Fiber types in red and white segments of rat sternomastoid muscle

Abstract

AbstractThe contractile, histochemical and morphological properties of fibers from red and white rat sternomastoid segments have been studied. Previous mechanical experiments suggested that both segments were fast‐twitch and the present experiments were done to confirm this observation and to determine other essential characteristics of the fibers.The ATP'ase and succinic dehydrogenase (SDH) activities of fibers were determined using standard histochemical techniques. Seventy‐six percent of fibers in the red segment had the high ATP'ase and high succinic dehydrogenase activity typical of fast‐twitch‐red fibers. Other fibers in the red segment were typically fast‐twitch‐white (13%) or slow‐twitch‐red (11%). The average cross‐sectional area of red fibers was 2,104 m̈m2. Fibers in the white segment were mostly (84%) fast‐twitch‐white with intermediate ATP'ase activity and low to intermediate SDH activity. The remaining fibers in the white segment were histochemically typical of fast‐twitch‐red fibers. The average cross‐sectional area of white fibers was 5,109 m̈m2. Morphologically, red fibers had three times as many mitochondria as white fibers and a Z‐line thickness 38% greater than that of white fibers.Although fibers from the red segment were histochemically and morphologically different from fibers in the white segment, the isometric tension characteristics, i.e., contraction time, post‐tetanic potentiation and fatigueability, were very similar and were typical of fast‐twitch‐fatigueable preparations. Thus, histochemical and morphological characteristics of fibers in the sternomastoid muscle are not indicative of the mechanical properties of the fibers.