Muscle Cells Obtain ATP From Several Sources

Although ATP is the immediate fuel for the contraction process, its concentration in the muscle cell is never high enough to sustain a long series of contractions. Most of the immediate energy supply is held in an "energy pool" of the compound creatine phosphate or phosphocreatine (PCr), which is in chemical equilibrium with ATP. After a molecule of ATP has been split and yielded its energy, the resulting ADP molecule is readily rephosphorylated to ATP by the high-energy phosphate group from a creatine phosphate molecule. The creatine phosphate pool is restored by ATP from the various cellular metabolic pathways. These reactions (of which the last two are the reverse of each other) can be summarized as follows:

ATP ^ ADP + P; (Energy for contraction) (1) ADP + PCr ^ ATP + Cr (Rephosphorylation of ATP) (2)

Because of the chemical equilibria involved, the concentration of PCr can fall to very low levels before the ATP concentration shows a significant decline. It has been shown experimentally that when 90% of PCr has been used, the ATP concentration has fallen by only 10%. This situation results in a steady source of ATP for contraction that is maintained despite variations in energy supply and demand. Creatine phosphate is the most important storage form of high-energy phosphate,- together with some other smaller sources, this energy reserve is sometimes called the creatine phosphate pool.

Two major metabolic pathways supply ATP to energy-requiring reactions in the cell and to the mechanisms that replenish the creatine phosphate pool. Their relative contributions depend on the muscle type and conditions of contraction. A simplified diagram of the energy relationships of muscle is shown in Figure 8.13. The first of the supply pathways is the glycolytic pathway or glycolysis. This is an anaerobic pathway- glucose is broken down without the use of oxygen to regenerate two molecules of ATP for every molecule of glucose consumed. Glucose for the glycolytic pathway may be derived from circulating blood glucose or from its storage form in muscle cells, the polymer glycogen. This reaction extracts only a small fraction of the energy contained in the glucose molecule.

The end product of anaerobic glycolysis is lactic acid or lactate. Under conditions of sufficient oxygen, this is converted to pyruvic acid or pyruvate, which enters another cellular (mitochondrial) pathway called the Krebs cycle. As a result of Krebs cycle reactions, substrates are made available for oxidative phosphorylation. The Krebs cycle and oxidative phosphorylation are aerobic processes that require a continuous supply of oxygen. In this pathway, an additional 36 molecules of ATP are regenerated from the energy in the original glucose molecule,- the final products are carbon dioxide and water. While the oxidative phos-phorylation pathway provides the greatest amount of energy, it cannot be used if the oxygen supply is insufficient, in this case, glycolytic metabolism predominates.

Glucose as an Energy Source. Glucose is the preferred fuel for skeletal muscle contraction at higher levels of exercise. At maximal work levels, almost all the energy used is derived from glucose produced by glycogen breakdown in muscle tissue and from bloodborne glucose from dietary sources. Glycogen breakdown increases rapidly during the first tens of seconds of vigorous exercise. This breakdown, and the subsequent entry of glucose into the glycolytic pathway, is catalyzed by the enzyme phosphorylase a. This enzyme is transformed from its inactive phosphory-lase b form by a "cascade" of protein kinase reactions whose action is, in turn, stimulated by the increased Ca2+ concentration and metabolite (especially AMP) levels associated with muscle contraction. Increased levels of circulating epinephrine (associated with exercise), acting through cAMP, also increase glycogen breakdown. Sustained exercise can lead to substantial depletion of glycogen stores, which can restrict further muscle activity.

Other Important Energy Sources. At lower exercise levels (i.e., below 50% of maximal capacity) fats may provide 50 to 60% of the energy for muscle contraction. Fat, the major energy store in the body, is mobilized from adipose tissue to provide metabolic fuel in the form of free fatty acids. This process is slower than the liberation of glucose

Atp Sourcers Skeletal Muscle

The major metabolic processes of skeletal muscle. These processes center on the supply of ATP for the actomyosin ATPase of the crossbridges. Energy sources are numbered in order of their proximity to the actual re-

actions of the crossbridge cycle. Energy is used by the cell in an A, B, and C order. The scheme shown here is typical for all types of muscle, although there are specific quantitative and qualitative variations.

The major metabolic processes of skeletal muscle. These processes center on the supply of ATP for the actomyosin ATPase of the crossbridges. Energy sources are numbered in order of their proximity to the actual re-

actions of the crossbridge cycle. Energy is used by the cell in an A, B, and C order. The scheme shown here is typical for all types of muscle, although there are specific quantitative and qualitative variations.

from glycogen and cannot keep pace with the high demands of heavy exercise. Moderate activity, with brief rest periods, favors the consumption of fat as muscle fuel. Fatty acids enter the Krebs cycle at the acetyl-CoA-citrate step. Complete combustion of fat yields less ATP per mole of oxygen consumed than for glucose, but its high energy storage capacity (the equivalent of 138 moles of ATP per mole of a typical fatty acid) makes it an ideal energy store. The depletion of body fat reserves is almost never a limiting factor in muscle activity.

In the absence of other fuels, protein can serve as an energy source for contraction. However, protein is used by muscles for fuel mainly during dieting and starvation or during heavy exercise. Under such conditions, proteins are broken down into amino acids that provide energy for contraction and that can be resynthesized into glucose to meet other needs.

Many of the metabolic reactions and processes supplying energy for contraction and the recycling of metabolites (e.g., lactate, glucose) take place outside the muscle, particularly in the liver, and the products are transported to the muscle by the bloodstream. In addition to its oxygen- and carbon dioxide-carrying functions, the enhanced blood supply to exercising muscle provides for a rapid exchange of essential metabolic materials and the removal of heat.

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Responses

  • Stefan
    What is crossbridge in skeletal muscle?
    4 years ago
  • Hannele
    Which processes use atp as an energy source in skeletal muscle?
    4 years ago
  • paulina
    How skeletal muscle attain ATP?
    11 months ago
  • SATU HAANP
    When will skeletal muscle utilize it source?
    3 months ago

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