Lactate dehydrogenase content related to cross-sectional area in muscle fibres

Cross-sectional areas and lactate dehydrogenase activity were determined of separate muscle fibres of mouse rectus femoris muscle in both ventral and dorsal parts of the muscle. The lactate dehydrogenase activity was measured quantitatively by microscope photometry using a tetrazolium staining in absence or presence of either vitamin K3 or phenazine methosulfate. If the observations are restricted to a relatively small area of the muscle cross section, one can distinguish categorically narrow ("red") and broad ("white) fibres with average values for cross sectional area and LDH content that depend on the part of the muscle chosen (higher LDH and smaller area in ventral part). However, if these data from the different parts or from the whole muscle are added together, no categorical distinction between the fibre types is allowed. On the basis of the observations a categorical division of muscle fibres within relatively large muscle areas or in a whole muscle is rather dubious. A relation is suggested between the subjective impression of different muscle fibre types in a field of observation with the number of motorunits present in that field.