Metabolism of skeletal muscle general features

Muscle cells differ according to their relative capacity for oxidative metabolism as opposed to anaerobic, glycolytic metabolism. If a cross-section of muscle is stained for one of the enzymes associated with aerobic metabolism (for instance, succinate dehydrogenase) then it can be seen that individual fibres differ in the extent of their staining (Fig. 4.11).

ADP3" ATP4"

ADP3" ATP4"

"OOC-CH2-N-C + h+ * Creatine X "OOC-CH2-N-C

I NH kinase I ^NH

OH3 CH3

Phosphocreatine Creatine

(creatine phosphate)

Fig. 4.10 The creatine kinase reaction in muscle. The reaction is referred to as the Lohmann reaction after the German biochemist who elucidated it. Creatine kinase operates near to equilibrium; therefore, as ATP is utilised rapidly at the beginning of contraction, the phosphocreatine pool is used to maintain the ATP concentration. In resting muscle, typical concentrations of ATP and phosphocreatine are 5 and 17 mmol per kg of muscle; therefore, the presence of phosphocreatine quadruples the ability to produce rapid contraction, before more ATP can be generated by other routes.

Fig. 4.11 Fibre-type composition of leg muscles in athletes. Different types of muscle fibre (muscle cell) are shown in a cross-section of muscle, by staining for the enzyme myosin-ATPase (which reflects fast-twitch muscles): dark-stained fibres are Type II, lighter fibres are Type I. Left, quadriceps muscle from a high-jumper; right, from a marathon runner. Reproduced from Jones, D.A. & Round, J.M. (1990) Skeletal Muscle in Health and Disease. A Textbook of Muscle Physiology, with permission of Manchester University Press.

Fig. 4.11 Fibre-type composition of leg muscles in athletes. Different types of muscle fibre (muscle cell) are shown in a cross-section of muscle, by staining for the enzyme myosin-ATPase (which reflects fast-twitch muscles): dark-stained fibres are Type II, lighter fibres are Type I. Left, quadriceps muscle from a high-jumper; right, from a marathon runner. Reproduced from Jones, D.A. & Round, J.M. (1990) Skeletal Muscle in Health and Disease. A Textbook of Muscle Physiology, with permission of Manchester University Press.

Broadly, there are two major types of muscle fibre (Table 4.1). Oxidative or red fibres are so-called because of their high content of myoglobin, a pigment related to haemoglobin, which assists the diffusion of oxygen into the muscle. They have a high density of capillaries perfusing them, and many mitochondria. These fibres use substrates, largely from the blood, and oxidise them to yield energy. Because the supply of substrate from the blood can be maintained for a long time - for instance, most of us have plenty of fat which can be supplied in this way - these muscle fibres are particularly important for sustained, but relatively low intensity, exercise such as walking or long-distance running. The oxidation of substrates from the blood requires time for diffusion of the substrate to the cell, diffusion of oxygen to the cell, and diffusion out of the cell of CO2. Therefore, contraction of this type of fibre, when it is stimulated, is relatively slow. These fibres are called red, slow-twitch or Type I fibres.

At the other extreme are the white fibres, lacking myoglobin. These fibres have fewer mitochondria, and are more equipped for anaerobic glycolysis than oxidative metabolism. Their main substrate for glycolysis is glucose 6-phos-phate produced by breakdown of glycogen stored within the same cells. The sequence of glycogen breakdown and generation of energy by glycolysis can be extremely rapid, since everything is 'on site'. Hence these are the fast-twitch fibres or Type II fibres (Table 4.1). Their role is to produce energy quickly, but

Table 4.1 Characteristics of fibre types in skeletal muscle.

Property

Type I (slow twitch) fibre Type II (fast twitch) fibre

Speed of contraction Myoglobin content Capillary density Myofibrillar ATPase activity Mitochondrial enzyme activity Glycogenolytic enzyme activity Glycogen content Triacylglycerol content Lipoprotein lipase activity

Slow

High High

High

High High

High

May be somewhat higher

High

Fast Low Low

Low Low

The terms 'high' and 'low' are relative. Type II fibres are adapted to fast work using their endogenous stores (ATP, phosphocreatine and glycogen, using anaerobic glycolysis), but have limited endurance. Type I fibres are adapted to slower work using energy generated by the complete oxidation of fuels (their own glycogen and triacylglycerol, and also glucose and non-esterified fatty acids taken up from the plasma).

because they largely depend upon stored substrate, they cannot maintain this for long. They are therefore particularly important in the rapid generation of energy over short periods, such as sprinting. A third type of muscle fibre is described as fast-twitch oxidative glycolytic or Type Ila, as distinct from the very fast, anaerobic glycolytic Type lib fibres.

In some animals, individual muscles are fairly uniform in their fibre type. In the rat, for example, there are some muscles which are composed almost entirely of red or white muscle fibres. For instance, the soleus muscle in the calf is used during movement such as running, and it is composed of consistently slow-twitch fibres. The adductor longus muscle in the thigh plays an intermittent role in maintaining posture and is a mainly fast-twitch muscle. In humans, most muscles are composed of a variety of fibre types. The composition of any particular muscle is not the same in everybody; some people have a preponderance of oxidative fibre types, some a preponderance of white, fast-twitch fibre types. This pattern is inherited to some extent. This is one reason why some people are naturally better than others at certain types of athletic events; for instance, someone with a preponderance of oxidative fibres will be better at endurance exercise than someone with more white, glycolytic fibres (Fig. 4.11).

Was this article helpful?

0 0
How To Boost Your Metabolism

How To Boost Your Metabolism

In The Next 45 Minutes You're Going To Discover How To Boost Your Metabolism And Lose Weight. Who Else Wants To Boost Their Metabolism And Shed Pounds Fast?

Get My Free Ebook


Post a comment