Insulin kinetics and insulin binding to adipocytes in physically trained and food-restricted rats

Endurance exercise training increases insulin responsiveness in isolated adipocytes through IRS/PI3-kinase/Akt pathway

Endurance exercise training promotes important metabolic adaptations, and the adipose tissue is particularly affected. The aim of this study was to investigate how endurance exercise training modulates some aspects of insulin action in isolated adipocytes and in intact adipose tissue. Male Wistar rats were submitted to daily treadmill running (1 h/day) for 7 wk. Sedentary age-matched rats were used as controls. Final body weight, body weight gain, and epididymal fat pad weight did not show any statistical differences between groups. Adipocytes from trained rats were smaller than those from sedentary rats (205 ± 16.8 vs. 286 ± 26.4 pl; P < 0.05). Trained rats showed decreased plasma glucose (4.9 ± 0.13 vs. 5.3 ± 0.07 mM; P < 0.05) and insulin levels (0.24 ± 0.012 vs. 0.41 ± 0.049 mM; P < 0.05) and increased insulin-stimulated glucose uptake (23.1 ± 3.1 vs. 12.1 ± 2.9 pmol/cm2; P < 0.05) compared with sedentary rats. The number of insulin receptors and the insulin-induced tyrosine phosphorylation of insulin receptor-β subunit did not change between groups. Insulin-induced tyrosine phosphorylation insulin receptor substrates (IRS)-1 and -2 increased significantly (1.57- and 2.38-fold, respectively) in trained rats. Insulin-induced IRS-1/phosphatidylinositol 3 (PI3)-kinase (but not IRS-2/PI3-kinase) association and serine Akt phosphorylation also increased (2.06- and 3.15-fold, respectively) after training. The protein content of insulin receptor-β subunit, IRS-1 and -2, did not differ between groups. Taken together, these data support the hypothesis that the increased adipocyte responsiveness to insulin observed after endurance exercise training is modulated by IRS/PI3-kinase/Akt pathway.

S(G), S(I), and EGP of exercise-trained middle-aged men estimated by a two-compartment labeled minimal model

To examine the effects of physical training on glucose effectiveness (S(G)), insulin sensitivity (S(I)), and endogenous glucose production (EGP) in middle-aged men, stable-labeled frequently sampled intravenous glucose tolerance tests (FSIGTT) were performed on 11 exercise-trained middle-aged men and 12 age-matched sedentary men. The time course of EGP during the FSIGTT was estimated by nonparametric stochastic deconvolution. Glucose uptake-specific indexes of glucose effectiveness (S(2*)(G) x 10(2): 0.81 +/- 0.08 vs. 0.60 +/- 0.05 dl. min(-1). kg(-1), P < 0.05) and insulin sensitivity [S(2*)(I) x 10(4): 24.59 +/- 2.98 vs. 11.89 +/- 2.36 dl. min(-1). (microU/ml)(-1). kg(-1), P < 0.01], which were analyzed using the two-compartment minimal model, were significantly greater in the trained group than in the sedentary group. Plasma clearance rate (PCR) of glucose was consistently greater in the trained men than in sedentary men throughout FSIGTT. Compared with sedentary controls, EGP of trained middle-aged men was higher before glucose load. The EGP of the two groups was similarly suppressed by approximately 70% within 10 min, followed by an additional suppression after insulin infusion. EGP returned to basal level at approximately 60 min in the trained men and at 100 min in the controls, followed by its overshoot, which was significantly greater in the trained men than in the controls. In addition, basal EGP was positively correlated with S(2*)(G) . The higher basal EGP and greater EGP overshoot in trained middle-aged men appear to compensate for the increased insulin-independent (S(2*)(G)) and -dependent (S(2*)(I)) glucose uptake to maintain glucose homeostasis.

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.

The effects of physical training on insulin secretion and effectiveness and on glucose metabolism in obesity and type 2 (non-insulin-dependent) diabetes mellitus

Obese subjects with normal glucose tolerance (n = 55), and, in another study, a group of patients with Type 2 (non-insulin-dependent) diabetes (n = 33), and controls (n = 13) matched for body weight and age but with normal glucose tolerance, participated in an individualized physical training program for 3 months. Under controlled dietary conditions, metabolic studies were performed before and in steady state after the last exercise session after training in the subjects showing signs of physical training in VO2 max and heart rate measurements. No changes occurred in body weight, body cell mass, body fat or adipose tissue cellularity. Oral glucose tolerance was improved in the patients with diabetes mellitus only. In both diabetic and control subjects initially elevated C-peptide concentrations decreased, while low C-peptide values increased and which was particularly pronounced in diabetic subjects with subnormal values. Peripheral insulin values did not change. Glucose disposal rate measured with the glucose clamp technique was similar in diabetic patients and control subjects. An improvement was seen at both submaximal and maximal insulin levels in both groups, correlating with improvement in glucose tolerance in the diabetic subjects. No changes were found in adipocytes in insulin binding or the antilipolytic effect of insulin at submaximal insulin levels, but there was a normalization of a decreased glucose incorporation into triglycerides. These results indicate that both insulin secretion and effectiveness are altered by physical training in different ways in different clinical entities. They suggest that in insulin resistant conditions with high insulin secretion (as indicated by high C-peptide concentrations) the increased peripheral insulin sensitivity is followed by a decreased insulin secretion. This is not associated with an improvement of glucose tolerance. In Type 2 diabetes with low insulin secretion, an increased insulin secretion results from physical training, perhaps due to accompanying sensitization of the autonomic nervous system. Peripheral insulin concentrations are not altered, suggesting that the extra insulin produced is captured by the liver. This mechanism, as well as the improved peripheral insulin responsiveness seen in the whole body and also seen at the cellular level, probably both contribute to an improvement in glucose tolerance.

Effects of exercise training on in vivo insulin action in individual tissues of the rat

It has previously been suggested that exercise training leads to increased whole body insulin sensitivity. However, the specific tissues and metabolic pathways involved have not been examined in vivo. By combining the euglycemic clamp with administration of glucose tracers, [3H]2-deoxyglucose (2DG), [14C]glucose, and [3H]glucose, in vivo insulin action at the whole body level and within individual tissues has been assessed in exercise-trained (ET, running 1 h/d for 7 wk) and sedentary control rats at four insulin doses. Whole body insulin sensitivity was significantly increased in ET. In addition, the skeletal muscles, soleus, red and white gastrocnemius, extensor digitorum longus (EDL), and diaphragm all showed increased sensitivity of insulin-stimulated 2DG uptake with training. With the exception of EDL, no significant difference in insulin-mediated glycogen synthesis between control and ET could be found. Therefore, the increased insulin-induced 2DG uptake observed in muscle following training is apparently directed towards glucose oxidation. In ET animals, adipose tissue exhibited a significant increase in insulin-mediated 2DG uptake and [14C]glucose incorporation into free fatty acids but there was no difference from control in any parameters measured in lung or liver. EDL and white gastrocnemius, which are not primarily involved during exercise of this type, also demonstrated increased insulin sensitivity following training. In conclusion, exercise training results in a marked increase in whole body insulin sensitivity related mainly to increased glucose oxidation in skeletal muscle. This effect may be mediated by systemic as well as local factors and is likely to be of therapeutic value in pathological conditions exhibiting insulin resistance.