INDUCTION OF A FAST-OXIDATIVE PHENOTYPE BY CHRONIC MUSCLE STIMULATION- MECHANICAL AND BIOCHEMICAL-STUDIES

We studied changes in the mechanical properties and myosin isoform com position of rabbit tibialis anterior muscles that were subjected to co ntinuous stimulation at 2.5 Hz for up to 12 wk. The effects of stimula tion at 2.5 Hz were less profound than those observed for the same dur ation of stimulation at 10 Hz (12). Stimulation at 10 Hz for 12 wk ind uced complete transformation to a slow-contracting muscle homogeneous in slow myosin isoforms; stimulation for the same period at 2.5 Hz res ulted in moderate changes in contractile speed and a very small increa se in the synthesis of slow myosin isoforms. On the other hand, the fa tigue resistance of muscles stimulated at 2.5 Hz was as great, in both isometric and dynamic fatigue tests, as that of the muscles stimulate d at 10 Hz. Thus entire fast skeletal muscles can be transformed to a state in which fast myosin isoforms continue to be synthesized, but th e oxidative capacity is sufficient to support sustained working at a h igher power output than that associated with slow muscle.