Phosphocreatine as a marker of contractile activity in single muscle fibres

ATP and phosphocreatine (PCr) were measured in randomly selected single fib res from control, 1- and 8-day low-frequency-stimulated rabbit tibialis ant erior muscles. The fibres were classified according to their myosin heavy c hain (MHC) complement as type I, IIA or IID. In 1-day stimulated muscle, wh ich has previously been shown to exhibit a steep decline in force output, t wo fibre populations could be distinguished according to either normal or m arkedly depressed PCr levels. The fibre population exhibiting normal PCr le vels encompassed a major fraction (65%) of type IID fibres and a minor frac tion (35%) of IIA fibres. The population with reduced PCr levels comprised type I fibres (congruent to 50% reduced), the majority of type IIA fibres ( congruent to 80% reduced), and a minor fraction of type IID fibres (congrue nt to 70% reduced). Levels of ATP were unaltered in type I and IIA fibres, but were congruent to 20% reduced in those IID fibres that exhibited low PC r levels. Assuming that those fibres that displayed reduced PCr levels were contracting, the IID and IIA fibres with normal PCr levels were regarded a s metabolically recovering, non-contracting fibres. As previously shown, th ese fibres are transiently refractory during the early phase of low-frequen cy stimulation. After 8 days of chronic low-frequency stimulation, when for ce was shown to rise again, most fibres appeared more uniform with regard t o reduced PCr and ATP levels. Our results suggest that PCr can be used as a sensitive measure of the degree of activity in single-fibre studies.