Effect of chronic uncontrollable stress on myosin heavy chain expression in rat masseter muscle.

This study has evaluated the influence of chronic uncontrollable painless stress on the expression of myosin heavy chain (MyHC) isoforms in deep and superficial parts of the rat masseter muscle. Muscle cross sections of three month old Wistar rats were processed for immunohistochemical staining. In control muscle the three adult fast MyHC isoforms (-2a, -2x/d, -2b), were nonhomogenously distributed: in the smaller anterior part of the muscle only fast MyHC isoforms were expressed. The major superficial and posterior part of the muscle fibres also contained MyHC-1 (10%) and only 7% expressed of MyHC-2b. Psychological stress significantly modified the expression of particular MyHC isoforms in both parts of the muscle. In the anterior part of the experimental masseter the proportion of MyHC-2b expressing fibres decreased and 10% of the muscle fibres expressed MyHC-1. The superficial and posterior part of the experimental muscle contained significantly more muscle fibres expressing MyHC-2b isoforms. Regarding whole cross-sectional area transition towards faster phenotype was evident due to a higher percentage of either pure MyHC-2b containing fibres or hybrid MyHC-2x/d and -2b expressing fibres. The results of this study demonstrate that psychological stress is accompanied by a modification of muscle phenotype which is region specific. Opposite changes in deep and superficial part of the muscle could be ascribed to differences in genetic programme, function or susceptibility to hormones in both parts of masseter.