Skeletal muscle is a heterogeneous tissue composed of different fiber types. Studies suggest that insulinmediated glucose metabolism is different between muscle fiber types. We hypothesized that differences are due to fiber type-specific expression/regulation of insulin signaling elements and/or metabolic enzymes. Pools of type I and II fibers were prepared from biopsies of the vastus lateralis muscles from lean, obese, and type 2 diabetic subjects before and after a hyperinsulinemiceuglycemic clamp. Type I fibers compared with type II fibers have higher protein levels of the insulin receptor, GLUT4, hexokinase II, glycogen synthase (GS), and pyruvate dehydrogenase-E1a (PDH-E1a) and a lower protein content of Akt2, TBC1 domain family member 4 (TBC1D4), and TBC1D1. In type I fibers compared with type II fibers, the phosphorylation response to insulin was similar (TBC1D4, TBC1D1, and GS) or decreased (Akt and PDH-E1a). Phosphorylation responses to insulin adjusted for protein level were not different between fiber types. Independently of fiber type, insulin signaling was similar (TBC1D1, GS, and PDH-E1a) or decreased (Akt and TBC1D4) in muscle from patients with type 2 diabetes compared with lean and obese subjects. We conclude that human type I muscle fibers compared with type II fibers have a higher glucose-handling capacity but a similar sensitivity for phosphoregulation by insulin.Skeletal muscle is important for whole-body insulinstimulated glucose disposal (1), and skeletal muscle insulin resistance is a common phenotype of obesity and type 2 diabetes (T2D) (2). Skeletal muscle is a heterogeneous tissue composed of different fiber types, which can be divided according to myosin heavy chain (MHC) isoform expression. Studies in rodents show that insulin-stimulated glucose uptake in the oxidative type I fiber-dominant muscles is higher than in muscles with a high degree of glycolytic type II fibers (3-6). Whether this phenomenon is due to differences in locomotor activity of individual muscles or a direct consequence of the fiber-type composition is largely unknown. In incubated rat muscle, insulin-induced glucose uptake was higher (;100%) in type IIa (oxidative/glycolytic) compared with IIx and IIb (glycolytic) fibers (7,8), suggesting that insulin-mediated glucose uptake is related to the oxidative capacity of the muscle fiber. In humans, a positive correlation between proportions of type I fibers in muscle and whole-body insulin sensitivity has been demonstrated (9-11). Furthermore, insulin-stimulated glucose transport in human muscle strips was associated with the relative type I fiber content (12). Thus, it is likely that human type I fibers are more important than type II fibers for maintaining glucose homeostasis in response to insulin. Indeed, a decreased proportion of type I fibers has been found in various insulin resistant states such as the metabolic syndrome (9), obesity (13,14), T2D in some (10,13,14) but not all (12,15) studies and following bedrest (16), as well as in tetraplegic patients (17), ...

Keypointsr AMP-activated protein kinase (AMPK) is an important regulator of cellular energy status during exercise.r Most human studies investigating skeletal muscle protein signalling have been performed in whole muscle biopsy samples, yet recent studies suggest muscle fibre type-specific AMPK expression with potential fibre type-specific regulation of AMPK during exercise.r This study provides novel and comprehensive data on human muscle fibre type-specific expression levels of AMPK subunits and downstream targets of AMPK.r We show a differentiated response to exercise of key metabolic signalling proteins in human type I and type II muscle fibres during interval exercise, not evident during continuous exercise. These differences between exercise types were not present in whole muscle biopsy samples.r Our findings highlight the importance of performing fibre type-specific measurements and the increased activation of AMPK in interval vs. continuous exercise could be important for exercise type-specific adaptations, i.e. metabolism, insulin sensitivity and mitochondrial density in human skeletal muscle.Abstract AMP-activated protein kinase (AMPK) is a regulator of energy homeostasis during exercise. Studies suggest muscle fibre type-specific AMPK expression. However, fibre type-specific regulation of AMPK and downstream targets during exercise has not been demonstrated. We hypothesized that AMPK subunits are expressed in a fibre type-dependent manner and that fibre type-specific activation of AMPK and downstream targets is dependent on exercise intensity. Pools of type I and II fibres were prepared from biopsies of vastus lateralis muscle from healthy men before and after two exercise trials: (1) continuous cycling (CON) for 30 min at 69 ± 1% peak rate of O 2 consumption (V O 2 peak ) or (2) interval cycling (INT) for 30 min with 6 × 1.5 min high-intensity bouts peaking at 95 ± 2%V O 2 peak . In type I vs. II fibres a higher β 1 AMPK (+215%) and lower γ 3 AMPK expression (−71%) was found. and ACC Ser221 ) or higher (GS 2+2a ). Exercise-induced glycogen degradation in type I vs. II fibres was similar (CON) or lower (INT). In conclusion, a differentiated response to exercise of metabolic signalling/effector proteins in human type I and II fibres was evident during interval exercise. This

. Enhanced insulin signaling in human skeletal muscle and adipose tissue following gastric bypass surgery.

Human muscle is studied during glycogen depletion and repletion to understand the influence of exercise and muscle glycogen on total ceramide content. In addition, fiber-type-specific ceramide storage is investigated. Ten healthy males (26.4 +/- 0.9 years, BMI 24.4 +/- 0.7 kg m(-2) and VO2max 57 +/- 2 mL O2 min(-1) kg(-1)) participated in the study. On the first day, one leg was glycogen-depleted (DL) by exhaustive intermittent exercise followed by low carbohydrate diet. Next day, in the overnight fasted condition, muscle biopsies were excised from vastus lateralis before and after exhaustive exercise from both DL and control leg (CL). Muscle glycogen was analyzed biochemically and total muscle ceramide content by 2D quantitative lipidomic approach. Furthermore, fiber-type ceramide content was determined by fluorescence immunohistochemistry. Basal muscle glycogen was decreased (P < 0.05) with 50 +/- 6% in DL versus CL. After exhaustive exercise, muscle glycogen was similar in CL and DL 139 +/- 38 and 110 +/- 31 mmol kg(-1), respectively. Total muscle ceramide 58 +/- 1 pmol mg(-1) was not influenced by glycogen or exercise. Ceramide content was consistently higher (P < 0.001) in type I than in type II muscle fibers. In conclusion, human skeletal muscle, ceramide content is higher in type I than in type II. Despite rather large changes in muscle glycogen induced by prior depletion, exercise to exhaustion and repletion, total muscle ceramide concentration remained unchanged.

Background and Aims Sodium glucose cotransporter 2 inhibitors (SGLT2i) reduce cardiovascular events and protects kidney function in type 2 diabetes (DM2) patients. These benefits may partly be related to improvement of vascular function. In this study we examined the effects of SGLT2i treatment on vasodilatory capacity in patients with DM2. Method Using a double-blind, randomized, placebo-controlled cross-over study design, we included 15 patients with DM2 and preserved kidney function (eGFR &gt; 60 ml/min/1.73 m2). Each participant received four weeks of SGLT2i treatment (empagliflozin 10 mg once daily) or matching placebo. After a two-week wash-out period, each participant was crossed over to four weeks of the opposite treatment. At the end of each treatment period, vascular function was evaluated by venous occlusion plethysmography. Forearm blood flow (FBF) was measured during intra-arterial infusion of increasing concentrations of acetylcholine (ACh) and sodium nitroprusside (SNP), assessing endothelium-dependent and independent vasodilation, respectively. Repeated measures two-way ANOVA was performed to compare absolute FBF between groups. Results 11/15 (73%) of participants were male. Mean age was 68±9 (SD) years (range 49-82), eGFR was 81±10 ml/min/1.73 m2 and duration of diabetes was 15.8±9 years. 11/15 (73%) had hypertension and 9/15 (60%) received treatment with an ACE-inhibitor or Angiotensin-II blocker. 4/15 (27%) had cardiovascular disease. The median albumin-to-creatinine ratio was 29 mg/g (range: 4-1293). Both ACh and SNP dose-dependently increased FBF (Figure 1 and 2). FBF during SNP infusion was significantly higher during empagliflozin treatment as compared to placebo (p = 0.004), whereas there was no difference between empagliflozin and placebo in FBF during ACh infusion (P = 0.399). Conclusion Empagliflozin improves endothelium-independent vasodilatation in patients with DM2, whereas no changes could be observed in endothelium-dependent vasodilation. These results suggest that SGLT2i positively affects vascular function and that the effect is not related to nitric oxide from the endothelium.