Effects of chronic stimulation on the metabolic heterogeneity of the fibre population in rabbit tibialis anterior muscle.

Chronic indirect stimulation (10 Hz) was performed on rabbit tibialis anterior muscle. Long-term stimulation (52-140 days) produced a transformation of the fast tibialis anterior into a slow red muscle as judged from the histochemistry of myofibrillar actomyosin ATPase, the pattern of myosin light chains and the thorough rearrangement of the enzyme activity pattern of energy metabolism. Activity levels of citrate synthetase (CS), malate dehydrogenase (MDH), succinate dehydrogenase (SDH), 3-hydroxy-acyl-CoA dehydrogenase (HAD), and lactate dehydrogenase (LDH) were determined quantitatively by either microbiochemical assays (CS, MDH, HAD and LDH) on microdissected, single fibres or by kinetic microphotometry on cross-sectioned fibres (SDH). The activity profiles of these enzymes displayed pronounced scattering in the fibre population of the unstimulated muscle. Despite a several fold increase in the activities of CS, MDH, SDH and HAD and a pronounced decrease in LDH, chronic stimulation failed to abolish the metabolic heterogeneity of the fibre population. It is possible that chronic indirect stimulation cannot produce uniformity of fibres because of continuing diverse natural activity of the motor units.