Histochemical fiber types characteristics in the normal and the persistent levator ani muscle of the rat

Morphological features of the persistent levator ani (LA) muscle of the female rat normally undergoing involution but maintained by application of testosterone propionate to newborn animals were compared with that of the normal LA muscle of untreated male rat. The two muscles differ in number and size of muscle fibers. Using histochemical methods for "myofibrillar" adenosine triphosphatase (ATPase), mitochondrial α-glycerophosphate dehydrogenase (α-GPDH) and succinic dehydrogenase (SDH) the distribution of these enzymes in individual muscle fiber types was studied. ATPase and α-GPDH activities show a homogenously positive reaction in the muscle fibers of the male rat, whereas a small portion of fibers with low activity is found in the persistent muscle of female rat. The most pronounced difference concerns SDH activity, i.e. two kinds of fibre types are barely discernible with prolonged incubation in LA muscle of male rat, but three basic fibre types (white, intermediate and red) are clearly distinguished in LA muscle of the female rat. The results are discussed in relation to neural and hormonal influences on histochemical features in cross-striated muscle.