alpha-Actinins are actin-binding proteins, and two isoforms (alpha-actinin-2 and -3) are major structural components of the sarcomeric Z line in mammalian skeletal muscle. Based on human and knockout mice studies, alpha-actinin-3 is thought to be associated with muscle force output and high contraction velocities. However, fiber-type specific expression of alpha-actinin isoforms is not well understood and may vary among species. In this study, we investigated the expression of alpha-actinin isoforms and the difference between fiber types in rat skeletal muscle and compared it with those of humans and mice from previous reports. Soleus and plantaris muscles were analyzed immunohistochemically to identify muscle fiber types and alpha-actinin protein expression. alpha-Actinin-2 was stained in all muscle fibers in both the soleus and plantaris muscles; i.e., all alpha-actinin-3 co-expressed with alpha-actinin-2 in rat skeletal muscles. The proportions of alpha-actinin-3 expression, regardless of fiber type, were significantly higher in the plantaris (75.8 +/- 0.6%) than the soleus (8.0 +/- 1.7%). No alpha-actinin-3 expression was observed in type I fibers, whereas all type Ilx+b fibers expressed a-actinin-3. alpha-Actinin-3 was also expressed in type Ila fibers; however, approximately 75% of type Ila fibers were not stained by alpha-actinin-3, and the proportion varied between muscles. The proportion of alpha-actinin-3 expression in type Ila fibers was significantly higher in the soleus muscle than the plantaris muscle. Our results showed that fiber-type specific expression of alpha-actinin isoforms in rats is more similar to that in humans compared to that of the mouse, whereas the proportion of alpha-actinin-3 protein varied between muscles. (C) 2012 Elsevier Inc. All rights reserved.