CYTOLOGICAL BASIS OF DECREASE IN MUSCLE STRENGTH DURING STARVATION

Clinical and histopathological features of myofibrillar myopathy in Warmblood horses

BACKGROUND

To report a novel exertional myopathy, myofibrillar myopathy (MFM) in Warmblood (WB) horses.

OBJECTIVES

To 1) describe the distinctive clinical and myopathic features of MFM in Warmblood horses and 2) investigate the potential inheritance of MFM in a Warmblood family.

STUDY DESIGN

Retrospective selection of MFM cases and prospective evaluation of a Warmblood family.

METHODS

Retrospectively, muscle biopsies were selected from Warmblood horses diagnosed with MFM and clinical histories obtained (n = 10). Prospectively, muscle biopsies were obtained from controls (n = 8) and a three generation WB family (n = 11). Samples were assessed for histopathology [scored 0-3], fibre types, cytoskeletal and Z disc protein aggregates, electron microscopic alterations (EM) and muscle glycogen concentrations.

RESULTS

Myofibrillar myopathy-affected cases experienced exercise intolerance, reluctance to go forward, stiffness and poorly localised lameness. Abnormal aggregates of the cytoskeletal protein desmin were found in up to 120 type 2a and a few type 2x myofibres of MFM cases. Desmin positive fibres did not stain for developmental myosin, α actinin or dystrophin. Scores for internalised myonuclei (score MFM 0.83 ± 0.67, controls 0.22 ± 0.45), anguloid atrophy (MFM 0.95 ± 0.55, controls 0.31 ± 0.37) and total myopathic scores (MFM 5.85 ± 2.10, controls 1.41 ± 2.17) were significantly higher in MFM cases vs.

CONTROLS

Focal Z disc degeneration, myofibrillar disruption and accumulation of irregular granular material was evident in MFM cases. Muscle glycogen concentrations were similar between MFM cases and controls. In the Warmblood family, desmin positive aggregates were found in myofibres of the founding dam and in horses from two subsequent generations.

MAIN LIMITATIONS

Restricted sample size due to limited availability of well phenotyped cases.

CONCLUSIONS

A distinctive and potentially heritable form of MFM exists in Warmblood horses that present with exercise intolerance and abnormal hindlimb gait. Muscle tissue is characterised by ectopic accumulation of desmin and Z disc and myofibrillar degeneration.

Muscle and Limb Mechanics

Understanding of the musculoskeletal system has evolved from the collection of individual phenomena in highly selected experimental preparations under highly controlled and often unphysiological conditions. At the systems level, it is now possible to construct complete and reasonably accurate models of the kinetics and energetics of realistic muscles and to combine them to understand the dynamics of complete musculoskeletal systems performing natural behaviors. At the reductionist level, it is possible to relate most of the individual phenomena to the anatomical structures and biochemical processes that account for them. Two large challenges remain. At a systems level, neuroscience must now account for how the nervous system learns to exploit the many complex features that evolution has incorporated into muscle and limb mechanics. At a reductionist level, medicine must now account for the many forms of pathology and disability that arise from the many diseases and injuries to which this highly evolved system is inevitably prone. © 2017 American Physiological Society. Compr Physiol 7:429-462, 2017.

Early postnatal food intake alters myofiber maturation in pig skeletal muscle

We examined the effects of undernutrition on muscle development during the first postnatal week in pigs. Eighteen piglets were subjected to three nutritional levels (300, 200 or 100 g/(kg body. d) of colostrum then milk) between birth and slaughter at 7 d of age. Longissimus lumborum (LL), a fast-twitch glycolytic muscle, and rhomboideus (RH), a mixed slow- and fast-twitch oxido-glycolytic muscle, were taken for myofiber typing and biochemical analyses. Enzyme activities of lactate dehydrogenase (LDH), citrate synthase (CS) and beta-hydroxy-acyl-CoA-dehydrogenase (HAD) were used as markers of glycolytic, oxidative and lipid beta-oxidation capacities, respectively. Undernutrition selectively decreased (P < 0.001) hypertrophy of the future fast-twitch glycolytic fibers in LL. Contractile and metabolic maturation was delayed in the later maturing LL, as reflected by a decrease in muscle protein concentration (P < 0.01), an increase (P < 0.05) in the percentage of myofibers still expressing the fetal myosin heavy chain (MyHC), a lower postnatal increase in LDH activity (P < 0.001) and a delayed decrease in the percentage of IIa MyHC positive fibers (P < 0.001). Otherwise, restriction tended (P < 0.10) to increase the percentage of slow type I MyHC containing fibers in both muscles and of alpha-cardiac MyHC positive fibers in RH (P < 0.05). The LDH/CS ratio decreased dramatically (P < 0.001) after restriction, to a greater extent in LL than in RH. These changes denoted a more oxidative metabolism using fewer carbohydrates and more lipids in restricted pigs, as suggested by the increased activity of HAD (P < 0.001) and decreased respiratory quotient (P < 0.001).

The effect of dietary restriction on muscle fibre length in mice

The dietary intake of male mice from a line selected for high body-weight was restricted to 60% by weight of the ad lib. intake of the control litter-mates between days 21 and 42 post partum. One group was killed and compared with controls at 42 d of age. A further group then resumed ad lib. feeding. Mice which had regained the control body-weight by 61 d of age were killed and also compared with controls. Muscle weight, fibre length, sarcomere length, sarcomere number per fibre, actin length and myosin length were measured in the biceps brachii and tibialis anterior muscles. Muscle weight, fibre length and the number of sarcomeres per fibre were significantly reduced in both muscles following dietary restriction, but regained their control values following a resumption of normal feeding. The other indices remained unchanged throughout the study. The pattern of fibre length changes, due entirely to alterations in sarcomere number, is similar to that reported previously during growth, senescence and immobilization and following selection for high and low body-weights.

Early life undernutrition in rats. 2. Some contractile properties of skeletal muscles from adult animals

1. The contractile properties of soleus and extensor digitorum longus muscles from animals at 12 months of age whose mothers had been undernourished during pregnancy and lactation (PU rats) have been compared with those of muscles from ge-matched controls. 2. Body-weight and muscle wet weight of PU rats was significantly reduced. Muscle:body-weight values were, however, no different from controls. 3. No significant alterations in whole muscle speeds of contraction or relaxation could be detected when compared with those of age-matched controls. 4. Twitch and tetanic forces of both SOL and EDL were greater per unit weight of muscle in PU rats. Apart from SOL twitch these differences were significant. The tetanus:twitch values were, however, not different.

The effect of acute dietary restriction on muscle fibre number in weanling rats

1. Male Sprague-Dawley rats were allocated at 100 g into either an ad lib.-fed control group or a food-restricted group. The restricted group was fed for 9 d at 25% of ad lib. intake. Controls were killed at a body-weight of 100 g and 29 d of age. 2. The effects of food restriction on muscle weight, fibre number, fibre diameter, DNA, and protein were examined in three skeletal muscles, the soleus, plantaris and extensor digitorum longus (EDL). 3. Acute dietary restriction caused body- and muscle-weight loss and a decrease in both the number and cross-sectional area of muscle fibres in each of the muscles. 4. The restriction halted growth-related increases in DNA in all muscles and decreased the protein:DNA value in the plantaris and EDL. 5. These results indicate that present theories describing cellular development are not adequate to define growth potential or growth retardation of skeletal muscle.

Changes in skeletal muscle cellularity in starved and refed young rats

1. All food was withdrawn from male weanling rats until a 40% loss of body-weight was attained. Another group of animals was treated similarly and then refed a stock diet until the original body-weight was attained. 2. The body-weight loss caused a significant reduction in the weight of the heart, kidney, liver and epididymal fat pads. Refeeding produced a return to the control weight of the heart and kidney, an increase in the weight of the liver and a deficit in the weight of the epididymal fat pads. 3. Body-weight loss caused a decrease in the weight of the three different muscles studied, and in the number and diameter of the fibres in each muscle. Refeeding restored the weight and cellularity of two of the three muscles to that of the control animals. The soleus muscle was heavier in the refed animals when compared to controls due to an increased fibre diameter. 4. It is concluded that the decrease in the number and diameter of muscle fibres during starvation in the rat can be restored on refeeding a stock diet.

The effects of pre- and perinatal undernutrition on the succinic dehydrogenase content of muscle fibres from fast and slow rat muscles

Male rats were subjected to early undernutrition by limiting the mothers' food supply by 50% during pregnancy and lactation. At age 36 weeks, quantitative cytochemical determinations of succinic dehydrogenase activity were made in muscle fibres from the anterior tibialis and soleus muscles. Marked decreases were found in the former muscle but relatively little decrease was seen in the latter. This response of the muscles to early undernutrition was discussed with reference to other studies on pre and post-natal starvation.

Effects of pre and perinatal malnutrition on muscle fibres from fast and slow rat muscles

Pre and perinatal malnutrition of rats was effected by means of limiting the mothers' food intake by 50% during pregnancy and lactation. Male offspring were sacrificed at 36 weeks of age and the anterior tibialis (ANTIB) and soleus (SOL) muscles were prepared for histochemical demonstration of Type I, IIA and IIB muslce fibre types using myosin ATPase and succinic dehydrogenase activity. Muscle weights and mean muscle fibre area determinations showed greater decreases in ANTIB than SOL, SOL muscle fibre areas being relatively unaffected by the undernutrition regimen. The proportions present of the muscle fibre types differed in ANTIB and to a small extent on SOL. In the former muscle, some decreases in area in certain of the fibres were associated with increases in the percentages present, showing a tendency to maintain the physiological cross-sectional area of the muscle.

Comparison of the chemical and biochemical composition of thirteen muscles of the rat after dietary protein restriction

1. The objective of this study was to determine whether the chemical and biochemical changes induced by muscle wasting caused by dietary protein restriction are different in various skeletal muscles. 2. Rats were fasted for 3 d and then fed on a 10 g protein/kg diet for 21 d. Thirteen muscles from the trunk, forelimb, and hind-limb regions were analysed for muscle weight, and the content of water, fat, cellular and extracellular protein, DNA and RNA. Results were compared to values for an 'initial' control group killed at the start of the experiment. 3. Weight loss was greatest in trunk muscles and least in the distal forelimb muscles. Water content decreased in most muscles, but increased in three forelimb muscles. A significant loss of lipid was found in the gastrocnemius, while the biceps brachii gained lipid. Changes in lipid content of the muscles did not form a distinctive pattern. 4. All muscles except the distal forelimb muscles lost a significant amount of cellular protein, while all muscles except the diaphragm gained extracellular protein. 5. DNA content was unchanged in all muscles. The value for cellular protein:DNA was significantly reduced in the rectus abdominis and the diaphragm. A significant loss of RNA was found in all muscles; the percentage change was greatest in trunk muscles and least in the distal forelimb muscles. The values for RNA:protein and RNA:DNA were significantly lower in all muscles except two distal forelimb muscles. 6. With the exception of the water and lipid content of the muscles, the directions of the changes in the experimental animals were the same for all muscles. The results suggested, however, that the magnitude of changes in certain chemical and biochemical indices of composition may depend to some extent on the anatomical location of the muscle: trunk muscles tended to show the greatest percentage change, while the distal forelimbs changed the least.

Muscle fiber types in women athletes and non-athletes

Muscle biopsies were obtained from the vastus lateralis muscle of 5 female collegiate field hockey players and 5 untrained female students. The fibers were classified histochemically as fast-twitch-oxidative-glycolytic (FOG), fast-twitch-glycolytic (FG) and slow-twitch-oxidative (SO). The fibers were found to be similar to those of males in distribution and histochemical properties, but were smaller. In the women athletes all 3 fiber types were larger than the respective fibers in the controls. Also, the athletes had a much higher percentage of oxidative fibers (SO + FOG), 83% vs. 46%. A direct relationship between fiber size and oxidative activity was observed in fast-twitch fibers, whereas the reverse was found in slow-twitch fibers.

Skeletal-muscle growth and protein turnover

Because of turnover, protein synthesis and breakdown can each be involved in the regulation of the growth of tissue protein. To investigate the regulation of skeletal-muscle-protein growth we measured rates of protein synthesis and breakdown in growing rats during development on a good diet, during development on a marginally low-protein diet and during rehabilitation on a good diet after a period of severe protein deficiency. Rates of protein synthesis were measured in vivo with a constant intravenous infusion of [14C]tyrosine. The growth rate of muscle protein was measured and the rate of breakdown calculated as breakdown rate=synthesis rate-growth rate. These measurements showed that during development on a good diet there was a fall with age in the rate of protein synthesis resulting from a fall in capacity (RNA concentration) and activity (synthesis rate per unit of RNA). There was a fall with age in the breakdown rate so that the rate was highest in the weaning rats, with a half-life of 3 days. There was a direct correlation between the fractional growth and breakdown rates. During rehabilitation on the good diet, rapid growth was also accompanied by high rates of protein breakdown. During growth on the inadequate diet protein synthesis rates were lesss than in controls, but growth occurred because of decreased rates of protein breakdown. This compression was not complete, however, since ultimate muscle size was only one-half that of controls. It is suggested that increased rates of protein breakdown are a necessary accompaniment to muscle growth and may result from the way in which myofibrils proliferate.

Work-induced hypertrophy in exercised normal muscles of different ages and the reversibility of hypertrophy after cessation of exercise

1. Groups of male hamsters of different ages were subjected to a weight-lifting exercise regimen, and the biceps brachii, soleus and extensor digitorum longus (EDL) muscles examined for structural changes occurring in response to the increased workload. In addition, two groups of adult hamsters were left to recover from the exercise stimulus for 5 and 15 weeks respectively.2. All the exercised muscles exhibited muscle fibre hypertrophy, and the extent of the hypertrophy was greater in the younger animals. In all age groups, the biceps brachii and EDL showed more hypertrophy than did the soleus. There was no significant increase in fibre number after exercise.3. In both groups allowed to recover from the exercise stimulus, the fibre dimensions reverted back to those of the control muscles; this appeared to be complete after 15 weeks recovery.4. Electron microscopical studies of fibres from exercised and control biceps brachii revealed no significant changes with exercise in the proportions present of myofibrillar, mitochondrial and tubular components within muscle fibres of the same size.

Muscle strength in boys of different ages

Isometric twitch contractions have been studied in extensor hallucis brevis muscles of 32 healthy male subjects aged between 3 and 25 years. The maximum twitch tensions were found to increase gradually with age and then to undergo marked enhancement at puberty. After puberty little further change occurred. In the youngest children examined the twitch contraction times were already within the adult range, though the half-relaxation times were prolonged.