Neural control of phenotypic expression in mammalian muscle fibers

Contractile properties and myosin isoenzymes of various kinds of Xenopus twitch muscle fibres

Single twitch muscle fibres have been isolated from various parts of the iliofibularis muscle of Xenopus laevis. After measurements of their isotonic contractile properties, myosin extraction was performed on individual fibres and the extract analysed by various forms of gel electrophoresis. In agreement with previous results three major fibre types, types 1, 2, and 3 could be discerned. Both mechanical data and native isomyosin patterns indicated a further subdivision of types 1 and 2 into subtypes (1n, 1s and 2f, 2n, respectively). Transitional forms between 2n and type 3 were also identified. Types 1 and 2 had the same kinds of light chains (LC1f, LC2, LC3f), but different heavy chains (HC) as observed on 7% SDS gels. Types 3 lacked LC3 and had a more slowly migrating LC1 (LC1s); their HC migration velocity was indistinguishable from that of type 2 HC. A comparison was made between LC1/LC3 ratio and contractile parameters for nine type 1n fibres and six type 2n fibres. No clear correlation was observed between light chain proportions on the one hand and force per cross-sectional area or shortening velocity on the other. It is concluded that myosin heavy chain composition is the major determinant for contractile performance in Xenopus skeletal muscle fibres.

Mechanical properties of rat skeletal muscle after hind limb suspension

It is clear that there are events which occur in response to neuromuscular activity that are essential to maintain normal muscle properties. Two factors though to influence muscle contractile properties are tension and electrical activity. To study these variables the hind limbs of 10 postpubertal female Sprague-Dawley rats were prevented from supporting the weight of the body for 4 weeks, then in situ isometric properties were determined and compared with 10 age-matched control rats. The soleus (slow plantarflexor), the medial gastrocnemius (fast plantarflexor), and the tibialis anterior (fast dorsiflexor) were studied. The suspended soleus wet weight was 42% lower and the maximum isometric twitch (Pt) and tetanic (P0) tensions were 62 and 69% lower respectively, than in the control rats. The suspended medial gastrocnemius was 18% lower in wet weight and 14 and 9% lower in P0 and Pt than the controls. No differences in wet weight, P0 or Pt were observed in tibialis anterior. Mean contraction time was 22% shorter in the soleus, 9% in the medial gastrocnemius, and unchanged in the tibialis anterior following suspension. Further, the percent of P0 attained during a 330-ms tetanus at 20 Hz was 15% lower in the suspended soleus and unchanged in the other two muscles. The fatigue index (ratio of tension after 2 min of stimulation at 40 Hz for 330 ms once per second to the maximum tension developed during the test) was unaffected by suspension in the soleus and tibialis anterior but was reduced from 49% to 36% in the medial gastrocnemius. The maximum rate of shortening of all three muscles was unaffected by suspension. These results indicate that suspension of the hind limbs selectively affects the mass and force-generating capabilities of the plantarflexors, particularly the predominantly slow soleus. In contrast, the fatigability of only the fast plantarflexor was increased whereas the slow plantarflexor was unaffected. These results, considered in light of collaborative studies, suggest that the chronic force-time levels in a muscle have an important influence on tension- and speed-related properties but not necessarily the fatigability of that muscle. Further, a marked difference in the sensitivity of the contractile elements of slow and fast muscle to this influence was evident.

Quantitative ultrastructure of histochemically identified avian skeletal muscle fiber types

A cryostat retrieval method and combined adenosine triphosphatase (ATPase) and acetylcholinesterase (AChase) method were used to study the ultrastructure and innervation of histochemically identified skeletal muscle fibers in different pigeon muscles. The Z-line structure and volume percentage sarcotubular system were analyzed from different muscles selected for their composition by fiber type. Histochemically, three main fiber types were investigated: slow tonic fibers with a moderate ATPase activity after preincubation at acid or alkaline pH; fast-twitch fibers that had high activity after alkaline treatment and low activity after acid preincubation; and a type considered to be slow-twitch that had low activity after alkaline, and high after acid preincubation. Both the slow tonic and slow-twitch fibers had multiple, en grappe innervation, while the fast-twitch fibers had robust, single end plates. The Z-line of the fast-twitch and slow-twitch fibers had a regular square lattice pattern, in contrast to the granular, nonlattice structure of the slow tonic Z-line. The volume percentage sarcotubular system of the slow-twitch fibers was intermediate between and significantly different from that of the fast-twitch and slow tonic fibers. These correlative analyses suggest that the avian muscles contain not only the fast-twitch and slow tonic fibers previously known, but also a slow-twitch fiber that appears to be intermediate between the tonic and the mammalian slow-twitch fiber type. Based on the abundance of the sarcotubular system, this fiber type appears to be fast-contracting and -relaxing, in spite of being multiply innervated.

Role of thyroid hormone in regulation of isomyosin composition, contractility, and size of heterotopically isotransplanted rat heart

The role of thyroid hormone on the heart in terms of contractility, induction of growth, and selective synthesis of cardiac isomyosins was studied. After transplanting rat hearts from inbred hypothyroid donors into the abdomen of hypothyroid recipients of the same strain, two hearts were obtained in the same animal, both having reduced heart rate (200-250 bpm), decreased maximum rate of force, and high predominance of V3 isomyosin. The heart in situ carried a full load, while the transplant was denervated, beat isovolumically with minimum external work. After surgery, the recipient rats were put either on normal diet only (controls) or injected with a daily dose of T3 (average 200 micrograms/kg), which increased the heart rate to 340 bpm in 3 days (euthyroid level) and to 450 bpm in 7 days (hyperthyroid level). In T3-treated rats, the contractility of both hearts normalized in 7 days and showed hyperthyroid pattern in 14 days, while the mass of the in situ hearts increased to normal values in 7 days (+130 mg) and hypertrophied in 14 days (+340 mg), in contrast to the transplanted heart, which underwent atrophy (-90 mg and -210 mg) similar to that of control group (-225 mg). The predominant V3 isomyosin was completely reversed to V1 in two weeks in both hearts. Thus, T3 can neither stimulate cardiac growth nor can it attenuate the rate of atrophy in the denervated "nonworking" heart in spite of its direct effect on contractility and synthesis of isomyosins, which was similar to that observed in the in situ heart.

Myosin polymorphism in single fibers of chronically stimulated rabbit fast-twitch muscle

Rabbit tibialis anterior (TA) muscles were indirectly stimulated (10 Hz, 24 h/d) for 30 d and 60 d and single fibers were analysed using a combined histochemical and biochemical technique (Staron and Pette 1986, 1987a, b). After 30 d of chronic stimulation there was a pronounced increase in the normally rare (0.5%) C fiber population (i.e., fibers containing slow- and fast-myosins in varying ratios). At this time, C fibers amounted to almost 60% of the total population. In the 60 d stimulated muscles, the major population (98%) consisted of an atypical type It fiber. This fiber type which was not detectable in normal TA muscle, differed histochemically and biochemically from type I fibers. It contained the slow-myosin light chains LC1s and LC2s, the heavy chain HCI, and, in addition, high concentrations of the fast-myosin alkali light chain LC1f and possibly traces of a heavy chain with an electrophoretic mobility comparable with that of the fast-myosin heavy chain HCIIa. These It fibers were occasionally observed in the unstimulated, contralateral TA muscles which also contained an increased population of C fibers (1.3-6.3%). Although the transformation even after 60 d of chronic stimulation was incomplete, these changes demonstrate the ability of muscle fibers to adapt in a specific manner to altered functional demands brought about by an altered stimulus pattern. In addition, the pronounced heterogeneity of the fiber population undergoing transformation indicates a nonuniform response to a uniform stimulus pattern.

The multiplicity of combinations of myosin light chains and heavy chains in histochemically typed single fibres. Rabbit soleus muscle

1. Six adult rabbit soleus muscles were analysed by isolating histochemically identified fibre pieces from freeze-dried serial cross-sections. 2. By the use of this method, four fibre types (I, IC, IIC and IIA) were identified and analysed micro-electrophoretically. 3. Type I fibres contained the slow myosin heavy chain HCI and the slow myosin light chains LC1s and LC2s. 4. Type IIA fibres contained the fast myosin HCIIa with the fast light chains and, in addition, either LC1s or both LC1s and LC2s. 5. The C fibres (IC and IIC) represented intermediate populations between types I and IIC (IC) and between IC and IIA (IIC). They contained varied ratios of HCI/HCIIa with both sets of fast and slow light chains. With regard to myosin composition and isoforms of other myofibrillar proteins (M- and C-proteins, alpha-tropomyosin, troponin I), IC fibres resembled type I and IIC fibres resembled type IIA. 6. The presence of various myosin light and heavy chains within a specific fibre suggests a multiplicity of isomyosins. Without consideration of LC1sa and LC1sb differences, at least 54 possible isomyosins can be derived: type I fibres contain one isomyosin, types IC and IIC 54 possible isomyosins, and type IIA up to 18.

Skeletal muscle as the potential power source for a cardiovascular pump: assessment in vivo

Skeletal muscle ventricles (SMVs) were constructed from canine latissimus dorsi and connected to a totally implantable mock circulation device. The SMVs, stimulated by an implantable pulse generator, pumped continuously for up to 8 weeks in free-running beagle dogs. Systolic pressures produced by the SMVs, initially of 139 +/- 7.2 mmHg and after 1 month of continuous pumping of 107 +/- 7 mmHg, were comparable to normal physiologic pressures in the adult beagles (114 +/- 21 mmHg). After 2 weeks of continuous pumping, the mean stroke work of the SMVs was 0.4 X 10(6) ergs, a performance that compares favorably with the animal's cardiac ventricles. This study shows that canine skeletal muscle which has not received prior training or electrical conditioning can perform sustained work at the high levels needed for an auxiliary cardiovascular pump. It might be possible eventually to use such muscle pumps in humans to assist the failing circulation and to provide support in children with certain types of congenital heart defects.

Developmental pattern of mouse skeletal myosin heavy chain gene transcripts in vivo and in vitro

We have studied the transcripts of the embryonic, perinatal, and adult fast myosin heavy chain (MHC) genes in mouse skeletal muscle in vivo before and after birth, and in vitro in myogenic cell lines. In vivo, in 15-day fetal muscle, embryonic and perinatal MHC mRNAs are both present, and the former is the major transcript. By 18 days the perinatal is predominant and the adult MHC mRNA appears. In beta-bungarotoxin-treated fetuses, a similar developmental pattern is detected, suggesting that it is nerve-independent and that primary myotubes alone undergo the same developmental changes. In vitro, in the absence of the nerve, embryonic, perinatal, and adult IIB MHC mRNAs accumulate. The level of the latter two isomRNAs is influenced by culture conditions.

Reversible inhibition of sarcoplasmic reticulum Ca-ATPase by altered neuromuscular activity in rabbit fast-twitch muscle

A 50% decrease in both the initial rate and the total capacity of Ca2+ uptake by the sarcoplasmic reticulum (SR) occurred 2 days after the onset of chronic (10 Hz) nerve stimulation in rabbit fast-twitch muscle. Prolonged stimulation (up to 28 days) did not lead to further decreases. This reduction, which was detected in muscle homogenates using a Ca2+-sensitive electrode, was reversible after 6 days cessation of stimulation and was not accompanied by changes in the immunochemically (ELISA) determined tissue level or isozyme characteristics of the SR Ca2+-ATPase protein. However, as measured in isolated SR, it correlated with a reduced specific activity of the Ca2+-ATPase. Kinetic analyses demonstrated that affinities of the SR Ca2+-ATPase towards Ca2+ and ATP were unaltered. Positive cooperativity for Ca2+ binding (h = 1.5) was maintained. However, a 50% decrease in Ca2+-dependent phosphoprotein formation indicated the presence of inactive forms of Ca2+-ATPase in stimulated muscle. The reduced phosphorylation of the enzyme was accompanied by an approximately 50% lowered binding of fluorescein isothiocyanate, a competitor at the ATP-binding site. In view of the unaltered affinity for ATP, this finding suggests that active Ca2+-ATPase molecules coexist in stimulated muscle with inactive enzyme molecules, the latter displaying altered properties at the nucleotide-binding site.

Evidence of myotonic origin of type 2B muscle fibre deficiency in myotonia and paramyotonia congenita

Available experimental and clinical evidence strongly suggests that deficiency in type 2B muscle fibres in generalized myotonia and paramyotonia congenita will develop consequent to lasting myotonic activity as its adequate stimulus. Type 2B fibre deficiency need not be regarded as the product of a hypothetical genetic factor.

Actin and myosin multigene families: their expression during the formation and maturation of striated muscle

The initial formation of skeletal muscle fibers is accompanied by the expression of muscle-type actin and myosin genes. During subsequent maturation of muscle fibers in vivo, developmental changes in the fetal/adult isoforms of these proteins occur. Skeletal muscle-specific transcripts coding for different myosin heavy chains accumulate sequentially both in vivo and in vitro. A genetic analysis demonstrates that these genes are clustered, implicating cis-acting regulatory factors. In contrast, actin and myosin light chain genes are dispersed in the mouse genome. These gene families show a different developmental "strategy": Genes expressed in adult cardiac tissue are coexpressed with the corresponding skeletal muscle sequence during fetal development. This phenomenon also occurs in adult tissue. Under conditions of cardiac overload, adult rat hearts accumulate skeletal actin mRNA and cardiac actin transcripts. In some mouse lines, a mutant cardiac actin gene locus is present. The presence of a second active upstream promoter at this locus depresses transcription of the bone fide gene, resulting in low levels of mature cardiac actin mRNA. In this situation skeletal actin gene transcripts accumulate. Genes expressed in the same fetal or adult muscle phenotype are not linked, suggesting that their coexpression is regulated by transacting factors. The promoter regions of such genes in the mouse have no common characteristics of primary structure with the exception of an E1A-type enhancer core sequence, which has a conserved 5' flanking element, seen for actin and myosin light chain genes. Reintroduction of these promoter regions into muscle cells provides a functional test for such potential regulatory sequences.

Responses of muscles of patients with Duchenne muscular dystrophy to chronic electrical stimulation

The effect of chronic low frequency stimulation on the tibialis anterior muscle of children with Duchenne muscular dystrophy was investigated. Baseline data from 16 boys established low values of maximum voluntary contraction which did not improve with age. Studies of the contractile properties revealed significant slowing (p less than 0.001) of mean relaxation time compared to that of normal children's muscles. There was no loss of force during fatigue testing, as in normal children, but in contrast to normal children, there was no potentiation at lower frequencies of stimulation. Intermittent chronic low frequency stimulation of muscles in six young ambulant children with Duchenne muscular dystrophy resulted in a significant increase (p less than 0.05) in mean maximum voluntary contraction compared with the mean forces exerted by the unstimulated control muscles of the contralateral leg.

Muscle lesions in beige (Chediak-Higashi syndrome) and heterozygous C57BL/6J mice

Muscles from male and female C57BL/6J Chediak-Higashi syndrome (CHS) and phenotypically normal mice with the bgJ allele were studied microscopically and histochemically for the presence of basophilic cytoplasmic structures seen by other investigators in muscles of CHS mice of the SB/Le strain. Triceps brachii, gastrocnemius, quadriceps femoris, and biceps femoris muscles were examined. Multiple basophilic cylindrical lesions were present in hematoxylin and eosin-stained muscle from all groups. Lesions were positive for esterase, Sudan black, and periodic acid-Schiff. Lesions were only seen in type II muscle fibers. Type I muscle cells comprised less than an estimated 5% of the total muscle fibers in the four muscles examined. Scores were assigned based on the presence or absence of lesions in each muscle. Male mice of both phenotypes had significantly more lesions (P less than 0.05) than female mice. When sexes were combined, lesions were significantly (P less than 0.05) more numerous in normal mice than CHS mice for all muscles except the gastrocnemius. Lesions were significantly (P less than 0.05) more numerous in the phenotypically normal male than the CHS male mice for the triceps and quadriceps muscles. There was no significant difference (P greater than 0.05) between lesions of phenotypically normal female and female CHS mice. Basophilic cytoplasmic structures did not prove to be a manifestation of the CHS trait.

The development of fibre types in grafts of a slow tonic avian muscle, the dorsocutaneous latissimus dorsi

The dorsocutaneous (DLD) and anterior (ALD) latissimus dorsii are both homogeneous slow tonic muscles. Autografts of mature DLD were attached onto the ALD of chickens to study regeneration of slow tonic muscle fibres innervated exclusively by slow tonic nerves. Fifty-three grafts were examined from 3 to 231 days after implantation for myosin ATPase, and for heavy chains of fast myosin. New muscle fibres in grafts were initially type 1 (slow) or type 2 (fast twitch). Tonic type 3 fibres were slow to differentiate and were not seen within 59 days. From 105 days many fibres were type 3A and type 1 were no longer apparent. However, type 2 fibres persisted and appeared to be present instead of type 3B fibres even after 8 months.

Type 1, 2A, and 2B myosin heavy chain electrophoretic analysis of rat muscle fibers

Mammalian skeletal muscles are mixture of three type of fibers: type 1, type 2A, and type 2B fibers. Immunological studies and proteolytic analysis of myosin heavy chains from the three type of fibers have demonstrated the presence of distinct myosin isoforms. By using typed single muscle fibers and improving an electrophoretic method we are able to resolve three distinct polypeptides which are demonstrate to correspond to type 1, 2A and 2B myosin heavy chain isoforms by using specific monoclonal antibodies. The analysis of single muscle fibers shows that different myosin heavy chain isoforms are frequently coexpressed in the same muscle fiber.

Enzyme activities in single electrophysiologically identified crab muscle fibres

The superficial muscle fibres in the proximal part of the closer muscle in the crab Eriphia can be separated into four fibre groups (I-IV) on the basis of electrophysiological and histochemical characteristics. The activity levels of glyceraldehydephosphate dehydrogenase (GAPDH), lactate dehydrogenase (LDH), citrate synthase (CS), NADP-isocitrate dehydrogenase (IDH) and 3-hydroxyacyl-CoA dehydrogenase (HAD), determined in single electrophysiologically identified fibres, differed significantly among the different fibre groups. In addition, fibres belonging to the same group, with similar electrophysiological characteristics, demonstrated variability with regard to metabolic enzyme activities. Nevertheless, comparison of absolute enzyme activities and enzyme activity ratios permitted the discrimination of at least three groups. These groups corresponded with those defined according to electrophysiological and histochemical characteristics. The group I fibres (tonic fibres) are intermediate in oxidative potential and show the lowest values of glycolytic enzymes. The group II and group III fibres can be regarded as fast oxidative fibres. The high ratio between activity levels of enzymes for glycolytic and oxidative metabolism found for group IV fibres (fast fibres) demonstrated that this group depends strongly on anaerobic metabolism.

Transmural distribution of glucose metabolizing enzymes across the left and the right ventricle heart walls in three different mammalian species

The transmural distribution of five glucose metabolizing enzymes (hexokinase; glucose-6-phosphate dehydrogenase; phosphofructokinase; aldolase; and lactate dehydrogenase) were explored in the left and in the right ventricle wall of rat, ox and pig hearts. The levels of most of these enzyme activities were different in the different animal species and (within the same species) in the two ventricles. Most of these enzyme activities were found to be non-uniformly distributed across the left (but not across the right) ventricle wall. Differences in the transmural distribution of enzyme activities were detected among the three examined mammalian species.

Purification and characterization of myosins from human and rabbit skeletal muscles by using specific monoclonal antibodies

By using immunoaffinity column chromatography slow (I) and fast (IIA, IIB) myosins were isolated from human (vastus lateralis) and rabbit (tibialis anterior, psoas and conoidal bundle) skeletal muscles. The peptide pattern revealed that slow (I) and fast (IIA, IIB) myosin heavy chains are quite distinct, as are those from pure slow (conoidal bundle) and fast (psoas) rabbit skeletal muscles. Unlike Billeter et al. (1981) the authors observed that fast human myosins were always associated with a small amount of slow myosin light chains. The fast myosins (IIA, IIB) from rabbit tibialis anterior muscle did not appear very distinct and contained only fast myosin light chains. These myosins were different from the IIB myosin from the psoas muscle. Ten per cent of the fibres revealed histochemically as fast IIA also reacted with an anti-slow myosin antibody. The classical histochemical techniques appear inadequate to demonstrate the existing differences among fibre types, but the monoclonal antibodies hold promise.