Are hybrid fibers a common motif of canine laryngeal muscles? Single-fiber analyses of myosin heavy-chain isoform composition.

BACKGROUND The canine lateral cricoarytenoid muscle contains a large proportion of muscle fibers that coexpress various combinations of myosin heavy-chain isoforms (ie, so-called hybrid fibers). OBJECTIVE To test the hypothesis that hybrid fibers are a common motif throughout laryngeal muscles. DESIGN The posterior cricoarytenoid, canine cricothyroid, and thyroarytenoid muscles were removed from 5 beagle dogs. The posterior cricoarytenoid and canine cricothyroid muscles were each dissected into horizontal, oblique, and rectus regions. The thyroarytenoid was separated into medial and lateral regions. Approximately 40 single fibers were microdissected from each region ( approximately 1800 total fibers were sampled) and placed into a denaturing sample buffer. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was then used to separate the individual myosin heavy-chain isoforms. RESULTS Each laryngeal muscle contained hybrid fibers; however, the types and proportions of hybrid fibers were clearly muscle specific. Within a given muscle, there were relatively minor regional differences in the types and proportions of hybrid fibers. CONCLUSION If the myosin heavy-chain isoform composition of a single fiber can be used as a "physiological marker," then the extent of hybridism may reveal the diversity of activity required of a given laryngeal muscle.