A histochemical study of lateral longissimus muscle in rat

Rat axial muscle previously has not been studied histochemically. We were interested in determining the fiber composition and fiber distribution in rat lateral longissimus (LL), the large epaxial dorsiflexor muscle active during sexual posturing in the female rat and to determine if estrogen replacement in ovariectomized rats would affect the histochemical profile. Staining for ATPase after acid preincubation at pH 4.5, pH 4.35, and after alkaline preincubation at pH 9.4 and staining for NADH-TR revealed that rat LL contains the three major types of fibers present in most mammalian hind limbs: fast-twitch glycolytic (FG); fast-twitch oxidative-glycolytic (FOG); and slow-twitch oxidative (SO). The muscle contains predominantly FG fibers; SO fibers are segregated superficially from L2--L6 where they comprise from 11 to 18% of the fiber population, and in an oxidative compartment in the medial deep region of L5 where they comprise 62% of all fibers. In the medial deep region of L5 most of the remaining fast fibers also contain oxidative enzyme. Spindles are most highly concentrated in this oxidative region of L5. Estrogen treatment did not affect the relative number, distribution, or diameter of the three muscle fiber types in rat LL. The concentration of SO and FOG fibers and spindles localized in the region of the lumbosacral joint is discussed by contrasting forceful movements (e.g., rump elevation during sexual behavior) with normal postural regulation.