Effect Of Unilateral Mastication On The Masseter Muscle Fiber Phenotype And MyHCs Gene Expression In Rats

ObjectiveThe present study was aimed to detect the changes of fiber phenotype and the gene levels of My HC isoforms(My HCs) in masseter muscle under the use of a rat unilateral mastication model, to analyze the correlation of the fiber phenotype with the changes of gene expressions of My HCs, and to clarify the unilateral mastication influence on masticatory muscle physiological characteristics. The results will provide experimental data to reveal the pathogenic mechanisms of masticatory muscle structure and functional disorders caused by abnormal occlusion. MethodsForty SD male rats fed in SPF environment were randomly divided into 8 groups, with 5 in each group. The rats were modeled for 2, 4, 6 and 8 week groups. The corresponding control groups were set up in the same period. For the experimental groups, all of the rats’ right upper molars were extracted by intraperitoneal injection anesthesia with 1 % sodium pentobarbital. ATPase staining method was used to detect the phenotypic characteristics of the rat masseter muscle fibers, including the distribution, orientation and expression in different subtypes. Imaging and SPSS statistical analysis software were used to quantify the data. The levels of gene expressions of My HCs in bilateral masseter muscles were evaluated by real-time fluorescent quantitative PCR, and the correlation analysis between the both was carried out. ResultsThe rat masseter muscle showed that the dominant proportion of ⅡA fiber type was about 64 %. In this area ⅡB fiber type contained about 27 %. Only about 9 % of typeⅠfiber was found in the deep masseter muscle. There were no obvious changes between different groups in the normal rats. After the establishment of unilateral mastication model, the proportions of all kinds of the fiber types and the gene levels of My HCs had been changed in bilateral masseter muscle. These changes appeared bilaterally asymmetry and dynamic with the time change. In 2 week group, the proportion of the typeⅠfiber and the level of My HC-Ⅰm RNA were raised, and these expression in non-extracted side were higher than theextracted side(P<0.05), while the composition ratio of the ⅡA fiber type was decreased. Transform from fast to slow contraction in muscle fiber types were realized. However, My HC-Ⅱa m RNA level had no obvious change. Over time, Compared with the control group, typeⅠfiber ratio and My HC-Ⅰgene level in the ipsilateral side with exodontia were continuously increased, with the declined trend of Ⅱ A fiber proportion(P<0.05). Conversely, the fiber phenotypic proportion and My HC gene levels in the contralateral side without exodontia were gradually recovered to the base line. There was a significant difference between the both side(P<0.05). Also, the alterations of ATPase staining were relatively consistent with the changes of My HC m RNA levels. ConclusionsRat masseter muscle appears to be the predominant composite type IIA fiber, with the characteristics of the rapid contraction and less fatigue. Unilateral mastication causes the change of masseter muscle fiber phenotypic properties and My HC gene levels, resulting in the transform of the ⅡA to theⅠfiber in partial fiber types. These changes with bilateral asymmetry under the action of unilateral mastication suggest that the bilateral imbalance between muscle function and masticatory muscle tension may play an important role in leading to masticatory muscle disorders.