Type I fibers

Tissue-Specific Expression. In adult rodents, PPAR.a is expressed in liver, kidney, intestine, heart, skeletal muscle, retina, adrenal gland, and pancreas. In adult human, PPARa is expressed in the liver, heart, kidney, large intestine, skeletal muscle (mostly slow-twitch oxidative type I fibers), and in cells of atherosclerotic lesions (endothelial cells, smooth muscle cells, and monocytes/macrophages). Therefore, regardless of... 

Figure 3. Top panel Whole muscle force (x) and single fiber PCr (a, a) and ATP ( , ) concentrations at rest and after 10 and 20 sec of intermittent electrical stimulation at 50 Hz. Open symbols denote type I fibers closed symbols denote type II fibers. Bottom panel Glycogenolytic rates in type I and II fibers during the 20 sec stimulation period. The open bar denotes type I fibers the closed bar denotes type II fibers.

Table 49-10. Characteristics of type I and type I fibers of skeletal muscle.

Some researchers reject the explanation that caffeine inhibits glycogen depletion during short term exercise54 but there is an increasing abundance of research supporting the notion of variable sensitivity to caffeine by muscle type. Muscles with higher ratios of type I fibers appear more sensitive than type II fibers, both in animal55-56 and human models.57... 

Cells from the type I fiber would be rich in mitochondria, whereas those of the type II fiber would have few mitochondria. 

The total tissue content of P-ARs was greater in the soleus, a muscle consisting almost entirely of slow-twitch (type I) fibers than in superficial white vastus lateralis, a muscle composed of greater than 95% fast-twitch (type lib) fibers (73). 

Type El fibers (fetal, reticular and vascular collagen) are delicate compared with type I fibers. In the fetus, type IE collagen is incorporated within the type I collagen to impart the greater flexibility critical for fetal development. After birth, the delicate type El collagen fibers contribute to reticular fibers and also type I collagen fibers that are present in cardiovascular and lymphoid tissues and also beneath epithelial basal cell layers, muscles, and nervous tissue Schwann cells. 

Yang L et al (2008) Mechanical properties of single electrospun collagen type I fibers. Biomaterials 29(8) 955-962... 

Ila (fast-twitch oxidative) and type I (slow-twitch oxidative) muscle fibers have greater oxidative capacity and are less dependent on the cycle than type Ilb (fast-twitch glycolytic) fibers. Thus, gradual exercise programs that lead to production of a greater proportion of type Ila and type I fibers might improve exercise tolerance in AMP deaminase deficiency. 

Skeletal muscle cells can be subdivided into type I and type IIfibers. Type I fibers are slow-twitch fibers that use primarily oxidative metabolism for energy, whereas the type 11 fibers (fast-twitch) use glycolysis as their primary energy-generating pathway. 

Measurements of twitch contraction times reflect the rates of Ca flow into and out of the myoplasm. The time to peak twitch force is influenced by the rate of increase in myoplasmic Ca . Conversely, half relaxation time of twitch force is influenced by the rate of decrease in myoplasmic Ca " (reflecting primarily SERCA activity). Values for time to peak twitch force and half relaxation time of twitch force also vary across muscles, such that muscles composed of predominantly type II fibers have shorter times compared to those composed of predominantly type I fibers (Larsson et al, 1994 Vedsted et al, 2003). 

EXAMPLE 13.20 Type I muscle fibers are often referred to as red muscle fibers because they are colored a distinct reddish brown by their relatively large numbers of mitochondria that contain colored cytochromes in the electron transport chain (Chap. 10 Fig. 10-17). They are also characterized by their association with a dense blood supply and a high concentration of the red-colored oxygen-binding protein, myoglobin. On arrival of a nerve impulse, these cells contract relatively slowly, so they are often referred to as slow-twitch muscle fibers. Thus, type I fibers have evolved for slow, endurance contractions such as are used for maintaining posture. 

EXAMPLE 13.22 The best example of muscles that are composed largely of type I fibers is the pectoral flight muscles of migratory birds these persist even in domestic chickens, namely, the white breast meat that many folk like to eat. Another familiar example is the lobster tail that sustains a single, or only a few, powerful bursts of contraction during the backward propulsion in the animal s escape response. 

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