Satoru Ato

Acute resistance exercise-induced IGF1 expression and subsequent GLUT4 translocation

Acute aerobic exercise (AE) is a major physiological stimulus for skeletal muscle glucose uptake through activation of 5′ AMP‐activated protein kinase (AMPK). However, the regulation of glucose uptake by acute resistance exercise (RE) remains unclear. To investigate the intracellular regulation of glucose uptake after acute RE versus acute AE, male Sprague–Dawley rats were divided into three groups: RE, AE, or nonexercise control. After fasting for 12 h overnight, the right gastrocnemius muscle in the RE group was exercised at maximum isometric contraction via percutaneous electrical stimulation (3 × 10 sec, 5 sets). The AE group ran on a treadmill (25 m/min, 60 min). Muscle samples were taken 0, 1, and 3 h after completion of the exercises. AMPK, Ca2+/calmodulin‐dependent protein kinase II, and TBC1D1 phosphorylation were increased immediately after both forms of exercise and returned to baseline levels by 3 h. Muscle IGF1 expression was increased by RE but not AE, and maintained until 3 h after RE. Additionally, Akt and AS160 phosphorylation were sustained for 3 h after RE, whereas they returned to baseline levels by 3 h after AE. Similarly, GLUT4 translocation remained elevated 3 h after RE, although it returned to the baseline level by 3 h after AE. Overall, this study showed that AMPK/TBC1D1 and IGF1/Akt/AS160 signaling were enhanced by acute RE, and that GLUT4 translocation after acute RE was more prolonged than after acute AE. These results suggest that acute RE‐induced increases in intramuscular IGF1 expression might be a distinct regulator of GLUT4 translocation.

Contraction mode itself does not determine the level of mTORC1 activity in rat skeletal muscle

Resistance training with eccentric contraction has been shown to augment muscle hypertrophy more than other contraction modes do (i.e., concentric and isometric contraction). However, the molecular mechanisms involved remain unclear. The purpose of this study was to investigate the effect of muscle contraction mode on mammalian target of rapamycin complex 1 (mTORC1) signaling using a standardized force‐time integral (load (weight) × contraction time). Male Sprague–Dawley rats were randomly assigned to three groups: eccentric contraction, concentric contraction, and isometric contraction. The right gastrocnemius muscle was exercised via percutaneous electrical stimulation‐induced maximal contraction. In experiment 1, different modes of muscle contraction were exerted using the same number of reps in all groups, while in experiment 2, muscle contractions were exerted using a standardized force‐time integral. Muscle samples were obtained immediately and 3 h after exercise. Phosphorylation of molecules associated with mTORC1 activity was assessed using western blot analysis. In experiment 1, the force‐time integral was significantly different among contraction modes with a higher force‐time integral for eccentric contraction compared to that for other contraction modes (P < 0.05). In addition, the force‐time integral was higher for concentric contraction compared to that for isometric contraction (P < 0.05). Similarly, p70S6K phosphorylation level was higher for eccentric contraction than for other modes of contraction (P < 0.05), and concentric contraction was higher than isometric contraction (P < 0.05) 3 h after exercise. In experiment 2, under the same force‐time integral, p70S6K (Thr389) and 4E‐BP1 phosphorylation levels were similar among contraction modes 3 h after exercise. Our results suggest that mTORC1 activity is not determined by differences in muscle contraction mode itself. Instead, mTORC1 activity is determined by differences in the force‐time integral during muscle contraction.

Effect of resistance exercise under conditions of reduced blood insulin on AMPKα Ser485/491 inhibitory phosphorylation and AMPK pathway activation

Insulin stimulates skeletal muscle glucose uptake via activation of the protein kinase B/Akt (Akt) pathway. Recent studies suggest that insulin downregulates AMP-activated protein kinase (AMPK) activity via Ser485/491 phosphorylation of the AMPK α-subunit. Thus lower blood insulin concentrations may induce AMPK signal activation. Acute exercise is one method to stimulate AMPK activation; however, no study has examined the relationship between blood insulin levels and acute resistance exercise-induced AMPK pathway activation. Based on previous findings, we hypothesized that the acute resistance exercise-induced AMPK pathway activation would be augmented by disruptions in insulin secretion through a decrease in AMPKα Ser485/491 inhibitory phosphorylation. To test the hypothesis, 10-wk-old male Sprague-Dawley rats were administered the toxin streptozotocin (STZ; 55 mg/kg) to destroy the insulin secreting β-cells. Three days postinjection, the right gastrocnemius muscle from STZ and control rats was subjected to resistance exercise by percutaneous electrical stimulation. Animals were killed 0, 1, or 3 h later; activation of the Akt/AMPK and downstream pathways in the muscle tissue was analyzed by Western blotting and real-time PCR. Notably, STZ rats showed a significant decrease in basal Akt and AMPKα Ser485/491 phosphorylation, but substantial exercise-induced increases in both AMPKα Thr172 and acetyl-CoA carboxylase (ACC) Ser79 phosphorylation were observed. Although no significant impact on resistance exercise-induced Akt pathway activation or glucose uptake was found, resistance exercise-induced peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1 α (PGC-1α) gene expression was augmented by STZ treatment. Collectively, these data suggest that circulating insulin levels may regulate acute resistance exercise-induced AMPK pathway activation and AMPK-dependent gene expression relating to basal AMPKα Ser485/491 phosphorylation.

Herbal supplement Kamishimotsuto augments resistance exercise-induced mTORC1 signaling in rat skeletal muscle.

OBJECTIVES Kamishimotsuto (KST) is a supplement containing 13 different herbs including Phellodendron bark, Anemarrhena rhizome and ginseng that have been shown to activate mammalian target of rapamycin complex 1 (mTORC1) and thereby increase muscle protein synthesis in vitro. However, the combined effect of KST and resistance exercise on muscle protein anabolism has not been investigated in vivo. Therefore, the purpose of this study was to investigate the effect of KST supplementation, resistance exercise on (mTORC1) signaling and subsequent muscle protein synthesis. METHODS Male Sprague-Dawley rats were divided into two groups: one group received KST (500 mg/kg/d in water) and the other group received placebo (PLA) for 7 d. After 12 h of fasting, the right gastrocnemius muscle was isometrically exercised via percutaneous electrical stimulation. Muscle samples were analyzed for muscle protein synthesis (MPS) and by western blotting analysis to assess the phosphorylation of p70S6K (Thr389), rpS6 (Ser240/244), and Akt (Ser473 and Thr308). RESULTS KST supplementation for 7 d significantly increased basal p-Akt (Ser473) levels compared with PLA, phosphorylation of the signaling proteins and MPS at baseline were otherwise unaffected. p-p70S6K and p-rpS6 levels significantly increased 1 h and 3 h after exercise in the PLA group, and these elevations were augmented in the KST group (P < 0.05). Furthermore, MPS at 6 h after resistance exercise was greater in the KST group than in the PLA group (P < 0.05). CONCLUSIONS While resistance exercise alone was able to increase p70S6K and rpS6 phosphorylation, Kamishimotsuto supplementation further augmented resistance exercise-induced muscle protein synthesis through mTORC1 signaling.

Panaxatriol derived from ginseng augments resistance exercised–induced protein synthesis via mTORC1 signaling in rat skeletal muscle

Resistance exercise activates muscle protein synthesis via the mammalian target of rapamycin complex 1 (mTORC1) pathway and subsequent muscle hypertrophy. Upstream components of the mTORC1 pathway are widely known to be involved in Akt and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling. Previous studies have shown that ginseng stimulated Akt and ERK1/2 signaling. Therefore, we hypothesized that panaxatriol (PT) derived from ginseng triggers mTORC1 signaling and muscle protein synthesis by activating both the Akt and ERK1/2 signaling pathways, and that PT additively stimulates muscle protein synthesis when combined with resistance exercise. The study included male Sprague-Dawley rats. The legs of the rats were divided into control, PT-only, exercise-only, and exercise + PT groups. The right legs were subjected to isometric resistance exercise using percutaneous electrical stimulation, whereas the left legs were used as controls. PT (0.2 g/kg) was administered immediately after exercise. The Akt and ERK1/2 phosphorylation levels were significantly higher in the exercise + PT group than in the exercise-only group 0.5 hour after exercise. The phosphorylation of p70S6K was significantly increased at both 0.5 and 3 hours after exercise, and it was higher in the exercise + PT group than in the exercise-only group at both 0.5 and 3 hours after exercise. Muscle protein synthesis was significantly increased 3 hours after exercise, and it was higher in the exercise + PT group than in the exercise-only group 3 hours after exercise. Our results suggest that PT derived from ginseng enhances resistance exercise-induced protein synthesis via mTORC1 signaling in rat skeletal muscle.

Resistance training recovers attenuated APPL1 expression and improves insulin-induced Akt signal activation in skeletal muscle of type 2 diabetic rats

Adapter protein containing Pleckstrin homology (PH) domain, phosphotyrosine-binding (PTB) domain, and leucine zipper motif 1 (APPL1) has been reported as a positive regulator of insulin-stimulated Akt activation. The expression of APPL1 is reduced in skeletal muscles of type 2 diabetic (T2D) animals, implying that APPL1 may be an important factor affecting insulin sensitivity. However, the regulation of APPL1 expression and the physiological interventions modulating these effects are unclear. Accordingly, we first confirmed that APPL1 expression and insulin-induced Akt phosphorylation were significantly attenuated in skeletal muscles of T2D rats. Additionally, we found that APPL1 expression levels were significantly correlated with fasting blood glucose levels. Next, we identified important signals involved in the expression of APPL1. APPL1 mRNA expression increased upon AMP-activated protein kinase, calcium, p38 mitogen-activated protein kinase, and insulin-like growth factor-1 signal activation. Moreover, acute resistance exercise in vivo significantly activated these signaling pathways. Finally, through in vivo experiments, we found that chronic resistance training (RT) increased APPL1 expression and activated insulin-induced Akt signaling in skeletal muscles of rats with T2D. Furthermore, variations in APPL1 expression (i.e., the difference between control and RT muscles) significantly correlated with variations in insulin-stimulated Akt phosphorylation under the same conditions. Therefore, chronic RT recovered attenuated APPL1 expression and improved insulin-stimulated Akt phosphorylation in skeletal muscles of T2D rats. Accordingly, APPL1 may be a key regulator of insulin resistance in skeletal muscle, and RT may be an important physiological treatment increasing APPL1 expression, which is attenuated in T2D.

The effect of different acute muscle contraction regimens on the expression of muscle proteolytic signaling proteins and genes

Previous studies have reported that different modes of muscle contraction (i.e., eccentric or concentric contraction) with similar contraction times can affect muscle proteolytic responses. However, the effect of different contraction modes on muscle proteolytic response under the same force−time integral (FTI: contraction force × time) has not been investigated. The purpose of this study was to investigate the effect of different contraction modes, with the same FTI, on acute proteolytic signaling responses. Eleven‐week‐old male Sprague–Dawley rats were randomly assigned to eccentric (EC), concentric (CC), or isometric contraction (IC) groups. Different modes of muscle contraction were performed on the right gastrocnemius muscle using electrical stimulation, with the left muscle acting as a control. In order to apply an equivalent FTI, the number of stimulation sets was modified between the groups. Muscle samples were taken immediately and three hours after exercise. Phosphorylation of FoxO3a at Ser253 was significantly increased immediately after exercise compared to controls irrespective of contraction mode. The mRNA levels of the ubiquitin ligases, MuRF1, and MAFbx mRNA were unchanged by contraction mode or time. Phosphorylation of ULK1 at Ser317 (positive regulatory site) and Ser757 (negative regulatory site) was significantly increased compared to controls, immediately or 3 h after exercise, in all contraction modes. The autophagy markers (LC3B‐II/I ratio and p62 expression) were unchanged, regardless of contraction mode. These data suggest that differences in contraction mode during resistance exercise with a constant FTI, are not factors in regulating proteolytic signaling in the early phase of skeletal muscle contraction.