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INsulin IN Touch

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INsulin IN Touch

Post by Canuck Singh on Mon Mar 08, 2010 2:33 am

What Does Insulin Do?

Insulin is a protein peptide hormone (around 50 odd amino acids linked in an exact sequence) secreted by the pancreas. It is secreted in response to ingestion of food. Different foods evoke different blood insulin levels. The job of insulin is to transport/drive nutrients into the cells of all organs in the body, including muscle and body fat.



It is difficult to determine precisely what effects insulin has on muscle in vivo (within the living body) because it also triggers a cascade of anabolic hormonal activity and other processes that, in turn, promote potent anabolic mechanisms within muscle. Also, the intrinsic biology of insulin and its related growth factors (such as IGF-I) dictates they possess the ability to bind to each other’s cell membrane receptors and that a hybrid of interchangeable IGF/insulin receptors also exist in muscle cells.4

Prior to its clinical availability, patients with Type I diabetes experienced continuous loss of protein from all tissues until they died. After its introduction it was apparent that treating of these patients with insulin dramatically restored muscle mass.4 Now as we discuss protein and muscle metabolism, it is important to keep in mind that both protein synthesis and degradation proceed simultaneously at all times. And that net protein metabolism (muscle gain or loss) is regulated by a increase/decrease of either or both of these processes.4

The Muscle Building Mechanisms of Insulin.

Recent human invivo research has revealed that insulin exerts its dramatic anabolic effect by inhibiting muscle breakdown/degradation and not stimulation of protein synthesis as invitro (isolated cell cultures ) studies once indicated.1,4,7,8 This process is believed to occur by the inhibition of the ubqitin-proteasome pathway (one of three major muscle degradation pathways in muscle cells). Research does show that the introduction of insulin stops muscle breakdown (proteolysis) dead in its tracks and reduces net protein loss to zero.8 Also, while insulin is driving amino acids and glucose into muscle cells, it appears it also prevents the "leaking out" of these nutrients from the muscle cells that usually occur in response to training or illness.1.7,8

The presence of insulin locks in all the vital components of muscle growth.1.7,8 Kinetic studies using amino acid tracers confirm insulin is a potent amino acid transporter. In the presence of insulin, blood plasma levels of amino acids disappear. They are being driven into the cell and thus little is detected in the blood.1,4,8 However, as soon as insulin is taken away, blood plasma amino acid levels increase fast!1,4,8 They are being sucked out of muscles and utilized by other organs to meet their ravenous metabolic demands.

Recent research reviews demonstrate that an optimal hormonal environment must be created after training for muscle deposition (muscle growth) to occur.2,3

Insulin concentrations close to upper physiological limits (above 100Mu/l) are required to maximize creatine uptake and no less than 100 grams of simple carbohydrates must be ingested with creatine to enhance the uptake!5

Recent research reviews indicate a protein/carb mix stimulates insulin and other anabolic hormones levels far greater than just carbohydrates alone.3 It appears insulin levels need to be pretty damn high!1,5,8,11

Second, we know insulin dramatically stimulates inward transport of amino acids and glucose into cells, but more importantly, this is shown to be a dose dependant phenomeon.8

Insulin and Muscle Cell Amino Acid Transporters.
There have been five amino acid transporter systems identified in skeletal muscle.6
* System A (stands for the amino acid alanine) transports small amounts of neutral amino acids - particularly alanine and glycine. It is a high affinity, but low capacity transporter - not real good.
* Systems S and C (stands for serine and cystine) are sodium dependant, but only moderate affinity and moderate capacity amino acid transporters.
* System L (stands for leucine) handles all BCAAs and aromatic amino acid transport. It is a high affinity, high capacity transporter, however, it is not sensitive to insulin.
* System Nm (stands for nitrogen) is, as far as muscle tissue goes, a highly unusual transporter. It is a sodium dependent, low affinity and high capacity transporter. These qualities make it the fastest amino acid transporter in muscle and it is responsible for producing dramatic concentration gradients between the muscle cell and extracellular fluid.10,11,12 This system is the amino acid glutamine's exclusive transporter. Most important, system Nm is highly sensitive to insulin.10,11,12

When a muscle cell increases its volume dramatic things happen - muscle growth is "turned on" and muscle breakdown is "turned off"!9

Reports demonstrate that as glutamine moves into the cell it creates rapid cell swelling. This cell swelling stimulates transport system Nm to keep increasing the rate of glutamine transport into the cell.10,11 This important finding by the scientists shows that in the presence of normal concentration of insulin, system Nm does not "down regulate" and return the cell to normal volume like other transport systems do.10,11 The cell remains in a swollen and continuous anabolic state as long as sufficient insulin levels are maintained!

The increases in cell volume we are discussing are miniscule, however, even the smallest increases in cell volume caused by glutamine trigger a potent muscle building response.12 Therefore, the unique glutamine transporter system Nm found in muscle cells appears to be a direct pathway to producing anabolism within the muscle cell and is activated dramatically by insulin.6, 11,13

Insulin Needs Amino Acids to Build Muscle
Research reviews have determined that insulin halts protein breakdown, but does little to stimulate increases in protein synthesis rates.1,4 Insulin research also demonstrates an abundant supply of amino acids must be present along with high insulin levels to produce net gains in muscle.8 In fact, the whole stimulatory effect of insulin on muscle anabolism is dependant on a large, plentiful amount of amino acids to stimulate protein synthesis rates that create increases in muscle!8

Once muscle cells are crammed with these important nutrients they have to be kept within the cell to continue to exert their powerful muscle building effects. This means two objectives have to be met. Insulin levels have to be kept constant and a continuous, easily assimilated supply of amino acids has to be present in the blood stream to meet the metabolic demands of all other organs in the body so they won't draw them from muscle tissue! This is an important job of a protein supplement most people don’t even consider. You have to give your body the right material all the time so it doesn't rob important nutrients from muscle tissue!

[size=85]References:
1. Grizard J et al. Insulin action on skeletal muscle protein metabolism during catabolic states.Reprod.Nutr.Dev.1999;39:61-74
2. Kraemer WJ, et al. Effects of heavy resistance training on hormonal response patterns in younger vs. older men. 1999. J.Appl.Physiol.87(3):982-992.
3. Kreider RB. Dietary supplements and the promotion of muscle growth with resistance exercise.1999.Sports Med 27(2):97-110.
4. Miers WR and Barrett EJ. The role of insulin and other hormones in the regulation of amino acid and protein metabolism in humans. Journal of Basic Clin Physiol & Pharm. 1998. Vol 9 #2-4:235-253.
5. Steenge GR, Lambourne J, Casey A, Macdonald IA, and Greenhaff PL. Stimulatory effect of insulin on creatine accumulation in human skeletal muscle. Am.J.Physiol. 1998. 275;38:E974-E979
6. Rennie MJ. Influence of exercise on protein and amino acid metabolism. Handbook of Physiology,Section 12 Exercise: Regulation and integration of multiple systems. Oxford, UK: University Press.1996:995-1035
7. Takala TO, et al. Insulin action on heart and skeletal muscle glucose uptake in weight lifters and endurance athletes.1999. Am.J.Physiol.276;39:E706-E7
8. Zhang X, Chinkes DL, Wolf S,E and Wolfe RR. Insulin but not growth hormone stimulates protein anabolism in skin wound and muscle. Am.J.Physiol.1999.276;39:E712-E720.
9. D.Haussinger et al. 1998 Functional significance of cell volume regulatory mechanisms. Phys. Rev.vol78.#1.p247-290
10. S.Y.Low et al. Signaling elements involved in amino acid transport reponses to altered muscle cell volume. FASEBJ. 1997 vol11.p1111-1117.
11. S.Y.Low et al. Intergrin and cytoskeletal involvement in signaling cell volume to glutamine changes in cell volume. J.Physiol.1998 vol512 #2 p481-485.
12. S.Y.Low et al. Response of glutamine transport in cultured rat skeletal muscle to osmotically induced changes in cell volume. J.Physiol.1996 vol492 #3 p877-885.
13. M.J.Rennie et al.Glutamine metabolism and transport in skeletal muscle and heart and their clinical relevance. J.Nutr.1996 vol126 p1142S-1149S
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