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The mammalian engine


Any muscle works like an engine. It has a size, structure, requires fuel to operate, produces force from the burning of that fuel, and needs to be controlled. If you are unfamiliar as to how muscles work, or quickly needs to brush up on the mechanics, click here.


Let’s take the Bicep brachii. It is made up of thousands of muscle cells or fibres, which lie parallel to one other. These fibres are connected to bone via tendons. Each muscle fibre is connected to a nerve (called innervation) and will only contract when this nerve fires. A number of these nerves coming from the fibres will fuse into one neurone and, together with the muscle fibres, will be known as a motor unit. Stimulation of this neurone will cause only those fibres to contract and produce force, whereas the other fibres will stay relaxed and not take part in contraction.

Each muscle fibre is made up of thousands of thick and thin filaments, also known as myosin and actin, respectively. With other proteins, they form the smallest contractile unit known as a sarcomere within a muscle fibre. It is also the myosin molecule, particularly the myosin heavy chain (MHC) protein, that determines the fibre type of the fibre. These myosin heads are able to move and are like the pistons of an engine that can produce force and power.

No engine can function without fuel. Adenosine triphosphate (ATP) is the main fuel source of many cells. It is this ATP that is needed for the myosin to work. ATP cannot be stored, but can be replenished from other more complex fuel sources. These complex fuel sources are phosphocreatine, blood glucose, glycogen and fat. The first three fuels can be converted to ATP without oxygen (O2). However, a more efficient way of generating ATP is when glucose, glycogen or fat are metabolised in the presence of with oxygen in the mitochondria. This yields a lot more ATP and the muscle fibre can contract for longer, and resist fatigue much better due to the continuous supply of ATP.

The breakdown of the four fuels yields byproducts that can lead to premature muscle fatigue. These include carbon dioxide (CO2), hydrogen ions (H+ – that lowers the pH making the environment more acidic) and lactate. All these byproducts are readily transported away from the muscle by the blood.

The next few sections will delve a little deeper into all these aspects to better understand muscle function.

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Contractile properties of skeletal muscle fibres and heart tissue

The permeabilised single muscle fibre system shown below consists of various components to accurately measure the contractile properties of a muscle fibre.
 
Properties of muscle that can be measured include:

☆ force
☆ elasticity
☆ shortening velocity
☆ power
☆ calcium sensitivity of fibres
☆ effects of drugs on contractility parameters

The upper limit of the force transducer is 5.1 mNewton, with a maximum recording frequency of 1000 Hz.

Muscle fibres can be submerged into 8 different temperature controlled baths, which can be set between 10˚C and 50˚C. Each bath can contain solutions that block contraction (e.g. inhibitors), activate the fibre (e.g. ATP, calcium) or enhance contraction (e.g. drugs).

The lever arm connected to the motor is able to control the length of the muscle fibre by stretching it or allowing the fibre to shorten during contraction. This lever can be programmed to produce rapid stretching or shortening (within 1 ms) or gradually over extended periods. The apparatus is mounted on an inverted microscope equipped with a pre-calibrated CMOS camera, allowing for the accurate measurement of the specimen length, diameter and sarcomere spacing.

 

Enzyme kinetics, metabolites and protein

For cells to live, ATP is required. This ATP is derived from the breakdown of more complex fuels, such a phosphocreatine, glucose, glycogen and fat (see Fuelling performance in Muscle 101). 

Each enzyme of the metabolic pathways, has a specific rate at which it can produce ATP from the above mentioned fuels, known as its activity. However, this rate can be altered acutely from either allosteric activation or inhibition, or interactions with substrates, or activation by other enzymes, or temperature. Additionally, various external stimuli can induce chronic adaptations that can alter this rate, either increasing the enzyme’s activity (e.g. endurance exercise training over 3 months) or decreasing the enzyme’s activity (e.g. bedridden for 3 months). 

Our lab uses a multitude of approaches, ranging from spectrophotometric and fluorometric assays by means of kinetic enzyme assays, histochemistry (e.g. NADH stain) or Western blotting. The former assays are very specific to the pathways and enzyme of interest. To illustrate, the graph shows the rate of activity of a particular enzyme related to exercise fatigue resistance – in this case citrate synthase. The slope of each line is calculated, normalised to the muscle weight (or protein concentration) and expressed as µmol/min/g tissue or protein. The slope of the lion is much lower than that of the human, indicating that the human has higher activity for CS, and thus, better endurance capacity than a lion. 

The following enzyme and metabolite assays are routinely performed in our lab, each providing information on the flux capacity through a specific pathway:


☆ 3-hydroxyacetyl Co A dehydrogenase
☆ creatine kinase
☆ citrate synthase
☆ lactate dehydrogenase
☆ phosphofructokinase
☆ phosphorylase

☆ muscle lactate
☆ muscle glycogen
☆ muscle creatine
☆ muscle soluble protein content


Assays for other enzymes in the tissues not listed above, can also be developed on request.

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