Laryngeal muscle fibre types.

The internal laryngeal muscles have evolved to subserve the highly specialized functions of airways protection, respiration, and phonation. Their contractile properties, histochemistry, biochemical properties, myosin heavy chain (MyHC) expression and their regulation by nerves and hormones are reviewed and compared with limb muscle fibres. Cricothyroid, the vocal cord tensor, is limb-like in MyHC composition and fibre type properties, while the vocal fold abductor and adductors are allotypically different, with capacity for expressing an isoform of MyHC that is kinetically faster than the fastest limb MyHC. In rats and rabbits the faster isoform is the extraocular (EO) MyHC, while in carnivores, it is the IIB MyHC. These adaptations enable the abductor and adductor muscles to remain always faster than the cricothyroid as the latter changes in speed during evolution to match changing metabolic and respiratory rates in relation to scaling with body mass. Such phylogenetic plasticity is vital to the airways protection and respiratory functions of these muscles. The posterior cricoarythenoid, the abductor muscle, is tonically driven during expiration, and consequently has a slower fibre type profile than the principal adductor, the thyroarythenoid. The human thyroarythenoid appears not to express EO or IIB MyHC significantly, but is unique in expressing the slow-tonic MyHC. The concepts of allotype and phylogenetic plasticity help to explain differences in fibre type between limb and laryngeal muscles and between homologous laryngeal muscles in different species. Laryngeal muscle fibres exhibit physiological plasticity as do limb muscles, being subject to neural and hormonal modulation.

Electrophoretic and immunochemical evidence showing that marsupial limb muscles express the same fast and slow myosin heavy chains as eutherians.

Limb muscles of eutherian (placental) mammals express a slow and three fast isoforms of myosin heavy chain (MyHC), but little is known about marsupial MyHCs. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of limb MyHCs from seven marsupial species, spanning two orders, revealed four components, each of which specifically cross-reacted in Western blots with a monoclonal antibody (mAb) against a corresponding eutherian MyHC. For all seven species, the relative mobility of the band identified by each mAb matched that in the rat, suggesting that the four are homologous to eutherian slow, 2B, 2X and 2A MyHCs, respectively, in the order of decreasing mobility. Immunohistochemical analysis of fast marsupial limb muscles identitied four different fiber populations whose relative fiber size spectra (IIA

Cross-bridge kinetics of rabbit single extraocular and limb muscle fibers.

PURPOSE: To gain insights into the functional significance of myosin heavy-chain (MyHC) heterogeneity by comparing the mechanical kinetic properties of single rabbit extraocular muscle (EOM) fibers with those of limb fibers. EOMs are known to contain developmental and EOM-specific MyHCs in addition to those present in limb muscles, and MyHCs profoundly influence muscle mechanics. METHODS: Isometric cross-bridge kinetics were analyzed in Ca(2+)-activated single glycerinated fibers from rabbit EOM and limb fast and slow muscles at 15 degrees C by means of mechanical perturbation analysis. The plots of stiffness and phase against frequency display a characteristic frequency, f(min), at which stiffness is minimum, and phase shift is zero. The value of f(min) is independent of Ca(2+) or force level but reflects the kinetics of cross-bridge cycling. RESULTS: Analysis of 121 limb fast fibers gave f(min) values ranging from 10 to 26 Hz. f(min) for the 10 slow soleus fibers was 0.5 Hz. Analysis of 170 EOM fibers gave f(min) values in the range for fast limb fibers, but in addition yielded f(min) values below (4-9 Hz) and above (27-33 Hz) this range. CONCLUSIONS: The wider range of mechanical kinetic characteristics in EOM fibers compared with limb fibers is likely due to the expression of developmental (low f(min)) and EOM-specific (high f(min)) MyHCs in addition to isoforms present in adult limb muscles. The considerable diversity of functional characteristics in EOM fibers is likely to be important for rotating the eyeball at various speeds during tracking and for executing saccades over a wide range of angles.

The distribution of myosin heavy chain isoforms among rat extraocular muscle fiber types.

PURPOSE: To determine the distribution of myosin heavy chain isoforms in each extraocular muscle (EOM) fiber type. METHODS: Serial sections of adult rat EOMs were stained with isoform-specific monoclonal antibodies against an array of myosin heavy chains. Immunofluorescent antibody staining of whole adult rat EOMs, examined by confocal microscopy, demonstrated the longitudinal variations of isoforms along individual fibers. RESULTS: Each global fiber type reacted predominantly with a single isoform-specific antibody and showed no longitudinal variation. Two major orbital fibers were defined, and both contained multiple myosin heavy chains. Both orbital singly and multiply innervated fibers stained proximal and distal to the neuromuscular junction with antibody to embryonic myosin heavy chain, but this isoform was sharply and completely excluded from the domain of the neuromuscular junction. Orbital singly innervated fibers also contained the EOM-specific isoform at the neuromuscular junction. Orbital multiply innervated fibers did not contain the EOM-specific isoform, but additionally contained a slow isoform along their entire length. CONCLUSIONS: Adult rat EOMs show unique fiber types with arrangements of myosin heavy chain isoforms not seen in other skeletal muscles. Moreover, unique cellular mechanisms must exist to target each isoform to its proper domain along individual orbital fibers.

Human extraocular muscles: unique pattern of myosin heavy chain expression during myotube formation.

PURPOSE: To study the myosin heavy chain composition of the human extraocular muscles (EOMs) during development. METHODS: EOMs from human fetuses of 8 to 22 weeks of gestation were studied with immunocytochemistry and gel electrophoresis. Antibodies specific against nine isoforms of myosin heavy chain (MyHC) were used in serial frozen sections. RESULTS: The developing EOMs had a delayed time course of myotube formation and a unique composition and distribution of MyHCs compared with human limb skeletal muscle. The primary myotubes coexpressed two developmental isoforms of MyHCI from the earliest stages. The third developmental MyHCI delineated the future orbital layer at 10 to 12 weeks of gestation. MyHC-slow tonic also appeared early, whereas MyHC alpha-cardiac and MyHC-extraocular, important components of adult EOM, were never detected at the gestational ages studied. CONCLUSIONS: The developmental features of the EOMs differed significantly from those reported for limb muscles of the corresponding ages. It is clear that the knowledge of limb muscle development does not fully apply to more specialized muscles, such as the eye muscles. The extreme complexity displayed by the EOMs probably reflects their distinct embryonic origin, innervation, and regulatory program of myogenesis.

Monospecific antibodies against the three mammalian fast limb myosin heavy chains.

Skeletal muscle fibres in mammalian limb muscles are of four types: slow, 2A, 2X, and 2B, each characterized by a distinct myosin heavy chain (MyHC) isoform. Existing monoclonal antibodies (mabs) against fast MyHCs lack fibre-type specificity across species and could not positively identify 2X fibres. In this work, mabs were raised against each of the fast MyHCs. These mabs were shown to be monospecific by Western blots and immunohistochemistry in the rat. The advantages of using these mabs for identifying the three fast fibre types and hybrid fibres expressing multiple isoforms were illustrated using rat tibialis anterior muscle. Immunohistochemical analyses confirmed the monospecificity of these mabs in the following additional species: mouse, guinea pig, rabbit, cat, and baboon. 2B fibres were absent in limb muscles of the cat and baboon. These mabs constitute a set of powerful tools for studying muscle fibre types and their transformations.

Jaw-closing muscles of kangaroos express alpha-cardiac myosin heavy chain.

The masseter muscle of eutherian grazing mammals typically express beta or slow myosin heavy chain (MyHC). Myosins in the masseter of 4 species of kangaroos and a slow limb muscle of one of them were compared with their cardiac myosin by pyrophosphate and sodium dodecyl sulphate (SDS) gel electrophoresis, immunoblotting and immunohistochemistry. It was found that ventricular muscle contains three isoforms homologous to V1 (alpha-MyHC homodimer), V2 (heterodimer) and V3 (beta-MyHC homodimer) of eutherian cardiac muscle, and that the masseter contained V1, with traces of V2 and V3, in great contrast to eutherian ruminants, which express only V3. A polyclonal antibody (anti-KJM) was raised in rabbits against red kangaroo masseter myosin. After cross-absorption against limb muscle myofibrils, anti-KJM specifically reacted in Westerns with MyHCs from masseter but not limb muscles, and immunohistochemically with masseter, but not limb muscle fibers. In pyrophosphate Western blots, anti-KJM reacted with V1 but not with V3. However, a monoclonal antibody specific for eutherian slow myosin stained all kangaroo slow muscle fibers but only weakly stained scattered fibers in the masseter. The SDS-PAGE revealed that light chain composition of masseter and ventricular myosins is identical, but isoforms of both light chains of kangaroo limb slow myosin were observed. These results confirm that kangaroo jaw muscle express alpha-MyHC rather than beta-MyHC. The difference in MyHC gene expression between marsupial and eutherian grazers may be related to the fact that kangaroos are not ruminants, and have only a single chance to comminute food into fine particles, hence the need for the greater speed and power of muscle contraction associated with V1 containing muscle fibers.

Mechanism of action of endothelin in rat cardiac muscle: cross-bridge kinetics and myosin light chain phosphorylation.

1. The molecular mechanism of inotropic action of endothelin was investigated in rat ventricular muscle by studying its effects on characteristics of isometric twitch, barium-induced steady contracture and the level of incorporation of 32Pi into myosin light chain 2. 2. Exposure of rat papillary muscle to endothelin caused an increase in isometric twitch force but did not alter the twitch-time parameters. 3. Endothelin did not significantly change the maximum contracture tension but did cause an increase in contracture tension at submaximal levels of activation, without changes in the tension-to-stiffness ratio and kinetics of attached cross-bridges. Kinetics of attached cross-bridges were deduced during steady contracture from complex-stiffness values, and in particular from the frequency at which muscle stiffness assumes a minimum value, fmin. Endothelin did not alter fmin. 4. Endothelin caused an increase in the level of incorporation of 32Pi into myosin light chain 2 without a concurrent change in the level of incorporation of 32Pi into troponin I. 5. We conclude that the inotropic action of endothelin is not due to an increase in the kinetics of attached cross-bridges, nor due to a change in the force per unit cross-bridge, but may result from an increased divalent cation sensitivity caused by elevated myosin light chain 2 phosphorylation, resembling post-tetanic potentiation in fast skeletal muscle fibres.

Extraocular fast myosin heavy chain expression in the levator palpebrae and retractor bulbi muscles.

PURPOSE: To determine whether the levator palpebrae superioris (LPS) and the retractor bulbi (RB) muscles, which share contractile characteristics with extraocular muscles (EOMs), express fast EOM-specific myosin heavy chain (MyHC) in the rabbit and other mammalian species. METHODS: Cryostat sections of rabbit eye and limb muscles were stained by indirect peroxidase immunohistochemical procedures using monoclonal antibodies (MAbs), including one (4A6) against EOM-specific fast MyHC. Myosin heavy chain isoforms from these muscles were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and the 4A6-reactive component was identified by immunoblotting. RESULTS: MAb 4A6 stained muscle fibers in rabbit LPS and RB. SDS-PAGE resolved a rabbit EOM-specific MyHC isoform (band 1) from two other components (bands 2 and 3) that comigrated with limb fast MyHCs. MAb 4A6 reacted only with band 1. Rabbit LPS and RB also displayed corresponding MyHC components with the same mobilities and immunoreactivities as bands 1 to 3 in the EOM. MAb 4A6 also stained muscle fibers in monkey and cat LPS, but it failed to stain muscle fibers in cat RB and rat LPS and RB. CONCLUSIONS: The expression of EOM-specific fast MyHC in EOM, LPS, and RB reflect their common developmental origin and similar contractile characteristics. These properties set them apart from other skeletal muscle groups. Eye muscles may constitute a distinct muscle group or allotype characterized by unique properties, including their propensity or resistance to disease.

Relationships between muscle membrane lipids, fiber type, and enzyme activities in sedentary and exercised rats.

Insulin resistance in skeletal muscle is associated with 1) relative increases in the proportion of glycolytic and fast-twitch muscle fibers and decreases in the proportion of more oxidative fibers and 2) a higher proportion of the saturated fatty acids in membrane structural lipids. Exercise is known to improve insulin action. The aims of the current studies were 1) to investigate the relationship between muscle fiber type and membrane fatty acid composition and 2) to determine how voluntary exercise might influence both variables. In sedentary Wistar rats in experiment 1, increased amounts of unsaturated fatty acids were found in the more oxidative insulin-sensitive red quadriceps and soleus muscles, whereas reduced levels of polyunsaturated fatty acids were found in primarily glycolytic white quadriceps muscles. In experiment 2, voluntary running-wheel exercise by adult female rats over 45 days resulted in reduced proportions of type IIb fibers (P = 0.01) and increased proportions of type IIa/IIx fibers (P = 0.03) in extensor digitorum longus muscle. The magnitude of these changes was related to the distance run (r = -0.73, P = 0.04; r = 0.79, P = 0.02, respectively). Exercise significantly increased oxidative capacity, as assessed by the proportion of intensely NADH-stained fibers (P = 0.0004) and citrate synthase (P = 0.003) and hexokinase (P = 0.04) activities. Citrate synthase activity was also increased by exercise in soleus muscle, where, as expected, no fiber type changes were detected. No significant differences in the fatty acid profile of soleus and extensor digitorum longus were found between groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of extraocular myosin heavy chain in rabbit laryngeal muscle.

The intrinsic laryngeal muscles of mammals are functionally heterogeneous, some of these muscles (e.g. the thyroarytenoid) contract extremely rapidly, like extraocular muscle, whilst others (e.g. the cricothyroid) contract as fast as limb fast muscle. The extraordinarily rapid contraction speed of extraocular muscles is associated with a fast myosin not found in limb muscles. In this work we explored the possibility that the thyroarytenoid muscle may also express this extraocular-specific fast myosin by raising a monoclonal antibody (mab 4A6) against its heavy chain. Electrophoretic separation of native isomyosins revealed that both the extraocular and the thyroarytenoid have two similar bands migrating ahead of bands found in limb fast or cricothyroid myosins. These two bands bound mab 4A6. The thyroarytenoid muscle can be divided into two divisions, a vocalis division which is important in phonation and an external division which functions in closing the glottis. Fibres in the vocalis are heterogeneous, some stain with mab 4A6, whilst others stain with mabs against limb myosin heavy chains. Fibres in the external division stain almost homogeneous with mab 4A6. The immunohistochemical staining pattern in the cricothyroid muscle resembled that of fast limb muscle: no fibres stained with mab 4A6. Thus, the high speed of contraction of the thyroarytenoid is associated with the same myosin heavy chain found in extraocular muscles, this characteristic is presumably an evolutionary adaptation for rapid closure of the glottis to enhance airway defense mechanisms.

Influence of myosin isoforms on tension cost and crossbridge kinetics in skinned rat cardiac muscle.

1. In attempting to consolidate the role of ventricular isomyosins in regulating the contractility of the myocardium, actomyosin ATPase and crossbridge kinetics were obtained at 24 degrees C in chemically skinned isometrically contracting cardiac muscles containing V1 and V3 isomyosins. 2. The ATPase activity was measured at various levels of Ca2+ activation by the enzymatic coupling of ATP hydrolysis with the conversion of NADH to NAD+. The crossbridge kinetics were inferred from small-amplitude perturbations of muscle length and muscle tension, and characterized by the frequency-domain parameter fmin. 3. The ATPase rates of V1 and V3 muscles obtained at various levels of Ca2+ activation were plotted against the corresponding proportional tensions. The ATPase vs tension plots were linear with slopes of 4.92 nmol/min-1 per mm per mN and 1.98 nmol/min-1 per mm per mN, respectively for, V1 and V3 muscles. Individual calculations of ATPase-to-tension ratios (nmol/min-1 per mm per mN) gave corresponding averages of 4.98 +/- 0.12 (s.e.m., n = 12) and 2.16 +/- 0.12 (s.e.m., n = 10). The myosin isoform induced proportional change in tension cost was accompanied by a similar change in fmin (4.1 +/- 0.1 Hz and 1.95 +/- 0.03 Hz, means +/- s.e.m., for V1 and V3 muscles, respectively). 4. The observations and other published kinetic data are discussed in the context of models of crossbridge cycling. It is suggested that the tension economy of V3 muscle arises principally from an increase in the fraction of time, during the crossbridge cycle, when the crossbridge is exerting force.

Isolation and structure of cat superfast myosin light chain-2 cDNA and evidence for the identity of its human homologue.

A full-length cDNA clone coding for superfast myosin light chain-2 (MyLC2) was isolated from an expression cDNA library prepared from cat masseter muscle and was fully characterized. The deduced amino acid sequence shares 58% overall identity with limb fast MyLC2, whereas homologies of the latter among higher vertebrates show 90% identity, indicating that superfast MyLC2 has diverged considerably from limb fast MyLC2 during evolution. Superfast MyLC2 cDNA has 89% nucleotide homology and 93% amino acid homology with a published novel human MyLC2 (MYL5), suggesting that MYL5 is a human homologue of the cat superfast MyLC2. Hybridization of a superfast MyLC2 isoform-specific probe reveals that expression of superfast MyLC2 in cat is confined to jaw-closing muscles. In conclusion, the present paper describes for the first time the cloning of a superfast myosin light chain coding sequence.

Muscle fiber types and function.

A major recent advance in the field of muscle fiber types has been the discovery that myogenic factors regulate fiber phenotypic properties. Myogenic influences occur in limb and trunk (somitic) muscle, but are particularly strong in jaw-closing muscles and extraocular muscles that express some unique myofibrillar proteins. In somitic muscles, a variant of fast fiber has been discovered, making four types of fibers: I, IIA, IIX, and IIB. These fibers express different isoforms of myofibrillar and other proteins. The speed and power of the four types of fibers are distinct and are controlled principally by their myosin heavy chains, which modulate the two regulatory steps in the crossbridge cycle, one controlling the rate of development of force, the other controlling the maximal velocity of shortening. Fast fibers have a higher threshold for Ca(2+)-activated force and a steeper force-pCa relation than do slow fibers. This difference is largely attributable to the cooperativity in the attachment of crossbridges and to the difference in Ca2+ binding capacity of their troponin C. Ca2+ initiates force development in muscle by increasing the rate of attachment of crossbridges. The phosphorylation of myosin light chain enhances this action. This effect of phosphorylation underlies the phenomenon of posttetanic potentiation of the isometric twitch in fast fibers.

Muscle sarcomere lesions and thrombosis after spaceflight and suspension unloading.

Spaceflight (flight) and tail suspension-hindlimb unloading (unloaded) produced significant decreases in fiber cross-sectional areas of the adductor longus (AL), a slow-twitch antigravity muscle. However, the mean wet weight of the flight AL muscles was near normal, whereas that of the suspension unloaded AL muscles was significantly reduced. Interstitial edema within the flight AL, but not in the unloaded AL, appeared to account for this apparent disagreement. In both experimental conditions, the slow-twitch oxidative fibers atrophied more than the fast-twitch oxidative-glycolytic and fast-twitch glycolytic fibers. Immunostaining showed that slow-twitch oxidative fibers expressed fast myosin, producing hybrid fibers containing slow and fast myosin isoforms. Two-dimensional gel electrophoresis of flight AL muscles revealed increased content of fast myosin light chains and decreased amounts of slow myosin light chains and fatty acid-binding protein. In the flight AL, absolute mitochondrial content decreased, but the relatively greater breakdown of myofibrillar proteins maintained mitochondrial concentration near normal in the central intermyofibrillar regions of fibers. Subsarcolemmal mitochondria were preferentially lost and reduced below normal concentration. Elevated fiber immunostaining for ubiquitin conjugates was suggestive of ubiquitin-mediated breakdown of myofibrillar proteins. On return to weight bearing for 8-11 h, the weakened atrophic muscles exhibited eccentric contraction-like lesions (hyperextension of sarcomeres with A-band filaments pulled apart and fragmented), tearing of the supporting connective tissue, and thrombosis of the microcirculation. Segmental necrosis of muscle fibers, denervation of neuromuscular junctions, and extravasation of red blood cells were minimal. Lymphocyte antibody markers did not indicate a significant immune reaction. The flight AL exhibited threefold more eccentric-like lesions than the unloaded AL; the high reentry G forces experienced by the flight animals, but not the unloaded group, possibly accounted for this difference. Muscle atrophy appears to increase the susceptibility to form eccentric contraction-like lesions after reloading; this may reflect weakening of the myofibrils and extracellular matrix. Microcirculation was also compromised by spaceflight, such that there was increased formation of thrombi in the post-capillary venules and capillaries. This blockage led to edema by 8-11 h after resumption of weight bearing by the COSMOS 2044 rats. The present findings indicate that defective microcirculation most likely accounted for the extensive tissue necrosis and microhemorrhages observed for COSMOS 1887 rats killed 2 days after landing.

Indirect myosin immunocytochemistry for the identification of fibre types in equine skeletal muscle.

The histochemical ATPase method for muscle fibre typing was first described by Brooke and Kaiser in 1970. However, problems have been found with the subdivision of type II fibres using this technique. To determine whether indirect myosin immunocytochemistry using anti-slow (5-4D), anti-fast (1A10) and anti-fast red (5-2B) monoclonal antibodies with cross reactivity for type I, II and IIa fibres, respectively, in a number of species, could identify three fibre types in equine skeletal muscle, data on fibre type composition and fibre size obtained using the two different techniques were compared. Results indicate that different myosin heavy chains can coexist in single equine muscle fibres. Type I and type II fibres were identified by immunocytochemistry, but subdivision of type II fibres was not possible. Although the percentage of type I and type II fibres was not significantly different for the two techniques, a few fibres reacted with both the 1A10 and 5-4D antibodies.

Basal lamina and superfast myosin expression in regenerating cat jaw muscle.

We investigated the possible role of extracellular matrix in specifying the expression of superfast myosin during cat jaw muscle regeneration. Equal proportions of muscle tissue from jaw and limb were minced together after killing cellular elements from one source. We allowed the mince to regenerate in the bed of a fast limb muscle. Regenerates were analyzed immunocytochemically at 71 to 294 days after operation. Fibers in control regenerates containing live cells from both sources expressed fast, superfast or slow myosins, or a mixture of these myosins. In regenerates containing only one type of live cells, we detected only myosins appropriate to the live cells. Our results suggest that during regeneration the original extracellular matrix of jaw-closing or limb muscle is unable to specify the expression of superfast or fast myosins, respectively; they point to the cellular elements, probably the satellite cells, as determinants of muscle specificity during regeneration.

Expression of superfast myosin in aneural regenerates of cat jaw muscle.

We investigated whether innervation is necessary for the expression of superfast myosin in regenerating cat jaw-closing muscle. Strips of jaw muscle were permitted to regenerate bilaterally in the beds of a fast limb muscle with innervation on one side being prevented surgically. Immunocytochemical analyses using anti-myosin heavy chain antibodies were done at various times postoperatively, the latest being after 78 days. We found little difference between innervated and uninnervated regenerates up to 27 days postoperatively. All regenerating myotubes expressed fetal myosin. In addition, some myotubes stained for superfast or slow myosin, while others stained for both superfast and slow myosins. Subsequently, uninnervated myotubes became atrophic but continued to express fetal, slow, and superfast myosins while innervated myofibers suppressed fetal and slow myosin expression. These results are consistent with the notion that satellite cells of jaw-closing muscles are committed to express superfast myosin during myogenesis, and that the expression of this program is independent of innervation.

Effects of dibutyryl cyclic AMP, ouabain, and xanthine derivatives on crossbridge kinetics in rat cardiac muscle.

In a previous communication, we showed that beta-adrenergic stimulation of cardiac muscles was associated with an increase in the rate of cycling of crossbridges as measured by perturbation analysis in the frequency domain. In this analysis, the frequency at which dynamic stiffness is a minimum (fmin) is taken as a measure of the rate of crossbridge cycling. In this paper, we test the hypothesis that the beta-adrenergic receptor-induced increase in crossbridge cycling rate is mediated by elevation of the intracellular level of cyclic AMP. The approach taken is to compare the effects on fmin in rat papillary muscles during Ba(2+)-activated contractures of 1) an agonist of cyclic AMP that can easily penetrate the cell, namely, dibutyryl cyclic AMP, 2) agents that block cyclic AMP phosphodiesterase, namely, the xanthine derivatives isobutylmethylxanthine and caffeine, and 3) an inotropic agent that does not affect the intracellular level of cyclic AMP, namely, ouabain. Our results showed that dibutyryl cyclic AMP at a dose of 5 mM has the same actions as beta-adrenergic stimulation: it potentiated the isometric twitch force, reduced the time to peak tension and time to half relaxation, and shifted fmin by a factor of 1.8 +/- 0.1 (n = 5). Isobutylmethylxanthine at up to 1.1 mM also acted in the same manner, increasing fmin by a factor of 1.8 +/- 0.2 (n = 6), but ouabain, at a dose (0.03 mM) sufficient to potentiate twitch force by 40 +/- 2% (n = 4), was without effect on the time course of the twitch nor was fmin changed (n = 4). Our findings support the hypothesis that a beta-adrenergic receptor-mediated increase in crossbridge cycling rate is due to an increase in intracellular cyclic AMP level and illustrate the usefulness of the frequency domain analysis approach in the analysis of the mechanism of action of inotropic agents.

Myogenic regulation of mammalian skeletal muscle fibres.

Motor nerves can only modify the phenotype of muscle fibres within a myogenically determined range. The particular range of a given fibre is an intrinsic property of that fibre, depending on the type of muscle (limb or jaw) and the specific tag imprinted upon it during development.