Myosin Light Chain Content Correlates to Shortening Velocity in Human Masseter Fibers

Objectives: When unusual myosin heavy chain (MyHC) proteins are found in mammalian jaw-closing muscles they often correlate to distinctive shortening velocities; as exampled by type II masticatory MyHC in carnivores & primates (Rowlerson et al., 1983) and a MyHC in rabbit (Sciote & Kentish, 1996). In the first study to test cellular kinetics in human masseter fibers, we recently demonstrated unusually slow unloaded shortening velocity in some human masseter fibers which express type I MyHC (Morris et al., J. Dent. Res. In press). The slowest fiber recorded had a maximum shortening velocity (V₀) value of 0.027 muscle lengths·s^-1, several times slower than the slowest type I fibers previously measured in humans. By contrast, human limb muscle controls produced V₀ measurements comparable to previously published results. Due to the lack of correlation between V₀ and MyHC isoform this study sought to determine if other proteins regulate contraction in type I fibers from human masseter. Methods: In single fibers from masseter, V₀ was determined using the slack test method and MyHC and Myosin Light Chain (MyLC) content were determined using glycerol-SDS-PAGE and standard discontinuous SDS-PAGE respectively. Results: Our results found that type I MyHC fibers with exceedingly slow unloaded shortening velocities also express the embryonic MyLC₁ isoform. Previously the tissue specific expression of this isoform was thought to be in developing muscle (embryonic MyLC₁) and cardiac muscle (cardiac MyLC₁). Conclusions: This new finding helps to explain the unusually slow V₀ in masseter type I fibers. Further it demonstrates for the first time in humans that relative content of MyLC isoforms does correlate to V₀, as has been demonstrated for rat hind limb muscle (Bottinelli et al., 1994).