Changes in myosin light chains in the rat soleus after thyroidectomy

Hormonal control of myosin heavy chain genes during development of skeletal muscles

A pattern of myosin heavy chain (MHC) switching is a hallmark of developing muscles. Factors responsible for these changes in gene expression include endogenous signals, motoneurons and hormones, especially thyroid hormones. After perturbing the innervation and/or thyroid hormone levels we have examined the neonatal-IIb MHC transition during rat development. First, denervation does not qualitatively affect the transition at either the transcriptional or translational level. Second, hypothyroidism prevents the appearance of IIb MHC and its mRNA in the innervated limb; in the denervated hypothyroid limb IIb MHC is synthesized at moderately high levels. Third, hyperthyroidism causes a precocious increase in IIb MHC in both innervated and denervated muscles. These results suggest that the transition from neonatal to adult IIb myosin synthesis is endogenously programmed during development, but is closely orchestrated by the changing neuronal and hormonal status of the animal. Thyroid hormone may exert its influence by effects both on the muscle fibre and on the developing motoneuron. In the guinea-pig the temporalis muscle is sexually dimorphic: it contains a fast-red MHC in the female but a fast-white MHC in the male. This dimorphism has been shown to be mediated by testosterone, since the castrated male synthesizes the fast-red MHC while the testosterone-supplemented female contains the fast-white MHC. During development male and female muscles initially synthesize the fast-red isoform. The male switches to the fast-white form at puberty.

Myopathy as the persistently isolated symptomatology of primary autoimmune hypothyroidism

Although disorders of thyroid function may cause a wide range of muscle disturbances, an overt myopathy has been rarely reported as an isolated clinical presentation of hypothyroidism. We observed 10 patients (5 males and 5 females) who had been referred to the department of neurology because of muscular fatigability, myalgia, cramps, or proximal weakness. Laboratory investigation showed that all patients had hypothyroidism due to Hashimoto's thyroiditis (atrophic variant in 9/10). Classic symptoms/signs of hypothyroidism such as lethargy, constipation, cold intolerance, myxedematous facies, and/or bradycardia were absent, as assessed independently by the three coauthoring thyroidologists. Muscular complaints improved greatly and then disappeared after substitutive levothyroxine treatment. Muscle biopsy revealed nonspecific changes. Nicotinamide adenine dinucleotide reductase (NADH-TR)-hyporeactive cores were present in two patients (10% and 90% of type 1 fibers). On electron microscopy, the core areas showed disorganized myofibrils, Z-band streaming, rod formation, and paucity of mitochondria and glycogen granules. Desmin intermediate filaments were overexpressed only in some cores. The similarity of the pattern of desmin expression between hypothyroid cores and target lesions of denervated fibers supports the hypothesis that, at least in some of our patients, myopathy was the result of an impaired nerve-mediated action of thyroid hormones on skeletal muscle. Our observations suggest that an isolated myopathy as the sole manifestation of hypothyroidism is not a rare event. We postulate that our cases may constitute a peculiar subgroup of Hashimoto's thyroiditis patients: (1) the strikingly abnormal F/M ratio of 1:1; (2) the relatively younger age; (3) the rarity of the goitrous variant; (4) the unusual finding of antithyroglobulin (Tg-Ab) > antithyroid peroxidase (TPO-Ab). Thorough evaluation of thyroid function is appropriate in patients with myopathy of uncertain origin.

Developmental modulation of myosin expression by thyroid hormone in avian skeletal muscle

It is well established that a rise in circulating thyroid hormone during the second half of chick embryo development significantly influences muscle weight gain and bone growth. We studied thyroid influence on differentiation in slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscles of embryos rendered hypothyroid by hypophysectomy or administration of an anti-thyroid drug. The expression of native myosins and myosin light chains (MLCs) was studied by electrophoretic analysis, and the myosin heavy chain (MHC) was characterized by immunohistochemistry. The first effects of hypothyroid status were observed at day 21 of embryonic development (stage 46 according to Hamburger and Hamilton). Analysis of myosin isoform expression in PLD muscles of hypothyroid embryos showed persistence of slow migrating native myosins and slow MLCs as well as inhibition of neonatal fast MHC expression, indicating retarded differentiation of this muscle. In ALD muscle, hypothyroidism maintained fast embryonic MHC and induced noticeable amounts of fast MLCs, thus delaying slow muscle differentiation. Our results suggest that thyroid hormones play a role in modulating the appearance of neonatal fast MHC and the disappearance of isomyosins transiently present during embryogenesis. However, T3 supplemental treatment would seem to compensate in part for the effects of hypothyroidism induced by hypophysectomy, suggesting that thyroid hormone might interfere with other factors also accounting for the observed effects.

The time course of thyroid-hormone-induced changes in the isotonic and isometric properties of rat soleus muscle

Rat thyroidectomy resulted in changes in a number of parameters used to characterise the mechanical and histochemical status of skeletal muscle. Thus thyroidectomy resulted in a prolongation of soleus slow-twitch muscle isometric contraction time and half-relaxation time with a reduced maximum velocity of shortening and maximum rate of development of tetanic tension but no significant change in twitch: tetanus ratio i.e. the ratio of twitch force/unit area to tetanic force/unit area. In addition the percentage of IIA fibres was reduced and the percentage of type I fibres increased. Triiodothyronine, administered to hypothyroid rats, brought about a speeding of these parameters again with no change in twitch: tetanus ratio. There was an increase in the percentage of IIA fibres with a concomitant reduction in the percentage of type I fibres. These changes were induced over 18 days and resulted in isotonic and isometric properties almost identical to those of soleus muscles from chronically hyperthyroid rats; speeding could be detected as early as 2 days after triiodothyronine had been given. Consideration is given to the possibility that changes in myosin isoforms and/or the kinetics of changes in intracellular calcium concentration in activation and relaxation could account for the time course of the observed changes.

Influence of hyperthyroidism on maximal shortening velocity and myosin isoform distribution in skeletal muscles

The objectives of this study were 1) to examine the effects of hyperthyroidism on the myosin isoform distribution in slow and fast skeletal muscle, 2) to explore how these effects were manifested with respect to the force-velocity relationship and maximal shortening velocity, and 3) to contrast two different techniques of measuring maximal shortening velocity under normal and hyperthyroid conditions. Adult female Sprague-Dawley rats were randomly assigned to one of two groups: control (n = 8) or hyperthyroid (n = 8). Hyperthyroidism was induced by injections of 3,3',5-triiodo-L-thyronine every other day for 20 wk. We found that hyperthyroidism produced a significant shift in the myosin isoform distribution of the soleus but not the plantaris. The relative amount of the slow myosin isoform was reduced from a control value of 93 to 69% in the hyperthyroid condition. In contrast, both the intermediate and fast myosin-3 isoform pools were substantially increased (P less than 0.001) by approximately fourfold. Hyperthyroidism produced an increase in the maximal shortening velocity of the soleus as measured either by the slack test (+57%; P less than 0.001) or by extrapolation of force-velocity data (+33%; P less than 0.001). The hyperthyroid condition did not, however, affect the mechanical properties of the plantaris.

Neonatal hyperthyroidism enhances growth of the heart but not of skeletal muscle

Comparison of growth and biochemical development of the heart and skeletal muscle (gastrocnemius) in normal rats and those treated with triiodothyronine (T3) (0.1 micrograms/g bw, s.c.) daily, from birth until weaning (day 25), revealed that hyperthyroidism selectively enhanced cardiac growth as shown by marked increases in total content (per heart) of DNA (25%), RNA (22%), protein (31%), and myofibrillar protein (30%), as well as the activity of myosin ATPase (11-20%). On the contrary, the above parameters were significantly decreased in the skeletal muscle of hyperthyroid rats as was body weight. Termination of T3 treatment on day 25 led to resumption of rapid body growth which was not complete even by day 90. Cardiac growth decelerated in rehabilitating rats, however, resulting in final reduction of all growth parameters, except DNA content which was equal in 90-day control and rehabilitated animals. Muscle growth in terms of DNA and RNA content failed to respond to rehabilitation and continued to be lower than control animals while protein synthesis was relatively enhanced. It is concluded that neonatal hyperthyroidism markedly enhanced growth of the heart but not of skeletal muscle. This condition not only inhibited growth of the muscle during the T3 treatment period but also led to permanently reduced DNA content and other growth parameters even two months after discontinuation of treatment.

Induction and stability of the adult myosin phenotype in striated muscles of dwarf mice after chronic thyroid hormone treatment

It is known that a deficiency in thyroid hormone delays the post-natal maturation of several mammalian tissues. In striated muscle tissue, hypothyroidism delays or inhibits some of the isoform transitions of myosin heavy chains which would occur during normal development. In this paper, using the mouse mutant dwarf, we demonstrate an influence of thyroid hormone on expression of the myosin phenotype in cardiac and skeletal muscle of dwarf mice. Myosin isoforms were identified by gel electrophoresis of native myosin, localised within muscle cells by indirect immunofluorescence and quantified using an ELISA technique. We show that an adult phenotype can be established in both cardiac and skeletal muscle following a treatment involving multiple injections of thyroxine although cardiac muscle responds more rapidly. The skeletal myosin phenotype remains stable until at least five weeks after the last injection. In contrast, the fetal form of cardiac myosin reaccumulates upon cessation of thyroxine treatment. Thus, cardiac and skeletal muscles are not only affected differently by the dwarf mutation but also they respond differently to thyroxine treatment and thyroxine withdrawal.

Myopathies associated with hypothyroidism: a review based upon 13 cases

The clinical and myopathological features of 13 patients with a myopathy occurring in association with hypothyroidism are presented. Seven patients had hypothyroid myopathy, including two with the Hoffmann syndrome and one with the Kocher-Debre-Semelaigne syndrome. Five patients had an inflammatory myopathy and one had polymyalgia rheumatica. Serum CK activity was elevated up to 12-fold in the patients with hypothyroid myopathy and returned to normal after treatment with thyroxine. Pathological changes in these cases included type 1 or type 2 fibre atrophy or hypertrophy, myofibre necrosis and regeneration in four cases and, in one case, prominent core-like areas containing amorphous granulo-filamentous material. The findings in this series of cases illustrate the clinical and histopathological heterogeneity of patients with hypothyroid myopathy and the need to consider other myopathies in hypothyroid patients who present with muscular symptoms.

Analysis of the Ca2+-binding parvalbumin in rat skeletal muscles of different thyroid states

The Ca2+-binding parvalbumin (PV) is possibly involved in the relaxation of fast-twitch muscle fibers and believed to be a marker for early muscular disturbances. The muscular content of parvalbumin has been shown to change with alterations of the relaxation speed that follow an experimentally changed nervous input. In hypo- and hyperthyroidism isometric twitch contraction and half-relaxation times are also altered, namely increased in hypothyroidism and decreased in hyperthyroidism. These changes are largely paralleled by modifications in the fiber type composition. Therefore we investigated the distribution and concentration of parvalbumin in extensor digitorum longus, soleus, and gastrocnemius muscles of rats by immunohistochemical and biochemical methods. The combined results of both procedures showed that parvalbumin distribution and concentration were largely unaffected in all thyroid states. This suggests that the expression of parvalbumin is neuronally controlled and not by thyroid hormones. Additionally our findings support the view that the changes in physiologic properties and fiber type composition are generated by a direct action of thyroid hormone on muscle fibers, and not via their nervous input.

Hyperthyroidism selectively increases oxidative metabolism of slow-oxidative motor units

The effects of thyroid hormone on the NADH-tetrazolium reductase activity (oxidative metabolism marker) of soleus (slow-oxidative) and tensor fascia lata (fast-glycolytic) motoneurons were determined and compared with changes in a variety of enzyme activities in the corresponding muscle fibers. Histochemical assays have demonstrated a selective and qualitative conversion in muscle fiber ATPase and quantitative increases of NADH-tetrazolium reductase (oxidative) and mitochondrial alpha-glycerophosphate dehydrogenase (glycolytic) activities in the soleus muscle. Paralleling the selective action upon the soleus slow muscle fibers was a selective central nervous system effect of thyroid hormone on oxidative enzymes of soleus slow-oxidative motoneurons. This indicates that either thyroid hormones act directly and specifically on slow motoneurons or that conversion of the muscle fibers by thyroid hormones produces secondary changes in the motoneuron. These data strengthen the hypothesis that oxidative enzyme activities in motoneurons are tightly matched with oxidative enzyme activities in muscle fibers.

Effect of hyperthyroidism on fibre-type composition, fibre area, glycogen content and enzyme activity in human skeletal muscle

Seven hyperthyroid patients were studied by repeated muscle biopsies (vastus lateralis) before and after a period of medical treatment which averaged 10 months. The biopsies were analysed with regard to fibre-type composition, fibre area, capillary density, glycogen content and enzyme activities representing the glycolytic capacity (hexokinase, 6-phosphofructokinase), oxidative capacity (oxoglutarate dehydrogenase, citrate synthase) and Ca2+- and Mg2+-stimulated ATPase in muscle. In the pretreatment biopsy (hyperthyroid state), there was a significantly lower proportion of type I fibres (30% vs. 41%), a higher capillary density (23%), lower glycogen content (33%), and higher hexokinase activity (32%) compared with the post-treatment biopsy. No significant changes in the activity of the remaining enzymes were observed. The present study indicates that hyperthyroidism induces a transformation from type I to type II fibres in human skeletal muscle. The increase in hexokinase activity probably reflects a higher glucose utilization by skeletal muscle in order to compensate partially for the reduced glycogen content.

Reflective densitometry of Western blots to quantitate the developmentally regulated accumulation of myosin light chain 3

We have employed a monoclonal antibody to fast myosin alkali light chains to study the accumulation of myosin light chain 3 (MLC3f) in the breast and limb musculature of developing quail embryos using quantitative densitometry of Western blots. Our analyses reveal that MLC3f is first detected in the breast muscle of 11 day embryos and accumulates at a constant rate until hatching at day 16. This data suggests, by extrapolation, that MLC3f accumulation is initiated at day 10 in embryonic breast muscle. MLC3f is also first detected in the limb muscle of 11 day embryos, but does not accumulate rapidly until after day 13. These results demonstrate the effective use of reflective densitometry in the study of developmental problems and in the quantitation of Western blots in general.

The effect of beta-adrenergic blockade on the myopathic changes in experimental hyperthyroidism in rats

Hyperthyroidism is frequently associated with myopathy. Beta-blocking agents, which are increasingly used in the treatment of hyperthyroidism, have been reported to have a possible beneficial effect on thyrotoxic myopathy. We have studied the myopathic changes in experimental hyperthyroidism in rats and the effect of the beta-blocking agents metoprolol and propranolol on these changes. During experimental hyperthyroidism, conversion of type 1 to type 2 fibres and accumulation of intracellular lipid, increased amounts of mitochondria, and reduced size of muscle fibers were noticed. None of these changes were prevented by metoprolol or propranolol. The results indicate that histochemical and morphological changes in skeletal muscle induced by thyroid hormone are not mediated by beta-receptors. The results of this investigation make dubious a beneficial effect of beta-blocking agents on thyrotoxic myopathy.

Myosin transitions in chronic stimulation do not involve embryonic isozymes

Chronic low-frequency stimulation of a fast skeletal muscle effects a transition of myosin isozymes from those characteristic of a fast muscle to those characteristic of a slow muscle. This transformation involves changes in both myosin heavy and light chains. During development, muscles usually change directly from embryonic to neonatal to fast or from embryonic to neonatal to slow isozymes. We have questioned whether chronic stimulation of the adult fast muscle results in a direct fast-to-slow isozyme shift or whether transformation requires reexpression of the developmental isozymes prior to the synthesis of adult slow isozymes. We have examined these alternatives in the chronically stimulated dog diaphragm using adenosine triphosphatase (ATPase) histochemistry, pyrophosphate gel electrophoresis of native isozymes, peptide mapping of myosin heavy chains, immunoblotting with an antibody specific to embryonic myosin heavy chain, and solid-phase radioimmunoassay. We have demonstrated that a transition from adult fast to adult slow isozymes during chronic stimulation does not involve a recapitulation of embryonic isozymes.

Postnatal histochemical fiber type differentiation in normal and hypothyroid rat soleus muscle

We have analyzed the effect of 6-propylthiouracil-induced hypothyroidism on neonatal fiber type differentiation in the rat soleus muscle. Body weight, total soleus fiber number, histochemical fiber type differentiation, and morphometry were determined at 7, 14, 21, and 28 days. Neonatal hypothyroidism (1) inhibits the apparent approximately 50% increase in soleus muscle fiber number occurring at 14-21 days, (2) blocks the transformation of type 2C to type 2A fibers occurring between 14 and 21 days, (3) preferentially inhibits the increase in type 2 fiber diameter, and (4) retards the development of sarcoplasmic reticulum. Immature muscle fibers reveal type 1 and type 2 fiber myosin adenosine triphosphatase (ATPase) differentiation at pH 10.3, which drops to pH 9.4 with maturation. No myosin ATPase differentiation was found at pH 9.4 in the hypothyroid animals, even at 28 days.

Muscular abnormalities in experimental hypothyroidism of rats with special reference to the mounding phenomenon

The mounding phenomenon was examined in vitro on the excised extensor digitorum longus or soleus muscle of hypothyroid rats. The mounding phenomenon was more prominent and lasted longer in hypothyroid muscles than in controls. In single skinned muscle fibers of both type 1 and type 2 from hypothyroid animals, the maximum tension produced by Ca ion was reduced and activity of Ca uptake by the sarcoplasmic reticulum was also decreased, judging from the response to caffeine. Decreased Ca uptake might retard the relaxation of contracture, thereby rendering the mounding more prominent. Morphological studies of both extensor digitorum longus and soleus muscles showed an increase of type 1 muscle fiber population as well as mild changes in arrangement of the internal membrane system. However, there was no muscle necrosis or other derangements. These results supported the idea that functional abnormality of the sarcoplasmic reticulum could be the basis of the exaggeration of the mounding phenomenon in hypothyroidism.

Muscle development in thyroidectomised chickens (Gallus domesticus)

The metabolic and contractile activity of muscle was determined in immature cockerels made hypothyroid by surgical thyroidectomy at 6 weeks of age. Four weeks after thyroidectomy the activity of Mg2+-activated myofibrillar ATPase and total phosphorylase was reduced in the fast-phasic, posterior latissimus dorsi (PLD) and scapulotriceps (ST) muscles. The activities of these enzymes were unaffected in the slow-tonic, anterior latissimus dorsi (ALD) muscle. Thyroidectomy had no effect on length of the muscles studied but reduced the weight of the ALD and ST muscles. These results suggest that hypothyroidism results in a "slowing down" of fast-phasic muscles, although it does not affect the activity of slow-tonic muscles.

Loss of fast-twitch isomyosins in skeletal muscles of the diabetic rat

By means of pyrophosphate electrophoresis the myosin isoenzyme pattern of two fast-twitch skeletal muscles (extensor digitorum longus, gastrocnemius) and one slow-twitch muscle (soleus) was investigated in control rats and was compared with that of rats 4 weeks after induction of diabetes mellitus by streptozotocin injection. In the fast-twitch muscles the isomyosin pattern consisting of FM1 (fast isomyosin 1), FM2 and FM3 was strongly affected by diabetes, resulting in an extensive loss of FM1 and a substantial decrease of FM2. These changes were also apparent when the light chains of the fast isomyosins were analysed by two-dimensional electrophoresis: LC3f (myosin light chain 3f) largely disappeared and LC2f was significantly diminished. In contrast, the isomyosin pattern in soleus muscle, consisting of SM1 (slow isomyosin 1) and SM2, was not affected by the diabetic state, and two-dimensional electrophoresis revealed a normal light-chain pattern of LC1sa, LC1sb and LC2s. These results indicate that the isomyosins of slow-twitch oxidative myofibres are more resistant to the hormonal and metabolic disorders during diabetes mellitus than are the isomyosins of fast-twitch fibres.

Effects of hypothyroidism on myosin isozyme transitions in developing rat muscle

Hypothyroidism was induced in young rats by methylthiouracil treatment of pregnant mothers from 18 days of gestation to 4 weeks after birth. Electrophoretic analysis of native myosin isozymes revealed a persistence of neonatal and embryonic myosin in developing fast and slow muscles up to at least 28 days after birth. The appearance of adult fast myosin was inhibited in 28-day old animals, however adult slow myosin was found in the soleus muscle. Immunocytochemical results on the soleus demonstrate a cellular heterogeneity in the response to hypothyroidism. About half fibers have a normal complement of slow myosin and do not contain neonatal myosin. Only the remaining fibers contain the large amounts of neonatal myosin demonstrated by electrophoresis.

Sexual dimorphism in the fibers of a "clasp" muscle of Xenopus laevis

When antibodies specific to fast-twitch, slow-twitch, and slow-tonic myosins were used to stain the clasp muscle, m. sternoradialis, of Xenopus laevis, three predominant fiber types were identified in both males and females. Three fiber types can also be distinguished by the diameters and conduction velocities of their motor nerves. Significant differences in the numbers of slow-tonic fibers were identified between the genders and between control and castrated male animals. This finding suggests that these slow-tonic fibers, which probably dominate the clasp reflex during mating, may be under hormonal control.

Denervation of newborn rat muscle does not block the appearance of adult fast myosin heavy chain

Several observations, both in vivo and in vitro, have indicated that the development and maturation of mammalian skeletal muscle fibres is influenced by nerve-muscle interactions. Morphological maturation of newly regenerated adult mouse muscle fibers in an organotypic nerve-muscle culture system depends on the presence of spinal cord neurones. Sciatic nerve transection in newborn rats has been shown to modify the development of the histochemical and contractile properties of the denervated muscles. In addition, neural influences are important for the appearance of certain of the myosin small subunits. It has been proposed that the nerve also controls the changes in myosin heavy chain isozymes appearing during development. One such transition occurs in rat muscle where the neonatal form of myosin heavy chain is replaced by the adult form during the second post-natal week. Here we demonstrate that innervation of the rat gastrocnemius muscle (a fast-contracting muscle in the adult) is not required for the appearance of the adult form of myosin heavy chain.

Effect of thyrotoxicosis on sarcoplasmic reticulum in rat skeletal muscle

The effect of thyrotoxicosis on the capacity of fragmented sarcoplasmic reticulum (FSR) and crude homogenate (CH) to sequester Ca2+ was determined in rat muscle for the slow-twitch type I soleus (SOL), the fast-twitch type IIA deep region of the vastus lateralis (DVL),and the fast-twitch type IIB superficial region of the vastus lateralis (SVL). The maximal rate of Ca2+ uptake (Vmax) and Km were determined in both the CH and FSR preparations, and the total Ca2+ uptake capacity of the CH was determined. In the slow SOL, thyrotoxicosis increased the Vmax (8.20 +/- 0.96 vs. 15.70 +/- 0.92 mumol Ca2+ . g wet muscle-1 . min-1) and the total Ca2+ uptake (17.62 +/- 1.30 vs. 27.13 +/- 2.16 mumol Ca2+ . g wet muscle-1) of the CH preparation. Thyrotoxicosis increased the FSR yield 2.3-fold in the slow-twitch SOL; however, the kinetic characteristics (Vmax and Km) of these vesicles were not altered. Thyrotoxicosis had no effect on the CH and FSR preparations in either the type IIA or type IIB sample. These results can be explained by a thyroid hormone-mediated increase in the quantity of the sarcoplasmic reticulum in type I muscle and suggest no effect on the hormone on the qualitative nature of the Ca2+-enzyme interaction.

Composition and muscle cellularity of Japanese Quail after selection for high body weight under an optimal or suboptimal nutritional environment

The purpose of this study was to determine if the compositional and cellularity changes associated with selection for rapid growth rate in an optimal and suboptimal nutritional environment were similar. The P-line Japanese quail had undergone long-term selection for high 4-week body weight and received a 28% protein diet. The unselected C-line quail received the same diet. T-line quail were selected by the same criteria as P-line quail but received a 20% protein diet containing .2% thiouracil. Body weights and muscle weights of P- and T-line quail were similar at 10 days of age. At 56 days T-line body weight and muscle weights were intermediate between C- and P-line quail, but whole body composition of P- and T-line quail was similar. The P- and T-line quail possessed a greater apparent number of muscle fibers than C line quail. Total muscle DNA content was lower in T-line muscles compared to P-line muscles at 56 days of age. No differences within age were observed among lines of myofiber diameter, fiber type composition, for concentrations of DNA, RNA and protein, or for protein/DNA and RNA/DNA. At 10 days the weight, length and diameter of the humerus and femur bones were greatest in T-line quail; these same parameters were intermediate between C- and P-line quail at 28 and 56 days of age. It was concluded that selection for rapid growth rate per se caused apparent fiber number and fiber length to increase while the effect of suboptimal nutritional environment was not evident until after 10 days of age. Selection under suboptimal conditions as exercised in this study prevented expression of the maximum number of muscle DNA units.

Effect of hyperthyroidism on the contractile and histochemical properties of fast and slow twitch skeletal muscle in the rat

Male Wistar rats were made hyperthyroid by intraperitoneal injections on alternate days for 1-6 weeks, of 200 micrograms/kg triiodothyronine (T3). The effects of this treatment on the contractile properties of the soleus, a slow twitch and the extensor digitorum longus (EDL), a fast twitch skeletal muscle, were studied in vivo in the anaesthetized animal. Post mortem, serial frozen sections of both muscles were stained histochemically for myosin ATPase, succinic dehydrogenase and phosphorylase. Muscle fibres were classified as either slow twitch (SO), fast twitch oxidative glycolytic (FOG) or fast twitch glycolytic (FG). Elevation of plasma T3 levels is associated with progressive alterations in the muscle fibre populations of both muscles. In the soleus there is conversion of SO to FOG fibres while in the EDL the main effect is FG to FOG conversion. There are also marked changes, mainly confined to the soleus muscle, in contractile properties; progressive increases in isometric twitch and tetanic tension and in the rates of contraction and relaxation during both twitch and tetanus. The effect of T3 on slow muscle contractility may be related to its effect on muscle phenotype. However, changes in the former precede detectable fibre population changes. T3 may influence properties such as the CA2+ binding activity of sarcoplasmic reticulum of existing slow twitch fibres before the later changes associated with the interconversion of fibre types occur.