Insulin responsiveness in metabolic syndrome after eight weeks of cycle training

Abstract

Introduction: Insulin resistance in obesity is decreased after successful diet and exercise. Aerobic exercise training alone was evaluated as an intervention in subjects with the metabolic syndrome.

Methods: Eighteen nondiabetic, sedentary subjects, 11 with the metabolic syndrome, participated in 8 wk of increasing intensity stationary cycle training.

Results: Cycle training without weight loss did not change insulin resistance in metabolic syndrome subjects or sedentary control subjects. Maximal oxygen consumption (V·O 2max), activated muscle AMP-dependent kinase, and muscle mitochondrial marker ATP synthase all increased. Strength, lean body mass, and fat mass did not change. The activated mammalian target of rapamycin was not different after training. Training induced a shift in muscle fiber composition in both groups but in opposite directions. The proportion of type 2× fibers decreased with a concomitant increase in type 2a mixed fibers in the control subjects, but in metabolic syndrome, type 2× fiber proportion increased and type 1 fibers decreased. Muscle fiber diameters increased in all three fiber types in metabolic syndrome subjects. Muscle insulin receptor expression increased in both groups, and GLUT4 expression increased in the metabolic syndrome subjects. The excess phosphorylation of insulin receptor substrate 1 (IRS-1) at Ser337 in metabolic syndrome muscle tended to increase further after training in spite of a decrease in total IRS-1.

Conclusions: In the absence of weight loss, the cycle training of metabolic syndrome subjects resulted in enhanced mitochondrial biogenesis and increased the expression of insulin receptors and GLUT4 in muscle but did not decrease the insulin resistance. The failure for the insulin signal to proceed past IRS-1 tyrosine phosphorylation may be related to excess serine phosphorylation at IRS-1 Ser337, and this is not ameliorated by 8 wk of endurance exercise training.

Conflict of interest statement

Disclosures. The authors have no conflicts of interest to disclose. The results of the present study do not constitute endorsement by the American College of Sports Medicine.

Figures

Figure 1. Insulin responsiveness quantified by euglycemic insulin clamps was not increased by eight weeks of supervised stationary cycle training

This graph plots the pre- and post-training steady state glucose infusion rates (SSGIR) for seven sedentary control subjects (open circles) and eleven subjects with the metabolic syndrome (black filled circles). The means and standard errors for the two groups before and after training are also plotted. The SSGIR of the metabolic syndrome group was 29% of the corresponding insulin responsiveness quantified in the control group both in the baseline study and after training.

Figure 2. Change in muscle fiber composition and in fiber size after eight weeks of cycle training

Panel A show the fiber composition of pre-training and post-training biopsies of vastus lateralis muscle in sedentary controls and in subjects with the metabolic syndrome. Shown here are the mean and SEM of the percent of each of the three principal fiber types as quantified by the slow-twitch/fast-twitch myosin antibody technique of Behan, et al (4). The asterix indicates significantly different post-training (p

Figure 3. The impact of exercise training on muscle expression of the insulin receptor, IRS-1, and GLUT4

Panels A, B, and C display images of examples of immunoblots of the insulin receptor beta subunit, IRS-1, and GLUT4, respectively, expressed in muscle from biopsies of our subjects. Each lane of polyacrylamide gels contained a sample with 10 μg protein from muscle homogenate. Each sample was run in four separate experiments. The mean expression for each subject's sample quantified on an arbitrary scale relative to a reference muscle sample was then averaged with the mean expression of the other subjects in the group. These data were then expressed in the graph of Panel D in proportion to the control baseline data for each of the three factors that were quantified. The asterix indicates significantly different from baseline (p

Figure 4. Fiber-specific changes in expression of activated AMPK and mitochondrial marker ATP synthase

Since it was anticipated that exercise that was primarily endurance training would impact mitochondrial biogenesis, muscle AMPK activation and ATP synthase expression were quantified. Sections of muscle from each subject pre- and post-training were incubated with a monoclonal antibody against slow-twitch myosin heavy chain to identify the type 1 fibers on the section. A second rabbit polyclonal antibody against either phospho-AMPK or ATP synthase was added and incubated overnight at 4° C. Fluorescent tagged anti-rabbit IgG and anti-mouse IgG were added after the primary incubation and dual color images were obtained using a Leica confocal microscope. The signal intensity generated with either the phospho-AMPK or the ATP synthase antibody for the identified fiber type was quantified using digital imaging software (Quantity One from BioRad). At least thirty fibers had the signal intensity quantified for each antigen. Panel A displays the data for the control and metabolic syndrome subjects before and after training in the type 1 fibers. The expression of phospho-AMPK tended to be lower in the type 2x fibers for both controls and metabolic syndrome subjects, but ATP synthase expression appeared higher in 2x fibers. The activated AMPK was 2-fold increased after training for both groups in both fiber types. The ATP synthase was significantly increased by training in type 2x only in the metabolic syndrome group, indicating training-induced increased mitochondrial production, albeit not nearly to the level of increased activated AMPK. An asterix indicates significant post-training increase in expression compared to corresponding baseline (p

Figure 5. Cycle training impact on tyrosine and serine phosphorylation of muscle IRS-1

Panels A, B, and C show typical immunoblot images of muscle homogenates probed with antibodies specific for phospho-Tyr896, phospho-Ser337, and phospho-Ser636, respectively. Immunoblot images like these were evaluated for signal strength using digital analysis software for each subject in four separate experiments. Panel D summarizes the results of these analyses. In the pre-training samples, Tyr896 phosphorylation was not different, but phosphorylation at Ser337 and Ser636 were about 50% higher in the metabolic syndrome muscle samples. The exercise training protocol did not decrease the amount of phosphorylation at Ser337 or Ser636, and there appears to be a trend toward increased phosphorylation at Ser337 in both controls and metabolic syndrome muscle. The plus sign on the graph indicates a difference from the corresponding control subject data (p