Bilateral effect of a unilateral occlusal splint on the expression of myosin heavy-chain isoforms in rat deep masseter muscle

Many studies have shown that various myosin isoforms are involved in muscle contraction. A search for specific antibodies directed against the myosin heavy chain (MHC) resulted in the identification of at least two main classes, referred to as MHC type I and type II. In this study, immunohistology and gel electrophoresis were used to determine the proportion of MHC isoforms in rat deep masseter muscle at different times after the insertion of an unilateral occlusal splint. An increasing proportion of MHC type I isoforms was found in both deep masseters soon after splinting, and this trend continued until 7 days after splint insertion. The type I fibres were clearly distributed on either side of the central axis of the muscle. At 15 days, a significant decrease in the percentage of the type IIb MHC isoform was observed on the occlusal splint side compared to the contralateral side. After 30 days of unilateral splinting, the proportion of type IIb fibres on the splint side returned to baseline whereas on the contralateral side there was an increase in the proportion of this type. The results suggest an initial adaptation after the unilateral occlusal disturbance in which muscles of both sides react in the same way; later, the muscles of each side adapt their expression of MHC isoforms according to altered functional demand.