Denervation enables the study of skeletal muscle properties in the absence of neural electrical input or the influence of nerve-derived trophic factors. The pattern of stimulatory electrical activity is of major importance in determining overall muscle properties in fast- and slow-type skeletal muscles, many contractile and biochemical properties of which revert to an intermediate state when neural influence is denied. One problem with studying denervated skeletal muscle is that experiments are usually carried out on bundles of intact fibres or on whole muscle preparations, meaning that events at the single fibre level can only be inferred. Here we have tracked changes in contractile properties, sarcoplasmic reticulum (SR) function and myosin heavy chain (MHC) isoform composition of mechanically skinned single muscle fibres taken from rat soleus and EDL muscles denervated for 0, 7, 21 or 50 days.

All experiments were carried out with approval from the Animal Ethics Committee of La Trobe University. Male Long Evans rats were anaesthetised with gaseous halothane, unilaterally denervated under aseptic conditions and allowed to recover. Untreated age-matched animals were used as controls. Animals were killed humanely by halothane overdose, and EDL and soleus muscles removed and placed under paraffin oil for single fibre isolation and mechanical skinning. Measurement of contractile activation parameters (pCa50, n_H, pSr50, etc.) was performed using previously described solutions and techniques (Fink et al. 1986). SR loading was determined as the ratio of Ca²⁺ released after a 2-min load in a solution of pCa 6.2 (0.5 mM EGTA) to that in a solution of pCa 7.10. MHC profiles of single muscle fibres and muscle homogenates were determined as previously reported (Bortolotto et al. 1999).

Results showed a significant (P < 0.05; Student’s unpaired t test) decrease compared to control in the SR loading ratio of EDL fibres following denervation, and an increase in that of soleus fibres such that the two became similar in 50-day denervated fibres. Compared to control, pCa50-pSr50 values for soleus fibres were more affected by denervation than were EDL fibres, with overall changes in soleus fibres towards fast-type fibre properties clearly apparent. The results can be explained in part by the increased number of hybrid muscle fibres after denervation.