RESUMO
Levels of calpain I and calpain II mRNAs were analyzed at different stages of rat skeletal myoblast differentiation using a competitive polymerase chain reaction method. The results provide evidence that only calpain II mRNAs were present in significant quantities on the second day while calpain I mRNAs were identified on the fourth day of differentiation. If there is no compelling reason to believe that synthesis of calpains I and II is regulated at the level of mRNA, our results suggest that calpain II will be more particularly involved in Ca(2+)-mediated events accompanying myoblast fusion. On the other hand, calpain I, because of its later appearance may probably act on specific substrates such as myofibrillar proteins, associated myofibrillar proteins or the control of enzyme metabolism. Added factors such as insulin, which is known to induce enhancement of myoblast growth or myoblast fusion, had a significant effect on the amounts of calpain I and II mRNAs. In the presence of TGF-beta, a potent inhibitor of myoblast fusion, calpain I and II mRNAs were decreased. These results confirm first that a Ca(2+)-dependent proteolytic system is positively correlated with myoblast fusion (via calpain II) and second, that transcriptional regulation of calpains I and II may be negatively modulated during myoblast differentiation.
