Fiber Type Changes Research Articles

Age and Exercise Effects on Myosin Heavy Chain Isoform mRNA Expression in Human Muscle

0389 Aging is associated with changes in the contractile and metabolic properties of human skeletal muscle. Among these changes is a shift in fiber type towards a predominance of Type I fibers. It was reported that mRNA levels of myosin heavy chain (MHC) isoforms, the determinant of contractile phenotype, are altered with age in a manner consistent with fiber type results, although two studies found no changes in MHC mRNA expression. Further, a predominance of Type I fibers has been observed in highly aerobically-trained athletes but the effect of aerobic training on MHC mRNA expression and the potential interaction with age remains to be determined. PURPOSE: To determine the effect of aging and a moderate intensity aerobic exercise training program on mRNA levels of MHC isoforms in human muscle. METHODS: Previously sedentary, healthy men and women aged 22–88 y were studied before and after 16 weeks of bicycle training (30–45 min at 70–80% peak heart rate, 3–4 days/week, N = 62) or control (stretching, N = 37) activity. Muscle biopsies of the vastus lateralis were analyzed for MHC mRNA isoforms I, IIA and IIX using real-time PCR. RESULTS: At baseline MHC-I mRNA was unchanged with age, while levels of IIA (r = −0.45) and IIX (r = −0.34) declined by 14% and 10% per decade, respectively (p < 0.001). After training, the mRNA levels of MHC-I and IIA increased 61 and 99%, respectively, while IIX decreased 50% (all p < 0.001). The increase in MHC-I mRNA was positively associated with age (r = 0.30, p < 0.01). There was no interaction between age and exercise responses for MHC-IIA or IIX, and MHC mRNA did not change in the control group. CONCLUSION: This study demonstrates that MHC isoform expression changes with aging consistent with reported changes in fiber type. Despite baseline differences, older muscle retains the ability to alter expression of MHC isoforms at the mRNA level in response to aerobic training. This agrees with previous data from this study demonstrating that training-induced enhancement of mitochondrial genes and functions are similar among younger and older people. Thus, skeletal muscle plasticity, at least at the transcript level, remains intact in older people. Future work will determine if training-induced changes in MHC mRNA are also reflected at the protein level. Supported by NIH AG-RO109531, MO1-RR00585 and Mayo Foundation.

Differences in Local Environment Determine the Site of Physiological Angiogenesis in Rat Skeletal Muscle

The specificity in location of angiogenesis to either glycolytic or oxidative fibre types, or muscle regions, was examined in the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles of rat. Angiogenesis was induced by mechanical means either with (chronic muscle stimulation) or without (muscle stretch by overload) changes in blood flow, treatments which invoked only minor changes in fibre type and fibre size. Proliferation estimated by PCNA labelling of cells co-localised with capillaries was very rare in control muscles, where it occurred mainly in the glycolytic regions, but was increased in both models of angiogenesis. However, when labelled capillaries were scored according to the type of surrounding fibres, only muscle stimulation significantly accentuated proliferation of capillaries surrounded by glycolytic fibres. We conclude that while mechanical stimuli are important for proliferation in glycolytic regions in both models, capillary growth occurs specifically around glycolytic fibres in that region when the angiogenic stimulus includes increased blood flow and/or increased metabolic demand.

Oxygen Cost of Exercise Is Increased in Heart Failure After Accounting for Recovery Costs

The oxygen cost during exercise has been reported to be decreased in patients with congestive heart failure (CHF), implying an increased efficiency (lower oxygen uptake [VO(2)] per Watt [VO(2)/W]); however, these studies ignored the oxygen debt that is increased in heart failure. The primary aim of this research was to evaluate the total oxygen cost (work VO(2)/W) during exercise and recovery in patients with heart failure as compared with healthy adults. We performed a retrospective analysis comparing the exercise VO(2)/W, the recovery VO(2)/W, the work VO(2)/W, and the VO(2)/W relationship above and below the ventilatory threshold (VT) in 11 healthy control subjects and 45 patients with CHF. The exercise VO(2)/W was decreased by 29% (p < 0.0001) in patients with CHF; however, the recovery VO(2)/W was increased by 167% (p < 0.0001) and the work VO(2)/W was increased by 14% in patients with CHF (p = 0.014). The VO(2)/W slope increased above the VT (+ 27%, p = 0.0017) in both normal subjects and patients with CHF, suggesting a decrease in efficiency above the VT. There was an inverse correlation (r = 0.646, p < 0.0001) between exercise VO(2)/W and recovery VO(2)/W, implying that subjects with a low exercise VO(2)/W were not efficient but rather accumulated a large oxygen debt that was repaid following completion of exercise. Heart failure is associated with lower exercise VO(2)/W; however, the patient with heart failure is not efficient, but rather accumulating a large oxygen debt (recovery VO(2)/W) that is repaid following exercise. In addition, the work VO(2)/W (including both exercise and recovery) is increased in patients with heart failure in comparison to control subjects, and correlates inversely with the percentage of predicted VO(2). The large recovery VO(2)/W is likely due to impaired oxygen delivery to exercising muscle during exercise. The increase in the work VO(2)/W is probably due to changes in skeletal muscle fiber type that occur in patients with heart failure (type I to type IIb).

Effects of age and nerve-repair grafts on reinnervation and fiber type distribution of rat medial gastrocnemius muscles

Aging is associated with a decline in skeletal muscle function. Previous research suggests that this decline in skeletal muscle function may, in part, be explained by an age-associated decline in the ability of skeletal muscle to reinnervate and/or age-associated changes in fiber types and distribution during reinnervation. This study used a nerve-repair graft model to investigate age-associated changes in the ability to reinnervate via expression of neural cell adhesion molecule (NCAM), a marker of denervated and recently reinnervated muscle fibers and changes in fiber type and Type I fiber grouping in medial gastrocnemius (MG) muscles of 6-, 12- and 24-month-old male Fischer 344 rats. Age had no effect on total MG muscle fiber number. Aging and nerve-repair grafting led to an increase in percent Type I and a decrease in percent Type IIB fibers. Aging and nerve-repair grafting led to an increase in NCAM positive fibers and an increase in the percentage of enclosed Type I muscle fibers, which was greatest in the 24 month nerve-repair grafted group. Thus, we conclude that diminished contractile function of aged and/or nerve-repair grafted MG muscle may be explained, in part, by an increase in the percentage of denervated fibers.

The effect of maternal undernutrition on muscle fibre type in the newborn lamb

Muscle fibre type can influence meat quality (Maltinet al1997). Muscle fibre formation occurs during gestation and in the sheep the total number of fibres in a muscle is essentially fixed at birth. (Ashmereet al1972). Postnatal growth of muscle is entirely due to elongation and widening of the existing muscle fibres. Therefore the gestational period is important in the long-term growth potential of the animal. By investigating changes in muscle fibre type, the aim of this study was to test the general hypothesis that the poor carcass quality sometimes seen in ruminant animals may be due to poor nutrition at strategic time points during the animal’s development. As agricultural practices continue to become more extensive, variation in the nutrient supply to the animal is becoming more common. Therefore it is important to understand the effect of any changes in nutrient supply to the mother, during gestation on the subsequent muscle development of the fetus and ultimately the effects on meat quality.

Age-related atrophy of rat soleus muscle is accompanied by changes in fibre type composition, bioenergy decline and mtDNA rearrangements.

A variety of techniques have been applied to investigate the interrelationship between age-related atrophy of rat soleus muscle and other signs of muscle aging, such as changes in muscle fibre type composition, decrease in bioenergy capacity and accumulation of mitochondrial DNA (mtDNA) arrangements. Age-related atrophy of rat soleus muscle was shown to start at the age of about 28 months. It was accompanied by a decrease in the number of slow twitch muscle fibres (type I) and an increase in the proportion of muscle fibres co-expressing slow and fast myosins (type Ic and IIc fibres). Bioenergy capacity of the soleus muscle, assessed by the level of measurable cytochrome c oxidase (COX) activity, was found to be decreased both in the middle age and old rats compared to the young animals. Muscle atrophy was also accompanied by a decrease in the amount of full-length mitochondrial DNA (FL-mtDNA) amplifiable by the extra-long PCR (XL-PCR) and the increase in the number of mtDNA deletions. The results of the study show that the decline in the bioenergy capacity of the rat soleus occurs by the middle age. It is followed by the onset of the age-related muscle atrophy that is accompanied by both fibre type changes and functional mtDNA degradation.

Serum cortisol, lactate and creatinine concentrations in Thoroughbred fillies of different ages and states of training

Exercise can be defined as normal stress stimulating body functions. Some reports suggest lactate as a stimulator of cortisol levels, while creatinine varies according to the amount of muscle tissue. In the present study we investigated the relationship between creatinine, serum lactate concentration and cortisol levels in training horses. Twenty-three Thoroughbred fillies were used, divided into 3 groups according to age and training protocol: G1, 1-2 years of age (N=7) on pasture, G2, 2-3 years (N=9) starting to be mounted, and G3, 3-4 years (N=7) racing at the Jockey Club. Blood samples were collected weekly during a six-month period at about 1:00 p.m. while the animals were resting. Cortisol was quantified with a commercial kit (Coat-a Count®) and serum creatinine and lactate were evaluated with an autoanalyzer with commercial reagents. Data were evaluated using non-parametric statistical tests, with the level of significance set at P< 0.05. Cortisol concentrations were 149a + 7, 126b + 6, and 101c + 5 nmol/l, lactate concentrations were 2.1a + 0.1, 2.0a + 0.1, and 1.75b + 0.1 mmol/l, and creatinine concentrations were 125a + 2, 132a + 2 145b + 3 mumol/l in G1, G2 and G3, respectively. Only G2 showed a low but significant positive correlation of cortisol with lactate and a negative correlation of cortisol with creatinine levels. It was possible to conclude that cortisol, lactate and creatinine varied during horse aging and physical conditioning. The decrease of cortisol concentration (G2) suggests that the better physical condition acquired during training led to the increase of creatinine concentration, possibly related to muscle mass. The lower cortisol and lactate concentrations observed in G3 animals may have been due to greater muscle mass inducing an increase in creatinine concentrations or changes in muscle fiber type during training.

Effect of chronic denervation and denervation-reinnervation on cytoplasmic creatine kinase transcript accumulation.

The extensor digitorum longus (EDL) and soleus muscles of adult mice were chronically denervated or denervated and allowed to reinnervate. Muscles were evaluated 1, 5, 14, 21, and 52 days after sciaticectomy. In terms of weight loss, myofiber atrophy, degeneration, and fibrosis, the soleus muscle was more affected than the EDL by chronic denervation. Fifty-two days after chronic denervation, the number of molecules of MCK/ng total RNA in both muscles (determined with competitive PCR) decreased, with the soleus muscle being more affected. At that stage, BCK mRNA levels in the denervated soleus were unchanged, but they were increased (>50%) in the EDL. Reinnervation restored MCK transcript accumulation in the EDL, whereas, in the soleus MCK, transcripts exceeded control values by 57%, approaching levels in the reinnervated EDL. Despite restoration of MCK mRNA levels, the number of molecules of BCK mRNA/ng total RNA was four- to fivefold higher in reinnervated versus control muscles, suggesting that the genes encoding the CK mRNAs are not coordinately regulated in adult muscle. The role of denervation induced, fiber type changes in regulating CK mRNA accumulation has been evaluated. Electron microscopic analyses have established that fibrosis is not a factor that determines BCK mRNA levels in the chronically denervated or denervated-reinnervated muscles. CK isozyme analyses support the hypothesis that a greater proportion of BCK mRNA found in 52 day chronically denervated and denervated-reinnervated muscles is produced in myofibers vs. nonmuscle cells than in control muscles.

A mutation in alpha-tropomyosin(slow) affects muscle strength, maturation and hypertrophy in a mouse model for nemaline myopathy.

Nemaline myopathy is a hereditary disease of skeletal muscle defined by a distinct pathology of electron-dense accumulations within the sarcomeric units called rods, muscle weakness and, in most cases, a slow oxidative (type 1) fiber predominance. We generated a transgenic mouse model to study this disorder by expressing an autosomal dominant mutant of alpha-tropomyosin(slow) previously identified in a human cohort. Rods were found in all muscles, but to varying extents which did not correlate with the amount of mutant protein present. In addition, a pathological feature not commonly associated with this disorder, cytoplasmic bodies, was found in the mouse and subsequently identified in human samples. Muscle weakness is a major feature of this disease and was examined with respect to fiber composition, degree of rod-containing fibers, fiber mechanics and fiber diameter. Hypertrophy of fast, glycolytic (type 2B) fibers was apparent at 2 months of age. Muscle weakness was apparent in mice at 5-6 months of age, mimicking the late onset observed in humans with this mutation. The late onset did not correlate with observed changes in fiber type and rod pathology. Rather, the onset of muscle weakness correlates with an age-related decrease in fiber diameter and suggests that early onset is prevented by hypertrophy of fast, glycolytic fibers. We suggest that the clinical phenotype is precipitated by a failure of the hypertrophy to persist and therefore compensate for muscle weakness.

Constancy of masseter muscle structure and function with age in F344 rats

Senile muscle atrophy is a characteristic feature of advancing age. Despite the growing body of knowledge about weight-bearing muscles in rodents and man, there is relatively little such information on the muscles of mastication. Therefore, the primary goal here was to develop a masseter muscle preparation from male Fischer 344 rats suitable for studying contractile characteristics in vitro. And, secondarily, the goal was to examine this preparation for evidence of age-related changes in muscle composition and function in rats aged 4–24 months. Histochemical analysis of the composition of the four anatomical regions (branches) of the masseter revealed a mixture of rapidly contracting, fatiguable and fatigue-resistant muscle fibres with no significant differences between branches. Fibre type and size were determined with myosin ATPase, NADH-TR and toluidine blue staining of quick- frozen muscle sections. No significant changes in fibre type or fibre area were found with increasing age. One branch of the masseter, the anterior deep masseter, is sufficiently thin (less than 0.8 mm thickness) for adequate diffusion of oxygen and nutrients in studies of isometric contractile properties in vitro. Contraction time, half-relaxation time, dry weight:wet weight ratio and maximum force per unit area were found to be similar in muscles of young and old rats. Our results demonstrate that the anterior deep masseter of the rat is a suitable preparation for investigating masseter function in vitro. The surprising absence of age-related changes in composition and function is consistent with some, but not all, data on ageing rodent limb muscles. The results suggest that masticatory muscle performance is preserved with age in rats.

Growth-Related Changes in Skeletal Muscle Fiber Type and Insulin Resistance in Diabetic Otsuka Long-Evans Tokushima Fatty Rats.

We investigated whether a relationship exists between the fiber type distribution in skeletal muscles and the insulin resistance of different ages of diabetic rats. The fiber type distributions of the slow soleus and fast plantaris muscles in 5-, 9-, and 21-week-old male Otsuka Long-Evans Tokushima Fatty (OLETF) rats as an animal model of spontaneous type 2 diabetes mellitus were examined and compared with those in age-matched non-diabetic Long-Evans Tokushima Otsuka (LETO) rats. In the soleus muscle, a higher percentage of type I fibers was observed in the 9- and 21-week-old OLETF rats compared with the age-matched LETO rats, and there were no type IIA fibers remaining in the soleus muscle of the OLETF rats. In the plantaris muscle, a lower percentage of type IIA fibers and a higher percentage of type IIB fibers were observed in the 9- and 21-week-old OLETF rats compared with the age-matched LETO rats. In addition, a lower percentage of type I fibers in the plantaris muscle was observed in the 9-week-old OLETF rats compared with the age-matched LETO rats, and there were no type I fibers in the plantaris muscle of the 21-week-old OLETF rats. All 20-week-old OLETF rats showed serum insulin concentrations of more than 20 ng/ml. The present study suggests that insulin resistance and impaired glucose tolerance from altered muscle fiber type are linked to the cause of diabetes.

Long-term metabolic and skeletal muscle adaptations to short-sprint training: implications for sprint training and tapering.

The adaptations of muscle to sprint training can be separated into metabolic and morphological changes. Enzyme adaptations represent a major metabolic adaptation to sprint training, with the enzymes of all three energy systems showing signs of adaptation to training and some evidence of a return to baseline levels with detraining. Myokinase and creatine phosphokinase have shown small increases as a result of short-sprint training in some studies and elite sprinters appear better able to rapidly breakdown phosphocreatine (PCr) than the sub-elite. No changes in these enzyme levels have been reported as a result of detraining. Similarly, glycolytic enzyme activity (notably lactate dehydrogenase, phosphofructokinase and glycogen phosphorylase) has been shown to increase after training consisting of either long (>10-second) or short (<10-second) sprints. Evidence suggests that these enzymes return to pre-training levels after somewhere between 7 weeks and 6 months of detraining. Mitochondrial enzyme activity also increases after sprint training, particularly when long sprints or short recovery between short sprints are used as the training stimulus. Morphological adaptations to sprint training include changes in muscle fibre type, sarcoplasmic reticulum, and fibre cross-sectional area. An appropriate sprint training programme could be expected to induce a shift toward type IIa muscle, increase muscle cross-sectional area and increase the sarcoplasmic reticulum volume to aid release of Ca(2+). Training volume and/or frequency of sprint training in excess of what is optimal for an individual, however, will induce a shift toward slower muscle contractile characteristics. In contrast, detraining appears to shift the contractile characteristics towards type IIb, although muscle atrophy is also likely to occur. Muscle conduction velocity appears to be a potential non-invasive method of monitoring contractile changes in response to sprint training and detraining. In summary, adaptation to sprint training is clearly dependent on the duration of sprinting, recovery between repetitions, total volume and frequency of training bouts. These variables have profound effects on the metabolic, structural and performance adaptations from a sprint-training programme and these changes take a considerable period of time to return to baseline after a period of detraining. However, the complexity of the interaction between the aforementioned variables and training adaptation combined with individual differences is clearly disruptive to the transfer of knowledge and advice from laboratory to coach to athlete.

Effects of high-intensity resistance training on untrained older men. II. Muscle fiber characteristics and nucleo-cytoplasmic relationships.

During growth and repair of skeletal muscle fibers, satellite cells become activated, undergo mitosis, and a daughter nucleus becomes incorporated into the muscle fiber to increase myonuclear numbers. An increase in myonuclei appears to be required for this postnatal growth. This study examined whether muscle fibers of elderly men can hypertrophy with strength training and, if so, whether they have the capacity to incorporate nuclei into the fibers. The sarcoplasmic area associated with each myonucleus was calculated in nine elderly men before and after 16 weeks of strength training, and compared to nine elderly control men. Muscle fiber type changes and myosin heavy chain composition were also compared. All major fiber types (I, IIA, IIB) became significantly larger after training, and a transition of type IIB fibers to IIA occurred with training. The area occupied by each fiber type correlated with myosin heavy chain percentage, and both of these changed similarly with strength training. The cytoplasm-to-myonucleus ratio increased, but not significantly (p = .07), with muscle fiber hypertrophy. Number of myonuclei per fiber and myonuclei per unit length of muscle fiber increased, but not significantly. Cross-sectional areas of the muscle fibers in untrained elderly men were much smaller than in untrained young men (when compared with our earlier studies). Training increased the sizes of the elderly muscle fibers to that of the untrained young men. This hypertrophy of muscle fibers by 30% with training resulted in no change in the cytoplasm-to-myonucleus ratio. This suggests that the myonuclear population continues to adapt to growth stimuli in the elderly muscles.

Eccentric Strength Training Prescription for Older Adults

This article describes changes in muscle fiber size and type, motor unit remodeling, altered recruitment patterns, and a sedentary lifestyle, which are often causes of muscle weakness in many aging adults. While some age-related losses of function appear unavoidable, it is also apparent that older persons can capitalize on their relatively higher eccentric strength reserve and use it advantageously in training protocols. Appropriate exercise prescription, such as gradual progression of intensity, an emphasis regarding safe body mechanics and posture, and frequent feedback on improvements, will help to optimize gains during resistance training, leading to increased maximal strength and endurance capacity for activities of leisure and daily living.

Differences of muscle fiber type and temporal change of K-value among parts toward depth of dorsal muscle in carp (cultured)

SUMMARY: In order to clarify the influence of the interposition of pink muscle fiber into the dorsal ordinary muscle on the post-mortem temporal change of K-value, using carp, the dorsal muscle was divided into three muscle parts toward depth with the naked eye, as follows: dark muscle part, intermediate muscle part, and deepest ordinary muscle part. The muscle fiber types in these parts were discriminated by the inactivation of actomyosin ATPase activity for both acid and alkaline pre-incubations, and the temporal changes of ATP related compounds in these parts were then measured at a kept temperature of 32°C. Three muscle parts were organized from the muscle fiber types as follows: dark muscle part was organized from only red muscle fiber type, intermediate part from only pink muscle fiber type in a thin layer and from two muscle fiber types of not only pink muscle fiber but also white muscle fiber of the IIa or IIb subtype in a region of mosaic pattern, and deepest ordinary muscle part from two muscle fiber types of white muscle fiber IIa or IIb. The temporal change of K-value among muscle fiber types was considerably different and remarkably faster in the order of red muscle fiber, pink muscle fiber, and white muscle fiber. From these results, it was considered that the interposition of pink muscle fiber into the dorsal ordinary muscle might accelerate the temporal change of K-value.

Adverse changes in fibre type and myosin heavy chain compositions of human jaw muscle vs. limb muscle during ageing.

This review shows that human jaw muscles not only have unique fibre type and myosin heavy chain (MyHC) compositions but also undergo muscle and region-specific changes in fibre composition during ageing. Alterations in the masseter and the lateral pterygoid muscles in the elderly are opposite to those reported for limb and trunk muscles, whereas changes in the anterior and posterior bellies of the digastric muscle resemble those of limb and trunk muscles. We conclude that age-related alterations in fibre type composition and MyHC expression are muscle and region specific, probably reflecting muscular differences in genetic programs and epigenetic influences.

Effect of weight loss on muscle fiber type, fiber size, capillarity, and succinate dehydrogenase activity in humans.

To examine the effects of weight loss on muscle oxidative properties, nine obese subjects (body mass index, 34 +/- 1.5) had muscle biopsies before and after weight loss and weight stabilization. Weight loss ranged from 13-32 kg and represented 20.8 +/- 2.1% of initial weight. After weight loss, there was no change in the proportions of oxidative (type I and type IIa) fibers and also no change in mean fiber cross-sectional area, whereas there was a small, but significant, decrease in the relative interstitial space (P < 0.05). However, weight loss resulted in a 32 +/- 6% (mean +/- SEM) increase in capillary/fiber ratio and a 54% increase in capillary density (P < 0.05). In addition, there was a 41 +/- 13% increase in succinate dehydrogenase (SDH) activity (P < 0.05). This increase in muscle capillarization and SDH activity was seen in all fiber types, even the relatively lower oxidative type IIx fibers. There was a strong correlation between the change in capillary/fiber ratio and the change in SDH activity (r = 0.82; P < 0.02). Thus, weight loss resulted in no change in muscle fiber type or cross-sectional area, but produced increases in capillary/fiber ratio, capillary density, and SDH activity, suggesting an increase in muscle oxidative capacity.

The effect of aging on skeletal muscle capillarization in a murine model.

Capillarization of skeletal muscle has been reported to be both maintained and reduced with advancing age. This conflict may represent methodological differences between biopsy studies. We have examined capillarization throughout two muscles, soleus and extensor digitorum longus (EDL), from a well-established colony of aging mice, and related this to fiber number (C/F ratio) and type. Labeling of muscle capillaries was performed with the biotinylated Griffonia (Bandeiraea) simplicifolia lectin (GSL 1) using immunochemistry. The results showed a significant increase in the C/F ratio in the aged mice when compared with the younger (6-month mice soleus = 1.296, 95% CI 1.226-1.366 vs 28-month mice soleus = 1.530, 95% CI 1.488-1.572, p <.001; 6-month mice EDL = 0.881, 95% CI 0.751-1.011 vs 28-month mice EDL = 1.124, 95% CI 1.028-1.220, p = .017). These differences could not be accounted for by changes in fiber type but may reflect loss of fibers. Alternatively, there may be increased angiogenic drive or a failure of downregulation of angiogenesis.

Age-related changes in muscle fiber types in the human thyroarytenoid muscle: An immunohistochemical and stereological study using confocal laser scanning microscopy

A decline in motor performance contributes to laryngeal dysfunction in the elderly, but the pathogenetic mechanisms are unknown. Quantitative 3-dimensional, age-related changes in the muscle fiber content of the human thyroarytenoid muscle were estimated from geometric probability (stereology) by use of a technique that provided a statistically unbiased sample of all possible section orientations and locations in the entire muscle volume. There was a preferential 27% age-related loss in the length density (L V type, muscle) of type 1 (slow) fibers in contrast to the selective type 2 (fast) fiber loss typical of aging limb muscles. In type 2 fibers there was no significant loss in the L V, but there was an age-related decrease ( P < 0.05) in the surface density (S V type, muscle) and an increase ( P < 0.05) in the atrophy factor, an index of the content of very small, atrophic fibers. There was also an age-related increase in the length fraction (L L type, all fibers) of muscle fibers that coexpress both fast and slow myosin heavy-chain isoforms ( P < 0.05). These findings demonstrate a type-specific fiber loss and atrophy that differs from that in aging limb muscles and an age-related increase in motor unit remodeling. (Otolaryngol Head Neck Surg 1999;121:441-51.)

Developmental changes in histochemical properties of intrinsic laryngeal muscles in rats

Objective: Using neonatal rats, the developmental changes in muscle fiber type of the intrinsic larygeal muscles were analyzed. The potential influence of two factors were also studied, that were predicted would influence developmental changes in muscle fiber type, denervation and hypothyroidism. Methods: Using the histochemical technique of myosin ATPase staining, postnatal changes in the ratio of muscle fiber types of each intrinsic laryngeal muscle were determined. In addition, to clarify factors influencing the development of the intrinsic laryngeal muscles, the same technique was employed to study recurrent laryngeal nerve–denervated rats and rats with experimentally-induced hypothyroidism. Results: In normal pups, type 2C fibers had almost disappeared by postnatal day (PND) 14. In denervated pups, differentiation to type 1 and 2A muscle fibers was not observed. In contrast, differentiation to type 2B muscle fibers was impaired in the hypothyroid pups. Conclusion: The differentiation of intrinsic laryngeal muscles occur earlier than that of hindlimb muscles. Each intrinsic laryngeal muscle exhibits a particular pattern of developmental changes in normal pups. The developmental changes in the intrinsic laryngeal muscles are affected by recurrent laryngeal nerve innervation and thyroid hormonal control. The findings suggest that both recurrent laryngeal nerve innervation and thyroid hormone play important roles in the differentiation of the intrinsic laryngeal muscles.