Expression of sarcoplasmic-endoplasmic reticulum Ca-ATPase isoforms in masticatory muscles.

The aim of this study was to characterize the sarcoplasmic-endoplasmic reticulum Ca-ATPase (SERCA) isoforms in rabbit masticatory muscles compared with those in fast-twitch muscle. It was hypothesized that combined expression of the SERCA isoforms in fast- and slow-twitch muscles accounts for lower Ca-ATPase activity. SERCA was isolated by differential centrifugation, the isoforms were determined by ELISA, and the activity of each isoform was measured using a colorimetric method. Activity was tested for significance by anova, and the distribution of isoforms was assessed using the chi-square test (P < 0.05) and correlated to SERCA activity using Spearman's rank correlation. SERCA1 was predominant (90.5%) in fast-twitch muscle, whereas a mixture of SERCA isoforms was found in masticatory muscles: 62-78% was SERCA2, 20-37% was SERCA1, and the SERCA3 content was negligible. Depressor muscles showed a significantly higher content (77.8%) of SERCA2, and elevator muscles showed a higher content (35.4%) of SERCA1. Elevator muscles showed higher expression of SERCA2a (58%), and depressor muscles showed higher expression of SERCA2b (20%). The SERCA1 content was mainly SERCA1a and significantly higher for elevator muscles (33%), whereas depressor muscles showed a higher content of SERCA1b (4%). The SERCA1 content of fast-twitch muscle was mainly SERCA1a (88.5%). It is concluded that the mixture of different SERCA isoforms, along with a substantial content of SERCA2b, in masticatory muscles would support lower Ca-ATPase activity and calcium transport.

The effects of increasing occlusal vertical dimension on the deep masseter of rat at different ages.

OBJECTIVE: To investigate the influence of increasing the occlusal vertical dimension (iOVD) on the fibre-type distribution and ultrastructure of deep masseter of rat at different ages. DESIGN: A total of forty-eight male Wistar rats were divided into two groups according to age: 'teenage' group (n=24, 1.5 months) and 'young adult' group (n=24, 8 months). Both the teenage and the young adult rats were then randomly divided into the control group (n=12) and the experimental group (n=12). The occlusal vertical dimensions of the rats in the experimental groups were increased by placing composite resin on all maxillary molars. The fibre-type distribution and ultrastructure of the deep masseter were subsequently observed on day 7 and day 14 after iOVD. RESULTS: In the teenage experimental group, the proportion of type IIa fibres increased, while the proportion of type IIb and type IIx fibres decreased by day 7 after iOVD (P<0.05). However, no significant fibre phenotype transformation was observed in the young adult experimental group until day 14 after iOVD. In addition, the proportion of type IIa in the teenage experimental group was higher than that of the young adult experimental group on day 7 and 14 (P<0.05). Under the transmission electron microscope, muscle fibre reconstruction and the compensatory increase in the number and volume of mitochondria appeared earlier in the teenage experimental group. The cellular traumatic reaction was less than that in the young adult experimental group. CONCLUSION: The teenage rat alters masseter muscle structure to a slower phenotype earlier and to a greater degree than that of the young adult rat when increasing the occlusal vertical dimension.

Effect of changes in food consistency on NADH-ubiquinone oxidoreductase activity and levels of mRNA for ND1, 51kDa, 75kDa and myosin heavy chain isoforms in two different portions of rat masseter muscle.

We investigated the effect of a change in food consistency on properties of the masseter muscle in 3-week-old rats fed a soft diet for 9 weeks (Group S) and fed a soft diet for 5 weeks followed by a hard diet for 4 weeks (Group S-H). The NADH-O2 oxidoreductase activity, levels of mRNAs transcribed from genes encoding NADH-ubiquinone oxidoreductase (Complex I: ND1, 51kDa, and 75kDa) and myosin heavy chain (MyHC) isoforms and the phenotype of the muscle fibers were measured in the superficial and deep portions of the muscle. In the period from 8 weeks to 12 weeks of age, NADH-O2 oxidoreductase enzyme activity in both the superficial and deep portions of the muscle showed similar patterns in Group S and Group S-H. In contrast, the ND1, 51kDa and 75kDa mRNA levels in the superficial and deep portions of the masseter muscle in the Group S-H were higher than those of Group S in the 12-week-old rats, except for the 51kDa mRNA in the superficial portion of the masseter muscle. MyHC-IIa and MyHC-IId/x mRNA levels in the superficial portion of the masseter muscle were higher in the Group S-H than in the Group S. These observations suggest that short-term feeding stress such as the transition from a soft diet to a hard diet causes changes in oxidative metabolism, in mRNA levels for the Complex I components ND1 and 75kDa, and the mRNA levels for the MyHC isoforms IIa and IId/x in the superficial portion of rat masseter muscle, but no changes in the composition of muscle fiber types.

Metabolic Changes in Masseter Muscle of Rats Submitted to Acute Stress Associated with Exodontia.

Clinical evidence has shown that stress may be associated with alterations in masticatory muscle functions. Morphological changes in masticatory muscles induced by occlusal alterations and associated with emotional stress are still lacking in the literature. The objective of this study was to evaluate the influence of acute stress on metabolic activity and oxidative stress of masseter muscles of rats subjected to occlusal modification through morphological and histochemical analyses. In this study, adult Wistar rats were divided into 4 groups: a group with extraction and acute stress (E+A); group with extraction and without stress (E+C); group without extraction and with acute stress (NO+A); and control group without both extraction and stress (NO+C). Masseter muscles were analyzed by Succinate Dehydrogenase (SDH), Nicotinamide Adenine Dinucleotide Diaphorase (NADH) and Reactive Oxygen Species (ROS) techniques. Statistical analyses and two-way ANOVA were applied, followed by Tukey-Kramer tests. In the SDH test, the E+C, E+A and NO+A groups showed a decrease in high desidrogenase activities fibers (P < 0.05), compared to the NO+C group. In the NADH test, there was no difference among the different groups. In the ROS test, in contrast, E+A, E+C and NO+A groups showed a decrease in ROS expression, compared to NO+C groups (P < 0.05). Modified dental occlusion and acute stress--which are important and prevalent problems that affect the general population--are important etiologic factors in metabolic plasticity and ROS levels of masseter muscles.

Coordinate expression of Ca2+-ATPase slow-twitch isoform and of beta calmodulin-dependent protein kinase in phospholamban-deficient sarcoplasmic reticulum of rabbit masseter muscle.

Modulation of sarcoplasmic reticulum (SR) Ca(2+) transport by endogenous calmodulin-dependent protein kinase II (CaM K II) involves covalent changes of regulatory protein phospholamban (PLB), as a common, but not the only mechanism, in limb slow-twitch muscles of certain mammalian species, such as the rabbit. Here, using immunofluorescent techniques in situ, and biochemical and immunological methods on the isolated SR, we have demonstrated that rabbit masseter, a muscle with a distinct embryological origin, lacks PLB. Accommodating embryological heterogeneity in the paradigm of neural-dependent expression of specific isogenes in skeletal muscle fibers, our results provide novel evidence for the differential expression in the SR of 72 kDa beta components of CaM K II, together with the expression of a slow-twitch sarcoendoplasmic reticulum Ca(2+)-ATPase isoform, both in limb muscle and in the masseter.

The influence of temperature on the distribution and intensity of the reaction product in rat muscle fibers obtained with the histochemical method for myosin ATPase.

The influence of temperature in the incubation medium on the localization and intensity of myosin ATPase was investigated in striated muscles from the rat using a conventional histochemical technique. It was found that the enzyme reaction was temperature-dependent since the activity in some fibers was raised and in others was depressed by alteration of the incubation temperature. There was no obvious correlation between the temperature sensitivity of ATPase in the muscle fibers and their activity for succinic dehydrogenase. It is proposed that the histochemical method for myosin ATPase can be used for demonstration of isoenzymes in striated muscle fibers.

Possible involvement of histamine in muscular fatigue in temporomandibular disorders: animal and human studies.

As an approach to clarifying the molecular basis of pain and fatigue in muscles involved in temporomandibular disorders, we examined the activity of histidine decarboxylase (HDC), the enzyme which forms histamine, in the masseter muscles of mice. In the resting muscle, HDC activity was very low. Direct electrical stimulation of the muscle markedly elevated HDC activity. HDC activity rose within 3 hrs of the electrical stimulation, peaked at 6 to 8 hrs, and then gradually declined. Intraperitoneal injection of a small amount of interleukin-1 (IL-1) (from 1 to 10 microg/kg) produced a similar elevation of HDC activity in the masseter muscle. We also examined the effect of an antihistamine, chlorphenylamine (CP), on temporomandibular disorders in humans and compared it with that of an anti-inflammatory analgesic, flurbiprofen (FB). Two groups received one or the other of the drugs daily for 7 days, and they were asked about their signs and symptoms before and after the treatment. A positive evaluation of their treatment was made by 74% of the CP group, but by only 48% of the FB group. Although the effects of CP on the limitation of mouth-opening and on joint noise were negligible, about 50% of the CP group answered positively concerning the drug's effect on spontaneous pain or pain induced by chewing or mouth-opening. The positive evaluation for CP (50%) in relieving associated symptoms (headache or shoulder stiffness) was significantly greater than for FB (13%). FB showed effectiveness similar to but sometimes weaker than that of CP on several symptoms. On the basis of these and previous results and the known actions of histamine, we propose that the histamine newly formed following the induction of HDC activity, which is itself mediated by IL-1, may be involved in inducing pain and, possibly, stiffness in muscles in temporomandibular disorders.

Histochemical fibre-type profile in the human masseter muscle.

A histochemical characterization of the masseter muscle was performed on biopsy samples of dentate subjects with normal occlusion. There was a continuum of ranges of oxidative and glycolytic capacities of the masseter muscle fibres. Besides the lightly-stained type I and the darkly-stained type II fibres, fibres with intermediate staining reactions for standard ATPase at pH 9.4, IM fibres, were seen in all biopsy samples. IM fibres had some staining characteristics in common with type I, i.e. the reaction for NADH-TR and for ATPase after preincubation at pH 4.6 and 4.2. Like type II fibres they showed strong reaction for menadione-linked alpha-glycerophosphate dehydrogenase and for phosphorylase. The ATPase reaction after preincubation at pH 4.6 did not generally reveal subtypes of type II. It is concluded that the masseter muscle in normal human subjects has a very special fibre composition, with ATPase-IM fibres being a part of the normal fibre population.

Further histochemical studies on masticatory muscles.

This paper describes a histochemical and histographic analysis of the masticatory muscles obtained from 78 early autopsy samples from subjects from 4 days to 87 years old. Five groups of muscles have been stuied: the temporalis, the medial and lateral pterygoid, the superficial bundle of the masseter and the mylohyoideus. All adult muscles have consistently shown a markedly increased number oftype II fibres and a disparity in the size of the two main fibre types, the average diameter of type II fibres being about half that of type I fibres. Fibres of intermediate size and stain were observed with myofibrillar ATPase at pH 9.40. A negative relation between the percentage of type II fibres and intermediate fibres was found, but not between the percentage of type I fibres and of intermediate fibres. Another negative relation was found between the number and the size of type II fibres, again not present in type I nor in intermediate fibres. In children, from 6 days old, an increased number of type II fibres and a definite disparity in the size of the two main fibre types were found. Intermediate fibres were present on the 17th day. Up to the age of 13 years, their diameter was greater than that of type I fibres. The analysis of the distribution and size modifications of the various fibre types seems to indicate a progressive adaptation of the masticatory muscles. This adaptation of the fibres to the successive reactions and to the various movements of the masticatory system is then discussed.

A quantitative histochemical study of the spatial distribution of intrafascicular fibre types in the porcine masseter and soleus muscles.

The distribution of intrafascicular type I and II muscle fibres together with proportions of edge- and centrally located type I and II fibres within whole fascicles were analysed by myosin ATP-ase histochemistry in 241 porcine masseter fascicles (six masseter muscles) and compared with result from 63 pig soleus fascicles (five soleus muscles). All fascicles were from 11 domestic pigs (1 yr old, 70-90 kg body weight, all female). The proportions of type I fibres (slow) and type II fibres (fast) on the edge of fascicles differed significantly from the proportions centrally. All the soleus fascicles had higher proportions of centrally located type I fibres. Only seven out of 241 (3%) masseter fascicles diverged in this respect and showed reversed intrafascicular fibre-type proportions with more edge-located type I fibres. Analysis of the fascicular distribution of type I and II fibres revealed that the porcine masseter had type II fibres as the predominant type. Between 68-87% of the total fibres were type II (p < 0.001). The intrafascicular content of type I fibres increased towards the deep part of the masseter. In four of five soleus muscles the type II fibre population was dominant (p < 0.01). However, one soleus revealed equal proportions of type I and II fibres.

Histochemical characterization of masseter muscle fibres in a biopsy study of normal young women.

Muscle fibres from biopsies of the anterior part of the masseter muscle (pars superficialis) were histochemically characterized in 13 healthy female students. They were 21-28 yr old with a full complement of teeth, and normal facial and occlusal relations. Before surgery, normal masseter muscle function was demonstrated by bite-force measurements and recordings of electromyographic activity. After staining for myosin ATPase activity, the relative mean areas of muscle fibres were: type I 82.9%, type IM 9.5% and type II 8.3%. The intraindividual (18-155% of mean) and interindividual (0-216% of mean) variation of the fibre size was large. The type I fibres had the largest diameter (10-80 microns, mean: 39 microns), the type II fibres (6-47 microns, mean: 21 microns) and the IM fibres (8-54 microns, mean: 28 microns) the smallest. The biopsy technique and the histochemical characterization will be suitable for reference in women with functional changes or diseases of the masseter muscle.

Changes in the pattern of SDH and PhR staining in fibres of rat masseter muscle following long-term functional stretch.

Histochemical changes in the three basic fibre types of the deep masseter muscle of the rat were studied following functional stretch due to increased vertical dimension of the occlusion. A large number of muscle fibres showed changing histochemical profiles for succinic dehydrogenase (SDH) and phosphorylase (PhR) during degeneration and subsequent regeneration of muscle fibres. On the other hand, a small number of muscle fibres with well defined sarcolemmas presented a normal pattern (i.e. low SDH and high or intermediate PhR activities) during this experiment. The ratio between the three basic fibre types remained unchanged after recovery of the muscle fibres. These results suggest that fast-twitch-oxidative-glycolytic and slow-twitch-oxidative fibres underwent degeneration and regeneration, while almost all the fast-twitch-glycolytic fibres retained normal SDH and PhR staining following functional stretch.

Histochemical enzyme profile of the masseter, temporal and lateral pterygoid muscles of the European hedgehog (Erinaceus europeaus).

In man, there are large differences in histochemical fibre-type composition, distribution and size between jaw and trunk muscles, probably related to the special functions of the human stomatognathic system. In the hedgehog, the influence of alkaline and acid pre-incubations on the reaction for myofibrillar ATPase was different from that in man, suggesting a different myosin structure; the fibre composition was different also. The masseter, the superficial portion of the temporal and the lateral pterygoid muscles all showed a homogeneous fibre type profile with almost 100 per cent alkali-stable fibres. In two animals, the deep temporal muscles showed an apparent heterogeneous fibre pattern with 81 per cent alkali-stable fibres, 4 per cent alkali-labile fibres and 15 per cent ATPase-intermediate fibres; in one animal 87 per cent alkali-stable fibres and 13 per cent ATPase-intermediate fibres. There was no difference in cross-sectional area between the three fibre types within each muscle, but the fibres of the lateral pterygoid were smaller than the alkali-stable and the alkali-labile fibres of the masseter and temporal muscles. The limb and trunk muscles showed reactions for myofibrillar ATPase similar to the jaw muscles, but had a heterogeneous fibre-type profile. There was no significant difference in cross-sectional fibre area between the jaw and the limb muscles. Thus the jaw and limb muscles of the hedgehog have similar fibre types and about equal fibre size.

Identification of matrix metalloproteinases and their tissue inhibitors type 1 and 2 in human masseter muscle.

Changes in masticatory muscle structure and function are either developmental, as seen in anomalies of facial form, or adaptive, as seen during procedures such as orthognathic surgery and functional-appliance orthodontic therapy. Remodelling of muscle extracellular matrix is pivotal in these processes. This turnover is mediated via members of the family of enzymes known as matrix metalloproteinases (MMP) and inhibited by the tissue inhibitors of metalloproteinases (TIMP). The aim here was to investigate the in vivo pattern of expression and distribution of MMPs and TIMPs in masseter muscle of humans with both normal and abnormal facial forms. Masseter muscle biopsies were taken from 10 patients, four with long-face syndrome and six normal controls as confirmed by cephalometry. Immunohistochemical techniques were used to show the pattern and distribution of MMPs and TIMP proteins in the muscle. Zymography of tissue extracts was used to determine the presence of MMP activity. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to detect the presence of MMP and TIMP-2 mRNA. MMP-1 was expressed around the individual muscle fibres, especially in those fibre surfaces in contact with the interstices of the connective tissue and around blood vessels. MMP-9 staining was less intense and was expressed in the interstices of the connective tissue and around blood vessels. Zymography of protein extracts confirmed that MMP-9 activity was present. MMP-2 and MMP-3 were not expressed in the samples, although MMP-2 mRNA could be detected by RT-PCR and its activity could be detected by zymography. Intense TIMP-1 staining was present around each muscle fibre, in the interstices of the connective tissue and surrounding blood vessels; TIMP-2 mRNA could be detected in all samples. These staining patterns were seen in all biopsies examined and were irrespective of the facial form of the donor. These findings provide evidence that the mechanisms required for matrix remodelling are present in the human masseter muscle.

Effects of 17 beta-oestradiol and 5 alpha-dihydrotestosterone on the expression of the muscle and heart types of lactate dehydrogenase isozymes in the masseter muscle of developing mice.

17 beta-oestradiol (E2) and/or 5 alpha-dihydrotestosterone (5 alpha-DHT) had no effect on the expression of isozymes of lactate dehydrogenase (LDH) in the masseter muscle of intact male mice. However, treatment with E2 restored the level of the muscle (M) type of LDH isozyme, which had been reduced by testectomy, to that found in intact male mice treated with vehicle only. Moreover, 5 alpha-DHT alone was more effective than E2 in increasing the relative level of this isozyme in testectomized mice. 5 alpha-DHT had a more significant effect on the increase in the relative level of the M-type LDH isozyme when combined with E2. These results suggest that androgens promote, in the presence of oestrogens, the postnatal changes in the characteristics of the masseter muscle of developing male animals.

Oxidative capacity of rat masseter muscle after implantation of thyrotropin-releasing hormone microspheres in proximity to trigeminal motoneurones.

Earlier work has shown that two important consequences of implanting thyrotropin-releasing hormone (TRH) microspheres near motoneurones within the trigeminal motor nucleus of actively growing rats are increased muscle mass and a darkening of the implant-side masticatory muscles. These phenomena have been associated with altered neuromuscular activity patterns and biomechanical forces that directly influence craniofacial growth and development. Now, whether the implantation of TRH microspheres in proximity to trigeminal motoneurones would affect the oxidative capacity of the implant-side masseter muscles was investigated. Cytochrome C oxidase (COX) assays were carried out for both implant- and non-implant-side masseters of TRH (n = 5) and blank microsphere (n = 6) Sprague-Dawley rats after stereotactic surgery at 35 days of age. Analyses of both groups at 14 days post-implantation revealed that the COX activity levels of implant-side masseters in TRH-implanted rats was significantly (P< or =0.05) greater than that of non-implant-side masseters; rats implanted with blank microsphere exhibited no significant difference between implant- and non-implant-side masseter COX activity levels. The stated null hypothesis was therefore rejected. These data suggest that TRH implants in proximity to trigeminal motoneurones effect increased oxidative capacity of the masseter muscle as measured by COX activity.

The correlation between histophotometrical and biochemical myosin-ATPase measurements in the myocardium and striated muscle of the rat.

To prove the correlation between histophotometrical and biochemical data of myosin-ATPase-activity, the change of enzyme activity during development was measured in the myocardium, masseter muscle, and tensor fasciae latae of rats. Both methods were applied to the same material. To avoid errors due to varying section thickness in each cutting and staining procedure, a reference material (e.g. heart muscle) was used, taken from one and the same animal. The correlation between histophotometrical and biochemical estimations was very good, histophotometrical arbitrary units being used. ATPase-activity in heart muscle during development, slightly decreases in masseter muscle and more in tensor fasciae latae. The ATPase-activity in the latter was higher than in masseter muscle. This is in good agreement with the physiological, biochemical, and histochemical classification.

Characterization of human oro-facial and masticatory muscles with respect to fibre types, myosins and capillaries. Morphological, enzyme-histochemical, immuno-histochemical and biochemical investigations.

This study provides a comparative characterization of four human oro-facial muscles, one masticatory muscle (the masseter) and two limb muscles, with respect to muscle fibre types, myosin isoforms and capillary supply. Enzyme-histochemical methods were used to evaluate the myofibrillar ATPase fibre type composition. Immuno-histochemical techniques were used to determine the expression of myosin heavy chain (MHC) isoforms in the different fibre types. The contents of MHCs and myosin light chains (MLC) in different muscles were analysed with electrophoretic methods. In addition, the capillary bed of the muscles was evaluated using both enzyme- and immuno-histochemical techniques. The fibre type compositions of the oro-facial and masseter muscles were found to be qualitatively and quantitatively different from each other and from those of limb muscles. In general, the oro-facial muscles contained a predominance of unusually high oxidative type II fibres, with a staining reaction for ATPase in between that of type IIA and type IIB fibres, termed type IIAB. In fact, one of the oro-facial muscles, the zygomatic minor, showed the highest type II fibre proportion ever reported in humans. This fibre type pattern is in contrast to that of the masseter muscle, which contains a majority of type I fibres, small diameter low oxidative type IIB fibres and a significant proportion of ATPase-intermediately stained fibres, termed IM, and IIC. Inter- and intra-muscular variability in fibre size and shape was considerable in both the oro-facial and masseter muscles. The oro-facial muscles were devoid of muscle spindles. The immuno-histochemical and biochemical analyses showed a characteristic myosin composition of each muscle. Notably, the results indicated the presence of a previously undetected fast MHC isoform in the oro-facial muscles, tentatively termed "fast F". The masseter contained unusual myosin isoforms, such as fetal and alpha-cardiac MHCs, and unique combinations of MHC isoforms which were not found in the limb or oro-facial muscles. The type IM and IIC fibres co-expressed slow and fast A MHCs in the oro-facial and limb muscles, but slow and a "fast B like" MHC in the masseter. Individual fibres in the oro-facial and limb muscles contained one or two MHC isoforms, whereas individual fibres in the masseter co-expressed up to four different MHC isoforms. On the basis of their pattern of expression of MHC isoforms, up to five fibre types could be distinguished in the oro-facial and limb muscles and eight in the masseter.(ABSTRACT TRUNCATED AT 400 WORDS)

Histopathologic features of masseter muscle in the distrophic hamster (UM-X7.1 Syrian hamster).

Dystrophic hamster has been regarded as the useful model animal for Severe childhood autosomal recessive muscular dystrophy (SCARMD). Although, many studies on Dystrophic hamster have utilized the muscular tissue of the trunk, however no study have been analyzed for the masticatory muscle. For this study, we used a Dystrophic hamster (UM-X7.1 Syrian hamster) to histochemically investigate the effect of muscular dystrophy on the masseter muscle. Large and small regenerated muscle fibers, and necrotic fibers were detected almost in all areas. Opaque fiber, hypertrophic fiber with fiber splitting structure and necrotic fiber filled up by mononuclear phagocytes were recognized. The region, in which the mononuclear phagocytic cells infiltrated, showed strong positivity to acid phosphatase, and lysosome enzyme. There were many muscle fibers with reduced levels of succinate dehydrogenase (SDH) activities in the muscle fiber. Some TUNEL-positive cells were confirmed in both necrotic and non-necrotic areas. It was suggested that a part of TUNEL-positive cells are the cells originated from the connective tissue or immunocytes. In this result, histopathologic changes of the masseter muscle of the UM-X7.1 Syrian hamster was similar to muscle of the body trunk in the past reports. As the result, it was suggested that jaw closing movements may be negatively affected caused by the decline of the masseter muscle twitch. And, the point of view by which apoptosis is the trigger for the muscle fiber collapse were not seen in the Dystrophic hamster masseter muscle. We suggest that apoptosis is a one step in the process of regeneration of muscle fibers.

Biochemical properties of the superficial masseter muscle in the aging rat.

In young (15 weeks) and senescent (2 years) rats, the metabolic enzyme activities and the composition of myosin heavy chain (MHC) isoforms of the superficial masseter muscle were assessed by biochemical analysis. The succinate dehydrogenase activity and phosphofructokinase activity of the masseter muscle were significantly lower in senescent rats than in young rats. A large portion of the masseter muscle was composed of MHC IIb and IId isoforms in both young and senescent rats. As compared with young rats, no significant change in the composition of MHC isoforms was found in the masseter muscle of the senescent rats. These findings indicate that aging has a significant effect on the energy supply of the superficial masseter muscle.