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Maximum Shortening Velocity and Myosin Heavy-chain Isoform Expression in Human Masseter Muscle Fibers

The Department of Orthodontics & Dentofacial Orthopedics, University of Pittsburgh, 3501 Terrace Street, Pittsburgh, PA 15261-1932, jjs6{at}pitt.edu

C.A. Branden

The Department of Orthodontics & Dentofacial Orthopedics, University of Pittsburgh, 3501 Terrace Street, Pittsburgh, PA 15261-1932

M.J. Horton

The Department of Orthodontics & Dentofacial Orthopedics, University of Pittsburgh, 3501 Terrace Street, Pittsburgh, PA 15261-1932

D.S. Carlson

Baylor College of Dentistry, Texas A&M University System Health Science Center

J.J. Sciote

The Department of Orthodontics & Dentofacial Orthopedics, University of Pittsburgh, 3501 Terrace Street, Pittsburgh, PA 15261-1932

While human masseter muscle is known to have unusual co-expression of myosin heavy-chain proteins, cellular kinetics of individual fibers has not yet been tested. Here we examine if myosin heavy-chain protein content is closely correlated to fiber-shortening speed, as previously reported in other human muscles, or if these proteins do not correlate well to shortening speeds, as has been demonstrated previously in rat muscle. Slack-test recordings of single, skinned human masseter fibers at 15°C revealed maximum shortening velocities generally slower and much more variable than those recorded in human limb muscle. 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 with previously published results. Analysis by gel electrophoresis found 63% of masseter fibers to contain pure type I MyHC and the remainder to co-express mostly type I in various combinations with IIA and IIX isoforms. Vo in masseter fibers forms a continuum in which no clear relationship to MyHC isoform content is apparent.

Key Words: V0 • slack test • skeletal muscle.

Journal of Dental Research, Vol. 80, No. 9, 1845-1848 (2001)
DOI: 10.1177/00220345010800091401


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