Effect of alterations in the thyroid state on the intrinsic contractile properties of isolated rat skeletal muscle

Dysphagia as a primary manifestation of hyperthyroidism: a case report

Myopathy effecting mainly skeletal muscles of the limbs are frequently seen in hyperthyroidism. Rarely bulbar muscles may also be involved, causing dysphagia, nasal speech, and aspiration. We report a 70-year-old woman with severe dysphagia and aspiration pneumonia. Clinical examination and laboratory tests showed an underlying Graves' disease. Her dysphagia improved dramatically by antithyroid therapy. Considering its excellent response to medical therapy, hyperthyroidism--being a very rare factor - is well-worth to remember for the unexplained dysphagia cases.

Disfunção muscular esquelética e composição corporal no hipertireoidismo

O objetivo deste artigo é revisar os aspectos da disfunção muscular esquelética e composição corporal no hipertireoidismo. O hipertireoidismo está associado a uma fraqueza muscular generalizada que é parte da manifestação clínica inicial de cerca de 80% dos pacientes, comprometendo a realização de tarefas cotidianas e a qualidade de vida. Um fator que contribui para a redução da força é a atrofia muscular, que tende a afetar mais comumente os grupos musculares proximais. Além disso, o hipertireoidismo é acompanhado de perda ponderal associada à depleção de massa muscular e tecido adiposo. Estudos demonstram que o tratamento medicamentoso é capaz de recuperar a força e mais lentamente a resistência, definida como a capacidade de sustentar cargas submáximas por períodos prolongados, e que o treinamento contra resistência associado ao tratamento medicamentoso, é capaz de promover um maior ganho de força e de resistência muscular nestes pacientes. Embora não tenha sido estabelecido um padrão de composição corporal na recuperação do peso após o tratamento da doença, sabe-se que pacientes submetidos ao treinamento de força apresentam recuperação de peso acompanhado principalmente de ganho de massa muscular.

Role of thyroid hormones in early postnatal development of skeletal muscle and its implications for undernutrition

Energy intake profoundly influences many endocrine axes which in turn play a central role in development. The specific influence of a short period of mild hypothyroidism, similar to that induced by undernutrition, in regulating muscle development has been assessed in a large mammal during early postnatal life. Hypothyroidism was induced by providing methimazole and iopanoic acid in the feed of piglets between 4 and 14 d of age, and controls were pair-fed to the energy intake of their hypothyroid littermates. Thyroid status was evaluated, and myofibre differentiation and cation pump concentrations were then assessed in the following functionally distinct muscles: longissimus dorsi (l. dorsi), soleus and rhomboideus. Reductions in plasma concentrations of thyroxine (T4; 32%, P < 0.01), triiodothyronine (T3; 48%, P < 0.001), free T3 (58%, P < 0.001) and hepatic 5'-monodeiodinase (EC 1.11.1.8) activity (74%, P < 0.001) occurred with treatment. Small, although significant, increases in the proportion of type I slow-twitch oxidative fibres occurred with mild hypothyroidism, in l. dorsi (2%, P < 0.01) and soleus (7%, P < 0.01). Nuclear T3-receptor concentration in l. dorsi of hypothyroid animals compared with controls increased by 46% (P < 0.001), a response that may represent a homeostatic mechanism making muscle more sensitive to low levels of circulating thyroid hormones. Nevertheless, Na+, K(+)-ATPase (EC 3.6.1.37) concentration was reduced by 15-16% in all muscles (l. dorsi P < 0.05, soleus P < 0.001, rhomboideus P < 0.05), and Ca(2+)-ATPase (EC 3.6.1.38) concentration was significantly reduced in the two slow-twitch muscles: by 22% in rhomboideus (P < 0.001) and 23% in soleus (P < 0.05). It is concluded that during early postnatal development of large mammals a period of mild hypothyroidism, comparable with that found during undernutrition, induces changes in myofibre differentiation and a down-regulation of cation pumps in skeletal muscle. Such changes would result in slowness of movement and muscle weakness, and also reduce ATP hydrolysis with a concomitant improvement in energetic efficiency.

Thyroid status and exercise tolerance. Cardiovascular and metabolic considerations

Both hypo- and hyperthyroidism are characterised by exercise intolerance. In hypothyroidism, inadequate cardiovascular support appears to be the principal factor involved. Insufficient skeletal muscle blood flow compromises exercise capacity via reduced oxygen delivery, and endurance through decreased delivery of blood-borne substrates. The latter effect results in increased dependence on intramuscular glycogen. Additionally, decreased mobilisation of free fatty acids from adipose tissue and, consequently, lower plasma free fatty acid levels compound the problem of reduced lipid delivery to active skeletal muscle in the hypothyroid state. In contrast, cardiovascular support is enhanced in hyperthyroidism, implicating other factors in exercise tolerance. Greater reliance on muscle glycogen appears to be the primary reason for decreased endurance. Biochemical changes with hyperthyroidism that would favour enhanced flux through glycolysis may account for this dependence on glycogen. Deviations from normal thyroid function, and the ensuing exercise tolerance, require appropriate medical therapy to attain euthyroid status.

Response of fast muscle innervation to hypothyroidism

The early period of motor innervation development is characterized by multiple innervation of muscle cells. This transitory state in rat extensor digitorum longus (edl) muscle is normally concluded at weaning when a 1:1 ratio between nerve endings and muscle cells is reached. Motor innervation of edl muscle in rats made hypothyroid after weaning was studied in three ways: electrophysiology (intracellular recordings of muscle postsynaptic potentials) was carried out to study neuromuscular transmission; silver impregnation of terminal axons to observe sprouting; force production in twitch and tetanus following direct muscle stimulation and nerve stimulation. A number of multiply innervated muscle cells was found in hypothyroid rats following two months of treatment. This finding seems to be related to the appearance of nodal sprouting in motor axons. No sign of denervated end-plates was found. Twitch and tetanus tension were smaller than in controls, but they were bigger when referred to unitary muscle mass. Time course of twitch, particularly half relaxation, was slowed in muscles of hypothyroid rats. These findings suggest that plastic processes occur in muscle innervation of rats made hypothyroid after weaning. Therefore, thyroid hormones play a role in stabilizing motor innervation not only during development, but also in adults.

Rabbit skeletal muscle sarcoplasmic reticulum Ca(2+)-ATPase activity: stimulation in vitro by thyroid hormone analogues and bipyridines

Sarcoplasmic reticulum-enriched membranes from rabbit skeletal muscle contained Ca(2+)-ATPase activity which was significantly enhanced (26% increase, P < 0.001) in vitro by physiological concentrations (10(-10) M) of L-thyroxine (T4) and 3,3',5-triiodo-L-thyronine (T3). In contrast, the biologically inactive iodothyronine analogues D-T4 and 3,3',5,5'-tetraiodothyroacetic acid (Tetrac) (10(-10) M) were without effect on enzyme activity. 3,5-Dimethyl-3'-isopropyl-L-thyronine (Dimit), a bioactive analogue, was highly effective as a Ca(2+)-ATPase stimulator, increasing enzyme activity by 43% (P < 0.02 vs. T4 effect). A bipyridine cardiac inotropic agent, milrinone, has been reported to be thyromimetic in a myocardial membrane Ca(2+)-ATPase system, and in concentrations from 10(-10) to 10(-5) M enhanced skeletal muscle SR membrane Ca(2+)-ATPase activity in vitro (P < 0.001). Milrinone analogues which have been previously shown to enhance rabbit myocardial membrane Ca(2+)-ATPase activity, and which have a twist relationship of the pyridine rings, were also striated muscle Ca(2+)-ATPase stimulators. We conclude that (1) striated muscle is a mammalian tissue in which physiological levels of biologically relevant thyroid hormone analogues, particularly Dimit, stimulate Ca(2+)-ATPase activity in vitro by a non-genomic mechanism; (2) cardiac bipyridine analogues which are thyromimetic in vitro in rabbit heart, and which have structural homologies with thyroid hormone, are stimulators of rabbit striated muscle sarcoplasmic reticulum Ca(2+)-ATPase activity.

Respiratory muscles in endocrinopathies

The aim of this review was to demonstrate that RM function is altered in various endocrinopathies and that RM weakness is a common finding. RM function has been well-studied in diseases such as thyroid dysfunction, and steroid induced RM myopathies. Less well documented reports on RM function were found in parathyroid dysfunctions, disorders of mineralocorticoids and pituitary disturbances. Controversial reports were found in diabetes mellitus. No report was found connecting RM function with androgens, pheochromocytoma or adrenaline deficiency in humans. These diseases could potentially cause RM impairment leading to severe respiratory failure (pump failure) putting life in great danger. Therefore, it is obvious that further studies are needed to investigate the performance of RMs in endocrinopathies. Such studies are extremely urgent in Cushing's and Addison's disease, acromegaly, disorders of the adrenal medulla, and in diabetes insipidus.

Recovery of muscle after different periods of denervation and treatments

Three aspects of reinnervation and recovery of skeletal muscle following various periods of denervation were investigated: (1) the effect of duration of denervation; (2) the effect of hyperthyroidism on recovery; and (3) whether the muscle or the nerve limits recovery. The rat medial gastrocnemius (MG) nerve was cut and then resutured after 0, 3, 7, 21, or 56 days. In a second group of animals, the MG muscle was denervated and, in addition, the animal received triiodothyronine (T3) supplementation during reinnervation. The third group of animals had the denervated MG muscle reinnervated by a larger number of newly transected foreign axons. The force produced by the reinnervated muscle depends on the period that the muscle was denervated. Recovery was impaired when the period of denervation exceeded 7 days. T3 treatment did not benefit the return of force production, nor did providing the muscle with a larger number of newly transected axons.

The effects of thyroxine treatment on slow- and fast-contracting skeletal muscle contractions of the cat and their cyclic AMP level

1. The effects of thyroxine treatment on soleus and extensor digitorum longus (EDL) muscle contractions and their cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels were examined in anaesthetized cats. 2. Thyroxine treatment decreased the tension of incomplete tetanic contractions of the soleus as well as the EDL muscles. The effect on tension of these muscles was not associated with an increase in the cyclic AMP level of the muscle as is the case with a beta 2-adrenoceptor agonist effect. 3. The results do not support the involvement of cyclic AMP in the tension depressant effect of thyroxine on contractions of skeletal muscle. 4. It is suggested that the muscle weakness and tremor observed in thyrotoxicosis and during administration of beta 2-adrenoceptor agonists are mediated by different mechanisms.

Thyroid hormone response of slow and fast sarcoplasmic reticulum Ca2+ ATPase mRNA in striated muscle

The thyroid status markedly influences the contractile function of muscle, and changes in the activity of the Ca2+ ATPase of the sarcoplasmic reticulum (SR) contribute to these alterations. Two separate genes encode the major isoforms of SR Ca2+ ATPase. In fast skeletal muscle, sarcoplasmic endoplasmic reticulum Ca2+ ATPase type 1 (SERCa1) presents the major isoform, whereas in slow skeletal muscle SERCa type 2 (SERCa2) predominates. Cardiac muscle contains only SERCa2. To examine the mechanisms responsible for changes in contractile function, we quantitated SERCa1 and SERCa2 mRNA levels in fast extensor digitorum longus muscle (EDL), slow soleus muscle, and cardiac muscle in rats of different thyroid status. Hypothyroidism led in soleus to a marked decrease in SERCa1 mRNA and SERCa2 mRNA levels, in cardiac muscle SERCa2 mRNA decreased markedly, as previously shown by us, and in EDL SERCa1 mRNA decreased. These findings are compatible with a hypothyroidism induced decrease in SR Ca2+ ATPase activity and a delay in muscle relaxation. In contrast, SERCa2 mRNA of EDL, representing only a small percent of total SERCa mRNA in this muscle, increased to 175% of control values. Muscle specific and SERCa gene specific changes also occur after acute triiodothyronine (T3) administration to hypothyroid rats. T3 does not induce a significant change in SERCa1 or SERCa2 mRNA levels in soleus, but in the heart SERCa2 mRNA increases about 3-fold. In EDL, T3 increases SERCa1 mRNA from a hypothyroid level of 59 +/- 6% to 138 +/- 4% of control values but SERCa2 mRNA is decreased to 75 +/- 5% of control levels.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

[Hypothyroid hypertrophic myopathy following mantle irradiation for Hodgkin's disease. A case]

The authors report a case of hypothyroid hypertrophic myopathy consecutive to mantle irradiation for Hodgkin's disease. A rise in TSH level is frequent after mantle irradiation and it justifies prolonged monitoring of these patients' thyroid function, in view of the risk of patent hypothyroidism and perhaps cancer. The patient's age, the pre-irradiation lymphography and the chemotherapy associated with radiotherapy are all factors that influence the incidence of thyroid dysfunction, but there is no agreement concerning their relative importance. Hypertrophic myopathies due to hypothyroidism are rare, and their dramatic clinical presentation contrasts with an almost normal muscle histology. Alterations of energy metabolism and changes in the properties of myosin induced by hormonal deficiency account for the muscular weakness of these patients. On the other hand, the mechanism of muscle hypertrophy remains controverted, the most probable theory being and increase in the number of myotubes. Following irradiation, notably for Hodgkin's disease, the frequency of hypothyroidism requires a regular and systematic laboratory follow-up. Replacement therapy must be instituted if the basal TSH level increases, even if the T4 level is normal.

Muscle strength in hyperthyroid patients before and after medical treatment

Ten hyperthyroid patients were assessed for muscle strength before and after a period of medical treatment that averaged 12 months. The subjects did not change their habitual level of physical activity between the two test occasions. Maximal voluntary isokinetic knee extensor muscle strength was determined during various concentric, eccentric and isometric conditions. Average increases in strength from before to termination of treatment ranged from 25 to 41% for the concentric and isometric tests (P less than 0.01, n = 10), and from 19 to 35% for the eccentric tests (P less than 0.01, n = 6). The present study demonstrates that medical treatment of hyperthyroid patients results in a marked increase in muscle strength.

Thyroxine induces transition of red towards white muscle in cultured heart cells

Thyroid hormones (TH) have previously been shown to alter the force and velocity of cardiac muscle contractions. To investigate the mechanism responsible for these alterations, excess amounts of thyroxine (T4, 1 microM) were applied on rat heart cells grown in cell culture. We found the following biochemical alterations: a) 40% decrease in the myoglobin content within 2 days; b) 25% increase in the rate of Ca-uptake into sacroplasmic reticulum (SR) in myocytes following chemical skinning; and c) a two-fold increase in Na-K-ATPase activity measured by 86Rb-uptake. These changes support our hypothesis that TH induce the transition of slow-twitch ("red") muscles towards the fast-twitch ("white") muscle type. This may explain the changes in contractile activity known to occur under TH influence.

The rate of tetanic relaxation is correlated with the density of calcium ATPase in the terminal cisternae of thyrotoxic skeletal muscle

The density of calcium ATPase was measured in the terminal cisternae of extensor digitorum longus (EDL) and soleus muscles from normal and thyrotoxic rats. The experiments tested the hypothesis that the rate of relaxation of these muscles following contraction, at temperatures above 22 C, is correlated with the density of calcium ATPase in the sarcoplasmic reticular membrane. In soleus fibres there was a progressive increase in calcium ATPase density, measured with immuno-electronmicroscopic techniques, of more than two-fold after 3 weeks of daily injections with triiodothyronine (T3). There was a parallel decrease in the relaxation time (from 80% to 20% of peak tension) of the tetanus: the parameters were closely correlated (r = 0.998) during the 3-week period. The rate of relaxation of the twitch also doubled and was correlated with the increase in gold particle density at the end of the 3-week injection period. However, twitch relaxation slowed during the 1st week of T3 injection and was not correlated with gold particle density at that time. The changes in calcium ATPase density and relaxation times in EDL fibres were small and largely insignificant. In contrast to relaxation, an increase in the rate of rise of tension is soleus was complete after only 2 weeks of T3 injection. The results show that the relaxation of tetanic tension is closely correlated with the calcium uptake capacity of the sarcoplasmic reticulum and that thyroid hormone acts more rapidly on factors regulating the rate of rise of tension than on those regulating tension relaxation and the density of calcium ATPase in the terminal cisternae.

Slow to fast alterations in skeletal muscle fibers caused by clenbuterol, a beta 2-receptor agonist

Chronic treatment of rats with clenbuterol, a beta 2-receptor agonist (8-12 wk), caused hypertrophy of histochemically identified fast- but not slow-twitch fibers within the soleus, while the mean areas of both fiber types were increased in the extensor digitorum longus (EDL). In contrast, treatment with the beta 2-receptor antagonist, butoxamine, reduced fast-twitch fiber size in both muscles. In the solei and to a lesser extent in the EDLs, the ratio of the number of fast- to slow-twitch fibers was increased by clenbuterol, while the opposite was observed with butoxamine. The muscle fiber hypertrophy observed in the EDL was accompanied by parallel increases in maximal tetanic tension and muscle cross-sectional area, while in the solei, progressive increases in rates of force development and relaxation toward values typical of fast-twitch muscles were also observed. Our results suggest a role of beta 2-receptors in regulating muscle fiber type composition as well as growth.

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.

Potassium contractures and asymmetric charge movement in extensor digitorum longus and soleus muscles from thyrotoxic rats

Potassium contractures and asymmetric charge movement were recorded from extensor digitorum longus (EDL) and soleus muscle from normal rats and rats that had been made thyrotoxic by daily intraperitoneal injections of triiodothyronine (150 micrograms kg-1) for two to three weeks. Potassium contracture tension (relative to tetanic tension) in thyrotoxic rats was greater in EDL muscles and smaller in soleus muscles than in normal rats. As the relationship between membrane potential and potassium concentration was unaltered by thyroid treatment, it was concluded that the changes in potassium contracture tension were due to changes in excitation-contraction coupling. In thyrotoxic rats there was an average negative shift of -5 mV in the voltage sensitivity of tension in EDL fibres and a positive shift of 5 mV in soleus. As a result, the tension-membrane potential curves for EDL and soleus fibres essentially coincided. There was a corresponding average negative shift of -4 mV in the voltage sensitivity of asymmetric charge in EDL fibres, and a positive shift of 4 mV in soleus fibres from thyrotoxic rats. The dependence of asymmetric charge movement on membrane potential became essentially the same in EDL and soleus fibres from thyrotoxic rats. The maximum asymmetric charge in soleus fibres increased from an average value of 6.5 nC microF-1 in normal rats (n = 33) to 8.9 nC microF-1 in thyrotoxic rats (n = 32; p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of thyroid hormone on T-tubules and terminal cisternae in rat muscles: an electrophysiological and morphometric analysis

Isometric twitches, passive electrical properties and the amounts of transverse (T) tubule system and terminal cisternae in extensor digitorum longus (EDL) and soleus muscle fibres were measured in normal rats and rats given daily injections of triiodothyronine (T3, 150 micrograms kg-1) for 15-25 days. Isometric twitches in both muscles were more rapid after the T3-treatment, particularly in soleus. Cable properties were measured using a three-microelectrode, end-of-fibre, voltage clamp technique. In order to increase the space constant of the T-tubule system, extracellular solutions were used that reduced ionic, particularly chloride, conductance. Fibre diameter was less than normal in the hyperthyroid rats. Membrane capacity, per cm2 of fibre surface, increased in both EDL and soleus muscles and there was a decrease in membrane resistance. The volume and surface area of the T-system and terminal cisternae were measured using standard morphometric techniques. Following T3-treatment the amount of T-tubule system per 100 micron3 of fibre volume, in both EDL and soleus fibres, was twofold higher than in normal fibres. The larger area of T-tubule membrane per unit volume was sufficient to account for the increase in membrane capacity. In contrast, the amount of terminal cisternae per 100 micron3 of fibre was unchanged in EDL following T3-treatment and there was only a small increase in soleus. As a consequence, the normal relationship between the T-tubules and terminal cisternae was changed in both muscles. There was an increase in the numbers of 'bare' T-tubules and an increased occurrence of diadic, pentadic and heptadic junctions between the membranes of the T-tubules and terminal cisternae. The results suggest that thyroid hormone has a differential effect on the synthesis of T-tubule and terminal cisternae membrane, resulting in a disproportionately large amount of T-tubule membrane.

Effects of the thyroid status on the sarcoplasmic reticulum in slow skeletal muscle of the rat

The effects of the thyroid status on the Ca++-transporting capabilities of rat slow skeletal muscle (m.soleus) were studied. The oxalate supported Ca++-uptake activity and Ca++-loading capacity of muscle homogenates from hyperthyroid rats showed an approximate 4.2 and 2.5 fold increase, respectively, as compared to values found in the hypothyroid group. Muscle homogenates of euthyroid rats gave intermediate values. The specific activity of oxalate supported Ca++ uptake, but not the Ca++-loading capacity, of membrane preparations enriched with respect to sarcoplasmic reticulum (SR) increased in proportion to the thyroid status. This was paralleled by a 3.5 fold increase in the amount of active Ca++ pumps in the SR preparations in the transition from hypothyroidism to hyperthyroidism as determined by measurement of Ca++-dependent 32P incorporation. These observations are not explained by differences in degree of purification of the examined SR preparations. Protein profiles of the membrane preparations obtained by gel electrophoresis indicated a thyroid-hormone dependent increase in Ca++-pump content relative to other SR proteins. The results suggest that thyroid hormone stimulates the proliferation of the SR and possibly also increases the Ca++-pump density in the SR membrane.

A freeze-fracture study of extensor digitorum longus and soleus muscle fibers from thyrotoxic rats

Freeze-fracture methods were used to study the sarcoplasmic reticulum and surface membranes in muscles from rats after chronic administration of triiodothyronine (150 micrograms/kg daily, for 1 to 20 days). The major effect of the hormone on the sarcoplasmic reticulum was to increase the numbers of indentations in the terminal cisternae in parallel with an increase in the speed of the isometric twitch. The indentations increased from 7.3 +/- 0.2 to 10.6 +/- 0.1 (mean +/- 1 SEM)/micron of terminal cisternae in the fast-twitch extensor digitorum longus (EDL) and from 0.9 +/- 0.1 to 4.4 +/- 0.1/micron in slow-twitch soleus fibers. The increase in indentation density in both types of muscle occurred within 10 days of the commencement of hormone injection. During the same period there was a small reduction in the density of intramembrane particles in the plasmalemma and a significant increase in the number of caveolae, from 14.6 +/- 0.25 to 20.4 +/- 0.3/micron2 in EDL fibers, and from 22.9 +/- 0.3 to 28.6 +/- 0.3/micron2 in soleus. The increase in caveolae density was coincident with an increase in the area of T-tubule membrane. The results provide further evidence that the indentations in the terminal cisternae play a functional role in muscle activation and that the caveolae are the surface openings of transverse tubules.

Contractile and fatigue properties of thyrotoxic rat skeletal muscle

The effects of thyrotoxicosis on the contractile properties and development of muscle fatigue in the slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles were examined in rats given 3 mg of L-thyroxine and 1 mg of L-triiodothyronine per kilogram of diet for 6 weeks. The hormone treatment produced significant decreases in the contraction time, one-half relaxation time, and twitch tension in the SOL, while the peak rate of tension development (+ dP/dt) and decline (- dP/dt) in this muscle were elevated. Additionally, the force-frequency curve was shifted to the right and, thus, resembled the curve of a normal fast-twitch muscle. In contrast, the contractile properties of the fast EDL were relatively unaltered by the hormone administration. Thyrotoxicosis also changed the SOL response to contractile activity as twitch tension, + dP/dt, and - dP/dt remained high, and a faster decline in muscle glycogen and an increase in lactate occurred compared to control muscles. These results clearly demonstrate a preferential effect of thyroid hormone on slow compared to fast skeletal muscle.

Effects of thyroid hormones on the biochemical specialization of human muscle fibers

The effects of thyrotoxicosis and of hypothyroidism on human muscle have been studied on single fiber preparations. In thyrotoxic muscle, the ratio between fibers showing the fast type of myofibrillar protein isoforms (fast fibers) and fibers showing the slow type (slow fibers) is increased, as is the percentage of fibers with incomplete segregation of fast and slow myosin (intermediate fibers). Furthermore, in fast fibers, the volume and, to a greater extent, the rate of Ca transport of sarcoplasmic reticulum (SR) are increased, without changes in the affinity for Ca2+ of the Ca-pump or in its sensitivity to the cyclic adenosine monophosphate (cAMP) dependent protein kinase system. These effects are completely reversed by the removal of thyroid hormones, as demonstrated by hypothyroid muscles. It is suggested that in human muscle cells thyroid hormones are critical for the expression of fast genes and for SR Ca transport.

Modulation of a major 30-kDa skeletal muscle protein by thyroid hormone

Thyroidectomy results in the transformation of type II fibres to type I in rat soleus muscle. In vitro translations containing polyribosomes indicate that the template activity of mRNA coding for a 30-kDa protein is increased in hypothyroid (6 months) rats. The cellular content of this protein is also increased in hypothyroid rats. The in vitro synthesis of the 30-kDa protein is not observed in thyroidectomized (10 weeks) rats that have been treated with triiodothyronine. The synthesis and accumulation of this protein are directly related to the proportion of type I fibres in rat skeletal muscle and appear to be modulated by thyroid hormone.

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.

Effects of thyroxine treatment on contractions of soleus muscles of anaesthetized cats

The effects of chronic thyroxine treatment on cat soleus muscle contractions were studied. Maximum twitch tension, contraction time, half relaxation time and tension-time integral of maximal twitches of the soleus muscles of thyroxine treated cats were significantly decreased. Consequently, there was a decrease in tension and degree of fusion of incomplete tetanic contractions of the soleus muscle. The maximum tetanic tension was not statistically significantly changed, suggesting that the effects may be due to a decrease in the duration of the active state of the muscle. Isoprenaline given intravenously during incomplete tetanic contractions of the soleus muscle caused a statistically significant depression of tension in the control group but not in the thyroxine treated group. This further suggests reduction in the duration of the active state of soleus muscles of thyroxine treated cats. Propranolol injected chronically with thyroxine reversed or prevented the depression of tension caused by thyroxine treatment, suggesting the involvement of beta-adrenoceptors in these effects. The decrease in tension and degree of fusion during incomplete tetanic contractions of the thyroxine treated soleus could be responsible, at least partly, for the muscle weakness and tremor of thyrotoxicosis. Cyclic AMP may possibly be the mediator of these effects.

The effects of thyroid status on some properties of rat fast-twitch muscle

The effects of different thyroid states on some histochemical and biochemical properties of fast-twitch muscle were studied using rat extensor digitorum longus (EDL) muscle. This muscle was found to be much less responsive to thyroidal influence than the slow-twitch soleus muscle. In EDL muscles of hypothyroid rats, fast leads to slow conversions were observed in fibre type composition, myosin ATPase activity and light chain pattern, and in the subunit composition of lactate dehydrogenase, while the only significant slow leads to fast conversion observed in thyrotoxicosis was a decrease in the proportion of slow-oxidative fibres. Denervation of the hypothyroid muscle produced the highest degree of fast leads to slow transformation. These findings support the view that denervation and dysthyreosis alter gene expression in muscle by independent mechanisms.

The effect of exercise, thyroid status and insulin-induced hypoglycaemia on the Achilles tendon reflex time in man

The effect of moderate exercise on the Achilles tendon reflex half-relaxation time (ART) was observed in 28 hypothyroid, 35 euthyroid, and 20 hyperthyroid subjects of comparable age and sex. Both the resting ART and the decrease after exercise (delta ART) were longer in hypothyroid patients and shorter in hyperthyroid patients as compared to the euthyroid group. delta ART was strongly related to the resting ART rather than to the free thyroxine index. In four hypothyroid subjects, the half-time of recovery of the ART after exercise was similar, approx. 24 min. Insulin-hypoglycaemia in eight resting endocrine patients also caused a fall in ART similar to the post-exercise delta ART in the same patients. A possible catecholamine effect is indicated by the smaller post-exercise delta ART in nine hypertensive patients receiving chronic propranolol treatment, 320 mg daily, than that of a control group matched for age, sex, and resting ART. These studies suggest that submaximal exercise largely removes the differential effect of thyroid status on skeletal muscle contractility as measured indirectly by the ART. The effects of exercise and induced insulin-hypoglycaemia on delta ART may be mediated in part by endogenous catecholamines.

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.

Thyrotoxicosis and lactate-producing angina pectoris with normal coronary arteries

Three patients with thyrotoxicosis are described, in whom the presenting symptom was severe cardiac pain at rest or on effort and who were admitted to hospital with suspected or proven myocardial infarction. All patients were studied by selective coronary arteriography and left ventriculography after thyroid function tests which confirmed thyrotoxicosis. There was no demonstrable disease of the major coronary arteries in any of the patients, yet myocardial infarction and left ventricular aneurysm were shown to be present in 1, and there was definite electrocardiographic evidence of ischaemia in all 3. In addition, under stress the myocardium of all 3 patients produced lactate. It is recommended that thyrotoxicosis be seriously considered in the differential diagnosis of cardiac pain, particularly in younger women. The cause of the pain seems related to the cellular effects of thyrotoxicosis on the myocardium and current views of these effects are summarised. Of the 3 patients, 1 died suddenly 6 months after becoming euthyroid, indicating that the disease may not be as benign as expected. A guarded prognosis and continued medical follow-up are recommended when thyrotoxicosis presents with angina pectoris even when normal coronary arteries have been demonstrated.

[In vivo incorporation of labeled palmitic and oleic acids into skeletal muscle lipids of normal and thyroidectomized rats during swimming]

The purpose of this work was to study the incorporation of [1-14C] palmitic and and [9,10(-3)H] oleic acids, after intravenous administration in the lipids of rat hind leg muscles. The animals were fasting or fed, at rest or swimming during 10 min before test, euthyroid or thyroidectomised. All these animals have been taking the same daily swimming training, during 15 days before the injection of labelled molecules. They were killed 10 min (+/-1)later. The lipidic muscle composition, the incorporation rate of labelled fatty acids in these lipids and the radioactivity distribution among the different lipids in the various cases have been determined. Moreover the plasmatic non-esterified acid radioactivity has been measured. These various values are affected by nutritional, hormonal state, and by physical activity of the animals. Particularly, it seems that supplementary energy spent during swimming test will be covered by the oxidation of different nutriments, according to the nutritional and hormonal state of animals.

Triiodothyronine effects on some electrogenic properties of frog sartorius

At 21 degrees C in vitro, 0.2 and 2.0 muM of triiodothyronine (T3) produced an increase in resting membrane potential (RMP) of Rana pipens sartorius when the pH of the external solution was 7.4. The RMP was increased by 2.0 muM T3 in the presence of 10(-4) and 10(-3) M ouabain but not in 10(-3) M of 2,4 dinitrophenol. Small increases in RMP were observed with 2.0 muM T3 in solutions with low external Na. At pH 7.1 0.2 muM T3 produced a small transient increase in RMP. Membrane resistance (Rm) was found to decline gradually during exposure to 0.2 muM at a pH of 7.4. Treatment with 2.0 muM T3 at pH 7.4 was accompanied by a transient reduction in Rm. Similar transient changes in Rm were produced by 0.2 and 2.0 muM T3 at pH of 7.1 T3 reduced membrane resistance in isotonic K2SO4 and tris-buffered Mn (20 mM) solutions indicating that T3 increases potassium permeability. Direct action potentials were studied at pH 7.1. Overshoot, amplitude and rate of rise of the action potential underwent a gradual decrease in the presence of 0.2 muM T3 while thresholds remained unchanged. Thresholds were increased during exposure to 2.0 muM T3 whereas overshoot, amplitude and rate of rise underwent transient decreases followed by a return toward control levels.

Hyperthyroid myopathy. Intracellular electrophysiological measurements in biopsied human intercostal muscle

Morphological and electrophysiological studies were performed on intercostal muscle biopsies from 2 thyrotoxic patients. The diseased fibers had numerous areas of subsarcolemmal glyogen accumulations and abnormal membranous projections. Both Type I and Type II muscle fibers were atrophied. Diseased fibers were substantially depolarized and when artifically hyperpolarized showed earlier inactivation of the sodium conductance as a function of membrane potential, and a critical depolarization potential more depolarized than in normal fibers. When stimulated at 20 pulses/sec, or faster, the diseased fibers could not generate normal action potentials due to membrane depolarization and the appearance of a marked after-hyperpolarization. Muscle weakness associated with hyperthyroidism is attributed to the reduced membrane excitability.