Effect Of Occlusal Disorders On The MyHC Isoforms Characteristics And Its Protein Expression Of Masseter Muscle In Rats

Objective:In the present study, the goal is to detect the protein expression of MyHC isoforms and the spatial distribution of masseter muscle in rats, and explore the effect of occlusal disorders on the MyHC isoforms characteristics and its protein expression, so as to reveal the changes in physiological characteristics of masticatory muscles induced by occlusal disorders, elucidate TMD pathogenic mechanism caused by abnormal occlusion, and offer theoretical and experimental evidence for TMD treatment which acts on regulating and blocking structure and function disorders of masticatory muscles.Methods:Eighty male SD rats were randomly set up into two occlusal disorder groups. The first one was subjected to the exodontia of the right upper molars, and then divided into 4 groups, including 2,4,8 and 12 week groups, respectively. For another one the rats were processed by putting a round metal tube with 1.5mm length and 0.75mm height metal pinhead on the right maxillary second molar, and then divided into 1,2,4,6 and 8 week groups as well as a removal interference group. Each group consists of five occlusal disorder rats and five sham-operated rats which were used as a control. The expression and distribution of MyHC isoforms in masseter muscle for both sides were analyzed by immunohistochemical technique and quantified by image analyzer. The level of MyHC isoforms protein expression in bilateral masseter muscles was valuated by Western blot.Results:The rat masseter showed dominant proportion of MyHC-â…¡fibers, and only a few MyHC-â… fibers can be seen in the deep masseter. The MyHC isoforms characteristics and its protein expression were changed in the masseter muscle of two kinds of abnormal occlusion model. The proportion and expression of MyHC-â… fibers in bilateral masseter muscle were increased in a 2 week group with exodontia. The fast type muscle fibers were transformed to the slow type, especially in deep masseter. Over time, the MyHC isoforms characteristics and its protein expression of the fibers in the contralateral side without exodontia gradually recovered to the original status, but MyHC-â… fibers in the ipsilateral side with exodontia was always higher than the contralateral side. The immunohistochemical staining in an occlusal interference group showed that MyHC-â… fibers in bilateral masster muscle had no significant changes expect for the lweek group, in which MyHC-â… fibers appeared to increase. Western blot analysis indicated that MyHC-â… expression gradually increased in the interference side, and reached a peak in the 6week group. But nothing in the contralateral side could be seen. There existed no change between the removal interference group and 8 week group, but they were higher than the control group.Conclusions:Rat masseter muscle appears to be the predominant composite MyHC-â…¡fibers, with rapid contraction and high tension output characteristics. MyHC isoforms characteristics can be changed by occlusal disorders, remodeling masseter structure. The MyHC isoforms of ipsilateral and contralateral masseters display an asymmetry change in the rat occlusal disorder models, suggesting the bilateral imbalance in muscle function and masticatory muscle tension output may play an important role in leading to the abnormal reconstruction or pathological degeneration of temporomandibular joint.