UNIQUE INTRAFUSAL AND EXTRAOCULAR MUSCLE FIBERS EXHIBITING DUAL ACTOMYOSIN ATPASE ACTIVITY

At least two forms of actomyosin adenosine triphosphatase (ATPase) have been demonstrated in mammalian skeletal muscles. One form is relatively alkalistabile: acid-labile, and is found in most muscle fibers of fast-twitch muscles, while the other is relatively acid-stabile: alkali-labile, and is present in the majority of fibers of slow-twitch muscles. Each singly efferented muscle fiber of the general somatic musculature usually possesses only one of the two enzyme forms. A histochemical survey of the intrafusal and extraocular musculature of the rat indicates that a substantial minority of the fibers comprising these muscles contain both forms of actomyosin ATPase (dual-activity). Many fibers of these specialized muscles are known to have complex innervations and to exhibit tonic, graded contractions. In light of the known neural regulation of actomyosin ATPase, it may be that dual enzyme activity is a consequence of dual innervation by efferents having qualitatively different regulatory capabilities. © 1969.