Metabolic modulation of muscle fiber properties unrelated to mechanical stimuli.

The effects of chronically increasing (creatine-fed) or decreasing (beta-guanidinopropionic acid [beta-GPA]-fed) high-energy phosphates for up to 8 weeks on daily voluntary activity levels, swimming endurance capacity, electromyogram (EMG) activity, and the morphological and metabolic properties of single fibers in the soleus and extensor digitorum longus (EDL) muscles in young rats were determined. High-energy phosphate, voluntary activity, and soleus-integrated EMG levels were lower in beta-GPA-fed rats than in control rats. Endurance capacity was higher at a relatively low intensity of swimming and lower at a relatively high intensity in beta-GPA-fed rats than in control rats. Muscle mass and fiber size were smaller, and the percentage of slow fibers was higher in the soleus and EDL of beta-GPA-fed rats than in control rats. Succinate dehydrogenase activity was higher in both the fast and slow fibers of the EDL of beta-GPA-fed rats than in control rats. Thus, a reduction in high-energy phosphates transformed some fast fibers toward a slow phenotype. Creatine supplementation had minimal effects: The only significant change was an increase in alpha-glycerophosphate dehydrogenase activity in the fast fibers of the EDL. These results indicate that the metabolic environment of a muscle fiber can influence the prominence of a given muscle fiber independent of the activity level of muscle.