Physiological factors influencing the growth of skeletal muscle

The growth of muscle can be regulated by developmental changes or by alterations in hormone levels or in the rate or amount of work demanded. The mechanisms and structures involved in growth processes can be studied by controlling these factors. The models used are chicken anterior latissimus dorsi (ALD) muscle under the influence of overloading and rabbit tibialis anterior (TA) muscle under the influence of chronic nerve stimulation. Both models involve changes in the isoform of myosin that is expressed. Methods of study include quantitative ultrastructural analysis, immunofluorescence and in situ mRNA hybridization. In overloaded chick ALD fibres polysomes are nonuniformly distributed between the myofibrils and in a peripheral annulus even though subcellular concentrations of the new isoform are not found. In normal rabbit muscle the highest concentration of myosin mRNA detected by in situ hybridization is found in the subsarcolemmal zone. In stimulated TA polysomes are found between myofibrils. It appears that the myosin mRNA accumulates at specific cell locations before translation; then diffusion of isomyosin and rapid exchange into myofibrils follows. Therefore, regulation of growth may be possible at the transcriptional, translational and assembly stages.

Ultrastructural distribution of myosin heavy chain mRNA in cardiac tissue: a comparison of frozen and LR White embedment

Electron microscopy (EM) in situ hybridization provides the higher resolution necessary to determine the spatial relationship between a specific mRNA and the organelle containing the protein encoded by that message. EM in situ hybridization was used to determine the subcellular myosin heavy chain (MHC) mRNA distribution with respect to the myofibril in normal cardiac tissue. Sections of frozen or acrylic-embedded tissue were compared for ultrastructural integrity and content of endogenous mRNA. Papillary muscles dissected from hearts of normal rabbits were aldehyde-fixed and either frozen or embedded in LR White. EM in situ hybridization with no riboprobe, vector sequence, same-sense, and anti-sense biotinylated riboprobes was detected by indirect immunocytochemistry. Labeling density using an antisense probe was highest over the intermyofibrillar space, with an average signal five times that of background. Background labeling by nonspecific sense probe was consistently low but not random, also having the highest density of gold clusters over the intermyofibrillar space. Ultracryomicrotomy yielded a higher absolute number of gold clusters, but sections were fragmented and disrupted striated muscle morphology. LR White embedment maintained ultrastructural integrity but gave a lower absolute signal. Fortunately, MHC mRNA is an abundant message and can tolerate the decreased sensitivity of LR White.

Correlates of fatigue resistance in canine skeletal muscle stimulated electrically for up to one year

In response to patterns of chronic electrical stimulation that increase its overall level of use, mammalian skeletal muscle becomes highly resistant to fatigue. The metabolic basis for this adaptation is well documented in the rabbit, but up to now it has not been possible to identify analogous changes in the dog. In this study, canine latissimus dorsi muscles were stimulated in situ for 2, 6 and 12 mo. Marked increases in fatigue resistance were consistently demonstrated. Citrate synthase and succinic dehydrogenase, conventionally used as markers of oxidative metabolism, did not increase in activity, but enzymes involved in major pathways supplying substrates to the tricarboxylic acid cycle increased up to threefold. Stimulation elevated the volume fraction of mitochondria 1.5-fold and that of lipid droplets 4.5-fold. After 6 mo of stimulation, mean fiber diameter had decreased by 30% and the area occupied by nonmuscle tissue had increased by 11%; these changes showed no further progression at 12 mo. Thus stimulated muscle becomes stably adapted to an increase in use, but the metabolic strategies for achieving increased fatigue resistance vary between species.

Expression of a fast myosin heavy chain mRNA in individual rabbit skeletal muscle fibers with intermediate oxidative capacity

In situ hybridization (ISH) of myosin heavy chain (MHC) mRNA, immunofluorescent detection of MHC protein, and oxidative enzyme histochemistry were performed on the same fibers in serially sectioned rabbit skeletal muscle. By combining these three techniques quantitatively, on a fiber-by-fiber basis, fibers that expressed mRNA complementary to a fast MHC cDNA pMHC24-79 of unknown subtype (Maeda et al., 1987) were classified into fiber types with respect to slow myosin expression and oxidative capacity. As expected, slow fibers had low hybridization to pMHC24-79. Fast fibers were divided into three subtypes. mRNA from the low oxidative fibers (fast-glycolytic, IIB) did not hybridize with pMHC24-79. Fast fibers whose mRNA hybridized best to pMHC24-79 were mainly in the intermediate range of oxidative capacity (probably IIX). The fast fibers with the highest oxidative capacity had low hybridization to this MHC mRNA (probably IIA). Thus, pMHC24-79 was identified as a clone of a fast isomyosin, tentatively designated as the fast IIX with intermediate oxidative capacity. The expression of more than a single species of fast and slow isomyosin mRNAs in classically defined fiber type was considered in interpreting these results.

Distribution of myosin heavy chain mRNA in normal and hyperthyroid heart

Hyperthyroid treatment produces rapid cardiac cell hypertrophy with all subcellular components increasing in an orderly manner. We compare normal and hyperthyroid tissue in order to relate changes in distribution of myosin mRNA during rapid assembly of myofibrils. At the light microscopic level, in situ hybridization of the ventricular cells shows myosin heavy chain mRNA to be distributed in a spoke-like pattern radiating from the nucleus. Electron microscopy provides the higher resolution necessary to determine mRNA distribution with respect to adjacent sarcomeric and cytoskeletal structures. Papillary muscles were removed from hyperthyroid and normal rabbits, aldehyde fixed, and embedded in LR white. Biotinated riboprobe transcribed from 0.5 kb in the coding region of terminal portion of the rod of alpha-myosin was hybridized and detected by immunocytochemical methods using 5 nm immunoglobulin G gold conjugates. Electron microscopy in situ hybridization runs with same-sense and anti-sense riboprobes were processed and ten micrographs randomly taken from each. Specific cytoplasmic densities of myosin mRNA were calculated by counting clusters of five or more gold particles over respective tissue components after subtraction of background counts. For both normal myocytes and hyperthyroid myocytes, the density of myosin mRNA was about 15 times higher in the cytoskeletal-rich inter-myofibrillar space than in the myofibrils. About half of the myosin mRNA in this inter-myofibrillar region is found within 10 nm of the peripheral filament, but no excess sarcomeric accumulation was seen beside the A-Band. It appears that most of the myosin is translated from mRNA within the inter-myofibrillar space along the entire length of the myofibril periphery. The emerging myosin heavy chain is not directly anchored to the thick filaments in either normal or rapidly growing cardiac cells.

Distribution of myosin mRNA during development and regeneration of skeletal muscle fibers

Myosin mRNA distribution among subcellular compartments of anterior tibialis muscles in rabbit is monitored by in situ hybridization. A high density of mRNA was widely distributed throughout myotubes from 29-day fetal muscle and from regenerating adult muscle. All cytoplasmic spaces contained mRNA except where scattered myofibrils and centrally located nuclei were found. In fibers from 22-week-old rabbits, myosin mRNA was concentrated under the sarcolemma and excluded from the consolidated myofibrils and peripheral nuclei. The dispersal of mRNA through the cytoplasm in myotubes suggests that translation of myosin is widespread and that rapid myofibril assembly can occur throughout the fiber.

Redistribution of myosin heavy chain mRNA in the midregion of stretched muscle fibers

Myosin mRNA distribution was compared to the distribution of striations, nuclei, and cytoskeletal components in normal fibers and in fibers undergoing growth and repair processes in response to stretch. Plantarflexion of rabbit lower hindlimb for 4 or 6 days resulted in a 35% increase in weight of the tibialis anterior muscle. Slow myosin expression in stretched fibers increased such that the proportion of fibers shifted from the fast type towards an intermediate type. Semi-quantitative in situ hybridization revealed a large increase in concentration of slow myosin mRNA in stretched fibers. Polysomes translating myosin heavy chain were excluded from the intact myofibrillar lattice. Significant increases of myosin mRNA concentration occurred only in the outer 8 microns subsarcolemmal annulus of these stretched fibers (P less than 0.001) where myofibril formation also was evident. In some fibers, stretch caused myofibrillar disorder where nuclei became centrally located, and focal concentrations of myosin mRNA also occurred. We discuss mechanisms for mRNA accumulation and favor free diffusion to loosely packed cytoplasmic regions where myosin is needed for myofibrillar growth and repair.

Myosin mRNA accumulation and myofibrillogenesis at the myotendinous junction of stretched muscle fibers

Myofiber growth and myofibril assembly at the myotendinous junction (MTJ) of stretch-hypertrophied rabbit skeletal muscle was studied by in situ hybridization, immunofluorescence, and electron microscopy. In situ hybridization identified higher levels of myosin heavy chain (MHC) mRNA at the MTJ of fibers stretched for 4 d. Electron microscopy at the MTJ of these lengthening fibers revealed a large cytoplasmic space devoid of myofibrils, but containing polysomes, sarcoplasmic reticulum and T-membranes, mitochondria, Golgi complexes, and nascent filament assemblies. Tallies from electron micrographs indicate that myofibril assembly in stretched fibers followed a set sequence of events. (a) In stretched fiber ends almost the entire sarcolemmal membrane was electron dense but only a portion had attached myofibrils. Vinculin, detected by immunofluorescence, was greatly increased at the MTJ membrane of stretched muscles. (b) Thin filaments were anchored to the sarcolemma at the electron dense sites. (c) Thick filaments associated with these thin filaments in an unregistered manner. (d) Z-bodies splice into thin filaments and subsequently thin and thick filaments fall into sarcomeric register. Thus, the MTJ is a site of mRNA accumulation which sets up regional protein synthesis and myofibril assembly. Stretched muscles also lengthen by the addition of myotubes at their ends. After 6 d of stretch these myotubes make up the majority of fibers at the muscle ends. Essentially all these myotubes repeat the developmental program of primary myotubes and express slow MHC. MHC mRNA distribution in myotubes is disorganized as is the distribution of their myofibrils.

Individual rabbit cardiac myocytes have different thresholds for alpha myosin heavy chain regulation by thyroid hormone

Myocytes in adult rabbit ventricle express and alpha and a beta form of myosin heavy chain (MHC). The alpha-MHC distribution detected with indirect immunofluorescence has been found in different proportions in adjacent myocytes producing a mosaic staining pattern. The basis for cell-specific expression of the alpha-MHC isoform is not known. Since thyroid hormone is a major regulator of myosin gene expression, we varied the plasma thyroid level and followed the alpha-MHC content within a population of myocytes. Ventricular myocytes were induced to become 100% beta-MHC by placing the rabbits on a 0.15% propylthiouracil diet for 70 days. L-triiodothyronine (LT3) over a dose range of 1 to 10 micrograms/kg/day was delivered by an osmotic minipump for 5 days, with actual serum levels confirmed by LT3 radioimmunoassay to be in the range of from 115 to 1,230 ng/dl. The amount of alpha-MHC that returned was estimated in randomly selected cells by measuring the relative intensity of the fluorescence-tagged secondary antibody. The normal mosaic pattern of alpha-MHC expression in the left ventricle returned with an LT3 dose of 2-5 micrograms/kg/day. The first myocytes to express alpha-MHC were in the subepicardium and did so at a LT3 serum level of 115 of ng/dl. All myocytes of the ventricular wall expressed alpha-MHC at serum levels above 1,230 ng/dl. These data are interpreted to show that the variation of myosin isoform content seen in the adult heart is indicative of heterogeneity of thyroid sensitivity, with the threshold for serum LT3 being between 115 and 370 ng/dl.

In situ hybridization and immunocytochemistry in serial sections of rabbit skeletal muscle to detect myosin expression

We performed in situ hybridization of myosin heavy-chain (MHC) mRNA on rabbit muscle using a biotin-labeled complementary RNA probe. An 1107-nucleotide fragment from an alpha-cardiac MHC cDNA was used to transcribe an RNA probe 97% similar to slow-twitch and 75% similar to fast-twitch sequences. Serial sections were used to identify slow-twitch fibers in medial gastrocnemius, soleus, and tibialis anterior by immunofluorescence of slow MHC and oxidative capacity by histochemistry. Slow-twitch fibers hybridized by the RNA probe stained heavily after detection with streptavidin-alkaline phosphatase (89% dark and 11% medium density). Fast-oxidative fibers stained intermediately (26% dark, 58% medium, and 16% light) and fast-glycolytic fibers stained lightly (12% medium and 88% light). Biotin-labeled probe and enzymatic detection allowed greater resolution of the subcellular location of the MHC mRNA, a distinct advantage over isotope labeling and autoradiography. A non-uniform distribution of MHC mRNA was recognized within an adult skeletal muscle fiber. High concentrations of MHC mRNA were found under the sarcolemma and between the myofibrils, suggesting the existence of a distribution mechanism. The combination of in situ hybridization and immunocytochemistry allows rapid subcellular localization of both MHC mRNA and its translated protein.

Nascent muscle fiber appearance in overloaded chicken slow-tonic muscle

The application of a weight overload to the humerus of chickens induces a hypertrophy of anterior latissimus dorsi (ALD) muscle fibers. This growth is accompanied by a rapid and almost complete replacement of one slow-tonic myosin isoform, SM-1, by another slow-tonic isoform, SM-2. In addition, a population of small fibers appears mainly in extrafascicular spaces and, concurrently, three additional myosin bands are detected by gel electrophoresis. Five antibodies against myosin heavy chain (MHC) isoforms were selected as immunocytochemical probes to determine the cellular location and nature of these myosins. The antibodies react with ventricular, fast skeletal muscle and either SM-1 or SM-2, or both the slow-tonic MHCs. The antifast and antiventricular antibodies react with myosin present in the 10-day embryonic ALD muscle but do not react with myosin in posthatch ALD muscle. The small fibers in overloaded muscle contain a myosin isoform characteristically expressed during the embryonic stage of ALD muscle development and therefore are named nascent myofibers. Some of the nascent myofibers do not react with the antibody to both slow-tonic MHCs, indicating the lack of the normal adult slow-tonic myosins which are expressed in 10-day embryos. In order to explore the origin of the nascent fibers, an electron microscopic study was performed. Stereological analysis of the existing fibers shows a stimulation of numbers and sizes of satellite cells. In addition, the volume occupied by nonmuscle and undifferentiated cells increases dramatically. Myotube formation with incipient myofibrils is seen in extrafascicular spaces. These data suggest that new muscle fiber formation accompanies hypertrophy in overloaded chicken ALD muscle and the process may involve satellite cell migration.

Incorporation of nascent myosin heavy chains into thick filaments of cardiac myocytes in thyroid-treated rabbits

A monoclonal antibody (mAb 37) specific for alpha-myosin heavy chain (alpha-MHC) is used to follow the spatial and temporal incorporation of alpha-MHC into rabbit left ventricular myocytes. The expression of the two adult cardiac MHC genes, alpha and beta, is regulated by manipulating the thyroid hormone level of the animal. 10 wk on a propylthiouracil diet down-regulates expression of alpha-MHC to near 0%. alpha-MHC gene expression is up-regulated by injecting L-triiodothyronine (100 micrograms/kg per d) for 1-4 d. This protocol provides a means by which to follow the redistribution pattern of alpha-MHC within the myocyte in vivo. A uniform distribution of immunofluorescent signal is seen within every myocyte throughout the left ventricle. Ultracryomicrotomy without fixation is used to obtain sections for immunogold-electron microscopy. To quantify the immunogold method the density of gold-labeled antibody per unit of area tissue is determined for various regions of the sarcomere. Tissue from normal and 2-wk baby has a uniform distribution of gold density along the length of the A band. The average gold density of the A band increases with days of thyroid injection from 38 +/- 4 grains/micron 2 (n = 2 animals) (mean +/- SE) at day 1 to 182 +/- 59 grains (n = 2 animals) at day 4. There is a nonuniform incorporation of the newly synthesized alpha-MHC within the A band of thyroid-treated animals since 50% more of the alpha-MHC is found at the end of the A band while the center of the A band has 40% less than the average alpha-MHC content (grains/micron 2, n = 7 animals). These results support a thick filament assembly model that allows every myosin in a thick filament to be exchanged with new myosin. However, in the intact functioning myocyte, there is greater exchange of new myosin at the ends than in the central region of the thick filament.

Isolated muscle cells as a physiological model

Summary of a symposium presented by the American Physiological Society (Cell and General Physiology Section and Muscle Group) at the 70th Annual Meeting of the Federation of American Societies for Experimental Biology, St. Louis, Missouri, April 15, 1986, chaired by M. Lieberman and F. Fay. This symposium reflects a growing interest in seeking new technologies to study the basic physiological and biophysical properties of cardiac, smooth, and skeletal muscle cells. Recognizing that technical and analytical problems associated with multicellular preparations limit the physiological significance of many experiments, investigators have increasingly focused on efforts to isolate single, functional embryonic, and adult muscle cells. Progress in obtaining physiologically relevant preparations has been both rapid and significant even though problems regarding cell purification and viability are not fully resolved. The symposium draws attention to a broad, though incomplete, range of studies using isolated or cultured muscle cells. Based on the following reports, investigators should be convinced that a variety of experiments can be designed with preparations of isolated cells and those in tissue culture to resolve questions about fundamental physiological properties of muscle cells.

Relationship of membrane systems in muscle to isomyosin content

The structures and functions of the various subdivisions of the membrane systems of muscle are reviewed. Morphometric data have been recalculated using functional definitions of the membranes as identified by their proteins. Thus, the junctional coupling between the sarcoplasmic reticulum and T system is separated from the remaining longitudinal sarcoplasmic reticulum that bears the calcium ATPase protein. In addition, the morphometry of the membrane systems is related to the various muscle fiber types as defined histochemically and by protein isoforms. The relation of isomyosin type and membrane quantities are compared for guinea pig, chicken, frog, and lobster skeletal muscles and rat and rabbit cardiac muscles. Fiber plasticity is considered in terms of the mixing and matching of amounts and kinds of membranes and proteins.

Noncardioplegic myocardial preservation

Despite the widespread use of K + cardioplegia, the optimal method of intraoperative myocardial preservation remains controversial. Since 1975, in more than 3,500 patients who have undergone myocardial revascularization at our institutions, intraoperative protection has been provided by a technique of cold-blood perfusion without K + cardioplegia. This report describes our technique, analyzes quantitative ultrastructural changes in four patients before and after bypass to assess cellular damage, and details the clinical and angiographic findings in a subgroup of 157 patients followed postoperatively for up to 6 years. In 100 patients in whom multiple vessels were bypassed, vessels were patent on restudy, thereby excluding technical failure as a cause of impaired left ventricular function. Postoperatively, global left ventricular function was unchanged in 80% and improved in 19%, as compared with preoperative function. Left ventricular function deteriorated in only one patient. We conclude that intraoperative oxygenation by cold-blood perfusion is a successful method of myocardial preservation.

Transmural distribution of isomyosin in rabbit ventricle during maturation examined by immunofluorescence and staining for calcium-activated adenosine triphosphatase

Mammalian ventricle contains two major isomyosins, V1 and V3, which differ in the primary structure of their heavy chains (HC alpha alpha and HC beta beta, respectively) and in their adenosine triphosphatase activity. The distribution of the HC alpha isomyosin in the left ventricle of the rabbit was followed as a function of age and transmural location. HC alpha was detected with a monoclonal antibody found to be specific for the hinge region of V1 myosin molecules when viewed in the electron microscope after low-angle rotary shadowing. Frozen sections were observed with indirect immunofluorescence developed to this anti-HC alpha hinge antibody. Serial sections were observed with the histochemical assay for calcium-activated myosin adenosine triphosphatase, using preincubation at various pH levels. Results show that all the ventricular myocytes in baby rabbits (2 weeks) are stained by the HC alpha-antibody from the epi- to endocardium. The isomyosin content of myocytes varies through the epi- to endocardium of the right ventricular wall of the adult (1-year-old) rabbit, with the HC alpha form predominating in the outer epicardial third of the wall and the lowest amount of HC alpha in the middle third of the wall. A mixture of stained and unstained myocytes is seen in the endo- and subendocardial regions. The spatial distribution of HC alpha in 4-month-old rabbits varies between that of the baby and adult. There is good agreement between myocyte classifications made by histochemical and antibody staining methods.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptability of ultrastructure in the mammalian muscle

All the skeletal muscle fibres taken from an adult mammal do not look alike. The structural differences are a result of adaptations which allow gradations in mechanical output to be achieved. The anatomy is described and the amounts of the subcellular components are measured by stereological techniques from electron micrographs. A population of normal, adult fibres is classified by the Z-line width, by the amounts of the mitochondria, T-system and terminal cisternae (TC), and by the isoforms of contractile proteins present. Classification of fibres by some of these ultrastructural components gives clusters named fast-twitch and slow-twitch types, but classification by other components gives a continuum of overlapping properties. Transformation from the fast- to the slow-twitch type or vice versa follows a specific alteration in the use of the fibre. The mechanical demand on the fibre is modified by changing the frequency of stimulation in the nerve with an implanted electrode. The time course of the changes in subcellular composition in the fibre during adaptation is followed for many weeks. Changes in the membrane systems begin within hours and are complete in days. Changes in the contractile proteins and metabolic systems begin in days and are complete in weeks. During these transitional phases of adaptation the fibres have an unusual complement of components never seen in a normal adult fibre. Extreme alterations, such as myofibril disassembly or supranormal amounts of mitochondria also result during some adaptive transitions. The aberrant appearance in the transitional fibres may be a result of doing the required mechanical work with a less than optimal set of proteins. At the end of the fibre type transformation, the fibre ultrastructure is indistinguishable from normal.

Classification and characterization of cardiac isomyosins

The structural properties of cardiac isomyosins from several species were compared using native gel electrophoresis, analysis of proteolytic digests, analysis of monoclonal antibody reactivity to specific proteolytic fragments on electroblots and S1 nuclease mapping with cDNA probes. The structure of specific regions of the myosin molecule was analyzed by reacting monoclonal antibodies with chymotryptic peptides of myosin separated by two-dimensional electrophoresis. The pattern of fragments reactive with antibody CCM-52 (epitope in LMM) was identical in all types of V3 isomyosin examined, and different in each type of V1 isomyosin. Peptides reactive with RCM-79 (epitope in HMM) were different from those reactive with CCM-52 and were also significantly different in each type of myosin examined. Thus, HC-alpha is structurally similar in the LMM portion of the molecule in all animals examined, while in the HMM region there are significant structural differences. HC-alpha differs from HC-beta, with structural differences in both LMM and HMM. We have also shown that atrial myosin HC and ventricular HC-alpha in the rabbit are indistinguishable both by RIA and peptide mapping analysis. The same conclusion was derived after analysis of the myosin HC mRNA expressed in rabbit atria and ventricles. Using cDNA probes specific for the alpha and beta myosin HC mRNA, we could not distinguish between the atrial myosin mRNA and ventricular HC alpha (V1 isomyosin) mRNA by S1 nuclease mapping experiments. Classification of different cardiac myosins is largely based on their mobility on native gel electrophoresis, immunological cross-reactivity, and ATPase activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle fiber termination at the tendon in the frog's sartorius: a stereological study

The force produced within skeletal muscle fibers is transmitted to the bone via a myotendinous junction. This junctional region was examined by light and electron microscopy in the sartorius muscles of three Rana temporaria. The muscle fibers tapered and inserted at an angle of about 25 degrees with the connective tissue fascia near the bone. The composition of the structures within the last 100 microns of the fiber was analyzed morphometrically. The T-system, terminal cisternae, and caveolae were the same as in the central region of the muscle fiber. However, the mitochondrial content was higher and the volume of longitudinal sarcoplasmic reticulum was lower than elsewhere in the fiber. The membrane at the end of the fiber had extensive villiform processes interdigitating with the tendon. The surface area of the membrane around the villiform processes was estimated with point-counting techniques and calculated from the stereological equations appropriate for partially anisotropic structures. The extra membrane involved in the myotendinous junction was about 32 times that of the cross-sectional area of the fiber. Part of this additional membrane contained specialized adherens junctions through which the contractile proteins of the muscle are anchored to collagen. The increased area at the myotendinous junction presumably provides greater mechanical strength than a flat termination. The high values of membrane capacitance and specific resistance measured electrophysiologically at the end of the fiber also can be attributed to the characteristics of the terminal membrane structure.

Restoration of fast muscle characteristics following cessation of chronic stimulation. The ultrastructure of slow-to-fast transformation

When fast-twitch skeletal muscles of the adult rabbit are subjected to continuous low-frequency activity by electrical stimulation of the corresponding motor nerves, the fibers undergo an ultrastructural transformation, so that after 6 weeks they have acquired an appearance typical of slow-twitch fibers. In the present study, stimulation was discontinued at this stage in order to follow the reverse transformation, in which the fibers recovered their original morphological characteristics under conditions of normal endogenous activity. Stereological techniques were used to assess the time course of this process over a period of 20 weeks in terms of fiber cross-sectional area, extent of T-system, thickness of the Z-band, and volume fraction of mitochondria in the fiber core. Fibers of transformed muscles were smaller than those of control muscles, but the differences were no longer evident after 9 weeks of recovery. After 2 weeks the T-system was still of limited extent, as is characteristic of slow-twitch fibers; it increased toward the amount typical of fast-twitch fibers between 2 and 4 weeks, and had reached its full extent by 12 weeks. The wide Z-bands characteristic of slow-twitch fibers were retained for 4 weeks, but the thickness had begun to decrease by 8 weeks and recovery was complete by 12 weeks. The mitochondrial volume did not increase during recovery, in contrast to the large increases which had been observed to take place between 2 and 6 weeks during the fast-to-slow transformation. Overall, the recovery of fast-twitch ultrastructural characteristics was complete, but followed a more extended time course, and involved less myofibrillar disruption at an intermediate stage, than the original fast-to-slow transformation.

Contributory etiologic factor for talipes equinovarus in congenital myotonic dystrophy: comparative biopsy study of intrinsic foot musculature and vastus lateralis in two cases

Biopsy specimens from the feet and proximal thigh musculature of two patients with congenital myotonic dystrophy with talipes equinovarus were compared. The findings were also compared with those reported in muscle biopsy of idiopathic clubfoot. It was concluded that structural change in intrinsic foot muscle can contribute to the force imbalance that produces clubfoot in this disease.

The ultrastructure of the cardiac Purkinje strand in the dog: a morphometric analysis

Purkinje strands from both ventricles of adult mongrel dogs were excised, and electrical properties were studied by the voltage-clamp technique. The strands were then examined with light and electron microscopy and structural properties were analysed by morphometric techniques. The canine Purkinje strand contains (by volume) about 28% myocyte and 55% dense outer connective tissue. The remainder of the volume is taken up by the inner shell of loosely packed connective tissue within 10 microns of a myocyte membrane. These volume fractions vary considerably from one strand to another. Clefts less than 10 microns wide occupy 18% of the myocyte volume and clefts less than 1 micron wide occupy 1%. The membrane surface area of the myocytes can be divided into three categories by reference to the size of the adjacent cleft. About 47.8% of the membrane surface area faces clefts wider than 1 micron, another 22.2% faces clefts between 0.1 and 1 micron wide, and the final 30% faces clefts less than 0.1 micron wide. The surface area facing the narrowest clefts (less than 0.1 micron wide) is divided between nexuses 3%, desmosomes 10%, and unspecialized membrane 17% (each figure is expressed as a percentage of the total surface area of myocyte membrane). The canine Purkinje strand has a more favourable anatomy than the sheep Purkinje strand for most physiological experiments. We expect that the complicating effects of series resistance and change in the concentration of extracellular ions will be much smaller than in sheep strands, but still not negligible.

The T-SR junction in contracting single skeletal muscle fibers

The junction between the T system and sarcoplasmic reticulum (SR) of frog skeletal muscle was examined in resting and contracting muscles. Pillars, defined as pairs of electron-opaque lines bounding an electron-lucent interior, were seen spanning the gap between T membrane and SR. Feet, defined previously in images of heavily stained preparations, appear with electron-opaque interiors and as such are distinct from the pillars studied here. Amorphous material was often present in the gap between T membrane and SR. Sometimes the amorphous material appeared as a thin line parallel to the membranes; sometimes it seemed loosely organized at the sites where feet have been reported. Resting single fibers contained 39 +/- 14.3 (mean +/- SD; n = 9 fibers) pillars/micrometer2 of tubule membrane. Single fibers, activated by a potassium-rich solution at 4 degrees C, contained 66 +/- 12.9 pillars/micrometer2 (n = 8) but fibers contracting in response to 2 mM caffeine contained 33 +/- 8.6/micrometer2 (n = 5). Pillar formation occurs when fibers are activated electrically, but not when calcium is released directly from the SR; and so we postulate that pillar formation is a step in excitation-contraction coupling.

The reorganization of subcellular structure in muscle undergoing fast-to-slow type transformation. A stereological study

Transformation of fast-twitch into slow-twitch skeletal muscle was induced in adult rabbits by chronic low-frequency stimulation and studied at the ultrastructural level. With the use of stereological techniques, a time course was established for changes in mitochondrial volume, sarcotubular system, and Z-band thickness for periods of stimulation ranging from 6 h to 24 weeks. T-tubules, terminal cisternae, and sarcoplasmic reticulum decreased at an early stage and reached levels typical of slow muscle after only 2 weeks of stimulation. Transformation of Z-band structure took place between 1 1/2 and 3 weeks after the onset of stimulation. Mitochondrial volume increased several fold over the first 3 weeks of stimulation, and fell rapidly after 7 weeks, although it still remained above the levels typical of slow muscle. Although there was no sign of degradation and regeneration of the muscle fibers themselves, considerable structural reorganization was evident at the subcellular level after 1 week of stimulation. The fibers passed through a less well organized transitional stage in which fibers could not be assigned to a normal ultrastructural category. After 3 weeks all of the stimulated fibers could be assigned to the normal slow-twitch category although some subcellular irregularities persisted even after 24 weeks. The ultrastructural alterations are discussed in relation to functional and biochemical changes in the whole muscle.

Disruption of the perineurium in amphibian peripheral nerve: morphology and physiology

Removal of a small piece of perineurium in amphibian nerve causes a lesion consistent with the presence of extensive demyelination. Conduction velocity is reduced by 30 to 40%, and most of the fibers that still conduct are labile, with abnormally low blocking temperatures and unusual susceptibility to conduction block induced by changes in extracellular electrolytes. Application of drugs that inhibit the delayed K+ conductance restores conduction to fibers blocked by temperature elevation and ionic changes. This preparation may prove useful in studies of the pathophysiology of demyelinating diseases.

Intracellular localization of markers within injected or cut frog muscle fibers

Many experimental procedures require drastic alterations of muscle fibers, such as cutting the fiber or injecting molecular probes through microelectrodes. We report the ultrastructure of similarly altered muscle fibers and the intracellular distribution of injected horseradish peroxidase (HRP). Cut fibers appear structurally normal at distances greater than 500 microM from the cut end, however, the structure deteriorates nearer to the cut. HRP diffuses longitudinally about 2,000 micrometer from the cut end and the concentration is uniform over the fiber's cross section. If HRP is introduced intracellularly either by pressure injection or through a nick in the sarcolemma, it distributes in a C-shaped annulus extending approximately 2,000 micrometer longitudinally and 1-20 micrometer radially. The ultrastructure of injected or nicked fibers appears normal. The HRP freely entered the junctional gap between T-system and sarcoplasmic reticulum (SR) but was excluded from either structure. Occasionally, a light pillar could be seen between T-system and SR; the space of these pillars suggest they are the central area of the "feet" appearing light against the dark marker.

Structural changes in single muscle fibers after stimulation at a low frequency

Direct stimulation of single muscle fibers from Xenopus laevis at a frequency of 1 Hz results in a decline of the peak isometric twitch tension after about 200 twitches. Fibers were chemically fixed in glutaraldehyde after a varying number of twitches and at several fatigue levels, and the ultrastructural appearance was compared with that of resting fibers treated by identical fixation methods. No gross structural abnormalities were observed but subtle changes occurred. The mitochondria of stimulated fibers contain granules of normal size and number. The inner crista width is constant but the matrix width is increased on stimulation. These changes would not compromise ATP production. The myofibrils are normal except for a slight swelling in the myosin lattice. The transverse system (T system) and sarcoplasmic reticulum are intact. The minor diameter of the transverse tubule (T tubule) is increased slightly in stimulated fibers. The gap between the T-TC membranes stays constant at about 110 A, but tiny connecting pillars are seen to cross this gap more frequently in stimulated fibers (21 +/- 5% triads) than in resting fibers (8 +/- 6%). In stimulated fibers there is a marked increase in the electron dense content of the terminal cisternae (TC). Inasmuch as the observed structural changes correlate with the number of twitches but not with the fatigue level, it is concluded that TC density and T-TC pillar formation are related to the normal mechanisms of excitation-contraction coupling.

Helicoids in the T system and striations of frog skeletal muscle fibers seen by high voltage electron microscopy

Reconstruction from thick serial transverse slices of frog skeletal muscle fibers stained with peroxidase and examined by high-voltage electron microscopy has revealed that the T system networks at successive sarcomeres are connected together in a helicoidal fashion. From zero to eight helicoids have been found in each of a group of 21 fibers reconstructed in cross section. Helicoids can have either right- or left-handed screw senses, and both senses can be found in one fiber cross section. Because the T system maintains a relatively precise alignment with the myofibrillar striations, it follows that the striations must also have a helicoidal arrangement. This has been found before, but has not been widely accepted in recent times. The presence of helicoids in the bands and membrane networks is not thought per se to alter very much our thinking about excitation and contraction mechanisms in skeletal muscle fibers.

Retrieval of cryostat section for comparison of histochemistry and quantitative electron microscopy in a muscle fiber

A method is presented that can be used to perform histochemical and morphometric analyses on the same muscle fiber. Freshly dissected fibers from medial gastrocnemius muscle of adult guinea pig were kept at a resting length and rapidly frozen. Serial frozen cross-sections were cut and reacted for myofibrillar adenosine triphosphatase and succinic dehydrogenase. The adjacent section, while still frozen, was immersed into 20 degrees C glutaraldehyde fixative to which EGTA was added to minimize artifactious contraction. The fixed section was processed for electron microscopy and the section rotated before thin sectioning to give longitudinal sections enabling study of sarcomeres. Ultrastructure was well-preserved despite slight disorganization of the contractile filaments and some vesiculation of the sarcoplasmic reticulum. The Z line width was measured and the mitochondrial volume fraction estimated by point counting morphometry from 89 fibers. The fibers with dark myofibrillar adenosine triphosphatase staining have Z widths of 547 +/- 165 A (n=69) and thoshosphatase staining have Z widths of 547 +/- 165 A (n=69) and those with light stain have 1023 +/- 113 A (n=20). The density of the succinic dehydrogenase reaction product in the fibers was divided into dark and light and the mitochondrial volume fractions were foud to be 4.3 +/- 2.1% (n=52) and 1.0 +/- 1.1% (n=37), respectively.

Sizes of components in frog skeletal muscle measured by methods of stereology

Stereological techniques of point and intersection counting were used to measure morphological parameters from light and electron micrographs of frog skeletal muscle. Results for sartorius muscle are as follows: myofibrils comprise 83% of fiber volume; their surface to volume ratio is 3.8 mum-1. Mitochondria comprise 1.6% of fiber volume. Transverse tubules comprise 0.32% of fiber volume, and their surface area per volume of fiber is 0.22 mum-1. Terminal cisternae of the sarcoplasmic reticulum comprise 4.1% of fiber volume; their surface area per volume of fiber is 0.54 mum-1. Longitudinal sarcoplasmic reticullum comprises 5.0% of fiber volume, and its surface area per volume of fiber is 1.48 mum-1. Longitudinal bridges between terminal cisternae on either side of a Z disk were observed infrequently; they make up only 0.035% of fiber volume and their surface area per volume of fiber is 0.009 mum-1. T-SR junction occurs over 67% of the surface of transverse tubules and over 27% of the surface of terminal cisternae. The surface to volume ratio of the caveolae is 48 mum-1; caveolae may increase the sarcolemmal surface area by 47%. Essentially the same results were obtained from semitendinosus fibers.

Size changes in single muscle fibers during fixation and embedding

During fixation of single muscles fibers with glutaraldehyde, the volume of the fiber shrinks 20%, recovers in rinse and osmium tetroxide to near normal volume and shrinks 20% again when staining with uranyl acetate. This suggest that osmotic properties of membranes may not have been completely lost during fixation, post-fixation and en bloc staining. Dehydration in ethanol and propylene oxide produces a further 10% shrinkage in volume. Infiltration and embedding with Epon causes an additional 15% change in volume. This gives a total shrinkage in volume of 45% which is nearly twice that of the apparent shrinkage in the volume of the myosin lattice as determined by electron microscopy.

Stereological analysis of mammalian skeletal muscle. I. Soleus muscle of the adult guinea pig

A quantitative analysis of the volumes, surface areas, and dimensions of the ultrastructural components in the soleus muscle fibers of the guinea pig was made by using point counting methods of stereology. Muscle fibers have structural orientation (anisotropy) and have spatial gradients of the structures within the fiber; therefore the standard stereological methods were modified where necessary. The entire analysis was repeated at two section orientations to test the modifications and identical results obtained from both. The volume of lipid droplets was 0.20 +/- 0.06% (mean +/- standard error, n = 5 animals) and the nuclei volume was 0.86 +/- 0.20% of the fiber volume. The total mitochondrial volume was 4.85 +/- 0.66% of the fiber volume with about one-third being found in an annulus within 1 microm of the sarcolemma. The mitochondrial volume in the remaining core of the fiber was 3.6 +/- 0.4%. The T system has a volume of 0.14 +/- 0.01% and a surface area of 0.064 +/- 0.005 microm(2)/microm(3) of the fiber volume. The surface area of the sarcolemma is 0.116 +/- 0.013 microm(2)/microm(3) which is twice the T system surface area. The volume of the entire sarcoplasmic reticulum is 3.52 +/- 0.33% and the surface area is 0.97 +/- 0.09 microm(2)/microm(3). The sarcoplasmic reticulum is composed of the terminal cisternae whose volume is 1.04 +/- 0.19% and surface area is 0.24 +/- 0.05 microm(2)/microm(3). The tubules of the sarcoplasmic reticulum in the I band and A band have volumes of 1.97 +/- 0.24% and 0.51 +/- 0.08%, and the surface areas of the I and A band reticulum are 0.56 +/- 0.07 microm(2)/microm(3) and 0.16 +/- 0.04 microm(2)/microm(3), respectively. The Z line width, myofibril and fiber diameters were measured.