section
2 1 .1
Muscle Systems
463
sections of muscle specimens can be incubated in solu-
tions at two different pH’s with ATP and Ca2+ and stained
for phosphate liberation. In the Brooke and Kaiser classi-
fication scheme, fibers that are light at pH >10 are type I;
those that are dark are called type II. This staining pat-
tern is reversed at pH <4.4. Fibers that are light at pH
4.4 but moderately dark at pH 4.6 are called type IIB;
those that are light both at pH <4.4 and at pH 4.6 are
type IIA. An additional type seen in small numbers in
human cells stains dark at both acid pH’s and at the al-
kaline pH, and is called IIC. Since the speed of shorten-
ing depends on which myosin is present, the relationship
between the Brooke and Kaiser fiber typing and the FT
vs. ST classification is quite consistent: type I fibers are
slow, IIB are the fastest, and IIA are intermediate in speed.
The I, IIA, IIB scheme has proven useful clinically and
in analysis of muscle performance, but does not convey
the near-continuum of myosin ATPase activity actually
present across fibers.
Metabolic Profile
Histochemical staining techniques enable the semi-
quantitative assessment of activity of enzymes in the en-
ergy pathways of cells, such as succinate dehydrogenase,
glyceraldehyde-3-phosphate dehydrogenase, and adeny-
late kinase (myokinase). By staining serial sections for
myosin ATPase typing and enzyme profiling, it has been
found that generally type I fibers are high in oxidative en-
zyme activity and low in glycolytic activity, while type IIB
fibers are the reverse. Type IIA fibers are moderately
high in both oxidative and glycolytic enzymes. Hence,
type I fibers are often called slow oxidative (SO), while
type IIB are called fast glycolytic (FG) fibers. Type IIA
are called FOG, or fast oxidative-glycolytic. However,
the Brooke and Kaiser typing and energy pathway pro-
files reflect quite different specializations of the cell, and
there is heterogeneity of enzyme profile within a muscle
unit.
The proportions of fibers within a muscle are deter-
mined by a combination of genetic and developmental
factors and by the pattern of recruitment of the muscle
unit. Due to this latter influence, the twitch and enzyme
characteristics of muscle fibers are somewhat malleable,
being influenced by training (especially endurance train-
ing) or by detraining (as in bed rest).
Multigene Families Encode Muscle Proteins
So far as we know, multiple isoforms of all of the myofib-
rillar proteins exist. These are encoded by families of genes
in probably all mammalian species. Expression of these
genes tends to be tissue-specific or fiber type-specific, and
for many there are fetal, adult, and (for some) neonatal
isoforms.
The human genome contains 20 or more actin genes
and
pseudogenes
(or large segments thereof), distributed
on several chromosomes. Apparently, actin genes were
frequently duplicated in the course of evolution. As men-
tioned earlier, six actin genes are expressed in significant
quantity in mammalian species in a tissue-specific man-
ner. Skeletal and myocardial actin genes are located on
chromosomes 1 and 15, respectively.
The type I muscle MHC gene is located on chromosome
14 in both humans and mouse. This gene also codes for
cardiac /1MHC, although they are not the identical protein.
The skeletal muscle type II MHCs are on chromosome 17
in humans (11 in mice). There is also a cardiac
a
MHC
and embryonic and neonatal cardiac and skeletal MHCs.
Similarly, there are multiple genes for the regulatory
proteins. In the case of troponin, for example, there are
two skeletal muscle genes for Tn I, one expressed in fast
and the other in slow fibers, and a cardiac Tn I specific
to myocardium. Fetal heart expresses this Tn I together
with slow skeletal Tn I. Cardiac Tn I is 30-32 amino acids
longer than either skeletal Tn I and thus, easily distin-
guished from them. In myocardial infarct (MI) patients,
Tn I appears in plasma about 4 hours post-MI and remains
elevated for approximately seven days. For Tn C, there are
also two skeletal genes and one cardiac gene. Tn T also has
two skeletal muscle forms, one fast and one slow. There
are also two isoforms of adult cardiac Tn T, called Tn Ti
and Tn T
2
, and two fetal cardiac Tn T isoforms. At each
age, the two forms are thought to result from alternative
RNA splicing. The predominant adult isoform is Tn T
2
,
and it has been found that serum Tn T
2
rises about four
hours post-MI and remains detectable for approximately
14 days. Although it has been reported that both Tn T and
Tn I are 90% (or more) sensitive and specific for MI, Tn I
has been found not to undergo ontogenic recapitulation
in tissue injury, which gives it an advantage over CKMB
(or Tn T) in suspected MI. Either Tn T or Tn I can be used
clinically, and the troponin assay has become part of the
standard of care in cases of suspected MI. In persons with
unstable angina, those with elevated cardiac troponins (es-
pecially Tn I and Tn T) are much more likely than others
to have a cardiac event in the coming months, so that the
troponin assay has predictive as well as diagnostic value.
Similarly, multiple genes, alternative RNA splicing, and
posttranslational modifications result in multiple essential
and regulatory light chains, tropomyosins, titins, and other
myofibrillar proteins. Energy pathway enzymes are differ-
entially expressed in various skeletal fiber types, in cardiac
and smooth muscle, and at different stages of development.
This also applies to Ca2+ regulatory proteins such as the
