Comparison of red and white muscles by cytophotometry of their muscle fibre populations.

Samples from two red muscles (vastus intermedius and vastus medialis) and two white muscles (biceps femoris and gluteus medius) were taken from four pigs. Serial transverse sections were reacted for ATPase and NADH oxidative activity. Sections were mapped with a projection microscope so that the staining intensity of individual fibres for the two reactions could be measured with a simple microscope photometer. Transmission values at 600 nm were converted to units of 0-10 for the range from darkest to lightest staining fibres on each section to cancel variation in staining intensity between sections. The aim of the study was to use simple cytophotometry instead of subjective judgement in the categorization of different histochemical types of muscle fibres. Cytophotometry enabled clear resolution of the major fibre types (types I and II using the ATPase reaction), partial resolution of more variable characteristics (NADH oxidative activity in type I and II fibres) and no resolution of subtle subtypes (IA and IB with the NADH oxidative reaction). However, between the major fibre types, cytophotometry revealed variable numbers of fibres with transitional characteristics. There were more of these fibres in red muscles. With sections reacted for ATPase, transmission values for low magnification fields containing 100 to 200 fibres were correlated (r = -0.91) with the ratio of type I:II fibres.