RESUMO
To further elucidate the pattern of MHC isoform expression in skeletal muscles of large mammals, in this study the skeletal muscles of brown bear, one of the largest mammalian predators with an extraordinary locomotor capacity, were analyzed. Fiber types in longissimus dorsi, triceps brachii caput longum, and rectus femoris muscles were determined according to the myofibrillar ATPase (mATPase) histochemistry and MHC isoform expression, revealed by a set of antibodies specific to MHC isoforms. The oxidative (SDH) and glycolytic enzyme (alpha-GPDH) capacity of fibers was demonstrated as well. By mATPase histochemistry five fiber types, i.e., I, IIC, IIA, IIAX, IIX were distinguished. Analyzing the MHC isoform expression, we assume that MHC-I, -IIa, and -IIx are expressed in the muscles of adolescent bears. MHC-I isoform was expressed in Type-I fibers and coexpressed with presumably -IIa isoform, in Type-IIC fibers. Surprisingly, two antibodies specific to rat MHC-IIa stained those fast fibers, that were histochemically and immunohistochemically classified as Type IIX. This assumption was additionally confirmed by complete absence of fiber staining with antibody specific to rat MHC-IIb and all fast fiber staining with antibody that according to our experience recognizes MHC-IIa and -IIx of rat. Furthermore, quite high-oxidative capacity of all fast fiber types and their weak glycolytic capacity also imply for MHC-IIa and -IIx isoform expression in fast fibers of bear. However, in adult, full-grown animal, only MHC-I and MHC-IIa isoforms were expressed. The expression of only two fast isoforms in bear, like in many other large mammals (humans, cat, dog, goat, cattle, and horse) obviously meets the weight-bearing and locomotor demands of these mammals.