Transformation of ultrastructural type of fast-twitch muscle fibres after cross-innervation by tetrodotoxin-blocked slow muscle nerve.


It is shown in rats that cross-innervation of the fast-twitch muscle extensor digitorum longus (EDL) by the nerve of the slow-twitch muscle soleus (SOL) can cause a complete transformation of the Z-band width from the fast to the slow muscle fibre type within two weeks and that it can still induce such a transformation, though usually less completely, even when impulse activity is excluded by chronical tetrodotoxin (TTX) block of the nerve. This provides a piece of clear-cut evidence for the presence of some activity-independent trophic factor in the neural determination of skeletal muscle fibre type.

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    ABSTRACT: In normal (untreated) rats the mean length ratio of postsynaptic to presynaptic membrane was 2.7 +/- 0.8 for neuromuscular junctions of slow-twitch soleus muscle fibres and 4.2 +/- 1.0 for neuromuscular junctions of fast-twitch extensor digitorum longus muscle fibres; this difference was significant (P less than 0.001). After experimental double innervation by fast and slow muscle nerves for four months, the ratio was (1) 2.9 +/- 0.8 for the original slow-twitch fibre end-plate and 2.8 +/- 0.8 for the newly established one, both not significantly different from that of the normal slow-twitch fibres; and (2) 2.2 +/- 0.5 for the original fast-twitch fibre end-plate and 2.2 +/- 0.7 for the newly established one, both significantly smaller than that of the normal fast-twitch fibres (P less than 0.001). This means that the double innervated slow-twitch muscle fibres retained their original neuromuscular junction type, whereas the doubly-innervated fast-twitch muscle fibres underwent a dramatic transformation of their neuromuscular junction from the fast-muscle to the slow-muscle type. In both doubly innervated fibres, the ultrastructural characteristics of neuromuscular junctions, whether altered or not, were identical at both end-plate regions.
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    ABSTRACT: The purpose of the present study was to analyse the changes of macromolecular organizations in nonjunctional sarcolemmas of different types of skeletal muscle fibres after cross-innervation. In normal rats the mean density of square arrays (6 nm particles organized in orthogonal arrays) was 9.02 / μm2 for the nonjunctional sarcolemmas of fast-twitch extensor digitorum longus (control EDL, CE) muscle fibres and 0.34 / μm2 for the nonjunctional sarcolemmas of slow-twitch soleus (control SOL, CS) muscle fibres. After cross-innervation between the fast-twitch EDL and slow-twitch SOL muscle fibres by slow and fast muscle nerves respectively for three months, the mean density was 0.45/ μm2 for the nonjunctional sarcolemmas of the operated EDL (OE) and 8.3/ μm2 for the nonjunctional sarcolemmas of the operated SOL (OS), This indicates that the cross-innervation causes a reciprocal transformation of the number and distribution of such macromolecular organizations in the electrically excitable nonjunctional sarcolemmas.
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