Sex differences in insulin regulation of skeletal muscle glycogen synthase and changes during weight loss and exercise in adults

OBJECTIVE

The authors sought to understand sex differences in muscle metabolism in 73 older men and women.

METHODS

Body composition, VO2max, and insulin sensitivity (M) by 3-hour hyperinsulinemic-euglycemic clamp with vastus lateralis muscle biopsies were measured.

RESULTS

Women had lower body weight, VO2max, and fat-free mass than men. Men had lower M, lower change (insulin minus basal) in muscle glycogen synthase (GS) activity, and lower change in AKT protein expression than women. M was associated with the change (insulin-basal) in GS activity and the change in AKT protein expression. Sex differences (n = 60) were tested with 6-month weight loss or 3×/week aerobic exercise training. The postintervention minus preintervention change (insulin-basal) (∆∆) in GS activity (fractional, independent, total) was higher in men than women in the weight loss group and ∆∆ in GS fractional activity was higher in women than men in the aerobic exercise group. In all participants, ∆∆ in GS fractional and independent activities was related to ∆∆ in AKT expression and glycogen content.

CONCLUSIONS

Sex differences in insulin sensitivity may be explained at the cellular muscle level, and to improve skeletal muscle insulin action in older adults, it may be necessary to recommend different behavioral strategies depending on the individual's sex.

Supplemental Oxygen Improves In Vivo Mitochondrial Oxidative Phosphorylation Flux in Sedentary Obese Adults With Type 2 Diabetes

Type 2 diabetes is associated with impaired exercise capacity. Alterations in both muscle perfusion and mitochondrial function can contribute to exercise impairment. We hypothesized that impaired muscle mitochondrial function in type 2 diabetes is mediated, in part, by decreased tissue oxygen delivery and would improve with oxygen supplementation. Ex vivo muscle mitochondrial content and respiration assessed from biopsy samples demonstrated expected differences in obese individuals with (n = 18) and without (n = 17) diabetes. Similarly, in vivo mitochondrial oxidative phosphorylation capacity measured in the gastrocnemius muscle via ³¹P-MRS indicated an impairment in the rate of ADP depletion with rest (27 ± 6 s [diabetes], 21 ± 7 s [control subjects]; P = 0.008) and oxidative phosphorylation (P = 0.046) in type 2 diabetes after isometric calf exercise compared with control subjects. Importantly, the in vivo impairment in oxidative capacity resolved with oxygen supplementation in adults with diabetes (ADP depletion rate 5.0 s faster, P = 0.012; oxidative phosphorylation 0.046 ± 0.079 mmol/L/s faster, P = 0.027). Multiple in vivo mitochondrial measures related to HbA_1c These data suggest that oxygen availability is rate limiting for in vivo mitochondrial oxidative exercise recovery measured with ³¹P-MRS in individuals with uncomplicated diabetes. Targeting muscle oxygenation could improve exercise function in type 2 diabetes.

Acute vitamin C improves cardiac function, not exercise capacity, in adults with type 2 diabetes

BACKGROUND

People with type 2 diabetes (T2D) have impaired exercise capacity, even in the absence of complications, which is predictive of their increased cardiovascular mortality. Cardiovascular dysfunction is one potential cause of this exercise defect. Acute infusion of vitamin C has been separately shown to improve diastolic and endothelial function in prior studies. We hypothesized that acute vitamin C infusion would improve exercise capacity and that these improvements would be associated with improved cardiovascular function.

METHODS

Adults with T2D (n = 31, 7 female, 24 male, body mass index (BMI): 31.5 ± 0.8 kg/m²) and BMI-similar healthy adults (n = 21, 11 female, 10 male, BMI: 30.4 ± 0.7 kg/m²) completed two randomly ordered visits: IV infusion of vitamin C (7.5 g) and a volume-matched saline infusion. During each visit peak oxygen uptake (VO₂peak), brachial artery flow mediated dilation (FMD), reactive hyperemia (RH; plethysmography), and cardiac echocardiography were measured. General linear mixed models were utilized to assess the differences in all study variables.

RESULTS

Acute vitamin C infusion improved diastolic function, assessed by lateral and septal E:E' (P < 0.01), but did not change RH (P = 0.92), or VO₂peak (P = 0.33) in any participants.

CONCLUSION

Acute vitamin C infusion improved diastolic function but did not change FMD, forearm reactive hyperemia, or peak exercise capacity. Future studies should further clarify the role of endothelial function as well as other possible physiological causes of exercise impairment in order to provide potential therapeutic targets.Trial registration NCT00786019. Prospectively registered May 2008.

Exenatide improves diastolic function and attenuates arterial stiffness but does not alter exercise capacity in individuals with type 2 diabetes

BACKGROUND

Exercise is recommended as a cornerstone of treatment for type 2 diabetes mellitus (T2DM), however, it is often poorly adopted by patients. Even in the absence of apparent cardiovascular disease, persons with T2DM have an impaired ability to carry out maximal and submaximal exercise and these impairments are correlated with cardiac and endothelial dysfunction. Glucagon-like pepetide-1 (GLP-1) augments endothelial and cardiac function in T2DM. We hypothesized that administration of a GLP-1 agonist (exenatide) would improve exercise capacity in T2DM.

METHODS AND RESULTS

Twenty-three participants (64±4years; mean±SE) with uncomplicated T2DM were randomized in a double-blinded manner to receive either 10μg BID of exenatide or matching placebo after baseline measurements. Treatment with exenatide did not improve VO_2peak (P=0.1464) or VO₂ kinetics (P=0.2775). Diastolic function, assessed via resting lateral E:E', was improved with administration of exenatide compared with placebo (Placebo Pre: 7.6±1.0 vs. Post: 8.4±1.2 vs. Exenatide Pre: 8.1±0.7 vs. Post: 6.7±0.6; P=0.0127). Additionally, arterial stiffness measured by pulse wave velocity, was reduced with exenatide treatment compared with placebo (Placebo Pre: 10.5±0.8 vs. Post: 11.5±1.1s vs. Exenatide Pre: 11.4±1.8 vs. Post: 10.2±1.4s; P=0.0373). Exenatide treatment did not improve endothelial function (P=0.1793).

CONCLUSIONS

Administration of exenatide improved cardiac function and reduced arterial stiffness, however, these changes were not accompanied by improved functional exercise capacity. In order to realize the benefits of this drug on exercise capacity, combining exenatide with aerobic exercise training in participants with T2DM may be warranted.

Plasminogen Activator Inhibitor-1, Body Fat and Insulin Action in Aging Women

Plasminogen activator inhibitor-1 (PAI-1) over-expression is linked to obesity, insulin resistance, and age. We hypothesized that aerobically trained women athletes would have reduced PAI-1 regardless of age compared to sedentary controls and levels would be associated with hyperinsulinemia. Plasma PAI-1 was measured in women athletes who were young (YA, n=19, VO_2max=53.7±1.1ml/kg/min) and older (OA, n=18, VO_2max=46.6±1.5ml/kg/min) and compared to 19 sedentary controls (YC, n=6, VO_2max=35.9±1.2ml/kg/min; OC, n=13, VO_2max=22.1±1.7ml/kg/min). PAI-1 levels did not differ between YA and OA but was 23% higher in OC compared to OA (P<0.05). PAI-1 was inversely associated with VO_2max, directly to %body fat, and subcutaneous abdominal fat, fasting leptin, insulin, and first-phase and second-phase insulin response during a hyperglycemic clamp. The current results suggest that older athletes have low PAI-1 levels possibly due to high levels of physical fitness, reduced body fat, and increased insulin action and may contribute to low atherothrombosis and improved cardiovascular health.