Meeting Abstract
Skeletal muscles play integral roles in a variety of physiological processes including locomotion, respiration, and thermoregulation. Our current model of muscle organization is that whole muscles are subdivided into discrete motor units, each contributing its specific physiological properties. This model conveys the idea that whole skeletal muscles are mosaics of different fiber types, but that the fiber types themselves are discrete. Hybrid muscle fibers, co-expressing two of more myosin heavy chain (MHC) isoforms complicate this view of muscle organization. Hybrid fibers are essential intermediates in fiber type transformations that result from use and disuse, and they are also recognized as being significant components of normal muscles. Given their common occurrence, there is surprisingly little known about many aspects of the basic organization of hybrids. For example, the spatial arrangement of multiple MHC isoforms within single fibers has important implications for muscle function. The principle of muscle contraction is based on hundreds of sarcomeres shortening together in unison, so if faster MHCs are segregated into limited regions of a fiber, that part of the fiber could potentially stretch out the slower sarcomeres before they can contract. In the current study, we focused on young rat skeletal muscles during a period of rapid growth to determine how MHC isoforms are spatially arranged within single hybrid fibers. During this period, a variety of hybrid fiber types are common within several muscles of the lower limbs. We present biochemical and anatomical data demonstrating that fiber type asymmetries are common at this stage of muscle development. These patterns suggest that these asymmetries may be a normal occurrence in skeletal muscles, and that they may be more prevalent in rapidly growing muscles.
