Influence of intermittent long-term stimulation on contractile, histochemical and metabolic properties of fibre populations in fast and slow rabbit muscles.

Slow (m.soleus) and fast (m.tibialis anterior) muscles of the rabbit were subjected to indirect long-term intermittent stimulation (3 weeks, 8 hrs daily) with a frequency pattern of 10 imp/sec. Whereas no changes were observed in case of the slow muscle, stimulation induced profound changes in the fast tibialis anterior muscle. These consisted in a rearrangement of the enzyme activity pattern of energy-supplying metabolism, e.g. decrease in glycogenolytic and glycolytic enzyme activities and severalfold increase in key enzymes of aerobic endoxidation of substrates in beta-oxidation and the citric acid cycle. Concomitant with the increase in aerobic oxidative capacity, there was an increased resistance to fatigue. Histochemical studies revealed a strong increase in mitochondria of all fibres. The bimodal distribution of fibre cross-sectional area in the normal tibialis anterior muscle was changed by stimulation into a more homogeneous population of fibres with a smaller cross-sectional area. Despite a 50% increase in time to peak of isometric twitch contraction no changes were observed in the fibre population with regard to myofibrillar ATPase reaction in quantitative evaluation of whole cross-sections of the muscles. The percentage of fibres histochemically classified as slow amounted to 2.8% and 3.1% in control and stimulated tibialis anterior muscle. Nevertheless the data suggest a transformation of the fibre population under the influence of long-term intermittent stimulation.