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Skeletal Muscle Fiber Types and The Influence on Glycogen Utilization and GLUT4 Abundance

Abstract on Skeletal Muscle Fiber

Skeletal muscle fibers are the group of mammalian body that is made of bundles of muscle fibers. The given study investigates the skeletal muscle fiber types, their metabolic profile, association between glycogen and GLUT4, effect of type II diabetes will be affected, and the effect of training and exercise on muscle fiber type, glycogen utilization and GLUT4 abundance. It has been reported that type I fibers have a higher rate of resynthesis by 25 ± 8% as compared to type II that is 41 ± 3 and between 3rd and 10th hour of recovery, resynthesis in type I muscle fibers got decreased by 60 ± 13 % (Casey et al., 1995) and exercise reduce the glycogen content of muscle by 43% approx and increases the capacity of muscle fibers for fat oxidation with the elevation in the abundance of oxidative and lipolytic enzymes in type I fiber. However, it has been evaluated that succinate dehydrogenase activity is comparatively higher in slow-twitch fiber than in fast-twitch fiber (Saltin et al., 2000). On account of Type II diabetes, it has been demonstrated that some alternative natural as well as potential synthetic compounds activate the GLUT4 pathways and helps in the emergence of target sources for type II diabetes drug development. Furthermore, we have shown that the upregulation of proteins, oxidation, and mobilization is coordinated by type I muscle fibers.

Introduction to Skeletal Muscle Fiber

Skeletal muscle fibers are the group of mammalian body that is made of bundles of muscle fibers; based on their movement rates, metabolic styles, and neural inputs they are classified into the types and are defined by particular isoforms of myosin heavy chain. Based on myoglobin content, skeletal muscle fibers are classified into red muscle fibers and white muscle fibers. Those having myoglobin in high content appear darker and refer to as red muscle fibers while those having low myoglobin content appears lighter in color and are termed as white muscle fibers. Comparatively, there are a large number of mitochondria in red fibers and are supplied by more blood capillaries. A fiber is categorized as either slow-twitch or fast-twitch, depending on the rapid action of ATPase in its myosin (Talbot & Maves, 2016). Skeletal muscle fibers, perform contraction as well as relaxation at a different pace, and vary according to the ATP generation and promptness of fatigue in metabolic reactions. The article reviews the differences in skeletal muscle fiber types, its influence on glycogen utilization, and GLUT4 abundance with or without exercise and consideration of how individuals with Type II diabetes will be affected.

Review of Skeletal Muscle Fiber

Fiber Types and Its Metabolic Profile

Synthesis of findings:

In Saltin, Henriksson, Nygaard, and Anderson’s article in 2000 shows the current study on skeletal muscle fibers, and their metabolic profile is explained. The authors have applied histochemical techniques for the identification of different fiber types in skeletal muscle along with the association of the characteristics of their role and metabolic properties in exercise. Histochemical techniques provide a semi-quantitative estimation of the metabolic profile of a particular type of fiber along with its twitch characteristics. For this, the muscle of man is mixed on account of the composition of the fiber. For quantitative determination of metabolic profile, authors have suggested to use a procedure where fibers can be dissected out and via Vivo 19 or invitro IR-zn contraction twitch characteristics (fast-twitch and slow-twitch) can be obtained.

Evaluation of findings:

The authors have determined different substrate and enzyme activities and determined the muscle tissue’s metabolic potential from histochemical techniques, it has been postulated that succinate dehydrogenase activity is comparatively higher in ST fiber than in FT fiber and in FT fiber, the oxidative potential is substantial while the glycolytic potential is more persistent in ST. However, authors have also demonstrated that after a mixed diet, at rest there is no significant difference in fiber types and the triglyceride pool is greater in ST as compared in FT (Saltin et al., 2000).

Effects of Training Status on Muscle Fiber Type Composition

Synthesis of findings:

Exercising enhances the capacity of muscle fibers for fat oxidation because of the high utilization of intramuscular lipids. Casey, Short, Hultman, and Greenhaff’s literature in 1995 quantitatively investigates the consequences of exercise training status on muscle fiber type composition a glycogen resynthesis in both type I and type II muscle fibers with the usage of biochemical methods for analysis. For evaluation of this authors have performed one-legged cycling exercise on 7 subjects and they were asked to consume 3g of glucose in the initial 2 hours of recovery and to consume an extremely high amount of carbohydrate [n = 7 subjects, glucose consumption = 3g] and biopsy samples of the muscles were obtained from both legs at the time of exhaustion and after 3, 10, and 24 hours of recovery from the exercised leg. The results were then obtained by comparing and evaluating the rate of resynthesis in type I muscle fibers and type II muscle fibers.

Evaluation of findings:

It has been evaluated that during the initial 2 hours of recovery, subjects when consumed 3 g of glucose, and a high amount of carbohydrate diet muscle biopsy samples were analyzed and obtained from both legs at the time of exhaustion, as well as from the exercised one consecutively after 3, 10, and 24 hours of recovery. In the first 3 h of recovery, it was found that type I fibers has a higher rate of resynthesis by 25 ± 8% as compared to type II that is 41 ± 3 and between 3rd and 10th hour of recovery, resynthesis in type I muscle fibers got decreased by 60 ± 13 %, while at the same time the rate of type II was maintained and a good deal was found when relating the mean concentration of type I fibers and type II fibers to the glycogen concentration of mixed fiber muscle (Casey, Short, Hultman & Greenhaff, 1995).

The Relationship of Glycogen and GLUT4

Synthesis of findings:

Murphy et al. in 2018 have demonstrated in her literature review that in previous studies, exercise enhances the insulin sensitivity of skeletal muscle that is associated with the increment of glucose transporter - 4 in the plasma membrane which is followed by the stimulation of insulin and linked with glycogen depletion in muscles but, in order to evaluate the potential direct linkage between the GLUT4 and glycogen, the authors have done an assessment when they took seven male subjects that were untrained and exercised them at approximately 75% VO2 peak for 60 minutes and the results were then obtained by the biopsy before and after the exercise.

Evaluation of findings:

By needle biopsy (percutaneous), muscle samples were obtained just before and after the exercise, and from this assessment, it has been evaluated that exercises reduce the glycogen content of muscle by 43% approx. With or without the amylase treatment, the total content of GLUT4 remained unchanged by exercise and it became difficult to detect GLUT4 in fractions of glycogen and it was reported that there was no dissimilarity in the amount of GLUT4 content in the fibers before and after the exercise which indicates that there is no direct linkage or association between glycogen and GLUT4 in skeletal muscle but alterations in translocation of GLUT4 is associated with the glycogen depletion in muscle via other mechanisms that are induced by exercise (Murphy et al., 2018).

The Effect of Fiber Types on Glycogen and GLUT4

Synthesis of findings:

Daugaard et al., 2000 investigated 8 male subjects to express the impact of fiber types on GLUT4 and glycogen. From the biopsy sample, single muscle fibers were obtained. These fibers were categorized into three groups: MHC I, II, and IIX based on the myosin heavy chain, and GLUT4 content was analyzed by immunological detection and SDS-PAGE that was a pool of 15-40 fibers.

Evaluation of findings:

With the help of biopsy, when single muscle fibers of those 8 subjects were obtained, It has been reported that the content of Glucose Transporter - 4 in muscle fibers that express MHC I was 20% higher than those expressing MHC IIA or IIX while no dissimilarity was found in GLUT4 pooled in the fibers that express MHC IIA or IIX (Daugaard et al., 2000).

The Effect of Exercise on Glycogen and GLUT4

Synthesis of findings:

In the literature review of Murphy et al in 2018, it has been demonstrated that exercises enhance insulin sensitivity of skeletal muscle that is associated with the increment of GLUT4 in the plasma membrane which is followed by the stimulation of insulin and in order to evaluate the effect of exercise on GLUT4 and glycogen, the authors have done an assessment on seven male subjects that were untrained, and subjected to exercise at approximately 75% VO2 peak for 60 minutes and the results were then obtained by the biopsy before and after the exercise for the evaluation of changes in GLUT4 and glycogen while exercising.

Evaluation of findings:

By Murphy et al., 2018 it has been evaluated that exercises reduce the glycogen content of muscle by 43% approx. while the total content of GLUT4 remained unchanged by exercise and it became difficult to detect the amount of GLUT4 in glycogen fractions.

The Effect of Exercise on Whole Muscle Fiber

Synthesis of findings:

Carroll et al. in 2019 in their literature review compared the physiological responses of skeletal muscle fibers when subjected to training programs. In order to evaluate this, the authors have taken two groups repetition maximum group or RM group and relative intensity group or RISR group. They took 8 subjects for the RISR group and 7 subjects for RM groups. The RM group were subjected to exercise until the muscle reach failure occurs on each exercise, while the RISR group throughout the intervention did not reach muscular failure till 10 weeks.

Evaluation of findings:

In findings of Carroll et al., 2019 it has been evaluated that when needle biopsy and ultrasonography of pre and post-training interventions were obtained from both of the groups, the RISR group showed greater adaptations in whole muscle fiber size and contractile proteins as compared to RM groups.

The Effect of Type II Diabetes on GLUT4

Synthesis of findings:

Type II diabetes is distinguished by resistance of insulin in peripheral and hepatic tissues. GLUT4 plays a vital role in type II diabetes’ pathophysiology. Alam, Islam, Khalil and Gan in 2016 have evaluated how glucose transporter 4 helps in the pathophysiology of type II diabetes and the effect of type II diabetes on GLUT4. In type II diabetic patients, the defective translocation of GLUT4 in the cell plasma membrane creates a hindrance for the glucose to enter into the cell for the production of energy. However, increasing the expression, concentration, and translocation of GLUT4 will help in the management of glucose metabolism of type II diabetic patients.

Evaluation of findings:

Alam et al., (2016) has evaluated that not only some conventional or traditional drugs, but there are also some alternative natural as well as potential synthetic compounds that can activate the GLUT4 (insulin-dependent as well as insulin-independent) pathways and helps in the emergence of target sources for type II diabetes drug development.

Summary of Influence on Glycogen Utilization and GLUT4 Abundance

The article reviews the differences in skeletal muscle fiber types, its metabolic profile, the effect of training and exercise on muscle fiber type, glycogen utilization, and GLUT4 abundance along with the association between glycogen and GLUT4, and consideration of how individuals with Type II diabetes will be affected. In order to evaluate all of the above the techniques that have been used are histochemical and biochemical methods along with the biopsy samples of muscle fibers. The findings that have been interpreted depict that exercising enhances the overall capacity of muscle fibers for fat oxidation because of the high utilization of intramuscular lipids and there is no direct linkage or association between glycogen and GLUT4 in skeletal muscle but alterations in translocation of GLUT4 is associated with the glycogen depletion.

Reference for Influence on Glycogen Utilization and GLUT4 Abundance

Alam, F., Islam, M. A., Khalil, M. I., & Gan, S. (2016). Metabolic control of type 2 diabetes by targeting the GLUT4 glucose transporter: Intervention approaches. Current Pharmaceutical Design, 22. 3034-3049.

Carroll, K. M. et al. (2019). Skeletal muscle fiber adaptations following resistance training using repetition maximums or relative intensity. Journal of Sports, 7(169), 1-12.

Casey, A., Short, A. H., Hultman, E., & Greenhafft, P. L. (1995). Glycogen resynthesis in human muscle fiber types following exercise-induced glycogen depletion. Journal of Physiology, 483(1), 265-271.

Daugaard, J. R., Nielsen, J. N., Kristiansen, S., Andersen J. L., Hargreaves, M., & Richter, E. A. (2000), Fiber Type–specific expression of GLUT4 in Human Skeletal Muscle. Influence of exercise training. Diabetes, 49 1092-1095.

Murphy, R. M., Opazo, M. F., Frankish, B. P., Garnham, A., Stapleton, D., & Hargreaves, M. (2018). No evidence of direct association between GLUT4 and glycogen in human skeletal muscle. Physiological Reports, 6(21), e13917.

Saltin, B., Henriksson, J., Nygaard, E., & Andersen, P. (1997). Fiber types and metabolic potentials of skeletal muscles in sedentary man and endurance runners. Annals of the New York Academy of Sciences, 301(1), 3-29.

Talbot, J. & Maves, L. (2016). Skeletal muscle fiber type: Using insights from Muscle developmental biology to dissect targets for susceptibility and resistance to Muscle Disease. Wiley Interdisciplinary Reviews Developmental Biology, 5(4), 518–534.

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