Altered balance of proteinase inhibitors in atrophic muscle after denervation.

Skeletal muscle denervation leads to an increase of proteolytic activity, which is also favoured by reduced levels of alpha1 antichymotrypsin and nexin II, two serine-proteinase inhibitors normally acting at the neuromuscular junction. In the present experiments we extended our investigation to other muscular proteinase inhibitors after denervation. In all muscles examined (soleus, plantaris, extensor digitorum longus) specific immunoreactivity for alpha2macroglobulin (alpha2M) and alpha1proteinase inhibitor (alpha-1-antitrypsin, ATI) was distributed in peri-endomysial structures as well as in small patches inside the fibres. By contrast, inter-alpha-trypsin inhibitor (ITI) was mainly localized in the extracellular matrix. These localization patterns did not change substantially in 15-days denervated muscles. Dot-blot analysis revealed a small decrease (about 15%) of alpha2M in 15-days denervated muscles, while ATI and ITI specific activities were substantially unchanged. RT-PCR allowed us to detect the above protease inhibitor mRNAs in normal muscle homogenates. Denervation atrophy induced by section of the sciatic nerve resulted in a remarkable reduction of (2macroglobulin mRNA (60%) and ITI (30%), but not ATI, as measured by computer-assisted semiquantitative densitometry of electrophoresed RT-PCR bands. The marked decrease of alpha2M we have detected in denervated muscle may be responsible, at least in part, for the proteolytic increase which is known to occur in skeletal muscle during denervation atrophy.