Adaptation of Guinea-pig superficial masseter muscle to an increase in occlusal vertical dimension

To study the effect of increased occlusal vertical dimension on the fibre phenotypes of the superficial masseter muscle, the composition of myosin heavy-chains (MHC), myosin light-chains (MLC) and tropomyosin was investigated by sodium dodecyl sulphate–polyacrylamide gel electrophoresis and two-dimensional gel electrophoresis in conjunction with densitometric analysis in normal (control) and bite-opened (5.7 mm increase in the vertical dimension for 1 week) guinea-pigs. The superficial masseter contained two fast-type MHC isoforms, II-1 and II-2, in both the bite-opened and control groups; their relative content (mean±SD, n=7) was 47.8±2.9% and 52.2±2.9%, in the bite-opened and 44.4±3.0% and 55.6±3.0% in control preparations, indicating no significant (p>0.05) changes in MHC composition in association with the bite opening. On the other hand, significant differences in MLC and tropomyosin composition were found between the two preparations. Although the MLC consisted of three components, LC1f, LC2f and LC3f, in both preparations, their relative content (mean±SD, n=7) was 37.1±2.4%, 49.6±1.6% and 13.2±3.2%, respectively, in the bite-opened and 28.1±3.1%, 50.9±1.6% and 21.0±3.5% in the control preparations, indicating that the bite opening induced a significant (p<0.0001) increase in the relative content of LC1f at the expense of that of LC3f. Although the tropomyosin consisted of two components, TM-α and TM-β, in both preparations, their relative content (mean±SD, n=7) was 91.8%±1.9% and 8.2±1.9%, respectively, in the bite-opened and 95.9±0.7% and 4.1±0.7% in the control preparations, showing a significant (p<0.001) increase in the relative content of TM-β in relation to the bite opening. These results indicate that in guinea-pigs an increase in occlusal vertical dimension for 1 week changes the composition of MLC and tropomyosin, with no significant change in MHC, in the masseter muscle. These changes might be required to meet altered functional demands.