Postexercise improvement in glucose uptake occurs concomitant with greater γ3-AMPK activation and AS160 phosphorylation in rat skeletal muscle

Phosphorylation of AS160-serine 704 is not essential for exercise-increase in insulin-stimulated glucose uptake by skeletal muscles from female or male rats

One exercise session can increase subsequent insulin-stimulated glucose uptake (ISGU) by skeletal muscle from rodents and humans of both sexes. We recently found that concurrent mutation of three key sites to prevent their phosphorylation (Ser588, Thr642, and Ser704) on Akt substrate of 160 kDa (AS160; also known as TBC1D4) reduced the magnitude of the enhancement of postexercise ISGU (PEX-ISGU) by muscle from male, but not female rats. However, we did not test the role of individual phosphorylation sites on PEX-ISGU. Accordingly, our current aim was to test whether AS160 Ser704 phosphorylation (pSer704) is required for elevated PEX-ISGU by muscle. AS160-knockout (AS160-KO) rats (female and male) were studied when either in sedentary or 3 h after acute exercise. Adeno-associated virus (AAV) vectors were used to enable muscle expression of wild-type AS160 (AAV-WT-AS160) or AS160 mutated Ser704 to alanine to prevent phosphorylation (AAV-1P-AS160). Paired epitrochlearis muscles from each rat were injected with AAV-WT-AS160 or AAV-1P-AS160. We discovered that regardless of sex 1) AS160 abundance in AS160-KO rats was similar in paired muscles expressing WT-AS160 versus 1P-AS160; 2) muscles from exercised versus sedentary rats had greater ISGU, and PEX-ISGU was slightly greater for muscles expressing 1P-AS160 versus contralateral muscles expressing WT-AS160; and 3) pAS160Thr642 was lower in muscles expressing 1P-AS160 versus paired muscles expressing WT-AS160. These results indicate that pAS160Ser704 was not essential for elevated PEX-ISGU by skeletal muscle from rats of either sex. Furthermore, elimination of the postexercise increase in pAS160Thr642 did not lessen the postexercise effect on ISGU.NEW & NOTEWORTHY The current study evaluated the role of Akt substrate of 160 kDa (AS160) phosphorylation on Ser704 in increased insulin-stimulated glucose uptake by skeletal muscle after exercise. Adeno-associated virus vectors were engineered to express either wild-type-AS160 or AS160 mutated so that it could not be phosphorylated on Ser704 in paired muscles from AS160-knockout rats. The results demonstrated that AS160 phosphorylation on Ser704 was not essential for exercise-induced elevation in insulin-stimulated glucose uptake by rats of either sex.

Independent and combined effects of calorie restriction and AICAR on glucose uptake and insulin signaling in skeletal muscles from 24-month-old female and male rats

We assessed the effects of two levels of calorie restriction (CR; eating either 15% or 35% less than ad libitum, AL, food intake for 8 weeks) by 24-month-old female and male rats on glucose uptake (GU) and phosphorylation of key signaling proteins (Akt; AMP-activated protein kinase, AMPK; Akt substrate of 160 kDa, AS160) measured in isolated skeletal muscles that underwent four incubation conditions (without either insulin or AICAR, an AMPK activator; with AICAR alone; with insulin alone; or with insulin and AICAR). Regardless of sex: (1) neither CR group versus the AL group had greater GU by insulin-stimulated muscles; (2) phosphorylation of Akt in insulin-stimulated muscles was increased in 35% CR versus AL rats; (3) prior AICAR treatment of muscle resulted in greater GU by insulin-stimulated muscles, regardless of diet; and (4) AICAR caused elevated phosphorylation of acetyl CoA carboxylase, an indicator of AMPK activation, in all diet groups. There was a sexually dimorphic diet effect on AS160 phosphorylation, with 35% CR exceeding AL for insulin-stimulated muscles in male rats, but not in female rats. Our working hypothesis is that the lack of a CR-effect on GU by insulin-stimulated muscles was related to the extended duration of the ex vivo incubation period (290 min compared to 40-50 min that was previously reported to be effective). The observed efficacy of prior treatment of muscles with AICAR to improve glucose uptake in insulin-stimulated muscles supports the strategy of targeting AMPK with the goal of improving insulin sensitivity in older females and males.

A novel genetic model provides a unique perspective on the relationship between postexercise glycogen concentration and increases in the abundance of key metabolic proteins after acute exercise

Some acute exercise effects are influenced by postexercise (PEX) diet, and these diet-effects are attributed to differential glycogen resynthesis. However, this idea is challenging to test rigorously. Therefore, we devised a novel genetic model to modify muscle glycogen synthase 1 (GS1) expression in rat skeletal muscle with an adeno-associated virus (AAV) short hairpin RNA knockdown vector targeting GS1 (shRNA-GS1). Contralateral muscles were injected with scrambled shRNA (shRNA-Scr). Muscles from exercised (2-hour-swim) and time-matched sedentary (Sed) rats were collected immediately postexercise (IPEX), 5-hours-PEX (5hPEX), or 9-hours-PEX (9hPEX). Rats in 5hPEX and 9hPEX experiments were refed (RF) or not-refed (NRF) chow. Muscles were analyzed for glycogen, abundance of metabolic proteins (pyruvate dehydrogenase kinase 4, PDK4; peroxisome proliferator-activated receptor γ coactivator-1α, PGC1α; hexokinase II, HKII; glucose transporter 4, GLUT4), AMP-activated protein kinase phosphorylation (pAMPK), and glycogen metabolism-related enzymes (glycogen phosphorylase, PYGM; glycogen debranching enzyme, AGL; glycogen branching enzyme, GBE1). shRNA-GS1 versus paired shRNA-Scr muscles had markedly lower GS1 abundance. IPEX versus Sed rats had lower glycogen and greater pAMPK, and neither of these IPEX-values differed for shRNA-GS1 versus paired shRNA-Scr muscles. IPEX versus Sed groups did not differ for abundance of metabolic proteins, regardless of GS1 knockdown. Glycogen in RF-rats was lower for shRNA-GS1 versus paired shRNA-Scr muscles at both 5hPEX and 9hPEX. HKII protein abundance was greater for 5hPEX versus Sed groups, regardless of GS1 knockdown or diet, and despite differing glycogen levels. At 9hPEX, shRNA-GS1 versus paired shRNA-Scr muscles had greater PDK4 and PGC1α abundance within each diet group. However, the magnitude of PDK4 or PGC1α changes was similar in each diet group regardless of GS1 knockdown although glycogen differed between paired muscles only in RF-rats. In summary, we established a novel genetic approach to investigate the relationship between muscle glycogen and other exercise effects. Our results suggest that exercise-effects on abundance of several metabolic proteins did not uniformly correspond to differences in postexercise glycogen.

AS160 expression, but not AS160 Serine-588, Threonine-642, and Serine-704 phosphorylation, is essential for elevated insulin-stimulated glucose uptake by skeletal muscle from female rats after acute exercise

One exercise session can increase subsequent insulin-stimulated glucose uptake (ISGU) by skeletal muscle in both sexes. We recently found that muscle expression and phosphorylation of key sites of Akt substrate of 160 kDa (AS160; also called TBC1D4) are essential for the full-exercise effect on postexercise-ISGU (PEX-ISGU) in male rats. In striking contrast, AS160's role in increased PEX-ISGU has not been rigorously tested in females. Our rationale was to address this major knowledge gap. Wild-type (WT) and AS160-knockout (KO) rats were either sedentary or acutely exercised. Adeno-associated virus (AAV) vectors were engineered to express either WT-AS160 or AS160 mutated on key serine and threonine residues (Ser588, Thr642, and Ser704) to alanine to prevent their phosphorylation. AAV vectors were delivered to the muscle of AS160-KO rats to determine if WT-AS160 or phosphorylation-inactivated AS160 would influence PEX-ISGU. AS160-KO rats have lower skeletal muscle abundance of the GLUT4 glucose transporter protein. This GLUT4 deficit was rescued using AAV delivery of GLUT4 to determine if eliminating muscle GLUT4 deficiency would normalize PEX-ISGU. The novel results were as follows: (1) AS160 expression was required for greater PEX-ISGU; (2) rescuing muscle AS160 expression in AS160-KO rats restored elevated PEX-ISGU; (3) AS160's essential role for the postexercise increase in ISGU was not attributable to reduced muscle GLUT4 content; and (4) AS160 phosphorylation on Ser588, Thr642, and Ser704 was not essential for greater PEX-ISGU. In conclusion, these novel findings revealed that three phosphosites widely proposed to influence PEX-ISGU are not required for this important outcome in female rats.

Greater Phosphorylation of AMPK and Multiple AMPK Substrates in the Skeletal Muscle of 24-Month-Old Calorie Restricted Compared to Ad-Libitum Fed Male Rats

AMP-activated protein kinase (AMPK), a highly conserved, heterotrimeric serine/threonine kinase with critical sensory and regulatory functions, is proposed to induce antiaging actions of caloric restriction (CR). Although earlier studies assessed CR's effects on AMPK in rodent skeletal muscle, the scope of these studies was narrow with a limited focus on older animals. This study's purpose was to fill important knowledge gaps related to CR's influence on AMPK in skeletal muscle of older animals. Therefore, using epitrochlearis muscles from 24-month-old ad-libitum fed (AL) and CR (consuming 65% of AL intake for 8 weeks), male Fischer-344 × Brown Norway F1 rats, we determined: (a) AMPK Thr172 phosphorylation (a key regulatory site) by immunoblot; (b) AMPKα1 and AMPKα2 activity (representing the 2 catalytic α-subunits of AMPK), and AMPKγ3 activity (representing AMPK complexes that include the skeletal muscle-selective regulatory γ3 subunit) using enzymatic assays; (c) phosphorylation of multiple protein substrates that are linked to CR-related effects (acetyl-CoA carboxylase [ACC], that regulates lipid oxidation; Beclin-1 and ULK1 that are autophagy regulatory proteins; Raptor, mTORC1 complex protein that regulates autophagy; TBC1D1 and TBC1D4 that regulate glucose uptake) by immunoblot; and (d) ATP and AMP concentrations (key AMPK regulators) by mass spectrometry. The results revealed significant CR-associated increases in the phosphorylation of AMPKThr172 and 4 AMPK substrates (ACC, Beclin-1, TBC1D1, and TBC1D4), without significant diet-related differences in ATP or AMP concentration or AMPKα1-, AMPKα2-, or AMPKγ3-associated activity. The enhanced phosphorylation of multiple AMPK substrates provides novel mechanistic insights linking AMPK to functionally important consequences of CR.

Time for Exercise? Exercise and Its Influence on the Skeletal Muscle Clock

Circadian rhythms drive our daily behaviors to coincide with the earth's rotation on an approximate 24-h cycle. The circadian clock mechanism present in nearly every cell is responsible for our circadian rhythms and is comprised of a transcriptional-translational feedback loop in mammals. The central clock resides in the hypothalamus responding to external light cues, whereas peripheral clocks receive signals from the central clock and are also sensitive to cues from feeding and activity. Of the peripheral clocks, the skeletal muscle clock is particularly sensitive to exercise which has shown to be an important time-cue with the ability to influence and adjust the muscle clock phase in response to exercise timing. Since the skeletal muscle clock is also involved in the expression of tissue-specific gene expression-including glucoregulatory genes-this might suggest a role for exercise timing as a therapeutic strategy in metabolic diseases, like type 2 diabetes. Notably, those with type 2 diabetes have accompanied disruptions in their skeletal muscle clock mechanism which may also be related to the increased risk of type 2 diabetes seen among shift workers. Therefore, the direct influence of exercise on the skeletal muscle clock might support the use of exercise timing to provide disease-mitigating effects. Here, we highlight the potential use of time-of-day exercise as a chronotherapeutic tool within circadian medicine to improve the metabolic profile of type 2 diabetes and support long-term glycemic control, potentially working through the skeletal muscle clock and circadian physiology.

Illumination of the Endogenous Insulin-Regulated TBC1D4 Interactome in Human Skeletal Muscle

Insulin-stimulated muscle glucose uptake is a key process in glycemic control. This process depends on the redistribution of glucose transporters to the surface membrane, a process that involves regulatory proteins such as TBC1D1 and TBC1D4. Accordingly, a TBC1D4 loss-of-function mutation in human skeletal muscle is associated with an increased risk of type 2 diabetes, and observations from carriers of a TBC1D1 variant associate this protein to a severe obesity phenotype. Here, we identified interactors of the endogenous TBC1D4 protein in human skeletal muscle by an unbiased proteomics approach. We detected 76 proteins as candidate TBC1D4 interactors. The binding of 12 of these interactors was regulated by insulin, including proteins known to be involved in glucose metabolism (e.g., 14-3-3 proteins and α-actinin-4 [ACTN4]). TBC1D1 also coprecipitated with TBC1D4 and vice versa in both human and mouse skeletal muscle. This interaction was not regulated by insulin or exercise in young, healthy, lean individuals. Similarly, the exercise- and insulin-regulated phosphorylation of the TBC1D1-TBC1D4 complex was intact. In contrast, we observed an altered interaction as well as compromised insulin-stimulated phosphoregulation of the TBC1D1-TBC1D4 complex in muscle of obese individuals with type 2 diabetes. Altogether, we provide a repository of TBC1D4 interactors in human and mouse skeletal muscle that serve as potential regulators of TBC1D4 function and, thus, insulin-stimulated glucose uptake in human skeletal muscle.

Exercise-Induced Improvement in Insulin-Stimulated Glucose Uptake by Rat Skeletal Muscle Is Absent in Male AS160-Knockout Rats, Partially Restored by Muscle Expression of Phosphomutated AS160, and Fully Restored by Muscle Expression of Wild-Type AS160

One exercise session can elevate insulin-stimulated glucose uptake (ISGU) in skeletal muscle, but the mechanisms remain elusive. Circumstantial evidence suggests a role for Akt substrate of 160 kDa (AS160 or TBC1D4). We used genetic approaches to rigorously test this idea. The initial experiment evaluated the role of AS160 in postexercise increase in ISGU using muscles from male wild-type (WT) and AS160-knockout (KO) rats. The next experiment used AS160-KO rats with an adeno-associated virus (AAV) approach to determine if rescuing muscle AS160 deficiency could restore the ability of exercise to improve ISGU. The third experiment tested if eliminating the muscle GLUT4 deficit in AS160-KO rats via AAV-delivered GLUT4 would enable postexercise enhancement of ISGU. The final experiment used AS160-KO rats and AAV delivery of AS160 mutated to prevent phosphorylation of Ser588, Thr642, and Ser704 to evaluate their role in postexercise ISGU. We discovered the following: 1) AS160 expression was essential for postexercise increase in ISGU; 2) rescuing muscle AS160 expression of AS160-KO rats restored postexercise enhancement of ISGU; 3) restoring GLUT4 expression in AS160-KO muscle did not rescue the postexercise increase in ISGU; and 4) although AS160 phosphorylation on three key sites was not required for postexercise elevation in ISGU, it was essential for the full exercise effect.

Prior AICAR induces elevated glucose uptake concomitant with greater γ3-AMPK activation and reduced membrane cholesterol in skeletal muscle from 26-month-old rats

Attenuated skeletal muscle glucose uptake (GU) has been observed with advancing age. It is important to elucidate the mechanisms linked to interventions that oppose this detrimental outcome. Earlier research using young rodents and (or) cultured myocytes reported that treatment with 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR; an AMP-activated protein kinase (AMPK) activator) can increase γ3-AMPK activity and reduce membrane cholesterol content, each of which has been proposed to elevate GU. However, the effect of AICAR treatment on γ3-AMPK activity and membrane cholesterol in skeletal muscle of aged animals has not been reported. Our purpose was to evaluate the effects of AICAR treatment on these potential mechanisms for enhanced glucose uptake in the skeletal muscle of aged animals. Epitrochlearis muscles from 26-27-month-old male rats were isolated and incubated ± AICAR, followed by 3 h incubation without AICAR, and then incubation with 3-O-methyl-[3 H] glucose (to assess GU ± insulin). Muscles were also analyzed for γ3-AMPK activity and membrane cholesterol content. Prior AICAR treatment led to increased γ3-AMPK activity, reduced membrane cholesterol content, and enhanced glucose uptake in skeletal muscle from aged rats. These observations revealed that two potential mechanisms for greater GU previously observed in younger animals and (or) cell models are also potentially relevant for enhanced GU by muscles from older animals.

Exercise effects on γ3-AMPK activity, Akt substrate of 160 kDa phosphorylation, and glucose uptake in muscle of normal and insulin-resistant female rats

Previous studies demonstrated that acute exercise can enhance glucose uptake (GU), γ3-AMP-activated protein kinase (AMPK) activity, and Akt substrate of 160 kDa (AS160) phosphorylation in skeletal muscles from low-fat diet (LFD)- and high-fat diet (HFD)-fed male rats. Because little is known about exercise effects on these outcomes in females, we assessed postexercise GU by muscles incubated ± insulin, delta-insulin GU (GU of muscles incubated with insulin minus GU uptake of paired muscles incubated without insulin), and muscle signaling proteins from female rats fed a LFD or a brief HFD (2 wk). Rats were sedentary (LFD-SED, HFD-SED) or swim exercised. Immediately postexercise (IPEX) or 3 h postexercise (3hPEX), epitrochlearis muscles were incubated (no insulin IPEX; ±insulin 3hPEX) to determine GU. Muscle γ3-AMPK activity (IPEX, 3hPEX) and phosphorylated AS160 (pAS160; 3hPEX) were also assessed. γ3-AMPK activity and insulin-independent GU of IPEX rats exceeded sedentary rats without diet-related differences in either outcome. At 3hPEX, both GU by insulin-stimulated muscles and delta-insulin GU exceeded their respective diet-matched sedentary controls. GU by insulin-stimulated muscles, but not delta-insulin GU for LFD-3hPEX, exceeded HFD-3hPEX. LFD-3hPEX versus LFD-SED had greater γ3-AMPK activity and greater pAS160. HFD-3hPEX exceeded HFD-SED for pAS160 but not for γ3-AMPK activity. pAS160 and γ3-AMPK at 3hPEX did not differ between diet groups. These results revealed that increased γ3-AMPK activity at 3hPEX was not essential for greater GU in insulin-stimulated muscle or greater delta-insulin GU in HFD female rats. Similarly elevated γ3-AMPK activity in LFD-IPEX versus HFD-IPEX and pAS160 in LFD-3hPEX versus HFD-3hPEX may contribute to the comparable delta-insulin GU at 3hPEX in both diet groups.NEW & NOTEWORTHY Glucose uptake (GU) and phosphorylated AS160 (pAS160) by insulin-stimulated muscles at 3 h postexercise (3hPEX) exceeded diet-matched controls in female low-fat diet-fed (LFD) or high-fat diet-fed (HFD) rats. GU with insulin for LFD-3hPEX exceeded HFD-3hPEX, whereas pAS160 was similar between these groups. γ3-AMPK immediately postexercise (IPEX) was similarly elevated in LFD and HFD, but only LFD-3hPEX had increased γ3-AMPK. These results suggest that greater γ3-AMPK at IPEX and pAS160 at 3hPEX may contribute to elevated GU with insulin, but greater γ3-AMPK at 3hPEX was dispensable for female HFD rats.

Reduced membrane cholesterol content in skeletal muscle is not essential for greater insulin-stimulated glucose uptake after acute exercise by rats

One exercise session can elevate insulin-stimulated glucose uptake (GU) by skeletal muscle, but it is uncertain if this effect is accompanied by altered membrane cholesterol content, which is reportedly inversely related to insulin-stimulated GU. Muscles from sedentary (SED) or exercised 3 h post-exercise (3hPEX) rats were evaluated for GU, membrane cholesterol, and phosphorylation of cholesterol regulatory proteins (pHMCGR^Ser872 and pABCA1^Ser2054). Insulin-stimulated GU for 3hPEX exceeded SED. Membrane cholesterol, pHMCGR^Ser872 and pABCA1^Ser2054 did not differ between groups. Novelty: Alterations in membrane cholesterol and phosphorylation of proteins that regulate muscle cholesterol are not essential for elevated insulin-stimulated GU in skeletal muscle after acute exercise.

Exercise effects on γ3-AMPK activity, phosphorylation of Akt2 and AS160, and insulin-stimulated glucose uptake in insulin-resistant rat skeletal muscle

One exercise session can increase subsequent insulin-stimulated glucose uptake (ISGU) by skeletal muscle. Prior research on healthy muscle suggests that enhanced postexercise ISGU depends on elevated γ3-AMPK activity leading to greater phosphorylation of Akt substrate of 160 kDa (pAS160) on an AMPK-phosphomotif (Ser⁷⁰⁴). Phosphorylation of AS160^Ser704, in turn, may favor greater insulin-stimulated pAS160 on an Akt-phosphomotif (Thr⁶⁴²) that regulates ISGU. Accordingly, we tested if exercise-induced increases in γ3-AMPK activity and pAS160 on key regulatory sites accompany improved ISGU at 3 h postexercise (3hPEX) in insulin-resistant muscle. Rats fed a high-fat diet (HFD; 2-wk) that induces insulin resistance either performed acute swim-exercise (2 h) or were sedentary (SED). SED rats fed a low-fat diet (LFD; 2 wk) served as healthy controls. Isolated epitrochlearis muscles from 3hPEX and SED rats were analyzed for ISGU, pAS160, pAkt2 (Akt-isoform that phosphorylates pAS160^Thr642), and γ1-AMPK and γ3-AMPK activity. ISGU was lower in HFD-SED muscles versus LFD-SED, but this decrement was eliminated in the HFD-3hPEX group. γ3-AMPK activity, but not γ1-AMPK activity, was elevated in HFD-3hPEX muscles versus both SED controls. Furthermore, insulin-stimulated pAS160^Thr642, pAS160^Ser704, and pAkt2^Ser474 in HFD-3hPEX muscles were elevated above HFD-SED and equal to values in LFD-SED muscles, but insulin-independent pAS160^Ser704 was unaltered at 3hPEX. These results demonstrated, for the first time in an insulin-resistant model, that the postexercise increase in ISGU was accompanied by sustained enhancement of γ3-AMPK activation and greater pAkt2^Ser474. Our working hypothesis is that these changes along with enhanced insulin-stimulated pAS160 increase ISGU of insulin-resistant muscles to values equaling insulin-sensitive sedentary controls.NEW & NOTEWORTHY Earlier research focusing on signaling events linked to increased insulin sensitivity in muscle has rarely evaluated insulin resistant muscle after exercise. We assessed insulin resistant muscle after an exercise protocol that improved insulin-stimulated glucose uptake. Prior exercise also amplified several signaling steps expected to favor enhanced insulin-stimulated glucose uptake: increased γ3-AMP-activated protein kinase activity, greater insulin-stimulated Akt2 phosphorylation on Ser474, and elevated insulin-stimulated Akt substrate of 160 kDa phosphorylation on Ser588, Thr642, and Ser704.

Insulin-induced membrane permeability to glucose in human muscles at rest and following exercise

KEY POINTS

Increased insulin action is an important component of the health benefits of exercise, but its regulation is complex and not fully elucidated. Previous studies of insulin-stimulated GLUT4 translocation to the skeletal muscle membrane found insufficient increases to explain the increases in glucose uptake. By determination of leg glucose uptake and interstitial muscle glucose concentration, insulin-induced muscle membrane permeability to glucose was calculated 4 h after one-legged knee-extensor exercise during a submaximal euglycaemic-hyperinsulinaemic clamp. It was found that during submaximal insulin stimulation, muscle membrane permeability to glucose in humans increases twice as much in previously exercised vs. rested muscle and outstrips the supply of glucose, which then becomes limiting for glucose uptake. This methodology can now be employed to determine muscle membrane permeability to glucose in people with diabetes, who have reduced insulin action, and in principle can also be used to determine membrane permeability to other substrates or metabolites.

ABSTRACT

Increased insulin action is an important component of the health benefits of exercise, but the regulation of insulin action in vivo is complex and not fully elucidated. Previously determined increases in skeletal muscle insulin-stimulated GLUT4 translocation are inconsistent and mostly cannot explain the increases in insulin action in humans. Here we used leg glucose uptake (LGU) and interstitial muscle glucose concentration to calculate insulin-induced muscle membrane permeability to glucose, a variable not previously possible to quantify in humans. Muscle membrane permeability to glucose, measured 4 h after one-legged knee-extensor exercise, increased ∼17-fold during a submaximal euglycaemic-hyperinsulinaemic clamp in rested muscle (R) and ∼36-fold in exercised muscle (EX). Femoral arterial infusion of N^G -monomethyl l-arginine acetate or ATP decreased and increased, respectively, leg blood flow (LBF) in both legs but did not affect membrane glucose permeability. Decreasing LBF reduced interstitial glucose concentrations to ∼2 mM in the exercised but only to ∼3.5 mM in non-exercised muscle and abrogated the augmented effect of insulin on LGU in the EX leg. Increasing LBF by ATP infusion increased LGU in both legs with uptake higher in the EX leg. We conclude that it is possible to measure functional muscle membrane permeability to glucose in humans and it increases twice as much in exercised vs. rested muscle during submaximal insulin stimulation. We also show that muscle perfusion is an important regulator of muscle glucose uptake when membrane permeability to glucose is high and we show that the capillary wall can be a significant barrier for glucose transport.

Fiber type-specific effects of acute exercise on insulin-stimulated AS160 phosphorylation in insulin-resistant rat skeletal muscle

Muscle is a heterogeneous tissue composed of multiple fiber types. Earlier research revealed fiber type-selective postexercise effects on insulin-stimulated glucose uptake (ISGU) from insulin-resistant rats (increased for type IIA, IIB, IIBX, and IIX, but not type I). In whole muscle from insulin-resistant rats, the exercise increase in ISGU is accompanied by an exercise increase in insulin-stimulated AS160 phosphorylation (pAS160), an ISGU-regulating protein. We hypothesized that, in insulin-resistant muscle, the fiber type-selective exercise effects on ISGU would correspond to the fiber type-selective exercise effects on pAS160. Rats were fed a 2-wk high-fat diet (HFD) and remained sedentary (SED) or exercised before epitrochlearis muscles were dissected either immediately postexercise (IPEX) or at 3 h postexercise (3hPEX) using an exercise protocol that previously revealed fiber type-selective effects on ISGU. 3hPEX muscles and SED controls were incubated ± 100µU/mL insulin. Individual myofibers were isolated and pooled on the basis of myosin heavy chain (MHC) expression, and key phosphoproteins were measured. Myofiber glycogen and MHC expression were evaluated in muscles from other SED, IPEX, and 3hPEX rats. Insulin-stimulated pAkt^Ser473 and pAkt^Thr308 were unaltered by exercise in all fiber types. Insulin-stimulated pAS160 was greater for 3hPEX vs. SED on at least one phosphosite (Ser⁵⁸⁸, Thr⁶⁴², and/or Ser⁷⁰⁴) in type IIA, IIBX, and IIB fibers, but not in type I or IIX fibers. Both IPEX and 3hPEX glycogen were decreased versus SED in all fiber types. These results provided evidence that fiber type-specific pAS160 in insulin-resistant muscle may play a role in the previously reported fiber type-specific elevation in ISGU in some, but not all, fiber types.

Effects of Nutrient Intake Timing on Post-Exercise Glycogen Accumulation and its Related Signaling Pathways in Mouse Skeletal Muscle

We investigated the effects of nutrient intake timing on glycogen accumulation and its related signals in skeletal muscle after an exercise that did not induce large glycogen depletion. Male ICR mice ran on a treadmill at 25 m/min for 60 min under a fed condition. Mice were orally administered a solution containing 1.2 mg/g carbohydrate and 0.4 mg/g protein or water either immediately (early nutrient, EN) or 180 min (late nutrient, LN) after the exercise. Tissues were harvested at 30 min after the oral administration. No significant difference in blood glucose or plasma insulin concentrations was found between the EN and LN groups. The plantaris muscle glycogen concentration was significantly (p < 0.05) higher in the EN group—but not in the LN group—compared to the respective time-matched control group. Akt Ser⁴⁷³ phosphorylation was significantly higher in the EN group than in the time-matched control group (p < 0.01), while LN had no effect. Positive main effects of time were found for the phosphorylations in Akt substrate of 160 kDa (AS160) Thr⁶⁴² (p < 0.05), 5′-AMP-activated protein kinase (AMPK) Thr¹⁷² (p < 0.01), and acetyl-CoA carboxylase Ser⁷⁹ (p < 0.01); however, no effect of nutrient intake was found for these. We showed that delayed nutrient intake could not increase muscle glycogen after endurance exercise which did not induce large glycogen depletion. The results also suggest that post-exercise muscle glycogen accumulation after nutrient intake might be partly influenced by Akt activation. Meanwhile, increased AS160 and AMPK activation by post-exercise fasting might not lead to glycogen accumulation.

Fiber type-selective exercise effects on AS160 phosphorylation

Earlier research using muscle tissue demonstrated that postexercise elevation in insulin-stimulated glucose uptake (ISGU) occurs concomitant with greater insulin-stimulated Akt substrate of 160 kDa (AS160) phosphorylation (pAS160) on sites that regulate ISGU. Because skeletal muscle is a heterogeneous tissue, we previously isolated myofibers from rat epitrochlearis to assess fiber type-selective ISGU. Exercise induced greater ISGU in type I, IIA, IIB, and IIBX but not IIX fibers. This study tested if exercise effects on pAS160 correspond with previously published fiber type-selective exercise effects on ISGU. Rats were studied immediately postexercise (IPEX) or 3.5 h postexercise (3.5hPEX) with time-matched sedentary controls. Myofibers dissected from the IPEX experiment were analyzed for fiber type (myosin heavy chain isoform expression) and key phosphoproteins. Isolated muscles from the 3.5hPEX experiment were incubated with or without insulin. Myofibers (3.5hPEX) were analyzed for fiber type, key phosphoproteins, and GLUT4 protein abundance. We hypothesized that insulin-stimulated pAS160 at 3.5hPEX would exceed sedentary controls only in fiber types characterized by greater ISGU postexercise. Values for phosphorylation of AMP-activated kinase substrates (acetyl CoA carboxylase^Ser79 and AS160^Ser704) from IPEX muscles exceeded sedentary values in each fiber type, suggesting exercise recruitment of all fiber types. Values for pAS160^Thr642 and pAS160^Ser704 from insulin-stimulated muscles 3.5hPEX exceeded sedentary values for type I, IIA, IIB, and IIBX but not IIX fibers. GLUT4 abundance was unaltered 3.5hPEX in any fiber type. These results advanced understanding of exercise-induced insulin sensitization by providing compelling support for the hypothesis that enhanced insulin-stimulated phosphorylation of AS160 is linked to elevated ISGU postexercise at a fiber type-specific level independent of altered GLUT4 expression.

Skeletal muscle fiber type-selective effects of acute exercise on insulin-stimulated glucose uptake in insulin-resistant, high-fat-fed rats

Insulin-stimulated glucose uptake (GU) by skeletal muscle is enhanced several hours after acute exercise in rats with normal or reduced insulin sensitivity. Skeletal muscle is composed of multiple fiber types, but exercise's effect on fiber type-specific insulin-stimulated GU in insulin-resistant muscle was previously unknown. Male rats were fed a high-fat diet (HFD; 2 wk) and were either sedentary (SED) or exercised (2-h exercise). Other, low-fat diet-fed (LFD) rats remained SED. Rats were studied immediately postexercise (IPEX) or 3 h postexercise (3hPEX). Epitrochlearis muscles from IPEX rats were incubated in 2-deoxy-[³H]glucose (2-[³H]DG) without insulin. Epitrochlearis muscles from 3hPEX rats were incubated with 2-[³H]DG ± 100 µU/ml insulin. After single fiber isolation, GU and fiber type were determined. Glycogen and lipid droplets (LDs) were assessed histochemically. GLUT4 abundance was determined by immunoblotting. In HFD-SED vs. LFD-SED rats, insulin-stimulated GU was decreased in type IIB, IIX, IIAX, and IIBX fibers. Insulin-independent GU IPEX was increased and glycogen content was decreased in all fiber types (types I, IIA, IIB, IIX, IIAX, and IIBX). Exercise by HFD-fed rats enhanced insulin-stimulated GU in all fiber types except type I. Single fiber analyses enabled discovery of striking fiber type-specific differences in HFD and exercise effects on insulin-stimulated GU. The fiber type-specific differences in insulin-stimulated GU postexercise in insulin-resistant muscle were not attributable to a lack of fiber recruitment, as indirectly evidenced by insulin-independent GU and glycogen IPEX, differences in multiple LD indexes, or altered GLUT4 abundance, implicating fiber type-selective differences in the cellular processes responsible for postexercise enhancement of insulin-mediated GLUT4 translocation.

Exercise training reduces the insulin-sensitizing effect of a single bout of exercise in human skeletal muscle

KEY POINTS

A single bout of exercise is capable of increasing insulin sensitivity in human skeletal muscle. Whether this ability is affected by training status is not clear. Studies in mice suggest that the AMPK-TBC1D4 signalling axis is important for the increased insulin-stimulated glucose uptake after a single bout of exercise. The present study is the first longitudinal intervention study to show that, although exercise training increases insulin-stimulated glucose uptake in skeletal muscle at rest, it diminishes the ability of a single bout of exercise to enhance muscle insulin-stimulated glucose uptake. The present study provides novel data indicating that AMPK in human skeletal muscle is important for the insulin-sensitizing effect of a single bout of exercise.

ABSTRACT

Not only chronic exercise training, but also a single bout of exercise, increases insulin-stimulated glucose uptake in skeletal muscle. However, it is not well described how adaptations to exercise training affect the ability of a single bout of exercise to increase insulin sensitivity. Rodent studies suggest that the insulin-sensitizing effect of a single bout of exercise is AMPK-dependent (presumably via the α₂ β₂ γ₃ AMPK complex). Whether this is also the case in humans is unknown. Previous studies have shown that exercise training decreases the expression of the α₂ β₂ γ₃ AMPK complex and diminishes the activation of this complex during exercise. Thus, we hypothesized that exercise training diminishes the ability of a single bout of exercise to enhance muscle insulin sensitivity. We investigated nine healthy male subjects who performed one-legged knee-extensor exercise at the same relative intensity before and after 12 weeks of exercise training. Training increased V ̇ O 2 peak and expression of mitochondrial proteins in muscle, whereas the expression of AMPKγ3 was decreased. Training also increased whole body and muscle insulin sensitivity. Interestingly, insulin-stimulated glucose uptake in the acutely exercised leg was not enhanced further by training. Thus, the increase in insulin-stimulated glucose uptake following a single bout of one-legged exercise was lower in the trained vs. untrained state. This was associated with reduced signalling via confirmed α₂ β₂ γ₃ AMPK downstream targets (ACC and TBC1D4). These results suggest that the insulin-sensitizing effect of a single bout of exercise is also AMPK-dependent in human skeletal muscle.