Skeletal Muscle Contains Slow Red Fast White Twitch Fibers

Different types of fibers have been detected in skeletal muscle. One classification subdivides them into type I (slow twitch), type IIA (fast twitch-oxidative), and type IIB (fast twitch-glycolytic). For the sake of simplicity, we shall consider only two types: type I (slow twitch, ox-

Table 49-9. Some physiologic functions and pathologic involvements of nitric oxide (NO).

• Vasodilator, important in regulation of blood pressure

• Involved in penile erection; sildenafil citrate (Viagra) affects this process by inhibiting a cGMP phosphodiesterase

• Neurotransmitter in the brain and peripheral autonomic nervous system

• Role in long-term potentiation

• Role in neurotoxicity

• Low level of NO involved in causation of pylorospasm in infantile hypertrophic pyloric stenosis

• May have role in relaxation of skeletal muscle

• May constitute part of a primitive immune system

• Inhibits adhesion, activation, and aggregation of platelets idative) and type II (fast twitch, glycolytic) (Table 49-10). The type I fibers are red because they contain myoglobin and mitochondria; their metabolism is aerobic, and they maintain relatively sustained contractions. The type II fibers, lacking myoglobin and containing few mitochondria, are white: they derive their energy from anaerobic glycolysis and exhibit relatively short durations of contraction. The proportion of these two types of fibers varies among the muscles of the body, depending on function (eg, whether or not a muscle is involved in sustained contraction, such as maintaining posture). The proportion also varies with training; for example, the number of type I fibers in certain leg muscles increases in athletes training for marathons, whereas the number of type II fibers increases in sprinters.

A Sprinter Uses Creatine Phosphate & Anaerobic Glycolysis to Make ATP, Whereas a Marathon Runner Uses Oxidative Phosphorylation

In view of the two types of fibers in skeletal muscle and of the various energy sources described above, it is of interest to compare their involvement in a sprint (eg, 100 meters) and in the marathon (42.2 km; just over 26 miles) (Table 49-11).

The major sources of energy in the 100-m sprint are creatine phosphate (first 4-5 seconds) and then anaerobic glycolysis, using muscle glycogen as the source of glucose. The two main sites of metabolic control are at glycogen phosphorylase and at PFK-1. The former is activated by Ca2+ (released from the sarcoplas-mic reticulum during contraction), epinephrine, and

Creatine phosphate

Creatine phosphate

Figure 49-16. The multiple sources of ATP in muscle.

AMP. PFK-1 is activated by AMP, P;, and NH3. Attesting to the efficiency of these processes, the flux through glycolysis can increase as much as 1000-fold during a sprint.

In contrast, in the marathon, aerobic metabolism is the principal source of ATP. The major fuel sources are blood glucose and free fatty acids, largely derived from the breakdown of triacylglycerols in adipose tissue, stimulated by epinephrine. Hepatic glycogen is degraded to maintain the level of blood glucose. Muscle glycogen is also a fuel source, but it is degraded much more gradually than in a sprint. It has been calculated that the amounts of glucose in the blood, of glycogen in the liver, of glycogen in muscle, and of triacylglycerol in adipose tissue are sufficient to supply muscle with en-

ergy during a marathon for 4 minutes, 18 minutes, 70 minutes, and approximately 4000 minutes, respectively. However, the rate of oxidation of fatty acids by muscle is slower than that of glucose, so that oxidation of glucose and of fatty acids are both major sources of energy in the marathon.

A number of procedures have been used by athletes to counteract muscle fatigue and inadequate strength. These include carbohydrate loading, soda (sodium bi-

Table 49-11. Types of muscle fibers and major fuel sources used by a sprinter and by a marathon runner.

Table49-10. Characteristics of type I and type II fibers of skeletal muscle.

Type I Slow Twitch

Type II Fast Twitch

Myosin ATPase

Low

High

Energy utilization

Low

High

Mitochondria

Many

Few

Color

Red

White

Myoglobin

Yes

No

Contraction rate

Slow

Fast

Duration

Prolonged

Short

Table 49-11. Types of muscle fibers and major fuel sources used by a sprinter and by a marathon runner.

Sprinter (100 m)

Marathon Runner

Type II (glycolytic) fibers are used predominantly.

Type I (oxidative) fibers are used predominantly.

Creatine phosphate is the major energy source during the first 4-5 seconds.

ATP is the major energy source throughout.

Glucose derived from muscle glycogen and metabolized by anaerobic glycolysis is the major fuel source.

Blood glucose and free fatty acids are the major fuel sources.

Muscle glycogen is rapidly depleted.

Muscle glycogen is slowly depleted.

carbonate) loading, blood doping (administration of red blood cells), and ingestion of creatine and an-drostenedione. Their rationales and efficacies will not be discussed here.

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Diabetes 2

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