[Show abstract][Hide abstract] ABSTRACT: Obesity is well known to be associated with a wide variety of illnesses, and is an increasing problem not only in developed countries but also in developing countries. It is well known that large bite size contributes to excess energy intake and obesity, whereas an increased number of chews before swallowing the food bolus is associated with suppression of obesity. However, the effect of food diameter on bite size per mouthful and on chewing behavior remains poorly understood. Here, we examined the effects of food diameter on bite size and chewing behavior using a masticatory counter during the mastication of stick-type biscuits having the same length (10 cm) and ingredients, but with four different diameters (3.0, 3.5, 4.0, and 8.0 mm). Bite length and bite weight per mouthful were similar among the 3.0, 3.5, and 4.0 mm groups. However, bite length in the 8.0 mm group was significantly smaller, whereas bite weight was significantly greater than in the 3.0/3.5 mm groups. Further, the number of chews gradually increased, whereas the number of chews per bite weight gradually decreased, with an increase of biscuit diameter. These results indicate that a smaller biscuit diameter is associated with a smaller bite weight per mouthful and a greater number of chews per bite weight. This is the first report to quantity the effect of food diameter on bite weight per mouthful and on chewing behavior; these results should be helpful in the design of effective, safe, and low-cost behavioral modification therapy to combat obesity.
Full-text · Article · Oct 2015 · The Journal of Physiological Sciences
[Show abstract][Hide abstract] ABSTRACT: Glucocorticoid has a direct catabolic effect on skeletal muscle, leading to muscle atrophy, but no effective pharmacotherapy is available. We reported that clenbuterol (CB) induced masseter muscle hypertrophy and slow-to-fast myosin heavy chain (MHC) isoform transition through direct muscle β2-adrenergic receptor stimulation. Thus, we hypothesized that CB would antagonize glucocorticoid (dexamethasone; DEX)-induced muscle atrophy and fast-to-slow MHC isoform transition.
We examined the effect of CB on DEX-induced masseter muscle atrophy by measuring masseter muscle weight, fiber diameter, cross-sectional area, and myosin heavy chain (MHC) composition. To elucidate the mechanisms involved, we used immunoblotting to study the effects of CB on muscle hypertrophic signaling (insulin growth factor 1 (IGF1) expression, Akt/mammalian target of rapamycin (mTOR) pathway, and calcineurin pathway) and atrophic signaling (Akt/Forkhead box-O (FOXO) pathway and myostatin expression) in masseter muscle of rats treated with DEX and/or CB.
Masseter muscle weight in the DEX-treated group was significantly lower than that in the Control group, as expected, but co-treatment with CB suppressed the DEX-induced masseter muscle atrophy, concomitantly with inhibition of fast-to-slow MHC isoforms transition. Activation of the Akt/mTOR pathway in masseter muscle of the DEX-treated group was significantly inhibited compared to that of the Control group, and CB suppressed this inhibition. DEX also suppressed expression of IGF1 (positive regulator of muscle growth), and CB attenuated this inhibition. Myostatin protein expression was unchanged. CB had no effect on activation of the Akt/FOXO pathway. These results indicate that CB antagonizes DEX-induced muscle atrophy and fast-to-slow MHC isoform transition via modulation of Akt/mTOR activity and IGF1 expression. CB might be a useful pharmacological agent for treatment of glucocorticoid-induced muscle atrophy.
[Show abstract][Hide abstract] ABSTRACT: The present study investigated the function of miR-1 and miR-133a during the postnatal development of mouse skeletal muscles. The amounts of miR-1 and miR-133a were measured in mouse masseter and gastrocnemius muscles between 1 and 12 weeks after birth with real-time polymerase chain reaction and those of HDACs, MEF2, MyoD family, MCK, SRF, and Cyclin D1 were measured at 2 and 12 weeks with Western blotting. In both the masseter and gastrocnemius muscles, the amount of miR-1 increased between 1 and 12 weeks, whereas the amount of HADC4 decreased between 2 and 12 weeks. In the masseter muscle, those of MEF2, MyoD, Myogenin, and MCK increased between 2 and 12 weeks, whereas, in the gastrocnemius muscle, only those of MRF4 and MCK increased. The extent of these changes in the masseter muscle was greater than that in the gastrocnemius muscle. The amounts of miR-133a, SRF, and Cyclin D1 did not change significantly in the masseter muscle between 1 and 12 weeks after birth. By contrast, in the gastrocnemius muscle, the amounts of miR-133a and Cyclin D1 increased, whereas that of SRF decreased. Our findings suggest that the regulatory pathway of miR-1 via HDAC4 and MEF2 plays a more prominent role during postnatal development in the masseter muscle than in the gastrocnemius muscle, whereas that of miR-133a via SRF plays a more prominent role in the gastrocnemius muscle than in the masseter muscle.
No preview · Article · May 2015 · Molecular and Cellular Biochemistry
[Show abstract][Hide abstract] ABSTRACT: Myocardial β-adrenergic receptor (β-AR) β1- and β2-subtypes are highly homologous, but play opposite roles in cardiac apoptosis and heart failure, as do cardiac adenylyl cyclase (AC) subtypes 5 (AC5) and 6 (AC6): β1-AR and AC5 promote cardiac remodeling, while β2-AR and AC6 activate cell survival pathways. However, the mechanisms involved remain poorly understood. We hypothesized that AC5 is coupled preferentially to β1-AR rather than β2-AR, and we examined this idea by means of pharmacological and genetic approaches. We found that selective inhibition of AC5 with 2’5’-dideoxyadenosine significantly suppressed cAMP accumulation and cardiac apoptosis induced by selective β1-AR stimulation, but had no effect on cAMP accumulation and cardiac apoptosis in response to selective β2-AR stimulation. The results of selective stimulation of β1-AR and β2-AR in neonatal cardiac myocytes prepared from wild-type and AC5-knockout mice were also consistent with the idea that β1-AR selectively couples with AC5. We believe these results are helpful for understanding the mechanisms underlying the different roles of AR subtypes in healthy and diseased hearts.
No preview · Article · Feb 2015 · Biochemical and Biophysical Research Communications
[Show abstract][Hide abstract] ABSTRACT: Cyclic adenosine monophosphate (cAMP) and Ca(2+) levels may oscillate in harmony within excitable cells; a mathematical oscillation loop model, the Cooper model, of these oscillations was developed two decades ago. However, in that model all adenylyl cyclase (AC) isoforms were assumed to be inhibited by Ca(2+), and it is now known that the heart expresses multiple AC isoforms, among which the type 5/6 isoforms are Ca(2+)-inhibitable whereas the other five (AC2, 3, 4, 7, and 9) are not. We used a computational systems biology approach with CellDesigner simulation software to develop a comprehensive graphical map and oscillation loop model for cAMP and Ca(2+). This model indicated that Ca(2+)-mediated inhibition of AC is essential to create oscillations of Ca(2+) and cAMP, and the oscillations were not altered by incorporation of phosphodiesterase-mediated cAMP hydrolysis or PKA-mediated inhibition of AC into the model. More importantly, they were created but faded out immediately in the co-presence of Ca(2+)-noninhibitable AC isoforms. Because the subcellular locations of AC isoforms are different, spontaneous cAMP and Ca(2+) oscillations may occur within microdomains containing only Ca(2+)-inhibitable isoforms in cardiac myocytes, which might be necessary for fine tuning of excitation-contraction coupling.
Preview · Article · Jan 2015 · The Journal of Physiological Sciences
[Show abstract][Hide abstract] ABSTRACT: The predominant isoform of β-adrenoceptor (β-AR) in skeletal muscle is β2-AR and that in the cardiac muscle is β1-AR. We have reported that Epac1 (exchange protein directly activated by cAMP 1), a new protein kinase A-independent cAMP sensor, does not affect cardiac hypertrophy in response to pressure-overload or chronic isoproterenol infusion. However, the role of Epac1 in skeletal muscle hypertrophy remains poorly understood. We thus examined the effect of disruption of Epac1, the major Epac isoform in skeletal muscle, on masseter muscle hypertrophy induced by chronic β2-AR stimulation with clenbuterol (CB) in mice (Epac1KO). The masseter muscle weight/tibial length ratio was similar in WT and Epac1KO at baseline and was significantly increased in WT after CB infusion, but this increase was suppressed in Epac1KO. CB treatment significantly increased the proportion of MHC IIb at the expense of that of MHC IId/x in both WT and Epac1KO, indicating that Epac1 did not mediate the CB-induced MHC isoform transition towards the faster isoform. The mechanism of suppression of CB-mediated hypertrophy in Epac1KO is considered to involve decreased activation of Akt signaling. In addition, CB-induced histone deacetylase 4 (HDAC4) phosphorylation on serine 246 mediated by calmodulin kinase II (CaMKII), which plays a role in skeletal muscle hypertrophy, was suppressed in Epac1KO. Our findings suggest that Epac1 plays a role in β2-AR-mediated masseter muscle hypertrophy, probably through activation of both Akt signaling and CaMKII/HDAC4 signalingThis article is protected by copyright. All rights reserved
No preview · Article · Oct 2014 · The Journal of Physiology
[Show abstract][Hide abstract] ABSTRACT: PKA phosphorylates multiple molecules involved in calcium (Ca2+) handling in cardiac myocytes and is considered to be the predominant regulator of β-adrenergic receptor-mediated enhancement of cardiac contractility; however, recent identification of exchange protein activated by cAMP (EPAC), which is independently activated by cAMP, has challenged this paradigm. Mice lacking Epac1 (Epac1 KO) exhibited decreased cardiac contractility with reduced phospholamban (PLN) phosphorylation at serine-16, the major PKA-mediated phosphorylation site. In Epac1 KO mice, intracellular Ca2+ storage and the magnitude of Ca2+ movement were decreased; however, PKA expression remained unchanged, and activation of PKA with isoproterenol improved cardiac contractility. In contrast, direct activation of EPAC in cardiomyocytes led to increased PLN phosphorylation at serine-16, which was dependent on PLC and PKCε. Importantly, Epac1 deletion protected the heart from various stresses, while Epac2 deletion was not protective. Compared with WT mice, aortic banding induced a similar degree of cardiac hypertrophy in Epac1 KO; however, lack of Epac1 prevented subsequent cardiac dysfunction as a result of decreased cardiac myocyte apoptosis and fibrosis. Similarly, Epac1 KO animals showed resistance to isoproterenol- and aging-induced cardiomyopathy and attenuation of arrhythmogenic activity. These data support Epac1 as an important regulator of PKA-independent PLN phosphorylation and indicate that Epac1 regulates cardiac responsiveness to various stresses.
Full-text · Article · Jun 2014 · Journal of Clinical Investigation
[Show abstract][Hide abstract] ABSTRACT: The influence of masticatory loading stimulus on mandibular development is not fully clear. In this paper, experimental alterations in the daily muscle use, caused by a changed diet consistency, were continuously monitored, while adaptations in bone and cartilage were examined. It is hypothesised that decreased muscular loading will result in a decrease in the growth factor expression and mandible growth. Fourteen 21-day-old Wistar strain male rats were randomly divided into two groups and fed on either a hard or soft diet for 14 weeks. An implanted radio-telemetric device recorded continuously muscle activity of the superficial masseter muscle. Chondroblast proliferation in the condylar cartilage was identified by insulin-like growth factor-1 receptor (IGF-1r) immunostaining. Furthermore, an X-ray was taken for cephalometric analysis. In the soft-diet group, the duty time of the superficial masseter muscle at higher activity levels was significantly lower than that in the hard-diet group. This decrease in muscular loading of the jaw system was accompanied by: a significant reduction in (i) articular cartilage thickness, (ii) expression of IGF-1r immunopositive cells and (iii) mandible ramus height. In conclusion, a decrease in masticatory demand during the growth period leads to insufficient mandibular development.
[Show abstract][Hide abstract] ABSTRACT: Stimulation of β-adrenergic receptors in cardiac myocytes activates cyclic AMP-dependent protein kinase A (PKA). PKA-mediated phosphorylation of myofibrils decreases their longitudinal stiffness, but its effect on transverse stiffness is not fully understood. We thus examined the effects of PKA treatment on the transverse stiffness of cardiac myofibrils by atomic force microscopy and determined the phosphorylation levels of myofibril components by SDS-PAGE. Transverse stiffness was significantly decreased by PKA treatment concomitantly with increased phosphorylation of troponin I, myosin-binding protein C, and titin (also called connectin). Subsequent treatment with protein phosphatase 1 abrogated these PKA-mediated effects. [Supplementary methods: available only at http://dx.doi.org/10.1254/jphs.13110SC].
No preview · Article · Oct 2013 · Journal of Pharmacological Sciences
[Show abstract][Hide abstract] ABSTRACT: Chronic administration of clenbuterol (CB), a lipophilic β2-adrenoceptor (β2-AR) agonist, induces skeletal muscle hypertrophy and slow-to-fast fiber-type transitions in mammalian species, but the mechanism and pathophysiological roles of these changes have not been explored. Here, we examined the effects of CB not only on masseter muscle mass, fiber diameter, and myosin heavy chain (MHC) composition, but also on daily muscle activity, a factor influencing muscle phenotype, by means of electromyogram analysis in rats. MHC transition towards faster isoforms was induced by 2-week CB treatment. In addition, daily duty time was increased at 1 day, 1 week, and 2 weeks after the start of CB treatment and its increase was greater at high activity level (6-fold) than at low activity level (2-fold). In order to examine whether these effects of CB were mediated through muscle or CNS β2-AR stimulation, we compared these effects of CB with those of salbutamol (SB), a hydrophilic β2-AR agonist. SB treatment induced masseter hypertrophy and MHC transition, like CB, but did not increase daily activity. These results suggest that CB-mediated slow-to-fast MHC transition with hypertrophy was induced through direct muscle β2-AR stimulation, but the increase of daily duty time was mediated through the CNS.
Full-text · Article · Aug 2013 · Journal of Pharmacological Sciences
[Show abstract][Hide abstract] ABSTRACT: To examine the effects of the Akt/mammalian target of rapamycin (mTOR) pathway on masseter muscle hypertrophy and myosin heavy chain (MHC) transition in response to mechanical overload, we analyzed the effects of bite-opening (BO) on the hypertrophy and MHC composition of masseter muscle of BO-rats treated or not treated with rapamycin (RAPA), a selective mTOR inhibitor. The masseter muscle weight in BO-rats was significantly greater than that in controls, and this increase was attenuated by RAPA treatment. Expression of slow-twitch MHC isoforms was significantly increased in BO-rats with/without RAPA treatment, compared with controls, but the magnitude of the increase was much smaller in RAPA-treated BO-rats. Phosphorylation of p44/42 MAPK (ERK1/2), which preserves fast-twitch MHC isoforms in skeletal muscle, was significantly decreased in BO-rats, but the decrease was abrogated by RAPA treatment. Calcineurin signaling is known to be important for masseter muscle hypertrophy and fast-to-slow MHC isoform transition, but expression of known calcineurin activity modulators was unaffected by RAPA treatment. Taken together, these results indicate that the Akt/mTOR pathway is involved in both development of masseter muscle hypertrophy and fast-to-slow MHC isoform transition in response to mechanical overload with inhibition of the ERK1/2 pathway and operates independently of the calcineurin pathway.
No preview · Article · Jul 2013 · Journal of Pharmacological Sciences
[Show abstract][Hide abstract] ABSTRACT: Although evidence has been presented that, at low ionic strength, myosin heads in relaxed skeletal muscle fibers form linkages with actin filaments, the effect of low ionic strength on contraction characteristics of Ca(2+)-activated muscle fibers has not yet been studied in detail. To give information about the mechanism of muscle contraction, we have examined the effect of low ionic strength on the mechanical properties and the contraction characteristics of skinned rabbit psoas muscle fibers in both relaxed and maximally Ca(2+)-activated states. By progressively decreasing KCl concentration from 125 mM to 0 mM (corresponding to a decrease in ionic strength μ from 170 mM to 50 mM), relaxed fibers showed changes in mechanical response to sinusoidal length changes and ramp stretches, which are consistent with the idea of actin-myosin linkage formation at low ionic strength. In maximally Ca(2+)-activated fibers, on the other hand, the maximum isometric force increased about twofold by reducing KCl concentration from 125 to 0 mM. Unexpectedly, determination of the force-velocity curves indicated that, the maximum unloaded shortening velocity Vmax, remained unchanged at low ionic strength. This finding indicates that the actin-myosin linkages, which has been detected in relaxed fibers at low ionic strength, are broken quickly on Ca(2+) activation, so that the linkages in relaxed fibers no longer provide any internal resistance against fiber shortening. The force-velocity curves, obtained at various levels of steady Ca(2+)-activated isometric force, were found to be identical if they are normalized with respect to the maximum isometric force. The MgATPase activity of muscle fibers during isometric force generation was found not to change appreciably at low ionic strength despite the two-fold increase in Ca(2+)-activated isometric force. These results can be explained in terms of enhancement of force generated by individual myosin heads, but not by any changes in kinetic properties of cyclic actin-myosin interaction.
[Show abstract][Hide abstract] ABSTRACT: Muscle contraction results from attachment-detachment cycles between myosin heads extending from myosin filaments and actin filaments. It is generally believed that a myosin head first attaches to actin, undergoes conformational changes to produce force and motion in muscle, and then detaches from actin. Despite extensive studies, the molecular mechanism of myosin head conformational changes still remains to be a matter for debate and speculation. The myosin head consists of catalytic (CAD), converter (CVD) and lever arm (LD) domains. To give information about the role of these domains in the myosin head performance, we have examined the effect of three site-directed antibodies to the myosin head on in vitro ATP-dependent actin-myosin sliding and Ca2+-activated contraction of muscle fibers. Antibody 1, attaching to junctional peptide between 50K and 20K heavy chain segments in the CAD, exhibited appreciable effects neither on in vitro actin-myosin sliding nor muscle fiber contraction. Since antibody 1 covers actin-binding sites of the CAD, one interpretation of this result is that rigor actin-myosin linkage is absent or at most a transient intermediate in physiological actin-myosin cycling. Antibody 2, attaching to reactive lysine residue in the CVD, showed a marked inhibitory effect on in vitro actin-myosin sliding without changing actin-activated myosin head (S1) ATPase activity, while it showed no appreciable effect on muscle contraction. Antibody 3, attaching to two peptides of regulatory light chains in the LD, had no significant effect on in vitro actin-myosin sliding, while it reduced force development in muscle fibers without changing MgATPase activity. The above definite differences in the effect of antibodies 2 and 3 between in vitro actin-myosin sliding and muscle contraction can be explained by difference in experimental conditions; in the former, myosin heads are randomly oriented on a glass surface, while in the latter myosin heads are regularly arranged within filament-lattice structures.
Full-text · Article · Jan 2013 · Biophysical Journal
[Show abstract][Hide abstract] ABSTRACT: Parkinson's disease (PD), a major neurological disease, is characterised by a marked loss of dopaminergic neurons in the substantia nigra. Patients with PD frequently show chewing and swallowing dysfunctions, but little is known about the characteristics of their stomatognathic functions. The purpose of this study was to evaluate the influence of PD on jaw muscle fibre and functions. PD model rats were made by means of the injection of 6-hydroxydopamine (6-OHDA) into the striatum of 8-week-old Sprague-Dawley male rats. Five weeks after the injection, a radio-telemetric device was implanted to record muscle activity continuously from the superficial masseter and anterior belly of digastric muscles. Muscle activity was recorded for 3 days and was evaluated by the total duration of muscle activity per day (duty time). After recording the muscle activities, jaw muscles were isolated for immunohistochemical and PCR analyses. In PD model rats, the following findings of the digastrics muscles verify that compared to the control group: (i) the higher duty time exceeding 5% of the peak activity level, (ii) the higher expression of the mRNA of myosin heavy chain type I, and (iii) the tendency for fast to slow fibre-type transition. With respect to the masseter muscle, there were no significant differences in all analyses. In conclusion, PD leads to the changes in the jaw behaviours, resulting in a PD-specific chewing and swallowing dysfunctions.
No preview · Article · Jan 2013 · Journal of Oral Rehabilitation
[Show abstract][Hide abstract] ABSTRACT: We previously demonstrated that type 5 adenylyl cyclase (AC5) functions in autonomic regulation in the heart. Based on that work, we hypothesized that pharmacological modulation of AC5 activity could regulate the autonomic control of the heart rate under micro- and hypergravity. To test this hypothesis, we selected the approach of activating AC5 activity in mice with a selective AC5 activator (NKH477) or inhibitor (vidarabine) and examining heart rate variability during parabolic flight. The standard deviation of normal R-R intervals, a marker of total autonomic variability, was significantly greater under micro- and hypergravity in the vidarabine group, while there were no significant changes in the NKH477 group, suggesting that autonomic regulation was unstable in the vidarabine group. The ratio of low frequency and high frequency (HF) in heart rate variability analysis, a marker of sympathetic activity, became significantly decreased under micro- and hypergravity in the NKH477 group, while there was no such decrease in the vidarabine group. Normalized HF, a marker of parasympathetic activity, became significantly greater under micro- and hypergravity in the NKH477 group. In contrast, there was no such increase in the vidarabine group. This study is the first to indicate that pharmacological modulation of AC5 activity under micro- and hypergravity could be useful to regulate the autonomic control of the heart rate.
No preview · Article · Jul 2012 · Journal of Pharmacological Sciences
[Show abstract][Hide abstract] ABSTRACT: Objectives:The composition of skeletal muscle fibers changes with general conditions like age. However, little is known about the aging-associated changes in the masticatory muscle fibers. The aim of this study is to investigate the effects of aging on rat masticatory muscle fibers by immunohistometric analysis.
Methods:Masseter, temporalis, and digastric muscles were taken from 10-week-old and 27-week-old Sprague-Dawley rats (n=6 for each age), and frozen in liquid-nitrogen-cooled isopentane. For each of the three muscles, a series of transverse sections of 10m were cut with a cryomicrotome. The fiber type composition was investigated by immunohistochemical staining according to their myosin heavy chain (MyHC) content. The fiber cross-sectional areas were measured using a custom-made program into a personal computer.
Results:In 10-week-old rats, most fibers were type II; especially IIA and IIX were dominant. Compared with 10-week-old rats, the older animals showed a lower proportion of type IIA, and a higher proportion of type IIB fibers in the temporalis muscle. Masseter and digastric muscles also revealed similar changes in muscle fiber composition. Furthermore, the fiber cross-sectional area tended to increase in all muscles with age, but increased significantly just in the temporalis muscle.
Conclusion:The present study suggests that there is a significant decrease in the proportion of type IIA muscles fibers. The fiber cross-sectional area in the temporalis muscle increased significantly. The aging-associated changes in muscle fiber characteristics may be dependent on the physiological demand for each muscle.
[Show abstract][Hide abstract] ABSTRACT: Objectives: Beta-agonists and glucocorticoids are frequently coprescribed for chronic asthma treatment. The combined effects of these drugs on fiber size and type of skeletal muscle have been reported to vary among muscles and the underlying mechanisms are less clear. In the present study we examined the effects of clenbuterol (CB), a beta-2-agonist, and dexamethasone (Dex), a glucocorticoid, on muscle mass and fiber type in masseter muscle of rats. Methods: Male Wistar rats, aged 12 weeks, were divided into four groups: control (n = 8), CB-treated (via the drinking water containing 30 mg/L, n = 8), Dex-treated (injected i.p. with 6 mg/kg body weight once every other day, n = 8) and CB + Dex-treated (n = 8) groups. After 2 weeks of each treatment, we measured both the muscle mass and the mRNA levels of myosin heavy chain (MyHC) isoforms using a real-time RT-PCR in the masseter muscle. Results: The masseter muscle mass was increased by CB but decreased by Dex. The CB-induced increase in the muscle mass was not antagonized by Dex. CB increased the MyHC Neo and IId/x mRNA levels but decreased IIa mRNA level both in the absence and presence of Dex. Dex decreased the MyHC IIa and IIb mRNA levels both in the absence and presence of CB. Conclusion: These results suggest that in rat masseter muscle CB and Dex produce opposite effects on the muscle mass and the fiber-type transition (CB induces an increase in muscle mass and a slow-to-fast transition, while the opposite is true by Dex) through different signaling pathways.
[Show abstract][Hide abstract] ABSTRACT: Objectives: We have previously reported that clenbuterol (CB), a beta-2-agonist, induces both hypertrophy and a slow-to-fast fiber-type transition in masseter muscle of rats. Although the CB-induced hypertrophy has been shown to be promoted through Akt/mTOR signaling pathway, the mechanism underlying the slow-to-fast transition is less clear. In the present study we examined the hypothesis that the slow-to-fast transition would be caused by changes in neuromuscular activity, which are well known to be a factor influencing fiber-type transitions. Methods: Daily electromyograms (EMGs) in masseter muscle of freely moving rats (n = 6) were recorded with a radio-telemetry system. Two 24-h periods of EMG (1 and 2 weeks after the implantation of the transmitter) were recorded, and served as control. Subsequently, the CB treatment (via the drinking water containing 30 mg/L) was initiated. Again, two 24-h periods of EMG (1 and 2 weeks after the initiation of the CB treatment) were recorded. Results: The CB treatment increased the duty times (i.e., the total duration of daily muscle activity) by about 1.5, 2.5, 4 and 5 times at activity levels exceeding 5, 20, 50 and 80% of the day's peak activity, respectively. This indicates the greater effects of CB on the duty time at the higher (50 and 80%) activity levels as compared with those at the lower (5 and 20%) activity levels. Conclusion: These results suggest that the CB treatment changes masseter muscle towards a faster phenotype by an increase in the duty time at the higher (50 and 80%) activity levels.
[Show abstract][Hide abstract] ABSTRACT: Objectives: To identify the mechanisms through which mechanical signals are converted to chemical signals that influence muscle growth and phenotype, Methods: we analyzed the effects of stretch in length (STR, 3 mm increase in the vertical dimension for 2 weeks) and/or a selective blocker of mTOR, rapamycin (RAPA, 1.2 mg/kg body weight, once daily for 2 weeks) on the muscle mass, the fiber diameter, the expression of myosin heavy chain (MHC) isoforms at mRNA and protein levels, and the mRNA levels of myocyte-enriched calcineurin (CaN)-interacting protein-1 (MCIP-1) as an indicator of CaN activity, peroxisome proliferator-activated receptor gamma co-activator-1a (PGC-1a), a key regulator of mitochondrial biogenesis, mitochondrial encoded cytochrome C oxidase (COX I), and the ubiquitin ligases (Atrogin-1 and MuRF-1) in rat masseter muscle. Results: STR increased the muscle mass and fiber diameter, which were not affected by RAPA. STR increased the MHC Neo, I, IIa, and IId/x mRNA levels but decreased the MHC IIb mRNA level. RAPA had no effects on these MHC mRNA level both in the absence and presence of STR. At the protein level, STR increased the MHC IIa and IId/x and decreased the MHC IIb both in the absence and presence of RAPA. STR increased the mRNA levels of MCIP-1, PGC-1a, and COX I both in the absence and presence of RAPA. RAPA had no effects on these mRNA levels. Both STR and RAPA had no effects on the mRNA levels of Atrogin-1 and MuRF-1. Conclusion: These results suggest that STR activates the CaN/NFAT pathway for promoting the muscle growth and fiber type switching towards slower phenotypes.
[Show abstract][Hide abstract] ABSTRACT: Mammalian skeletal muscles change their contractile-protein phenotype in response to mechanical loading and/or chronic electrical stimulation, implying that the phenotypic changes in masticatory muscles might result from new masticatory-loading conditions. To analyze the effects of increased occlusal vertical dimension (OVD) on daily activities and fibre-type compositions in jaw muscles, we measured the total duration of daily activity (duty time) and the myosin heavy chain (MyHC) compositions in the masseter and digastric muscles of freely moving control and bite-opened rats. In the control state, the duty time of the digastric muscle was higher than that of the masseter muscle at activity levels exceeding 5 and 20% of the day's peak activity. The opposite was true at activity levels exceeding 50 and 80% of the day's peak activity. The MyHCs consisted of a mixture of fast and slow types in the digastric muscle. The masseter consisted of mostly fast-type MyHC. The increment of OVD increased not only the duty time at activity levels exceeding 5, 20, 50 and 80% of the day' peak activity in both muscles but also the proportion of MyHC IIa in the masseter muscle and MyHC I in the digastric muscle at the expense of that of MyHC IIb. These results suggest that the increment of OVD changes masseter and digastric muscles towards slower phenotypes by an increase in their daily activities.
No preview · Article · Sep 2009 · Archives of oral biology