Bilateral effect of a unilateral occlusal splint on the expression of myosin heavy-chain isoforms in rat deep masseter muscle
Introduction
Mammalian skeletal muscle fibres contain myosin, an important molecule that exhibits the calcium-dependent ATPase activity responsible for the mechanical force generated by the whole muscle. As the muscle cell matures, different selections of isoforms are produced, adapted to the changing demands for speed, strength, and endurance in the foetus, the newborn and the adult. Within a single adult muscle, velocity is correlated with activity (Barany, 1967). Some myofibrils contract slowly but exhibit much shortening and therefore can produce large movement. In contrast, others contract rapidly and produce short but intense movement. These characteristics suggest distinct functional properties of the different myosin heavy-chain isoforms that are indicated by the muscle fibre type.
The expression of these different isoforms (for review, see Schiaffino and Reggiani, 1994) has been largely explored and their regulation during development is known to be programmed and muscle-specific (Whalen et al., 1981, Whalen et al., 1981, d'Albis et al., 1986). Immature and regenerating fibres express two developmental isoforms whereas mature fibres express four major adult isoforms, one slow (type I) and three fast (types IIa, IId/x, IIb).
Many studies have reported the potential of masseter muscle to adapt its particular structure to specific functional demands. The proportions of various myosin heavy-chain isoforms as a function of the diet of different species is one striking example (for review, see Rowlerson, 1990). In addition, particularly in rodents, masseter muscle displays a marked sexual dimorphism, observed in terms of isoform expression (d'Albis et al., 1989, Rowlerson, 1990), and various types of myosin heavy-chain are coexpressed within a single muscle fibre (Thornell et al., 1984).
Disturbances in masticatory function, from the diet or the wearing of bilateral appliances to increase the occlusal vertical dimension are well documented (Kiliaridis et al., 1988, Maeda et al., 1987) but less attention has been paid to the expression of myosin heavy-chain isoforms induced by a unilateral occlusal disorder in the masseter.
By using the myosin–ATPase reaction, Ulrici et al. (1985) report differences in the fibre types present on both the occlusal-splint side and the contralateral side of the masseter of miniature pigs. An electromyographic study by Zhang et al. (1994) demonstrates the preference of pigs to chew food on the bite-splint side. The pig was chosen for these studies as its masticatory apparatus is similar to that of humans. Yaffe et al. (1991) demonstrated that bilateral bite-raising splints reduced electromyographic activity in the rat deep masseter, but did not modify the muscle fibre types. In humans with temporomandibular joint disturbance syndrome, an occlusal splint reduced masseter electromyographic activity and induced a decrease in masseter muscle activity (Shi and Wang, 1991). The effect of an occlusal splint on masticatory muscles was then proposed to exert a therapeutic effect. More recently, Sfondrini et al. (1996) demonstrated, in rats subjected to a protusive appliance, a decrease in the numbers of type IIb fibres and an increase in types IId/x or IIa fibres in the superficial masseter. In addition, the diameters of all muscle fibres were decreased by diet treatment (Maeda et al., 1987).
Our aim now was to investigate physiopathological and morphological changes occurring bilaterally in the deep masseter muscles after fixing a unilateral occlusal splint to the mandibular molars in rats.We measured the variations in the proportions of five (four major adult and one developmental) myosin heavy-chain isoforms, using immunohistochemistry and electrophoresis.
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Animals and muscles
The experiments were performed on 9-week-old male Wistar rats from a colony maintained in the University of Montpellier, France. All animals were provided with standard rodent food ad libitum and housed in an air-conditioned protected unit at a constant temperature of 20°C with a 12/12 h light/dark cycle. The rats were anaesthetized with an intraperitoneal injection of a solution containing ketamine (40 mg/kg), xylazine (8 mg/kg) and acepromazine (1.2 mg/kg). Next, they were fitted with a l.5
Body and muscle weight
Differences were observed in the body weights of each rat group (Fig. 1). The body weights of all experimental rats were less than those of the standard reference population. A standard curve for body-weight variation was obtained from the supplier of the Wistar rats (IFFA CREDO, les Oncins, 69592 L'Arbresle, France). The body weights of splint wearers changed little during the first week, then resumed normal growth to standard values during the remainder of the experimental period.
There were
Discussion
Our histological observations showed no inflammation in any of the sections studied. These findings are in agreement with those of Akagawa et al. (1983a), who did not detect any inflammatory or degenerative changes in the posterior part of the deep masseter after 4 days of bilateral splint wearing. In their study, inflammation was more apparent after 12 h to 2 days of occlusal disturbance.
Rowlerson (1990) reports that the few (<l%) slow fibres are concentrated in the deepest layers of the rat
Acknowledgements
We are grateful to Laurence Maurin for high-quality animal care; Nicole Lautredou, Georges Peyriere, Patrick Atger, Gérard Gauffier and Alain Sahuquet for technical assistance; Alain Westphal for his advice in the fixing of splints, Francis Bacou for helpful discussion; Nicole Helaine-Marshall for reviewing the manuscript. We thank the Centre de Recherche en Imagerie Cellulaire (CRIC, Montpellier, France) for granting access to their installations. This research was supported by grants from
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