Stapedius muscle fibre composition in the rat

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Stapedius muscle fibre characterization in the noise exposed and auditory deprived rat

In skeletal muscle, interventions that unload the muscle cause slow-to-fast myosin heavy chain (MHC) conversions, whereas fast-to-slow conversions are seen when the muscles are engaged in resistance training and endurance exercise. The stapedius muscle (SM) is reported to prevent cochlear damage by noise. This theory may be supported by showing comparable changes of muscle fibre composition when ears are exposed to longstanding noise (SM training). Comparable changes after sound deprivation (SM unloading) would suggest that the SM needs a certain degree of daily activity evoked by environmental sound to sustain its normal composition. We investigated the difference in myosin composition of SM fibres from rats exposed to noise, from auditory deprived rats and from rats exposed to low level ambient noise (control group). Consecutive complete SM cross-sections were processed by enzymehistochemistry to determine acid/alkali lability of myofibrillar adenosine triphosphatase (mATPase) and by immunohistochemistry using MHC antibodies. Fibres were assigned to mATPase type I, IIA, IIX or 'Miscellaneous' categories. Per mATPase category, the fibres were attributed to groups with specific MHC isoform compositions. Auditory deprivation lasting nine weeks was accomplished by closure of the external meatus at the age of three weeks. A slow-to-fast shift was seen in these rats when compared to the control group. The noise exposed group was exposed to 65-90dB sound pressure level during a period lasting nine weeks from the age of three weeks onwards. A shift from an overwhelming presence of type mATPase IIX, as seen in the control group, to type mATPase IIA occurred in the noise exposed group. Also, more MHC IIA/IIX hybrid fibres were found in the mATPase IIX category. An adaptive response to the acoustic environment in the characteristics of the fibres of the SM, comparable to the response in skeletal muscles on unloading and training activity, can be ascertained. This supports the theory that the SM plays an active role in modulating external acoustic energy on entry to the cochlea. Our results are also in favour of another postulated function of the SM, the unmasking of high-frequency signals in low-frequency background noise.

Stapedius muscle fiber characterization during postnatal development in the rat

The stapedius muscle (SM) is reported to prevent cochlear damage by noise. Functional demands are then the ability of fast contraction with long endurance. At the end of the third postnatal week, the middle ear of the rat is completely pneumatized and according to electrophysiological data, the auditory function starts to match the adult. We investigated the developmental changes in myosin composition of SM fibres using consecutive complete SM cross-sections (taken from rats on post natal day (PND) 7, 14, 16, 21, 28, 42 and 84) which were processed by enzymehistochemistry to determine acid/alkali lability of myofibrillar adenosine triphosphatase (mATPase) and by immunohistochemistry using myosin heavy chain (MHC) antibodies (mAb). Fibres were assigned to mATPase type I, IIA, IIB, IIX or 'Miscellaneous' categories. Per mATPase category, the fibres were attributed to groups with specific MHC isoform compositions. Neonatal MHC expression could not be documented with the mAb used. However, embryonal (Emb) MHC was expressed at PND 7, very little at PND 14; at later PND fibres did not show Emb MHC. In general, the mATPase-based classification did not show large alterations after PND 21. Expression of MHC IIB, which was present in almost 50% of the fibres at PND 7 and 14, diminished to 3% at PND 84. A decrease in number of fibres expressing more than one MHC isoform was found. These results show that the SM is a precociously developing muscle compared to limb muscles and even to the diaphragm. Moreover, it is shown that the expression of the adult MHC isoform phenotype coincides with the onset of auditory function in the third postnatal week.

Myosin heavy chain composition of rat middle ear muscles

OBJECTIVE

To quantitatively analyze myosin heavy chain (MHC) mRNA composition in two rat middle ear muscles (the tensor tympani and stapedius) using competitive polymerase chain reaction (PCR).

MATERIAL AND METHODS

An exogenous template, including oligonucleotide sequences specific for the seven rat MHCs (2A, 2B, 2X, 2L/EOM, embryonic, neonatal and beta-cardiac) as well as beta-actin, was constructed and used as the competitor.

RESULTS

The tensor tympani and stapedius contained all MHC isoforms except 2L. The tensor tympani contained approximately equal proportions of 2X (40.4% +/- 6.5%) and 2A (34.0% +/- 1.3%) MHCs, with a smaller percentage of 2B (16.6% +/- 1.5%) and neonatal (7.5% +/- 0.6%) MHCs, while beta-cardiac and embryonic MHCs were minimally expressed. The stapedius contained predominantly 2X (58.0% +/- 4.2%) and 2A (32.3% +/- 6.7%) MHCs, with a smaller percentage of 2B (7.4% +/- 0.2%) and beta-cardiac (1.9% +/- 0.1%) MHCs. Neonatal and embryonic MHCs were detected at very low levels.

CONCLUSION

These results suggest that two middle ear muscles, which are mainly composed of two fast-twitching myosins (2X and 2A MHCs), contract fast and are fatigue-resistant.

Muscle fiber type IIX atrophy is involved in the loss of fat-free mass in chronic obstructive pulmonary disease

BACKGROUND

Although the loss of peripheral muscle mass has been shown convincingly in chronic obstructive pulmonary disease (COPD), the underlying pathogenesis remains unclear.

OBJECTIVE

The aim of the present study was to determine the relations between skeletal muscle fiber types, fiber cross-sectional area (CSA), enzyme activities, and fat-free mass (FFM) in patients with COPD and in control subjects.

DESIGN

In 15 patients with COPD and 15 healthy, age-matched control subjects, FFM was determined by dual-energy X-ray absorptiometry and bioelectrical impedance analysis. In biopsy specimens from the vastus lateralis fiber types, fiber CSA and activities of cytochrome oxidase (EC 1.9.3.1), succinate dehydrogenase (EC 1.3.99.1), and glycogen phosphorylase (EC 2.4.1.1) were examined immunohistochemically and histochemically.

RESULTS

Compared with control subjects, patients with COPD had less FFM (49 compared with 59 kg, P = 0.030) and lower mean fiber CSA (3839 compared with 4647 microm(2), P = 0.037). A strong correlation (r = 0.87, P < 0.001) was observed between the FFM measured by bioelectrical impedance analysis and mean fiber CSA in patients with COPD. Within fiber-type categories the mean CSA of only the IIA/IIX and IIX fiber types was lower in patients than in control subjects [3358 compared with 4428 microm(2) (P = 0.022) and 2566 compared with 4248 microm(2) (P = 0.003), respectively]. In COPD, 20% of the type IIX fibers lacked stainable activities of cytochrome oxidase, succinate dehydrogenase, and glycogen phosphorylase, and this proportion correlated negatively with type IIX fiber CSA (r = -0.65, P = 0.012).

CONCLUSIONS

Muscle fiber atrophy occurs in the vastus lateralis in patients with COPD and contributes to the loss of muscle mass in COPD. Atrophy is specific to fiber types IIA/IIX and IIX and is associated with a disturbed metabolic capacity.

Hybrid skeletal muscle fibres: a rare or common phenomenon?

1. The main aim of the present review is to raise awareness of the molecular complexity of single skeletal muscle fibres from "normal" and "transforming" muscles, in recognition of the many types of hybrids that have been observed in vertebrate skeletal muscle. The data used to illustrate various points made in the review were taken from studies on mammalian (mostly rat) and amphibian muscles. 2. The review provides a brief overview of the pattern and extent of molecular heterogeneity in hybrid muscle fibres and of the methodological problems encountered when attempting to identify and characterize such fibres. Particular attention is given to four types of skeletal muscle hybrids: (i) myosin heavy chain (MHC) hybrids; (ii) mismatched MHC-myosin light chains (MLC) hybrids; (iii) mismatched MHC-regulatory protein hybrids; and (iv) hybrids containing mismatched MHC-sarcoplasmic reticulum protein isoforms. 3. Some of the current ideas regarding the functional significance, origin and cognitive value of hybrid fibres are examined critically.