The effect of reduced dietary consistency on the fiber properties of rabbit jaw muscles

The dynamic nature of muscle fibers enables them to adapt to altered functional requirements by changing their myosin heavy chain (MyHC) isoform and cross-sectional area. Although these changes may occur under various conditions, muscular activity plays an essential role in modulating the phenotypic properties of muscle fibers. In jaw muscles, muscular activity is influenced by the consistency of the available food. It has been shown that the continuous intake of a liquid diet, which eliminates masticatory effort, reduces muscular activity and, in turn, the functional capacity of jaw muscles. However, little is known about how jaw muscles respond to a reduction in dietary consistency within the normal range of compressive strengths of foods. Therefore, the present study investigated the effect of the long-term intake of such a diet on the MyHC composition and the fiber cross-sectional area of jaw muscles in the rabbit. Male juvenile rabbits were randomly divided into two groups, which were raised on diets of different consistency from weaning to puberty. The experimental group was fed pellets requiring significantly reduced peak loadings, and thus lower level of jaw-muscle activity, to break the pellet in comparison with the standard pellets fed to the control group. At puberty, the MyHC composition and the corresponding cross-sectional area of fibers in the superficial masseter, superficial temporalis, medial and lateral pterygoid, and digastric muscles of both groups were determined using immunohistochemistry. The proportion and cross-sectional area of fibers co-expressing MyHC-I and MyHC-cardiac alpha were significantly smaller in the masseter muscles of the animals that had been fed the soft pellets than in those of the controls. In contrast, the proportions and cross-sectional areas of the various fiber types in the other jaw muscles did not differ significantly between the groups. These findings suggest that the long-term reduction in the dietary consistency contributes to selective disuse resulting in structural adaptation of the masseter muscle, the jaw-closing muscle primarily responsible for generating occlusal force during chewing, reflected in decreases in the proportion and cross-sectional area of its slow fibers. At the same time, it appears that the reduced activity during mastication of a diet with the consistency of soft foods normally eaten by rabbits is sufficient to prevent disuse atrophy in the other jaw muscles. Overall, the rabbit masticatory system seems to be relatively rigid, manifesting few diet-specific changes in the fiber properties of jaw muscles.