Evidence for three fast myosin heavy chain isoforms in type II skeletal muscle fibers in the adult llama (Lama glama)

Skeletal muscle fiber types classified on the basis of their content of dif ferent myosin heavy chain (MHC) isoforms were analyzed in samples from hind limb muscles of adult sedentary llamas (Lama glama) by correlating immunohi stochemistry with specific anti-MHC monoclonal antibodies, myofibrillar ATP ase (mATPase) histochemistry, and quantitative histochemistry of fiber meta bolic and size properties. The immunohistochemical technique allowed the se paration of four pure (i.e., expressing a unique MHC isoform) muscle fiber types: one slow-twitch (Type I) and three fast-twitch (Type II) phenotypes. The same four major fiber types could be objectively discriminated with tw o serial sections stained for mATPase after acid (pH 4.5) and alkaline (pH 10.5) preincubations. The three fast-twitch fiber types were tentatively de signated as IIA, IIX, and IIB on the basis of the homologies of their immun oreactivities, acid denaturation of their mATPase activity, size, and metab olic properties expressed at the cellular level with the corresponding isof orms of rat and horse muscles. Acid stability of their mATPase activity inc reased in the rank order IIA > IIX > IIB. The same was true for size and gl ycolytic capacity, whereas oxidative capacity decreased in the same rank or der IIA > IIX > IIB. In addition to these four pure fibers (I, IIA, IIX, an d IIB), four other fiber types with hybrid phenotypes containing two (I + I IA, IIAX, and IIXB) or three (IIAXB) MHCs were immunohistochemically deline ated. These frequent phenotypes (40% of the semitendinosus muscle fiber com position) had overlapped mATPase staining intensities with their correspond ing pure fiber types, so they could not be delineated by mATPase histochemi stry. Expression of the three fast adult MHC isoforms was spatially regulat ed around islets of Type I fibers, with concentric circles of fibers expres sing MHC-IIA, then MHC-IIX, and peripherally MHC-IIB. This study demonstrat es that three adult fast Type II MHC isoproteins are expressed in skeletal muscle fibers of the llama. The general assumption that the very fast MHC-I IB isoform is expressed only in small mammals can be rejected.