Muscle and Muscle Fiber Type Transformation In Clawed Crustaceans

SYNOPSIS. The first pair of thoracic limbs in many crustaceansis elaborated into claws in which the principal muscle is thecloser. Changes in the fiber composition of the closer muscleduring claw development, regeneration and reversal are reviewedhere and the hypothesis is advanced that such changes are nerve-dependent.In adult lobsters, Homarus amencanus, the paired claws and closermuscles are bilaterally asymmetric, consisting of a minor orcutter claw with predominantly fast fibers and a small ventralband of slow and a major or crusher claw with 100% slow fibers.Yet in the larval and early juvenile stages the paired clawsand closer muscles are symmetric consisting of a central bandof fast fibers sandwiched by slow. Differentiation into a cutteror crusher muscle during subsequent juvenile development isby appropriate fiber type transformation. Experimental manipulationof the claws or the environment in early juvenile stages whenthe claws are equipotent revealed that the determination ofclaw and closer muscle asymmetry is dependent on the convergenceof neural input from the paired claws: the point of convergencemost likely being the CNS. Bilaterally symmetrical input resultsin the development of paired cutter claws while bilaterallyasymmetric input gives rise to dimorphic, cutter and crusherclaws. In the northern crayfish, Orconectes rusticus, wherethe paired claws are bilaterally similar, the closer muscletransforms its central band of fast fibers to slow, both duringprimary development and regeneration. Whether these fiber typetransformations are nerve-dependent is unknown. In adult snappingshrimps, Alpheus sp., the paired claws and closer muscles areasymmetric: the minor or pincer claw has a central band of fastfibers flanked by slow while the major or snapper claw has 100%slow fibers. Claw reversal occurs with removal of the snapperresulting in the transformation of the existing pincer to asnapper and the regeneration of a new pincer at the old snappersite. Transformation of the closer muscle from pincer to snappertype is by degeneration of the fast fiber band and hypertrophyof the slow fibers. Claw transformation can be either preventedif the pincer nerve is sectioned at the time of snapper removalor promoted if the snapper nerve is sectioned: both resultsimplicating a neural basis for muscle transformation.     

Electrical properties and transmitter output associated with growth and transformation in shrimp muscle fibers

Summary Comparisons were made of the passive electrical properties of closer muscle fibers in the dimorphic claws of snapping shrimp,Alpheus armillatus. During claw transformation the small fibers of pincer claws grow to become much larger snapper claw fibers. As muscle fibers grow, the relationship of fiber input resistance (R ₀) to fiber diameter (d) is predicted by the proportionality,R ₀d ^–3/2. Muscle fiber membrane resistance,R _m, is independent of fiber diameter, but membrane capacitance,C _m, grows with diameter. This results in a 40 to 50 fold reduction in fiber input impedance as fiber diameter enlarges during transformation. Reductions of muscle fiber impedance are partially compensated by 2–5 fold increases in quantal content at excitatory synapses on snapper muscle fibers. However, changes in quantal content during transformation apparently are independent of fiber diameter per se. Excitatory junction potentials in both pincer and snapper muscle fibers have equal amplitude. Because fiber input impedance decreases precipitously during transformation, and in view of the relatively small compensatory changes in quantal content at excitatory synapses, additional pre- or post-synaptic modifications must supplement increased quantal content to maintain synaptic efficacy in transformed muscle fibers.Abbreviations ejp excitatory junctional potential - epp endplate potential - mepp miniature endplate potential     

Polarized distribution of γ interferon-stimulated MHC antigens and transferrin receptors in a clonal cell line isolated from Fisher rat thyroid (FRT cells)

Summary The fine structure of single identified muscle fibers and their nerve terminals in the limb closer muscle of the shore crab Eriphia spinifrons was examined, using a previous classification based on histochemical evidence which recognizes a slow (Type-I) fiber and three fast (Type-II, Type-III, Type-IV) fibers. All four fiber types have a fine structure characteristic of crustacean slow muscle, with 10–12 thin filaments surrounding each thick filament and sarcomere lengths of 6–13 m. Type-IV fibers have sarcomere lengths of 6 m while the other three types have substantially longer sarcomeres (10–13 m). Structural features of nerve terminals revealed excitatory innervation in all four fiber types but inhibitory innervation in Type-I, Type-II, and Type-III fibers only. Thus fibers with longer sarcomeres receive the inhibitor axon but those with shorter sarcomeres do not. Amongst the former, synaptic contact from an inhibitory nerve terminal onto an excitatory one, denoting presynaptic inhibition, was seen in Type-I and Type-II fibers but not in Type-III and Type-IV fibers. Inhibitory innervation of the walking leg closer muscle is therefore highly differentiated: some fibers lack inhibitory nerve terminals, some possess postsynaptic inhibition, and some possess both postsynaptic and presynaptic inhibition.     

Snapping behaviour in intraspecific agonistic encounters in the snapping shrimp (Alpheus heterochaelis)

During intraspecific agonistic encounters in snapping shrimp (Alpheus heterochaelis) the behaviour of the snapper, emitting a fast water jet by very rapid closure of the large modified snapper claw, and the receiver was analysed by single frame video analysis before, during, and after the snap. During snapping the opponents usually face each other. Snapping is most frequently preceded by touch of frontal appendages. The snapping animal keeps its snapper claw slightly across the midline, shielding frontal body parts, and its tailfan bent downwards. The mean claw cocking duration (generating muscle tension) before snapping amounts to about 500 ms. In 58% of the snaps, the snapper claw pointed at the opponent, its claws, densely covered with sensory hairs, representing the main target of the water jet. The mean distance for these directed snaps was 0.9 cm, while undirected snaps were emitted from larger distances of on average 3.4 cm. The snapper usually withdraws immediately after snapping, the receiver approaches. Initial snaps are often answered by return snaps and both are emitted from smaller distances and hit more often than subsequent snaps.     

Muscle Restructuring in Crustaceans: Myofiber Death, Transfiguration and Rebirth

Research on the dimorphic claws of the snapping shrimp Alpheushas revealed moult-associated changes in structure and biochemicalcomposition—including atrophy and biochemical modification—ofclaw muscle fibers during morphological transformation of aclaw from a pincer to a snapper. Electrophysiology, SDS-PAGEgel electrophoresis, and immunocytochemistry were used to analyzechanges in claw closer muscle function and composition duringthe transformation process. Remodification of closer muscleduring claw transformation, involving the complete loss of acentral section of fast-contracting fibers and their replacementthrough enlargement of existing slowly-contracting segmentsof the muscle, may mimic similar muscle modifications duringinitial claw development. Exposure of intact animals to environmentalecdysteroid hormones accelerated the rate of these changes.These processes appear to be a product of a remarkable trophicplasticity of crustacean skeletal muscle first discovered bySkinner.     

Enhanced reappearance of fast fibers in regenerating crayfish claw closer muscles

In the pristine claws of adult crayfish the muscle fibers of the closer are all of slow type as judged by sarcomere lengths of greater than 6 micron, and a uniform degree of myofibrillar ATPase activity. In regenerating claws of mature and immature crayfish, the muscle has a central band of fast type fibers as characterized by shorter sarcomeres (less than 6 micron) and a higher degree of ATPase activity than the surrounding slow fibers. During primary development, the closer muscle has a fiber composition similar to that of the regenerating muscle except for a smaller proportion of fast fibers. Thus the reappearance of fast fibers during regeneration recapitulates ontogeny while their enhanced proportions may reflect epigenetic influences such as restriction of nerve-mediated muscle activity in the limb bud.     

Two fast-type fibers in claw closer and abdominal deep muscles of the Australian freshwater crustacean, Cherax destructor, differ in Ca2+ sensitivity and troponin-I isoforms

One type of fast fiber and two types of slow (slow-twitch, S1 and slow-tonic, S2) fibers are found in decapod crustacean skeletal muscles that differ in contractile properties and myofibrillar protein isoform compositions. In this study the structural characteristics, protein isoform compositions, and Ca2+-activation properties of fast fibers in the claw closer (F1) and abdominal deep flexor (F2) muscles of Cherax destructor were analyzed. For comparison, myofibrillar protein isoform compositions of slow (long-sarcomere) fibers from claw and abdomen were also determined; our results indicate that the slow fibers in the claw closer were the slow-twitch (S1) type and those in the abdominal superficial flexor were primarily slow-tonic (S2) type. F1 fibers had shorter resting sarcomere lengths (2.93 microm in unstretched fibers and 3.06 microm in stretched fibers) and smaller fiber diameter (256 microm) than F2 fibers (sarcomere lengths 3.48 microm in unstretched and 3.46 microm in stretched; 747 microm diameter). Moreover, F1 fibers showed a narrower range in sarcomere lengths than F2 fibers (2.81 to 3.28 microm vs. 2.47 to 4.05 micro m in unstretched fibers). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting showed that the fast fibers from claw and abdomen differed in troponin-I composition; F1 fibers expressed two isoforms of troponin-I (TnI1 and TnI2) in approximately equal amounts, whereas F2 fibers expressed primarily TnI3 and lower levels of TnI1. F1 fibers were more sensitive to Ca2+, as shown by higher pCa values at threshold activation (pCa(10)=6.50+/-0.07) and at 50% maximum force (pCa(50)=6.43+/-0.07) than F2 fibers (pCa(10)=6.12+/-0.04 and pCa(50)=5.88+/-0.03, respectively). F1 fibers also had a greater degree of co-operativity in Ca2+ activation, as shown by a higher maximum slope of the force-pCa curve (n(Ca)=12.98+/-2.27 vs. 4.34+/-0.64). These data indicate that there is a greater fast fiber-type diversity in crustacean muscles than was previously supposed. Moreover, the differences in activation properties suggest that the TnI isoform composition influences the Ca2+ sensitivity of the contractile mechanism.     

Contractile proteins of fast and slow fibers during differentiation of lobster claw muscle

Contractile protein populations were determined, using gel electrophoresis, during development of the claw closer muscles of the lobster Homarus americanus. In the adult the paired claw closer muscles are asymmetric, consisting of a crusher muscle with all slow fibers and a cutter muscle with a majority of fast and a few slow fibers. The electrophoretic banding pattern of these adult fast and slow fibers shows a similarity in the major proteins including myosin, actin, and tropomyosin which are common to both fiber types. Paramyosin is slightly heavier in fast fibers than in slow. However, fast fibers have three proteins and slow fibers have four proteins which are unique to themselves. Several of these unique proteins belong to the regulatory troponin complexes. In juvenile 4th stage lobster, where the paired closer muscles are undifferentiated, the banding pattern reveals the presence of proteins common to both fiber types including myosin, actin, and tropomysin but the conspicuous absence of all unique fast fiber proteins as well as one unique slow fiber protein. By the juvenile 10th stage most of these unique proteins are present except for one unique slow fiber protein. Thus lobster fast and slow fiber differentiation entails coordinate gene activation to add unique contractile proteins.     

Composition of external setae during regeneration and transformation of the bilaterally asymmetric claws of the snapping shrimp,Alpheus heterochelis

The paired thoracic chelipeds or claws of adult snapping shrimp, Alpheus heterochelis, are bilaterally asymmetric, consisting of an enlarged and elaborate, sound-producing major (snapper) claw and a much smaller minor (pincer) claw. These paired claws vary in the composition of their external sensilla. Both possess long serrulate and simple short setae but the snapper also have plumose setae and long serrulate setae on the plunger. The pincers differ in having short serrulate setae and, in males alone, a prominent fringe of plumoserrate setae. During regeneration of each claw type, these setal structures are gradually added over three molts to reach the pristine condition. The long serrulate and simple short setae appear first, being seen in intermolt limb buds and commonly in both claws. Setae exclusive to each claw, i.e., plumoserrate and short serrulate in the pincer and plumose and long serrulate on the plunger in the snapper, appear sparsely in either the regenerated 1st or 2nd postmolt claw, they proliferate in the subsequent 2nd or 3rd postmolt claw. Transformation of the pincer claw to the snapper type begins in the 1st postmolt stage with the loss of pincer setae and addition of snapper setae and is completed by the 3rd postmolt stage. Since changes in composition of the external sensilla are restricted to postmolt stages, the underlying hypodermis is presumably being remodeled during proecdysis.     

The claw closer muscle of Neohelice granulata (Grapsoidea,Varunidae): a morphological and histochemical study

Longo, M.V., Goldemberg, A.L. and Díaz, A.O. 2011. The claw closer muscle of Neohelice granulata (Grapsoidea, Varunidae): a morphological and histochemical study. —Acta Zoologica (Stockholm) 92 : 126–133. The claw closer muscle of Neohelice granulata was studied according to histological, histochemical, and morphometrical criteria. Adult male crabs in intermoult stage were collected from Mar Chiquita Lagoon (Buenos Aires, Argentina). Muscle fibers show evident striations and oval‐elongated nuclei with loose chromatin. The loose connective tissue among muscle fibers consists of cells and fibers embedded in an amorphous substance. Muscle histochemistry reveals two slow fiber types: ‘A’ and ‘B’. Prevailing A fibers are larger, and they usually show, with respect to B type, a weaker reaction to whole techniques. Fibers with short (SS), intermediate (IS), and long sarcomeres (LS) appear in the claw closer muscle, being the LS fibers predominant. Concluding, the histochemical and morphometrical characteristics of the claw closer muscle fibers of N. granulata are indicative of slow fibers. The slow A type (low resistant to fatigue) prevails.     

Development of high frequency multiple shoot formation in Persian clover (<Emphasis Type="Italic">Trifolium resupinatum</Emphasis> L.)

An efficient and reliable micropropagation system for Persian clover (Trifolium resupinatum L.) was developed using different explants and media. Node, hypocotyl and cotyledonary node explants were cultured on Murashige and Skoog (MS) medium supplemented with combinations of either 6-benzyladenine (BA) and indole-3-butyric acid (IBA) or BA, Kinetin (KIN) and IBA. Direct multiple shoots developed within 6weeks in all explants in most media tested. The best shoot multiplication capacity was obtained from cotyledonary node explants on MS medium containing 7.1M BA and 1M IBA or 14.1M BA and 1M IBA. Elongated shoots were rooted on either MS medium alone or combination with different concentrations of indole-3-butyric acid (IBA), indole-3-acetic acid (IAA) and -naphthaleneacetic acid (NAA). High rooting was achieved in half strength MS medium containing 8M IBA.     

Post-autotomy claw regrowth and functional recovery in the snapping shrimp Alpheus angulosus

The ability to regenerate lost tissues, organs or whole body parts is widespread across animal taxa; in some animals, regeneration includes transforming a remaining structure to replace the one that was lost. The transformation of one limb into another involves considerable plasticity in morphology, physiology and behavior, and snapping shrimp offer excellent opportunities for studying this process. We examined the changes required for the transformation of the small pincer to a mature snapping claw in Alpheus angulosus. First molt claws differ from mature claws in overall shape as well as in morphology related to snapping function; nonetheless, shrimp with first molt claws do produce snaps. While most shape variables of second molt claws do not differ significantly from mature claws, the plunger (structure required for snap production) does not reach mature size until the third molt for females, or later for males. Thus, the pincer claw can be transformed into a functional snapping claw in one molt, although both the underlying morphology and superficial shape are not fully regenerated at this stage. The rapid production of a functional snapping claw that we observe in this study suggests that this particular function is of significant importance to snapping shrimp behavior and survival.     

Tumor necrosis factor-α in macrophages of heart,liver, kidney,and in the pituitary gland

The relationship between myonuclear number, cellular size, succinate dehydrogenase activity, and myosin type was examined in single fiber segments (n=54; 9±3 mm long) mechanically dissected from soleus and plantaris muscles of adult rats. One end of each fiber segment was stained for DNA before quantitative photometric analysis of succinate dehydrogenase activity; the other end was double immunolabelled with fast and slow myosin heavy chain monoclonal antibodies. Mean±S.D. cytoplasmic volume/myonucleus ratio was higher in fast and slow plantaris fibers (112±69 vs. 34±21x10³ m³) than fast and slow soleus fibers (40±20 vs. 30±14x10³ m³), respectively. Slow fibers always had small volumes/myonucleus, regardless of fiber diameter, succinate dehydrogenase activity, or muscle of origin. In contrast, smaller diameter (<70 m) fast soleus and plantaris fibers with high succinate dehydrogenase activity appeared to have low volumes/myonucleus while larger diameter (>70 m) fast fibers with low succinate dehydrogenase activity always had large volume/myonucleus. Slow soleus fibers had significantly greater numbers of myonuclei/mm than did either fast soleus or fast plantaris fibers (116±51 vs. 55±22 and 44±23), respectively. These data suggest that the myonuclear domain is more limited in slow than fast fibers and in the fibers with a high, compared to a low, oxidative metabolic capability.     

Changes in myofibrillar gene expression during fiber-type transformation in the claw closer muscles of the snapping shrimp, Alpheus heterochelis

Isotopes of a number of crustacean myofibrillar proteins have been identified with sodium dodecyl sulfate-polyacrylamide electrophoresis, and their distribution in muscles of the snapping shrimp has been examined. Fast-slow differences in distribution have been observed for myosin light chains and tropomyosin. In contrast, three troponin T subunits have been resolved, each specific to one of the three muscles examined. This result suggests that expression of crustacean contractile proteins is not accomplished by a simple coexpression of a battery of slow or fast isotopes. In addition, the expression of these proteins was examined during the quasi-developmental fiber-type transition of the main claw closer muscle during the reversal of claw asymmetry in response to the loss of the large snapper appendage. The changes observed appear similar to the cross-innervation induced changes in gene expression of vertebrate muscle.     

Claw asymmetry in lobsters: case study in developmental neuroethology.

An enduring debate in the study of development is the relative contribution of genetic and epigenetic factors in the genesis of an organism, that is, the nature vs. nurture debate. The behavior of the paired claws in the lobster offers promising material for pursuing this debate because of the way they develop. The paired claws and their closer muscles are initially symmetrical; both are slender in appearance and have a mixture of fast and slow fibers in their closer muscles. During a critical period of development, they become determined into a major (crusher) and minor (cutter) claw and during subsequent development acquire their final form and behavior: The crusher becomes a stout, molar-toothed claw capable of closing only slowly because its closer muscle has 100% slow fibers while the cutter becomes a slender, incisor-toothed claw capable of closing rapidly because its closer muscle has 90% fast fibers. Our initial hypothesis was that the more active claw became the crusher and its less active counterpart the cutter. Presumably, nerve activity would influence muscle transformation, which in turn would influence the exoskeleton to which they attach and hence claw morphology. Curtailing nerve activity to the claw prevented crusher development, while reflex activation of a claw promoted its development; both results support the notion that nerve activity directly regulates claw form and function. This is not, however, the case, for when both claws were reflexly exercised neither formed a crusher, signifying rather that bilateral differences in predominantly mechanoreceptive input to the paired claws somehow lateralized the claw ganglion [central nervous system (CNS)] into a crusher and cutter side. The side experiencing the greater activity becomes the crusher side while the contralateral side becomes the cutter and is also inhibited from ever becoming a crusher. This initial lateralization in the CNS is expressed, via as yet unknown pathways, at the periphery in claw morphology, muscle composition, and behavior. The critical period defines a time when the CNS is susceptible to being lateralized into a crusher and cutter side. Such lateralization is dependent upon experience of the environment in the form of mechanoreceptive input. In the absence of such experience, the CNS is not lateralized and paired cutter claws develop. Thus, while the critical period for crusher determination is genetically determined the actual trigger is influenced by experience.     

Forced MyHCIIB expression following targeted genetic manipulation of conditionally immortalized muscle precursor cells

The ability to carry out gene targeting in somatic stem cells while maintaining their stem cell characteristics would have important implications for gene therapy and for the analysis of gene function. Using mouse myoblasts, we have explored this possibility by attempting to alter the promoter of a myosin heavy chain gene (MyHCIIB) characteristic of physiologically "fast" muscle so as to force its unscheduled expression in physiologically "slow" muscle fibers. Conditionally immortalized muscle precursor cells were transfected with a gene targeting construct designed to replace the MyHCIIB promoter with that for the carbonic anhydrase III gene (CAIII), which is highly expressed in slow muscle. A potentially targeted clone was isolated and differentiated in culture to form myotubes which expressed MyHCIIB. Cells from the same clone were injected into both slow and fast muscle of host mice, where they contributed to fiber formation. In slow muscle, the fibers derived from this clone did not express MyHCIIB; this may reflect an instability of the targeted MyHCIIB locus and/or a failure of the hybrid promoter to function in slow fibers in vivo. Nonetheless, we have demonstrated that a "promoter knock-in" gene targeting procedure can be used to generate unique MyHCIIB-expressing myotubes in culture and that conditionally immortalized myoblasts can be subjected to extensive passaging and genetic manipulation without losing their ability to form fibers in culture and in vivo.     

The membrane systems of the cardiac muscle cell of Euchaeta norvegica Boeck (Crustacea,Copepoda)

Summary The membrane systems of the cardiac muscle cell of the copepod Euchaeta norvegica Boeck are described. The heart wall, which is between 0.12 and 1.36 m thick, consists of an epicardium and a single layer of muscle cells. Invaginations of the sarcolemma forming transverse tubules have been found at all levels of the sarcomere with the exception of the H-band level. The longitudinal tubules of the same system are closely associated with the sarcoplasmic reticulum to form interior couplings at the A-I level of the sarcomere. Triadic couplings at the Z band level were not seen in E. norvegica, but peripheral couplings were demonstrated. Nexuses were found in the intercalated discs.     

The evolution of armament strength: evidence for a constraint on the biting performance of claws of durophagous decapods

Performance data for the claws of six sympatric species of Cancer crabs confirmed a puzzling pattern reported previously for two other decapod crustaceans (stone crabs, Menippe mercenaria, and lobsters, Homarus americanus): Although biting forces increased, maximum muscle stresses (force per unit area) declined with increasing claw size. The negative allometry of muscle stress and the stress at a given claw size were fairly consistent within and among Cancer species despite significant differences in adult body size and relative claw size, but were not consistent among decapod genera. Therefore, claw height can be used as a reliable predictor of maximum biting force for the genus Cancer, but must be used with caution as a predictor of maximum biting force in wider evolutionary and biogeographical comparisons of decapods. The decline in maximum muscle stress with increasing claw size in Cancer crabs contrasts with the pattern in several other claw traits. Significantly, three traits that affect maximal biting force increased intraspecifically with increasing claw size: relative claw size, mechanical advantage, and sarcomere length of the closer muscle. Closer apodeme area and angle of pinnation of the closer muscle fibers varied isometrically with claw size. The concordant behavior of these traits suggests selection for higher biting forces in larger crabs. The contrast between the size dependence of muscle stress (negative allometry) and the remaining claw traits (isometry or positive allometry) strongly suggests that an as yet unidentified constraint impairs muscle performance in larger claws. The negative allometry of muscle stress in two distantly related taxa (stone crabs and lobsters) further suggests this constraint may be widespread in decapod crustaceans. The implications of this performance constraint for the evolution of claw size and the "arms-race" between decapod predators and their hard-shelled prey is discussed.     

Up-regulation of the vitamin C transporter SVCT2 upon differentiation and depolarization of myotubes

In addition to its role as a strong antioxidant, vitamin C regulates the differentiation of several cell lineages. In vertebrate skeletal muscle, the vitamin C transporter SVCT2 is preferentially expressed in slow muscle fibers. To gain insights into the possible involvement of intracellular vitamin C on early myogenesis, we investigated the regulation of SVCT2 expression in cultures of chick fetal myoblasts. SVCT2 expression increases in cultures of both, slow and fast muscle-derived myoblasts, as they fuse to form mainly fast myotubes. Interestingly, we found that SVCT2 could be positively modulated by potassium-induced depolarization of myotubes. These findings suggest that SVCT2-mediated uptake of vitamin C could play diverse roles on skeletal muscle development and physiology.     

Tension relaxation after stretch in resting mammalian muscle fibers: stretch activation at physiological temperatures.

Tension responses to ramp stretches of 1-3% Lo (fiber length) in amplitude were examined in resting muscle fibers of the rat at temperatures ranging from 10 degrees C to 36 degrees C. Experiments were done using bundles of approximately 10 intact fibers isolated from the extensor digitorum longus (a fast muscle) and the soleus (a slow muscle). At low temperatures (below approximately 20 degrees C), the tension response consisted of an initial rise to a peak during the ramp followed by a complex tension decay to a plateau level; the tension decay occurred at approximately constant sarcomere length. The tension decay after a standard stretch at approximately 3-4.Lo/s contained a fast, an intermediate, and a (small amplitude) slow component, which at 10 degrees C (sarcomere length approximately 2.5 microns) were approximately 2000.s-1, approximately 150.s-1, and approximately 25.s-1 for fast fibers and approximately 2000.s-1, approximately 70.s-1 and approximately 8.s-1 for slow fibers, respectively. The fast component may represent the decay of interfilamentary viscous resistance, and the intermediate component may be due to viscoelasticity in the gap (titin, connectin) filament. The two- to threefold fast-slow muscle difference in the rate of passive tension relaxation (in the intermediate and the slow components) compares with previously reported differences in the speed of their active contractions; this suggests that "passive viscoelasticity" is appropriately matched to contraction speed in different muscle fiber types. At approximately 35 degrees C, the fast and intermediate components of tension relaxation were followed by a delayed tension rise at approximately 10.s-1 (fast fibers) and 2.5.s-1 (slow fibers); the delayed tension rise was accompanied by sarcomere shortening. BDM (5-10 mM) reduced the active twitch and tetanic tension responses and the delayed tension rise at 35 degrees C; the results indicate stretch sensitive activation in mammalian sarcomeres at physiological temperatures.