Histochemical profiles of rat soleus intrafusal fibres after chronic exercise.

Intrafusal fibres from the rat soleus were investigated for representative histochemical profiles in sedentary animals and animals chronically exercised for 17 weeks on a treadmill. The pattern of myosin adenosine triphosphatase (ATPase) activity in the polar region revealed three intrafusal fibre types: (1) myosin ATPase-dark (MD) fibres, alkali- and acid-stabile; (2) myosin ATPase-light (ML) fibres, alkali- and acid-labile; and (3) myosin ATPase-reversible (MR) fibres, alkali-stabile and acid-labile. The three fibre types were correlated with the level of reduced NADH diaphorase activity, with MR, ML and MD fibres staining dark, moderate and light, respectively. In the equatorial region the morphological features of representative ML and MD fibres revealed that they were nuclear bag fibres, while representative MR fibres were identified as nuclear chain fibres. The MR fibres in the exercised animals had higher levels of myosin ATPase alkaline stability and acid lability than MR fibres in the sedentary animals, suggesting the MR fibre profiles are selectively influenced by chronic exercise. The mean cross-sectional area of MR fibres from the exercised animals was significantly less than the MR fibres from the sedentary animals. In contrast to the effect of endurance training on NADH diaphorase activity in extrafusal muscle fibres, there was evidence of less activity in the MD fibres of the exercised animals.