Glycogen depletion in intrafusal fibres in rats during short-duration high-intensity treadmill running

Abstract

Aim: The recruitment patterns of the intrafusal and extrafusal fibres in the soleus (SOL) and extensor digitorum longus (EDL) muscles of rats were investigated during brief‐intensity exercise by assaying their glycogen content histochemically. Methods: Six adult male rats were assigned to each of four groups that ran up a 6° incline on a motor‐driven treadmill, at 40 m min⁻¹ for either 0.5, 1, 2, or 4 min. Six adult male rats in the control group did not run. Extrafusal and intrafusal fibres were classified by myosin ATPase staining. Optical densities for glycogen content were evaluated in serial periodic acid Schiff (PAS) stained‐sections from the B and C regions of intrafusal fibres. Results: The glycogen content of type IIA fibres in the SOL and EDL muscles decreased significantly in the early phase of exercise whereas the glycogen content of type I fibres in these muscles decreased later than that of type IIA fibres. The glycogen content of bag₂ fibres decreased after 1 min of exercise in the SOL muscle and after 2 min of exercise in the EDL muscle. On the other hand, the glycogen content of bag₁ and chain fibres decreased significantly after 2 min in the SOL muscle but not in the EDL muscle. Conclusion: The results suggest that during brief‐intensity exercise, as the glycogen content of type IIA fibres is reduced earlier than that of type I fibres, bag₂ fibres are most important early in this type of exercise.