Is fast fiber innervation responsible for increased acetylcholinesterase activity in reinnervating soleus muscles?

Molecular forms of acetylcholinesterase in the rat extensor digitorum longus and soleus muscles regenerating from notexin-induced necrosis

The activity of acetylcholinesterase molecular forms were examined after separation on sucrose gradients during notexin-induced necrosis and the following regeneration in rat extensor digitorum longus (EDL) and soleus (SOL) muscles. All forms dropped rapidly in both muscles in the first few days after single notexin injection. After a delay small globular forms (G1+G2) started to regenerate from day 7 and larger forms (G4 and A12) from day 10 in EDL. The A8 form which cannot be detected in normal EDL was present between day 7 and day 28. In SOL the recovery of AChE forms begun already on day 3. The small globular forms displayed a more rapid increase between day 3 and day 7 then the other forms. In SOL we observed a temporary overshooting peak at day 7 in the activity of all molecular forms. Both muscles recovered their normal AChE pattern by that time when muscle fibres regained their normal diameter (day 28). Most of the events of regeneration of AChE forms resembled those of normal myogenesis.

Effect of thyroid hormones on acetylcholinesterase mRNA levels in the slow soleus and fast extensor digitorum longus muscles of the rat

In the rat, the level of acetylcholinesterase messenger RNA in the typical slow soleus muscles is only about 20-30% of that in the fast extensor digitorum longus muscles. The expression of contractile proteins in muscles is influenced by thyroid hormones and hyperthyroidism makes the slow soleus muscle faster. The influence of thyroid hormones on the levels of acetylcholinesterase messenger RNA level in the slow soleus and fast extensor digitorum longus muscle of the rat was studied in order to examine the effect of thyroid hormones on muscle acetylcholinesterase expression. Hyperthyroidism was induced in rats by daily thyroid hormone injection or thyroid hormone releasing tablet implantation. Hind-limb suspension was applied to produce muscle unloading. Muscle denervation or reinnervation was achieved by sciatic nerve transection or crush. Acetylcholinesterase messenger RNA levels were analyzed by Northern blots and evaluated densitometrically. Hyperthyroidism increased the levels of acetylcholinesterase messenger RNA in the slow soleus muscles close to the levels in the fast extensor digitorum longus. The effect was the same in the unloaded soleus muscles. Acetylcholinesterase expression increased also in the absence of innervation (denervation), in the presence of changed nerve activation pattern (reinnervation), and under enhanced tonic neural activation of the soleus muscle (electrical stimulation). However, the changes were substantially smaller than those observed in the control soleus muscles. Enhancement of acetylcholinesterase expression in the soleus muscles by the thyroid hormones is, therefore, at last in part due to hormonal effect on the muscle itself. On the contrary, increased level of the thyroid hormones had no influence on acetylcholinesterase expression in the normal fast extensor digitorum longus muscles. However, some enhancing influence was apparent whenever the total number of nerve-induced muscle activations per day in the extensor digitorum longus muscle was increased. Thyroid hormones seem to be an independent extrinsic factor of acetylcholinesterase regulation in the slow soleus muscle.

Fibre type specific expression of Leu19-antigen and N-CAM in skeletal muscle in various stages after experimental denervation

Leu-19 antigen, which seems to be identical with neural cell adhesion molecule (N-CAM), plays a major role in the innervation of muscle cells, and in adult muscle appears after denervation and during regeneration of muscle fibres, where it acts as part of a signalling system increasing the probability of re-innervation. This combined enzyme-histochemical and immunohistochemical study examined whether this signalling process was regulated in a uniform or differential pattern for type 1 and type 2 muscle fibres. The subscapular nerve of 18 rabbits was transsected with subsequent complete denervation of the supraspinatus muscle. Leu-19 and N-CAM immunohistochemistry was performed 2 to 64 days after surgery. Whereas in normal muscle there are virtually no Leu-19/N-CAM positive muscle fibres; from day 2 after denervation an increasing proportion of fibres expressed Leu-19/N-CAM, prior to any neurogenic atrophy. In the early stage of denervation Leu19/N-CAM expression was confined to type 1 fibres. After 11 days nearly all fibres were Leu19/N-CAM positive irrespective of their fibre type. Sixty-four days after denervation type 1 fibres became Leu19/N-CAM negative, while atrophic type 2 fibres showed intensive staining. Thus, expression of Leu-19 antigenicity is differently regulated in both fibre types.

Fiber type and non-endplate acetylcholinesterase in normal and experimentally altered muscles

The non-endplate (sarcoplasmic) acetylcholinesterase (AChE) was investigated in eight different muscles of the rat. Serial consecutive sections were stained for AChE, myofibrillar ATPase (after alkaline and acid preincubation), and cytochrome C-oxidase. The following general correlation could be established: within a given muscle the sarcoplasmic AChE was highest in type IIB fibers, lowest in type I and intermediate in type IIA. Additionally, the intensity of the reaction was directly proportional to the size of the type IIA fibers. The distribution of sarcoplasmic AChE was correlated to the ATPase fiber types but was complementary to the cytochrome C-oxidase staining pattern. In single fiber preparations, accumulation of AChE at the myotendinous junction was found to occur in "cap-like" form exclusively in fibers with very low or absent sarcoplasmic AChE. To study the role of innervation in the expression of the sarcoplasmic AChE, we cross-reinnervated the extensor digitorum longus (EDL) muscle with the soleus (SOL) nerve and vice versa (X-EDL, X-SOL). In the X-EDL the sarcoplasmic AChE was transformed to that of a normal SOL as were also the ATPase and the cytochrome oxidase. Surprisingly, in the X-SOL the high AChE activity typical for a normal EDL was present after 3 weeks but decreased steadily to very low levels lacking any correlation with ATPase and cytochrome oxidase. The results suggest that the cytoplasmic AChE of the SOL muscle depends more on the load-bearing function of the muscle than on the imposed impulse pattern. There is additional evidence for a retrograde effect of the X-SOL upon its motoneurons.