Control of Myosin Isozymes during Myogenesis in the Rat
Like most contractile proteins, the myosin molecule exists in a number of polymorphic forms which show specificity among muscles (365, 366) and among muscle fibers (367, 368) with different physiological and metabolic properties. For example, fast and slow twitch fibers contain both different myosin heavy chain and myosin light chain complements; and among the fast twitch fibers, IIa and IIb fibers contain myosins with qualitatively distinct myosin heavy chains, but similar myosin light chains (369, 370). These distinct properties within individual adult fast and slow fibers are partly the result of innervation by different types of motoneurons, since cross innervation of fast and slow muscles causes a reciprocal transformation of the muscles’ properties (371). Eccles et al (372) have demonstrated that fast and slow motoneurons have different frequencies of impulse activity; and in fact, the effects of cross innervation of a fast muscle with a slow motoneuron can be mimicked by chronic stimulation of the fast muscle’s own intact motoneurons at 5–10 Hz, the frequency of activity of a slow motoneuron (373, 374).
KeywordsIodine Electrophoresis Testosterone Hypothyroidism Deconvolution
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