Decreased acetylcholine receptor content in denervated skeletal muscles infused with nerve extract

Putative effects of nerve extract on carbonic anhydrase III expression in rat muscles

Carbonic anhydrase III (CA III), the predominant CA isoform in skeletal muscle is very sensitive to neuronal influences. We aimed to determine whether CA III expression could be influenced by neurotrophic factor(s) present in sciatic nerve extract (SNE). Intact muscles were thus compared with denervated soleus (SOL), extensor digitorum longus (EDL), and tibialis anterior (TA) muscles injected daily for 7 days with saline solution (SS) or with SNE. CA III activity was significantly increased in SS-treated EDL and TA muscles compared to control (CTR), while SNE injections partially prevented this increase. There was no significant difference for CA III activity in the SOL between CTR, SS, and SNE groups. The CA III mRNA increase observed in response to denervation was reduced by 40% in SNE-treated EDL and TA muscles. While SOL CA III mRNA level was not affected by denervation, a 52% decrease was observed with SNE. We concluded that neuronal modulation of CA III expression in type II fibers may involve a neurotrophic component.

Acetylcholine receptor gene expression in skeletal muscle of chronic ethanol-fed rats

The expression of the neuromuscular acetylcholine receptor (AChR) alpha-subunit gene was evaluated in soleus muscles from an animal model of chronic alcoholism. At 8 weeks of age, test rats were placed on a nutritionally complete liquid diet containing 6.7% ethanol (v/v). Age- and weight-matched control rats were pair-fed an isocaloric liquid diet. After a 16-week diet period, soleus muscles were obtained and total RNA and poly(A)+ RNA were isolated. Muscle RNA levels from ethanol-fed and control rats were comparable. AChR alpha-subunit mRNA was detected by hybridization of muscle poly(A)+ RNA with a 32P-labeled, complementary riboprobe. The steady-state level of AChR alpha-subunit mRNA was reduced by 39% (p less than 0.001) in soleus muscles from the ethanol-fed rats as compared to pair-fed controls. These results suggest that the expression of the AChR alpha-subunit gene is down-regulated after chronic ethanol exposure at a transcriptional or posttranscriptional level.

Temporal relationship between nerve-stump-length-dependent changes in the autophosphorylation of a cyclic AMP-dependent protein kinase and the acetylcholine receptor content in skeletal muscle

The acetylcholine receptor (AChR) content and the autophosphorylation of the regulatory subunit of cyclic AMP-dependent protein kinase type II (R-II) were evaluated in rats soleus muscles at 24, 30 and 66 hr after surgical denervation by cutting the nerve at a short distance (short-nerve-stump) and at a long distance (long-nerve-stump) from the muscle. AChR content was based on the specific binding of [125I]alpha-bungarotoxin (BUTX); changes in the autophosphorylation of R-II were based upon the predominant in vitro 32P-phosphorylation of a 56-Kd soluble protein in cytosolic fractions of solei. The AChR content and the 32P-autophosphorylation of R-II were increased in samples from short-nerve-stump solei, but not from long-nerve-stump solei, after a denervation-time of 30 hr. This nerve-stump-length dependency indicates that the two denervation effects are not related to the immediate halt of impulse-evoked muscle contractility. Furthermore, the results show that alterations in the 32P-autophosphorylation of R-II occurred before, as well as whenever, increases in the AChR content were found. Speculatively, this temporal relationship may be significant with respect to the potential role of R-II in gene expression.

Role of cholinergic neuromuscular transmission in the neuroregulation of the autophosphorylatable regulatory subunit of cyclic AMP-dependent protein kinase type II and the acetylcholine receptor content in skeletal muscle

Previously, we found that the in vitro [32P]-autophosphorylation of the regulatory subunit of cyclic AMP-dependent protein kinase type II in rat soleus muscles is subject to a nerve-stump-length-dependent neuroregulation which indicates that this event is dependent upon some neural signal other than the impulse-directed release of acetylcholine. In this investigation, tetrodotoxin and alpha-bungarotoxin were also administered to further differentiate the effect of impulse-directed and spontaneously released acetylcholine upon this event and also upon the appearance of new acetylcholine receptors as measured by the binding of radioiodinated bungarotoxin. A 24 h blockade of cholinergic transmission with either neurotoxin did not change the phosphorylation level of the regulatory subunit, while a significant increase is observed when solei are surgically denervated for this period. The phosphorylation level and also the acetylcholine receptor content were increased only after more prolonged (48-96 h) muscle inactivity was produced with the neurotoxins. However, then their effects may not be solely related to alterations in cholinergic transmission. Taken together, our results do not support a trophic role for spontaneously released acetylcholine with respect to the two neurotrophic events studied.