Correspondences Between Physiological-Metabolic Fibre Typing, ATPase-Fibre Typing and Differentiation of Myosin Isoforms
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The cytophotometrical measurements of enzyme activities presented in this study were performed in defined muscle fibres which were physiologically-metabolically classified. Changes in enzyme activities are related to a defined fibre type of this classification system. Besides this physiological-metabolic typing, there is the ATPase-typing which is based on different myosin isoforms in different muscle fibres. As already mentioned in Chap. 3, such ATPase-typed fibres are not clearly metabolically defined. Changes of myosin isoforms, but not of enzyme activities, can be clearly related to ATPase-fibre types. It is a matter of two different classification systems, whose fibre type designations are not changeable in any way. Each of the two systems characterizes specific properties of muscle fibres. Comparing metabolic properties and the content of myosin isoforms of a fibre type, correspondences between the different designations of a fibre type can be detected. It was already shown, see Chap. 3, that SO fibres are identical with the ATPase-fibre type I. In contrast, FOG and FG fibres were not clearly exchangeable with ATPase-fibre types IIA and IIB. However, our own studies and studies of Schiaffino et al. (1990) and Larsson et al (1991) suggest: If we consider the phenomenon of heterogeneity of FOG fibres on the one hand and of heterogeneity of IIB fibres on the other hand, it should be possible to find further correspondences between the two classification systems. In detail, the studies of Schiaffino et al. (1990), Bottinelli and Reggiani (2000) and Larsson et al. (1991) revealed three populations of type II fibres which differ in their heavy chains myosin isoforms (MHC). This means that three fast MHC isoforms were detected on the protein level.