ATYPICAL MYOSIN HEAVY-CHAIN IN RAT LARYNGEAL MUSCLE

The myosin content of rat posterior cricoarytenoid and thyroarytenoid muscles was described by means of histochemical, immunohistochemical, and electrophoretic techniques. Laryngeal muscles were dissected and f rozen, together with other muscles (extraocular, diaphragm, extensor d igitorum longus, and soleus) for comparative purposes, then sectioned serially and stained: 1) histochemically for myofibrillar adenosine tr iphosphatase reactivity and 2) immunohistochemically for myosin heavy chain (MHC) content with six different antibodies. Other portions of t he muscle samples were electrophoresed by a glycerol sodium dodecyl su lfate polyacrylamide gel electrophoresis technique that separates the MHC protein into its specific isoforms. In electrophoretic comparison to limb muscles, the laryngeal muscles contained an additional MHC ban d we designated as type III, (type II laryngeal) MHC. On histochemical and immunohistochemical staining, no fibers from the thyroarytenoid m uscle and few fibers from the posterior cricoarytenoid muscle could be classified according to the standard fiber type categories establishe d for limb muscles (types I, IIA, IIB, and IIX). These laryngeal muscl e fibers appear to represent an atypical fiber type.