THE ANATOMICAL RECORD 228:23-30 (1990)

Composition of Myofiber Types in Limb Muscles of the House Shrew (Suncus murinus):

Lack of Type I Myofibers ATSUSHI SUZUKI

Laboratory of Animal Morphology, Faculty of Agriculture, Tohoku University, Sendai 981, Japan

ABSTRACT Postural muscles have many type I myofibers, which reacted strongly for acid-stable myosin ATPase and were unreactive for alkali-stable my- osin ATPase (Ariano et al., J. Histochem. Cytochem., 2151-55, 1973; Armstrong et al., Am. J. Anat., 163.87-98, 1982; Smith et al., J. Neurophysiol., 40.503-513, 1977). House shrews (Suncus murinus) keep abducting their limbs in locomotion and hardly lift their trunk off the ground. The limb muscles of Suncus were ex- amined by histochemical methods to determine whether the locomotory and pos- tural behavior is related to the proportion of type I myofibers. The observation of whole cross sections from the triceps surae, flexor digitorum superficialis, quad- riceps femoris, and caudally situated muscles in the thigh showed that all myofi- bers of these muscles were unreactive for acid-stable myosin ATPase and strongly reactive for alkali-stable myosin ATPase: Those were classified a s type I1 myofi- bers. Type I1 myofibers showed a weak (type IIB), moderate (type IIAB), or strong (type IIA) reaction for NADH tetrazolium reductase. Part of type IIA myofibers reacted weakly to moderately for menadione-linked glycerol-3-phosphate dehydro- genase (m-GPD), which predominated in the soleus muscle. Type IIAB, type IIB, and the remainder of type IIA myofibers reacted strongly for m-GPD. The limb muscles contained subtypes of type I1 myofibers but no type I myofibers. In Suncus murinus, type I myofibers specialized for a postural maintenance may not be re- quired because all myofibers function exclusively for propulsion.

Skeletal muscles of vertebrates have two or more myofiber types that differ in histochemical properties. On the basis of myosin ATPase and dehydrogenase re- activity, mammalian myofibers are classified mainly into type I (Brooke and Kaiser, 1970) or slow-twitch/ oxidative (SO) myofibers (Peter et al., 1972), type IIA or fast-twitch/oxidative/glycolytic (FOG) myofibers, and type IIB or fast-twitch/glycolytic (FG) myofibers. Tonic myofibers of chickens (Shafiq et al., 1971; Ovalle, 1978), amphibians (Smith and Ovalle, 19731, and rep- tiles (Gleeson et al., 1980) are distinguishable from twitch myofibers by differences in myosin adenosine triphosphatase (ATPase) reactivity.

In mammals, postural or antigravity muscles have more type I (SO) myofibers, whereas propulsive or lo- comotory muscles have many type I1 (FOG and FG) myofibers (Ariano et al., 1973; Armstrong et al., 1982; Smith et al., 1977; Walmsley et al., 1978; Burke, 1981). The biceps femoris and semimembranosus muscles of European hares, cottontail rabbits (Schnurr and Tho- mas, 1984), tree shrews, and lesser bushbabies (Sickles and Pinkstaff, 1981), which move quickly, have large percentages of type I1 (FOG and FG) myofibers, whereas those of slow loris as seen in slow-climbing quadrupeds have equal percentages of type I and type I1 myofibers. Chameleons move slowly and spend long periods of time standing motionless on their perches. The iliofibularis of chameleons has more tonic myofi-

0 1990 WILEY-LISS, INC

bers than those of lizards that can run a t high speeds (Abu-Ghalyun et al., 1988). Such a variation in the composition of myofiber types in analogous muscles is considered to reflect locomotory patterns in the ani- mals.

Suncus murinus, which are terrestrial and live among ground litter, continually abduct their limbs both during walking and at rest (Kondo, 1985). It al- ways touches part of its trunk to something, and hardly lifts its trunk off the ground against gravity. In the laboratory, the animal was not observed to assume a sitting posture or to hold its trunk elevated in an oblique or vertical position as is seen in the mouse. Therefore, limb muscles of S. murinus with their sprawling posture were presumed to have fewer type I myofibers involved in postural activity than those of animals holding their trunk elevated off the ground against gravity. The present study was undertaken to determine whether the proportion of type I myofibers in limb muscles of S. murinus is associated with its locomotory and postural behavior.

MATERIALS AND METHODS Four male and one female adult house shrews (S .

murinus) were used in this study. The animals have

Received May 22, 1989; accepted December 19, 1989

24 A. SUZUKI

been bred to be used as laboratory animals in Japan. The males weighed 41.5-59.5 gm and the female 21.5 gm. M. quadriceps femoris and m. triceps surae, including m. flexor digitorum superficialis (m. planta- ris), were removed immediately after the animals were anesthetized with ether and killed by cervical disloca- tion. M. biceps femoris, m. semitendinosus, m. semi- membranosus, m. adductor magnus, and m. gracilis were removed together because the individual muscles were too small to remove separately. In addition, m. triceps brachii was removed from two animals. For comparison, m. triceps surae with m. flexor digitorum superficialis were removed from two adult male mice (ICR strain) weighing 32.0-33.4 gm.

The muscle samples, frozen in a mixture of dry ice and acetone, were cut serially on a cryostat. Sections were obtained from the region where the whole cross section of each muscle was observed in the muscle group removed. Unfixed cross sections (10 pm) were incubated for demonstration of myosin ATPase reac- tion after preincubation at pH 4.3, 4.4, 10.4, 10.5, and 10.6 (Brooke and Kaiser, 1970; Suzuki and Cassens, 1980a). Other sections were stained with NADH tetrazolium reductase (NADH-TR) and menadione- linked glycerol-3-phosphate dehydrogenase (m-GPD) (Lojda et al., 1979). Sections from Suncus and the mice were always incubated together in the identical staining jar. Myofibers were classified into types I, IIA-S, IIA-W, IIAB, and IIB myofibers by differences in reactivity for myosin ATPase after acid and alkaline preincubation and activity for NADH-TR and m-GPD.

The photomicrographs were taken of cross sections stained with NADH-TR to determine the percentage of myofiber types. The middle to deep portions of the medial and lateral head of gastrocnemius and the middle portion of the soleus, flexor digitorum superfi- cialis, vastus intermedius, and caudally situated muscles were photographed. The superficial and deep portions were photographed in the rectus femoris, vastus lateralis, and vastus medialis muscles. A total of 450-638 myofibers per muscle section were classi- fied and examined; diameters of 50 myofibers per myofiber type in the shortest dimension were mea- sured with a n ocular micrometer. The means of diameter measurements and percentages between myofiber types were compared by Student's t test. Differences between means were considered signifi- cant at P < 0.05.

RESULTS Reactivity of Myosin ATPase After Acid and

Alkaline Preincubation In Suncus, all myofibers of the muscles examined

were unreactive for myosin ATPase after preincuba- tion at pH 4.3, very weakly reactive at pH 4.4, and strongly reactive at pH 10.4 and 10.5, thus correspond- ing to the type I1 myofibers of Brooke and Kaiser (1970) (Figs. la,b, 2a,b; Table 1). Numerous myofibers of the soleus and a few myofibers of the other muscles reacted strongly for myosin ATPase after preincubation at pH 10.6, whereas the remaining myofibers were unreac- tive or weakly reactive. The type I myofibers of Brooke and Kaiser (19701, which reacted strongly for acid- stable myosin ATPase and were unreactive for alkali- stable myosin ATPase, were not found in any muscles examined.

In mice, the soleus and gastrocnemius muscles had both type I and type I1 myofibers. Type I myofibers reacted strongly for myosin ATPase after acid preincu- bation and were unreactive for myosin ATPase after alkaline preincubation (Fig. 3a,b).

Histochemical Activity of NADH-TR and rn-GPD and Myofiber Typing

In Suncus, type I1 myofibers reacted weakly to strongly for NADH-TR (Figs. lc, 2c). The myofibers with strong NADH-TR activity were classified as type IIA, the myofibers with moderate NADH-TR activity as type IIAB (Burke, 19811, and the myofibers with weak NADH-TR activity as type IIB (Fig. 2c). Part of the type IIA myofibers reacted strongly for myosin AT- Pase after preincubation at pH 10.6 (Table 1). Type IIA myofibers reacted weakly to strongly for m-GPD (Figs. Id, 2d). Type IIA myofibers were subdivided into two types: type IIA-W, with a weak to moderate activity for m-GPD, and type IIA-S, with a strong activity for m- GPD (Fig. 2c,d). Types IIAB and IIB myofibers reacted strongly for m-GPD (Fig. 2d).

In mice, type I myofibers reacted strongly for NADH- TR and weakly to moderately for m-GPD (Fig. 3c,d). Type I1 myofibers were classified into types IIA, IIAB, and IIB myofibers by differences in activity for NADH- TR (Table 1). Type IIAB and IIB myofibers reacted strongly for m-GPD. Many type IIA myofibers reacted strongly for m-GPD, and the remainder reacted weakly to moderately for m-GPD. Type IIA myofibers were subdivided into types IIA-S and IIA-W myofibers in the mice.

TABLE 1. Histochemical enzyme activities of msofiber types in limb muscles of Suncus rnurinus and mouse

Myosin ATPase Myofiber type 4.3' 4.4 10.4 10.5 10.6 N A D H - T R m-GPD IIA-W - + + + + + + + + + - t o + + + + + + + t o + + + + + t o + + IIA-S - + + + + + + + + + - t o + + + + + + + t o + + + + + + + t o + + + + IIAB - + + + + + + + + + - t o + + + + + + + IIB - + + + + + + + + + - to + + + + + + 13 + + + + + + + + - - __ + + + + t o + + 'pH of preincubation. '-, Unreactive; + , weak; + + , moderate; + + + to + + + + , strong. 3Type I myofibers are found in muscle of mouse but not in muscle of Suncus. Types IIA-W, IIA-S, IIAB, and IIB myofibers of Suncus and mouse are similar in reactivity.

2

MYOFIBER TYPES OF SUNCUS MUSCLES 25

Composition of Myofiber Types In Suncus, all the muscles had types IIA, IIAB, and

IIB myofibers, although there is great variability in fiber type percentages (Table 2). The soleus muscle had numerous type IIA myofibers, predominantly type IIA- W. Other muscles had small percentages of type IIA-W myofibers. The superficial portions in the rectus femo- ris, vastus lateralis, and vastus medialis had larger percentages of type IIB myofibers than of other myofi- ber types (P < 0.051, whereas the deep portions had larger percentages of type IIA-S myofibers or showed a tendency to contain more type IIA-S myofibers. A sim- ilar tendency of distribution of type IIB myofibers was observed in the gastrocnemius, semitendinosus, semi- membranosus, and triceps brachii muscle. The adduc- tor magnus muscles tended to have more type IIA my- ofibers in the craniomedial portion than in the caudolateral portion. Types IIA, IIAB, and IIB myofi- bers were distributed uniformly in the flexor digitorum superficialis, biceps femoris, vastus intermedius, and gracilis muscles.

The semimembranous and gracilis muscles had larger percentages of type IIA-S myofibers than of other myofiber types (P < 0.05). The vastus interme- dius muscle tended to have more type IIA-S myofibers than other myofiber types. The biceps femoris and semitendinosus muscles had smaller percentages of the type IIAB myofibers than of types IIA-S and IIB myo- fibers (P < 0.05). Differences in percentages of type IIAB myofibers appeared smaller than those of type IIA or type IIB myofibers among the other muscles or their subdivisions.

In mice, the soleus muscle had equal percentages of types I and I1 myofibers, consisting of types IIA-W and

IIA-S myofibers, but no types IIAB and IIB myofibers were present. The gastrocnemius muscle contained all myofiber types.

Diameters of Myofiber Types In Suncus, types IIAB and IIB myofibers of a female

were smaller in diameter than those of males, whereas diameters of type IIA myofibers in the female were similar to those of the males. The body weight of the female was the lowest among the animals used. Sex difference in body weight has been shown to exist in Suncus (Kondo, 1985). The differences in diameter be- tween the male and female may be associated with sex differences in body weight. The mean values of diam- eters in myofiber types were determined from male an- imals (Table 3).

Type IIB myofibers were larger in diameter than type IIA-S myofibers in the gastrocnemius caput me- diale, biceps femoris, semitendinosus, semimembrano- sus, and adductor magnus muscles (P < 0.05) but did not differ from type IIAB myofibers in diameter. No differences in diameter among four myofiber types were found within other muscles. In all muscles, the average diameter of type IIA-W myofibers (36.8 * 3.6 pm; mean * S.D.) was similar to that of type IIA-S myofibers (37.9 * 2.9 pm). The average diameter of type IIAB myofibers (43.5 2 2.3 pm) did not differ from that of type IIB myofibers (47.0 5.1 pm). Type IIA (IIA-W and IIA-S) myofibers were smaller in average diameter than types IIAB and IIB myofibers (P < 0.05).

In mice, types IIA-W and IIA-S myofibers appeared smaller in diameter than types IIAB and IIB myofi- bers. The type IIA-W myofibers of Suncus were similar

TABLE 2. Percentages of myofiber types in limb muscles of Suncus murinus and mouse

Myofiber types (%)

Muscle N1 Portion I IIA-W IIA-S IIAB IIB Suncus

Soleus 5 Mid2 0 63.6 t 3.53 29.4 t 7.0 6.0 t 3.8 1.0 t 1.4 Gastrocnemius

23.5 t 5.2 Caput laterale 5 Mid-D 0 0.1 t 0.2 52.7 t 19.7 23.7 t 8.5 Caput mediale 5 Mid-D 0 0.2 t 0.4 41.4 t 11.0 17.3 t 1.8 41.1 t 9.4

Flexor digitorum 5 Mid 0 2.7 2 4.4 51.1 t 23.9 27.2 t 12.9 19.0 ? 21.7 superficialis

Rectus femoris 5 s 0 0 16.3 t 5.6 11.3 t 2.4 72.4 t 7.7 D 0 9.8 t 7.7 67.0 t 10.2 14.9 t 4.5 8.3 2 8.8

Vastus lateralis 5 L ( S ) 0 0 7.5 + 3.8 12.6 t 3.0 79.9 t 5.3 M (D) 0 6.7 t 7.6 45.0 t 7.1 19.7 t 6.2 28.6 2 15.4

Vastus medialis 5 M (S) 0 0 18.1 t 8.9 16.7 2 10.1 65.2 t 16.1 L (D) 0 11.3 ? 9.8 44.9 t 7.5 24.4 * 11.7 19.4 + 17.9

Vastus intermedius 5 Mid 0 16.5 t 7.8 49.5 + 8.4 22.3 t 12.5 11.7 2 11.4 Biceps femoris 4 Mid 0 0.5 t 0.6 39.7 t 5.2 10.9 2 3.3 48.9 + 5.2 Semitendinosus 4 Mid 0 0.3 + 0.5 38.6 t 5.9 16.5 t 8.3 44.6 t 11.2 Semimembranosus 4 Mid 0 6.7 2 4.4 57.9 2 10.3 20.6 2 10.8 14.8 2 11.8

45.1 t 8.6 21.9 2 9.4 27.3 t 13.5 Adductor magnus 4 Mid 0 5.7 t 4.1 Gracilis 4 Mid 0 1.0 t 1.9 78.6 t 9.2 15.3 2 3.9 5.1 2 6.2

Soleus 2 Mid 51.9 t 12.2 47.1 t 12.0 1.0 t 0.6 0 0 Gastrocnemius 2 M ( S ) 0 0 8.3 t 7.1 11.1 + 4.0 80.6 2 11.0

Mouse

caput mediale L (D) 6.8 t 5.1 5.8 2 2.0 46.2 t 5.7 20.3 t 12.1 20.9 2 0.7

'Number of animals "D, deep; L, lateral; M, medial; Mid, middle; Mid-D, middle to deep; S, superficial. "Mean ? S.D.

26 A. SUZUKI

Fig. 1

MYOFIBER TYPES OF SUNCUS MUSCLES

TABLE 3. Diameters of myofiber types in limb muscles of Suncus rnurinus and mouse

27

Muscle N1 Myofiber type diameters (pm)

I IIA-W IIA-S IIAB IIB Suncus

Soleus Gastrocnemius

Caput laterale Caput mediale

Flexor digitorum superficialis

Rectus femoris Vastus lateralis Vastus rnedialis Vastus intermedius Biceps femoris Semitendinosus Semimembranosus Adductor magnus

Soleus Gastrocnemius

Mouse

4

4 4 4

4 4 3 3 4 4 4 4

2 2

36.6 t 5.@

-

- 29.6 t 1.0

36.3 t 4.6 36.1 2 3.8 35.4 t 4.6 40.0 ? 5.0 - -

40.2 2 1.6 40.4 t 3.7

46.5 2 2.1 43.7 t 3.0

caput mediale 'Number of animals. 2Mean 2 S.D.

to those of mice in average diameter. Types IIA-S, IIAB, and IIB myofibers of Suncus were smaller in av- erage diameter than those of mice (P < 0.05).

DISCUSSION The muscles of S. murinus were composed exclu-

sively of type I1 myofibers similar to type I1 myofibers of the mouse in histochemical properties. The histo- chemical characteristics of Suncus myofibers show that type IIA-S myofibers correspond to FOG myofibers and type IIB myofibers to FG myofibers. Type IIAB myofi- bers may belong in the FOG category. On the other hand, type IIA-W myofibers correspond to FO myofi- bers, as demonstrated in the bat (Armstrong et al., 1977), rat (Nemeth et al., 19791, and chicken (Suzuki et al., 1982). Murine muscle contained type I myofibers corresponding to SO myofibers in addition to these four types. Oxidative (type IIA-W) and oxidative/glycolytic (type IIA-S) myofibers were smaller in diameter than glycolytic myofibers (type IIB) in Suncus.

The soleus muscle of mice had about 50% type I my- ofibers. This percentage agrees with the results of Wirtz et al. (1983). Type I myofibers predominate in the soleus muscle of rats (Ariano et al., 1973; Pullen, 1977; Armstrong and Phelps, 1984), kangaroo rats (Williamson and Frederick, 19771, guinea pigs (Ariano et al., 19731, cats (Ariano et al., 1973; Burke et al., 1974), rabbits (Lobley et al., 1977), striped skunks

Fig. 1. Histochemical profiles of limb muscles in Suncus rnurinus. The lateral (GL) and medial (GM) head of gastrocnemius, soleus (S), and flexor digitorum superficialis (F) muscles have only myofibers unreactive for myosin ATPase after preincubaton a t pH 4.3 (a) and strongly reactive for myosin ATPase after preincubation at pH 10.4 (b). These myofibers react weakly to strongly for NADH tetrazolium reductase ( c ) and for menadione-linked glycerol-3-phosphate dehydro- genase (d). Arrows indicate connective tissue septa and tendons sep- arating the four muscles. Asterisk indicates a nerve fascicle. a 4 x 44.

36.2 t 7.7

34.5 t 5.7 35.0 2 2.6 35.0 t 3.8

41.0 t 5.7 41.1 2 4.8 38.5 ? 2.7 39.8 t 3.8 33.9 t 1.9 37.6 2 2.7 40.8 i 1.0 41.4 i 2.5

45.5 t 1.0 38.1 i 1.1

41.9 2 2.7

41.4 t 6.2 40.5 i 2.6 40.3 t 5.0

46.1 t 8.2 44.1 i 6.2 44.1 2 3.5 42.5 i 6.1 43.3 t 2.0 44.9 ? 2.8 47.9 t 3.6 44.4 t 3.5

-

50.4 t 0.2

36.4 t 8.7

45.5 ? 6.2 47.9 i 1.7 37.6 2 7.2

53.1 2 7.5 49.1 i 7.6 47.2 2 4.5 46.2 t 6.3 51.1 t 3.3 50.0 2 2.7 50.5 t 2.8 49.3 i 3.0

- 63.2 t 1.2

(Frederick and Goslow, 19761, lesser bushbabies, slow loris (Ariano et al., 1973; Sickles and Pinkstaff, 1981), and tree shrews (Sickles and Pinkstaff, 1981). The vas- tus intermedius muscle has numerous type I myofibers in guinea pigs, cats (Ariano et al., 1973), slow loris (Ariano et al., 1973; Sickles and Pinkstaff, 1981), tree shrews (Sickles and Pinkstaff, 19811, dogs (Armstrong et al., 1982), pigs (Suzuki and Cassens, 1980b), and sheep (Suzuki and Tamate, 1988). The vastus interme- dius and soleus muscles extend the knee joint and the tarsal joint, respectively, during standing. The muscles used for postural support have been shown to have many type I myofibers (Smith et al., 1977; Walmsley et al., 1978; Dum and Kennedy, 1980).

The muscles of aquatic animals a s well a s terrestrial animals contain two and more myofiber types distin- guishable by myosin ATPase reactivity. The iliofibu- laris muscles of lizards that move vigorously and fast (Gleeson et al., 1980; Gleeson and Harrison, 1986; Put- nam et al., 1980; Abu-Ghalyun et al., 1988) and of cha- meleons, which move slowly (Abu-Ghalyun et al., 1988), contain tonic myofibers that stained lightly with alkali-stable myosin ATPase. The tonic myofibers function to hold the body elevated off the ground or in postural support on arboreal perches. Rattlesnakes contain five myofiber types distinguishable with the acid-stable and alkali-stable myosin ATPase reaction (Schultz et al., 1980). Amphibian muscles have two my- ofiber types different in myosin ATPase reactivity (Smith and Ovalle, 1973). In chickens, the type I cate- gory predominates in the m. latissimus dorsi pars cra- nialis, which holds the wing in its normal position (Ovalle, 1978), and in the puboichiofemoralis pars me- dialis, a postural muscle (Suzuki et al., 1985). The long- issimus muscles of dolphins (Suzuki et al., 1983) and the lateral musculature of fishes (Mosse and Hudson, 1977; Carpene et al., 1982) have type I myofibers and myofibers resembling type I myofibers, respectively, suitable for prolonged and slow swimming.

28 A. SUZUKI

Figs. 2 and 3

MYOFIBER TYPES OF SUNCUS MUSCLES 29

Armstrong et al. (1977) have reported that the acces- sory flight muscles of little brown bats have three types of FO myofibers: FOh myofibers stained more darkly with NADH-TR and myosin ATPase, F01 myo- fibers stained more lightly with the two enzymes, and FOm myofibers with staining intensity intermediate between FOh and F01 myofibers, but there were no typical SO myofibers. On the other hand, Foehring and Hermanson (1984) have shown that the accessory flight muscles contained SO and FO myofibers in free-tailed bats and hypothesized that the SO myofibers subserve a tonic force for wing stabilization. The FO myofibers have been assumed to be adapted for rapid and power- ful movement, because the primary downstroke muscle (pectoralis) has FO myofibers nearly exclusively (Arm- strong et al., 1977; Foehring and Hermanson, 1984). Suncus type IIA-W myofibers, which have histochem- ical properties similar to those of bat FO myofibers, may be involved in rapid and powerful locomotion. However, large percentages of type IIA-W myofibers in the soleus muscle imply that the soleus muscle differs from the other muscles in function.

In the tree shrew, an arboreal-terrestrial quadruped, locomotory muscles have large percentages of type I1 myofibers, and the deep postural muscles have many type I myofibers (Sickles and Pinkstaff, 1981). When the tree shrew stays on a tree, type I myofibers may function in keeping its balance to prevent it from fall- ing. S. murinus weighs less than tree shrews. Animals with very low body weights may not require sustained activity for supporting a standing position. Neverthe- less, the mice with lighter body weights than Suncus had type I myofibers in the triceps surae muscle. Thus the lower body weights do not seem to be associated with lack of type I myofibers.

S. murinus had no type I myofibers in its limb mus- cles. In the cat, motor units composed of type I myofi- bers have been shown to be recruited for maintaining a standing position (Smith et al., 1977; Walmsley et al., 1978; Dum and Kennedy, 1980; Burke, 1981). Mice can assume a sitting posture and hold the cranial portion of their trunk elevated off the ground. In mice, the motor units of type I myofibers seem to be recruited for the maintenance of such an antigravity posture. Inasmuch as Suncus moves with its fore- and hindlimbs abduct- ing and its trunk close to the ground and does not as- sume a sitting and erect posture, it may not need to

Figs. 2, 3. Myofiber types of the triceps surae (Fig. 2) in Suncus murinus and the soleus (Fig. 3) of the mouse. Type I1 (11) myofibers are unreactive for myosin ATPase after preincubation a t pH 4.3 (2a, 3a) and are strongly reactive for myosin ATPase after preincubation a t pH 10.4 (2b, 3b). Type I (I) myofibers react strongly for acid-stable myosin ATPase (3a) and are unreactive for alkali-stable myosin AT- Pase (3b). Types IIA-S (AS) and IIA-W (AW) myofibers are strong in NADH tetrazolium reductase activity (2c, 3121, type IIAB (AB) myo- fibers are moderate, and type IIB (B) myofibers are weak (2c). Suncus type IIA-S, type IIAB, and type IIB myofibers are strong and type IIA-W myofibers weak in menadione-linked glycerol-3-phosphate de- hydrogenase (m-GPD) activity (2d). Murine type HA-W myofibers show a moderate activity for m-GPD (3d). The soleus muscle of the mouse has no type IIAB or type IIB myofibers (3c, 3d). In Figure 2, arrows indicate a connective tissue septum separating the lateral head of gastrocnemius (left portion) from the soleus muscle (right portion). In Figure 3, V indicates blood vessels. 2 a 4 and 3 a 4 x 264.

support its body against gravity for a long period of time in its mode of life. Suncus appears not to require motor units composed of type I myofibers specialized for postural maintenance.

S. murinus moves vigorously, scrambles, and jumps (Kondo, 1985). These movements can be performed with type I1 myofibers alone. Motor units composed of types IIA (IIA-S plus IIA-W) and IIAB myofibers ap- peared to be recruited for walking and running, whereas motor units of type IIB myofibers appear to be recruited for running and jumping. In addition, types IIA and IIB myofibers are also recruited for these ac- tivities, as shown in cats (Smith et al., 1977; Walmsley et al., 1978; Dum and Kennedy, 1980; Burke, 1981). The relationship of myofiber type composition to loco- motory behavior in Suncus is consistent with the con- cept that the variation in composition of myofiber types in analogous muscles in various animals is related to their locomotory patterns (Sickles and Pinkstaff, 1981; Schnurr and Thomas, 1984; Abu-Ghalyun et al., 1988).

ACKNOWLEDGMENT I thank Dr. Tetsuya Matsuzaki (Central Institute for

Experimental Animals, Kawasaki City) for providing the animals for this study.

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