Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 15 Skeletal Muscle Function
and Arabic numbers will be used for myosin isoforms.
Immunohistochemical staining for myosin isoforms reveals
that the true number of distinctly identified muscle fiber
types supersedes the number recognized by histochemistry
alone. Myosin isoforms include neonatal and slow-twitch
myosin isoforms as well as five distinct fast-twitch isoforms
( Rubenstein and Kelly, 2004 ). Type 2a, type 2b, and type 2x
are the most widely expressed skeletal muscle isoforms in
the body with 2 m fibers found in the jaw muscles of carnivores
and 2eom in extraocular muscles. Unfortunately,
type IIB fibers distinguished histochemically by myosin
ATPase activity do not correspond to type 2b fibers distinguished
by immunhistochemical staining for myosin heavy
chains. Rather type IIB fibers correspond more closely with
type 2x fibers (also called 2d) found in many mammalian
species whereas 2b fibers correspond to myosin found in
rapidly contracting muscle fibers found in rodents and camelids
( Gorza, 1990 ). Moreover patterns of contractile protein
isoforms and enzyme activities recognized in developing or
pathological muscles do not correlate with standard histochemically
derived fiber types ( Linnane et al. , 1999 ). Hence,
though useful as a screening tool for pathologists and physiologists,
histochemical techniques have limitations.
Most limb muscles are “ mixed ” and contain variable
proportions of type 1, type 2a, and type 2x myofibers.
Intermediate fiber types called type IIC by myosin ATPase
histochemistry or neonatal by immunohistochemistry are
normally rare in mature muscle. These fibers presumably
represent fibers capable of transitioning between type 1
and type 2 myofibers ( Brooke and Kaiser, 1970 ).
c . Histoenzymic Properties Associated with Aerobic
and Anaerobic Energy Metabolism
Histochemical studies show that type 1 myofibers have
higher activities of oxidative enzymes such as succinate
dehydrogenase (SDH), reduced nicotinamide adenine dinucleotide
tetrazolium reductase (NADH-TR) ( Fig. 15-6 ),
and reduced nicotinamide adenine dinucleotide phosphatetetrazolium
reductase (NADPH-TR) than type 2 fibers
( Dubowitz and Brooke, 1973 ). In conjunction with electron
microscopic observations, the activities of these enzymes
have been localized to mitochondria, which are present in
abundance in type I fibers. Associated with the large mitochondrial
volume of type 1 myofibers are lipid inclusions.
Type II fibers in general stain darkly with glyco(geno)lytic
stains such as phosphorylase or phosphofructokinase
activity. Because various intermediate histochemical reactions
of myofibers also exist, a classification system that
describes type 1, type 2 oxidative, and type 2 glycolytic is
often used.
d . Relationships with Functional Properties
Each motor unit is homogeneous with respect to its myofiber-type
composition. Motor units with slow-twitch fibers
OXIDATIVE METABOLISM
TABLE 15-1 Metabolic Properties of Muscle Fiber
Types in an Untrained Animal
SLOW-TWITCH
Glycogen
G-1-P
G-6-P
Pyruvate
Acetyl-CoA
TCA
cycle
UDPG
HMP shunt
NAD
respiratory
chain
NADH
Type 1 Type 2a Type 2x
Speed of Contraction Slow Intermediate Fast
Myoglobin Content High Intermediate Low
Fatigue Resistance High Intermediate Low
Oxidative Capacity High Intermediate Low
Fat Content High Intermediate Low
Glycolytic Capacity Low High High
Glycogen Content Low High High
Glucose
Lactate
Fatty acids
Amino acids
FAST-TWITCH
UDPG
HMP shunt
O 2
NAD
respiratory
chain
H 2 O NADH
Glycogen
G-1-P
G-6-P
Pyruvate
Acetyl-CoA
TCA
cycle
FIGURE 15-7 Schematic representation of some differences in energyyielding
metabolic pathways of slow-twitch and fast-twitch muscles. In
slow-twitch muscles, energy for contraction is derived primarily by oxidative
phosphorylation resulting from the oxidation of fatty acids, carbohydrates,
and perhaps amino acids via the tricarboxylic acid cycle (TCA).
Fast-twitch muscles derive their energy primarily via anaerobic glycogenolysis
and glycolysis through the degradation of glycogen and glucose to
lactate. Aerobic glycolysis via the HMP (hexose monophosphate) shunt is
a minor pathway in both types of muscle.
are fatigue resistant and corresponded to type 1 fibers in
histochemical stains. Two types of fast-contracting motor
units exist; fast-twitch fatigue resistant, corresponding to
type 2a fibers, and fast-twitch rapidly fatigable corresponding
to type 2x fibers identified histochemically ( Burke,
1975 ).
The physiological and histochemical properties and
classifications of myofibers are summarized in ( Table 15-1 )
and illustrated in Figure 15-6 and Figure 15-7 .
ANAEROBIC GLYCOLYSIS