AMER. ZOOL., 27:1067-1077 (1987)
Muscle Fiber Types in Crabs: Studies on
Single Identified Muscle Fibers1
WERNER RATHMAYER AND LOTHAR MAIER
Fakultdt fur Biologie, Universitdt Konstanz, Postfach 5560, D-7750 Konstanz,
Federal Republic of Germany, and Stazione Zoologica, Naples, Italy
SYNOPSIS. Based on electrophysiological and histochemical data, four types of muscle
fibers (types I, II, III and IV) can be identified in the closer of the crab Eriphia. Although
characteristics used for typing vary among the fibers of a particular type, the combination
of several parameters permits an assignment. Of particular significance for typing is the
myosin ATPase activity and its stability after preincubation at different pH levels. The
fiber types defined for the closer muscle can also be found in the other leg muscles of
Eriphia.
Single, electrophysiologically identified fibers of each type were quantitatively analyzed
for several key enzymes of oxidative and glycolytic energy metabolism (GAPDH, LDH,
CS, IDH, HAD). Despite the variations found, different metabolic types can be defined.
The typing derived from biochemical studies correlates well with that obtained electrophysiologically and histochemically. The variability of the biochemical properties, however, seems to be considerably larger.
The type I fibers can be regarded as slow oxidative, the type II and III fibers as fast
oxidative glycolytic, and the type IV fibers as fast glycolytic.
intermediate (for review see Govind and
Several attempts have been made to clas- Atwood, 1982).
Little is known about distribution and
sify crustacean muscle fibers based mainly
on structural, contractile and electrophys- activity of enzymes important for energy
iological criteria (for review see Govind and metabolism in crustacean muscles. Studies
Atwood, 1982). Recently, histochemical so far have been limited to whole muscles
measurements, particularly of the myofi- or muscle fiber groups, and histochemical
brillar actomyosin ATPase activity (Silver- determinations of enzyme activities (Hajek
man and Charlton, 1980; Govind et al, et al., 1973; Govind et al, 1981; Parsons
1981; Tse et al, 1983) and biochemical and Mosse, 1982; Costello and Govind,
studies on crustacean muscle fibers added 1983; Tse et al, 1983). Correlation of data
further information (Leferovich and Ste- on energy metabolism with known physphens, 1982; Costello and Govind, 1983). iological parameters of single muscle fibers
From these investigations, two conclusions should be of great importance for any fiber
can be drawn. 1. Crustacean motor units typing. For the closer muscle of the crab
are far more heterogenous with regard to Eriphia spinifrons a preparation was develtheir fiber composition and fiber proper- oped recently, where individual fibers with
ties than vertebrate muscles. 2. Typing of characteristic and distinct physiological
crustacean muscle proved to be difficult properties can be located at identical posibecause the fiber properties exhibit a wide tions from one preparation to the next
spectrum of variations. On one end a slow (Rathmayer and Erxleben, 1983). These
fiber type, on the other end a fast fiber fibers can be classified into four types on
type can be easily recognized. Fibers not the basis of their electrophysiological
fitting into these two categories were often, responses. Since it is also possible to deterby lack of specific criteria, simply termed mine activities of selected key enzymes of
glycolytic and aerobic energy metabolism
in single electrophysiologically identified
1
From the Symposium on Muscle Fiber Typing as a muscle fibers after microdissection (Maier
Bioassay of Nerve-Muscle Interaction: Comparison of et al., 1986), a comparison of fiber types
Arthropod and Vertebrate Systems presented at the Annual described from physiological and histoMeeting of the American Society of Zoologists, 27- chemical measurements with types
INTRODUCTION
30 December 1985, at Baltimore, Maryland.
1067
1068
W. RATHMAYER AND L. MAIER
FCE:
fibre group
SCE
IV
FIG. 1. Schematic representation of the proximal half of the closer muscle in Eriphia. Nine superficial fibers
are identified by numbers. Fibers of the same type are indicated by identical symbols. They are arranged in
characteristic groups. Innervation through FCE, SCE and CI is shown.
obtained from enzyme activity measurements is feasible. This approach has been
adopted for the first time for crustacean
muscles (Maier et al., 1986).
RESULTS AND DISCUSSION
Electrophysiological typing of identified
closer muscle fibers
The superficial dorsal layer of muscle
fibers in the two proximal heads of the
closer in the first three pairs of walking
legs in Eriphia is composed of 9-12 fibers
(Fig. 1). All of them, as well as the other
300 fibers of the closer muscle, are innervated through the fast closer excitor (FCE).
Many fibers are additionally innervated by
the slow closer excitor (SCE) and a branch
of the common inhibitor (CI). It can be
shown, that with regard to their electrophysiological and histochemical differences, these superficial fibers resemble by
and large all the fibers composing the closer
(Rathmayer and Erxleben, 1983; Maier et
al., 1984).
On the basis of differences in passive
electrical membrane properties, innervation pattern and junctional responses (Fig.
2), the muscle fibers of the closer can be
grouped into four types (Rathmayer and
Erxleben, 1983; Maier et al., 1986, where
details are given). They are summarized in
Table 1. Although the characteristics used
for typing vary among the fibers of a particular type, the combination of several
parameters permits an assignment.
When the facilitation properties of both
fast and slow ejps are taken into account
(Rathmayer and Hammelsbeck, 1985),
additional differences become apparent
among the fibers. Amount, time course and
frequency dependence of facilitation of
slow and fast ejps differ characteristically
(Fig. 3), but are similar within fibers
belonging to the same type. The most strik-
1069
MUSCLE FIBER TYPES IN CRABS
SCE
fiber 1
fiber 2
type 4
fiber 3
fiber 4
type 1
fiber 5a
fiber 5b
type 2-
fiber 6
type 3
fiber 7
fiber 8
— type 4 —
FIG. 2. Slow and fast ejps from nine superficial closer muscle fibers (numbers 1 to 8 of Fig. 1) upon stimulation
of SCE and FCE separately with six pulses at 90 Hz. Fiber 5b is another type II fiber, which does not appear
on the surface in the plane of section shown in Figure 1.
ing difference is exhibited by type I fibers.
Facilitation of slow ejps increases moderately with time of stimulation up to frequencies of 16 Hz to 32 Hz. After an initial
increase, facilitation often does not increase
further with prolonged stimulation. At frequencies above 32 Hz, facilitation increases
steeply at first, but declines to smaller val-
TABLE 1. Summary of parameters used for the definition of four fiber types in different leg muscles of the crab Eriphia.
Type I
Electrophysiol. parameters
Membrane potential
Time constant
Input resistance
Synaptic output
Slow axon
Fast axon
Ejp facilitation
Slow ejps
Fast ejps
CI inhibition
Innervation pattern
Closer
Bender
Extensor
Histochemical parameters
ATPase activity
Glycogen content
SDH activity
Contractile type
Enzyme activities
GAPDH
LDH
CS
IDH
HAD
Metabolic type
Type II
Type III
Type IV
long
high
medium
medium
medium
high
short
high
short
low
low
medium
medium
small
medium-large
large
small
medium
negative
strong
large
medium-little
weak
little
medium
slow, fast, CI
slow, fast, CI
slow, fast, CI
slow, fast, CI
slow, fast, CI
slow, fast, CI
fast
fast
fast
fast
fast
fast
low
low
high
high
high
fast
high
high
high
fast
high
low-medium
low-high
fast
medium
medium
high
high
high
high
high
high
high
high
medium—high
high
FOG
FOG
low
low-medium
slow
low
low
medium
medium
medium
SO
low
low
low
FG
1070
W. RATHMAYER AND L. MAIER
40
SCE 16
15
30
2 10
20
c
o
10
Urns)
-1
500
FCE 64
Kms)
FCE 64
FCE 64
2-
500
tCms)
500
Kms)
FIG. 3. Facilitation of ejps upon stimulation of SCE and FCE separately with 8, 16 and 64 Hz. The data
have been obtained from fiber 2 (a), fiber 5 (b), fiber 6 (c) and fiber 8 (d), which represent types I, II, III and
IV, respectively.
ues during prolonged stimulation. The
depolarization obtained remains on a pla- Histochemical typing of closer
teau. Fast ejps in type I fibers usually show muscle fibers
no facilitation; on the contrary, they exhibit
The fiber typing based on electrophysantifacilitation (negative values for f, Fig. iological findings in the closer muscle is
3a) at frequencies above 4-8 Hz.
corroborated by histochemical studies of
In fiber types II, III and IV, an inverse myofibrillar actomyosin ATPase activity
relationship between facilitation values and (Maier et al., 1984). When studied at pH
size of slow and fast ejps exists. This has 9.4 (Padykula and Hermann, 1955), type I
also been shown for other crustacean mus- fibers are characterized by low myosin
cles (Atwood and Bittner, 1971; Sherman ATPase activity levels, whereas fibers of
and Atwood, 1972). The small, slow ejps types II, III and IV always show high activof type II fibers always facilitate better than ities. The low myosin ATPase activity in
the larger fast ejps in these fibers (Fig. 3b). type I fibers is in agreement with their slow
Facilitation of fast ejps in type IV fibers contraction. They resemble slow fibers of
(Fig. 3d) exceeds that of large amplitude other crustacean muscles (Ogonowski and
fast ejps in type III fibers (Fig. 3c), but is Lang, 1979; Costello and Govind, 1983;
smaller than that of slow ejps of similar Govind etai, 1981; T s e ^ al., 1983). Their
small amplitude in type II fibers. In gen- myosin ATPase tolerates the widest pH
eral, however, high output synapses show range, from 4.6 to 10.4. After preincupoor facilitation, whereas transmission at bation at these pH values, only type I fibers
low output synapses is strongly facilitated stain. The fast fiber types, exhibiting high
myosin ATPase levels, can be further dif(Table 1).
MUSCLE FIBER TYPES IN CRABS
fiber type
la
1071
IV
FIG. 4. Cross sections through the muscles in the propodite (top), carpopodite (lower left) and meropodite
(lower right) of a walking leg of Eriphia at proximal levels. Muscle fibers were typed according to differences
in myosin ATPase activity after preincubation at different pH. Abbreviations as in Figure 5. Calibration bar:
500 Mm.
ferentiated by preincubation at different
pH levels. With preincubation at pH 5.0,
in addition to type I fibers, type II fibers also
stain. Type III fibers stain weakly at this
pH, type IV fibers not at all. At still higher
preincubation pH, the myosin ATPase of
type IV fibers is active and therefore is
stained.
Whereas the fiber classification according to different levels of myosin ATPase
activity and to pH stability shows a good
correlation with the typing on the basis of
electrophysiological data, the histochemical determination of glycogen content and
SDH activity does not permit a similar
unequivocal grouping. Considerable differences exist among fibers showing similar electrophysiological criteria and also
similar myosin ATPase activities. Despite
the heterogeneity, it is safe to say that type
II fibers always show high, type III fibers
very high, and type I fibers usually little
glycogen content. Among type IV fibers a
wide spectrum from low to medium gly-
1072
W. RATHMAYER AND L. MAIER
•xcitor
excitor + Cl
excitor
excitor + Cl
B
S
F
-
\\W\\\
FIG. 5. Effects of Cl on fibers of type I (two right columns) and on another fiber type (type II for the closer,
bender and accessory flexor) in Eriphia. C = closer, O = opener, B = bender, S = stretcher, AF = accessory
flexor, F = flexor, E = extensor. Calibration pulse: 2 mV, 10 ms.
cogen content exists. With regard to SDH
activity, type II and III fibers exhibit the
highest values, but again, differences occur
among fibers we believe belong to the same
type. This is particularly evident in fibers
of type IV, which show low to high SDH
activity, but also in type I, where low to
medium SDH activity is found. In all fibers,
however, a direct correlation between SDH
activity and glycogen content exists (see
Table 1).
Comparison with other leg muscles
Determination of myosin ATPase activity levels combined with preincubation at
different pH values permits in the closer
the discrimination of fiber types which correspond to those defined electrophysiolog-
ically. Therefore, staining for myosin
ATPase activities was employed to investigate the other leg muscles of Eriphia in a
comparative analysis (Maier, in preparation). All fiber types found in the closer
muscle are also present in the opener,
stretcher, bender, flexor, accessory flexor
and extensor muscle (Fig. 4). At least one
additional type, not seen in the closer, could
be discriminated in several other muscles
(Fig. 4). Its myosin ATPase shows a similar
pH stability to that of type I fibers. The
overall activity, however, is higher than in
type I, but lower than in the fast fibers.
Although we do not know the contractile
or electrophysiological responses of this
fiber type, we have tentatively termed it as
type la. As in the closer, muscle fibers of
a particular type are grouped together.
1073
MUSCLE FIBER TYPES IN CRABS
constriction
pipette .,
single fiber
<5-300ug),
glass bead
sonication
(7min,0°C)
fiber
homogen
addition of homogenate
aliquots (1-5pl)
microcuvettes
containing
reaction mixture
(87-98ul)
start of reaction
by adding substrate
<5-Bul)
fluorimetnc determination
of reaction kinetics
FIG. 6. Procedure for determination of enzyme reaction kinetics in single identified muscle fibers. The
numbers in ii\ indicate the range of volumes used for different enzyme tests.
Electrophysiological recordings from
several hundred muscle fibers from each
of the six leg muscles other than the closer
permit again a correlation with the results
from histochemical myosin ATPase staining. Although the evaluation is not yet
complete (Rathmayer, Wiens, and Maier,
in preparation), it is clear that in the bender
and in the extensor, the neuromuscular
responses are similar to those described for
the closer. These two muscles are innervated by two excitors each, a slow and fast
neuron, and by a branch from CI. The
fibers receiving triple innervation are of
types I and II (Fig. 5). Type I is characterized by a) low ATPase activity, which is
stable over a wide pH range, b) long time
constant, c) slow ejps with moderate facilitation, d) fast ejps with antifacilitation, e)
strong pre- and postsynaptic inhibition
through CI. Type II fibers exhibit a) high
ATPase activity, which is not active after
preincubation at pH 4.6, but at 5.0, b) short
time constant, c) low output slow synapses
with large facilitation, d) facilitating fast
ejps, the amount depending on their initial
size, e) presence of CI, but weak inhibition.
The type III found in histochemical stainings according to its weak ATPase activity
with preincubation at pH 5, has not yet
been found electrophysiologically. Type IV
is, as in the closer, the most common fiber
type. It has only fast innervation. As in the
closer, the fast axon innervates all fibers in
these two muscles, the facilitation of its ejps
is poor to medium.
For the remaining muscles, which differ
in their innervation pattern from the closer,
bender and extensor, the electrophysiological typing is not yet complete. In the
opener and stretcher muscles, which share
a single excitor, all fibers are additionally
innervated by a specific inhibitor, and some
also by CI. In both muscles, CI is again
very powerful in type I fibers, less in type
II fibers (Fig. 5). The accessory flexor muscle, which is innervated by a single excitor
and by CI (Wiens and Rathmayer, 1985),
and also the main flexor muscle, which
receives complex innervation from at least
four excitors and CI, contain fibers of the
types described above (Fig. 5). CI is most
powerful on type I fibers (Rathmayer,
Wiens, and Maier, in preparation), but produces little effect in other fiber types. The
different types of fast fibers seen in the
myosin ATPase stains have not yet been
systematically investigated with regard to
1074
W. RATHMAYER AND L. MAIER
1 ft
+
i
1
h
h
i
(H
2
3 4
5
6
1
II
III
7
6
9 10 I
IV
I,
f
2 3 1
5
6
1
II
III
7 8 9
10 I
IV
1
•S 1
ft
I
I/)
0)
I
G
2
3 4
6
1
III
7 S 9 I 0 I
2 3 4
1
IV
5
6
II
HI
7 8 9
*
10 I
IV
!
2 3 4
1
5
e
II
III
7 8 9
10 I
IV
FIG. 7. Absolute enzyme activities (/tmoles x min~' x g-' muscle) in ten muscle fibers (numbers 1-9 in Fig.
1. Number 10 lies beside number 9. It is not yet present at the sectioning level shown in Fig. 1). The bars
give SE of the mean.
MUSCLE FIBER TYPES IN CRABS
1075
their electrophysiological characteristics.
It could very well be that additional types
will be obtained.
2 15
Enzyme measurements in single
electrophysiologica lly identified
muscle fibers
8
For any fiber typing it is desirable to know
the metabolic characteristics of the indif=0.88
vidual muscle fiber. Recent advances in
microdissection techniques and quantitative microchemistry enable the determi- E l m IS
nation of enzymes in single, individually
dissected fibers or even segments of them
(Nemeth et al., 1981; Pette, 1985). By
0
20
(0
SO
80
100
Gtyceraldehydephosphate dehydrogenase (U/g)
employing these techniques and adapting
them to the special requirements of crustacean muscle fibers, it was possible to
quantitatively determine for each of the
ten superficial muscle fibers in the closer a 20
(Fig. 1) the activities of a number of o
enzymes involved in either anaerobic or
aerobic energy metabolism (Maier et al.,
1986). The activities of glyceraldehydphosphate-dehydrogenase (GAPDH) and CJ 10
lactate-dehydrogenase (LDH), key enzymes
for anaerobic metabolism of carbohydrates during glycolysis, were measured.
As indicators for the oxidative capacities
of fibers, the activity of citrate-synthetase
(CS) and NADP-isocitrate-dehydrogenase
0
20
40
60
60
100
(IDH), both involved in the citrate cycle,
Glyceraldehydephosphate dehydrogenase (U/g)
have been determined. Finally, as an indicator for the capacity of fibers to use fatty
20
acids for energy metabolism, the activity
of 3-hydroxyacyl-CoA-dehydrogenase
(HAD) was determined.
r=09S
Single fibers, which were first identified
according to their electrophysiological
responses, were intracellularly dye-marked
by injection of 15 mM nitroblue-tetrazolium chloride (NBT) from dye-filled
microelectrodes. After freeze-drying the
muscle, the marked fiber was dissected,
weighed and subjected to a procedure
shown in Figure 6 (for details see Maier et
al., 1986). For each type, identified fibers
were dissected from five to nine closer
0
5
10
15
20
preparations. From each fiber three sam3-hydroxyacyl-CoA dehydrogenase (U/g)
ples were analyzed, yielding 15 to 27 values
Fie. 8. Ratios between different enzyme activities
for a particular fiber.
(Mmoles x min"1 x g~' muscle). The numbers refer
The enzyme activities show considerably to the fibers of Figures 1 and 7.
larger variability within a fiber type than
t,
r
1076
W. RATHMAYER AND L. MAIER
any of the above mentioned parameters.
However, significant, type specific differences are also obvious (Fig. 7), which match
with the previously defined types and support the concept. With regard to GAPDH,
type I fibers show two- to threefold lower
activities than the fast fibers. Fibers 5 and
6, which represent types II and III, but
in particular type IV fibers exhibit high
GAPDH activities. This activity varies,
however, by a factor of 4 among type IV
fibers (Fig. 7), one fiber (number 9) even
exhibiting activities in the range of type I
fibers.
T h e LDH activity parallels that of
GAPDH. Again, type I fibers, despite some
heterogeneity, display lower activities than
the fast fibers.
With regard to the enzymes characteristic for the citrate cycle and for fatty acid
metabolism and therefore representative
of oxidative capacities, the fast type IV
fibers exhibit clearly lower activities than
other fast fibers (5 and 6) or the slow type
I fibers. This suggests, that fibers 5 and 6
have high oxidative capacities, but they
have also glycolytic potencies. The slow
type I fibers show oxidative potential, which
is lower than in fibers 5 and 6, higher, however, than in type IV fibers. Glycolytic
activity is also present.
The question arises, whether the single
fibers investigated for various enzyme
activities represent different metabolic
types. To answer this, the ratios of enzyme
activities for glycolysis and citrate cycle
(GAPDH/IDH, Fig. 8, upper part), glycolysis and oxidative fatty acid metabolism
(GAPDH/HAD, fig. 8, center), and oxidative fatty acid metabolism and citrate
cycle (HAD/IDH, Fig. 8, lower part),
respectively, were plotted. Enzyme activities for O2-dependent pathways (HAD and
IDH) are directly correlated in all ten fibers
studied. Fibers with high activity in enzymes
of the citrate cycle show also high activity
for fatty acid metabolism and vice versa.
With regard to the ratios of GAPDH/IDH
and GAPDH/HAD, the data from the
individual fibers can be described by two
correlation lines of significantly different
steepness. The lower line represents all
fibers belonging to the previously defined
type IV. The upper line comprises the type
I fibers and fibers 5 and 6. The different
steepness of the two correlation lines suggests different metabolic pathways used by
the fibers. Type IV fibers represent a separate metabolic type, which depends mainly
on glycolytic anaerobic metabolism. They
are different from type I fibers and fibers
5 and 6. The latter share a metabolic concept with type I fibers; their oxidative
capacities, however, are distinctly higher
than those of type I fibers. Using the terminology of Peter et al. (1972) type I fibers
can be regarded as slow oxidative (SO),
type II and type III fibers as fast oxidative
glycolytic (FOG), and type IV fibers as fast
glycolytic (FG).
NOTE ADDED IN PROOF
The biochemical data reported here were
obtained from 10 fibers, which lie on the
surface of the closer muscle and therefore
can be easily identified. Only 3 fibers of
type I, 5 fibers of type IV and 1 each of
types II and III have been analyzed.
Although the data agree with the typing
based on electrophysiological and histochemical criteria, the variability of enzyme
activities (which can be taken as differences
in enzyme concentrations) found within the
groups of type I and type IV fibers, suggests the existence of a wide spectrum of
metabolic properties among the muscle
fibers. This makes the muscle a very heterogenous tissue, in which each fiber might
represent a typical, separate metabolic
entity.
ACKNOWLEDGMENTS
Some of the experiments were performed at the Zoological Station at Naples.
We thank the director and Dr. A. de Santis
for hospitality and support. We have also
to thank Dr. T. J. Wiens for permission to
refer to unpublished results and M. A. Cahill for correcting our English. The technical assistance of Birgitt Rapp, Gaby Westhoff and Dietrich Ruhrmann is gratefully
acknowledged. The work has been supported by the Deutsche Forschungsgemeinschaft (SFB 156).
MUSCLE FIBER TYPES IN CRABS
1077
evidence for enzyme differences in crustacean
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REFERENCES