Changes in rodent muscle fibre types during post-natal growth, undernutrition and exercise.

1. Using histochemical staining methods for myosin ATPase oxidative and glycolytic enzymes, three major types of muscle fibre could be identified in the skeletal muscle of hamsters and mice. 2. Muscle fibre counts showed that the proportions of the different fibres were not entirely stable with age. In the hamster biceps brachii which is predominantly composed of ATPase-high fibres there was a decrease in the number of ATPase-low fibres. In the soleus muscle which is predominantly composed of ATPase-low fibres there was a decrease in ATPase-high fibres with age. 3. Although there was a change in the proportions of fibre types there was no change in the total number of fibres within the muscles with age. It is suggested that some reinnervation may take place during growth and that this is why the less dedominant fibre type decreases. 4. The response of the different fibre types to partial starvation was studied. The ATPase-high fibres showed the greatest decrease in size. Of these, the ATPase-high glycolytic type responded more than the ATPase-high oxidative type. The effects of the under-nutrition on the different fibre types were found to be completely reversible. Starvation did not affect the total number of fibres or the numbers of any fibre type. 5. The response of the different types to high intensity exercise (weight lifting) was studied. This type of exercise resulted in hypertrophy of all three major fibre types. However, the extent of the response varied according to the fibre type and the exact nature of the exercise. In most cases the ATPase-high fibres underwent hypertrophy more readily than the ATPase-low fibres. Where distinction was made between the two types of ATPase-high fibres, the ATPase-high glycolytic were found to hypertrophy more than the ATPase-high oxidative fibres. The effects of post exercise recovery (return to relative inactivity) were also studied and the changes in size of the fibres were found to be completely reversible.