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

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Chapter | 15 Skeletal Muscle Function
kinase (AMPK), which monitors astonishingly small shifts
in the cellular AMP:ATP ratio ( Hardie, 2004 ). The activation
of AMPK—either by allosteric interactions of AMP,
phosphorylation by AMPK kinase, or both—triggers a shift
from energy-consuming pathways, such as glycogen, fatty
acid, and cholesterol synthesis, to ATP-generating pathways
via the phosphorylation of key regulatory enzymes ( Carling,
2004 ). AMPK activity may be necessary for contractionstimulated
glucose transport. AMPK has also been implicated
in regulating gene transcription and, therefore, may
function in some of the cellular adaptations to training.
F . Muscle Fiber Composition
Most muscles in domestic animals contain a mixture of
muscle fiber types ( Fig. 15-6 ). The muscle fiber composition,
the percentage of type 1, 2a, and 2x fibers, and
muscle fiber cross-sectional areas vary greatly among species,
muscle groups, individuals, and breeds. When comparing
the fiber-type composition of different individuals,
a standardized site must be used as fiber-type proportions
vary along the length and depth of a muscle. Locomotor
muscles in most domestic animals have a combination of
type 1, 2a, and 2x fibers (or I, IIA, and IIB depending on
the technique used for fiber typing). Locomotor muscles
in dogs contain type I and type IIA fibers and no type 2B
fibers using ATPase stains for fiber typing; however, histochemical
type IIA fiber types appear to correspond to
both type 2a and hybrid type 2a/x MHC isoforms ( Strbenc
et al. , 2004 ). Camelid muscles contain an unusual mixture
of 1, 2a, 2b, and 2x fibers ( Graziotti et al. , 2001 ). Horses
have a high proportion of type 2a and 2x fibers relative to
type 1 fibers in their locomotor muscles. Breed differences
have been extensively studied in horses ( Snow and Valberg,
1994 ). In general, quarter horses and Thoroughbreds have
the highest percentage of fast-twitch muscle fibers, 80% to
90%; standardbreds have an intermediate number, 75%; and
donkeys have the lowest percentage of fast-twitch fibers in
locomotor muscles ( Snow and Valberg, 1994 ).
Fiber-type composition is not constant, as growth and
training can alter the fiber-type composition and fiber size
in the same muscle over time. With growth and training,
there is a change in the length and breadth of a fiber as
well as a change in the proportion of fiber types rather than
an increase in the number of muscle fibers. Growth and
training at speed results in an increase in the proportion of
type IIA (2a) fibers and a concomitant decrease in type IIB
(2x) fibers ( Eto et al. , 2003, 2004 ).
G . Muscle Fiber Recruitment
When a muscle contracts during exercise, it does so in
response to a predetermined recruitment of particular muscle
fibers. This orderly recruitment of muscle fibers leads
to smooth, coordinated movement. As exercise begins, a
select number of motor units are recruited to provide the
power to advance the limb. Motor units are recruited with
respect to their contractile speed and oxidative capacities
( Burke, 1975 ; Valberg, 1986 ). At slow exercise intensities,
type I and a small number of type II fatigue-resistant
muscle fibers are stimulated. The force produced by any
muscle is proportional to the cross-sectional area that is
active. As the speed or duration of exercise increases, more
muscle fibers are recruited, and this occurs in the order of
their contractile speed from type I to type IIA and type IIB
( Lindholm et al. , 1974 ; Valberg, 1986 ). With moderate
intensity, type I and type IIA myofibers are preferentially
recruited, whereas moderate intensity of long duration or
maximal exercise intensity is required for recruitment of
type IIB myofibers.
More recent studies suggest that muscle fiber recruitment
may be regulated by the central nervous system, with
the subconscious brain producing an anticipated regulated
response governed by peripheral feedback mechanisms and
predetermined patterns of recruitment acquired from training
and modulated by conscious motivation ( Noakes et al. ,
2004 ).
IV . ORIGINS OF FIBER DIVERSITY
The origins of muscle fiber-type composition appear to lie
in lineage directives that developing embryonic myoblasts
obtain from their progenitors, which limit to some extent
the plasticity of the adult myofibers ( Rubenstein and Kelly,
2004 ). Specification to become slow- or fast-twitch fibers
appears to exist already in the myoblast stage and manifests
when myotubes express one or the other MHC isoform.
Subtypes of fast-twitch fibers become established following
the commitment to the fast-twitch phenotype and occur in
concert with the development of thyroid function ( Russell
et al. , 1988 ). In the embryo, the primordial myoblasts migrate
to their position in the limb of the embryo where their fiber
type is further influenced by temporal and positional factors,
synaptogenesis, imposed neuronal activity, and activation
of specific signal transduction pathways. At least two signal
transduction pathways may contribute to fiber-specific synthesis
of slow myosin. These include calcineurin, a calciumregulated
serine/threonine phosphatase, and Ras ( Rubenstein
and Kelly, 2004 ). A mosaic of fiber types subsequently
forms with fiber-type predominance programmed in certain
muscles or portions of muscles.
In mature muscle, the nerve has important trophic influences
on the innervated muscle, which regulate its structural
and metabolic properties. Motor units contain fibers of the
same type. When motor neurons that normally innervate
slow muscles are cross-innervated to supply muscles that are
normally fast, and motor neurons that normally innervate
fast muscles come to innervate muscles that are normally