulin resistance which occurs after elevation of plasma FFA levels does not seem to be explained by changes in the amount of IRGT protein in either oxidative or glycolytic skeletal muscle. Thus fatty acid infusion appears to be associated with a defect in IRGT translocation to the plasma membrane, fusion with the membrane, or intrinsic activity.

Glucose processing during the intravenous glucose tolerance test

The impact of the dynamic changes in plasma glucose and insulin levels observed during a frequently sampled intravenous (IV) glucose tolerance test (FSIGT) on whole-body glucose processing and muscle glycogen metabolism is not known. Paired randomized FSIGTs were performed in eight healthy subjects (age, 31 years; range, 28 to 35; BMI, 25.4 kg/m2; range, 22.3 to 32.1), one with muscle biopsy samples and one without. The mean time average (0- to 40- and 0- to 120-minute) insulin levels during the test were 26.6 and 11.4 mU/1, respectively. Glucose oxidation increased following the IV glucose bolus (basal 1.34 +/- 0.21 v mean value at 0 to 120 minutes 2.09 +/- 0.22 mg/kg fat-free mass [FFM]/min, P < .02). In contrast, fractional glucose-6-phosphate [G-6-P]) (0.1/10 mmol/L) skeletal muscle glycogen synthase activity in muscle biopsies obtained before and following the IV glucose bolus (-30, 30,60, and 120 minutes, respectively) were unchanged (38.1% +/- 2.3%, 38.3% +/- 2.9%, 38.1% +/- 2.3%, 35.4% +/- 2.3%, NS). Skeletal muscle glycogen concentration decreased slightly (449 +/- 54, 439 +/- 55, and 383 +/- 29, and 438 +/- 48 mmol/kg dry weight, P =.05), indicating no net storage of glucose into glycogen during the FSIGT. G-6-P decreased (0.77 +/- 0.08, 0.64 +/- 0.07, 0.66 +/- 0.07, and 0.54 +/- 0.04 mmol/kg dry weight, P < .05). Levels of the insulin-regulatable glucose transporter, GLUT-4, were unchanged. Insulin sensitivity (Si), glucose effectiveness, and insulin secretion parameters (01 and 02) were not affected by the muscle biopsy procedure. In conclusion, the FSIGT is associated predominantly with increased whole-body glucose oxidation with no apparent activation of muscle glucose storage as glycogen. Thus, the Si measured by the FSIGT, although similar in magnitude to the clamp-derived parameter, represents primarily glucose oxidation, in contrast to the euglycemic clamp, which involves glucose oxidation and storage.

Effect of insulin on GLUT-4 mRNA and protein concentrations in skeletal muscle of patients with NIDDM and their first-degree relatives

We examined whether insulin resistance, i.e. impaired insulin stimulated glucose uptake in NIDDM patients and their first-degree relatives is associated with alterations in the effect of insulin on the expression of the GLUT-4 gene in skeletal muscle in vivo. Levels of GLUT-4 mRNA and protein were measured in muscle biopsies taken before and after a euglycaemic insulin clamp from 14 NIDDM patients, 13 of their first-degree relatives and 17 control subjects. Insulin stimulated glucose uptake was decreased in the diabetic subjects (19.8 +/- 3.0 mumol.kg LBM-1.min-1, both p < 0.001) compared with control subjects (44.1 +/- 2.5 mumol.kg LBM-1.min-1) and relatives (39.9 +/- 3.3 mumol.kg LBM-1.min-1). Basal GLUT-4 mRNA levels were significantly higher in diabetic subjects and relatives compared to control subjects (99 +/- 8 and 108 +/- 9 pg/micrograms RNA vs 68 +/- 5 pg/micrograms RNA; both p < 0.01). Insulin increased GLUT-4 mRNA levels in all control subjects (from 68 +/- 5 to 92 +/- 6 pg/micrograms RNA; p < 0.0001), but not in the diabetic patients (from 99 +/- 8 to 90 +/- 8 pg/micrograms RNA, NS), or their relatives (from 94 +/- 9 to 101 +/- 11 pg/micrograms RNA, NS). In the relatives, individual basal GLUT-4 mRNA concentrations varied between 55 and 137 pg/micrograms RNA.(ABSTRACT TRUNCATED AT 250 WORDS)

GLUT 4 and insulin receptor binding and kinase activity in trained human muscle

1. Physical training enhances sensitivity and responsiveness of insulin-mediated glucose uptake in human muscle. This study examines if this effect of physical training is due to increased insulin receptor function or increased total concentration of insulin-recruitable glucose transporter protein (GLUT 4). 2. Seven healthy young subjects carried out single leg bicycle training for 10 weeks at 70% of one leg maximal oxygen uptake (VO2,max). Subsequently biopsies were taken from the vastus lateralis muscle of both legs. 3. Single leg VO2,max increased for the trained leg (46 +/- 3 to 52 +/- 2 ml min-1 kg-1 (means +/- S.E.M., P < 0.05), and cytochrome c oxidase activity was higher in this compared to the untrained leg (2.0 +/- 0.1 vs. 1.4 +/- 0.1 nmol s-1 (mg muscle)-1, P < 0.05). Insulin binding as well as basal- and insulin-stimulated receptor kinase activity did not differ between trained and untrained muscle. The concentration of GLUT 4 protein was higher in the former (14.9 +/- 1.9 vs. 11.6 +/- 1.0 arbitrary units (micrograms protein)-1 in crude membranes, P < 0.05). The training-induced increase in GLUT 4 (26 +/- 11%) matched a previously reported increase in maximum insulin-stimulated leg glucose uptake (25 +/- 7%) in the same subjects, and individual values of the two variables correlated (correlation coefficient (r) = 0.84, P < 0.05). 4. In conclusion, in human muscle training induces a local contraction-dependent increase in GLUT 4 protein, which enhances the effect of insulin on glucose uptake. On the other hand, insulin receptor function in muscle is unlikely to be affected by training.