Page 319 - Small Animal Clinical Nutrition 5th Edition

326 Small Animal Clinical Nutrition

The fiber composition varies between muscles and between
VetBooks.ir individuals. High-power athletes such as racing greyhounds
have a higher proportion of Type II fibers, whereas endurance
athletes have a higher proportion of Type I fibers (Table 18-4).
Because the work performed by most intermediate athletes
resembles that done by endurance athletes, but is of shorter
duration, the muscle fiber-type profile of intermediate athletes
should resemble that of endurance athletes more than that of
sprint athletes. Muscle fiber type is a function of genetics and
dictates the type of exercise for which an individual is best suit-
ed. However, some modification is possible through training.
Endurance training increases the number and volume of mito-
chondria and increases capillary density in all fiber types
(Åstrand, 1986).

Muscle Energetics
Exercise requires the transfer of chemical energy into physical
work. Chemical energy stored in high-energy phosphate bonds
of adenosine triphosphate (ATP) is the sole source of energy for
muscle contraction. ATP is cleaved to ADP during contrac-
tion. The amount of ATP used is proportional to the amount
of work performed (i.e., Fenn effect). ATP is vital not only for
the events of contraction but also for relaxation and mainte-
Figure 18-2. Summary of major energy-generating pathways used
during exercise. Key: ADP = adenosine diphosphate, ATP = adeno- nance of important ion gradients (Box 18-2). Normal excitabil-
sine triphosphate, TCA = tricarboxylic acid, NADH = reduced form ity of nerve and muscle is due to an electrochemical gradient
of nicotinamide dinucleotide, FADH = reduced form of flavin-adenine maintained by the sodium-potassium pump at the expense of
dinucleotide.
ATP. The calcium pump uses ATP to maintain a low concen-
tration of calcium in the muscle cell in the relaxed state. An
during exercise and how they may affect nutrient needs and 4) estimated one-third of the basal energy requirement is used to
the energy cost of running, which dictates dietary energy needs. maintain electrolyte concentration gradients across cellular
All of these factors are important to nutritional assessment of membranes (Blaxter, 1989; Pivarnik, 1994).
canine athletes and form the basis for a good feeding plan. Although ATP is the high-energy compound that cells use
as fuel to perform work, the energy required for exercise can
Muscle Metabolism ultimately come from a variety of sources. Because the concen-
Muscle Fiber Types tration of ATP in muscle cells is relatively low in comparison to
Muscles are not homogeneous. They are composed of fibers the cell’s need during exercise, ATP must be replenished from
with different contractile and metabolic characteristics. Muscle other fuel sources. These metabolic fuels are stored in muscle
fibers are classified into two groups based on contractile proper- (endogenous) and at other body sites (exogenous). The metab-
ties and histochemical staining: Type I or slow twitch and Type olism of these fuels occurs either with oxygen (aerobic) or with-
II or fast twitch. Type I fibers have high oxidative capacity and out oxygen (anaerobic). The anaerobic pathways (i.e., the crea-
endurance.These fibers are smaller than Type II fibers and have tine phosphate shuttle and glycolysis) occur in the cytoplasm,
high capillary density and high numbers of mitochondria.They whereas the aerobic pathways (i.e., complete oxidation of glu-
are low in glycolytic ability and low in staining for myofibrillar cose, fatty acids and amino acids) take place in mitochondria.
ATPase, an enzyme associated with fast contraction and relax- Figure 18-2 shows an overview of these pathways.The propor-
ation. Conversely, Type II fibers are high in myofibrillar tion of each pathway used is determined by the duration and
ATPase, larger, contain more glycolytic enzymes and have intensity of the task performed and by the conditioning and
greater strength. In most species, Type II fibers can be further nutritional status of the animal (Blaxter, 1989; Nadel, 1985;
subdivided into Type IIa and Type IIb. Contraction characteris- Williams, 1985; Kronfeld and Downey, 1981; Kronfeld et al,
tics are similar for Type IIa and Type IIb fibers, but Type IIa 1977; Hammel et al, 1977). Table 18-5 lists metabolic fuels,
fibers have greater oxidative capacity than Type IIb fibers; the their uses and storage sites.
latter are more fatigable. However, dogs and perhaps other The concentration of ATP is tightly regulated, although it is
members of their genus and subfamily, appear not to have clas- rapidly consumed during exercise (Blaxter, 1989; Nadel, 1985;
sic Type IIb fibers but, instead, two other kinds of Type II fibers Stryer, 1988; Rusko et al, 1986). Resting muscle cells have only
that are more oxidative (called Type IIDog and Type IIC).This enough ATP to fuel muscle contraction for a few seconds. If
fits with the general observation that dogs are tireless runners work continues beyond this point, ATP must be regenerated
(Snow et al, 1982; Latorre et al, 1993; Rivero et al, 1994). from other metabolic fuels at a rate comparable to that at which