Comparative study of two classifications of muscle fibres: Consequences for the photometric determination of glycogen according to fibre type in red and white muscle of the pig.

The present experiment was designed to compare two classifications of myofibres in samples of muscles longissimus lumborum (LL) and semispinalis capitis (SC) taken at slaughter from 56 Large White pigs. Comparisons were made for the percentage and histological characteristics of the different types of myofibres and for the photometric determination of glycogen using periodic acid schiff (PAS) staining. Computerized image analysis was used (i) to classify the fibres on the basis of myosin ATPase activity [Brooke and Kaiser (1970) Arch. Neurol.23, 3669; types I, IIA and HB], or by combination of myosin ATPase and succino-dehydrogenase (SDH) activities [Ashmore and Doerr (1971) Exp. Neurol.31, 408: types βR, αR, and αW] and (ii) to compute the PAS staining luminance in individual fibres. Glycogen and related metabolites were determined in muscle homogenates using enzymatic techniques. The LL showed significantly higher glycogen and lactate contents than the SC and a significantly lower average PAS staining luminance (indicative of higher glycogen content). The LL was composed predominantly of fast-twitch (89%) and white (77.3%) fibres. The SC contained mainly red (61.1%) and fast-twitch (59.3%) fibres. In both muscles, the comparison of the two classifications indicates that all type I fibres showed high SDH activity and were therefore typed as βR in Ashmore and Doerr's classification. However, in the LL, approximately 7% of the type IIB fibres showed a marked SDH activity and were thus typed as αR in Ashmore and Doerr's classification. Furthermore, significant differences were found between these two types for the cross sectional area of the fibres. In the SC, no significant differences were found for the pairwise comparisons of corresponding types between the two classifications. According to the photometric determination, fast-twitch glycolytic fibres of the LL showed significantly higher glycogen content than red (fast- and slow-twitch) fibres, whereas in the SC, slow-twitch fibres had significantly lower glycogen levels than fast-twitch (red and white fibres). Corresponding types between the two classifications showed similar glycogen levels.